Big questions about condensed matter (part 3)

More questions asked by Ross McKenzie's son about the culture/history of condensed matter physics:

3.  What are the most interesting historical anecdotes?  What are the most significant historical events?  Who were the major players?

The first couple of these are hard to address in anything resembling an unbiased way.  For events that happened before I was in the field, I have to rely on stories I've read or things I've heard.  Certainly the discovery of superconductivity by Onnes is a good example - where they thought that they had an experimental problem with their wiring, until they realized that their voltmeter reading dropping to zero (trying to measure the voltage drop across some mercury in the presence of a known current) happened at basically the same temperature every time.  (Pretty good for 1911!).  Major experimental results very often have fun story components.  From my thesis adviser, I'd heard lots of stories about the discovery of superfluidity in 3He, including plugging a leaky vacuum flange using borax; thinking up the experiment while recovering from a broken leg skiing accident; the wee-hour phone call to the adviser.   He also told me a story about this paper, where he and Gerry Dolan came up with a very clever way to see tiny deviations away from a mostly linear current-voltage curve, an observation connected with weak localization that paved the way for a lot of mesoscopic physics work.   

There are fun theory stories, too.  Bob Laughlin figuring out the theory of the fractional quantum Hall effect while stuck in a trailer at Livermore because his clearance paperwork hadn't come through yet.

Other stories I've read in books.  Strong recommendations for Crystal Fire; the less popular/more scholarly Out of the Crystal Maze.  Ohl's discovery of the photovoltaic effect in silicon.  The story about how Bell Labs and IBM researchers may or may not have traded hints poolside in Las Vegas about how to get field-effect transistors really working.  Shockley's inability to manage people eventually resulting in Silicon Valley.  

These aren't necessarily the best anecdotes, but they have elements of interest.  I'm sure there are many out there who could tell fun stories.

As for the major players, it seems that everyone mentioned on Prof. McKenzie's post and in the comments are theorists.  That seems limiting.  It's fair to talk about theorists if you're concentrating on theoretical developments, but experimentalists have often opened up whole areas.  Onnes liquefied helium and discovered superconductivity.  Laue invented x-ray diffraction.  Brattain made transistors.  Nick Holonyak was an inventor of the light emitting diode, which has been revolutionary.   Binnig and Rohrer invented the STM.  Bednorz and Muller discovered the cuprates.  

Big questions about condensed matter (part 2)

Continuing, another question asked by Ross McKenzie's son:

2. Scientific knowledge changes with time. Sometimes long-accepted ``facts''  and ``theories'' become overturned? What ideas and results are you presenting that you are almost absolutely certain of? What might be overturned?

I think this question is framed interestingly.  Physics in general and condensed matter in particular is a discipline where the overturning of long-accepted ideas has often really meant a clearer appreciation and articulation about the limits of validity of models, rather than a wholesale revision of understanding. 

For example, the Mermin-Wagner theorem is often mentioned as showing that one cannot have true two-dimensional crystals (this would be a breaking of continuous translational symmetry).  However, the existence of graphene and other atomically thin systems like transition metal dichalcogenides, and the existence of magnetic order in some of those materials, are experimentally demonstrated.  That doesn't mean that Mermin-Wagner is mathematically incorrect.  It means that one must be very careful in defining what is meant by "truly two-dimensional". 

There are many things in condensed matter that are as "absolutely certain" as anything gets in science.  The wave nature of x-rays, electrons, and neutrons plus the spatial periodicity of matter in crystals leads to clear diffraction patterns.  That same spatial periodicity strongly influences the electronic properties of crystals (Bloch's theorem, labeling of states by some wavevector-like parameter \(\mathbf{k}\), some energy dependence of those states \(E(\mathbf{k})\)).   More broadly, there are phases of matter that can be classified by symmetries and topology, with distinct macroscopic properties.  The macroscopic phases that are seen in equilibrium are those that correspond to the largest number of microscopic configurations subject to any overall constraints (that's the statistical physics basis for thermodynamics).  Amazingly, knowing the ground state electronic density of a system everywhere means its possible in principle to calculate just about everything about the ground state.

Leaving those aside, asking what might be overturned is a bit like asking where we might find either surprises or mistakes in the literature.  Sometimes seemingly established wisdom does get upset.  One recent example:  For a couple of decades, it's been thought that Sr₂RuO₄ is likely a spin-triplet superconductor, where the electron pairs are p-wave paired (have net orbital angular momentum), and is an electronic analog to the A phase of  superfluid ³He.  Recent results suggest that this is not correct, and that the early evidence for this is not seen in new measurements.   There are probably more things like this out there, but it's hard to speculate.  Bear in mind, though, that science is supposed to work like this.  In the long run, the truth will out.

Big questions about condensed matter physics (pt 1)

Ross McKenzie, blogger of Condensed Concepts, is working on a forthcoming book, a Very Short Introduction about condensed matter physics.  After reading some sample chapters, his son posed some questions about the field, and Prof. McKenzie put forward some of his preliminary answers here.  These are fun, thought-provoking topics, and I regret being so busy writing other things that I haven't had a chance to think about these as much as I'd like.  Still, here are some thoughts.

1. What do you think is the coolest or most exciting thing that CMP has discovered? 

Tricky.  There are some things that are very intellectually profound and cool to the initiated that would not strike an average person as cool or exciting.  The fractional quantum Hall effect was completely surprising.  I heard a story about Dan Tsui looking at the data coming in on a strip chart recorder (Hall voltage as a function of time as the magnetic field was swept), roughly estimating the magnetic field from the graph with the span of his fingers, realizing that they were seeing a Hall plateau that seemed to imply a charge of 1/3 e, and saying, jokingly, "Quarks!"  In fact, there really are fractionally charged excitations in that system.  That's very cool, and but not something any non-expert would appreciate. 

Prof. McKenzie votes for superconductivity, and that's definitely up there.  In some ways, superfluidity is even wilder.  Kamerlingh Onnes, the first to liquefy helium and cool it below the superfluid transition, somehow missed discovering superfluidity, which had to wait for Kapitsa and independently Allen in 1937.  Still, it is very weird - fluid flowing without viscosity through tiny pores, and climbing walls (!).   While it's less useful than superconductivity, you can actually see its weird properties readily with the naked eye.

ACS symposium - Chemistry in Real Space and Time

On Sunday I was fortunate enough to be able to speak at the first day of a week-long symposium at the American Chemical Society national meeting in San Diego, titled "Chemistry in Real Space and Time".  This symposium was organized by Ara Apkarian, Eric Potma, and Venkat Bommisetty, all from the UC Irvine NSF-supported center for Chemistry at the Space-Time Limit.  During its span as a center, CaSTL has been at the forefront of technique development, including integrating ultrafast optics-based time-resolved measurements with the atomic-scale precision of scanning tunneling microscopy.  The center is sun-setting, and the symposium is a bit of a valedictory celebration.

