DOE ECMP PI meeting, day 1 - things I learned

Yesterday was an extremely dense meeting day.  Many talks, many posters.  By its nature, this meeting is far more technical than the Packard meeting, so the bullet points below are going to be more obscure to the nonexpert.  The program is here.  Among the things I learned yesterday:

• Harold Hwang continues to do very interesting physics at the interface between LaAlO3 and SrTiO3, looking at fundamental issues like the limits of charge mobility in the 2d electron gas there, and how to make delta-doped bilayers.
• Many other people are playing with oxide and pnictide MBE, making pnictide superlattices, strain-controlled pnictides, multiferroic films, etc.
• It is possible to use the elastic deformation of VO2 at the metal-insulator transition to alter the magnetic coercivity of an overlying Ni layer. 
• In strained films, it is possible to see through x-ray techniques that one can decouple the electronic transition in VO2 from the structural transition.  
• Real progress has been made recently in using engineered structures of nanomagnetic patterns to model complex systems like spin ice.
• Nd2Fe14B, the rare-earth hard magnet, can take up hydrogen into its open structure, and when it does, the lattice expands, which greatly softens the magnetic response.
• Mott insulating materials can be synthesized that exhibit quantum criticality at zero magnetic field and as-made.
• Iridates are interesting and complicated.
• Investing in developing a particular technique (in this case, NMR of unusual elements like oxygen, sodium, and arsenic) can pay big long-term dividends in terms of unique experimental insights (e.g., there are no "static loop currents" flowing in the cuprate superconducting state).

DOE experimental condensed matter physics principal investigator meeting

I am spending the next 2.5 days at the DOE's experimental CMP principal investigator meeting in the Washington DC area.  I'll try to blog a few highlights over the course of the meeting.  The basic idea is supposed to be to get all of the PIs together to talk about their latest stuff, and ideally to foster new collaborations and activities.  Judging from the people sitting around me, it looks like this will be an extremely strong meeting in terms of science.

Things I learned this week at the Packard meeting

For the 25th anniversary of the amazingly awesome David and Lucille Packard Foundation fellowships, I was fortunate (and as always very grateful) for the opportunity to go to their annual meeting and listen to talks from incoming and outgoing Fellows.  These meetings are tremendous - a very rare chance to hear 20 minute talks on topics across science, engineering, and math, aimed at the technically literate non-expert.  Back in the dim past of this blog, I've posted about these before (here, here, and here).  Here are some take-away facts I learned this time around:

• By very narrow targeting of specific pathogens, it might be possible to remove some of the evolutionary pressure (exerted by horizontal gene transfer [something I'd never learned about] from your gut bacteria) that leads to antibiotic resistant strains.
• It's possible to use ideas from superresolution microscopy and principal component analysis to improve structure determination in materials characterization.
• Using small molecule dyes, it is possible to use optical processes to turn the tables on some chemical reactions, favoring "anti-Markovnikov" selection, rather than Markovnikov rules (where reaction sites are determined by permanent dipole moments of bonds).
• Sometimes cells can recognize themselves (and distinguish between themselves and close relatives) using proteins based only on one or two genes.
• I'm used to thinking about coupling two (identical) resonators and getting an energy splitting (like bonding/antibonding orbitals).  I hadn't realized that using an effectively imaginary coupling means you can get a lifetime splitting (one long-lived, one short-lived mode).
• You can tie vortex rings in knots.  Watch the videos!
• Greenland has not been ice-free for at least 350,000 years, and radioactive dating based on dust captured in the ice makes it possible to untangle even faulted or folded ice cores.
• Monsoons are complicated, even if you model a completely water-covered idealized planet.
• Every time a pair of neutron stars collide, they produce about one Jupiter mass worth of Au, while a core-collapse supernova makes about one lunar mass worth of Au.  As a result, even though colliding neutron stars are rare, half of the gold out there came from them.  (In case you were wondering, in all of human history we have mined about 165,000 tons of Au.)

I've left out many others.  As always, very cool.

Ionic liquids and gating - how much is chemistry?

I've written before (here, here and here) about the use of ionic liquids in condensed matter physics investigations.  These remarkable liquid salts, with small organic molecules playing the roles of both positive and negative ions, can be used in electrochemical applications to generate extremely large surface charge densities near electrode interfaces.  Many experiments have been published in the last few years in which ionic liquids are meant to induce (via capacitive coupling) large densities of mobile charge carriers within interesting solids at the solid/ionic liquid interface. 

One concern in these experiments has been the role of surface chemistry.  While the molecular ions themselves are intended to be stable over a large range of electrochemical conditions, the ionic liquids can dissolve more reactive species (like water).  Likewise, recent experiments by Stuart Parkin of IBM Research have shown that in some systems (vanadium oxide in particular), under certain electrochemical conditions it would appear that ionic liquids can favor the formation of oxygen vacancies in the adjacent solid.  Since oxygen vacancy defects in many oxide materials can act as dopants, changing the concentration of charge carriers, one must be extremely careful that any measured changes in electronic properties are really from electrostatics rather than effective chemical doping. 

These concerns can only be ratcheted higher by the simultaneous online publication of two more papers from the Parkin lab, this one in Nano Letters (on SrTiO₃) and this one in ACS Nano (on TiO₂).  In both systems, the authors again find evidence that changes in oxygen stoichiometry (rather than pure electrostatic charging) can be extremely important in generating apparently metallic 2d surface layers.  

This is a very subtle issue, and the gating experiments remain of great interest.  Unraveling the physics and chemistry at work in all the relevant systems is going to be a big job, with a strong need for in situ characterization of buried solid-liquid interfaces.  Fun, challenging stuff that shows how tricky this area can be.

New big science prizes - time for nominations and opinions

There are large, endowed prizes in a number of disciplines.  The most famous of all are the Nobel prizes, of course, and in the sciences at least (chemistry, physics, and medicine), being awarded a Nobel is a singular crowning achievement.  A huge amount has been written about the Nobels - if you want to learn how they came to be, and at the same time become extremely disillusioned about the process for the early awards (my goodness I hope it's better these days - it seems like it must be), I recommend The Politics of Excellence.   The purpose of the Nobel is to reward a major, transformative (to use the NSF's favorite word) intellectual achievement.  The money is not meant to be a research grant.  (Similar in spirit is the Fields Medal for mathematics, though that is much less money and purposefully directed at younger researchers.)

