This year's Nobel in physics - Thouless, Kosterlitz, Haldane

Update:  well, I was completely wrong!  Topology ruled the day.  I will write more later about this, but Congratulations to Thouless, Kosterlitz, and Haldane!

Real life is making this week very busy, so it will be hard for me to write much in a timely way about this, and the brief popular version by the Nobel Foundation is pretty good if you're looking for an accessible intro to the work that led to this.  Their more technical background document (clearly written in LaTeX) is also nice if you want greater mathematical sophistication.

Here is the super short version.  Thouless, Kosterlitz, and Haldane had major roles to play in showing the importance of topology in understanding some key model problems in condensed matter physics.

Kosterlitz and Thouless (and independently Berezinskii) were looking at the problem of phase transitions in two dimensions of a certain type.  As an example, imagine a huge 2d array of compass needles, each free to rotate in the plane, but interacting with their neighbors, so that neighbors tend to want to point the same direction.  In the low temperature limit, the whole array will be ordered (pointing all the same way).  In the very high temperature limit, when thermal energy is big compared to the interaction between needles, the whole array will be disordered, with needles at any moment randomly oriented.  The question is, as temperature is increased, how does the system get from ordered to disordered?  Is it just a gradual thing, or does it happen suddenly in a particular way?  It turns out that the right way to think about this problem is in terms of vorticity, a concept that comes up in fluid mechanics as well (see this wiki page with mesmerizing animations).  It's energetically expensive to flip individual needles - better to rotate needles gradually relative to their neighbors.  The symmetry of the system says that you can't spontaneously create a pattern to the needles that has some net swirliness ("winding number", if you like).  However, it's relatively energetically cheap to create pairs of vortices with opposite handedness (vortex/antivortex pairs).  Kosterlitz, Thouless, and Berezinskii showed that these V/AV pairs "unbind" collectively at some finite temperature in a characteristic way, with testable consequences.  This leads to a particular kind of phase transition in a bunch of different 2d systems that, deep down, are mathematically similar.  2d xy magnetism and superconductivity in 2d are examples.  This generality is very cool - the microscopic details of the systems may be different, but the underlying math is the same, and leads to testable quantitative predictions.

Thouless also realized that topological ideas are critically important in 2d electronic systems in large magnetic fields, and this work led to understanding of the quantum Hall effect.  Here is a nice Physics Today article on this topic.   (Added bonus:  Thouless also did groundbreaking work in the theory of localization, what happens to electrons in disordered systems and how it depends on the disorder and the temperature.)

Haldane, another brilliant person who is still very active, made a big impact on the topology front studying another "model" system, so-called spin chains - 1d arrangements of quantum mechanical spins that interact with each other.  This isn't just a toy model - there are real materials with magnetic properties that are well described by spin chain models.  Again, the questions were, can we understand the lowest energy states of such a system, and how those ordered states go away as temperature is increased.  He found that it really mattered in a very fundamental way whether the spins were integer or half-integer, and that the end points of the chains reveal important topological information about the system.  Haldane has long contributed important insights in quantum Hall physics as well, and in all kinds of weird states of matter that result in systems where topology is critically important.  (Another added bonus:  Haldane also did very impactful work on the Kondo problem, how a single local spin interacts with conduction electrons.)

Given how important topological ideas are to physics these days, it is not surprising that these three have been recognized.   In a sense, this work is a big part of the foundation on which the topological insulators and other such systems are built.


Original post:  The announcement this morning of the Nobel in Medicine took me by surprise - I guess I'd assumed the announcements were next week.  I don't have much to say this year; like many people in my field I assume that the prize will go to the LIGO gravitational wave discovery, most likely to Rainer Weiss, Kip Thorne, and Ronald Drever (though Drever is reportedly gravely ill).    I guess we'll find out tomorrow morning!

Mapping current at the nanoscale - part 1 - scanning gates

Inspired by a metaphor made by our colloquium speaker, Prof. Silke Paschen, this past week, I'd like to try to explain to a general audience a couple of ways that people have developed for mapping out the flow of charge in materials on small scales.

Eric Heller's art piece "Dendrite", based
on visualization of branching current flow.

Often we are interested in understanding how charge flows through some material or device.  The simplest picture taught in courses is an analogy with water flowing through a pipe.  The idea is that there is some input for current, some output for current, and that in the material or device, you can think of charge moving like a fluid flowing uniformly along.  Of course, you could imagine a more complicated situation - perhaps the material or device doesn't have uniform properties; in the analogy, maybe there are obstacles that block or redirect the fluid flow.  Prof. Eric Heller of Harvard is someone who has thought hard about this situation, and how to visualize it.  (He's also a talented artist, and the image at right is an example of artwork based on exactly this issue - how the flow of electrons in a solid can branch and split because of disorder in the material.)

There's a different analogy that might be more useful in thinking about how people actually map out the flow of current in real systems, though.  Suppose you wanted to map out the roads in a city.  These days, one option would be to track all GPS devices (especially mobile phones) moving faster than, say, a few km/h.  If you did that you would pretty quickly resolve a decent map of the streets of a city, and you'd find where the traffic is flowing in high volume and at what speed.  Unfortunately, with electronic materials and devices, we generally don't have the option of tracking each individual mobile electron.  

Some condensed matter experimentalists (like Bob Westervelt, for example) have developed a strategy, however.  Here's the traffic analogy: You would set up traffic cameras to monitor the flow of cars into and out of the city.  Then you would set up road construction barrels (lanes blocked off, road closures) in known locations in the city, and see how that affected the traffic flow in and out of town.  By systematically recording the in/out traffic flow as a function of where you put in road closures, you could develop a rough map of the important routes.  If you temporarily close a road that hardly carries any cars, there won't be any effect on the net traffice, but if you close a major highway, you'd see a big effect.  

The experimental technique is called scanning gate microscopy.  Rather than setting up traffic cones, the experimentalists take a nanoscale-sharp conductive tip and scan it across the sample in question, mapping the sample's end-to-end conduction as a function of where the tip is and what it's doing.  One approach is to set the tip at a negative potential relative to the sample, which would tend to repel nearby electrons just from the usual like-charges-repel Coulomb interaction.  If there is no current flowing near the tip, this doesn't do much of anything.  If the tip is right on top of a major current path, though, this can strongly affect the end-to-end conduction.   It's a neat idea, and it can produce some impressive and informative images.  I'll write further about another technique for current mapping soon.

Nanovation podcast

 Michael Filler is a chemical engineering professor at Georgia Tech, developing new and interesting nanomaterials.  He is also the host of the outstanding Nanovation podcast, a very fun and informative approach to public outreach and science communication - much more interesting than blogging :-) .  I was fortunate enough to be a guest on his podcast a couple of weeks ago - here is the link.  It was really enjoyable, and I hope you have a chance to listen, if not to that one, then to some of the other discussions.

Deborah Jin - gone way too soon.

As was pointed out by a commenter on my previous post, and mentioned here by ZapperZ, atomic physicist Deborah Jin passed away last week from cancer at 47.   I don't think I ever met Prof. Jin (though she graduated from my alma mater when I was a freshman) face to face, and I'm not by any means an expert in her subdiscipline, but I will do my best to give an overview of some of her scientific legacy.  There is a sad shortage of atomic physics blogs....  I'm sure I'm missing things - please fill in additional information in the comments if you like.