The start of our semester made it necessary for me to return to Houston, but a couple of highlights from the first day:

• Pri Narang spoke about her group's efforts to do serious combined quantum electrodynamics calculations and microscopic nanostructure modeling.  If one wants to try to understand strong coupling problems between matter and light in nanostructures nonperturbatively, this is the direction things need to go.  An example.
• Erik Nibbering talked about ultrafast proton transport - something I'd never thought about that depends critically on the positioning and alignment, say, of water molecules, so that hydrogens can swap their oxygen bonding partners.  His group uses a combination of photoacids (for optical control over when protons are released) and time-resolved infrared spectroscopy to follow what's going on.
• Ji-Xin Cheng showed some impressive results of applying plasmon-enhanced stimulated Raman spectroscopy, basically tagging living systems with plasmonically active nanoparticles and performing pump-probe stimulated Raman to follow biological processes in living tissue. Very impressive hyperspectral imaging.  
• My colleague Stephan Link showed some nice, clean results (related to these) in understanding chirality effects (trochoidal dichroism) in scattering of light by curved nanostructures, where the longitudinal component of the electric field (only happens at surfaces) is critically important.
• Frank Hegmann and Tyler Cocker spoke about various aspects of THz-based STM.  I can't really do this justice in a brief blurb, but check out this paper and this paper for a flavor.  Similarly, Dominik Peller spoke about this paper and this paper, and Hidemi Shigekawa showed what you can do when you can achieve phase control over the THz light pulse.  Combining STM with femtosecond time resolution lets you see some impressive things.
• While not STM, but none-the-less very cool, Yichao Zhang showed movies from the Flannigan group taken by time-resolved transmission electron microscopy, so that you can actually see the propagation of sound waves.  

Wish I could've stayed to see more - I felt like I was learning a lot.

Pairs in the cuprates at higher energies than superconductivity

I've been asked by student readers over the years about how scientists come up with research ideas.  Sometimes you make an unanticipated observation or discovery, and that can launch a research direction that proves fruitful.  One example of that is the work our group has done on photothermoelectric effects in plasmonic nanostructures - we started trying to understand laser-induced heating in some of our plasmonic devices, found that the thermoelectric response of comparatively simple metal nanostructures was surprisingly complicated, and that's led to some surprising (to us) insights and multiple papers including a couple in preparation.

In contrast, sometimes you have a specific physics experiment in mind for a long time, aimed at a long-standing problem or question, and getting there takes a while.  That's the case with our recent publication in Nature.

I've written a bit about high temperature superconductivity over the years (here, here, here, here).  For non-experts, it's hard to convey the long historical arc of the problem of high temperature superconductivity in the copper oxides.

Superconductivity was first discovered in 1911 in elemental mercury, after the liquefaction of helium made it possible to reach very low temperatures.  Over the years, many more superconductors were discovered, metallic elements and compounds.  Superconductivity is a remarkable state of matter and it took decades of study and contributions by many brilliant people before Bardeen, Cooper, and Schrieffer produced the BCS theory, which does a good job of explaining superconductivity in many systems.  Briefly and overly simplified, the idea is that the ordinary metallic state (a Fermi liquid) is often not stable.  In ordinary BCS, electrons interact with phonons, the quantized vibrations of the lattice - imagine an electron zipping along, and leaving behind in its wake a lattice vibration that creates a slight excess of positive ionic charge, so that a second electron feels some effective attraction to the first one.  Below some critical temperature \(T_{c}\), electrons of opposite spin and momenta pair up.   As they pair up, the paired electrons essentially condense into a single collective quantum state.  There is some energy gap \(\Delta\) and a phase angle \(\phi\) that together make up the "order parameter" that describes the superconducting state.  The gap is the energy cost to rip apart a pair; it's the existence of this gap, and the resulting suppression of scattering of individual carriers, that leads to zero electrical resistance.  The collective response of the condensed state leads to the expulsion of magnetic flux from the material (Meissner effect) and other remarkable properties of superconductors.  In a clean, homogeneous traditional superconductor, pairing of carriers and condensation into the superconducting state are basically coincident.

In 1986, Bednorz and Muller discovered a new family of materials, the copper oxide superconductors.  These materials are ceramics rather than traditional metals, and they show superconductivity often at much higher temperatures than what had been seen before.  The excitement of the discovery is hard to overstate, because it was a surprise and because the prospect of room temperature superconductivity loomed large.  Practically overnight, "high-Tc" became the hottest problem in condensed matter physics, with many competing teams jumping into action on the experimental side, and many theorists offering competing possible mechanisms.  Competition was fierce, and emotions ran high.  There are stories about authors deliberately mis-stating chemical formulas in submitted manuscripts and then correcting at the proof stage to avoid being scooped by referees.   The level of passion involved has been substantial.   Compared to the cozy, friendly confines of the ultralow temperature physics community of my grad days, the high Tc world did not have a reputation for being warm and inviting.

As I'd mentioned in the posts linked above, the cuprates are complicated.  They're based on chemically (by doping) adding charge to or removing charge from materials that are Mott insulators, in which electron-electron interactions are very important.  The cuprates have a very rich phase diagram with a lot going on as a function of temperature and doping.  Since the earliest days, one of the big mysteries in these materials is the pseudogap (and here), and also from the earliest days, it has been suggested (by people like Anderson) that there may be pairs of charge carriers even in the normal state - so-called "preformed pairs".  That is, perhaps pairing and global superconductivity have different associated energy and temperature scales, with pair-like correlations being more robust than the superconducting state.  An analogy:  Superconductivity requires partners to pair up and for the pairs to dance in synch with each other.  In conventional superconductors, the dancing starts as soon as the dancers pair up, while in the cuprates perhaps there are pairs, but they don't dance in synch.

Moreover, the normal state of the cuprates is the mysterious "strange metal".  Some argue that it's not even clear that there are well-defined quasiparticles in the strange metal - pushing the analogy too far, perhaps it doesn't make sense to even think about individual dancers at all, and instead the dance floor is more like a mosh pit, a strongly interacting blob.

