tag:blogger.com,1999:blog-138699032024-03-18T04:48:53.796-05:00nanoscale viewsA blog about condensed matter and nanoscale physics. Why should high energy and astro folks have all the fun?Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.comBlogger1348125tag:blogger.com,1999:blog-13869903.post-13402910796304358612024-03-07T13:27:00.002-06:002024-03-07T13:44:50.993-06:00APS March Meeting 2024, Day 4 and wrap-up<p>Because of the timing of my flight back to Houston, I really only went to <a href="https://meetings.aps.org/Meeting/MAR24/Session/S62" target="_blank">one session</a> today, in which my student spoke as did some collaborators. It was a pretty interesting collection of contributed talks. </p><p></p><ul style="text-align: left;"><li>The work that's been done on spin transport in multiferroic insulators is particularly interesting to me. A relevant preprint is <a href="https://arxiv.org/abs/2306.02185" target="_blank">this one</a>, in which electric fields are used to reorient \(\mathbf{P}\) in BiFeO3, which correspondingly switches the magnetization in this system (which is described by a complicated <a href="https://doi.org/10.1002/adma.202003711" target="_blank">spin cycloid order</a>) and therefore modulates the transmission of spin currents (as seen in ferromagnetic resonance). </li><li>Similarly <a href="https://meetings.aps.org/Meeting/MAR24/Session/S62.2" target="_blank">adding</a> a bit of La to BiFeO3 to favor single ferroelectric domain formation was a neat complement to this.</li><li>There were also <a href="https://meetings.aps.org/Meeting/MAR24/Session/S62.12" target="_blank">multiple</a> <a href="https://meetings.aps.org/Meeting/MAR24/Session/S62.9" target="_blank">talks</a> showing the utility of the spin Hall magnetoresistance as a way to characterize spin transport between magnetic insulators and strong spin-orbit coupled metals.</li></ul><div>Some wrap-up thoughts:</div><div><ul style="text-align: left;"><li>This meeting venue and environment was superior in essentially every way relative to last year's mess in Las Vegas. Nice facilities, broadly good rooms, room sizes, projectors, and climate control. Lots of hotels. Lots of restaurants that are not absurdly expensive. I'd be very happy to have the meeting in Minneapolis again at some point. There was even a puppy-visiting booth at the exhibit hall on Tuesday and Thursday.</li><li>Speaking of the exhibit hall, I think this is the first time I've been at a meeting where a <a href="https://www.qblox.com/" target="_blank">vendor</a> was actually running a dilution refrigerator on the premises. </li><li>Only one room that I was in had what I would describe as a bad projector (poor color balance, loud fan, not really able to be focused crisply). I also did not see any session chair this year blow it by allowing speakers to blow past their allotted times.</li><li>We really lucked out on the weather. </li><li>Does anyone know what happens if someone ignores the "Warning: Do Not Drive Over Plate" label on the 30 cm by 40 cm yellow floor plate in the main lobby? Like, does it trigger a self-destruct mechanism, or the apocalypse or something?</li><li>Next year's combined March/April meeting in Anaheim should be interesting - hopefully the venue is up to the task, and likewise I hope there are good, close housing and food options.</li></ul><div><br /></div></div><p></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com2tag:blogger.com,1999:blog-13869903.post-88542458981510325332024-03-06T22:21:00.002-06:002024-03-07T06:30:09.755-06:00APS March Meeting 2024, Day 3My highlights today are a bit thin, because I was fortunate enough to spend time catching up with collaborators and old friends, but here goes:<div><ul style="text-align: left;"><li><a href="https://en.wikipedia.org/wiki/Pedram_Roushan" target="_blank">Pedram Roushan</a> from Google gave an interesting <a href="https://meetings.aps.org/Meeting/MAR24/Session/M24.1" target="_blank">talk</a> about noisy intermediate-scale quantum experiments for simulation. He showed some <a href="https://arxiv.org/abs/2306.09333" target="_blank">impressive data</a> looking at the propagation of (simulated) magnons in the 1D Heisenberg spin chain.</li><li>In the same session, <a href="https://qutech.nl/lab/vandersypen-lab/" target="_blank">Lieven Vandersypen</a> from Delft <a href="https://meetings.aps.org/Meeting/MAR24/Session/M24.2" target="_blank">presented</a> their recent results using gate-defined Ge/SiGe quantum dot arrays to simulate a small-scale version of the Hubbard model. <a href="https://arxiv.org/abs/2307.02401" target="_blank">Looking</a> at exciton formation and propagation in a Hubbard ladder while being able to tune many parameters, the data are pretty neat, though I have to say it seems like scaling this up to large arrays will be extremely challenging in terms of layout and tuning. He also showed some in-preparation work on spin propagation in similar arrays - neat.</li><li>In a completely different session, <a href="https://institut-curie.org/personne/jacques-prost" target="_blank">Jacques Prost</a>, recipient of <a href="https://aps.org/programs/honors/prizes/prizerecipient.cfm?last_nm=Prost&first_nm=Jacques&year=2024" target="_blank">this year's Onsager Prize</a>, gave an interesting <a href="https://meetings.aps.org/Meeting/MAR24/Session/M57.3" target="_blank">talk</a> about broken symmetries and dynamics of living tissue. This included cell motion <a href="https://www.science.org/doi/full/10.1126/sciadv.abn5406" target="_blank">driven by nematicity</a> (living tissue as liquid crystal....) and how in a cylindrical environment this can <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.118001" target="_blank">lead to rotation</a> of growing tissue. These sorts of interactions in "active matter" can be related to how tissue grows and differentiates in living systems.</li><li>My colleague <a href="https://scuseria.rice.edu/" target="_blank">Gustavo Scuseria</a> is this year's recipient of the <a href="https://www.aps.org/programs/honors/prizes/prizerecipient.cfm?last_nm=Scuseria&first_nm=Gustavo&year=2024" target="_blank">Aneesha Rahman Prize</a>, and he gave a good explanation of his group's <a href="https://doi.org/10.1063/5.0125124" target="_blank">recent work</a> on using dualities to map strongly correlated models onto more tractable (polynomial-growth rather than exponential growth in problem size) equivalent weakly correlated models.</li><li>In a session on quantum spin liquids, <a href=" https://occamy.chemistry.jhu.edu/" target="_blank">Tyrel McQueen</a> of Johns Hopkins <a href="https://meetings.aps.org/Meeting/MAR24/Session/Q22.1" target="_blank">spoke</a> about two examples of his group's recent work. Chemical substitution <a href="https://doi.org/10.1103/PhysRevB.108.064433" target="_blank">can help tune</a> interactions in a Kitaev spin liquid candidate, and they've also <a href="https://arxiv.org/abs/2303.02218" target="_blank">examined</a> the controlled interplay of charge density waves and magnetic order. The talk did a great job of conveying a taste of the breadth and depth of the space of quantum magnets.</li><li>Lastly, <a href="https://www.mxic.com.tw/en-us/about/management-team/Pages/Chih-Yuan-Lu.aspx" target="_blank">Chih-Yuan Lu</a>, recipient of this year's <a href="https://www.aps.org/programs/honors/prizes/prizerecipient.cfm?last_nm=Lu&first_nm=Chih-Yuan&year=2024" target="_blank">George E. Pake Prize</a>, gave a very nice historical overview of the development of semiconductor electronics from the integrated circuit to the present frontiers (of <a href="https://semiengineering.com/knowledge_centers/integrated-circuit/transistors/3d/gate-all-around-fet/" target="_blank">gate-all-around transistors</a> and <a href="https://www.digitimes.com/news/a20230118VL209/macronix-memory-chips.html" target="_blank">3D integrated NAND memory</a>).</li></ul><div>Two other notes not directly germane to the APS meeting:</div></div><div><ul style="text-align: left;"><li>The <a href="https://www.aaas.org/news/fy-2024-rd-appropriations-dashboard" target="_blank">AAAS appropriations tracker</a> shows how outlays for the coming year are shaping up for NSF and the other agencies. </begin rant>Can someone explain to me why the conference NSF budget allocation for research ends up -8.5%, when the House pushed +0.3% and the Senate pushed -2.9%? Also, cutting the STEM education budget (which includes GRFP) by <b><i>28%</i></b> seems terrible. Griping about US STEM competitiveness and the need for developing the next-generation technical workforce, while simultaneously cutting research training resources: Congress in action. Once again, they feel good about supporting the <a href="https://new.nsf.gov/chips" target="_blank">authorization of doubling the NSF budget over five years</a>, but don't actually want to appropriate the funds to do it. </end rant></li><li>Purely by random chance (ahem), I want to point to <a href="https://talkingpointsmemo.com/edblog/is-your-iphone-cooler-than-you-think" target="_blank">this column</a>.</li></ul></div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com2tag:blogger.com,1999:blog-13869903.post-6795609959221245052024-03-05T21:07:00.006-06:002024-03-05T21:07:45.935-06:00APS March Meeting 2024, Day 2<p>A decent part of today was spent in conversation with friends and colleagues, but here are some high points of scientific talks:</p><p></p><ul style="text-align: left;"><li>The <a href="https://meetings.aps.org/Meeting/MAR24/Session/F06" target="_blank">DMP prize session</a> was excellent. The first talk was by <a href="https://web.stanford.edu/group/hyhwang/" target="_blank">Harold Hwang</a>, this year's awardee of the <a href="https://www.aps.org/programs/honors/prizes/mcgroddy.cfm" target="_blank">McGroddy Prize</a>. He gave a very compelling review of his group's accomplishments doing epitaxial growth of perovskite oxides, ranging over <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.75.914" target="_blank">early work</a> on colossal magnetoresistance compounds, <a href="https://www.nature.com/articles/nature00977" target="_blank">designing conducting interfaces</a>, understanding <a href="https://doi.org/10.1103/PhysRevB.18.4402" target="_blank">polar catastrophes</a>, attempts at <a href="https://www.nature.com/articles/nature08566" target="_blank">delta doping</a>, creating <a href="https://www.nature.com/articles/nmat4749" target="_blank">free-standing oxide membranes</a>, and most recently "de-intercalating" to create <a href="https://doi.org/10.1038/s41586-023-06129-x" target="_blank">nickelate superconductors</a>. I also appreciated the quote from <a href="https://en.wikipedia.org/wiki/Herbert_Kroemer" target="_blank">Herbert Kroemer</a>, "If, in discussing a semiconductor problem, you cannot draw an energy band diagram, this shows that you don't know what you are talking about."</li><li>The next talk in that session was by <a href="https://sites.psu.edu/samarthgroup/" target="_blank">Nitin Samarth</a>, this year's awardee of the <a href="https://aps.org/programs/honors/prizes/prizerecipient.cfm?last_nm=Samarth&first_nm=Nitin&year=2024" target="_blank">David Adler lectureship</a>, all about semiconductor spintronics. Again, a great collection of topics, including Mn-doped <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.67.3824" target="_blank">II-VI materials</a>, the inspiration of the <a href="https://doi.org/10.1063/1.102730" target="_blank">Datta-Das spin transistor idea</a>, <a href="https://doi.org/10.1103/PhysRevB.66.012408" target="_blank">Ga(Mn)A</a>s as a ferromagnetic semiconductor, optically injected spin transport in non-magnetic semiconductors, topological insulators for <a href="https://doi.org/10.1038/nature13534" target="_blank">spin-charge conversion</a>, <a href="https://www.nature.com/articles/s41563-023-01622-0" target="_blank">quantized anomalous Hall</a> response, and <a href="https://doi.org/10.1126/science.adk1270" target="_blank">topology+superconductivity</a>.</li><li>Then <a href="https://mitrano.physics.harvard.edu/people/matteo-mitrano" target="_blank">Matteo Mitran</a>o, winner of the IUPAP prize, gave a nice <a href="https://meetings.aps.org/Meeting/MAR24/Session/F06.3" target="_blank">talk</a> about time-resolved x-ray measurements, particularly of <a href="https://arxiv.org/abs/2310.05201" target="_blank">photo-induced metastable states of correlated materials</a>. These included time-resolved resonant inelastic x-ray scattering (<a href="https://doi.org/10.1038/s42005-020-00447-" target="_blank">trRIXS</a>) and time-resolved x-ray absorption spectroscopy (<a href="https://doi.org/10.1103/PhysRevX.12.011013" target="_blank">trXAS</a>).</li><li>In another session, I saw a very nice <a href="https://meetings.aps.org/Meeting/MAR24/Session/F24.5" target="_blank">talk</a> by <a href="https://sites.krieger.jhu.edu/chienlab/" target="_blank">Chia-Ling Chien</a> about spin Seebeck response in the non-collinear antiferromagnet <a href="https://doi.org/10.1103/PhysRevLett.129.117202" target="_blank">LuFeO3</a>, including <a href="https://doi.org/10.1038/s41567-022-01608-w" target="_blank">results</a> on spin-swapping, when the spin current direction and the spin index orientation can swap, <a href="https://doi.org/10.1103/PhysRevLett.103.186601" target="_blank">predicted theoretically</a> 15 years ago.</li><li>I also was able to see <a href="https://www-thphys.physics.ox.ac.uk/people/SteveSimon/" target="_blank">Steve Simon</a>'s <a href="https://meetings.aps.org/Meeting/MAR24/Session/G44.4" target="_blank">presentation</a> about unusual "<a href="https://doi.org/10.1103/PhysRevX.11.041063" target="_blank">spiral Kekule order</a>" in bilayer graphene, and <a href="https://www.physics.purdue.edu/people/faculty/manfra.php" target="_blank">Mike Manfra</a>'s <a href="https://meetings.aps.org/Meeting/MAR24/Session/G44.5" target="_blank">talk</a> about anyon interferometry (<a href="https://www.nature.com/articles/s41567-020-1019-1" target="_blank">here</a> and <a href="https://doi.org/10.1103/PhysRevX.13.041012" target="_blank">here</a>) in 2D electron gases. </li></ul><div>More tomorrow....</div><p></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com6tag:blogger.com,1999:blog-13869903.post-84284325546477541722024-03-04T22:37:00.002-06:002024-03-04T22:37:20.366-06:00APS March Meeting 2024, Day 1<p>There is no question that the meeting venue in Minneapolis is superior in multiple ways to last year's meeting in Las Vegas. The convention center doesn't feel scarily confining, and it also doesn't smell like a combination of cigarettes and desperation.</p><p>Here are a few highlights from the day:</p><p></p><ul style="text-align: left;"><li>There was an interesting <a href="https://meetings.aps.org/Meeting/MAR24/Session/A42" target="_blank">session</a> about "polar materials", systems that have the same kind of broken inversion symmetry within a unit cell as <a href="https://en.wikipedia.org/wiki/Ferroelectricity" target="_blank">ferroelectrics</a>; this includes "<a href="https://doi.org/10.1146/annurev-matsci-080921-105501" target="_blank">polar metals</a>" which host mobile charge carriers. One polar material family involving <a href="https://en.wikipedia.org/wiki/Multiferroics" target="_blank">multiferroic</a> insulators was <a href="https://meetings.aps.org/Meeting/MAR24/Session/A42.1" target="_blank">presented</a> by Daniel Flavián, in which dielectric (capacitance) measurements can show magnetic quantum critical phenomena, as in <a href="https://doi.org/10.1103/PhysRevResearch.3.033053" target="_blank">here</a> and <a href="https://doi.org/10.1103/PhysRevLett.130.216501" target="_blank">here</a>. Both sets of materials examined, Rb2Cu2Mo3O12 and Cs2Cu2Mo3O12, show remarkable dielectric effects due to fluctuating electric dipoles, connected to quantum critical points at B-field driven transitions between magnetic ordered states.</li><li><a href="https://sites.google.com/view/drichkolab/home" target="_blank">Natalia Drichko</a> from Johns Hopkins <a href="https://meetings.aps.org/Meeting/MAR24/Session/A42.2" target="_blank">showed</a> Raman spectroscopy data on an organic Mott insulator, in which melting charge order <a href="https://dx.doi.org/10.1126/science.aan6286" target="_blank">is connected</a> to spin fluctuations.</li><li><a href="https://pavolkov.com/" target="_blank">Pavel Volkov</a> from U Conn <a href="https://meetings.aps.org/Meeting/MAR24/Session/A42.3" target="_blank">discussed</a> doped strontium titanate (STO), an example of an incipient polar metal, and looking at how polar fluctuations might be connected with the mechanism behind the unusual superconductivity of STO. </li><li>The last <a href="https://meetings.aps.org/Meeting/MAR24/Session/A42.4" target="_blank">talk</a> of that session that I saw was <a href="https://physics.mit.edu/faculty/pablo-jarillo-herrero/" target="_blank">Pablo Jarillo-Herrero</a> giving a characteristically clear presentation about <a href="https://www.science.org/doi/full/10.1126/science.abd3230" target="_blank">sliding ferroelectricity</a>. Taking a material like hBN and trying to stack a bilayer with perfect A-A alignment is not energetically favored - it's lower in energy if the two layers shift relative to each other by a third of a lattice parameter, resulting in an out-of-plane electric dipole moment, pointing either up or down depending on the direction of the shift. Applying a sufficiently large electric field perpendicular to the plane can switch the system - this works <a href="https://doi.org/10.1038/s41565-021-01059-z" target="_blank">on TMDs</a> as well. Putting a moire bilayer in the mix, and you can get some neat <a href="https://arxiv.org/abs/2306.03922" target="_blank">charge ratcheting effects</a>. </li><li>The <a href="https://meetings.aps.org/Meeting/MAR24/Session/B07" target="_blank">session</a> on transport in non-Fermi liquids was fun and informative. I thought the <a href="https://arxiv.org/abs/2403.00062" target="_blank">discussion</a> of possible intrinsic nonlinear transport in strange metals was intriguing.</li><li>I also saw a couple of interesting invited talks (<a href="https://meetings.aps.org/Meeting/MAR24/Session/D21.1" target="_blank">here</a> and <a href="https://meetings.aps.org/Meeting/MAR24/Session/D21.4" target="_blank">here</a>) about experiments that try to use electronic transport in adjacent layers to probe nontrivial magnetic properties of adjacent spin ices. Very cool.</li></ul><div>More tomorrow....</div><p></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com0tag:blogger.com,1999:blog-13869903.post-50153107203840412502024-03-03T10:52:00.001-06:002024-03-03T10:52:14.231-06:00APS March Meeting 2024 - coming soonThis week I'm going to be at the <a href="https://march.aps.org/" target="_blank">APS March Meeting</a> in Minneapolis. As I've done in past years, I will try to write up some highlights of talks that I am able to see, though it may be hit-or-miss. If readers have suggestions for sessions or talks that they think will be particularly interesting, please put them in the comments.Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com0tag:blogger.com,1999:blog-13869903.post-75011158790473001082024-02-25T08:25:00.003-06:002024-02-25T08:28:13.822-06:002024 version: Advice on choosing a graduate school<p>It's been four years since I posted the previous version of this, so it feels like the time is right for an update.</p><p>This is written on the assumption that you have already decided, after careful consideration, that you want to get an advanced degree (in physics, though much of this applies to any other science or engineering discipline). This might mean that you are thinking about going into academia, or it might mean that you realize such a degree will help prepare you for a higher paying technical job outside academia. Either way, I'm not trying to argue the merits of a graduate degree - let's take it as given that this is what you want to do.<br /></p><ul><li><span style="font-size: small;">It's ok at the applicant stage not to know exactly what research area you want to be your focus. While some prospective grad students are completely sure of their interests, that's more the exception than the rule. I do think it's good to have narrowed things down a bit, though. If a school asks for your area of interest from among some palette of choices, try to pick one (rather than going with "undecided"). We all know that this represents a best estimate, not a rigid commitment.</span></li><li><span style="font-size: small;">If you get the opportunity to visit a school, you should go. A visit gives you a chance to see a place, get a subconscious sense of the environment (a "gut" reaction), and most importantly, an opportunity to talk to current graduate students. Always talk to current graduate students if you get the chance - they're the ones who really know the score. A professor should always be able to make their work sound interesting, but grad students can tell you what a place is really like.