The MacArthur Fellowships are another well-known set of awards.  These are known in popular parlance as "Genius Grants", and unlike the Nobels are (apparently) intended not so much as a financial reward, but as a liberating resource, a grant that can provide the winner with the financial freedom to continue to excel.  In some disciplines (the arts and the humanities in particular) this can completely change the financial landscape for the winners.   Awards that go directly toward furthering the creative ends of the recipients are clearly great things.

In recent years, a couple of new, very large awards have been created, and it's interesting to consider whether this is a good thing.  The Kavli Foundation is awarding prizes every other year in Neuroscience, Astrophysics, and Nanoscience.  To nominate someone, see here.   In spirit, these seem much like the Nobels, with awards so far going to extremely well regarded people, and not meant to function as direct research support.

In more flamboyant style, Yuri Milner has endowed the Fundamental Physics Prizes, also not meant to function as research grants.    What really distinguishes these latest, apart from the sheer magnitude of the awards ($3M each), is that they have largely gone to high energy physics theorists whose work has not been confirmed by experiment (in contrast to theoretical physics Nobel awards).  More recently there has been a special award to the LHC experimentalists, and some related prizes to condensed matter theorists.  However, the idea of giving very large prizes for unconfirmed theoretical work is controversial.  In essence, is something a "scientific breakthrough" if it's not confirmed by experiment, or is it very exciting math?  Perhaps this is just a labeling issue, but it is hard not to be unsettled by the willingness of some to try to detach science from experimental tests.

Is the scientific community better off from having more of these kinds of prizes?  Certainly it makes sense to consider awards for fields not recognized by the Nobel Foundation.  Nobels have gravitas because of their long established history, but that does not mean that there shouldn't be an analogous prize for, e.g., computer science.  Likewise, anything positive about the sciences that gets public attention is probably a net good.  However, prizes will lose their meaning if there are too many, and making some of them destabilizingly large amounts of money is not necessarily great.   It's also not clear quite what the point is if the same people win multiple large prizes for the same work.   For example, it's credible that Alan Guth could win a Nobel in addition and a Kavli astrophysics prize in addition to the Fundamental Physics prize.    I always tell would-be scientists not to get into this if they're after the big prize at the end - that's not the point of the enterprise, and I'd hate to see that change.  It's also hard for me to believe that the existence of these prizes is going to get the public or students materially more interested in the sciences.   Somehow prizes that go toward helping people continue their work or recognize a career of achievement seem more sound to me, but I remain ambivalent.

How to: Carry on a scientific collaboration

I'm writing this at the suggestion of a commenter on my previous how-to post, who was specifically interested in experiment/theory interactions.  Collaborations, as a fundamentally personal endeavor, are as varied as the people who collaborate.  Over the years I have collaborated with a number of theorist colleagues as well as fellow experimentalists, and generally it's been a very positive set of experiences, both scientifically as well as personally.  The main recommendations I can make about collaboration:

• Discuss and plan the ground rules at the beginning.  How is the collaboration going to work?  Is this the sort of collaboration that requires regular discussions and updates?  Are physical samples being sent by one party to another?  Which people are going to be responsible for what tasks?  What are peoples' expectations of authorship (recognizing that occasionally work may take an unanticipated turn, and someone's contribution may grow or shrink along the way)?  Are there restrictions about the samples or data?  (For example, a materials grower might collaborate with person A and person B on different projects; it could be very awkward if person A took samples and then on the side started working on the same project as person B!)
• Collaborate with people who have a similar approach to research projects as you, in terms of rigor, timeliness, and seriousness.  This is true whether those people are your own group, or outside collaborators. 
• Make sure to understand what your collaborators are actually doing.  Collaborations are a chance for you to learn something, since presumably you're working with these people because they bring something to a project that you can't do your self.  Sometimes asking what might seem at first glance a silly or naive question can lead to discussion that is informative for everyone.
• Have realistic expectations.  On the sociological level, realize that no one is going to retool their entire research enterprise or retask several people for your sake.  On the scientific side, know what can and can't be done by your collaborators and their techniques.
• Be communicative.  Keep your collaborators in the loop and up to date on what's going on.  If there is a big delay on your end for some reason, let them know.  You'd want them to do the same.  If you have decided that you don't think the project is going to work, or it's not working as anticipated, bring this up and don't let it sit.
• Be a finisher.  The most successful grad students are the ones who actually finish tasks and projects.  In the same way, don't let things slide.  If your collaborator wants you to read through a draft, or you promised to get some data to them in time for some deadline, follow through. 

Online access to papers + university libraries - info wanted

Now that we live in the Information Age, where I am reliably told that Information Wants to be Free, I'm confused by a trend that is coming in terms of how university researchers access electronic versions of journals.  (For those of you under 30, there was once a time when journal articles were published in an arcane format that predates pdf called "paper".)   The electronic availability of journals, including historical archives, has largely been an enormous boon to scientific progress.  It is far easier and faster now than ever before to do proper literature research when writing a paper or a proposal.  If I'm using google scholar or Web of Knowledge or Scopus or any other reference crawling aid, I can now find and (and if my institution subscribes or the content is available free) download copies of relevant references very quickly and efficiently.  If anything, the technology to provide this content is continually becoming cheaper and faster, since providing print content has far lower bandwidth requirements than the streaming video demands that are really driving innovation.

That is why I am concerned and confused by a trend popping up in the perpetually-financially-stressed university libraries around the country (and the world, presumably).  We all know that commercial publishers have been cranking up prices and applying annoying/evil tactics like bundling one high impact title with a dozen expensive, low-impact journals in forced package deals.  (Wiley, Elsevier, Taylor and Francis, that's you.)  Now, though, there is this idea being pushed that it would somehow be cheaper for university libraries to actually drop their subscriptions (!!) and instead use Get It Now, a product of the Copyright Clearance Center (those people you have to contact if you want permission to use a figure in a review article).  The problem is, Get It Now is misnamed; really it's Get It In Seven Minutes.  Needless to say, if you are trying to trace references and write a paper or proposal, having to wait seven minutes for every article you want to examine (which could easily number in the dozens while proposal writing) would be a major mess.   

Given that the publishers have the capability to provide content essentially instantly, and that the infrastructure to support that capability is steadily getting cheaper, and that the publishers could quite readily track download statistics (and could charge per download if they really wanted to), I don't understand how Get It Now is a positive step.  Surely if per-article billing was an economically viable approach, the publishers would do it themselves, right?  The publishes are going to recoup their costs somehow, passing them along to CCC, and CCC will pass those along to the universities, so it's hard for me to see how interposing a middleman like CCC can really do anything except slow down researchers and make money for CCC.  This idea seems to go directly against the trend of open access, public archives, etc. 