The advent of optical trapping and laser cooling (relevant Nobel here) transformed atomic physics from what had been a comparatively sleepy specialty, concerned with measuring details of optical transitions and precision spectroscopy (useful for atomic clocks), into a hive of activity, looking at the onset of new states of matter that happen when gases become sufficiently cold and dense that their quantum statistics start to be important.  In a classical noninteracting gas, there are few limits on the constituent molecules - as long as they don't actually try to be in the same place at the same time (think of this as the billiard ball restriction), the molecules can take on whatever spatial locations and momenta that they can reach.  However, if a gas is very cold (low average kinetic energy per molecule) and dense, the quantum properties of the constituents matter - for historical reasons this is called the onset of "degeneracy".  If the constituents are fermions, then the Pauli principle, the same physics that keeps all 79 electrons in an atom of gold from hanging out in the 1s orbital, keeps the constituents apart, and keeps them from all falling into the lowest available energy state.   In contrast, if the constituents are bosons, then a macroscopic fraction of the constituents can fall into the lowest energy state, a process called Bose-Einstein condensation (relevant Nobel here); the condensed state is a single quantum state with a large occupation, and therefore can show exotic properties.

Prof. Jin's group did landmark work with these systems.  She and her student Brian DeMarco showed that you could actually reach the degenerate limit in a trapped atomic Fermi gas.  A major challenge in this field is trying to avoid 3-body and other collisions that can create states of the atoms that are no longer trapped by the lasers and magnetic fields used to do the confinement, and yet still create systems that are (in their quantum way) dense.  Prof. Jin's group showed that you could actually finesse this issue and pair up fermionic atoms to create trapped, ultracold diatomic molecules.  Moreover, you could then create a Bose-Einstein condensate of molecules (since a pair of fermions can be considered as a composite boson).  In superconductors, we're used to the idea that electrons can form Cooper pairs, which act as composite bosons and form a coherent quantum system, the superconducting state.  However, in superconductors, the Cooper pairs are "large" - the average real-space separation between the electrons that constitute a pair is big compared to the typical separation between particles.  Prof. Jin's work showed that in atomic gases you could span between the limits (BEC of tightly bound molecules on the one hand, vs. condensed state of loosely paired fermions on the other).  More recently, her group had been doing cool work looking at systems good for testing models of magnetism and other more complicated condensed matter phenoma, by using dipolar molecules, and examining very strongly interacting fermions.   Basically, Prof. Jin was an impressively creative, technically skilled, extremely productive physicist, and by all accounts a generous person who was great at mentoring students and postdocs.   She has left a remarkable scientific legacy for someone whose professional career was tragically cut short, and she will be missed.

Alan Alda Center for Communicating Science, posting

Tomorrow I'll be a participant in an all-day workshop that Rice's Center for Teaching Excellence will be hosting with representatives from the Alan Alda Center for Communicating Science - the folks responsible for the Flame Challenge, a contest about trying to explain a science topic to an 11-year-old.  I'll write a follow-up post sometime soon about what this was like.

I'm in the midst of some major writing commitments right now, so posting frequency may slow for a bit.  I am trying to plan out how to write some accessible content about some recent exciting work in a few different material systems. 

 

Professional service

An underappreciated part of a scientific career is "professional service" - reviewing papers and grant proposals, filling roles in professional societies, organizing workshops/conferences/summer schools - basically carrying your fair share of the load, so that the whole scientific enterprise actually functions.  Some people take on service roles primarily because they want to learn better how the system works; others do so out of altruism, realizing that it's only fair, for example, to perform reviews of papers and grants at roughly the rate you submit them; still others take on responsibility because they either think they know best how to run/fix things, or because they don't like the alternatives.   Often it's a combination of all of these.

More and more journals proliferate; numbers of grant applications climb even as (in the US anyway) support remains flat or declining; and conference attendance continues to grow (the APS March Meeting is now twice as large as in my last year of grad school).  This means that professional demands are on the rise.  At the same time, it is difficult to track and quantify (except by self-reporting) these activities, and reward structures give only indirect incentive (e.g., reviewing grants gives you a sense of what makes a better proposal) to good citizenship.  So, when you're muttering under your breath about referee number 3 or about how the sessions are organized nonoptimally at your favorite conference (as we all do from time to time), remember that at least the people in question are trying to contribute, rather than sitting on the sidelines.

Conference for Undergraduate Women in Physics!

Over January 13-15, 2017, Rice is going to be hosting one of the American Physical Society's Conferences for Undergraduate Women in Physics.  Registration is now open - please click on the link in the previous sentence, and you will be taken to the meeting website.  This is one of about 10 regional CUWiP meetings, and our region encompasses Texas, Mississippi, Alabama, Florida, Arkansas, and Louisiana.  Many thanks to my faculty colleagues Prof. Marj Corcoran and Prof. Pat Reiff for leading the way on this, and to our staff administrator and our excellent SPAS undergraduates for their efforts.

Gulf Coast Undergraduate Research Symposium!

Rice University's schools of Natural Sciences and Engineering want to make sure that when talented science and engineering undergraduates in the US are deciding where to apply for graduate school, we are on their radar, so to speak.  To that end, we are hosting our second annual Gulf Coast Undergraduate Research Symposium.  To quote the webpage,

The Gulf Coast Undergraduate Research Symposium (GCURS) is a forum for undergraduate researchers to present original research discoveries.... GCURS fosters intercollegiate interactions among students and faculty who share a passion for undergraduate research. We expect several hundred speakers from about half of the states. The event also offers a friendly and supportive environment to students who would be giving their first formal research presentation, and faculty will provide written constructive feedback.

The registration deadline is Sept. 29.  Breakfast, lunch, and dinner will be provided on Saturday, and travel expenses for students (hotel, mileage, airfare if preapproved) will be covered by Rice's Office of Graduate and Postdoctoral Studies.  Please pass this along - it's a fun time.  If you want more details and contact information either for our department's role or the meeting as a whole, please let me know.

Amazon book categories are a joke

A brief non-physics post.  Others have pointed this out, but Amazon's categorizations for books are broken in such a way that they almost have to be designed to encourage scamming.  As an example, my book is, at this instant (and that's also worth noting - these things seem to fluctuate nearly minute-to-minute), the number 30 best seller in "Books > Science & Math > Physics > Solid State Physics".  That's sounds cool, but it's completely meaningless, since if you click on that category you find that it contains such solid state physics classics as "Ugly's Electrical References, 2014 ed.", "Barron's 500 Flash Cards of American Sign Language", "The Industrial Design Reader", and "Electrical Motor Controls for Integrated Systems", along with real solid state books like Kittel, Simon, and Ashcroft & Mermin.  Not quite as badly, the Nanostructures category is filled "Strength of Materials" texts and books about mechanical structures.  Weird, and completely fixable if Amazon actually cared, which they seem not to.

Proxima Centauri's planet and the hazards of cool animations

It was officially announced today that Proxima Centauri has a potentially earthlike planet.  That's great, especially for fans of science fiction.  Here is a relevant video by Nature:

Did you spot the mistake?  The scientists discovered the planet by seeing the wobble in the star's motion (measured by painstaking spectroscopy of the starlight, and using the Doppler shift of the spectrum to "see" the tiny motion of the star).  The animation tries to show this at 0:55-1:12.  The wobble is because the star and planet actually orbit around a common center of mass located on the line between them.  Instead, the video seems to show the center of mass of the star+planet tracing out a circle around empty space.  Whoops.   Someone should've caught that.  Still an impressive result.