I've been thinking for a long while about how one might test for this.  One natural approach is to look at shot noise (see here).  When charge comes in discrete amounts, this can lead to fluctuations in the current.  Imagine rain falling on your rooftop.  There is some average arrival rate of water, but the fluctuations about that average rate are larger if the rain comes down in big droplets than if the rain comes falls as a fine mist.  Mathematically, when charges of magnitude \(e\) arrive at some average rate via a Poisson process (the present charge doesn't know when the last one came or when the next one is coming, but there is just some average rate), the mean square current fluctuations per unit bandwidth are flat in frequency and are given by \(S_{I} = 2 e I\), where \(I\) is the average current.  For electrons tunneling from one metal to another, accounting for finite temperature, the expectation is \(S_{I} = 2 e I \coth (eV/(2 k_{\mathrm{B}}T) )\), which reduces to \(2 e I\) in the limit \(eV >> k_{\mathrm{B}}T\), and reduces (assuming an Ohmic system) to Johnson-Nyquist noise \(4 k_{\mathrm{B}}T/R\)  in the \(V \rightarrow 0\) limit, where \(R = V/I\).

TLDR:  Shot noise is a way to infer the magnitude of the effective charge of the carriers.

In our paper, we use tunnel junctions made from La_2-xSr_xCuO₄ (LSCO), an archetypal cuprate superconductor (superconducting transition around 39 K for x near 0.15), with the tunneling barrier between the LSCO layers being 2 nm of the undoped, Mott-insulating parent compound, La₂CuO₄.   We could only do these measurements because of the fantastic material quality, the result of many years of effort by our collaborators.   We looked at shot noise in the tunneling from LSCO through LCO and into LSCO, over a broad temperature and voltage range.

The main result we found was that the noise in the tunneling current exceeded what you'd expect for just individual charges, both at temperatures well above the superconducting transition, and at bias voltages (energy scales) large compared to the superconducting gap energy scale.  This strongly suggests that some of the tunneling current involves the transport of two electrons at a time, rather than only individual charges.  (I'm trying to be very careful about wording this, because there are different processes whereby charges could move two at a time.)  While there have been experimental hints of pairing above Tc for a while, this result really seems to show that pairing happens at a higher energy scale than superconductivity.  Understanding how that relates to other observations people have made about the pseudogap and about other kinds of ordered states will be fun.   This work has been a great educational experience for me, and hopefully it opens the way to a lot of further progress, by us and others.

"Seeing" chemistry - another remarkable result

I have written before (here, here) about the IBM Zurich group that has used atomic force microscopy in ultrahigh vacuum to image molecules on surfaces with amazing resolution.  They've done it again.  Starting with an elaborate precursor molecule, the group has been able to use voltage pulses to strip off side groups, so that in the end they leave behind a ring of eighteen carbon atoms, each bound to its neighbor on either side.  The big question was, would it be better to think of this molecule as having double bonds between each carbon, or would it be energetically favorable to break that symmetry and have the bonds alternate between triple and single.  It turns out to be the latter, as the final image shows a nonagon with nine-fold rotational symmetry.  Here is a video where the scientists describe the work themselves (the video is non-embeddable for some reason).  Great stuff.

APS Division of Condensed Matter Physics invited symposium nominations

While I've rotated out of my "member-at-large" spot on the APS DCMP executive committee, I still want to pass this on.  Having helped evaluate invited symposium proposals for the last three years, I can tell you that the March Meeting benefits greatly when there is a rich palette.

---

The DCMP invited speaker program at March Meeting 2020 is dependent on the invited sessions that are nominated by DCMP members. All invited speakers that we host must be nominated in advance by a DCMP member.

The deadline to submit a nomination is coming up soon: August 23, 2019.

Please take a moment to submit an invited session nomination.

Notes regarding nominations:

• All nominations must be submitted through ScholarOne by August 23, 2019.
• In ScholarOne, an invited session should be submitted as an "Invited Symposium Nomination".
• An invited session consists of 5 speakers. You may include up to 2 alternate speakers in your nomination, in the event that one of the original 5 does not work out.
• While no invited speakers will be guaranteed placement in the program until after all nominations have been reviewed, please get a tentative confirmation of interest from your nominated speakers. There will be a place in the nomination to indicate this.
• A person cannot give technical invited talks in two consecutive years. A list of people that gave technical invited talks in 2019, and are therefore ineligible for 2020, can be found on this page.
• Nominations of women, members of underrepresented minority groups, and scientists from outside the United States are especially encouraged. 

Be sure to select a DCMP Category in your nomination. DCMP categories are:
07.0 TOPOLOGICAL MATERIALS (DCMP)
09.0 SUPERCONDUCTIVITY (DCMP)
11.0 STRONGLY CORRELATED SYSTEMS, INCLUDING QUANTUM FLUIDS AND SOLIDS (DCMP)
12.0 COMPLEX STRUCTURED MATERIALS, INCLUDING GRAPHENE (DCMP)
13.0 SUPERLATTICES, NANOSTRUCTURES, AND OTHER ARTIFICIALLY STRUCTURED MATERIALS (DCMP)
14.0 SURFACES, INTERFACES, AND THIN FILMS (DCMP)
15.0 METALS AND METALLIC ALLOYS (DCMP)

Thank you for your prompt attention to this matter.

Daniel Arovas, DCMP Chair, and Eva Andrei, DCMP Chair-Elect

More brief items

Writing writing writing.  In the meantime:

• This is a very solid video about physics careers - especially at around the 10 minute mark.  
• J. Robert Schrieffer has passed away.  Truly a scientific pioneer.
• I came upon this article yesterday in Nat Rev Physics, and it seems like an exceptionally clear review of strong coupling physics between light and matter, beyond the usual Purcell factor cavity QED effects.  
• SpaceX really proceeds in testing at a remarkable pace.
• The scanning Josephson microscope is a very impressive piece of kit!

Ferromagnetic droplets

Ferromagnets are solids, in pretty nearly every instance I can recall (though I suppose it's not impossible to imagine an itinerant Stoner magnet that's a liquid below its Curie temperature, and here is one apparent example). There's a neat paper in Science this week, reporting liquid droplets that act like ferromagnets and can be reshaped. 

The physics at work here is actually a bit more interesting than just a single homogeneous material that happens to be liquid below its magnetic ordering temperature.  The liquid in this case is a suspension of magnetite nanoparticles.  Each nanoparticle is magnetic, as the microscopic ordering temperature for Fe₃O₄ is about 858 K.  However, the individual particles are so small (22 nm in diameter) that they are superparamagnetic at room temperature, meaning that thermal fluctuations are energetic enough to reorient how the little north/south poles of the single-domain particles are pointing.  Now, if the interface at the surface of the suspension droplet confines the nanoparticles sufficiently, they jam together with such small separations that their magnetic interactions are enough to lock their magnetizations, killing the superparamagnetism and leading to a bulk magnetic response from the aggregate.  Pretty cool!  (Extra-long-time readers of this blog will note that this hearkens waaaay back to this post.)