Do any of my readers work at institutions that use this service?   How does it work for you?  Is it as annoying as it sounds?  Does it actually enable your university to save money (that is, provide more or better content for the same actual cost) relative to the old approach?  A major challenge faced by universities in budgeting is that libraries don't sound as exciting as new buildings or major initiatives, and yet libraries and their services are essential to the scholarly mission of the institution.

How to: Write a response to referees

I think I'm going to start a periodic series of "how to" posts.  First up, how to write a decent "response to referees" document.  While this is pretty much common sense, it's not bad to think about it a bit in the abstract, rather than in the heat of the moment of having just received some kind of (perceived) searing blast of criticism.  In brief, assuming you get some collection of referee reports, at least one or two of which are not particularly positive, and you intend to revise and resubmit:

• Read the reports, and then put them aside for a day, as your white-hot rage over the terrible injustice that has befallen you fades, and in the cold light of reflection you realize that perhaps the manuscript you'd sent in is not, in fact, the greatest non-fiction prose writing since Churchill's six volume history of the Second World War.
• Now that you're in a less annoyed frame of mind, read through the reviews again, carefully, trying to understand (a) what the reviewers are actually saying, and (b) what the reviewers want you to do (assuming that's not "dry up and blow away").  Often the answers to (a) will reveal either that the reviewers did not properly understand the main point or some subsidiary point of the paper.  Much as we like to grumble about referees, you may have to admit that the fault could lie in your presentation.  Were your figures unclear?  Did the abstract and the intro make your main point explicit, or did you bury the lede somewhere down in the conclusions?  Remember, scientific papers are not mystery stories.  Springing the cool observation on the reader after a lot of setup risks the reader not realizing that the observation is cool.  Moreover, often the answers to (a) will reveal that the reviewer has thought of a possible concern or objection that you either didn't consider, or you did consider but dismissed without pointing it out and explaining your reasoning.  An extremely important part of the response process is figuring out what the main point of the referee is, and realizing that frequently it's worthy of consideration.
• Regarding (b) above, write down and make a list of what you think the referees want you to do, or what you think it would take to address the points that they raise.    Then consider whether you want to or should do all of those things.  Sometimes the referees can be very demanding.  (We've all seen this.)  You have to use your judgment, and remember that referees are not generally gratuitously mean.  I'd say the default position should be to do what they want, unless what they want is really considered unreasonable by you and your coauthors.  This list, by the way, is a headstart on the eventual "list of changes" that you'll need to provide when you resubmit.
• When you sit down to write your response, have the referee remarks right there.  In fact, it's a good idea to use copy/paste to intersperse your point-by-point responses.  That way you can be sure you didn't miss anything, and you are forced to write your response in an order that will seem logical to the referee.  
• Always (always) thank the referees for your time.  Seriously.  You know what refereeing is like, and you'd like to be thanked, admit it.  
• Point out that after this process you believe the paper is much improved (it will be, too, assuming the referees were really on point and not just asking you to cite their seminal work on the topic at hand), and if possible explain why.  (e.g., we believe that our main point is now much clearer)
• Always be polite and professional.  If you fly off the handle in your response, even if the referee is overtly hostile, it won't do you any favors with other referees or the editor.  Similarly, just as tone is difficult to convey in email, I suggest avoiding attempted jokes or sarcasm.  This is a professional communication - keep it that way.
• Try to be timely about revisions.  It's much better to get revisions done while everything is fresh in your mind, rather than letting things linger.  (Don't write them in the heat of the moment, though.)

In your accompanying cover letter when you resubmit, make sure that you emphasize the changes you made in response to the referees.  Also, it doesn't hurt to point out to the editor if you think a referee either missed the mark or seems not to be objective, but it would be best to do so in a very professional way.  Calling the referee an idiot won't win you any friends, particularly since the editor likely chose the referee.  Still, if you really think the referee made serious mistakes, or was not competent, or didn't read the paper, you should bring that to the editor's attention in a professional way.  Again, this kind of response is an important part of your repertoire of professional communications, so it's best to get in the habit of writing them well.

That's it for now.  I'm sure I've left out points - please feel free to bring them up in the comments.

Rankings and metrics - yet again

I (along with my departmental colleagues) was very happy to see this.  My department does extremely well in a particular ranking scheme (described in the original paper here and implemented online here, though you need to ask for password access) that asks, essentially, what fraction of the papers published by a department fall into the top 10% in terms of impact in an area.  We can debate about the flaws of any ranking scheme (hint:  they're all imperfect, because quantifying scientific quality and impact in a single number is fundamentally wrong-headed).  Still, it is nice to see an approach that agrees well with much intuition (that is, the usual top-10 suspects all look pretty good; schools that don't do much research in an area rank lower) where Rice does well. 

A new kind of solid - why "q-glass" really is weird and interesting

As long-time readers here know, I'm not a big fan of hype and press releases.  While it is very important to let people know what academic scientists and engineers are doing, not everything needs to be trumpeted from the rooftops as a huge breakthrough or a paradigm-altering thunderbolt.  However, this new result (the actual paper is here) is genuinely weird, unexpected, and exciting (at least to me).  The authors claim to have discovered a truly new kind of solid.  Let me break down what this means and why it's surprising.

A solid is a material that resists shear deformation - if you exert a certain force horizontally across the top surface of the material, the material will deform a bit until it's internal forces balance your applied force, and then deformation will reach some constant amount.  (In contrast, a fluid will keep deforming continuously!)  The most ordinary solids people know about are either crystalline (this includes polycrystalline materials made up of many crystal grains) or glasses.  In a crystalline solid, the atoms have taken on highly symmetric spatial arrangements.  That is, the atoms aren't separated by random distances, but integer multiples of certain particular spacings; similarly, crystals are not isotropic - there are particular directions along which atoms are arranged.   In contrast, simple liquids are isotropic, and except for some typical nearest-neighbor distance set by the atomic or molecular size, there is no other spatial ordered arrangement.   When a solid crystallizes from a liquid, it is a collective phenomenon, a phase transition, and this happens on cooling when the free energy of solid phase becomes lower than that of the liquid phase.  Quasicrystals (see 2011 Nobel for Chemistry) are in these senses crystals - their symmetries are just more subtle than those of ordinary crystals.

Glasses (including those made from polymers) are different.  They resist shear, too, but they do not have the long-range, periodic/anisotropic arrangement of constituents seen in crystals.  Instead, upon cooling, glasses become solid (meaning that their viscosity diverges toward infinity) because the constituents become "kinetically hindered".  At the risk of dragging up controversy, the simple description is that there is no true glass phase in the thermodynamic sense - glasses are rigid because the constituents can't readily move out of each others' ways, not because there is some true collective thermodynamic stability (involving free energies) at work. 