Update:  The makers of the video have updated with a link to a more accurate animation of the Doppler approach:  https://youtu.be/B-oZYm3L1JE.

Statistical and Thermal Physics

Eight years ago I taught Rice's undergraduate Statistical and Thermal Physics course, and now after teaching the honors intro physics class for a while, I'm returning to it.   I posted about the course here, and I still feel the same - the subject matter is intellectually very deep, and it's the third example in the undergraduate curriculum (after electricity&magnetism and quantum mechanics) where students really need to pick up a different way of thinking about the world, a formalism that can seem far removed from their daily experience.

One aspect of the course, the classical thermodynamic potentials and how one goes back and forth between them, nearly always comes across as obscure and quasi-magical the first (or second) time students are exposed to it.  Since the last time I taught the course, a nice expository article about why the math works has appeared in the American Journal of Physics (arxiv version).  

Any readers have insights/suggestions on other nice, recent pedagogical resources for statistical and thermal physics?  

Updated - Short items - new physics or the lack thereof, planets and scale, and professional interactions

Before the start of the new semester takes over, some interesting, fun, and useful items:

Update:. This is awesome.  Watch it.

• The lack of any obvious exotic physics at the LHC has some people (prematurely, I suspect) throwing around phrases like "nightmare scenario" and "desert" - shorthand for the possibility that any major beyond-standard-model particles may be many orders of magnitude above present accelerator energies.  For interesting discussions of this, see here, here, here, and here.  
• On the upside, a recent new result has been published that may hint at something weird.  Because protons are built from quarks (and gluons and all sorts of fluctuating ephemeral stuff like pions), their positive charge has some spatial extent, on the order of 10^-15 m in radius.  High precision optical spectroscopy of hydrogen-like atoms provides a way to look at this, because the 1s orbital of the electron in hydrogen actually overlaps with the proton a fair bit.  Muons are supposed to be just like electrons in many ways, but 200 times more massive - as a result, a bound muon's 1s orbital overlaps more with the proton and is more sensitive to the proton's charge distribution.  The weird thing is, the muonic hydrogen measurements yield a different size for the proton than the electronic hydrogen ones.  The new measurements are on muonic deuterium, and they, too, show a surprisingly smaller proton than in the ordinary hydrogen case.  Natalie Wolchover's piece in Quanta gives a great discussion of all this, and is a bit less hyperbolic than the piece in ars technica.
• Rumors abound that the European Southern Observatory is going to announce the discovery of an earthlike planet orbiting in the putative habitable zone around Proxima Centauri, the nearest star to the sun.  However, those rumors all go back to an anonymously sourced article in Der Spiegel.  I'm not holding my breath, but it sure would be cool.
• If you want a great sense of scale regarding how far it is even to some place as close as Proxima Centauri, check out this page, If the Moon were One Pixel.
• For new college students:  How to email your professor without being annoying.
• Hopefully in our discipline, despite the dire pronouncements in the top bullet point, we are not yet at the point of having to offer the physics analog of this psych course.
• The US Department of Energy helpfully put out this official response to the Netflix series Stranger Things, in which (spoilers!) a fictitious DOE national lab is up to no good.  Just in case you thought the DOE really was in the business of ripping holes to alternate dimensions and creating telekinetic children.

Why is desalination difficult? Thermodynamics.

There are millions of people around the world without access to drinkable fresh water.  At the same time, the world's oceans contain more than 1.3 million cubic kilometers of salt water.  Seems like all we have to do is get the salt out of the water, and we're all set.   Unfortunately, thermodynamics makes this tough.  Imagine that you have a tank full of sea water and magical filter that lets water through but blocks the dissolved salt ions.    You could drag the filter across the tank - this would concentrate the salt in one side of the tank and leave behind fresh water.  However, this takes work.  You can think about the dissolved ions as a dilute gas, and when you're dragging the membrane across the tank, you're compressing that gas.  An osmotic pressure would resist your pushing of the membrane.  Osmotic effects are behind why red blood cells burst in distilled water and why slugs die when coated with salt.  They're also the subject of a great Arthur C. Clarke short story.

In the language of thermodynamics, desalination requires you to increase the chemical potential of the dissolved ions you're removing from the would-be fresh water, by putting them in a more concentrated state.   This sets limits on how energetically expensive it is to desalinate water - see here, slide 12.   The simplest scheme to implement, distillation by boiling and recondensation, requires coming up with the latent heat of the water and is energetically inefficient.  With real-life approximations of the filter I mentioned, you can drive the process, called reverse osmosis, and do better.  Still, the take-away message is, it takes energy to perform desalination for very similar physics reasons that it takes energy to compress a gas.

Interestingly, you can go the other way.  You know that you can get useful work out of a gas reservoirs at two different pressures.  You can imagine using the difference in chemical potential between salt water and fresh water to drive an engine or produce electricity.  In that sense, every time a freshwater stream or river empties into the ocean and the salinity gradient smooths itself by mixing of its own accord, we are wasting possible usable energy.  This was pointed out here, and there is now an extensive wikipedia entry on osmotic power.

Ask me something.

I realized that I haven't had an open "ask me" post in almost two years.  Is there something in particular you'd like me to write about?  As we head into another academic year, are there matters of interest to (grad or undergrad) students?

Dark matter, one more time.

There is strong circumstantial evidence that there is some kind of matter in the universe that interacts with ordinary matter via gravity, but is otherwise not readily detected - it is very hard to explain things like the rotation rates of galaxies, the motion of star clusters, and features of the large scale structure of the universe without dark matter.   (The most discussed alternative would be some modification to gravity, but given the success of general relativity at explaining many things including gravitational radiation, this seems less and less likely.)  A favorite candidate for dark matter would be some as-yet undiscovered particle or class of particles that would have to be electrically neutral (dark!) and would only interact very weakly if at all beyond the gravitational attraction.

There have been many experiments trying to detect these particles directly.  The usual assumption is that these particles are all around us, and very occasionally they will interact with the nuclei of ordinary matter via some residual, weak mechanism (say higher order corrections to ordinary standard model physics).  The signature would be energy getting dumped into a nucleus without necessarily producing a bunch of charged particles.   So, you need a detector that can discriminate between nuclear recoils and charged particles.  You want a lot of material, to up the rate of any interactions, and yet the detector has to be sensitive enough to see a single event, and you need pure enough material and surroundings that a real signal wouldn't get swamped by background radiation, including that from impurities.  The leading detection approaches these days use sodium iodide scintillators (DAMA), solid blocks of germanium or silicon (CDMS), and liquid xenon (XENON, LUX, PandaX - see here for some useful discussion and links).

I've been blogging long enough now to have seen rumors about dark matter detection come and go.  See here and here.  Now in the last week both LUX and PandaX have reported their latest results, and they have found nothing - no candidate events at all - after their recent experimental runs.  This is in contrast to DAMA, who have been seeing some sort of signal for years that seems to vary with the seasons.  See here for some discussion.  The lack of any detection at all is interesting.  There's always the possibility that whatever dark matter exists really does only interact with ordinary matter via gravity - perhaps all other interactions are somehow suppressed by some symmetry.  Between the lack of dark matter particle detection and the apparent lack of exotica at the LHC so far, there is a lot of head scratching going on....