Brief items

I just returned from some travel, and I have quite a bit of writing I need to do, but here are a few items of interest:

• No matter how many times I see them (here I discussed a result from ten years ago), I'm still impressed by images taken of molecular orbitals, as in the work by IBM Zurich that has now appeared in Science.  Here is the relevant video.
• Speaking of good videos, here is a talk by Tadashi Tokieda, presently at Stanford, titled "Science from a Sheet of Paper".  Really nicely done, and it shows a great example of how surprising general behavior can emerge from simple building blocks.
• It's a couple of years old now, but this is a nice overview of the experimental state of the problem of high temperature superconductivity, particularly in the cuprates.
• Along those lines, here is a really nice article from SciAm by Greg Boebinger about achieving the promise of those materials.  
• Arguments back and forth continue about the metallization of hydrogen.
• And Sean Carroll shows how remunerative it can be to be a science adviser for a Hollywood production.

Science and a nation of immigrants

It was very distressing to read this news article in Nature about the treatment of scientists of Chinese background (from the point of view of those at MIT).  Science is an international enterprise, and an enormous amount of the success that the US has had in science and technology is due to the contributions of immigrants and first-generation children of immigrants.  It would be wrong, tragic, and incredibly self-defeating to take on a posture that sends a message to the international community that they are not welcome to come to the US to study, or that tells immigrants in the US that they are suspect and not trusted. 

In any large population, there is always the occasional bad actor - the question is, how does a bureaucracy react to that?  One example:  Clearly some small percentage of medical researchers in the US have behaved unethically, taking money from medical and pharmaceutical companies in ways that set up conflicts of interest which they have hidden.  That's wrong, we should try to prevent it from happening, and those who misbehave should be punished.  The bureaucratic response to this has been that basically nearly every faculty member at a research university in the US now has to fill out annual disclosure and conflict of interest forms.   The number of people affected by the response dwarfs the number of miscreants by probably a factor of 1000, though in this case the response is only at the level of an inconvenience, so the consequences have not been dire.

Reacting to the bad behavior of a tiny number of people by taking wholesale measures that make an entire population feel threatened, unwelcome, and presumed guilty, is wrong and lazy.  The risk of long term negative impacts far beyond the scale of any original bad behavior is very real. 

Magic hands, secret sauce, and tricks of the trade

One aspect of experimental physics that I've always found interesting is the funny, specialized expertise that can be very hard to transcribe into a "Methods" section of a paper - the weird little tricks or detailed ways of doing things that can make some processes work readily in one lab that are difficult to translate to others.   This can make some aspects of experimental work more like a craft or an art, and can lead to reputations for "magic hands", or the idea that a group has some "secret sauce". 

An innocuous, low-level example:  My postdoctoral boss had basically a recipe and routine for doing e-beam lithography on an old (twenty+ years), converted scanning electron microscope, plus thermal evaporation of aluminum, that could produce incredibly fine, interdigitated transducers for surface acoustic waves.  He just had it down cold, and others using the same kind of equipment would have had a very tough time doing this at that resolution and with that reliability, even with all the steps written down, because it really was a skill.

Another possible example:  I was talking today with an atomic physics colleague, and he mentioned that there is a particular isotope that only one or two AMO groups in the world have really been able to use successfully in their ultracold atom setups.  The question was, how were they able to get it to work, and work well, when clearly other groups had tried and decided that it was too difficult? 

Any favorite examples out there from readers? 

The physics subject GRE and grad school

As I've mentioned before, there is a lot of discussion lately about the physics subject GRE.  The exam is intended to cover a typical undergrad physics curriculum in terms of content, and is in the format of about 100 multiple-choice questions in about 170 minutes.  The test is put together with input from a committee of physics faculty, and there is presently a survey underway by ETS to look at undergrad curriculum content and subscores as a way to improve the test's utility in grad admission.  The issue out there is to what extent the test should be a component in admissions decisions for doctoral programs. 

The most common argument for requiring such a test is that it is a uniform, standardized approach that can be applied across all applicants.  Recommendation letters are subjective; undergraduate grades are likewise a challenge to normalize between different colleges and universities.  The subject exam is meant to allow comparisons that avoid such subjectivity.  ETS points to studies (e.g., this one) that argue meaningful correlations between subject test scores and first-year graduate GPA. 

At the same time, there has been a growing trend away from emphasizing the test.  The astronomy and astrophysics community has been moving that way for several years - see here.  There have been recent studies (e.g. this one, with statistics heavily criticized here and relevant discussion here) arguing that the test scores are not helpful in actually predicting success (degree completion, for example) in doctoral programs.  In our own graduate program, one of my colleagues did a careful analysis of 17 years worth of data, and also found (to the surprise of many) basically no clear correlation between the subject test score and success in the program.  (Sampling is tricky - after all, we can only look at those students that we did choose to admit.)  At the same time, the tests are a financial obligation, and as mentioned here scores tend to be systematically lower for women and underrepresented minorities due to educational background and access to opportunities. 

Our program at Rice has decided to drop the physics subject GRE.  This decision was a result of long consideration and discussion, and the data from our own program are hard to argue.  It all comes down to whether the benefits of the test outweigh the negatives.  There is no doubt that the test measures proficiency at rapidly answering those types of questions.  It seems, however, that this measurement is just not that useful to us, because many other factors come into play in making someone an effective doctoral student.   Similarly, when people decide to leave graduate school, it is rare that the driving issue is lack of proficiency in what the test measures. 

I'm on a mailing list of physics department chairs, and it's been very interesting to watch the discussion back and forth on this topic and how much it mirrored our own.  It takes years to see the long term effects of these decisions, but it will definitely be something to watch. 

Round-up of various links

I'll be writing more soon, but in the meantime, some items of interest:

• A cute online drawing utility for making diagrams and flowcharts is available free at https://www.draw.io/.
• There is more activity afoot regarding the report of possible Au/Ag superconductivity.  For example, Jeremy Levy has a youtube video about this topic, and I think it's very good - I agree strongly with the concerns about heterogeneity and percolation. The IIS group also has another preprint on the arxiv, this one looking at I-V curves and hysteresis in these Au/Ag nanoparticle films.  Based on my prior experience with various "resistive switching" systems and nanoparticle films, hysteretic current-voltage characteristics don't surprise me when biases on the scale of volts and currents on the scale of mA are applied to aggregated nanoparticles.  
• Another group finds weird effects in sputtered Au/Ag films, and these have similar properties as those discussed by Prof. Levy.  
• Another group finds apparent resistive superconducting transitions in Au films ion-implanted with Ag, with a transition temperature of around 2 K.  This data look clean and consistent - it would be interesting to see Meissner effect measurements here.  
• For reference, it's worth noting that low temperature superconductivity in Au alloys is not particularly rare (pdf here from 1984, for example, or this more recent preprint).   
• On a completely different note, I really thought this paper on the physics of suction cups was very cute.
• Following up, Science had another article this week about graduate programs dropping the GRE requirement.
• This is a very fun video using ball bearings to teach about crystals - just like with drought balls, we see that aspects of crystallinity like emergent broken symmetries and grain boundaries are very generic.