The authors of this new work have found something special that they have termed a "q-glass" while looking at what happens in the solidification of a molten mixture of aluminum, iron, and silicon.  In the resulting solids, they find nodules of a new material (Al₉₁Fe₇Si₂, approximately) that is definitely not crystalline or polycrystalline (no preferred lattice spacings; completely isotropic).  At the same time, the material does form out of the melt through a genuine first-order phase transition (!), and therefore appears to be highly ordered in some sense (both distinguishing it from a glass).  It will be very interesting to learn exactly what is going on here, and whether there are other materials that have these peculiar features.

Peer review, tone, and common courtesy

I'm back, though now I'm in the writing-six-things-before-the-term mode, so blogging will likely continue to be sparse.   Several of my friends pointed out this article in the Chronicle of Higher Education regarding the tone of correspondence in the peer review process.  In short, some fraction (from my own experience, I'd say maybe 15%) of "negative" reviews go beyond pointing out issues that need to be corrected to improve the paper and instead are genuinely hostile and nasty - basically tone and phrasing that the reviewer would very likely never have the nerve to use face to face.  Interestingly, those of us who experience the peer review process beat the rest of the world to the observation of a behavior pattern that is now realized to be common on the internet.

I wish I knew the solution to this.  Removing the blindness of the review process is one possibility, though I do worry that the same petty, vindictive people who write reviews like this will then engage in additional unprofessional behaviors toward people that they perceive as slighting them. 

The point of the review process in science is to make sure that correct, clear, original science results get disseminated in the literature.  We are all (allegedly) on the same side.  If people would just adhere to that, then reviews could be much more constructive in tone (e.g., instead of "The authors are just plain wrong", wouldn't it be better to say "I'm concerned that there are some problems with steps 1 through 4"?).

I am worried that there is a general erosion in common courtesy as well.  I know this makes me sound like a grumpy old man, but again there are some people who use electronic communications in general as an excuse for rudeness.  Taking the time to say "please" and "thank you" is never time poorly spent.

Online physics lecture notes

Next week is going to be largely internet-free for me, so don't expect much excitement here until early August.  I do have a few things I want to discuss (thermoelectric effects, the great mess that was San Jose State's experiment in MOOCs, etc.), but that will wait.  In the mean time, I wanted to point out some nice lecture notes I'd found lately on the arxiv and elsewhere.  If people have their own favorite notes to point out online, please do so in the comments.

Neri Merhav has produced a couple of nice sets of notes, written from the perspective of trying to teach very physicsy concepts to electrical engineering students.  This past week he put up these notes about statistical mechanics, and previously he had written this set about the connections between information theory and statistical physics.  I found them both very readable.

Doron Cohen's notes on statistical mechanics and mesoscopics are a bit more mathy and closer to notes than a textbook-style discourse.

Not on the arxiv, but Yoshi Yamamoto's online notes regarding noise and noise processes are great.

Printing at the 180 nm scale??

I stumbled across this post, where it is asserted (with no link, and my google-fu is inadequate) that some group or collaboration at Berkeley is working on the ability to make 180 nm critical dimension transistors via printing (and therefore over really large areas, rather than only on dinner-plate-sized Si wafers).  Can a reader out there point me to who is really doing this, or is this a case of a press office distorting things (e.g., taking a layer thickness and claiming it's a lateral feature size)?

Physics is hard - how much should that worry us?

I'm a bit late to the party, but there have been discussions lately about the number of undergrad STEM majors, including physics, with some gnashing of teeth about overall difficulty. For example, a report by the National Bureau of Economic Research has been interpreted as saying that the main reason students bail on science majors is poor grades. That is, students go in, knowing that science will require more work than majoring in something fluffy, but when many receive tough grades even though they work hard, that's too much for them and they change fields. Chad Orzel does his usual thorough job looking into what the study really says, and it does seem true that tough grading drives some people out of STEM pursuits.

Similarly, there is a new report from the National Academy of Sciences called "Adapting to a Changing World: Challenges and Opportunities in Undergraduate Physics Education". I found the content rather disappointing, in the sense that it didn't seem to say much new. We all know that some approaches can be better under some circumstances than traditional lecture. However, many of those are very labor intensive, and I'm sure that my 50 person class would benefit if it were instead five ten-person classes. More to the point, though, the report specifically claims that hard grades are a major factor in the low participation of women and underrepresented groups in the physics major.

So, is physics unnaturally harsh in its grading, to its detriment? Or is this a question of high school preparation on the one hand, and grade inflation in nonscience majors on the other? I lean toward the latter.

(Note that the NSF has proven that science is hard. Also, here is the paper featured in that article - it's actually very interesting.)

(One other note: no one commented on my three part post about the physics of contacts, and the hit rate on those posts was very low. At the same time, in one 15 minute interval last week my post about "whiskey stones" got nearly 500 page views after it was mentioned in an argument about whiskey on reddit. Guess I should write about other things besides physics if I want more readership:-).

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Contacts III: The search for measurements

In the last two posts I've talked a bit about contact resistances, but I haven't said much of anything about how to infer these experimentally. 

In some sense, the best, most general way to understand contact voltages is through scanning potentiometry.  For example, this paper (pdf - sorry for the long URL) in Fig. 10 uses a conductive AFM tip to look at the local electrostatic potential as a function of position along an organic transistor under bias.  When done properly, this allows the direct measurement of the potential difference between, e.g., the source electrode and the adjacent channel material.  If you know the potential difference and the current flowing, you can calculate the contact resistance.  Even better, this method lets you determine the \( I-V \) characteristic of the contact even if it is non-Ohmic, because you directly measure \(V\) while knowing \(I\).   The downside, of course, is that not every device (particularly really small ones) has a geometry amenable to this kind of scanned probe characterization.

A more common approach used by many is the transmission line method.  In the traditional version of this, you have a whole series of (otherwise identical) devices of differing channel lengths.  You can then plot the resistance of the device as a function of \(L\).  For Ohmic contacts and an Ohmic device, the slope of the \(R-L\) plot gives the channel resistance per unit length, while the intercept at \(L \rightarrow 0\) is the total contact contribution.  This does not tell you how the contact resistance is apportioned between source/channel and channel/drain interfaces (this can be nontrivial - see the figure I mentioned above, where most of the voltage is dropped at the injecting contact, and a smaller fraction is dropped at the collecting contact).  Related to the transmission line approach is the comparison between two- and four-terminal measurements of the same device.   The four-terminal measurement, assuming that no current flows in the voltage contacts and that the voltage probes are ideal, should tell you the contribution of the channel.  Comparison with the two-terminal resistance measurement should then let you get some total contact resistance.  I should also note that, if you know that the channel is Ohmic and that one contact dominates the resistance, you can still use length scaling to infer the \( I-V \) characteristic of the contact even if it is non-Ohmic.