Impact factors and academic "moneyball"

For those who don't know the term:  Moneyball is the title of a book and a movie about the 2002 Oakland Athletics baseball team, a team with a payroll in the bottom 10% of major league baseball at the time.   They used a data-intensive, analytics-based strategy called sabermetrics to find "hidden value" and "market inefficiencies", to put together a very competitive team despite their very limited financial resources.   A recent (very fun if you're a baseball fan) book along the same lines is this one.  (It also has a wonderful discussion of confirmation bias!)

A couple of years ago there was a flurry of articles (like this one and the academic paper on which it was based) about whether a similar data-driven approach could be used in scientific academia - to predict success of individuals in research careers, perhaps to put together a better department or institute (a "roster") by getting a competitive edge at identifying likely successful researchers.

The central problems in trying to apply this philosophy to academia are the lack of really good metrics and the timescales involved in research careers.  Baseball is a paradise for people who love statistics.  The rules have been (largely) unchanged for over a hundred years; the seasons are very long (formerly 154 games, now 162), and in any game an everyday player can get multiple opportunities to show their offensive or defensive skills.   With modern tools it is possible to get quantitative information about every single pitched ball and batted ball.  As a result, the baseball stats community has come up with a huge number of quantitative metrics for evaluating performance in different aspects of the game, and they have a gigantic database against which to test their models.  They even have devised metrics to try and normalize out the effects of local environment (baseball park-neutral or adjusted stats).

Fig. 1, top panel, from this article.  x-axis = # of citations.
The mean of the distribution is strongly affected by the outliers.

In scientific research, there are very few metrics (publications; citation count; impact factor of the journals in which articles are published), and the total historical record available on which to base some evaluation of an early career researcher is practically the definition of what a baseball stats person would call "small sample size".   An article in Nature this week highlights the flaws with impact factor as a metric.  I've written before about this (here and here), pointing out that impact factor is a lousy statistic because it's dominated by outliers, and now I finally have a nice graph (fig. 1 in the article; top panel shown here) to illustrate this.  

So, in academia, the tantalizing fact is that there is almost certainly a lot of "hidden value" out there missed by traditional evaluation approaches.  Just relying on pedigree (where did so-and-so get their doctorate?) and high impact publications (person A must be better than person B because person A published a paper as a postdoc in a high impact glossy journal) almost certainly misses some people who could be outstanding researchers.  However, the lack of good metrics, the small sample sizes, the long timescales associated with research, and enormous local environmental influence (it's just easier to do cutting-edge work at Harvard than at Northern Michigan), all mean that it's incredibly hard to come up with a way to find these people via some analytic approach.  

Keeping your (samples) cool is not always easy.

Very often in condensed matter physics we like to do experiments on materials or devices in a cold environment.  As has been appreciated for more than a century, cooling materials down often makes them easier to understand, because at low temperatures there is not enough thermal energy bopping around to drive complicated processes.  There are fewer lattice vibrations.  Electrons settle down more into their lowest available states.  The spread in available electron energies is proportional to \(k_{\mathrm{B}}T\), so any electronic measurement as a function of energy gets sharper-looking at low temperatures.

Sometimes, though, you have to dump energy into the system to do the study you care about.  If you want to measure electronic conduction, you have to apply some voltage \(V\) across your sample to drive a current \(I\), and that \(I \times V\) power shows up as heat.  In our case, we have done work over the last few years trying to do simultaneous electronic measurements and optical spectroscopy on metal junctions containing one or a few molecules (see here).   What we are striving toward is doing inelastic electron tunneling spectroscopy (IETS - see here) at the same time as molecular-scale Raman spectroscopy (see here for example).   The tricky bit is that IETS works best at really low temperatures (say 4.2 K), where the electronic energy spread is small (hundreds of microvolts), but the optical spectroscopy works best when the structure is illuminated by a couple of mW of laser power focused into a ~ 1.5 micron diameter spot.

It turns out that the amount of heating you get when you illuminate a thin metal wire (which can be detected in various ways; for example, we can use the temperature-dependent electrical resistance of the wire itself as a thermometer) isn't too bad when the sample starts out at, say, 100 K.  If the sample/substrate starts out at about 5 K, however, even modest incident laser power directly on the sample can heat the metal wire by tens of Kelvin, as we show in a new paper.  How the local temperature changes with incident laser intensity is rather complicated, and we find that we can model this well if the main roadblock at low temperatures is the acoustic mismatch thermal boundary resistance.  This is a neat effect discussed in detail here.  Vibrational heat transfer between the metal and the underlying insulating substrate is hampered (like \(1/T^3\) at low temperatures) by the fact that the speed of sound is very different between the metal and the insulator.   There are a bunch of other complicated issues (this and this, for example) that can also hinder heat flow in nanostructures, but the acoustic mismatch appears to be the dominant one in our case.   The bottom line:  staying cool in the spotlight is hard.  We are working away on some ideas on mitigating this issue.  Fun stuff.

(Note:  I'm doing some travel, so posting will slow down for a bit.)

The critical material nearly everyone overlooks

Condensed matter physics is tough to popularize, and yet aspects of it are absolutely ubiquitous in modern technologies.  For example:  Nearly every flat panel display, from the one on your phone to your computer monitor to your large television, takes advantage of an underappreciated triumph of materials development, a transparent conducting layer.  Usually, when a material is a good conductor of electricity, it tends to be (when more than tens of nm thick) reflective and opaque.   Remember, light is an electromagnetic wave.  If the electric field from the light can make the mobile charge in the material move, and if that charge can keep up with the rapid oscillations (10¹⁴ Hz and faster!) of the electric field, then the light tends to be reflected rather than transmitted.  This is why polished aluminum or silver can be used as a mirror.

The dominant technology for transparent conductors is indium tin oxide (ITO), which manages to thread between two constraints.  It's a highly doped semiconductor.  The undoped indium oxide material has a band gap of 3 eV, meaning that violet light with a shorter wavelength than about 350 nm will have enough energy to be absorbed, by kicking electrons out of the filled valence band and into the conduction band.  Longer wavelength light (most of the visible spectrum) doesn't have enough energy to make those transitions, and thus the material is transparent for those colors.   ITO has had enough tin added to make the resulting material fairly conducting at low frequencies (say those relevant for electronics, but much lower than the frequency of visible light).  However, because of the way charge moves in ITO (see here or here for a nice article), it does not act reflective at visible frequencies.   This material is one huge enabling technology for displays!  I remember being told that the upper limit on LCD display size was, at one point, limited by the electrical conductivity of the ITO, and that we'd never have flat screens bigger than about a meter diagonal.  Clearly that problem was resolved.

Indium isn't cheap.  There are many people interested in making cheaper (yet still reasonably transparent) conducting layers.  Possibilities include graphene (though even at monolayer thickness it does absorb about 2% in the visible) and percolative networks of metal nanowires (or nanotubes).    Unfortunately, because of the physics described above, it would appear that transparent aluminum  (in the sense of having true bulk metal-like properties but optical transparency in the visible) must remain in the realm of science fiction.