Brief items

A number of interesting items:

• The slides for most of the talks at the Shoucheng Zhang memorial workshop are now available - there are pdf links on the schedule page.
• Charles Kittel has passed away.  I have not been able to find an obituary online anywhere yet - I will update this accordingly when one appears.  (The closest I've been able to find is here.) He was a great physicist - he is one of the Ks in the RKKY interaction, and had a big impact on physics pedagogy through his books, which include: the mechanics volume of the Berkeley Physics Course; the grad level Quantum Theory of Solids; the undergrad level Introduction to Solid State Physics; and Thermal Physics with Herb Kroemer.
• Much more has been written in the last couple of days about Murray Gell-Mann passing away.  See the NY Times, the Washington Post, and Science News, for example.
• Back in January, a study was published in Science Advances that makes a compelling case that the physics subject GRE is basically useless as a predictor of performance in grad school. 
• The NSF has a video with their advice on how to review proposals.
• The National Academy has released its decadal survey, Frontiers of Materials Research.  You can download a free pdf copy (at least, from within the US you can; not sure about non-US readers).  Some controversy from one of the sponsors....
• As pointed out by a commenter on the previous post, there is an updated version of the paper from last year was a report of superconductivity in Ag-decorated Au nanoparticles dropcast as a film.  An optimistic discussion is here.  As I said at the time, extraordinary claims require extraordinary evidence - at least, independent verification.

Publons?

I review quite a few papers - not Millie Dresselhaus level, but a good number.  Lately, some of the electronic review systems (e.g., manuscriptcentral.com, which is a front end for "Scholar One", a product of Clarivate) have been asking me if I want to "receive publons" in exchange for my reviewing activity. 

What are publons?  Following the wikipedia link above is a bit informative, but doesn't agree much with my impressions (which, of course, might be wrong).   My sense is that the original idea here was to have some way of recording and quantifying how much effort scientists were putting into the peer review process.  Reviewing and editorial activity would give you credit in the form of publons, and that kind of information could be used when evaluating people for promotion or hiring.   (I'm picturing some situation where a certain number of publons entitles you to a set of steak knives (nsfw language warning).)

The original idea now seems to have been taken over by Clarivate, who are the people that run Web of Science (the modern version of the science citation index) and produce bibliographic software that continually wants to be upgraded.  Instead of just a way of doing accounting of reviewing activity, it looks like they're trying to turn publons into some sort of hybrid analytics/research social network platform, like researchgate.  It feels like Clarivate is trying to (big surprise here in the modern age of social media) have users allow a bunch of data collection, which Clarivate will then find a way to monetize.  They are also getting into the "unique researcher identifier" game, apparently in duplication of or competition with orcid.

Maybe it's a sign of my advancing years, but my cynicism about this is pretty high.  Anyone have further insights into this?

Magnets and energy machines - everything old is new again.

(Very) long-time readers of this blog will remember waaaay back in 2006-2007, when an Irish company called Steorn claimed that they had invented a machine, based on rotation and various permanent magnets, that allegedly produced more energy than it consumed.  I wrote about this here, here (complete with comments from Steorn's founder), and here.  Long story short:  The laws of thermodynamics were apparently safe, and Steorn is long gone.

This past Friday, the Wall Street Journal published this article (sorry about the pay wall - I couldn't find a non-subscriber link that worked), describing how Dennis Danzik, science and technology officer for Inductance Energy Corp, claims to have built a gizmo called the Earth Engine.  This gadget is a big rotary machine that claims it spins two 900 kg flywheels at 125 rpm (for the slow version), and generates 240V at up to 100A, with no fuel, no emissions, and practically no noise.  They claim to have run one of these in January for 422 hours generating an average 4.4 kW.  If you want, you can watch a live-stream of a version made largely out of clear plastic, designed to show that there are no hidden tricks. 

To the credit of Dan Neil, the Pulitzer-winning WSJ writer, he does state, repeatedly, in the article that physicists think this can't be right.  He includes a great quote from Don Lincoln:  "Perpetual motion machines are bunk, and magnets are the refuge of charlatans." 

Not content with just violating the law of conservation of energy, the claimed explanation relies on a weird claim that seemingly would imply a non-zero divergence of \(\mathbf{B}]) and therefore magnetic monopoles:  "The magnets IEC uses are also highly one-sided, or 'anisotropic,' which means that their field is stronger on one face than the other - say 85% North and 15% South". 

I wouldn't rush out and invest in these folks just yet.

Light emission from metal nanostructures

There are many ways to generate light from an electrically driven metal nanostructure.  

The simplest situation is just what happens in an old-school incandescent light bulb, or the heating element in a toaster.  An applied voltage \(V\) drives a current \(I\) in a wire, and as we learn in freshman physics, power \(IV\) is dissipated in the metal - energy is transferred into the electrons (spreading them out up to higher energy levels within the metal than in the undriven situation, with energy transfer between the electrons due to electron-electron interactions) and the disorganized vibrational jiggling of the atoms (as the electrons also couple to lattice vibrations, the phonons).  The scattering electrons and jiggling ions emit light (even classically, that's what accelerating charges do).  If we look on time scales and distance scales long compared to the various e-e and e-lattice scattering processes, we can describe the vibrations and electron populations as having some local temperature.  Light is just electromagnetic waves.  Light in thermal equilibrium with a system (on average, no net energy transfer between the light and the system) is distributed in a particular way generically called a black body spectrum.  The short version:  current heat metal structures, and hot structures glow.  My own group found an example of this with very short platinum wires.  

In nanostructures, things can get more complicated.  Metal nanostructures can support collective electronic modes called plasmons.  Plasmons can "decay" in different ways, including emitting photons (just like an atom in an excited state can emit a photon and end up in the ground state, if appropriate rules are followed).  It was found more than 40 years ago that a metal/insulator/metal tunnel junction can emit light when driven electrically.  The idea is, a tunneling electron picks up energy \(eV\) when going from one side of the junction to the other.   Some fraction of tunneling electrons deposit that energy into plasmon modes, and some of those plasmon modes decay radiatively, spitting out light with energy \(\hbar \omega \le eV\).

This same thing can happen in scanning tunneling microscopy.  There is a "tip mode" plasmon where the STM tip is above the conducting sample, and this can be excited electrically.  That tip plasmon can decay optically and spit out photons, as discussed in some detail here back in 1990. 