The length scaling argument to infer contact resistances has also been used to great effect in molecular junctions.  There, for non-resonant transport, the usual assumption is that the bulk of the molecule (whatever that means) acts as an effective tunneling barrier, so that conductance should fall exponentially with increasing molecular length (assuming the barrier height does not change with molecular length, an approximation most likely to be true in saturated as opposed to conjugated molecules).  Thus, one can plot \(log G\) as a function of molecular length, and expect a straight line, with an intercept that tells you something about the contact between the molecule and the metal electrodes.  This has been done in molecular layers (see here, for example), and in single molecule junctions (see here, for example).  These kinds of contact resistances can then be related, ideally, to realistic electronic structure calculations looking at overlap between electronic states in the metal and those of the linking group of the molecule.

Hopefully these three posts have clarified a little the issue of contact effects in electronic devices - why they are not trivial to characterize, and how they may actually tell you interesting things.

Contacts, part deux

I will make an argument now that contact resistances are much maligned, and instead of rigorously trying to avoid worrying about them, we should instead look for opportunities (with well defined, reproducible contact interfaces) when they can actually tell us something. I'll punctuate this with some papers from our own group and areas I happen to know, but that's only because those are the examples that come to my mind.

What happens when you try to inject charge from a metal into a hopping conductor - a material with some energy-dependent density of localized states? Many organic semiconducting polymers are such systems. In this situation, an injected charge carrier faces a competition between diffusion away into the channel by hopping, and an attraction to its own image charge in the metal. The rather odd result is that this contact often tends to be Ohmic (in the sense that the contact voltage is directly proportional to the current), but the contact resistance ends up being inversely proportional to the mobility of the charge in the channel. This is true even when the metal Fermi level lies somewhere in the tail of the band (a situation where you would expect a Schottky contact in a nonhopping semiconductor). We ran into this here, and systematically varied the contact resistance by using surface chemistry to adjust the energetic alignment.

In correlated materials, the situation may seem tantalizing yet hopeless. On the one hand, you know something interesting must happen when charge is injected into the material - carriers in the metal are boring, electron-like quasiparticles, while charge excitations in the correlated system could in principle be very different, with fractional charge or spin-charge separation. On the other hand, depending on the bulk properties and ability to make reproducible contacts, it can be very hard to extract useful information from contact resistances in these systems. We did get lucky, and found that in magnetite conduction in both the high temperature (short range ordered) state and in the low temperature (long range ordered) state seems to be through hopping, similar to the description above. I definitely think that there is a lot more to be done in such materials by using contact effects as a tool rather than avoiding them.

In the world of molecular junctions, often one is in the limit where the device is "all contact", in the sense that the "bulk" is only a couple of nanometers and a few atoms. Next time I'll talk about some great measurements by others in these systems, as part of a discussion on how one can measure contact resistance.

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Contacts - annoying or an opportunity

In condensed matter physics, often we are interested in the flow of charge from a "source" electrode, through some material (the "channel", probably a different material than the source), and away into a "drain" electrode.  When the source, channel, and drain are all metals, life is simple.  While there might be some mismatch in the electrical conductivities of the materials, in the end an electron can go from some delocalized (extended, wavelike) state in the source, into such a state in the channel, and then into such a state in the drain, smoothly.   Because of the discontinuity in electronic band structure and dielectric properties, there is some reflection at the interface, leading to a contact resistance.  That is, some fraction of the applied voltage, linearly proportional to the applied voltage, is dropped across the source-channel contact, and some across the channel-drain contract.  This is an example of an ohmic contact.

However, the situation can be more complicated.  If the channel is a crystalline semiconductor, the Fermi level of the metal usually winds up sitting somewhere in the band gap.  If the is appropriate band bending takes place, there can then be an energy barrier (a Schottky barrier) for injection if charge from the metal into the semiconductor.   The spatial width of the barrier depends on the level of doping in the semiconductor, with higher doping leading to a narrower (though not necessarily shorter) barrier.   In this case, the current-voltage characteristics of the contact is not Ohmic, and looks instead like a diode, because the applied bias changes the shape of the barrier.  To avoid this in transistors, the regions of the channel where the source and drain contact it are very highly doped.  Still, in this case we are still assuming that the actual electronic states are extended, delocalized things.

The situation gets more complicated when the channel does not have delocalized states near the Fermi level.  

Usually experiments are designed to mitigate contact effects, either by avoiding measurements of the contact voltages (so-called four terminal measurements) or by making the contact contribution negligible compared to the bulk channel.  However, it turns out that sometimes contact effects can provide valuable insights into charge transport properties in the bulk.  I'll write more soon about this.

Timescales, averaging, and baseball

Please pardon the summer blogging slowdown - it's been a surprisingly busy couple of weeks, between an instructor search, working on papers, proposal stuff, and trying to write more on my big long-term project. 

Thanks to an old friend for pointing me to this link, which does a great job looking at why a knuckleball is so erratic in its flight from pitcher to batter.  For non-Americans:  In baseball, a pitcher throws a ball to a catcher, while a batter attempts to hit the ball.  There are several types of pitches, depending on the pitcher's grip on the ball (which has seams due to the stitching that holds the leather cover on), the throwing motion, and the release.  A fastball can reach speeds in excess of 100 mph (161 kph) and typically spins more than 1000 rpm.  In contrast, a knuckleball can drift by the batter at a leisurely 70 mph yet be nearly unhittable because of its erratic motion.   A knuckleball barely spins, so that it may complete only 1-2 revolutions from leaving the pitcher's hand to reaching the batter.  This means that the positioning of the seams is absolutely critical to determing the aerodynamics of the motion, and no two knuckleballs move the same way.  In physics lingo, a knuckleball has almost none of the orientational averaging that happens in basically every other pitch.  I propose the definition of a new dimensionless parameter, the Wakefield number, \(W\), that is the ratio of the ball's period of revolution to its time-of-flight from pitcher to batter.   A knuckleball is a pitch with \(W \sim 1\).

Come on, PRL editors.