</span></li><li>International students may have a very challenging time being able to visit schools in the US, between the expense (many schools can help defray costs a little but cannot afford to pay for airfare for trans-oceanic travel) and visa challenges. Trying to arrange zoom discussions with people at the school is a possibility, but that can also be challenging. I understand that this constraint tends to push international students toward making decisions based heavily on reputation rather than up-close information. </li><li><span style="font-size: small;">Picking an advisor and thesis area are major decisions, but it's important to realize that those decisions do not define you for the whole rest of your career. I would guess (and if someone had real numbers on this, please post a comment) that the very large majority of science and engineering PhDs end up spending most of their careers working on topics and problems distinct from their theses. Your eventual employer is most likely going to be paying for your ability to think critically, structure big problems into manageable smaller ones, and knowing how to do research, rather than the particular detailed technical knowledge from your doctoral thesis. A personal anecdote: I did my graduate work on the ultralow temperature properties of amorphous insulators. I no longer work at ultralow temperatures, and I don't study glasses either; nonetheless, I learned a huge amount in grad school about the process of research that I apply all the time.</span></li><li><span style="font-size: small;">Always go someplace where there is more than one faculty member with whom you might want to work. Even if you are 100% certain that you want to work with Prof. Smith, and that the feeling is mutual, you never know what could happen, in terms of money, circumstances, etc. Moreover, in grad school you will learn a lot from your fellow students and other faculty. An institution with many interesting things happening will be a more stimulating intellectual environment, and that's not a small issue.</span></li><li><span style="font-size: small;">You should not go to grad school because you're not sure what else to do with yourself. You should not go into research if you will only be satisfied by a Nobel Prize. In both of those cases, you are likely to be unhappy during grad school. </span></li><li><span style="font-size: small;">I know grad student stipends are low, believe me. However, it's a bad idea to make a grad school decision based purely on a financial difference of a few hundred or a thousand dollars a year. Different places have vastly different costs of living - look into this. Stanford's stipends are profoundly affected by the cost of housing near Palo Alto and are not an expression of generosity. Pick a place for the right reasons.</span></li><li><span style="font-size: small;">Likewise, while everyone wants a pleasant environment, picking a grad school largely based on the weather is silly. </span></li><li><span style="font-size: small;">Pursue external fellowships if given the opportunity. It's always nice to have your own money and not be tied strongly to the funding constraints of the faculty, if possible. (It's been brought to my attention that at some public institutions the kind of health insurance you get can be complicated by such fellowships. In general, I still think fellowships are very good if you can get them.)</span></li><li><span style="font-size: small;">Be mindful of how departments and programs are run. Is the program well organized? What is a reasonable timetable for progress? How are advisors selected, and when does that happen? Who sets the stipends? What are TA duties and expectations like? Are there qualifying exams? Where have graduates of that department gone after the degree? Are external internships possible/unusual/routine? Know what you're getting into! Very often, information like this is available now in downloadable graduate program handbooks linked from program webpages. </span></li><li>When talking with a potential advisor, it's good to find out where their previous students have gone and how long a degree typically takes in their group. What is their work style and expectations? How is the group structured, in terms of balancing between team work to accomplish goals vs. students having individual projects over which they can have some ownership? </li><li>Some advice on what faculty look for in grad students: Be organized and on-time with things. Be someone who completes projects (as opposed to getting most of the way there and wanting to move on). Doctoral research should be a collaboration. If your advisor suggests trying something and it doesn't work (shocking how that happens sometimes), rather than just coming to group meeting and saying "It didn't work", it's much better all around to be able to say "It didn't work, but I think we should try <i>this</i> instead", or "It didn't work, but I think I might know why", even if you're not sure. </li><li><span style="font-size: small;">It's fine to try to communicate with professors at all stages of the process. We'd much rather have you ask questions than the alternative. If you don't get a quick response to an email, it's almost certainly due to busy-ness, and not a deeply meaningful decision by the faculty member. For a sense of perspective: I get 50+ emails per day of various kinds not counting all the obvious spam that gets filtered. </span> </li></ul><p><span style="font-size: small;">There is no question that far more information is now available to would-be graduate students than at any time in the past. Use it. Look at departmental web pages, look at individual faculty member web pages. Make an informed decision. Good luck!</span></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com3tag:blogger.com,1999:blog-13869903.post-76232605632802333742024-02-13T08:17:00.001-06:002024-02-13T08:17:35.737-06:00Continuing Studies course, take 2<p></p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6ckZOgKYBtKWWqxJhSSJkxDCrukWozUq-_XrNcSeEkkyRrcO_eaWJs56QyiFVueEKYBl5BQzPqKPa6G4ZK7_zunFyjoyaPLcFBA1EtQddCA0dCWvBRaXQzEyJ-k7rS95mF5X-8nmX8Crcq1odXJmktQqjLafhFpS-n38RKeepCamfihTnmJwlsA/s940/S24_CLE_I.C._Asset_Physics.png" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" data-original-height="788" data-original-width="940" height="268" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6ckZOgKYBtKWWqxJhSSJkxDCrukWozUq-_XrNcSeEkkyRrcO_eaWJs56QyiFVueEKYBl5BQzPqKPa6G4ZK7_zunFyjoyaPLcFBA1EtQddCA0dCWvBRaXQzEyJ-k7rS95mF5X-8nmX8Crcq1odXJmktQqjLafhFpS-n38RKeepCamfihTnmJwlsA/s320/S24_CLE_I.C._Asset_Physics.png" width="320" /></a></div>A year and a half ago, I <a href="https://nanoscale.blogspot.com/2022/09/coming-next-month.html" target="_blank">mentioned</a> that I was going to teach a course through Rice's <a href="https://glasscock.rice.edu/" target="_blank">Glasscock School of Continuing Studies</a>, trying to give a general audience introduction to some central ideas in condensed matter physics. Starting in mid-March, I'm doing this again. <a href="https://bit.ly/3uovwiU" target="_blank">Here is a link</a> to the course registration for this synchronous online class. This course is also intended as a potential continuing education/professional development offering for high school teachers, community college instructors, and other educators, and thanks to the generous support of the NSF, the Glasscock School is able to offer a limited number of full scholarships for educators - <a href="https://gscs.formstack.com/forms/s24_everyday_physics_teacher_scholarship_application" target="_blank">apply here</a> by February 27 for consideration. <br /><p></p><p>(I am aware that the cost of the course is not trivial; at some point in the future I will make the course materials available broadly, and I will be sure to call attention to that at the time.)</p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com3tag:blogger.com,1999:blog-13869903.post-77030494166320002512024-02-07T14:09:00.001-06:002024-02-07T14:09:44.791-06:00A couple of links + a thought experiment about spin<p>A couple of interesting things to read:</p><p></p><ul style="text-align: left;"><li>As someone interested in lost ancient literature and also science, I really liked <a href="https://www.nature.com/articles/d41586-024-00346-8" target="_blank">this news article from Nature</a> about progress in reading scrolls excavated from <a href="https://en.wikipedia.org/wiki/Herculaneum" target="_blank">Herculaneum</a>. The area around the Bay of Naples was a quite the spot for posh Roman families, and when <a href="https://en.wikipedia.org/wiki/Eruption_of_Mount_Vesuvius_in_79_AD" target="_blank">Vesuvius erupted in 79 CE</a>, whole villas, complete with their libraries of books on papyrus scrolls, were buried and flash-cooked under <a href="https://en.wikipedia.org/wiki/Pyroclastic_flow" target="_blank">pyroclastic flows</a>. Those scrolls now look like lump charcoal, but with modern x-ray techniques (CT scanning using the beam from <a href="https://www.diamond.ac.uk/Home.html" target="_blank">a synchrotron</a>) plus machine learning, it is now possible to virtually unroll the scrolls and decipher the writing, because the ink has enough x-ray contrast with the carbonized papyrus to be detected. There is reason to believe that there are more scrolls out there still buried, and there are lots of other books and scrolls out there that are too delicate or damaged to be handled and read the normal way. It's great to see this approach starting to succeed.</li><li>I've written about <a href="https://nanoscale.blogspot.com/2016/11/lenses-from-metamaterials.html" target="_blank">metalenses</a> before - using nanostructured surfaces for precise control of optical wavefronts to make ultrathin optical elements with special properties. This <a href="https://seas.harvard.edu/news/2024/01/metalens-meets-stars" target="_blank">extended news item</a> from Harvard about <a href="https://pubs.acs.org/doi/full/10.1021/acsnano.3c09462" target="_blank">this paper</a> is a nice piece of writing. With techniques now developed to make dielectric metalenses over considerably larger areas (100 mm silica wafers), these funky lenses can now start to be applied to astronomy. Nifty.</li></ul>And now the gedanken experiment that I've been noodling on for a bit. I know what the correct answer must be, but I think this has done a good job at reminding me how what constitutes a measurement is a very subtle issue in quantum mechanics.<p></p><p>Suppose I have a single electron roughly localized at the origin. It has spin-1/2, meaning that, if there are no other constraints, if I choose to make a measurement of the electron spin along some particular axis, I will find that with 50/50 probability the component of the angular momentum of the electron is \(\pm \hbar/2\) along that axis. Suppose that I pick a \(z\) axis and do the measurement, finding that the electron is "spin-up" along \(z\). Because the electron has a magnetic dipole moment, that means that the magnetic field at some distance \(r\) away from the origin should be the field from a magnetic dipole along \(z\). </p><p>Now suppose I make another measurement of the spin, this time along the \(x\) axis. I have a 50/50 chance of finding the electron spin up/down along \(x\). After that measurement, the magnetic field at the same location \(r\) away from the origin should be the field from a magnetic dipole along \(x\). It makes physical sense that the magnetic field at location \(r\) can only "know" that a measurement was done at the origin on a timescale \(r/c\). (Note: A truly correct treatment of this situation would seem to require <a href="https://en.wikipedia.org/wiki/Quantum_electrodynamics" target="_blank">QED</a>, because the spin is entangled with the electromagnetic field via its magnetic moment; likewise one would really need to discuss in detail what it means to measure the spin state at the origin and what it means to measure the magnetic field locally. Proper descriptions of detectors and measurements are really necessary.)</p><p>To highlight how subtle the situation is, suppose the spin at the origin is initially half of an <a href="https://en.wikipedia.org/wiki/Bell_state" target="_blank">EPR pair</a>, so that it's in a spin singlet with a second spin near Alpha Centauri, so that the total spin of the two is zero. Now a measurement of \(s_{z}\) at the origin determines the state of \(s_{z}\) at Alpha Centauri, and the magnetic field near that dipole at Alpha Centauri should be consistent with that. Thinking about all of the subtleties here has been a good exercise for me in remembering how the seemingly simple statements we make when we teach this stuff can be implicitly very complicated.</p><p><br /></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com5tag:blogger.com,1999:blog-13869903.post-84973539556198816912024-02-03T13:13:00.002-06:002024-02-03T13:13:53.187-06:00Large magnetic fields as a scientific tool<p>When I was at Berkeley at the beginning of the week to give a seminar, I was fortunate enough to overlap with their departmental physics colloquium by <a href="https://en.wikipedia.org/wiki/Gregory_S._Boebinger" target="_blank">Greg Boebinger</a>, an <a href="https://scholar.google.com/citations?user=ZnL7orIAAAAJ&hl=en&oi=ao" target="_blank">accomplished scientist</a> who is also an extremely engaging and funny speaker. Since 2004 he has been the director of the <a href="https://nationalmaglab.org/" target="_blank">National High Magnetic Field Lab</a> in Tallahassee, Florida, the premier user facility for access to large magnetic fields for scientific research. He gave a great talk that discussed both the challenges in creating very large magnetic fields and a sampling of the cool science that can be done using these capabilities.</p><p>Leaving aside spin for a moment, magnetic fields* in some reference frame are generated by currents of moving charges and changing electric fields, as in Ampère's law, \(\nabla \times \mathbf{B} = \mu_{0}\mathbf{J} + \epsilon_{0}\mu_{0}\partial_{t}\mathbf{E}\), where \(\mathbf{J}\) is the current density. Because materials have collective responses to magnetic fields, generating within themselves some magnetization (magnetic dipole moment per volume \(\mathbf{M}\)), we can think of the magnetic field as a thermodynamic variable, like pressure. Just as all kinds of interesting physics can be found by using pressure to tune materials between competing phases (because pressure tunes interatomic spacing, and thus things like the ability of electrons to move from atom to atom, and hence the magnitude of <a href="https://en.wikipedia.org/wiki/Exchange_interaction" target="_blank">magnetic exchange</a>), a magnetic field can tune materials across phase transitions. </p><p>It's worth remembering some physically relevant scales. The earth's magnetic field at the surface is around 30-50 microTesla. The magnetic field at the surface of a rare earth magnet is around 1 Tesla. The field in a typical MRI machine used for medical imaging is 1.5 or 3 T. The energy levels for the spin of an electron in a magnetic field are set by the <a href="https://en.wikipedia.org/wiki/Zeeman_effect#Theoretical_presentation" target="_blank">Zeeman effect</a> and shift by an amount around \(\mu_{\mathrm{B}}B\), where \(\mu_{\mathrm{B}}\) is the Bohr magneton, \(9.27 \times 10^{-24}\) J/T. A 10 T magnetic field, about what you can typically get in an ordinary lab, leads to a Zeeman energy comparable to the thermal energy scale at about 6.7 K, or compared to an electron moving through a voltage of 0.6 mV. In other words, magnetic fields are weak in that it generally takes a lot of current to generate a big field, and the associated energies are small compared to room temperature (\(k_{\mathrm{B}}T\) at 300 K is equivalent to 26 mV) and the eV scales relevant to chemistry. Still, consequences can be quite profound, and even weak fields can be very useful with the <a href="https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance" target="_blank">right</a> <a href="https://en.wikipedia.org/wiki/Magnetic_resonance_imaging" target="_blank">techniques</a>. (The magnetic field at the surface of a <a href="https://en.wikipedia.org/wiki/Magnetar" target="_blank">neutron star</a> can be \(10^{11}\) T, a staggering number in terms of energy density.)</p><p>Generating large magnetic fields is a persistent technological challenge. <a href="https://en.wikipedia.org/wiki/Superconducting_magnet" target="_blank">Superconductors</a> can be great for driving large currents without huge dissipation, but they have their own issues of critical currents and critical fields, and the mechanical forces on the conductors can be very large (see <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10065139/" target="_blank">here</a> for a recent review). The largest steady-state magnetic field that has been achieved with a (high-Tc) superconducting coil combined with a resistive magnet is <a href="https://www.nature.com/articles/s41586-019-1293-1https://www.nature.com/articles/s41586-019-1293-1" target="_blank">around 45.5 T</a> (see <a href="https://phys.org/news/2022-08-china-world-strongest-steady-magnetic.html" target="_blank">here</a> as well). At the Los Alamos outpost of the Magnet Lab, they've achieved non-destructive pulsed fields as large as 101 T (see <a href="https://www.youtube.com/watch?v=N0R8dyyXtTo" target="_blank">this video</a>). A huge limiting factor is the challenge of making joints between superconducting wires, so that the joint itself remains superconducting at the very large currents and fields needed. </p><p>The science that can be done with large fields extends well beyond condensed matter physics. One <a href="https://nationalmaglab.org/magnet-academy/read-science-stories/science-simplified/whats-in-an-oil-drop/" target="_blank">example</a> from the talk that I liked: Remarkable resolution is possible in ion cyclotron resonance mass spectroscopy, so that with a single drop of oil, it is possible to identify the contribution of the many thousands of hydrocarbon molecules in there and "fingerprint" where it came from. </p><p>Fun stuff, and a great example of an investment in technology that would very likely never have been made by private industry alone.</p><p><br /></p><p><span style="font-size: x-small;">* I know that \(\mathbf{B}\) is technically the magnetic induction or magnetic flux density in SI units, but colloquially everyone calls it the magnetic field, so I'll do the same here.</span></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com3tag:blogger.com,1999:blog-13869903.post-59350064650442184252024-01-26T11:17:00.006-06:002024-02-03T10:58:25.421-06:00Generative AI and scientific images/diagrams<p>Generative AI for image generation is a <a href="https://en.wikipedia.org/wiki/Artificial_intelligence_art#Criticism,_issues_and_controversy" target="_blank">controversial topic</a> for <a href="https://www.nytimes.com/interactive/2024/01/25/business/ai-image-generators-openai-microsoft-midjourney-copyright.html?" target="_blank">many reasons</a>. Still, as someone who doesn't have a staff of graphic artists on hand to help make scientific illustrations, it has certainly been tempting to see whether it might be a useful tool. My brief experiments are based using bing's integrated engine (which I believe is <a href="https://openai.com/dall-e-3" target="_blank">DALL-E 3</a>) since Rice has a license. The short summary: Trying to make scientific illustrations this way is great at showing how this technology is not close to useful yet for this application. While natively integrated w/ chatGPT, it still cannot parse prompts well, indicating that it doesn't really "know" the meaning of a lot of basic terms and how those relate to its image training data.</p><p>Let me give you an example. When I asked it "Make me an image of a benzene molecule showing the chemical bonds between the carbon atoms", this is the response: </p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjX_JzieQ_mWeVxxry_zylrfwBldlujldJB0TU3VHXCldtgI8yLPynWnaXAfnwToC8QuL9aq2VNMhKhehExediWhUxBgrcf7TfJmWHTPVAmR9MdFkfenypveA8OyC-G2mXfTfOK0msM9EKsDyAQ8zNRtiKMTXcP3LhP80FDQgLZ2RNP4QfKmFRIrQ/s504/benzenedalle1.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="504" data-original-width="468" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjX_JzieQ_mWeVxxry_zylrfwBldlujldJB0TU3VHXCldtgI8yLPynWnaXAfnwToC8QuL9aq2VNMhKhehExediWhUxBgrcf7TfJmWHTPVAmR9MdFkfenypveA8OyC-G2mXfTfOK0msM9EKsDyAQ8zNRtiKMTXcP3LhP80FDQgLZ2RNP4QfKmFRIrQ/s320/benzenedalle1.jpg" width="297" /></a></div><p>When I tried to refine this by typing "Make an image of a benzene molecule.", it responded by showing me image search results with sources, which were all correctly benzene, but not an original image. When I responded "Please create a new image (not from an existing source) of a benzene molecule.", it responded with "Ok, I will create a new image from scratch showing a benzene molecule." and showed me this:</p><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgvRCbvVCgeYhx2JuifbtCInOEFqgdtilKym5pd3JXx40ZPQTQBytiqpr4LEfdFGvPYBe4S4168n7pxo7f4lLG-TUSvRDxpOxveV6mcKaoiv-3S354EkEyL1qJlzcxq5uTjDirVoa3cgWfAqoR6xc2sN5D4ecwo_TMlsTC3AM9iRbtwJBBZTaOqFg/s508/benzenedalle2.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="508" data-original-width="459" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgvRCbvVCgeYhx2JuifbtCInOEFqgdtilKym5pd3JXx40ZPQTQBytiqpr4LEfdFGvPYBe4S4168n7pxo7f4lLG-TUSvRDxpOxveV6mcKaoiv-3S354EkEyL1qJlzcxq5uTjDirVoa3cgWfAqoR6xc2sN5D4ecwo_TMlsTC3AM9iRbtwJBBZTaOqFg/s320/benzenedalle2.jpg" width="289" /></a></div>I then tried more specificity in the prompt: "Create (from scratch) an image of a benzene molecule, which consists of six carbon atoms (black spheres) linked in a ring, with one hydrogen atom (a smaller red sphere) attached to each carbon atom." The result was this:<div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjXwgvSrVLJQQ1sKq7tE2nOqPQoTxIWmgpp7QzCmHAJP6GgiZdDmdM9iD_mA3ncN4L7i8nPOl3OQS3wvAJOu_T00Nve7f1mLwQjoC4y3U7fKYOD-WBYf_TA1Hz_CkM4dKunxu1yA5MF-XrMVYWAJRzskMy8G4vqWCfuLtpQkYV2HEubIUbYl3jv2A/s504/benzenedalle3.