Short items

Here are a few items:

• This is fantastic.  Eric Schlaepfer, a hardware engineer at Google, has built a "disintegrated circuit", making a 6502 processor (the CPU from the Apple II and also used in one of my favorite undergrad courses back when I took it) out of surface-mount transistors.  It can't run at MHz clock speeds because of the stray capacitance of the traces on the circuit board, but it's still amazing.  If you want a metric for modern processors, if you made a version of the processor for the iPad Air 2, it would cover 82000 m².
• This is a bit "meta", but here is Peter Woit's recent Quick Items link.  I've steered clear from the whole multiverse discussion, but wow, I find it very disturbing how much recent mass publicity has been given to an idea that is described, at best, as an extremely speculative notion.  It's like having Bayesian arguments about how many angels can dance on the head of a pin.
• Speaking of absurdist speculative garbage, Michio Kaku in recent days has claimed that we will shortly be able to create avatars that will live after us based on uploaded memories, and that we are living in The Matrix, which proves the existence of God.   How has this person become one of the well-known faces of science popularization?
• American Ninja Warrior really is a good way to illustrate some fun physics.
• Geekwrapped has highlighted this blog as one of the 20 best science blogs out there.  Thanks!

Frontiers in Quantum Materials and Devices 2016 - day 2

Continuing with my very brief (and necessarily incomplete) summary of the FQMD 2016 meeting at RIKEN at the beginning of this week:

• Eric Heller of Harvard gave a very interesting and provocative talk about two topics, Raman scattering in graphene and then the onset of optical absorption in semiconductors.  Regarding the former (see here), he makes a strong case that the "double resonance" theoretical treatment of Raman scattering in graphene that has been highly cited since 2000 is not the right way to think about the problem.  Rather, one should use the Kramers-Heisenberg-Dirac theory of Raman scattering c. 1925-27, and keep in mind the important role played by (crystal) momentum conservation, as explained in the paper linked above.   Regarding the latter topic, he went on to argue (persuasively, in my view) that the textbook approach (literally - I described it in my own book) to the onset of optical absorption in direct-gap semiconductors as the photon energy exceeds the band gap is incomplete and gets the functional dependence on frequency wrong.  This work isn't published yet, and it wouldn't be appropriate for me to present his argument before he does, but I will definitely be keeping an eye out for this.
• Denis Maryenko of RIKEN spoke about measurements of the anomalous Hall effect in the 2d electron gas that is present at the interface between ZnO and MnZnO.  This system is pretty impressive, with disorder so small that it supports very clean fractional quantum Hall effect, but with larger Coulomb and Zeeman energies than the more traditional GaAs/AlGaAs interface because of the different dielectric functions and g factors, respectively of the ZnO system.   Interesting (not yet published) magnetic physics appears to be taking place at the interface due apparently to point defects that support unpaired spins.
• Pertti Hakonen from Aalto presented a nice talk about the quantum Hall effect in suspended graphene.  They have (not yet published) measurements in suspended structures made in the Corbino geometry, where there is an electrode in the center of a disk, and a second contact around the disk's perimeter.  As you might imagine, making a structure like that where the graphene disk is suspended in space, yet there is a nice contact to the central electrode without disrupting the disk, is quite a fabrication tour de force, based on an approach from here.
• Vincent Bouchiat from CNRS, Grenoble talked about using tin-decorated graphene as a system to explore the nature of the superconductor-insulator transition.  It's a flexible material system, in that you can control the coverage of the tin (the size and distribution of tin islands), the disorder in the graphene via damage, and the carrier density in the graphene via electrostatic gating.   An earlier paper is here, and a more recent one is here.
• Steven Richardson of Howard University spoke about the challenges of trying to make germanene, the germanium analog to graphene.  One approach that has been used in graphene growth has been to start with small, polycyclic carbon ring molecules as seeds.  Doing this in germanium has proven difficult, and Prof. Richardson's group does quantum chemistry calculations with DFT to establish the relative energetic stability and properties of candidate molecules.  From his talk I learned something I had not appreciated, that treating dispersion forces (van der Waals interactions) in DFT is really nontrivial.  
• James Analytis of Berkeley gave a very nice talk about Weyl fermions, where I actually felt like I had a grasp of this for a few minutes.  Up to now, most of the experiments on materials that are supposed to support Weyl-like band structure have been based on photoemission, rather than actual transport.  Prof. Analytis showed particular transport signatures (quantum oscillations of resistance as a function of magnetic field) that are consistent with what one would expect from electrons actually tracing out Weyl-expected trajectories (in both real space and reciprocal space).  This work relies on impressive nanofabrication, where a focused ion beam is used to carve Cd₃As₂ into nanostructures + leads without killing the material quality.
• Yoshinori Tokura from RIKEN surveyed his group's results looking at the interplay of magnetism, the quantum Hall effect, and the quantum anomalous Hall effect, built on high quality epitaxial structures based on a topological insulator (Bi_1-xSb_x)₂Te₃ and its Cr-doped relative.  Relevant papers are here, here, and here.   This is a great example of how much scientific activity can spring forth when it becomes possible to grow a new material system with very high quality.
• Jagadeesh Moodera from MIT presented work that is similar in spirit, involving Cr doping of Bi₂Se₃, and then V doping of Sb₂Te₃.  In systems like this it is possible to see robust, ballistic transport via chiral edge states over millimeters.  Again, excellent material quality + interesting choices of materials = impressive science.
• Joe Checkelsky of MIT spoke about exploring electronic materials with magnetically frustrated lattices.  Many systems with magnetic frustration (where magnetic moments at different lattice sites have competing interactions so that it's not possible to satisfy all of them) are insulators.  In conducting versions of these systems, there can be really funky effects where the magnetic states interact with the electrons through mechanisms like Berry curvature.  This work is in press right now and I will come back and update this once it's available online.
• Hajime Okamoto from NTT gave a neat talk about optomechanical effects (see here for a review) - where photogenerated carriers in an AlGaAs/GaAs cantilever can couple (via the piezoelectric properties of the material) to the mechanical oscillations of the cantilever.  This makes it possible to do an interesting kind of optical driving and optical cooling of such structures.   See here and here, for example.

Whew.  Overall, a fun, interesting, and dense two days!

Frontiers in Quantum Materials and Devices 2016 - day 1

There were a number of really interesting talks at the Harvard/MIT sponsored, RIKEN-co-sponsored FQMD workshop this week.   I'm very grateful for the invitation to come and present.  It was a very dense two days!  I have to be a bit careful in what I write, given that some of the work is not yet published.  Here are some highlights.  I'll try to use links to the arxiv versions of the papers so that people without paid access can see them.