I rarely criticize papers.  I write this not to single out the authors (none of whom I know), nor to criticize the actual science (which seems very interesting) but to ask pointedly:  How did the editors of PRL, a journal that allegedly prizes readability by a general physics audience, allow this to go through in its current form?  This paper is titled "Poor Man’s Understanding of Kinks Originating from Strong Electronic Correlations".  A natural question would be, "Kinks in what?".  Unfortunately, the abstract doesn't say.  Worse, it refers to "the central peak".  Again, a peak in what?!   Something as a function of something, that's for sure. 

Come on, editors - if you are going to let articles be knocked from PRL contention because they're "more suitable for a specialized journal", that obligates you to make sure that the papers you do print at least have titles and abstracts that are accessible.  I'm even a specialist in the field and I wasn't sure what the authors were talking about (some spectral density function?) based on the title and abstract.

The authors actually do a good job explaining the issue in the very first sentence of the paper:  "Kinks in the energy vs. momentum dispersion relation indicate deviations from a quasiparticle renormalization of the noninteracting system."   That should have been the first sentence in the abstract.  In a noninteracting system, the relationship between energy and momentum of particles is smooth.  For example, for a free electron, \( E = p^{2}/2m \) where \(m\) is the mass.  In an ordinary metal (where Fermi liquid theory works), you can write a similar smooth relationship for the energy vs. momentum relationship of the quasiparticles. Kinks in that relationship, as the authors say, "provide valuable information of many-body effects".  

Academic self-sabotage

Ordinarily I wouldn't just post a link, but this article ("Self-Sabotage in the Academic Career: 15 ways in which faculty members harm their own futures, often without knowing it") from the Chronicle of Higher Education is exceptionally good advice for new faculty members.  While several of the points are specific to academia, some are generalizable to any career within a moderately large organization. 

The state of "molecular electronics"

For those interested in the history and current state of "molecular electronics", I refer you to the latest focus issue of Nature Nanotechnology.  Good news for those without subscription access - some good articles are available free of charge:

• Mark Ratner has an historical perspective on the field here.
• Emanuel Lörtscher discusses the challenges of making an actual technology out of these systems. 
• A variety of experts (including me) weigh in with blurbs about where things are and where they are going.
• Sri Aradhya and Latha Venkataraman present an excellent up-to-date review of the field, emphasizing the evolution of measurements beyond just collecting current-voltage characteristics.

Rescheduled, Workshop on Surface Plasmons, Metamaterials, and Catalysis

Three of my colleagues and I are helping to organize a workshop at Rice University on October 21-23, 2013.   (This had originally been planned for May, but sequester-related travel restrictions on the government participants among other things forced a rescheduling.)  The goal of this ARO-sponsored workshop is to explore the opportunities for chemical catalysis arising from recent advances in the fields of metamaterials and plasmonics.  The workshop will bring together scientists from the disciplines of electrochemistry, catalysis, and plasmonics, which have not traditionally had a common platform.





The confirmed invited speakers are:

Rick Van Duyne - Northwestern University
Paul Bohn - University of Notre Dame
Martin Moskovits - University of California, Santa Barbara
Katherine Willets - University of Texas at Austin
Jennifer Dionne - Stanford University
Mark Brongersma - Stanford University
Louis Brus - Columbia University
Harry Atwater - California Institute of Technology
John Yates - University of Virginia
Mengyan Shen - University of Massachusetts, Lowell
Suljo Linic - University of Michigan
Mostafa El-Sayed –- Georgia Tech
Tom Mallouk –- Penn State

Topics include:

• The state of the art in plasmonics, metamaterials, and chemical catalysis
• Areas of catalysis that could benefit from enhanced optical/electromagnetic concepts
• Concepts for nanophotonic- and metamaterials-driven catalysis and heat generation
• Surface nanoengineering to merge nanophotonics and catalysis
• Quantum plasmonics
• Hot electrons driving chemistry
• Chemical sensing using nanophotonic and plasmonic concepts
• Nanophotonic characterization of catalytic structures:  Where do the reactions happen, and how fast?

The deadline for abstract submission is July 21, 2013, and space is limited.  The workshop website is here:  http://PlasEnhCat2013.rice.edu  .

Please feel free to distribute this information to people that would be interested!

What does "heating" mean at the nanoscale?

I've talked before about what physicists mean when they talk about "temperature", and work has me thinking about this a lot these days. Temperature is inherently a statistical concept.  It doesn't really make sense to talk about the temperature of a single electron.  The electron has some momentum (and therefore some kinetic energy), but temperature is not a meaningful concept for a single particle in isolation.  Now, if you have a whole bunch of electrons, you can talk about how many of them have a certain amount of energy.  That distribution of electrons as a function of energy takes on a particular form when the electron system is in thermal equilibrium.  (That is, if the electron system is weakly coupled somehow to an energy reservoir so that energy can be exchanged freely between the reservoir and the electrons.)  When the electron system is in thermal equilibrium with the reservoir, on average no net energy is transferred as a function of time between the electrons and the reservoir; this is what we mean when we say that the electrons and the reservoir have the same temperature.

The situation gets really tricky when a system is driven out of equilibrium.  For example, you can use a battery to drive electrons through some nanoscale system.  When you do that, and you look at different points within the nanoscale system, you will find that, in general, the distribution of the electrons as a function of energy doesn't necessarily look much like the thermal equilibrium case.  So, is there a sensible way to generalize the idea of temperature to quantify how "hot" the electrons are?  The problem is, there are many ways you might want to do this - you are trying to take a potentially very complicated distribution function and essentially summarize it by a single number, some local effective temperature.   A natural direction to go is to consider a thought experiment:  what if you took a reservoir with a well defined equilibrium temperature, and allowed it to exchange energy with the nonequilibrium system at a location of interest.  What reservoir temperature would you have to pick so that there is no net average energy transfer between the system and the reservoir in steady state?   That is one sensible way to go, but in the nano limit the situation can be very tricky, even in the thought experiment.  The details of how the imagined energy exchange takes place can affect the answers you get.  Tough stuff.

Instructor opening at Rice

RICE UNIVERSITY

Wiess Instructorship in Physics and Astronomy

The Physics and Astronomy Department at Rice University invites applications for a one-year instructorship position teaching introductory physics, commencing July/August 2013.  The teaching load is equivalent to two courses per semester.  There would also be opportunities to develop innovative teaching methods and pursue independent research or collaborations with existing research programs (see web page http://physics.rice.edu).  Evaluation of applications will begin immediately and continue until the position is filled. Applicants should send a curriculum vitae, a statement of teaching and research interests, and a list of publications as a single PDF file, and should arrange for three letters of reference to be sent to: vcall@rice.edu with subject line "Wiess Instructorship" (pdf format preferred), or by postal mail to Wiess Instructorship Search, c/o Valerie Call, Physics and Astronomy Department-MS61, Rice University, 6100 Main Street, Houston, TX 77005-1892. Applicants must have a PhD and be eligible to work in the U.S. Rice University is an affirmative action/equal opportunity employer.