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="504" data-original-width="457" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjXwgvSrVLJQQ1sKq7tE2nOqPQoTxIWmgpp7QzCmHAJP6GgiZdDmdM9iD_mA3ncN4L7i8nPOl3OQS3wvAJOu_T00Nve7f1mLwQjoC4y3U7fKYOD-WBYf_TA1Hz_CkM4dKunxu1yA5MF-XrMVYWAJRzskMy8G4vqWCfuLtpQkYV2HEubIUbYl3jv2A/s320/benzenedalle3.jpg" width="290" /></a></div>Closer, but I'm still surprised at how poorly any of these resemble the stock images of benzene that it correctly found through image searching on the internet. That last one is particularly close, so I tried "The fourth image is close, but it should have only one hydrogen attached to each carbon, not three." In classic chatGPT form, the response was "I’m sorry, I made a mistake. I’ll try to create a new image of a benzene molecule with only one hydrogen attached to each carbon. Please wait a moment." It then produced this:</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEimTA58ReGNkTgsGLblOtrucb8J8p9rKDYsrF3ehfjzkrNd0n9YeuUrcnETi-mfoxvu0NzB5pxpTXkAlOofrNfjzPwOaOu7VdE8oA1Hyne5uEmyoLSwMvVND_6U0AkGKFDj8Ty2WmH_ME050O9iDj3eqUeS9l8-lozyjP97bTWGNCX12NFyCbItfg/s498/benzenedalle4.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="498" data-original-width="460" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEimTA58ReGNkTgsGLblOtrucb8J8p9rKDYsrF3ehfjzkrNd0n9YeuUrcnETi-mfoxvu0NzB5pxpTXkAlOofrNfjzPwOaOu7VdE8oA1Hyne5uEmyoLSwMvVND_6U0AkGKFDj8Ty2WmH_ME050O9iDj3eqUeS9l8-lozyjP97bTWGNCX12NFyCbItfg/s320/benzenedalle4.jpg" width="296" /></a></div>Clearly it just cannot understand or follow the instructions yet. <div><br /></div><div>Similar things happen if you ask it to make an image of a lattice. If I ask "Make me an image from scratch of a simple cubic lattice.", the verbal and image responses are quite encouraging. "I’ll try to create that. Here is a new image of a simple cubic lattice that I generated from scratch. A simple cubic lattice is a type of crystal structure where the atoms are arranged in a regular pattern of cubes. Each atom is located at the center of a cube and has six nearest neighbors. A simple cubic lattice is one of the simplest and most symmetric types of lattices."</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVkOQ3RcFVrUXf2_EeSa3hnMl6xj1Nbve1Nf1bETCpTGXUdZhWGBd9Nn9F-rpth0ZA5ZiL72zbiMkKTSiVcwPVeLS6tHP7pY2g9FIWdxcOIJNeMwuK8hDyvgmdNUPg4Q-uq058_9Onu6gqDHRhpCz67I5qX8eXX6UNslzfbOw7AqmtAbuE6lcj_w/s501/simplecubicdalle.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="501" data-original-width="446" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhVkOQ3RcFVrUXf2_EeSa3hnMl6xj1Nbve1Nf1bETCpTGXUdZhWGBd9Nn9F-rpth0ZA5ZiL72zbiMkKTSiVcwPVeLS6tHP7pY2g9FIWdxcOIJNeMwuK8hDyvgmdNUPg4Q-uq058_9Onu6gqDHRhpCz67I5qX8eXX6UNslzfbOw7AqmtAbuE6lcj_w/s320/simplecubicdalle.jpg" width="285" /></a></div><br /><div>This is quite close, though all of these have problems (non-orthogonal angles, uneven spacings). Things go off the rails badly, though, if you want a face-centered cubic lattice. It can say the definition of an FCC lattice reasonably well: "I’ll try to create that. Here is a new image of a face-centered cubic lattice that I generated from scratch. A face-centered cubic lattice is a type of crystal structure where the atoms are arranged in a regular pattern of cubes. Each atom is located at the center of a cube and at the center of each face of the cube. A face-centered cubic lattice has 12 nearest neighbors for each atom and is one of the most common and dense types of lattices." However, the images are, ummm, artistic.</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmd0UrJs7OCbAbgPDFqSsK4rhe2I3oluxk1ITlndt-UVpwsv5smVb-jX9rCMt7OTZUZTXQzKmgQH2YpF1cNKgRtR9TE8rcBHqrEecSXVl6xU0GPs8LS1gFhjkWhH4p07dCXHt1MjIf0eH3qMRomOfo1ViMC_P4zZO6wCRUWCNLMFEhGl6Z6pRWpA/s503/fccdalle.jpg" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="503" data-original-width="450" height="320" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmd0UrJs7OCbAbgPDFqSsK4rhe2I3oluxk1ITlndt-UVpwsv5smVb-jX9rCMt7OTZUZTXQzKmgQH2YpF1cNKgRtR9TE8rcBHqrEecSXVl6xU0GPs8LS1gFhjkWhH4p07dCXHt1MjIf0eH3qMRomOfo1ViMC_P4zZO6wCRUWCNLMFEhGl6Z6pRWpA/s320/fccdalle.jpg" width="286" /></a></div><div>These are quite different from the simple image search of the internet for "face-centered cubic lattice", which does a good job if you don't mind using someone else's image. The bottom line: These kinds of image generation tools have enormous potential to save us time, but right now the parsing technology and training methods just aren't there yet. If a simple image search turns up the right targets, but the generation tool can't make images that look like those search results, then training has failed somewhere. </div><div><br /></div><div>If anyone out there has found some flavor of these image generation capabilities that really does work well for helping to make scientific illustrations, please let me know.</div><div><br /></div><div><b>Update</b>: Google just released <a href="https://aitestkitchen.withgoogle.com/tools/image-fx" target="_blank">their free image generation tool</a>, and while it still has problems like those above, it's pretty impressive. For example, here is what it made from the prompt "drawing of a benzene molecule, consisting of six black carbon atoms arranged in a ring, with each carbon atom connected to one red hydrogen atom":</div><div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjFRJm6JjfmC03YTdWhip359mpsiscSMWcIlwE8PSyX_rLjhuBPKQRQfZULB6OeHXJIJ8BVAToliLBlDWsFzeUINd9PQ6mi5KJZbUWWU-zoMK1DLAFTYricoa6L0przA32ZCtnRcTfCjIwMU0z9XgEsWllab2Qo9J3_v7XO8pPjDeaZSD2GOfaMyw/s1536/googlebenzene.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="1536" data-original-width="1536" height="250" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjFRJm6JjfmC03YTdWhip359mpsiscSMWcIlwE8PSyX_rLjhuBPKQRQfZULB6OeHXJIJ8BVAToliLBlDWsFzeUINd9PQ6mi5KJZbUWWU-zoMK1DLAFTYricoa6L0przA32ZCtnRcTfCjIwMU0z9XgEsWllab2Qo9J3_v7XO8pPjDeaZSD2GOfaMyw/w250-h250/googlebenzene.png" width="250" /></a></div><br /><div>That's certainly better than anything DALL-E made. Of course, the other three images that came along with that one were all screwed up. Still, progress.</div><div><br /><div><br /> <br /><p><br /></p></div></div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com6tag:blogger.com,1999:blog-13869903.post-39510006176171362332024-01-16T11:23:00.005-06:002024-01-25T14:43:02.398-06:00Materials characterization techniques – a brief glossary<span id="docs-internal-guid-f1acf3d4-7fff-9d3b-fedc-4084791780e1"><span><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: left;"><span style="white-space-collapse: preserve;">Suppose someone has synthesized or found what they think is a new material. How do people studying materials (condensed matter physicists, materials scientists, materials chemists) figure out what they have and understand its properties? That's the puzzle-solving aspect of working with materials: In general, solid matter involves an enormous number of interacting particles, and determining even something as basic as its structure and underlying excitations is not simple. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: left;"><span style="white-space-collapse: preserve;">There are many, many materials characterization techniques available, each with its own peculiarities and limitations. (I think that the alphabet-soup collection of acronyms associated with these is part of condensed matter's general perception as complicated, obscure, and full of jargon, but the need for a variety of techniques is clear in practice.) For the class I'm teaching, I wrote up a brief glossary of these. Apologies for undoubtedly leaving out someone's favorite. Please let me know in the comments what I've missed or mis-stated. Wikipedia already does a creditable job explaining many of these, including with diagrams and citations to key literature. Hopefully sticking a lot of these in one place will be useful to some. -- DN</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: left;">PS - the fact that there are so many different techniques that can be applied just to determine material structure and composition is a hint why trying to automate materials characterization in <a href="https://nanoscale.blogspot.com/2023/12/aiml-and-condensed-matter-materials.html" target="_blank">AI/ML-based materials synthesis and discovery</a> has a long way to go.</p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: left;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"><br /></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: left;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"><br /></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: center;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Materials characterization techniques – a brief glossary</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Microscopy</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Optical_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Optical microscopy</span></a><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">provides optical information about structure on scales > </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">∼</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> 1 μm</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Electron microscopy and related</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_electron_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning electron microscopy (SEM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">electron beam (1-40 keV) rastered across sample; secondary electrons knocked out of the sample are detected as a function of beam position to create an image. Sensitive to surface conditions, works best on conductive materials, larger signals from high </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">Z</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> materials. Beam spot size typically nm scale; lateral resolution down to 1-2 nm possible. Penetration depth into solid of 10s of nm, more with higher electron beam energy. Typically requires sample in vacuum (or at least detector closer to sample than electron mean free path in background gas). Best with conductive samples to avoid charging.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Electron_backscatter_diffraction" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Back-scatter electron diffraction (BSED)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: back-scattered electrons from the beam used to create diffraction patterns from the surface crystal structure.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Energy (X-ray) dispersive spectroscopy (EDS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: x-ray fluorescence excited by electron beam is detected; can be used for elemental compositional analysis.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Electron_microprobe" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Electron microprobe analysis (EMPA)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: carefully calibrated cousin of EDS, allows precise elemental analysis.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Cathodoluminescence" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Cathodoluminescence (CL)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: optical photons collected from e-beam excited sample as a function of beam position. Can detect excitations of material like plasmons, excitons.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Transmission_electron_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Transmission electron microscopy (TEM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> and</span><a href="https://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopy" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">scanning transmission electron microscopy (STEM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: sub-nm spot size electron beam (typically 100 keV and higher) passed through thin (</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">∼</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> 100 nm thick) sample into a detector. Can detect atomic-scale structural information. EDS, CL can be performed as well. “</span><a href="https://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopy#Bright-field" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Bright field</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">” and “</span><a href="https://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopy#Annular_dark-field" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">dark field</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">” imaging modes possible. Sample in vacuum. Special sample holders available to allow measurements as a fn of temperature, strain, electronic biasing,</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Selected_area_diffraction" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Selective-area electron diffraction (SAED)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> – get electron diffraction from portions of the sample.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Electron_energy_loss_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Electron energy loss spectroscopy (EELS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> – measure energy loss of transmitted electrons, can infer excitations (e.g. plasmons) within the material. Energy resolution down to sub-100 meV possible.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://www.jeol.com/words/emterms/20121023.110857.php#gsc.tab=0" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Lorentz electron microscopy (LEM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> – can infer magnetic domain patterns from deflection of transmitted electron beam</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h3 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #1f3763; font-size: small; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Electron diffraction:</span></h3><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Reflection_high-energy_electron_diffraction" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Reflection high energy electron diffraction (RHEED)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: diffraction using grazing incidence electrons (10-30 keV). Extremely sensitive to surface conditions, used for </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">in situ</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> characterization of thin film growth in molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) systems. Requires vacuum.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Low-energy_electron_diffraction" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Low energy electron diffraction (LEED)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: low energy (20-300 eV) electrons diffracted in reflection off surfaces. This is the original electron diffraction discovered by Davisson and Germer back in 1924. Requires vacuum, very surface sensitive (nm scales), vulnerable to magnetic fields. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Auger_electron_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Auger electron spectroscopy (AES)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: use keV electrons to knock out core electrons; as electron drops down to fill core hole, excess energy kicks out less bound electron, whose energy is measured. Very surface sensitive. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Low-energy_electron_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Low energy electron microscopy (LEEM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"> and spin-polarized LEEM (SPLEEM)</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Doing electron microscopy using < 100 V electrons; extremely surface sensitive, SPLEEM good for local magnetic structure.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><a href="https://en.wikipedia.org/wiki/Scanning_probe_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanned probe microscopy (SPM)</span></a></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">Category of microscopy methods that involves moving a sharp tip in close proximity to a material surface. Typically involves piezoelectric transducers for sample/tip relative motion and scanning. Examples:</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Atomic_force_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Atomic force microscopy (AFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A sharp tip (down to a few nm in radius) at the end of a cantilever or tuning fork structure is moved relative to the sample surface. In contact mode, changes in surface topography cause deflection of the cantilever, which is typically detected optically. In non-contact (tapping) mode, the tip is oscillated at the cantilever resonance frequency. The short-range interaction between tip and sample alters the frequency and phase of the cantilever motion. Feedback of tip height above sample is used to maintain tip-sample separation and map topography. Can be performed in ambient conditions. If performed in vacuum, with molecule-functionalized tips, it is possible to perform atomic-resolution imaging and “see” molecular orbitals. Versions of AFM may be performed in fluid environments as well.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikibooks.org/wiki/Nanotechnology/AFM#Lateral_Force_Microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Lateral force microscopy (LFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: looks at sideways forces on tip as it is scanned over the sample surface; sensitive to changes in local friction and elastic properties.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Piezoresponse_force_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Piezoresponse force microscopy (PFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: uses a conductive tip and an applied ac current to map piezoelectric response of sample.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Conductive_atomic_force_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Conducting probe AFM</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: In contact mode, allows mapping of electronic properties of the sample, though care is required for interpretation.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_capacitance_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning capacitance microscopy (SCM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Using conductive tip as effective capacitor plate, maps capacitance of sample. Useful for mapping carrier concentration in semiconductor materials.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Magnetic_force_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Magnetic force microscopy (MFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Uses a ferromagnetically coated tip. Scanning a line in close non-contact mode to get topography, and rescanning back over the line with tip elevated a fixed amount so that long-range magnetic forces are mapped. One challenge: magnetic field from tip can perturb magnetic domains in sample. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Electrostatic_force_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Electrostatic force microscopy (EFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Conductive AFM tip is used and held at a particular potential relative to the sample. As in MFM, mapping at a fixed tip-sample distance can reveal local electric field forces between tip and sample.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Kelvin_probe_force_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Kelvin probe force microscopy (KPFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Feedback is performed, so that the conductive AFM tip potential is adjusted to null out any long-range electric field forces between tip and sample. This can be used to map out the local contact potential or work function difference between tip and sample. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Magnetic_resonance_force_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Magnetic resonance force microscopy (MRFM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Uses radio frequency (RF) excitation and a magnetic tip to drive magnetic resonance (either electron spin resonance or nuclear magnetic resonance) of spins in the sample, detected via the cantilever motion. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Near-field scanning optical microscopy (NSOM or SNOM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Using AFM-like control, a tip is brought into close proximity (nm to tens of nm) of the sample surface. Near-field optical interactions are then mapped as a function of tip position. Tip can be a tapered optical fiber or a contain a hole/waveguide, so that light travels through the tip to the sample surface. Scattered light can be detected back through the tip or in the far field. Alternately, light can be shined in via the far field and scattered into the tip or into another far-field detector. Key idea is that the very small tip and tip-sample distance can scatter sub-diffraction-limit information into the far field.