The situation is tricky, though.  When it comes down to atomic-scale tunneling and all the details, there are deep connections between light emission and shot noise.  Light emission is often seen at energies larger than \(eV\), implying that there can be multi-electron processes at work.  In planar tunneling structures, light emission can happen at considerably higher energies, and it really looks there like there is radiation due to the nonequilibrium electronic distribution.  It's a fascinating area - lots of rich physics.

 

Updated: CM/nano primer - aggregated posts

Here is an updated and slightly reorganized (since 2017) listing of posts I've made over the years trying to explain some key concepts in condensed matter and nanoscale physics.  Please feel free to suggest topics that should be added. 


What is temperature?
What is chemical potential?
What is mass?
Fundamental units and condensed matter

What are quasiparticles?
What is effective mass?
What is a phonon?
What is a plasmon?
What are magnons?
What are skyrmions?
What are excitons?
What is quantum coherence?
What are universal conductance fluctuations?
What is a quantum point contact?  What is quantized conductance?
What is tunneling?

What are steric interactions?
(effectively) What is the normal force?
(effectively) What is jamming?
(effectively) What is capillary action?
What are liquid crystals?
What is a phase of matter?
About phase transitions....
(effectively) What is mean-field theory?

About reciprocal space....  About spatial periodicity.
What is band theory?
What is a "valley"? 
What is a metal?
What is a bad metal?  What is a strange metal?
What is a Tomonaga-Luttinger liquid?

What is a crystal?
What is a time crystal?
What is spin-orbit coupling?
What is Berry phase?
What is (dielectric) polarization?

About graphene, and more about graphene
Why twisting materials is interesting

About noise, part one, part two (thermal noise), part three (shot noise), part four (1/f noise)
What is inelastic electron tunneling spectroscopy?
What is demagnetization cooling?
About memristors....
What is thermoelectricity?
What are "hot" electrons?

What is a functional?  (see also this)
What is density functional theory?  Part 2  Part 3

What are the Kramers-Kronig relations?
What is a metamaterial?
What is a metasurface?
What is the Casimir effect?

About exponential decay laws
About hybridization
About Fermi's Golden Rule

The 1993 Stanford physics qual

Graduate programs in physics (and other science and engineering disciplines) often have some kind of exam that students have to take on the path to doctoral candidacy.  Every place is a bit different.  When I was an undergrad there, Princeton had a two-tiered exam system, with "prelims" largely on advanced undergrad level material and "generals" on grad-course-level content.  Rice has an oral candidacy exam with subfield-specific expectations laid out in our graduate handbook.  Stanford, when I went there in fall of 1993, had a written "qual", two days, six hours each day, ostensibly on advanced undergrad level material. 

There are a couple of main reasons for exams like this:  (1) Assessment, so that students learn the areas where they need to improve their depth of knowledge; (2) Synthesis -  there are very few times in your scientific career when you really have to sit down and look holistically at the discipline.  Students really do learn in preparing for such exams.

I've written about this particular exam experience here.  Thanks to an old friend whose handwriting decorates some of the pages, here (pdf) is a copy of that exam (without the solutions).   Wow.  Brings flashbacks. 

Liquid droplets with facets

One essential concept in condensed matter physics is spontaneous symmetry breaking - the idea that the collective response of many components acting together results in a situation that has less symmetry than the underlying system.  Crystalline solids are a classic example.  The universe itself has (to high precision) "continuous translational symmetry" - the laws of physics governing some isolated system are the same as the laws of physics if you slide that system over a bit.  Space has continuous rotational symmetry - reorienting your isolated system doesn't change anything.  A collection of atoms, though, can assemble into a crystal, and the crystal structure itself has lower symmetry.  For, e.g., an electron within the crystal, there is now discrete translational symmetry, meaning that the electron's environment is the same if the crystal is shifted not by arbitrary amounts, but by precise multiples of the lattice spacing of the crystal.  Similarly, only specific discrete rotations of the lattice about particular axes leave the electron's environment unchanged.

We tend to think of liquids as not breaking continuous rotational or translational symmetry.  If you consider timescales long compared to the jostling motion of atoms or molecules in a liquid, all positions look about the same, as do all directions in space.  (It is possible to have intermediate situations, with liquids made from non-spherical molecules, and these can have directionality and local clustering.  Such liquid crystals are used in the display you're most likely using to read this.)  The lack of translational and rotational symmetry breaking in liquids is one reason that droplets (and bubbles) tend to be spherical.  If it is energetically expensive to have an interface between, say, oil in water, for a fixed volume of oil, then the lowest energy situation is to have a spherical oil droplet - that minimizes the surface area of interface.

Fig. 1 from Guttman et al., PNAS 113,  493-496 (2016).
Faceted oil droplets!

Yesterday I stumbled upon this paper, and that sent me down a literature rabbit-hole.   This Nature paper (archived version here) led me to this PNAS paper, where I grabbed Fig. 1 at right.  It turns out that it is possible to form faceted liquid droplets of certain oils (alkanes) in aqueous suspensions.  The outermost layer of the alkanes acts rather like a lipid membrane, and it is possible to sit at a temperature where that layer crystallizes (the molecules in it spontaneously break translational and rotational symmetry) while the bulk of the droplet remains a liquid.  What picks out the orientation of the resulting faceted shape?  Spontaneous symmetry breaking.  Tiny fluctuations or attributes of the local environment.  Wild!

Brief items, + "grant integrity"

As I have been short on time to do as much writing of my own as I would like, here are links to some good, fun articles:

• Ryan Mandelbaum at Gizmodo has a very good, lengthy article about the quest for high temperature superconductivity in hydrogen-rich materials.
• Natalie Wolchover at Quanta has a neat piece about the physics of synchronization. 
• Adam Mann in Nat Geo has a brief piece pointing toward this PNAS paper arguing the existence of a really weird state of matter for potassium under certain conditions.  Basically the structure consists of a comparatively well-defined framework of potassium with 1D channels, and the channels are filled with (and leak in 3D) liquid-like mobile potassium atoms.  Weird.

Not fun:  US Senator Grassley is pushing for an "expanded grant integrity probe" of the National Science Foundation.   The stated issue is concern that somehow foreign powers may be able to steal the results of federally funded research.  Now, there are legitimate concerns about intellectual property theft, industrial espionage, and the confidentiality of the grant review process.  The disturbing bit is the rhetoric about foreign actors learning about research results, and the ambiguity about whether international students fall under that label in his or other eyes.  Vague wording like that also appeared in a DOE memo reported by Science earlier in the year.  International students and scholars are an enormous source of strength for the US, not a weakness or vulnerability.  Policy-makers need to be reminded of this, emphatically.