Ask me something.

I realized I've never really tried having my readers just post questions for me.   Have at it!

A Scientist Laureate position for the US?

This is at least thought-provoking.  Lamar Smith (R-Texas, chair of the US House science committee, famous for things like this) and Zoe Lofgren (D-CA, about as far from Lamar Smith as I can imagine with the possible exception of Nancy Pelosi) are co-sponsoring a bill that would create a position called Scientist Laureate of the United States.  This person would be appointed by the President following nomination by the National Academy of Sciences, and would be meant to act as an inspirational figure, making public appearances and furthering the cause of science.  This could be a good thing, provided (1) an actual accomplished scientist is chosen, not someone who has to satisfy a political agenda; and (2) the person chosen is charismatic and able to use the bully pulpit effectively.  The Science Laureate should do more than show up at middle schools - they should get major exposure (e.g., late night talk shows; hosting a science program on a major network with actual resources to make it good; having the ear of Congress, perhaps even the limited ability to insist on speaking at a hearing of the House or Senate science-related committees).  (Halftime at the Superbowl is probably out of line.)

While I applaud scientists with great public outreach track records (Neil deGrasse Tyson just spoke at our commencement), that should not be the sole criterion.  If this passes, hopefully Congress will keep in the bit about the NAS making the choice.  Suggestions are invited in the comments.

MOOC followup

Very briefly, here is an open letter by the San Jose State University philosophy department regarding MOOCs (one in particular).   Food for thought.

update:  my colleague Moshe Vardi pointed out his own editorial on this topic.

I don't agree with everything in either of these documents.  I do think it's worth thinking hard about the purpose of MOOCs.  Are they about idealistically providing access to fantastic educational opportunities at very low cost to the student for millions of potential pupils who have an internet connection?  Are they about cynically slashing the operating costs of universities by restructuring the educational experience and potentially eliminating large numbers of faculty jobs?  These are not mutually exclusive.

Fun with single atoms

IBM Almaden research has produced "A Boy and His Atom", a stop-motion movie where the frames are scanning tunneling microscope images of carbon monoxide molecules on the Cu(111) surface.  Here is the "Making of..." movie as well.  Fun stuff.  I was particularly amused by Andreas Heinrich's comment that "if I can get a thousand kids to join science rather than go into law school, I'd be super happy."  Amen :-)

Can Congress please not screw up the NSF? Please?

I just read this article at Science's blog, describing how Lamar Smith, chair of the House science committee, basically wants to gut peer review at the NSF and replace it with something more to the liking of the House Republicans.  This would be catastrophically bad for a large number of reasons.  If they do this, you know it's only a matter of time before they decide to undermine peer review at NIH and DOE Office of Science also.  Gahh.  Can't blog - too incoherently angry.

update:  For what it's worth, this would presumably have a hard time passing the Senate and getting signed into law by the President, though given Congress' tendency to lump zillions of unrelated bills together into giant omnibus legislation, you never know for sure.   NSF is probably not the real long-term target of these types.  Picture what would happen to research if big pharma lobbyists get to have Congress decide what NIH grants should be funded, or if big energy lobbyists do the same for DOE grants (to say nothing of de-funding anything they find politically unacceptable).

Cryogenic dark matter detection, redux

About 3.5 years ago, I posted about the technology used by the CDMS collaboration to look for dark matter using clever solid-state detectors.  These folks have some news that is, as is often the case in any novel particle detection experiment, intriguing but not yet definitive.  This paper is probably the best place to see a summary of the results.  In their CDMS II run, the team had 19 germanium-based and 11 silicon-based detectors (cooled to 0.04 K!) running for five years (2003-2008) in an old mine in Minnesota (to cut down on cosmic ray background).  This paper reports results from the Si detectors, where after a lengthy blind analysis they see three events that look interesting.  Since germanium has a higher atomic number than Si, the idea of running the two materials in parallel was to have a cross-check and provide some information about how the searched-for weakly interacting massive particles (WIMPs) might interact with ordinary matter as a function of energy.  I should also note that the collaboration is now running "SuperCDMS" with a larger mass of germanium (9 kg) since 2011, and will eventually expand up to 200 kg of Ge running in Soudan in Ontario.  It's interesting that their earlier analysis from their Ge detectors reported no candidate events (as far as I can tell), while the analysis of the Si detectors shows three candidate events.  My understanding is that this could have to do with the mass range of the WIMPs, but I would be happy if someone would provide more context in the comments below.  Either way, I think it's great to see how condensed matter physics (and in particular cool device fabrication, as in the superconducting transition-edge sensors used here) can have an impact on Big Questions like dark matter.

MOOCs and online education

Massive open online courses (MOOCs) and concerns about online education are all the rage these days at universities.  There is a growing recognition of a few key points:  The cost of undergraduate education (in the US at least) continues to increase much more rapidly than inflation; online capabilities are sufficiently advanced now that it is possible, for comparatively little investment, to distribute educational content to many thousands of people at very low cost, in principle having a major pedagogical impact (see, e.g., the Khan Academy, to say nothing of MIT's opencourseware); more than one major concern is springing up trying to guide online education at the university level (see, e.g., coursera and edX).  [Note to self:  find some demo as cool as the thermite reaction to hook people into any online course I ever teach.]  There is clearly a major sense of urgency on the part of university administrators.  To belabor an overused analogy, they are worried that the online education train is leaving the station, and they fear the consequences of getting left behind. 

All of these things are true, and I understand the concern.  However, a few points have occurred to me about this, and I'd be happy for some discussion in the comments if people are interested.

1. Many people do not really have the self-discipline to learn in an online-only environment.  I like to think I was a pretty dedicated student (no smart comments from my former classmates, please), and I'm not sure I would have the self-discipline to watch online-only lecture material and do online-only assignments for an entire semester.  Some people do have the personality for this, but I have a hunch that many of them are the same folks who really can check a book out of the library and teach themselves a new subject ab initio.  Most 18 year olds are not like that, and the peer pressure/social environment of having friends physically going to scheduled classes is a major motivator.  Bill Press, when he visited Rice and we chatted about this, pointed out that many people pay real money to take Microsoft online certification courses, and complete them at a high rate.  That's true, but it's also a particular case where the financial benefits of completing that particular course are often very clear to the student, and it's also true that there's a difference between university study and vocational training.
2. It only makes sense to develop online courses where your institution really adds value.  Does anyone think it would be a good idea for every major university to develop their own MOOC for Introductory Calculus?  We could do that, but in the end there will likely be a small handful of truly innovative, extremely well done calc courses.  The market will drive toward some kind of mix-and-match mode of operation (unless the content providers constrain things greatly).
3. The sense of urgency is not unreasonable, but early innovators don't necessarily win the day.  For example, Lycos and Alta Vista were early to the scene in "search", yet comparative latecomer google crushed them.