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://doi.org/10.1126/science.276.5312.579" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning single-electron transistor microscopy (SSETM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A tip is prepared (e.g., on a drawn optical fiber) with a single-electron transistor (SET, a device based on “Coulomb blockade”, consisting of a metal “island” with tunnel junctions to a source and a drain electrode, sometimes with an additional “gate” electrode that is capacitively coupled to the island) at the tip apex. The tip is positioned close to the sample using AFM-like techniques to avoid crashing into the surface. The electronic transport through the SET as a function of biasing conditions and the tip position. The surface potential of the sample acts as a “gate” that modulates conduction through the island in the Coulomb blockade regime. By modulating the tip position and biasing conditions, can be used to measure local charge density and electronic compressibility. Typical spatial resolution 10s of nm at best, because of diameter of island and positioning precision. Requires cryogenic temperatures to operate.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_SQUID_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning SQUID microscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A superconducting quantum interference device (SQUID) is fabricated on a tip (e.g., on a drawn optical fiber). The tip is again positioned near the sample using AFM-like techniques. The SQUID, consisting of a superconducting loop with Josephson junction weak links, is used to detect magnetic flux from the sample. This can be used to map current distributions in operating devices. Requires cryogenic temperatures to operated, does not work well with magnetic fields.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_Hall_probe_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning Hall probe microscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A 2D electronic system is patterned into a Hall configuration on some kind of tip and positioned (using AFM-like methods) close to a sample of interest, to act as a magnetic field detector. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Nitrogen-vacancy_center#Applications" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning NV center microscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A nitrogen-vacancy center in a diamond crystal has optical transitions that are highly sensitive to local magnetic fields. Incorporating NV centers into diamond films on SPM tips enables high resolution (tens of nm) measurements of local fields including direction, and the inference of current distributions.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://arpes.stanford.edu/research/tool-development/microwave-impedance-microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Microwave Impedance Microscopy (MIM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A microwave resonator is made and incorporated so that a conductive AFM-like tip is part of the resonant circuit. Scanning the tip over a device changes the </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">Q</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> of the resonator, allowing mapping (with 10s of nm resolution) of the microwave frequency (say hundreds of MHz to GHz) dielectric properties of the sample. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_thermal_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning thermal microscopy (SThM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">:</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">Scanning a special temperature-sensitive probe tip over a sample to assess local thermal conduction properties or local temperature. Several variants depending on the type of thermally sensitive probe used (e.g. thermocouple, phase change material, optical defect center with </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">-dependent lifetime).</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_tunneling_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning tunneling microscopy (STM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Tunneling current between metallic tip (sometimes Pt, W) and conductive sample used for </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">z</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">-positioning feedback. Because of the exponential distance dependence of tunneling, atomic resolution is possible. Can be performed at ambient conditions, but by far the best results are obtained in vacuum and at low temperatures. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Scanning_tunneling_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Scanning tunneling spectroscopy (STS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: At each tip position over the sample, </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">z</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> feedback is turned off and tunneling </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">-</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> curves are obtained at a nominally fixed tip height (usually including d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs. </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> and sometimes d</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs. </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">). The d</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs. </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> data is used to perform inelastic electron tunneling spectroscopy (IETS), and can detect local excitations like vibrations.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">Quasiparticle interference (QI): From STS maps, spatial Fourier transforms of the (fixed energy) maps of conductance vs. position are performed. For itinerant quasiparticles that can move around on the sample surface, quantum interference between trajectories that bounce off scattering sites and the tip mean that the QI transforms make it possible to infer </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">E</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">(</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">k</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">) for the surface states of the sample. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Spin-polarized_scanning_tunneling_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Spin-polarized STM (SPSTM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Requires a magnetic/spin-polarized tip. Can reveal local magnetic information due to spin-dependent tunneling between tip and sample.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">X-ray methods</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/X-ray_crystallography" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">X-ray diffraction (XRD)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: gives crystal structure (spatial frequencies of atomic stacking) of materials via coherent scattering of x-rays. Powder XRD = gives bright rings as a function of angle away from forward scattering (linear combination of many spots). Obeys Bragg condition. Single-crystal XRD = gives discrete spots. A Laue single-crystal diffractometer can be used to find the crystal orientation of single crystals.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/X-ray_reflectivity" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">X-ray reflectometry (XRR)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: rather like optical ellipsometry; looking at x-ray reflections at grazing incidence with respect to a multilayered surface. Can be used to infer layer thicknesses (assuming there is x-ray contrast between different layers)</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/X-ray_absorption_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">X-ray absorption spectroscopy (XAS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"> and</span><a href="https://en.wikipedia.org/wiki/X-ray_absorption_fine_structure" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">x-ray absorption fine structure (XAFS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Using tunable x-ray sources (e.g. beam from a synchrotron), it is possible to measure x-ray absorption in detail, allowing determination of chemical structure and valence in materials. Also related:</span><a href="https://en.wikipedia.org/wiki/X-ray_absorption_near_edge_structure" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">x-ray absorption near edge structure (</span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">XANES</span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, gives more detailed chemical information.</span></p><p dir="ltr" style="line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><b><a href="https://en.wikipedia.org/wiki/X-ray_scattering_techniques#Inelastic_X-ray_scattering_(IXS)" target="_blank">Inelastic x-ray scattering (IXS)</a></b>: Angle- and energy-resolved x-ray scattering, allowing measurement of absorption edges and detection of excitations launched in the material at some known energy and momentum transfer. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Resonant_inelastic_X-ray_scattering" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Resonant inelastic x-ray scattering (RIXS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Angle- and energy-resolved x-ray scattering where the incident wavelength is chosen to be close to an x-ray line of an element in the target. Needs tunable x-ray source (free electron laser (FEL), e.g.) Since it is sensitive to electron density, it can be used with small sample volumes, and can be used to look for dispersive excitations in the material. There is hope that RIXS can be used to detect magnetic excitations as an alternative to neutron scattering for small amounts of sample material. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/X-ray_magnetic_circular_dichroism" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 700; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">X-ray magnetic circular dichroism (XMCD)</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">: Difference of XAS between left- and right-circularly polarized x-ray beams. Can be used to infer magnetic moments of atoms in the sample. Can be resonantly enhanced if x-rays are chosen to be at transitions of the core electrons of the magnetic atoms in the material. Typically needs a synchrotron to get high brightness beams.</span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit;"><span style="background-color: transparent; color: black; font-style: italic; font-variant: normal; font-weight: 700; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">Related </span><span style="white-space-collapse: preserve;">→</span></span></p></span></span><blockquote style="border: none; margin: 0px 0px 0px 40px; padding: 0px;"><span><span><p style="line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt; text-align: left;"><span style="font-family: inherit; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span id="docs-internal-guid-a68c6b7b-7fff-32cd-d049-6f5b7c97c2fc"><a href="https://www-ssrl.slac.stanford.edu/dichroism/md.html" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline;">X-ray magnetic linear dichroism (XMLD)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline;">: Difference of XAS between x- and y-polarized x-ray beams. Closely related to XMCD, useful for looking at charge order and orbital order in magnetic materials.</span></span></span></p></span></span></blockquote><span><span><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: small; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Photoemission</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">X-ray photoemission spectroscopy (XPS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> and</span><a href="https://en.wikipedia.org/wiki/Ultraviolet_photoelectron_spectroscopy" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">ultraviolet photoemission spectroscopy (UPS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Uses x-ray or UV light to eject electrons from sample and analyzes the energy of the ejected electrons. This gives the energies of the core levels of the constituents relative to the vacuum, which encodes the valence state of the elements. Sample in vacuum. Surface-sensitive, very useful for determining chemical composition. Can be combined with etching to do depth profiling of composition. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> →</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Inverse_photoemission_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Inverse photoemission spectroscopy (IPES)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Low energy (< 20 eV) electrons interact with low-lying unoccupied electronic states, sometimes generating emitted photons. Probes states above the Fermi level of materials. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Photoemission_electron_microscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Photoemission electron microscopy (PEEM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: With a scannable optical source, it is possible to map spatial nonuniformity in photoemitted electrons.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Angle-resolved_photoemission_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Angle-resolved photoemission spectroscopy (ARPES)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Uses incident x-rays or UV at precisely known energy and momenta to eject electrons from sample; <a href="https://en.wikipedia.org/wiki/Hemispherical_electron_energy_analyzer" target="_blank">hemispherical analyzer</a> is used to measure energy and momenta of ejected electrons with high precision (energy resolution can be as sharp as 1 meV in synchrotron facilities). Sample in ultrahigh vacuum, typically requires surfaces cleaved in vacuo. This is the primary technique for measuring electronic band structure. Like all photoemission techniques, it works best on conductive samples to avoid charging problems. Variations include spin-polarized ARPES (polarization of detected electrons is found) and time-resolved ARPES (optical pump followed by time-delayed x-ray/UV pulse to do the photoemission). There is also a related technique in terms of hardware called </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">momentum-resolved EELS</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, where incident electrons of known energy and momentum are bounced off the material of interest and their final energy and momenta are measured.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Neutrons</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Neutron_diffraction" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Neutron diffraction</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Neutron scattering, requires beam of monoenergetic neutrons (prepared from a reactor via moderation + diffraction off a known crystal to act as a monochromator) (or broad-band neutrons but with time-of-flight to assess neutron energy). Sensitive to lattice structure (nuclei). Magnetic dipole interactions with electrons allows neutron diffraction to be sensitive to magnetic order. Variations: cold neutrons (prepared by scattering off cryogenic material) for higher sensitivity to magnetic systems; polarized neutrons, with polarized detection for higher sensitivity to magnetic systems. Because neutron scattering cross-sections are generally small, neutron scattering historically requires large quantities (many milligrams) of material, and single-crystal diffraction is typical (with magnetic structure measurements requiring careful alignment of sample material via XRD first). High brightness sources are improving the situation. Another challenge: some elements and isotopes have large absorption cross-sections for neutrons and thus cannot readily be measured via neutron scattering. A positive flipside of this is that neutron scattering is very sensitive to hydrogen and lithium, of interest in batteries and other energy-related applications.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Neutron_scattering#Inelastic_neutron_scattering" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Inelastic neutron scattering (INS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Momentum- and energy-resolved neutron scattering, with change in neutron energy and momentum recorded. Similar in spirit to ARPES, for mapping out dispersion relations of excitations within the sample material. This is the primary method of tracing out phonon dispersions in solids, as well as the means of identifying and quantifying magnons. Spin-polarized INS is possible, though any neutron scattering technique that requires preparation or detection of neutrons in particular spin states is more demanding (takes longer, requires higher initial flux) because of loss of neutrons during preparation and detection. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/Neutron_reflectometry" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 700; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">Neutron reflectometry</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">: Diffraction of reflected neutrons, rather analogous to EBSD, though also sensitive to magnetic scattering.</span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><span style="font-family: inherit; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span id="docs-internal-guid-342cd108-7fff-a30e-6600-9c03fcd04a6e"><a href="https://en.wikipedia.org/wiki/Small-angle_neutron_scattering" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline;">Small-angle neutron scattering (SANS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline;">: Analogous to SAXS, but with grazing-incidence neutrons. Strongly sensitive to light elements (because they have bigger neutron scattering cross-sections) and magnetic structure.</span></span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Optical spectroscopy</span></h2><p dir="ltr" style="line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;">Note that many optical techniques can be combined with microscopy to achieve spatial resolution and mapping of responses over sample surfaces. A good review article on some of these is <a href="https://doi.org/10.1146/annurev-physchem-052516-050703" target="_blank">this</a>.</p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">UV/Vis/IR absorption</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A sample is illuminated in a transmission geometry with broadband light, and by measuring the transmitted spectrum, electronic transitions can be identified and band structure can be constrained. Selection rules constrain what transitions can be seen.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Fourier-transform_infrared_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Fourier transform infrared (FTIR) spectroscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"> and microscopy</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Using a broadband mid- to far-IR light source and incorporating the sample into one arm of an interferometer, it is possible to measure absorption out to longer wavelengths (10 μm, e.g.). Good for identifying “infrared active” (e.g. involving polar displacements) low energy vibrational modes in solids.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Ellipsometry" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Ellipsometry</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;"> and spectroscopic ellipsometry</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Incident light of known wavelength, measuring reflected light from a surface as a function of angle of incidence (and wavelength of incident light, in the spectroscopic case). Allows determination of dielectric function/index of refraction, interpretation through modeling. Great for quantifying layer thicknesses for dielectric multilayers.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Terahertz_spectroscopy_and_technology" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">THz spectroscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Using THz sources and detection, can look at transmission and reflection in the mm-wave (very far IR; not quite the microwave). Great for identifying vibrational modes, low-energy excitations as in superconductivity and some magnetic states. CW sources now exist for THz using quantum cascade lasers. Time-resolved THz (</span><a href="https://en.wikipedia.org/wiki/Terahertz_time-domain_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">THz time-domain spectroscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">) is often used, as broadband THz pulses can be created using pulsed lasers and photoconductive antennas. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Optical conductivity</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: By measuring real and imaginary parts of the dielectric function (through light scattering, ellipsometry, absorption measurements) and using the Kramers-Kronig relations, it is possible to infer the frequency-dependent conductivity σ(ω), which can reveal a lot about dynamics of charged excitations.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Faraday_rotator" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Faraday rotation</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: In transmission, the polarization of light can be rotated due to magnetization of the sample. Provides information about magnetic structure of materials.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Magneto-optic_Kerr_effect" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Magneto-optic Kerr effect (MOKE)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: In reflection, the polarization of light can be rotated due to magnetization of the sample. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Raman_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Raman spectroscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: This is inelastic light scattering, often applied to molecules or optical phonons in solids. An incoming photon of angular frequency ω</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">0</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">. Elastic scattering is called Rayleigh scattering. If the photon excites a vibration or another excitation of energy </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">ℏω</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, the (“Stokes”) scattered photon comes out with frequency ω</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">0</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> – ω. If the system is already excited, the (“anti-Stokes”) scattered photon can grab energy from the excitation and come out with frequency ω</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">0</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> + ω. Raman scattering can take place if the polarizability tensor of the system α depends on the displacements of the atoms. In Raman spectroscopy of solid crystalline materials, with polarization control of the incoming light and known incident angle vs. the crystallographic orientation, it is possible to gain insight into dispersion of excitations. Detection is usually done with a grating spectrometer + CCD or CMOS camera. Variation: magnetoRaman, where sample is in an applied magnetic field.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Brillouin_scattering" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Brillouin light scattering</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Inelastic light scattering at quite low energy transfers, better suited for looking at acoustic phonons, magnons, etc. in solids. Energy transfers are sufficiently small that detection is usually done with an interferometer.</span></p><p dir="ltr" style="line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><b><a href="https://en.wikipedia.org/wiki/Photoluminescence" target="_blank">Photoluminescence (PL)</a></b>: Optical spectroscopy in which incident light electronically excites the sample, and the sample then emits photons of energies characteristic of the electronic excitations. This is a standard way to characterize excitons and related excitations in semiconductors. Variations include time-resolved PL (to look at dynamics of excitations and their lifetimes) using pulsed excitation and timed detection; and two-photon PL (TPPL), in which high intensity lower energy excitation is used to nonlinearly excite the sample. (Nonlinear optical processes depend critically on symmetries of the underlying material.) When applied to molecular systems (or semiconductor nanocrystals) in the context of chemistry, PL is often referred to as <a href="https://en.wikipedia.org/wiki/Fluorescence_spectroscopy" target="_blank">fluorescence spectroscopy</a>.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Electronic transport</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Current%E2%80%93voltage_characteristic" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">I-V characterization</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Measuring the current as a function of voltage (or voltage as a function of current). Depending on the material involved, considerable information may be inferred from such data.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Magnetoresistance/magnetoconductance</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Measuring electrical resistance or conductance as a function of applied magnetic field and temperature. Conductance measurements = source a voltage, measure a current. Resistance measurements = source a current, measure a voltage. Best practice, if possible, is to perform a 4-terminal (or more) measurement, with current sourced via two leads and voltages measured with other leads. Since an ideal voltage probe draws no current, contact resistances do not interfere with the voltage measurement. </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Differential conductance/differential resistance</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: For differential conductance (d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">), the applied bias includes a small ac voltage in addition to an applied dc voltage </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, and an ac measurement (via a</span><a href="https://en.wikipedia.org/wiki/Lock-in_amplifier" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">lock-in amplifier</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">) allows the detection of the ac contribution to the current; this allows measurement of d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> as a function of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">. Similarly, for differential resistance (d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">), the applied bias includes a small ac current in addition to an applied dc current </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, and an ac measurement via lock-in allows detection of the ac contribution to the voltage; this allows measurement of d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> as a function of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">. Note that differential resistance measurements are appropriate for examining candidate superconductors, when it is possible that the sample may support nonzero current with zero voltage.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Hall_effect" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Hall effect</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: By measuring longitudinal and transverse resistance (</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">R</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">xx</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> ≡ </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">xx</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">x</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">R</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">xy</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> ≡ </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">xy</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">x</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">) in the presence of a perpendicular magnetic field </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">B</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">z</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, it is possible to infer the sign of the charge carriers, charge mobility, and carrier density (assuming an isotropic single-band conductor). </span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Tunneling spectroscopy</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: In a tunnel junction (between a conducting sample and a normal metal probe electrode), at zero temperature the differential tunneling conductance d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> is proportional to the electronic density of states of the probe at its Fermi energy and the density of states of the sample at </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">E</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> = </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">E</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">F,sample</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">-e</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, where </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> is the bias voltage of the probe relative to the sample. (For a superconducting probe, the probe density of states is very sharp but is also shifted relative to the normal state </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">E</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">F</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> because of the superconducting energy gap.)</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Inelastic_electron_tunneling_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Inelastic electron tunneling spectroscopy (IETS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Conventionally, in tunneling spectroscopy, when the bias energy scale </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">eV</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> crosses the energy </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">ℏω </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">required to inelastically excite an excitation of the sample, this adds a possible path for electron transport. The result is a kink in </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">-</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, equivalently a step in </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">dI</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs. </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, and therefore a peak in d</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> (at positive </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">) at </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">=</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">ℏ</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">ω/</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">e</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">.</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">A real excitation of the sample should result in antisymmetric d</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">I</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: super;">2</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> features at </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">dc</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">=±</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">ℏ</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">ω/</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">e</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">. This approach has been used to identify vibrations in molecules, optical phonons in solids, and also magnetic excitations in solids. The IETS features are broadened by the finite electronic temperature (</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">k</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">B</span></span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">), so cryogenic temperatures are best suited for this technique.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Thermodynamic and thermal measurements</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Specific_heat_capacity" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Specific heat</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Adding a small amount of thermal energy to a sample via a heater and measuring the temperature rise of the sample using a local thermometer. Because of the relationship between specific heat and entropy (</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">C</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">p</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> = (1/</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">)(∂</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">S</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/∂</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">)|</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">p</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">), the specific heat as a function of temperature may be used to infer entropy. First-order phase transitions show up as a huge feature in specific heat vs temperature, since the entropy is discontinuous across a first-order transition. Second-order phase transitions show up as a singular feature (discontinuity) in heat capacity vs temperature because (∂</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">S</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/∂</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">) is discontinuous across such a transition, and will show critical fluctuations approaching the transition temperature. Specific heat of metals is linear in </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> at low temperatures and is used to infer the electronic density of states at the Fermi level.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Related</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-left: 36pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Differential_scanning_calorimetry" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Differential scanning calorimetry (DSC)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Temperature is measured as heat input to the sample is scanned. Intended to reveal phase changes within the material.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Thermal_conductivity_measurement" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Thermal conductivity</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A known thermal energy current is applied through a sample, and the temperature drop across the sample is measured using local thermometers. This is a measure of the transport of energy by all mobile excitations in the material. In conductors, charge carriers are expected to transport an amount of energy proportional to their specific heat, leading in metals to the</span><a href="https://en.wikipedia.org/wiki/Wiedemann%E2%80%93Franz_law" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Wiedemann-Franz</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> relation.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Thermal_conductivity_measurement" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Thermal expansion</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Changes in sample dimensions as a function of temperature are measured, giving insights into material structure and bonding. Typically, thermal expansion relates to the anharmonicity of the interatomic potential, and it is related therefore to nonlinearities in the properties of phonons (see the</span><a href="https://en.wikipedia.org/wiki/Gr%C3%BCneisen_parameter" style="text-decoration-line: none;"><span style="color: black; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Grüneisen parameter</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">).</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Seebeck_coefficient" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Thermopower/Seebeck coefficient</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Absolute Seebeck response = the change in voltage across a sample is measured as a function of the temperature difference imposed across the sample. Electronic excitations (and phonons) tend to diffuse away from the hot side. Seebeck response sign generally depends on sign of the charge carriers (electron-like or hole-like). The Seebeck response in a conductor is proportional to the energy dependence of the conductivity (and hence the mean free path) of the carriers.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Nernst_effect" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Nernst-Ettingshausen effect</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: In a Hall-like geometry, the transverse voltage across a sample </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">V</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">xy</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> is propertional to the temperature gradient along the sample d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">x</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> and the mutually perpendicular magnetic field </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">B</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">z</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, so that the Nernst coefficient is defined as ν = (</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">E</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">xy</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">B</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span style="vertical-align: sub;">z</span></span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">)/(d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">/d</span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">x</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">). This gives information about the transverse scattering of heat-carrying excitations in the presence of a magnetic field.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-size: 11pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Magnetic measurements</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">Magnetization</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Measurements of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">M</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">H</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> may be performed using SQUID-based and other magnetometers, though knowledge of sample dimensions and geometry are required. Characteristic features of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">M</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> are expected for certain material types. For example, near zero field, a superconductor is expected to show perfect diamagnetism. Often measurements are also made of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">M</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">T</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> at fixed </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">H</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">, comparing field-cooled and zero-field-cooled responses. Saturation of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">M</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> vs </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">H</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> at low temperatures and high fields can reveal the magnetic state of elements hosting local magnetic moments.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Vibrating-sample_magnetometer" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Vibrating sample magnetometry</span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;"> </span><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">(VSM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: a particular type of magnetometer that vibrates the sample back and forth through pickup coils.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Magnetic_susceptibility#In_the_frequency_domain" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">AC susceptibility</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: An oscillating component of </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">H</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> is applied and the change in </span><span style="font-style: italic; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; vertical-align: baseline; white-space-collapse: preserve;">M</span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> is measured.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 700; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">Nuclear magnetic resonance (NMR)</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">: liquid (for molecules) or solid-state. Applied magnetic field provides Zeeman energy splitting for spin states of nuclei, radio frequency pulse sequences (and continuous wave methods) used to determine nuclear spin properties (and because of hyperfine couplings, provides information about electronic states). Specific effects in superconductors (</span><a href="https://en.wikipedia.org/wiki/Knight_shift" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 400; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">Knight shift</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">). Care must be taken with conducting samples, as microwaves don’t necessarily penetrate into the bulk of the material.</span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 700; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">Electron paramagnetic resonance (EPR) or electron spin resonance (ESR)</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">: Applied magnetic field provides Zeeman energy splitting for spin states of electrons, microwave pulse sequences (and continuous wave methods) are applied to do spectroscopy of these. Best in insulating materials with unpaired electrons. Particularly handy in determining the </span><span style="background-color: transparent; color: black; font-style: italic; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">g</span><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;"> factors for local magnetic moments, which is affected by crystal fields (local chemical bonding environment) at the local spin-carrying atoms. </span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/Ferromagnetic_resonance" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 700; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">Ferromagnetic resonance (FMR)</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">: Conventionally, a radio frequency/microwave drive is applied to make the ferromagnetic magnetization </span><span style="background-color: transparent; color: black; font-style: italic; font-variant: normal; font-weight: 700; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">M</span><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;"> of a material precess around an external magnetic field. Gives information about the magnetization dynamics and damping. Recently, FMR in small devices has been driven via spin currents (from the spin Hall effect/</span><a href="https://www.nist.gov/programs-projects/theory-spin-orbit-torque" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 400; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">spin-orbit torques</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;"> or </span><a href="https://en.wikipedia.org/wiki/Spin-transfer_torque" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 400; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">spin transfer torques</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">).