• Ania Bleszynski-Jayich of UCSB spoke about her group's impressive nanoscale magnetic imaging using single nitrogen-vacancy centers in diamond AFM tips.   The N-V centers are defects in the diamond lattice, where a N atom is substituted for a C atom, directly adjacent to a C-atom vacancy.  These defects play host to a single unpaired electronic spin and can be probed through optically detected magnetic resonance.  Brendan Shields at Basel gave a talk later in the day on this technique as well - impressive imaging of domains in antiferromagnetic (!) structures.
• Naoto Nagaosa of RIKEN gave an overview of his group's work on nonlinear and nonreciprocal electronic and optical responses in special (topological) materials - see here, here, and here for examples.  The last of these is an example where because of funky topological band structure, you can have a material that is rectifying (resistance \( R(I) \neq R(-I)\) ) where the rectification is controlled by a magnetic field.
• Dylan Maher of Bristol, most recently in the spotlight for cool quantum optics work with Aephraim Steinberg, gave a great overview of the impressive integrated photonics capabilities at Bristol - see here, here, and here. 
• Satoshi Iwamoto of Tokyo showed some neat results involving 3d chiral photonic materials (that is, materials with optical helicity built into their structure).  The wild thing here is that these materials in particular are constructed by manually stacking (!) individual nanoscale-thickness layers, using manipulation within an electron microscope - see here for an example.
• Jason Petta from Princeton presented some really technically beautiful work involving SiGe quantum dots coupled to (and via) superconducting resonators.  These are gate-defined dots, where metal electrodes are used as capacitor electrodes to "suck in" and confine electrons.  It's hard to explain to a non-expert just how technically impressive the multiple gate structures are that they've developed.  See here.   Figure 1 just doesn't do it justice.
• Makoto Kohda of Tohoku spoke very clearly about spin-orbit effects in GaAs 2d electron gas and in the layered semiconductor GaSe.  He showed very cool stuff - this paper showing coherent motion and precession of spin over long distances, and gate-controlled switching between weak localization and weak antilocalization in tape-exfoliated GaSe.
• Bill Wilson, executive director of Harvard's CNS, gave an overview of their nanofab facility.  Truly, it is amazing how much internal investment Harvard has made in that facility, and I'm not even talking about the construction of the building itself.  It's very hard not to be jealous.  As often comes up when talking about Harvard, we again see that having a $40B endowment simply makes many problems faced by mere mortals simply evaporate.

Quantum materials workshop followup and preview

At the beginning of last month, the Rice Center for Quantum Materials hosted a workshop "Interacting Quantum Systems Driven Out of Equilibrium", which I reported here and here.  As promised, the slides from the talks are now available here if you click on the names of the speakers.

I am currently attending this workshop at RIKEN, sponsored by the Harvard/MIT NSF-supported Center for Integrated Quantum Materials.  I will be posting a limited summary of this workshop as well, once I recover from jet lag.

The 2016 Kavli Prize in Nanoscience

Every two years the Kavli Foundation awards three large scientific prizes, in astrophysics, neuroscience, and nanoscience.  This year's nanoscience prize goes to Gerd Binnig, Christoph Gerber, and Cal Quate, for the invention and development of the atomic force microscope (AFM).

The AFM is a great example of one of those inventions that seems elegant and simple, yet could only come into being after the stage had been set through the development of several other enabling technologies.  (My former faculty colleague Prof. Cyrus Mody does an excellent job telling this story in his book, which I heartily recommend.)

The atomic force microscope idea is very simple in concept.  Take a very sharp stylus on a flexible cantilevered arm, and scan it in a controlled way over a surface.  If the stylus tip is actually in contact with the sample surface, changes in surface topography will be detectable through the deflection of the cantilever, which can be measured optically (e.g. deflection of a laser) or by other means (e.g., changes in the electrical resistance of the cantilever as it is strained).  This is basically an extrapolation to the very small scale of the profilometer.  Alternately, you don't need the tip to be in hard contact with the surface - it just needs to get close enough to detect the short-range forces between the tip atoms and the surface.  Oscillating the cantilever/tip up and down at or near its tuning-fork-like mechanical resonance can give you benefits in terms of detection sensitivity.  Unlike STM, AFM has the benefit of working on insulating surfaces.

To implement this requires a number of building blocks:  fabrication of tips with nm-scale sharpness; precise (nm-scale or better) control at the nanoscale of the tip position relative to the sample; computerized data acquisition to map out the tip response as a function of tip position.  These are similar to the necessary requirements for scanning tunneling microscopy, and it is no coincidence that Binnig was associated with STM as well.  Widespread adoption of AFM (as discussed in Mody's book) required these building blocks to be widely available.

AFM has turned out to be incredibly versatile.  These devices can be used to measure extremely tiny local forces.  Once you know the topography, you can withdraw the tip a little, scan back over the surface and measure longer-ranged forces (electrostatics, magnetic forces if you have a magnetic tip).  Lateral deflection of the tip can tell you about frictional interactions between the tip and the sample.  A conducting tip may be used as a local potentiometer, or as a scanning "gate" electrode.  Functionalizing the tip and high frequency techniques have enabled AFM to image surfaces and even molecular orbitals with better-than-atomic resolution.  AFM has been an incredible enabling technology with utility far beyond the original vision of its pioneers.  That's exactly the kind of achievement that big prizes are meant to recognize.

Journal costs - what's the answer?

Sorry for the brief break in posting - real life obligations sometimes make it tough to blog as frequently as I would like.

The Nature Publishing Group is going to launch another five journals this year.  University library subscription costs for each of these are going to be around $5K/yr.   Other journal publishers are making similar moves - the ACS has launched three new journals this year, including an open access journal that sounds like it's meant to be a direct competitor to NPG's Scientific Reports.

On the one hand, these journals wouldn't be launched if publishers didn't think they could at least break even, meaning that someone somewhere has done a marketing study suggesting that there is sufficient demand out there both from authors and would-be subscribers.  On the other hand, it's hard for me to believe that the market can really sustain continuous growth in the number of journals, especially when this implies a similar growth in the number of requests to review papers (for free of course) from all of these editorial boards.  

What is the endpoint of this proliferation of journals, especially when many university library budgets simply make it impossible for those schools to pay for institutional subscriptions, and the pool of qualified reviewers is not similarly expanding?  In the long term, it seems like services like the arxiv have to win, perhaps with some kind of post-publication review/commentary.  However, the reward structures in place (i.e., the emphasis on particular "high impact" journal publications in hiring and promotion) put in place a huge barrier to change in that direction.  This is another area where I worry about the inevitability of a greater bifurcation into "have" and "have not" institutions, something that has a certain internal consistency but is probably long-term bad for creativity in research.

Research blogging: Magnetism in layered materials

Following on from graphene, there has been enormous interest in other layered materials for the last few years, such as transition metal dichalcogenides (TMDs) like MoS₂.   Depending on the constituents and particular structure, these materials can be semiconductors, superconductors, charge density wave compounds, etc., and can have properties that vary strongly as the number of layers in the material is reduced toward one.  You can expand the palette further by substitutionally doping different elements into the chalcogenide layers, or you can intercalate other atoms between the layers.  There are a huge number of possible compounds and variations.  (Fun note:  TMDs have been studied intensely before.  See here for a review from almost 50 years ago!  And magnetism in intercalated TMDs was examined by people like Stuart Parkin and Richard Friend almost 40 years ago.   The resurgence now is due to a combination of improved growth and characterization techniques, interest in low-dimensionality materials, and theoretical appreciation for the richness of possible states in these systems.)

Recently, collaborating with my colleague Jun Lou, we had some fun examining a related material, V₅S₈, which you can also think of as (V_0.25)VS₂.  There are vanadium disulfide layers, and intercalated between them are additional vanadium atoms in an ordered pattern.  The bulk version of this material was found in the 1970s to be an antiferromagnet - below the Neel temperature T_N ~ 32 K, the spins of the unpaired electrons on the intercalated vanadium atoms spontaneously order into the arrangement shown in the upper panel at right.   If an external magnetic field bigger than about 4 T is applied perpendicular to the planes of the material, the spins flop over into the arrangement shown in the bottom panel - this is called a spin flop transition. 

