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.