</span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span id="docs-internal-guid-146d0a4d-7fff-a344-7374-9b4f4880201b"><a href="https://en.wikipedia.org/wiki/M%C3%B6ssbauer_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline;">Mossbauer spectroscopy</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline;">: This is really a nuclear physics-based technique, but given that the most famous Mössbauer material is iron, it has relevance for magnetism. Gamma-ray spectroscopy using the Mössbauer effect (collective recoil or lack thereof of the entire lattice rather than individual atoms), gives extremely precise energetic information about nuclear environment of the particular isotopes, including hyperfine interactions.</span></span></span></p><p dir="ltr" style="line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline;"><span><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><b><a href="https://en.wikipedia.org/wiki/Muon_spin_spectroscopy" target="_blank">Muon spin spectroscopy (μSR)</a></b>: Muons produced via an accelerator are implanted or transmitted through a material of interest. Decay of positive muons leads to emission of positrons, with directional asymmetry of emission related to the spin state of the muon. These measurements this give information about the magnetic environment within the material. Does not require pulsed fields. </span></span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"> </span></p><h2 dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt;"><span style="color: #2f5496; font-size: 13pt; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 400; vertical-align: baseline; white-space-collapse: preserve;">Other techniques to assess composition</span></h2><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Secondary_ion_mass_spectrometry" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Secondary ion mass spectrometry (SIMS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Material is sputtered away from the sample, and the fragments are analyzed using mass spectrometry (e.g., ionized fragments are accelerated and curved in a magnetic field for detection, to determine their charge to mass ratio).</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Inductively_coupled_plasma_mass_spectrometry" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Inductively coupled plasma mass spectrometry (ICP-MS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Using an inductively coupled plasma source to ionize sample material for MS.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Atomic_emission_spectroscopy" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Atomic emission spectroscopy (AES)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Material is heated or otherwise excited, and the emission spectra of the products is measured. Modern version of old approach of looking at the color of flame produced by a bit of material.</span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><a href="https://en.wikipedia.org/wiki/Rutherford_backscattering_spectrometry" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Rutherford backscattering spectrometry (RBS)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: Ions (protons, alpha particles) are fired at the sample <span style="font-family: inherit;">material and back-scattered ions are detected; can give depth-dependent compositional information.</span></span></p><p dir="ltr" style="font-family: inherit; line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;"><span id="docs-internal-guid-d7262bec-7fff-b0ce-5033-d87ecfc6c343"><a href="https://en.wikipedia.org/wiki/Thermogravimetric_analysis" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline;">Thermogravitic analysis (TGA)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline;">: Destructive technique. The sample is placed in a sensitive balance and heated through its decomposition, and the sample is weighed as the temperature is swept. Different breakdown products will be produced at different temperatures. Often combined with mass spectrometry to determine the molecular weight of the evolved products.</span></span></span></p></span></span><p><br /></p><h2 style="line-height: 1.38; margin-bottom: 0pt; margin-top: 2pt; text-align: left;"><span style="background-color: transparent; color: #2f5496; font-size: 13pt; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;"><span style="font-family: inherit;">Other surface characterization methods</span></span></h2><p dir="ltr" style="line-height: 1.38; margin-bottom: 3pt; margin-top: 3pt;"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/Helium_atom_scattering" style="text-decoration: none;"><span style="background-color: transparent; color: #0563c1; font-style: normal; font-variant: normal; font-weight: 700; text-decoration-skip-ink: none; text-decoration: underline; vertical-align: baseline; white-space: pre-wrap;">Helium atom scattering (HAS)</span></a><span style="background-color: transparent; color: black; font-style: normal; font-variant: normal; font-weight: 400; text-decoration: none; vertical-align: baseline; white-space: pre-wrap;">: Diffraction of helium atoms off surfaces. Extremely surface sensitive.</span></span></p><p><span id="docs-internal-guid-88c70d63-7fff-6141-7fa2-cd8176257bab"><span style="font-family: inherit;"><a href="https://en.wikipedia.org/wiki/Field_ion_microscope" style="text-decoration-line: none;"><span style="color: #0563c1; font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; font-weight: 700; text-decoration-line: underline; text-decoration-skip-ink: none; vertical-align: baseline; white-space-collapse: preserve;">Field ion microscopy (FIM)</span></a><span style="font-variant-alternates: normal; font-variant-east-asian: normal; font-variant-numeric: normal; font-variant-position: normal; vertical-align: baseline; white-space-collapse: preserve;">: A sharp tip is biased up to a high voltage. Gas molecules impinge on the tip, ionize due to the strong electric field, and are repelled away to a detection screen. Amazingly, this can give atomically precise information about the configuration of atoms at the tip.</span></span></span></p><br /><p></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com15tag:blogger.com,1999:blog-13869903.post-11101384050706318482024-01-13T21:06:00.003-06:002024-01-15T11:31:55.437-06:00Dye-sensitized solar cells - an idea whose time has finally come?Dyes are generally small molecules that have electronic transitions with energies corresponding to the visible spectrum of light (around 1-3 eV). Around 35 years ago, the idea was put forward, particularly by <a href="https://en.wikipedia.org/wiki/Michael_Gr%C3%A4tzel" target="_blank">Michael Grätzel</a> and <a href="https://en.wikipedia.org/wiki/Brian_O%27Regan_(chemist)" target="_blank">Brian O'Regan</a>, to couple dye molecules to semiconductors and electrolytes, so that when the dye molecules are excited by light, the electrons/holes can be captured and used for photovoltaic power. This is the concept behind <a href="https://en.wikipedia.org/wiki/Dye-sensitized_solar_cell" target="_blank">dye-sensitized solar cells</a>, as demonstrated early on <a href="https://www.nature.com/articles/353737a0" target="_blank">here</a>. I <a href="https://nanoscale.blogspot.com/2011/05/nano-for-solar.html" target="_blank">wrote a little about this</a> a long while ago. <table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEibE5XB_nGxFG0uZBL9kDzaXpgDtEYHIL4tM9fc-QJcysDIrMFJSBsvl_qnSnZVUU-ZoHELsADgXD6WfWXi4yvM5oxZQ39HFt5dAFyZXkZgVgMozqSfEXsCaTdZy5m0av0awMuDgLhggGfN-P4Tt4qGORRy2yiH-hYTnX3FVJF8tv280_oZZk7lcw/s778/d0cs01336f-f3.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="652" data-original-width="778" height="268" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEibE5XB_nGxFG0uZBL9kDzaXpgDtEYHIL4tM9fc-QJcysDIrMFJSBsvl_qnSnZVUU-ZoHELsADgXD6WfWXi4yvM5oxZQ39HFt5dAFyZXkZgVgMozqSfEXsCaTdZy5m0av0awMuDgLhggGfN-P4Tt4qGORRy2yiH-hYTnX3FVJF8tv280_oZZk7lcw/s320/d0cs01336f-f3.jpg" width="320" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;">Energy diagram of a dye-sensitized photovoltaic<br />cell, from <a href="https://doi.org/10.1039/D0CS01336F" target="_blank">this paper</a>. </span></td></tr></tbody></table><br /><div><br /></div><div>This is a compelling idea, and a main selling point is the hope that devices based on this could be cheap and much less energy-intensive in their manufacturing, since they could be made with materials that don't require high temperature synthesis or high purity, like Si solar cells. After many years of effort, end-to-end power conversion efficiencies are up around 13% for outdoor solar illumination-type conditions. Here (<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8591630/" target="_blank">link to NIH free version</a>) is a good review from 2021 that is very complete in summarizing progress.</div><div><br /></div><div>So, 13% is nice, but it's hard to see that being competitive with Si for bulk photovoltaics, and <a href="https://en.wikipedia.org/wiki/Perovskite_solar_cell" target="_blank">perovskites</a>, also solution-processable, are up over 25%, similar to Si. Still, outdoor solar is not the only application! <a href="https://www.nature.com/articles/nphoton.2017.60" target="_blank">This paper from 2017</a> showed that it is possible for dye-sensitized cells to get power conversion efficiencies up to around <b>30%</b> for <i>indoor</i> lighting conditions (much lower intensity, different spectra than solar illumination). That seems to be the basis for <a href="https://www.wsj.com/tech/personal-tech/what-if-you-never-had-to-charge-your-gadgets-again-955ea960" target="_blank">this story in today's Wall Street Journal</a>, pointing out that there are actual consumer products coming to market that have dye-sensitized cells for indoor operations. Very cool to see this product really start to make it out of the lab! If one of my readers has a good, clear explanation of <i>why</i> the power conversion efficiency is so much better for indoor lighting conditions, please leave it in the comments. </div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com7tag:blogger.com,1999:blog-13869903.post-19642672042282312122024-01-10T09:26:00.003-06:002024-01-10T09:26:50.473-06:00Items of interestFor the first post of the new calendar year, here are a few items that I thought were interesting:<div><ul style="text-align: left;"><li><a href="https://www.science.org/content/article/ghostly-quasiparticle-rooted-century-old-mystery-unlock-quantum-computings-potential" target="_blank">Here</a> is a feature article in Science that talks about the experimental quest for detecting <a href="https://en.wikipedia.org/wiki/Majorana_fermion" target="_blank">Majorana fermions</a> in solid state systems, bookended by the story of <a href="https://en.wikipedia.org/wiki/Ettore_Majorana" target="_blank">Majorana</a>'s <a href="https://en.wikipedia.org/wiki/Ettore_Majorana#Disappearance" target="_blank">disappearance</a>. </li><li><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right; margin-left: 1em; text-align: right;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwJvVxbbLxwB_t59unyRcL4I_A4-DGhV9jybf2k9wPpANm085KcLaPO38s2xQ8sK-PV6AEk3kefvsyQrg_k_01ow-NkVgHMF2SSwFxqO_zJg8JTuX28sv2IrIjjvFh27zzMSU-QaDXZ6RZljyDgOx7LhGsp0LSZYavmyH3I0jwXAa-R5BQODeY0Q/s500/altermag.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" data-original-height="154" data-original-width="500" height="107" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhwJvVxbbLxwB_t59unyRcL4I_A4-DGhV9jybf2k9wPpANm085KcLaPO38s2xQ8sK-PV6AEk3kefvsyQrg_k_01ow-NkVgHMF2SSwFxqO_zJg8JTuX28sv2IrIjjvFh27zzMSU-QaDXZ6RZljyDgOx7LhGsp0LSZYavmyH3I0jwXAa-R5BQODeY0Q/w346-h107/altermag.png" width="346" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;">Adapted from <a href="https://doi.org/10.1103/PhysRevX.12.040501" target="_blank">PRX <b>12</b>, 045501 (2022)</a></span></td></tr></tbody></table>This <a href="https://physics.aps.org/articles/v17/4" target="_blank">brief column</a> is a good starting point with references if you want to learn about altermagnetism. For a lengthier, more technical discussion, see <a href="https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.040501" target="_blank">this PRX paper</a>. Quickly: in local-moment <a href="https://en.wikipedia.org/wiki/Ferromagnetism" target="_blank">ferromagnetism</a>, the spins (and therefore magnetic moments) of electrons on lattice sites in a material spontaneously align (at least, in a single <a href="https://en.wikipedia.org/wiki/Magnetic_domain" target="_blank">domain</a>). Shifting over one lattice site leads to the same pattern of magnetic moments, so the ferromagnetic ordered state is "invariant under a lattice translation". In a local moment <a href="https://en.wikipedia.org/wiki/Antiferromagnetism" target="_blank">antiferromagnet</a>, the spins alternate up and down on neighboring sites. Shifting over one lattice site + a 180 degree rotation gets back the same pattern of magnetic moments, so the antiferromagnetic ordered state is invariant under a one lattice site translation plus 180 degree rotation. It turns out, there is a third possibility: in an "altermagnet", neighboring spins alternate up and down, but their local environments are rotated by 90 degrees, so that the ordered state is invariant under a spin flip plus a 90 degree rotation. This has neat consequences for band structure and could lead to technological applications.<br /></li><li>A statement in the press this week caused me to realize that I've never written a nicely accessible post about magnetism and how it works. Thinking about how to do that brought me back to this <a href="https://www.youtube.com/watch?v=Q1lL-hXO27Q" target="_blank">classic video with Richard Feynman</a>, explaining why this can be very challenging. It seems necessary to ask a general reader simply to accept certain postulates - for example, that electrons, which are nominally point particles, have angular momentum called "spin", and that associated with that spin is a magnetic moment, so that electrons act in some sense like little magnetic dipoles. That really is remarkable, and in trying to find a way to think about this that is more accessible, I found <a href="https://doi.org/10.1119/1.14580" target="_blank">this classic paper</a> (pdf <a href="https://physics.mcmaster.ca/phys3mm3/notes/whatisspin.pdf" target="_blank">here</a>) by <a href="https://www.uvm.edu/cems/physics/profiles/hans-ohanian" target="_blank">Ohanian</a>. My conclusion: I still need to think about this further, particularly the connection between the classical dipolar \(1/r^{3}\) field from a magnetic dipole and the fact that spin for an electron is a quantum mechanical quantity that follows its particular rules.</li><li><a href="https://en.wikipedia.org/wiki/N._David_Mermin" target="_blank">N. David Mermin</a> posted a neat little <a href="https://arxiv.org/abs/2401.04711" target="_blank">autobiographical essay</a> on the arxiv yesterday. Fun to read, especially if you are familiar with his writing.</li></ul></div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com7tag:blogger.com,1999:blog-13869903.post-35208709789657398072023-12-31T16:53:00.002-06:002023-12-31T16:53:36.408-06:00Very brief end of the year round-up<p>It's hard to believe that it's already the end of 2023. It's been a busy year for condensed matter; it's unfortunate that two of the biggest stories (problems with high pressure superconductivity papers; the brief excitement about LK99, the not-actually-a-superconductor) were probably the field's highest profile events. Still, hopefully the latter at least had the effect of bringing to the public a little bit of the excitement and potential of how condensed matter and materials physics affects our lives. Physics World summarizes some of their picks for big materials-related stories of 2023 <a href="https://physicsworld.com/a/materials-physics-highlights-of-2023/" target="_blank">here</a>. Similarly, <a href="https://www.quantamagazine.org/the-biggest-discoveries-in-physics-in-2023-20231221/" target="_blank">here</a> are Quanta's choices for biggest physics stories of the year, and <a href="https://physics.aps.org/articles/v16/213" target="_blank">these are the choices</a> from the editors of APS's Physics. </p><p>It's been a busy year personally, with lots going on and too much proposal writing, but at least my blog posting was more frequent than in 2022. It's still surprising to me that I've been writing this since mid-2005, enough to see almost the entire lifecycle of blogging. Happy New Year to my readers, and if there are any particular topics about which you think I should write, please let me know in the comments. I'm always looking for CM/materials concepts that I can try to explain on a non-specialist accessible level. Still looking for the time and appealing perspective to write that popular book....</p><p>Anyway, I hope you have a very happy new year, and best wishes for a great 2024.</p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com6tag:blogger.com,1999:blog-13869903.post-60477950941313997792023-12-21T17:08:00.001-06:002023-12-21T17:08:06.258-06:00New paper - plasmons, excitons, and steering energy <p>We have a <a href="https://doi.org/10.1021/acs.nanolett.3c04684" target="_blank">new paper out in Nano Letters</a> (arxiv version <a href="https://arxiv.org/abs/2302.00023" target="_blank">here</a>), and I wanted to explain a bit about it and why I think it's a really cool result. </p><p>I've <a href="https://nanoscale.blogspot.com/2021/10/the-purcell-effect-still-mind-blowing.html" target="_blank">written before</a> about the <a href="https://en.wikipedia.org/wiki/Purcell_effect" target="_blank">Purcell Effect</a>. When we study quantum mechanics, we learn that the rates of processes, like the spontaneous emission of light from an atom, are actually malleable. The rate of a particular process is usually proportional to the number of ways that process can happen - this is quantified in something called <a href="https://nanoscale.blogspot.com/2014/05/recurring-themes-in-condensed.html" target="_blank">Fermi's Golden Rule</a>. When we are talking about something like emission of light from an atom, the rate is proportional to the number of possible final states of the photon. We know how to count those states in a given energy range in free space, and Purcell pointed out that by placing that atom in an optical cavity, we alter the density of final states as a function of frequency, \(\rho(\omega)\) from its empty space value, and hence can change the rate of emission. Pretty wild that placing a system in a cavity can alter the flow of energy in that system away from what it would otherwise be.</p><p>I've also <a href="https://nanoscale.blogspot.com/2014/04/recurring-themes-in-condensed.html" target="_blank">written before</a> about what happens we take two resonators and couple them together - we get "hybridization" or "new normal modes". If you take a mass on a spring (natural frequency \(\omega_0 = \sqrt{k/m}\)) and couple it mechanically to another identical mass on an identical spring, the coupled system will now have two resonances, one above and one below \(\omega_{0}\). The chemistry analog of this is, bonding two hydrogen atoms (each with 1<i>s</i> orbitals) together leads to two \(\sigma\) orbitals, one bonding and one antibonding. </p><p>In the new paper, we start with a little metal tunnel junction that hosts plasmonic resonances, like the junctions I wrote about <a href="https://nanoscale.blogspot.com/2020/06/how-do-hot-electrons-get-hot.html" target="_blank">here</a>. We showed in that paper and subsequent work that it is possible to use an applied voltage and current to get some of the electrons, right near where the electrodes almost touch, to become effectively so hot that they glow (emitting light at energies larger than the applied voltage), while the atomic lattice itself remains cold. The light emission process here is the radiative recombination of hot electrons and holes in the metal, where an electron drops down in energy to fill in a hole and spit out a photon. The plasmon resonances of the bare metal act like a sort of cavity, shaping the density of photon states \(\rho(\omega)\), as we also showed <a href="https://nanoscale.blogspot.com/2022/10/getting-light-out-of-plasmonic-tunnel.html" target="_blank">here</a>. The plasmons, set by the metal shape and electronic properties, actually affect the rate at which the electrons and holes in that same metal radiatively combine.