Prof. Lou's group has figured out how to grow V₅S₈ by chemical vapor deposition, so that we were able to make measurements on single crystals of a variety of thicknesses, down to about 10 nm.  We found a couple of cool things, as reported here.   

First, we found a previously unreported first-order (in the thicker crystals) phase transition as a function of externally applied magnetic field.   The signature of this is hysteresis in the electrical resistance of the material as a function of the magnetic field, H.  Just below T_N, the hysteresis appears near zero magnetic field.  As T is lowered, the magnetic field where the hysteresis takes place increases dramatically - in a thick crystal, it can go from basically 0 T to taking place at 9 T when the temperature is lowered by only three Kelvin!  Indeed, that's probably one reason why the transition was missed by previous investigators:  If you take data at only select temperatures, you could easily miss the whole thing.   This kind of a transition is called metamagnetic, and we think that large applied fields kill the antiferromagnetism (AFM), driving the material into a paramagnetic (PM) state.  We suggest a phase diagram shown in the table-of-contents figure shown here.  The transition extrapolates to a finite value of H at zero temperature.  That implies that it ends up as a quantum phase transition.

Second, we found that there are systematic changes in the magnetic properties as a function of the thickness of the crystals.  In thinner crystals, the antiferromagnetism appears to be weaker, with T_N falling.  Moreover, the hysteresis in the field-driven transition vanishes in thinner crystals, suggesting that the metamagnetic transition goes from first-order to second order in the thin limit.   

This work was a lot of fun.  As far as I know, it's the first example of a systematic study of magnetic properties in one of these layered materials as a function of material thickness.  I think we've just scratched the surface in terms of what could be possible in terms of magnetism in this layered material platform. 

Interacting Quantum Systems Driven Out of Equilibrium - day 2

Continuing into day 2 of our workshop:

• Bryce Gadway of the University of Illinois spoke about using cold atoms in an optical lattice to simulate topological and disordered systems.  His group has implemented an optical lattice constructed not by interference of retroreflected lasers, but by interference between a laser and counterpropagating beams frequency shifted by precise, controlled amounts.  As a non-atomic physics person I'm a bit fuzzy on the details, but the point is that this allows his group to put in place precise control of the on-site potential of each lattice site and to dial in designer phase shifts associated with tunneling between adjacent sites, on demand.  This means it is possible to study transport problems (like Bloch oscillations) as well as introducing designer, time-varying, site-specific disorder if desired.  
• I spoke about my group's work on heating and dissipation in atomic- and molecular-scale junctions driven out of equilibrium (and into a steady state) by electronic bias.  I framed the discussion in terms of how hard it is to obtain truly local information about vibrational and electronic distributions in such driven systems.  On the vibration side, if you're interested, I suggest looking here, here, and here, with a recent related result here.  On the electronic front, I talked about published (here and here) and some unpublished data looking at electronic shot noise at high biases in atomic-scale metal junctions.  
• Eugene Demler from Harvard (my grad school classmate) gave a nice talk that addressed nonequilibrium aspects of both cold atoms and electrons.   For example, he and collaborators have developed some theoretical machinery for looking at a cold atom version of the orthogonality catastrophe - what happens if you suddenly "turn on" interactions between a cold Fermi gas and a single impurity, and watch the dynamics.  These same theoretical techniques can be applied to solid state systems as well.  (This is just a subset of what was presented.)
• Ryo Shimano from Tokyo University gave a very pretty talk about optical manipulation and driving of the Higgs mode inside superconductors.  You can hit a superconductor with THz radiation as a pump, and then probe at some delay with additional THz radiation.  If the pump is at the right frequency (energy half the superconducting gap, in the s-wave case), you can excite collective sloshing of the condensate (see here and scroll down to the first example).  As you might imagine, things get more rich and complicated with more exotic superconductors (multiband or unconventional).
• Emil Yuzbashyan from Rutgers presented a look at the fundamental issues involved in non-thermal steady states of ensembles of quantum particles at long times after a quench (a sudden change in some parameter).  As I wrote in the first-day discussion, the interesting question here is when does the system evolve seemingly coherently (i.e., the particles slosh around in recurring patterns, just as a Newton's cradle ticks back and forth), and when does the system instead tend toward a long-time state that looks like a randomized, thermalized condition?   To see how this relates to classical mechanics, see these articles (here and here) that I need to find time to read.  
• Lastly, my colleague Matt Foster from Rice spoke about quenched BCS superfluids, topology, spectral probes, and gapless (topological) superconductivity under intense THz pumping.  This was a neat pedagogical talk about this work.  It touches some of the same issues as the Shimano talk above.  One aspect that I found interesting to consider:  You can have a system where a quench drives some collective oscillations, and those collective oscillations act as a Floquet perturbation, changing the effective band structure and giving rise to nonlinearities that continue the oscillations.  Wild stuff - here are the slides.  

We then had a lunch that segued smoothly into a relaxed poster session for students and postdocs.  Overall, it was a great workshop and a good chance to get people from diverse areas of CM and AMO physics together over common interests in nonequilibrium response of quantum systems.  Thanks to everyone who was able to come, to our staff organizer who made everything actually come together, and of course to our sponsors (NSF DMR, ICAM-I2CAM, the Gordon and Betty Moore Foundation, Lakeshore, Advantest, and Coherent)!

Interacting Quantum Systems Driven Out of Equilibrium - day 1 (updated - complete)

Our workshop was fun and interesting.   There are multiple ways to drive physical systems out of equilibrium - you can take some system and push on it with some force, for example.  In the case of a condensed matter system (whether solid state or trapped cold atoms), you can apply a bias - some difference in population (or chemical potential or pressure) that drives the system, either by adding kinetic energy to it or encouraging the flow of matter and/or charge.  You can apply a temperature difference across the system, driving some average flow of energy through the system's degrees of freedom.  You can shine light on the system, adding energy and momentum either at a steady rate or in a sudden pulse.  One favorite piece of vocabulary these days is a quench - suddenly (compared with relaxation rates of the system) changing some condition like the potential energy of the particles, and then watching the response of the system's degrees of freedom.  Does the system "thermalize"?  That is, do the microscopic pieces of the system interact with each other and redistribute energy so that there seems to be some effective temperature?  Or does the system fail to thermalize, and instead slosh around in some non-thermal configuration for a long time?  There are many open issues.

We had 13 talks on the first day, and I don't want to write exhaustive summaries of all of them.  We will eventually be posting pdf files of the relevant slides.  That being said, I will give a super-brief description of each, and link to a relevant paper or two so that you can see what was discussed.  Here are the 13 talks we had on the first day.