So, are MOOCs really going to sweep through and destroy the modern university system?  Are physical universities going to become like specialty bookshops and online providers like Amazon?  Let me know what you think.

Book review: Alsos

I just found and read a great book, Alsos, by Samuel Goudsmit. The Alsos mission was the Allied dual scientific/military intelligence gathering expedition following the Normandy Invasion, tasked with learning the status of the German atomic program and rounding up German nuclear scientists. Goudsmit, who with Uhlenbeck helped convince people like Pauli of the usefulness of the concept of spin (the intrinsic angular momentum of particles like the electron), was a Dutch Jew, and while he was in the States working on radar, his parents were sent to a concentration camp and killed. The book is fascinating. It's split between the story of the actual mission (which discovered relatively quickly and much to the relief of all involved that the Germans never even got a nuclear pile to go critical) and an indictment of science and industry in a totalitarian regime. It is quite the cautionary tale of the politicization of scientific research and the arrogance of some physicists (Heisenberg fares particularly poorly, to the surprise of no one), told with a wry sense of humor. Highly recommended.

 

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Workshop on "Electronic Properties of Carbon-based Nanostructures"

I'm on my way back to the US from this workshop at the Universität Regensburg.  It was a fun and interesting meeting, and the quality of the invited talks was uniformly high.  The city was also very neat.  I'd had no idea that it had managed to escape (almost completely) Allied bombing during WWII, so as a result it has many buildings dating back to the Middle Ages.  

On the science side, it was particularly nice to hear some talks from and meet a number of people that whose work I've seen over the years but I'd never met face-to-face before.  For example, Steven Louie (linking to wikipedia since all of the Berkeley servers are inexplicably slow right now) spoke about ways to accurately calculate the optical properties of graphene (including electron-hole interactions properly). Philip Collins showed how it's possible to look at single-molecule biophysics (like the functioning of individual enzyme molecules) by anchoring the molecules of interest to single-walled carbon nanotubes, where the action is transduced into changes in the conductance.  Adrian Bachtold gave a nice overview of their work on optomechanics of nanotubes, which has enabled them to do mass sensing at the resolution of a single atomic mass unit (10^-27 kg) and force sensing with similarly impressive sensitivity.  Richard Berndt from Kiel discussed his group's work where they argue that light emission from STM tips shows the signature of shot noise in the current at optical frequencies.  Wolfgang Wernsdorfer, grand poobah of molecular magnetism, presented new results showing amazing control and detection of individual electronic spin lifetimes (in Tb-containing molecules).  For a spin to flip spontaneously, the molecule has to transfer angular momentum to the rest of the world somehow.  In the new experiment, this happens by dumping angular momentum into a carbon nanotube to which the molecule is anchored.  Since the allowed vibrational states of the nanotube can be controlled, this in turn tunes the spin flip rates.  Finally, Klaus Müllen gave an overview of ways to rationally synthesize, by chemical means, graphene flakes, ribbons, and other shapes.  The chemistry is just unreal.

Spin Hall physics

As I mentioned during the APS meeting, Dan Ralph presented some beautiful work (for example) on spin torque devices (where the flow of spin-polarized electrons is able to rotate the magnetization of some "free" ferromagnetic layer of material).  This spin torque business is a fairly mature idea, and the early demonstrations of this effect made use of layered structures (ferromagnet/normal metal/ferromagnet), with the current flowing perpendicular to the layers.  That is, if electrons flow from FM1, some of them are spin-polarized because of the magnetization of FM1, and those polarized electrons traverse the normal layer into FM2.  That works fine, but the most angular momentum you're ever going to transfer that way is \( \hbar/2 \) per electron, and that assumes that the electrons from FM1 are perfectly polarized.   Suppose you could do better than this.  Is there some way, for a given amount of charge current that you flow, to get more angular momentum transferred?

The answer is "yes", and the key is to leverage the spin Hall effect.  (For a good summary of spin Hall physics, see this paper by one of the progenitors of the field - I'll briefly summarize.)  In the regular Hall effect, we think about charge current flow in a plane in the presence of a perpendicular magnetic field.  The charge carriers experience a Lorentz force from the magnetic field that pushes them in the plane transverse to the direction of the (longitudinal) charge current.  Net charge of one sign piles up at one transverse edge of the sample, and net charge of the other sign piles up at the opposite edge, until the force from the resulting transverse electric field balances the Lorentz force.  (Glad to see wikipedia has fixed the figure in this article.  A few years ago they had the direction of the Lorentz force backward.)  In the spin Hall effect, we again think about current flow in a plane.  However, there is no external magnetic field.  Instead, we have the current flowing in a material with strong spin-orbit scattering (that is, in the reference frame of the moving electron, the effective charge current due to the nuclei seemingly moving by produces enough of a magnetic field in that frame to couple significantly to the spin of the electron.  Fundamentally this is a relativistic effect!).  Because of the coupling of spin to orbital motion, if the charge carriers scatter, the spins self-polarize; spin-"up" electrons will pile up on one transverse edge of the sample, while spin-"down" electrons will tend to pile up on the opposite edge.  The extent to which this happens is determined mostly by the strength of the spin-orbit coupling, which is larger in heavier atoms.

So, Ralph and coworkers have used this effect to great advantage.  Instead of the FM/N/FM layered structure, they make a structure that looks like SO/FM/N/FM, where SO is a strong spin-orbit material, such as tungsten or platinum.  They can flow a current within the plane of the SO layer.  Through the spin Hall effect, this can pump polarized spins perpendicular to the plane, into the adjacent FM layer.  (The electrical resistance vertically through the FM/N/FM stack is a way of monitoring the relative alignment of the FM layers, thanks to the giant magnetoresistance.)  This is particularly clever, because for strong SO coupling in the SO layer, thanks to the large contact area at the SO/FM interface, they can get more like 10 \( \hbar \) of angular momentum per electron flowing within the SO layer.   Fascinating to realize that these effects (because they originate from SO physics) are really dramatic experimental proof of the way electric and magnetic fields obey special relativity!