</p><p><table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjJywCMkCIADxuWam6_XH3gsxsy-BHZ85OtJAslzH6perUSYE0yRhjPlSRZM4J34FsFVWCgmlDddyTtv_Yzi1Qhc3nddKamHbcQbM7ORtQswVxfF-NUxap0KTmIH5_BM4tZxfyEN-xqZqHAmTRO_NuJyFVe5PbMFAanDtLqRE1yAhoE-ZmpyRAwnQ/s756/plasmonwse2.png" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="476" data-original-width="756" height="201" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjJywCMkCIADxuWam6_XH3gsxsy-BHZ85OtJAslzH6perUSYE0yRhjPlSRZM4J34FsFVWCgmlDddyTtv_Yzi1Qhc3nddKamHbcQbM7ORtQswVxfF-NUxap0KTmIH5_BM4tZxfyEN-xqZqHAmTRO_NuJyFVe5PbMFAanDtLqRE1yAhoE-ZmpyRAwnQ/s320/plasmonwse2.png" width="320" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;"><span style="font-size: x-small;">Left: A thin flake of WSe2 is placed on a plasmonic<br />Au junction. Right: Overbias light emission from the<br />device at a particular emitted polarization shows a big <br />peak splitting right around where the exciton resonance <br />is of the WSe2 (orange curve). Adapted from the <br />SI of <a href="https://doi.org/10.1021/acs.nanolett.3c04684" target="_blank">this paper</a>.</span></td></tr></tbody></table><br />The wrinkle in the new paper is that we couple that metal plasmonic junction with a thin (few nm) layer of 2D semiconductor by placing the semiconductor on top of the metal. The semiconductor can host <a href="https://nanoscale.blogspot.com/2010/06/excitons.html" target="_blank">excitons</a>, bound electron-hole pairs, and if the semiconductor is excited with enough energy to create them, the excitons can radiatively annihilate, leading to a comparatively narrow resonance at an energy that overlaps the plasmon resonances of the metal junction. Thanks to hybridization between the plasmons in the metal and the excitons in the semiconductor, the photon density of states now has a split peak structure ("upper and lower plexciton polariton resonances" if you are an expert). Light emission in this device is still due to recombination of electrons and holes in the metal, but now the recombination dynamics of those electrons "feels" the strong coupling between the excitons and plasmons. (The polarization of the emitted light is rather complicated because of the polarization properties of the plasmon resonances). </p><p>There are a lot of interesting possibilities on where to go from here, but it's always amazing to me to see how this physics comes together. In this case, by changing the optical environment of a metal structure, we can alter the fate of energy stored in the electrons of that metal. Really neat.</p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com2tag:blogger.com,1999:blog-13869903.post-75616343353603143922023-12-12T11:41:00.002-06:002023-12-12T11:41:27.563-06:00AI/ML and condensed matter + materials science<div>Materials define the way we live. That may sound like an exaggeration that I like to spout because I'm a condensed matter physicist, but it's demonstrably true. Remember, past historians have given us terms like "Stone Age", "Bronze Age", and "Iron Age", and the "Information Age" has also been called the "<a href="https://en.wikipedia.org/wiki/Silicon#:~:text=The%20%22Silicon%20Age%22%20refers%20to,century%20to%20early%2021st%20century." target="_blank">Silicon Age</a>". (And who could forget <a href="https://www.youtube.com/watch?v=eMtLdE5Zq-8" target="_blank">plastics</a>.)</div><div><br /></div><div>Perhaps it's not surprising, then, that some of the biggest, most wealthy companies in the world are turning their attention to materials and the possibility that AI approaches could lead to disruptive changes. As I mentioned last week, there have been recent papers (<a href="https://www.nature.com/articles/s41586-023-06735-9" target="_blank">back</a> to <a href="https://www.nature.com/articles/s41586-023-06734-w" target="_blank">back</a> in Nature) by the <a href="https://deepmind.google/" target="_blank">Google Deep Mind</a> group on this topic. The idea is to use their particular flavor of AI/machine learning to identify potential new compounds/solids that should be thermodynamically stable and synthesizable, and make predictions about their structures and properties. This is not a new idea, in that the <a href="https://www.mgi.gov/" target="_blank">Materials Genome Initiative</a> (started in 2011) has been working in this direction, compiling large amounts of data about solid materials and their properties, and the <a href="https://next-gen.materialsproject.org/" target="_blank">Materials Project</a> has been pushing on efficient computational methods with the modest goal of computing "the properties of all inorganic materials and provid[ing] the data and associated analysis algorithms for every materials researcher free of charge".</div><div><br /></div><div>In addition to the Google work, Microsoft has released <a href="https://arxiv.org/abs/2312.03687" target="_blank">on the arxiv</a> their effort, <a href="https://www.microsoft.com/en-us/research/blog/mattergen-property-guided-materials-design/" target="_blank">MatterGen</a>, which uses a generative AI approach to try to predict new stable materials with desirable properties, such as a target symmetry or chemical composition or mechanical/electronic/magnetic response. An example from their paper is to try to find new magnetic materials that have industrially useful properties but do not involve rare earths. </div><div><br /></div><div>There is a long way to go on any of these projects, but it's easy to see why the approach is enticing. Imagine saying, I want a material that's as electrically conductive and mechanically strong and workable as aluminum, but transparent in the visible, and having software give you a credible approach likely to succeed (rather than having to rely on a <a href="https://www.youtube.com/watch?v=90eg_erObDo" target="_blank">time-traveling Mr. Scott</a>). </div><div><br /></div><div>I'd be curious to know readers' opinions of what constitute the biggest obstacles on this path. Is it the reliability of computational methods at predicting formation energies and structures? Is it the lack of rapid yet robust experimental screening approaches? Is it that the way generative AI and related tools work is just not well-suited to finding truly new systems beyond their training sets?</div><div><br /></div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com10tag:blogger.com,1999:blog-13869903.post-86304409949766519752023-12-01T17:22:00.005-06:002023-12-02T16:00:45.266-06:00Intriguing papers - exquisite thermal measurements + automated materials discovery/synthesisIt's a busy time, but I wanted to point out a couple of papers from this past week.<div><br /></div><div>First, I want to point to <a href="https://arxiv.org/abs/2311.15787" target="_blank">this preprint</a> on the arxiv, where the Weizmann folks do an incredibly technically impressive thing. I'd <a href="https://nanoscale.blogspot.com/2023/08/what-is-thermal-hall-effect.html" target="_blank">written recently</a> about the thermal Hall effect, when a longitudinal heat current (and temperature gradient) in the presence of a magnetic field results in a <i>transverse</i> temperature gradient as well as the usual longitudinal one. One of the most interesting ways this can happen is if there are edge modes, excitations that propagate around the perimeter of a 2D system and can carry heat (even if they are neutral and don't carry charge). Unsurprisingly, to measure thermal transport requires putting thermometers at different places on the sample and carefully measuring temperature differences. Well, these folks have done just exquisitely nice measurements of <a href="https://nanoscale.blogspot.com/2009/12/noise-ii.html" target="_blank">Johnson-Nyquist noise</a> in particular contacts for thermometry, and they can see the incredibly tiny heat currents carried by rather exotic edge modes in some unusual fractional quantum Hall states. It's just a technical tour de force.</div><div><br /></div><div>Second, on a completely unrelated note, there are back to back papers in Nature this week from the Google deep mind folks - their own <a href="https://deepmind.google/discover/blog/millions-of-new-materials-discovered-with-deep-learning/" target="_blank">write-up is here</a>. The <a href="https://www.nature.com/articles/s41586-023-06735-9" target="_blank">first paper</a> uses their methods to predict a large number of what are expected to be new stable crystal structures. The <a href="https://www.nature.com/articles/s41586-023-06734-w" target="_blank">second paper</a> talks about how they used an automated/robot-driven lab to try to synthesize a bunch of these in an automated way and characterize the resulting material. This is certainly thought-provoking. It is worth noting that detailed characterization (including confirming that you've made what you were trying to make) and optimized synthesis of new materials is very challenging and of concern <a href="https://twitter.com/Robert_Palgrave/status/1730358675523424344" target="_blank">here</a>. <b>Update</b>: there is further discussion of the characterization <a href="https://www.linkedin.com/feed/update/urn:li:activity:7136540178843652097/?updateEntityUrn=urn%3Ali%3Afs_feedUpdate%3A%28V2%2Curn%3Ali%3Aactivity%3A7136540178843652097%29" target="_blank">here</a> (on LinkedIn by the authors) as well, and more on Twitter <a href="https://twitter.com/SchoopLab/status/1730802839222358251" target="_blank">here</a> and <a href="https://twitter.com/Robert_Palgrave/status/1730882978664911329" target="_blank">here</a>.</div><div><br /></div><div>Third, <a href="https://doi.org/10.1103/PhysRevX.13.041036" target="_blank">this paper</a> looks extremely interesting. It’s long been a staple of condensed matter theory to try to capture complex materials with effective low energy models, like suggesting the Hubbard model as a treatment of the essential physics of the cuprate superconductors. The authors here report that they’ve done a more orbital-based/<i>ab initio</i> version of this, solved these models numerically, and state that they can reproduce details of the phase diagram of four of the cuprates spanning a big range of superconducting transition temperatures. Seems like this may bode well for gaining insights into these systems.</div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com1tag:blogger.com,1999:blog-13869903.post-50244189104967740382023-11-27T14:29:00.000-06:002023-11-27T14:29:49.457-06:00Noise in a strange metal - pushing techniques into new systems<p>Over the holiday weekend, we had a <a href="https://doi.org/10.1126/science.abq6100" target="_blank">paper</a> come out in which we report the results of measuring charge <a href="https://nanoscale.blogspot.com/2009/12/noise-iii.html" target="_blank">shot noise</a> (see <a href="https://en.wikipedia.org/wiki/Shot_noise" target="_blank">here</a> also) in a <a href="https://nanoscale.blogspot.com/2014/09/what-is-bad-metal-what-is-strange-metal.html" target="_blank">strange metal</a>. Other write-ups of the work (<a href="https://phys.org/news/2023-11-physicists-evidence-exotic-quantum-material.html" target="_blank">here</a> and especially <a href="https://www.quantamagazine.org/meet-strange-metals-where-electricity-may-flow-without-electrons-20231127/" target="_blank">this nice article in Quanta here</a>) do a good job of explaining what we saw, but I wanted to highlight a couple of specific points that I think deserve emphasis. </p><p>In thermal equilibrium at some temperature \(T\), there are current and voltage fluctuations in a conductor - this is called <a href="https://nanoscale.blogspot.com/2009/12/noise-ii.html" target="_blank">Johnson-Nyquist noise</a> - and it is <a href="https://en.wikipedia.org/wiki/Fluctuation-dissipation_theorem" target="_blank">unavoidable</a>. Shot noise in electrical current results from the granularity of charge and, as shown in its <a href="https://ia600708.us.archive.org/view_archive.php?archive=/28/items/crossref-pre-1923-scholarly-works/10.1002%252Fandp.19183622102.zip&file=10.1002%252Fandp.19183622304.pdf" target="_blank">original incarnation</a> (pdf is in German), from the statistical variation in the arrival times of electrons. Shot noise is an "excess" noise that appears in addition to this, only when a conductor is driven out of equilibrium by an applied voltage and carries a net current.</p><p>While the idea of shot noise is tunnel junctions and vacuum tubes had been worked out a long time ago (see the above 1918 paper by <a href="https://en.wikipedia.org/wiki/Walter_H._Schottky" target="_blank">Schottky</a>), it was in the 1990s when people really turned to the question of what one should see in noise measurements in small metal or semiconductor wires. Why don't we see shot noise in macroscopic conductors like your house wiring? Well, shot noise requires some deviation of the electrons from their thermal equilibrium response - otherwise you would just have Johnson-Nyquist noise. The electrons in a metal or semiconductor are coupled to the vibrations of the atoms (<a href="https://en.wikipedia.org/wiki/Phonon" target="_blank">phonons</a>) - the clearest evidence for this is that the decrease in scattering of the electrons by the phonons explains why metals become more conductive as temperature is decreased. In conductors large compared to the (temperature-dependent) electron-phonon scattering length, the electrons should basically be in good thermal equilibrium with the lattice at temperature \(T\), so all that should be detected is Johnson-Nyquist noise. To see shot noise in a wire, you'd need the wire to be small compared to that e-ph length, typically on the order of a micron at low temperatures. In the 1980s and 1990s, it was now possible to make structures on that scale.</p><p></p><table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhR_LIpP_3tvKE8m-v7JJcW-cGMbDx0rK1OtNMDpnrvcjMZTeWgfK5CyGHb9VTNqKwanvupDQC8zq747j57RZx7RyG_WQVEU8TOxjgNO9rDW6PO4xaAHaMTFpdstsfDZu84fbQjwZfgE4IF2VIfIZjemLhaCDbG2sUuh8HrKZnHzdr_sqAfimhdxQ/s1073/yrsfig4.png" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="428" data-original-width="1073" height="157" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhR_LIpP_3tvKE8m-v7JJcW-cGMbDx0rK1OtNMDpnrvcjMZTeWgfK5CyGHb9VTNqKwanvupDQC8zq747j57RZx7RyG_WQVEU8TOxjgNO9rDW6PO4xaAHaMTFpdstsfDZu84fbQjwZfgE4IF2VIfIZjemLhaCDbG2sUuh8HrKZnHzdr_sqAfimhdxQ/w393-h157/yrsfig4.png" width="393" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">Fig. 4 from the paper</td></tr></tbody></table>The theory of what should be seen was worked out in a couple of different ways, initially assuming that it is safe to describe the conductor as a Fermi gas (ignoring electron-electron interactions). One approach started from the conduction-as-wave-transmission picture of <a href="https://en.wikipedia.org/wiki/Rolf_Landauer" target="_blank">Landauer</a> (see <a href="https://doi.org/10.1103/PhysRevB.46.1889" target="_blank">here</a> and <a href="https://doi.org/10.1103/PhysRevB.46.12485" target="_blank">here</a> for two examples). A complementary approach (see <a href="https://doi.org/10.1103/PhysRevB.52.4740" target="_blank">here</a>) calculated noise from the electronic distribution functions and got the same answer for non-interacting electrons, that the current noise should be 1/3 of the classic Schottky result. That factor of 1/3 is called the Fano factor, \(F\). If electron-electron interactions are "turned on", allowing the electrons to exchange energy amongst themselves but <i>not</i> lose energy to the lattice, the noise is actually a bit larger, \(F \rightarrow \sqrt{3}/4\). It turns out that these values were verified in experiments in gold wires (see <a href="https://doi.org/10.1103/PhysRevLett.76.3806" target="_blank">here</a> and <a href="https://doi.org/10.1103/PhysRevB.59.2871" target="_blank">here</a>, though one has to be careful in experimental design to see \(F \rightarrow \sqrt{3}/4\)). This confirmation is a great triumph of our understanding of physics at these mesoscopic scales. (Interestingly, similar results are expected even with a <i>non-degenerate</i> electron gas - see <a href="https://doi.org/10.1103/PhysRevLett.80.2901" target="_blank">here</a> and <a href="https://doi.org/10.1103/PhysRevLett.82.2761" target="_blank">here</a>.)<div><br /></div><div>We applied these same experimental approaches to nanowires we made from exquisite films of a strange metal, YbRh2Si2, and we found that the noise is much reduced from the usual result seen in Au wires (which we also confirmed). We tested whether phonons could be responsible for the noise suppression, applying the same approach as had been done in the '90s (<a href="https://doi.org/10.1103/PhysRevB.59.2871" target="_blank">measurements</a> on wires tens of microns long, where e-ph scattering should be important), and found (in addition to further confirming the e-ph energy loss results in Au from the '90s) that energy loss to phonons can't explain what we see in YbRhsSi2. </div><div><br /></div><div>Some further points of interest:<p></p><p></p><ul style="text-align: left;"><li>Until recently there really has not been much attempt to push the theoretical analysis of these kinds of measurements beyond the 1990s/early 2000s results. My <a href="https://qmsi.rice.edu/about-qimiao-si/" target="_blank">colleague</a> Qimiao Si and his group have <a href="https://arxiv.org/abs/2211.11735" target="_blank">looked</a> at whether strong Fermi liquid corrections affect the expected noise, and the answer is "no". Of course, there are all kinds of additional complications that one could imagine.</li><li>This work was only possible because of the existence of high quality thin films of the material, and the ability to fabricate nanostructures from this stuff without introducing so much disorder or chemical change as to ruin the material. My collaborator <a href="https://www.ifp.tuwien.ac.at/paschen" target="_blank">Silke Bühler-Paschen</a> and her group have spent years learning how to grow this and related materials, and long-term support for materials growth is critically important. My student, the lead author on the study, did great work figuring out the fabrication. It's really not trivial. </li><li>I think it's worthwhile to consider pushing older techniques into new regimes and applying them to new materials systems. The heyday of mesoscopics in the 1990s doesn't need to be viewed as a closed, somewhat completed subfield, but rather as a base from which to consider new ways to characterize the rich variety of materials and phases that we have to play with in condensed matter. </li></ul><p></p></div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com10tag:blogger.com,1999:blog-13869903.post-14656259183588058732023-11-16T11:55:00.000-06:002023-11-16T11:55:09.798-06:00Faculty positions at Rice - follow-up<p>I had mentioned about 6 weeks ago that my department at Rice is searching in the quantum/AMO space for experiment and theory. Now I want to put the larger context of this out there - Rice has four quantum-related searches going on right now:</p><div>Quantum theory (PHYA): <a href="https://apply.interfolio.com/131379">https://apply.interfolio.com/131379</a></div><div>Quantum experiment (PHYA): <a href="https://apply.interfolio.com/131378">https://apply.interfolio.com/131378</a></div><div>Quantum engineering (ECE): <a href="https://apply.interfolio.com/133316">https://apply.interfolio.com/133316</a></div><div>Quantum materials (MSNE): <a href="https://apply.interfolio.com/135086">https://apply.interfolio.com/135086</a></div><div><br /></div><div>Interested candidates, we hope you will apply! It's an exciting time here, and our quantum initiative folks can help make sure applications end up in the right place. </div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com1tag:blogger.com,1999:blog-13869903.post-85283990323427181132023-11-16T11:47:00.000-06:002023-11-16T11:47:20.280-06:00Postdoctoral opportunities at Rice<p>I will be sending some emails shortly, but I wanted to point out postdoctoral opportunities here at Rice University.</p><p>The <a href="https://sci.rice.edu/" target="_blank">Smalley-Curl Institute</a> is having a <a href="https://sci.rice.edu/postdoctoral-research-associates" target="_blank">competition</a> for two two-year postdoctoral fellow slots. Click on the link for the details. The requirements for a candidate:</p><p></p><ul style="text-align: left;"><li>Nomination by current <a href="https://sci.rice.edu/sci-members" target="_blank">SCI faculty member</a></li><li>Ph.D. in a field related to an SCI focus areas</li><li>Successful Ph.D. thesis defense before start of appointment</li><li>Ph.D. completed no more than three years before the start of the appointment</li></ul>I would be happy to work with an interested, competitive candidate on this, and the deadline for applying is December 31. Research areas in my lab these days include: nanostructure-based studies of correlated quantum materials, including noise-based measurements; studies of spin transport and thermally driven spin effects in insulating magnets, from basic science to applications in low-power electronics; plasmon-based nanophotonic light sources and plasmonic junctions for physical chemistry. If you're a student finishing up and are interested, please contact me, and if you're a faculty member working with possible candidates, please feel free to point out this opportunity. <p></p><p>Rice also has a university-wide endowed honorific postdoctoral program called the <a href="https://riceacademy.rice.edu/application-information" target="_blank">Rice Academy of Fellows</a>. Like all such things, it's very competitive, and it similarly has a deadline of January 3, 2024. Again, applicants have to have a faculty mentor, so in case someone is interested in working with me on this, please contact me via email. </p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com0tag:blogger.com,1999:blog-13869903.post-53384891876152606212023-11-11T14:04:00.001-06:002023-11-11T14:04:13.868-06:00Scientific publishing - where are we going?