This week in the arxiv

A fun paper jumped out at me from last night's batch of preprints on the condensed matter arxiv.

arXiv:1904.06409 - Ivashtenko et al., Origami launcher
A contest at the International Physics Tournament asked participants to compete to see who could launch a standard ping-pong ball the highest using a launcher made from a single A4 sheet of paper (with folds).  The authors do a fun physics analysis of candidate folded structures.  At first, they show that one can use idealized continuum elasticity to come up with a model that really does not work well at all, in large part because the act of creasing the paper (a layered fibrous composite) alters its mechanical properties quite a bit.  They then perform an analysis based on a dissipative mechanical model of a folded crease matched with experimental studies, and are able to do a much better job at predicting how a particular scheme performs in experiments.  Definitely fun.

There were other interesting papers this week as well, but I need to look more carefully at them.

Brief items

A few brief items as I get ready to write some more about several issues:

• The NY Times posted this great video about using patterned hydrophobic/hydrophilic surfaces to get bouncing water droplets to spin.  Science has their own video, and the paper itself is here.  
• Back in January Scientific American had this post regarding graduate student mental health.  This is a very serious, complex issue, thankfully receiving increased attention.
• The new Dark Energy Spectroscopic Instrument has had "first light." 
• Later this week the Event Horizon Telescope will be releasing its first images of the supermassive black hole at the galactic center.
• SpaceX is getting ready to launch a Falcon Heavy carrying a big communications satellite.  The landing for these things is pretty science-fiction-like!

The physics of vision

We had another great colloquium last week, this one by Stephanie Palmer of the University of Chicago.  One aspect of her research looks at the way visual information is processed.  In particular, not all of the visual information that hits your retina is actually passed along to your brain.  In that sense, your retina is doing a kind of image compression. 

Your retina and brain are actually anticipating, effectively extrapolating the predictable parts of motion.  This makes sense - it takes around 50 ms for the neurons in your retina to spike in response to a visual stimulus like a flash of light.  That kind of delay would make it nearly impossible to do things like catch a hard-thrown ball or return a tennis serve.  You are able to do these things because your brain is telling you ahead of time where some predictably moving object should be.  A great demonstration of this is here.  It looks like the flashing radial lines are lagging behind the rotating "second hand", but they're not.  Instead, your brain is telling you predictive information about where the second hand should be.

People are able to do instrumented measurements of retinal tissue, looking at the firing of individual neurons in response to computer-directed visual stimuli.  Your retina has evolved both to do the anticipation, and to do a very efficient job of passing along the predictable part of visualized motion while not bothering to pass along much noise that might be on top of this.  Here is a paper that talks about how one can demonstrate this quantitatively, and here (sorry - can't find a non-pay version) is an analysis about how optimized the compression is at tossing noise and keeping predictive power.  Very cool stuff.

The statistical mechanics of money

Slow posting recently because of many real-life things going on after the March Meeting.  We had a very engaging colloquium this week by Victor Yakovenko, a condensed matter theorist from the University of Maryland.   A number of years ago, he got into "econophysics", applying insights from physics to the economy.  A great review is here. 

A classic example is in his highly cited paper, with the same title as this post.  Make some simple assumptions:  Money is a conserved quantity, and the rates of transactions don't depend on the financial direction of the transactions.  Take those assumptions, start with everyone having the same amount of money, and allow randomized transactions between pairs of people.  The long-time result is an exponential (Boltzmann-like) distribution of wealth - the probability of having a certain amount of money \(m\) is proportional to \(\exp(-m/\langle m \rangle)\), where \(\langle m \rangle \) is the average wealth, a monetary "temperature".  The take-away:  complete equality is unstable just because of entropy, the number of possible transactions.

Apparently similar arguments can be applied to income, because it would appear that you can describe the distribution of incomes in many countries as an exponential distribution (more than 90% of the population).  Basically, for a big part of the population, it seems like income distribution is dominated by these transactional dynamics, while the income distribution for the top 3-ish% of the population follows a power-law distribution, likely because that income comes from returns on investments rather than wages.  The universality is quite striking, largely independent of governmental policies on managing the economy.

Yakovenko would be the first to say not to over-interpret these results, but the power of statistical arguments familiar from physics is impressive.  Now all we have to do is figure out the statistical mechanics of people....

APS March Meeting wrapup

I spent the lion's share of today talking w/ my collaborators.  This was great scientifically, but meant that I only went to a couple of talks. 

• This one was pretty slick.  If you look at the conduction properties of a Josephson junction as a function of magnetic field through it, you see a Frauenhofer pattern as a function of the enclosed flux (see this pdf, fig 2).  In principle, taking the inverse Fourier transform of this should reveal the real-space current distribution as a function of the distance along the width of the junction.  This group made Josephson junctions using oriented thin pieces of WTe2.  When the current flowed along one direction, they found that the Josephson current was mostly flowing near the edges of the strip of material.  When current flowed along a different direction in the plane, the current distribution was much more uniform.  
• Similarly evocative, this talk presented work using magnetic focusing plus scanning gate microscopy plus collimating contacts to look at the real-space paths of electrons in a graphene-hBN bilayer w/ a Moire superlattice.  They could then infer the shape of the Fermi surface in momentum-space, confirming that the Moire superlattice results in a roughly triangular (miniband) Fermi surface.  Cooler than my jargon-heavy description sounds.
• I greatly regret that I was unable to attend the invited session in honor of Millie Dresselhaus.  If one of my readers who did make it could please describe it in a comment, I'd appreciate it.
• One other random note:  I did actually speak to the APS person who was in charge of the trade show, and I asked what the heck was up with the two weird "pain relief" booths, which seemed borderline late-night-infomercial/much more like something you'd see at a cheesy shopping mall.  This was apparently an experiment in allowing local vendors in, and it sounds very unlikely that it'll ever happen again.  

If I missed a big story from the meeting, please let me know in the comments.

APS March Meeting Day 3

A handful of semi-random highlights (broken up by my conversations w/ colleagues and catching up on work-related issues):

• Laura Heydermann from ETH spoke about "artificial" magnetic systems, where mesoscopic, lithographically patterned arrays of magnetic islands can yield rich response.  A couple of representative papers are here and here, and recently they've been moving into 3D fabrication and magnetically sensitive imaging.  Very neat stuff.  
• Christian Glatti from Saclay showed a very interesting result, analogous to the ac Josephson effect, but in fractional quantum Hall edge-state tunneling.  The relevant paper, just out in Science, is here.  This idea is, measure electronic shot noise as a function of bias voltage.  Ordinarily this has a minimum at zero bias, and the noise sits at the Johnson-Nyquist level there.  Now shine microwaves of frequency f on the device.  With photon-assisted tunneling, the net result is a change in the noise that has kinks at voltages of +/- hf/e*, where h is Planck's constant, and e* is the effective charge of the low-energy excitations.  Do this in the fractional quantum Hall regime, and you see fractional charge.  
• On a related topic, Michael Pepper from Cambridge showed a very recent result.  In quantum point contacts at very low charge carrier densities, they see quantized conductance at some very surprising rational fractions of the usual conductance quantum 2e²/h.  I still need to digest this.  
• I spent much of the afternoon at the big Kavli Symposium, on topics spanning from unit cell all the way to biological cells.  All excellent speakers.  I won't try to summarize this - rather, when the talks become available streaming, I will put the link here.  (Claudia Felser did bring donuts for the audience to talk about topology, always a crowd-pleaser.)