• Nadya Mason from UIUC spoke about her group's work on engineered superconducting/normal metal structures in magnetic fields.  These devices allow studies of current-driven motion of trapped magnetic flux.  In some sense this is an old, established problem, but traditional models actually do a poor job of reproducing the experimental data.  The experiments are here, and it looks like it's important to include some "delayed friction" to understand vortex motion.
• Jonathan Bird from Buffalo spoke about his group's studies of quantum point contacts in semiconductors, where it's long been known how to measure electronic conduction down to the limit of discrete quantum channels, where the devices act like waveguides for the electrons.   His group has developed some high speed techniques for making sub-ns electronic measurements, and what really gets interesting is when systems are driven hard, so that the electronic bias is the largest energy scale in the problem - you have to worry quite a bit about exciting phonons and what they do.  A key result is the apparent formation of a specific, somewhat heating-immune transport mode when such a point contact is driven really hard.
• David Goldhaber-Gordon from Stanford spoke about his group's recent experiments looking at quantum dots, some building on work looking at the so-called two-channel Kondo effect.  An unpaired electron is placed in the position of trying to couple to two (carefully tuned to be) independent baths of electrons.  Some of the not-yet-published results look at interesting scaling as one tunes through the accessible regimes, and involved some stunningly pretty device fabrication done at the Weizmann Institute.  Other experiments looked at the apparent emergence of symmetry in systems comprising two quantum dots.
• Tilman Esslinger of ETH presented his group's great work on using cold atoms to look at systems rather analogous to the ones Prof. Bird had mentioned.  They can create blobs of fermionic cold atom fluids of unequal populations, and link them by a carefully controlled constriction, and then they can image transport.  If they squeeze the contact to be effectively one dimensional, they can see quantized conductance of atoms (just as solid state folks can do with charge in a quantum point contact).  They can use atomic physics methods to dial around the interactions between the particles, and can then look at how this affects dissipation in the out of equilibrium situation.  Gorgeous stuff.
• Takashi Oka of the Max Planck Institutes in Dresden talked about Floquet theory and using lasers to control the topology of the band structure of materials.  There was a lot to this talk, and it's not easy to summarize.  In Floquet theory, you apply a periodic driving potential to a quantum system.  Just like a spatially periodic potential energy picks out certain spatial periodicities and gives you a compact way of looking at band structure, temporal periodicity creates what you could call replicas of the band structure but shifted in energy by multiples of \( \hbar \omega\), where \(\omega\) is the driving frequency.  If you do this right, the driven system can have topological edge states.  You can also use periodic driving to reorient the magnetization of materials as if you had a whopping huge effective magnetic field.
• Andrew Millis of Columbia University has worked on many relevant topics, and in this case chose to speak about theory he and collaborators have done regarding a recent experiment looking at vanadium dioxide.  That material has a structural phase transition at 65 C that separates a low temperature, monoclinic, insulating state from a high temperature, tetragonal, metallic state.  In the experiment, optical excitation puts the material into a metallic state without actually leaving the monoclinic crystal structure.  The theory suggests that this is a correlation effect - scoop electrons out of the lower Hubbard band and drop them into the upper band, and interorbital interaction effects can stabilize a new, metastable electronic structure that's a metal.
• Alessandra Lanzara of Berkeley gave a really nice talk about her group's work on time-resolved angle-resolved photoemission.  You hit a material of interest with an ultrafast, time-resolved pump pulse of near-infrared light (1.5 eV photons), and then at some known delay you smack the system with a 6 eV probe pulse at a particular polarization and orientation, and measure the energy and momentum distribution of the electrons that get kicked out.  This lets you measure the transient electronic structure.  They've been able to use this approach to study the dynamics of quasiparticles in cuprate superconductors, how Cooper pairs respond to such pumping, etc.
• N. Peter Armitage at Johns Hopkins articulated nicely three reasons to "go nonequilibrium":  to learn about elementary excitations of an equilibrium phase; to access "phases" not possible in equilibrium material configurations; and to look for new "phases" that have no equilibrium analog.  He then gave a fun talk about using optical spectroscopy techniques to look at many-body relaxations (older paper here) in the Coulomb glass phase of lightly doped semiconductors - when there are strongly interacting, localized electrons in a disordered configuration so that screening is poor.  Interestingly, these systems relax more slowly when the carrier densities get higher, in physics related to the orthogonality catastrophe. 
• My faculty colleague Jun Kono from Rice spoke about so-called Dicke phenomena (such as superradiance, superfluorescence) in semiconductors.  These effects are great examples of nonequilibrium physics, when a driven system (say a semiconductor in a magnetic field illuminated by THz radiation that spans the energy scale of the cyclotron resonance, \(\omega_{\mathrm{c}} = e B/m^{*}\)) spontaneously develops coherence among the many electron-hole excitations in the system.  You can put such a system in a clever kind of 1d optical cavity, and approach the "strong coupling" regime so that the energetic coupling between the charge carriers and the photons in the cavity is comparable to the cyclotron energy.
• Christof Weitenberg from Hamburg then spoke about exciting results in simulating condensed matter systems using cold atoms in optical lattices.  One piece of physics that's very in vogue right now because of the rise of topology and various 2d materials is Berry curvature.  It's hard to explain this in brief - if you look at how the energy bands of a material as a function of crystal momentum \(E(\mathbf{k})\) are curved, the wavefunction of a particle traversing some closed trajectory in \(\mathbf{k}\)-space can pick up a phase factor related to that curvature.  In Weitenberg's experiments, cleverly arranged laser beams can create designer lattices.  Shaking the lasers periodically as a function of time can lead to the same Floquet physics discussed above, changing the effective band structure for atoms confined in those lattices, and through cool imaging techniques the experimentalists can reconstruct the Berry curvature that they have designed into that effective band structure.
• Another colleague Kaden Hazzard from Rice gave a nice theoretical talk about different nonequilibrium collective phenomena in ultracold atomic matter.  One aspect involved dilute molecules with electric dipoles (KBr) trapped in an optical lattice.  Because of their dipole moments, the molecules interact with each other over long ranges (dipole-dipole interactions scale like \(1/r^{3}\)), and their relaxation after getting dinged is governed by many-body interaction effects.  Another system is trapped Rydberg atoms, where dipolar interactions scale like the principal quantum number to the eleventh power (!).  
• Andrea Cavalleri from the Max Planck in Hamburg (and also spending time at Oxford) spoke about his group's very high profile work that I've already described here.  The central question here is really can driving a quantum material stabilize collective states like superconductivity that have coherence, correlations, and remarkable physical properties that would be absent without the drive.  Both Cavalleri and Oka made reference to this video, which shows how driving a classical pendulum can render the inverted position of the pendulum stable.  The experiments themselves are truly remarkable.
• In the last talk of Day 1, Sarang Gopalakrishnan of Cal Tech gave a theory talk again examining the response of driven many-body quantum systems, focusing particularly on the issue of many-body localization.  That is, when do the quantum dynamics of a many-body system lead to a real breakdown of quantum ergodicity, so that the degrees of freedom get "stuck", having large variability of local observables (instead of things being smoothed out and looking thermally smeared) and comparatively weak entanglement (which grows more slowly with system size than in the effectively thermal case).  He pointed out experimental challenges, that experiments probe dynamics rather than quantum eigenstates and that everything really is coupled (however weakly) to some thermal "bath", but argued that these issues aren't fatal to the interesting physics.

Updates coming - Interacting Quantum Systems Driven Out of Equilibrium

As I'd advertised, the Rice Center for Quantum Materials is hosting a two-day workshop on interacting quantum systems driven out of equilibrium.  This event brings together people from roughly three different perspectives:  people who worry about (solid state) systems driven out of equilibrium by electrical bias; people who worry about quantum systems driven out of equilibrium by light (often ultrafast and/or very intense); and people who leverage the amazing cleanliness and tunability of cold atom systems to examine driven quantum many-body systems.   I've been taking notes, and after the workshop wraps up today I'll post some highlights.