<p>I think it's safe to say that anyone involved in scientific publishing will tell you that it's a mess and the trends are worrisome. This week, <a href="https://phys.org/news/2023-11-avalanche-published-academic-articles-erode.html#google_vignette" target="_blank">this news release/article</a> came out about <a href="https://arxiv.org/abs/2309.15884" target="_blank">this preprint</a> which shows a number of the issues. In brief (not all of this is in the preprint; some is me editorializing):<table cellpadding="0" cellspacing="0" class="tr-caption-container" style="float: right;"><tbody><tr><td style="text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRyOlj3Fyaf0ZTKvQonuhkpp8dv0wobrVhvfuIiXxMoKJtk7CCoggS6dJ_3l2AKHttlGStfhHPdywXMmbygiskqk4g6FsSxBzrpXTRa9Xp-rXCp25sygy3ydBIS_rx4TsgrMFTkzb366drOFD1z-1YuNe0futOf1saD9cPfPGJV8eXSzLLMtmtcw/s964/publishingdata.jpg" imageanchor="1" style="clear: right; margin-bottom: 1em; margin-left: auto; margin-right: auto;"><img border="0" data-original-height="964" data-original-width="710" height="442" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRyOlj3Fyaf0ZTKvQonuhkpp8dv0wobrVhvfuIiXxMoKJtk7CCoggS6dJ_3l2AKHttlGStfhHPdywXMmbygiskqk4g6FsSxBzrpXTRa9Xp-rXCp25sygy3ydBIS_rx4TsgrMFTkzb366drOFD1z-1YuNe0futOf1saD9cPfPGJV8eXSzLLMtmtcw/w326-h442/publishingdata.jpg" width="326" /></a></td></tr><tr><td class="tr-caption" style="text-align: center;">Figure 1 from <a href="https://arxiv.org/abs/2309.15884" target="_blank">this preprint</a></td></tr></tbody></table><br /></p><p></p><ul style="text-align: left;"><li>The number of scientific papers being published is growing at a rate that looks completely unsustainable. In my opinion, it's problematic on multiple levels. There aren't enough reviewers (though that doesn't bother all publishers) and the average paper gets smaller and smaller readership (raising the question of why bother to publish papers that no one reads). Does it make sense that the number of papers is skyrocketing while the number of PhDs granted is falling?</li><li>Some publishers (especially <a href="https://www.frontiersin.org/" target="_blank">Frontiers</a>, <a href="https://www.hindawi.com/" target="_blank">Hindawi</a>, <a href="https://www.mdpi.com/" target="_blank">MDPI</a>) have boosted this by drastically cranking up the number of papers that they publish, through launching specialized journals with "special issues" designed to have super-short review times (assuming that review is even truly part of the process). Lest you think this is only the provenance of publishers previously accused of being <a href="https://en.wikipedia.org/wiki/Predatory_publishing" target="_blank">predatory</a>, this week alone I have received five different "special issue" announcements from <a href="https://publishing.aip.org/" target="_blank">AIP journals</a>.</li><li>Why do people do this? To try to game the <a href="https://en.wikipedia.org/wiki/Impact_factor" target="_blank">impact factor</a> calculations. I've aired my grievances <a href="https://nanoscale.blogspot.com/2012/08/impact-factors-and-damage-they-do.html" target="_blank">before</a> <a href="https://nanoscale.blogspot.com/2016/07/impact-factors-and-academic-moneyball.html" target="_blank">about</a> why journal impact factor is a lousy metric. </li><li>Why do people want to inflate impact factors? Because that's how journals keep score, and some countries put in place big-time incentives tied to impact factor. A publisher worries that if its journal's impact factor falls below some threshold, then the government of China, for example, will no longer view that journal as important, and then <i>thousands</i> of authors will stop submitting....</li><li>Open access is a complicating factor, with some publishers charging absolutely sky-high charges, while at the same time having very high profit margins. In the US, at least, those charges can be much larger than what grants will support.</li><li>Over all of this is the concern that massively inflating the amount of scientific literature lowers its quality and undermines the credibility of science in general. </li></ul>Coincidentally, this week we hosted <a href="https://science.psu.edu/astro/people/sxs540" target="_blank">Steinn Sigurðsson</a> for a colloquium. He is now the scientific director of the <a href="https://arxiv.org/" target="_blank">arxiv</a>, the preprint archive that went from a quick and dirty preprint sharing site in 1991 to an enormously important part of the global scientific enterprise. In his talk he hit on some wild numbers. The arxiv is up to around 20,000 papers per month now (!) (in part because new disciplines like quantitative biology are using the arxiv). Thankfully the arxiv has recently landed some <a href="https://blog.arxiv.org/2023/10/20/arxiv-receives-10-million-in-gifts-and-grants-from-simons-foundation-and-national-science-foundation/" target="_blank">good support</a>. Their annual operating budget is around $3.5M, and this is an enormous bargain by any measure. The arxiv is partnering with volunteer developers who are <a href="https://info.arxiv.org/labs/showcase.html" target="_blank">adding some neat functionality</a>. Unsurprisingly, generative AI is a serious concern, even more so than for the publishing houses. <p></p><p>It's a transformative time, for sure. Maybe what we are seeing is analogous to the fluctuations that happen when approaching a 2nd order phase transition, and we are headed for a real change in the way publishing works. It's hard to see how the current trends can continue unabated.</p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com15tag:blogger.com,1999:blog-13869903.post-76082373699158758462023-11-01T20:54:00.005-05:002023-11-01T20:54:44.256-05:00Strategic planning + departmental reviews<p>It's been a while since I've written a post about the ways of academia, so I thought it might be time, though it's not exactly glamorous or exciting. There are certain cycles in research universities, and two interrelated ones are the cycle of departmental strategic planning and the cycle of external departmental reviews.</p><p>Strategic planning can be extremely important, as it allows departments to take stock of where things are, what opportunities exist for improvement (in terms of research, teaching, departmental operations), and how the department aspires to move forward. Often this can involve a hiring plan, based on demographic trends in the department (e.g., how many faculty lines are expected to be available in the next, say, five to seven years?), rising field/school/university research priorities (e.g., there is likely to be enormous investment in AI/ML in the coming years). Discussions for strategic planning can be frought, since even maintaining departmental faculty size means alloting new hires between different possible research areas in a zero sum. Still, arriving at a departmental plan is often expected at one level up (that of a School or College, depending on the university's org chart labeling scheme), and having a plan that department members know and understand is helpful in transparency of how decisions get made that shape the future of the department. It doesn't make sense to do reformulate these plans at too rapid a frequency, since the ability to implement the plan can be strongly perturbed by, e.g., economic events, global pandemics, or big changes in university leadership.</p><p>Very often, deans (or provosts) also value periodic reviews of departments by an external visiting committee. The visiting committee is typically put together with input from the department (research areas that should be represented, suggestions of possible reviewers) and invited to come for a couple of days of interviews and departmental presentations. These reviews are typically very broad, looking at research, teaching, departmental climate, staffing levels and organization, infrastructure and space needs, etc. It's important to talk to all stakeholders (departmental leadership, TT and NTT faculty, staff, undergrad and grad students, postdocs, and of course the dean or equivalent who is the intended recipient of the report). The expected output of these visits is a report to the dean (or provost). Such a report can be very helpful for the department to get feedback on their plans and operations, and to serve as a way of putting priorities forward to the dean/provost level. Similarly, often deans find these things valuable as a way to make certain arguments up to higher levels. It seems to be human nature that a statement made by a nominally objective external committee can get more traction than the same statement made by locals. Like strategic plans, it only makes sense to do external reviews on a timescale sufficiently long that the department would have a chance to address issues raised from the previous visit before the next one. For both of these things, every five years is on the edge of being too frequent, and every ten years would definitely be too long an interval.</p><p>Participating in external visits takes time, but I've found it to be a very valuable experience. It's allowed me to meet and work with faculty from a variety of places, and it can be very helpful to see how other institutions do things (even at a level of learning about tools like <a href="https://www.stellic.com/" target="_blank">software that can be useful for tracking degree progress</a>, or organizations that work to facilitate <a href="https://www.erdosinstitute.org/" target="_blank">career placement at the graduate level</a>). </p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com3tag:blogger.com,1999:blog-13869903.post-18066869599114172962023-10-27T14:26:00.004-05:002023-10-27T14:27:54.478-05:00Reading material - orders of magnitude and difficult times<p>Over the past couple of weeks (and more) I have found a number of things to read that I wanted to pass on. First, if you'd like a break from the seemingly continual stream of bad news in the world and enjoy good "think like a physicist"/dimensional analysis/order of magnitude estimate/<a href="https://en.wikipedia.org/wiki/Fermi_problem" target="_blank">Fermi problem</a> discussions, I suggest:</p><p></p><ul style="text-align: left;"><li><a href="https://www.astro.princeton.edu/~burrows/classes/542/papers/Patryk.1975_Weisskopf.pdf" target="_blank">This paper</a> (<a href="https://www.jstor.org/stable/1739660" target="_blank">jstor</a> link here) by <a href="https://en.wikipedia.org/wiki/Victor_Weisskopf" target="_blank">Weisskopf</a>, which does a great job at explaining quite a bit about matter, like the heights of mountains, e.g. I had previously recommended this <a href="https://nanoscale.blogspot.com/2018/05/coming-attractions-and-short-items.html" target="_blank">back in 2018</a>. </li><li>That led me to a collection of Weisskopf's series of articles in the <a href="https://pubs.aip.org/aapt/ajp" target="_blank">American Journal of Physics</a> from back in the day, all under the name "<a href="https://scholar.google.com/scholar?hl=en&as_sdt=0%2C44&q=weisskopf+%22search+for+simplicity%22&oq=weissk" target="_blank">Search for Simplicity</a>". Here is a <a href="https://minotwiki.physics.oregonstate.edu/lib/exe/fetch.php?media=weisskopf_simplicity_long.pdf" target="_blank">link</a> to a pdf file from the <a href="https://minotlab.physics.oregonstate.edu/content/lab" target="_blank">Minot Lab</a> at Oregon State, and they have a bunch of <a href="https://minotwiki.physics.oregonstate.edu/doku.php?id=fermi_estimates" target="_blank">other content</a> along those lines. </li><li>The famed <a href="https://en.wikipedia.org/wiki/Edward_Mills_Purcell" target="_blank">Edward Purcell</a> also wrote a bunch of content for AJP about similar estimates; a page with links to those is <a href="https://www.aapt.org/Publications/AJP/Readers/back_of_the_envelope.cfm" target="_blank">here</a> from the <a href="https://www.aapt.org/" target="_blank">AAPT</a>.</li><li>The amazingly versatile and articulate <a href="https://www.kitp.ucsb.edu/zee" target="_blank">Anthony Zee</a> has written a wonderful textbook in this vein, called <i><a href="https://www.kitp.ucsb.edu/zee/books/fly-by-night-physics" target="_blank">Fly By Night Physics</a></i>. It's great - highly recommended.</li></ul><div>On a much more sobering note, I was saddened to learn of the grave illness of <a href="https://en.wikipedia.org/wiki/Jan_Zaanen" target="_blank">Prof. Jan Zaanen</a>, who has terminal cancer. A colleague brought my attention to an essay (<a href="https://www.lorentz.leidenuniv.nl/zaanen/wordpress/wp-content/uploads/2023/10/musingsb-1.pdf" target="_blank">link here</a>) that Prof. Zaanen has written in the hopes that it will be widely read, and I pass it along. </div><div><br /></div><div>More soon.</div><p></p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com4tag:blogger.com,1999:blog-13869903.post-18362704882406639502023-10-18T13:26:00.002-05:002023-10-18T13:26:20.509-05:00Scientific travel Particularly in these post-pandemic, climate-change-addled, zoom-enabled times, I appreciate the argument that it's always worth asking, "Is this trip really necessary?" We are in the age of remote work and zoom seminars that are attended by people from all over the world. Is there sufficient benefit to in-person visits to justify travel for work? I just got back from my first really lengthy science trip in a number of years, and it was definitely very valuable, with experiences and knowledge transfer that just could not have happened nearly as readily any other way. <div><br /></div><div>I was fortunate enough to be able to attend and speak at a (beginning of October) <a href="https://www.q-ms.org/events-seminars/" target="_blank">summer school</a> at <a href="https://ista.ac.at/en/home/" target="_blank">ISTA</a>, which is a large and growing scientific institute in Klosterneuberg, outside of Vienna, and I was also able to visit my collaborator's lab at TU Wien as well as the <a href="https://www.ifp.tuwien.ac.at/wien-mu-k/" target="_blank">Vienna MicroKelvin Laboratory</a>. I spend the following week visiting the <a href="https://www.lps.u-psud.fr/en/home-english/" target="_blank">Laboratoire de Physique des Solides</a> in Orsay, hanging out with the quantum electronics group. Many thanks to my hosts for helping to organize these trips and for making me feel so welcome.</div><div><br /></div><div>In-person visits allow for longer, extensive, interactive conversations - standing at a whiteboard, or having coffee, or pointing at actual apparatus. It's a completely different experience than talking to someone over zoom or over the phone. I think I did a better job explaining our work, and I definitely think that I learned a lot more about diverse topics than if I'd only had brief virtual interactions. As an experimentalist, it can be very valuable to learn details about how some measurements are actually done, even including which bits of equipment and instrumentation are preferred by others. (LPS has a ton of <a href="https://www.rohde-schwarz.com/us/home_48230.html" target="_blank">Rohde and Schwarz</a> equipment, which I've really not seen to that extent in the US. I'd also never heard of <a href="https://www.mycryofirm.com/" target="_blank">mycryofirm</a> and their closed cycle cryostats.)<div class="separator" style="clear: both; text-align: center;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi9fyXM0tD2XLbuEk4RCPbhDL361YLxDbBTJH5I4QRl0aA3PG942lsyUzq0oqaVicxQEQIuyUT2pavbKa8UA3p1KufRiH38xRTs2thxx84nBFklKtm6bjTpNsTchDFWPgyOarMrXFcDMWeSAObZk4N6DZwYgDzOR3rMwyM8ROpOsyD3puM_XwYaQA/s4032/curie_geiger.jpg" imageanchor="1" style="clear: right; float: right; margin-bottom: 1em; margin-left: 1em;"><img border="0" data-original-height="3024" data-original-width="4032" height="240" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi9fyXM0tD2XLbuEk4RCPbhDL361YLxDbBTJH5I4QRl0aA3PG942lsyUzq0oqaVicxQEQIuyUT2pavbKa8UA3p1KufRiH38xRTs2thxx84nBFklKtm6bjTpNsTchDFWPgyOarMrXFcDMWeSAObZk4N6DZwYgDzOR3rMwyM8ROpOsyD3puM_XwYaQA/s320/curie_geiger.jpg" width="320" /></a></div></div><div><br /></div><div>As an added bonus, I got to visit the <a href="https://musee.curie.fr/" target="_blank">Musée Curie</a> in Paris and see Marie Curie's lab. Here is a photograph of a Geiger counter that they'd made c. 1930. Hand-soldered, uninsulated wires. The biggest tube is the actual Geiger-Müller tube which produces current pulses when ionizing radiation zips through it. The other two tubes make an amplifier to crank up the current pulses enough to turn an electric motor that drives a mechanical counter on the far right outside the box.</div><div><br /></div><div>Hopefully I will finish up some writing and be posting more soon.</div>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com13tag:blogger.com,1999:blog-13869903.post-42084788711043825142023-10-05T07:21:00.002-05:002023-10-05T07:21:24.083-05:00The Nobels, physics and chemistry<p>As you undoubtedly know, the 2023 Nobel in physics has been awarded to <a href="https://en.wikipedia.org/wiki/Pierre_Agostini" target="_blank">Pierre Agostini</a>, <a href="https://en.wikipedia.org/wiki/Ferenc_Krausz" target="_blank">Ferenc Krausz</a>, and <a href="https://en.wikipedia.org/wiki/Anne_L%27Huillier" target="_blank">Anne L'Huillier</a>, for the development of techniques associated with <a href="https://en.wikipedia.org/wiki/Attosecond" target="_blank">attosecond-scale</a> optical pulses. <a href="https://www.nobelprize.org/uploads/2023/10/popular-physicsprize2023.pdf" target="_blank">Here</a> is the more popular write-up about this (including a good handwave of how attosecond pulses can be made) from the Nobel Foundation, and <a href="https://www.nobelprize.org/uploads/2023/10/advanced-physicsprize2023.pdf" target="_blank">here</a> is the more technical version. A number of people (including friends and relatives) have asked me in the last couple of days about this, including what discoveries have these techniques led to, and how is this work different than preceding Nobel prizes (like the <a href="https://www.nobelprize.org/prizes/chemistry/1999/press-release/" target="_blank">1999 chemistry prize</a> for femtosecond chemistry, the <a href="https://www.nobelprize.org/prizes/physics/2005/9807-the-nobel-prize-in-physics-2005-2005-6/" target="_blank">2005 prize</a> in physics for frequency combs, and the <a href="https://www.nobelprize.org/prizes/physics/2018/summary/" target="_blank">half of the 2018 physics prize</a> for femtosecond pulsed lasers). This isn't really my area of expertise, but my impression from talking with people is that the attosecond work is thus far more of a technical achievement than a technique that has led to a series of groundbreaking scientific results or technologies. </p><p>Scientifically, the attosecond regime is very fast compared to the dynamics of, e.g., solids. That said, attosecond techniques have been used to characterize condensed matter systems, as described <a href="https://doi.org/10.1038/nature06229" target="_blank">here</a>. Crudely speaking, the relevant energy scale associated with 100 as is \(h/10^{-16} s \sim\) 40 eV, the kind of energy (in the deep ultraviolet range) associated with photoemission. It makes sense that some of the results highlighted in the Nobel citation have to do with using these methods to measure time delays associated with photoemission - like seeing that 4<i>f</i> electrons take longer to photoemit than <i>s</i> and <i>p</i> electrons in other bands. If readers can point to a great explanation that goes deeper than this, please leave it in the comments.</p><p>The 2023 chemistry prize has been awarded to <a href="https://en.wikipedia.org/wiki/Moungi_Bawendi" target="_blank">Moungi Bawendi</a>, <a href="https://en.wikipedia.org/wiki/Louis_E._Brus" target="_blank">Louis Brus</a>, and <a href="https://en.wikipedia.org/wiki/Alexey_Ekimov" target="_blank">Alexey Ekimov</a>, for the discovery and development of semiconductor nanocrystals now popularly called <a href="https://en.wikipedia.org/wiki/Quantum_dot" target="_blank">quantum dots</a>. These systems are absolutely great platforms to demonstrate <a href="https://en.wikipedia.org/wiki/Potential_well#Quantum_confinement" target="_blank">quantum confinement</a>. By taking a bulk semiconductor and carving it up into pieces so small that the electronic wavefunctions get squeezed by the boundaries - this generally increases the energy spacings between levels, including the energy associated with the gap between the valence band and the conduction band. That is, a semiconductor that might fluoresce in the red in the bulk can be chopped into pieces that fluoresce in the green or the blue (higher energies). The story of these materials (their growth, how to make them uniform and stable without bad defects at their surfaces) is very cool. Quantum dots are now widely used as <a href="https://en.wikipedia.org/wiki/Quantum_dot_display" target="_blank">luminescent materials in display devices</a>, and they are also <a href="https://doi.org/10.1146/annurev-anchem-060908-155136" target="_blank">broadly employed as fluorophores</a> for biological imaging and related applications. (Louis Brus is a Rice alumnus - huzzah! I've never met Dr. Ekimov, but in my experience, Brus and Bawendi are both very nice, down-to-earth people whose groups write clear, non-hype-ridden papers.)</p>Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.com1