APS March Meeting, Day 2

A random selection from Day 2:

• Thomas Silva at NIST gave a fun talk about some experiments using the linac coherent light source.  Using pump/probe time-resolved x-ray diffraction, they discovered some surprising acoustic modes in thin, polycrystalline metal films, with systematics suggesting that they might be seeing localization of phonons due to scattering off grain boundaries.
• Along those lines, Gang Chen of MIT spoke about seeing reductions in thermal conductivity due to phonon localization.  His group was working with semiconductor superlattices, with little ErAs nanodots embedded in a disordered way at the superlattice interfaces.  They see systematics in the thermal conductivity that suggest that they are seeing Anderson localization of the heat-carrying phonons.  
• I stopped by the session on conveying physics to a popular audience, and caught most of the talk by Allison Eck chock full of advice for would-be science writers, and a skyped-in talk by Sean Carroll about podcasting.  The depressing truth: If I really want to expand my audience, I should probably join twitter.  (The problem is, that's a conversational medium and I don't see how I could do it well given everything else.)
• Abe Nitzan gave a prize talk that was a nice overview of the last decade's work on understanding electrons, photons, and phonons in molecular junctions.
• I spent much of the afternoon at this session about the copper oxide superconductors.  Dan Dessau's talk primarily about this paper showed the capabilities of a new technique in analyzing angle-resolved photoemission data, to figure out the actual spatial shape of Cooper pairs in these systems. My collaborator Ivan Bozovic spoke (similar to this), showing the power of his tremendous MBE growth approach, able to create epitaxially perfect materials smoothly and systematically spanning the whole doping range.  The other talks in the session were also very interesting.

APS March Meeting, Day 1

A few things I saw at the APS Meeting today, besides 10 inches of fresh, wet snow on the ground this morning (disclaimer:  for various reasons I was session-hopping quite a bit, so this is rather disjointed):

• Ignacio Franco at Rochester spoke about some experiments (here) that I'd not remembered, where carefully controlled, intense femtosecond light pulses were used to turn on a transient current in SiO₂, normally one of the best insulators out there.  The theory is interesting, and made me start thinking about possible opportunities in this area.
• A focus topic session on 2D magnetic materials was extremely crowded - so much so that I literally couldn't get in the room for the first talk.  Interesting talks, including Yujun Deng from Fudan presenting this work; Masaki Nakano from the University of Tokyo spoke about growing epitaxial films of V₅Se₈, a cousin of a material with which we've worked; and Boyi Zhou at Washington University in St. Louis presented this work, which seems to show nontrivial electronic conduction in (ordinarily Mott insulating) monolayer RuCl₃ layered on graphene.  Lots of interesting activity going on here, many fun ideas.
• Naomi Ginsberg at Berkeley talked about some impressive imaging techniques used to follow energy flow in complex materials.  Combining super-resolution methods, interferometry, and time-resolved techniques is a heck of an enabling technique!
• Peter Abbamonte at Illinois presented some remarkable measurements using an angle-resolved electron energy loss technique (M-EELS) to look at the strange metal state of a cuprate superconductor.  The main result is that this material seems to support a very broad plasmon mode with a lot of properties that are inconsistent with what you'd expect in a Fermi liquid, and may make connection with more exotic pictures of strange metals.  
• Wojciech Zurek's talk about the foundations of quantum mechanics (based on this article) was very engaging (and apparently in a superposition of all possible fonts), though again the room was so full that people were sitting on the floor in the aisles and lining three walls.  The session also was running about 10 minutes ahead of schedule, which definitely was not great for people who ended up missing the beginning of Zurek's talk or Rovelli's before it.

The unwieldy size of the meeting is increasingly clear, with lines in the restrooms, and local fastfood places unable to handle the lunch crowd.  

APS March Meeting, Day 0

A brief summary of topics/reading material/things I learned today during DCMP and joint DCMP/DMP executive committee meetings:

• As usual, this will be the biggest March Meeting ever, with 11500 registrants ahead of time.  This is still increasingly problematic in terms of organization and availability of sites.
• New APS Strategic Plan
• New APS report on the Impact of Industrial Physics on the US Economy
• DOE Basic Energy Sciences report (pdf) on the impact of the BES at its 40th anniversary
• The upcoming privatization of the US Strategic Helium Reserve looks depressingly unavoidable.  Sounds like changing this is a non-starter in Congress.

Michelle Simmons and Si-based quantum computing

A last tidbit before the March Meeting.

Earlier this week, Prof. Michelle Simmons came to Rice for our Chapman Lecture series and gave a great talk about her team's project to develop quantum computing in a silicon platform, with individual phosphorus donor atoms as the qubits.   This idea goes back more than twenty years to this proposal by Bruce Kane.   Actually implementing this approach requires overcoming many technical challenges, including positioning individual phosphorus atoms inside single-crystal Si with nearly atomic precision, and similarly fabricating control and read-out electrodes in registry with those.  

Prof. Simmons' group has made truly remarkable progress in this direction.  The key enabling technique is using a scanning tunneling microscope (STM) as a lithography tool.  Single-crystal Si surfaces are prepared in ultrahigh vacuum and terminated with a hydrogen.  The STM tip is then used to strip off the hydrogen atoms with atomic precision.  (This is a serial technique, and so scaling up to the production numbers of the present-day Si industry would require something different, but for now it's fine.)  Phosphine gas decomposes in a particular way when exposed to the dangling bonds left behind by stripping the hydrogen, placing P atoms in particular locations.  This approach can also be used to make highly conductive wires and gates by doping, enabling transport measurements through single dopant atoms.   Growing more single-crystal Si on top of the dopants without having the dopants move around is another success story, making possible 3D fabrication schemes.  With isotopically pure Si, encapsulating the donors can give long coherence times.

There are many competing platforms for possible quantum computer implementations, and this approach is undoubtedly difficult.  In terms of technical achievement, though, this effort has shown the power of sustained support - progress has been truly impressive.