More brief items

Writing writing writing.  In the meantime:

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

Ferromagnetic droplets

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

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

Brief items

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

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

Science and a nation of immigrants

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

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

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

Magic hands, secret sauce, and tricks of the trade

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

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

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

Any favorite examples out there from readers? 

The physics subject GRE and grad school

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

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

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

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

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

Round-up of various links

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

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

Brief items

A number of interesting items:

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

Publons?

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

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

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

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

Magnets and energy machines - everything old is new again.

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

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

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

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

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

Light emission from metal nanostructures

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

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

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

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

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

 

Updated: CM/nano primer - aggregated posts

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


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

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

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

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

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

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

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

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

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

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

The 1993 Stanford physics qual

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

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

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

Liquid droplets with facets

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

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

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

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

Brief items, + "grant integrity"

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

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

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

This week in the arxiv

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

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

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

Brief items

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

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

The physics of vision

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

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

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

The statistical mechanics of money

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

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

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

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

APS March Meeting wrapup

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

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

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