Sunday, July 23, 2017

Several items - the arxiv, "axial-gravitational" fun, topology

Things have been a bit busy, but here are a few items that have popped up recently:
  • Symmetry magazine is generally insightful and well-written.   Recently they posted this amusing article looking at various fun papers on the arxiv.  Their first example reminds me of this classic.
  • Speaking of the arxiv, it's creator, Paul Ginsparg, posted this engaging overview recently.  It's not an overstatement to say that the arxiv has had an enormous impact on science over the last 25 years.
  • There has been a huge amount of media attention on this paper (arxiv version).  The short version:  In high energy physics there is a certain conservation principle regarding chiral (meaning that the particle spin is directed along its momentum) massless fermions, so that ordinarily these things are produced so that there is no net excess of one handedness of spin over the other.  There is a long-standing high energy theory argument that in curved spacetime, the situation changes and you can get an excess of one handedness - a "chiral anomaly".  It is difficult to see how one could test this directly via experiment, since in our daily existence spacetime curvature is pretty minimal, unlike, say, near the event horizon of a small blackhole.  However, solid state materials can provide a playground for some wild ideas.  The spatial arrangement of atoms in a crystalline solid strongly affects the dispersion relation, the relationship between energy and (the crystal analog of) momentum.  For example, the linear dispersion relation between energy and momentum in (neutral) graphene makes the electrons behave in some ways analogous to massless relativistic particles, and lets people do experiments that test the math behind things like Klein tunneling.  As a bonus, you can add in spin-orbit coupling in solids to bring spin into the picture.  In this particular example, the electronic structure of NbP is such that, once one accounts for the spatial symmetries and spin-orbit effects, and if the number of electrons in there is right, the low-energy electronic excitations are supposed to act mathematically like massless chiral fermions (Weyl fermions).  Moreover, in a temperature gradient, the math looks like that used to describe that gravitational anomaly I'd mentioned above, and this is a system where one can actually do measurements.  However, there is a lot of hype about this, so it's worth stating clearly:  gravity itself does not play a role in NbP or this experiment.  Also, I have heard concerns about the strength of the experimental interpretation, because of issues about anisotropy in the NbP material and the aspect ratio of the sample.  
  • Similarly, there is going to be a lot of media attention around this paper, where researchers have combined a material ((Cr0.12Bi0.26Sb0.62)2Te3) that acts like a kind of topological insulator (a quantum anomalous Hall insulator, to use the authors' particular language) and a superconductor (Nb).  The result is predicted to be a system where there is conduction around the edges with the low energy current-carrying excitations act like Majorana fermions, another concept originally invented in the context of high energy physics.  
  • Both of these are examples of a kind of topology mania going on in condensed matter physics these days, as described here.  This deserves a longer discussion later.  

Sunday, July 16, 2017

A thermoelectric surprise in metals

Earlier this year I'd described what thermoelectricity is, and I'd also discussed recent work of ours where we used a laser as a scan-able heat source, and were then able to see nicely the fact that changing the size of a nanoscale metal structure can vary the material's thermoelectric properties, and make a thermocouple out of a single metal.

With this same measurement technique, we found a result that we thought was rather strange and surprising, which we have written up here.   Take a moderately long wire, say 120 nm wide and several microns long, made by patterning a 15 nm thick Au film.  Hook up basically a volt meter to the ends of the wire, and scan the laser spot along the length of the wire, recording the voltage as a function of the laser position.  If the wire is nice and homogeneous, you'd expect not to see to much until you get to the ends of the wire where it widens out into bigger contacts.  (There the size variation should make the skinny/wide junction act like a thermocouple.)   Instead, we see the result shown here in the figure (fig. 2 of the paper).  There is a great deal of spatial variability in the photothermoelectric voltage, like the wire is actually made up of a whole bunch of little thermocouples!

Note that your eye tends to pick out a spatial scale in panel (a) comparable to the 1 micron scale bar.  That's a bit misleading; the spot size of the laser in our system is about 1.8 microns, so this measurement approach would not pick up much smaller spatial scales of variation.

The metal wire is polycrystalline, and if you look at the electron microscope images in panels (c, d, e) you can make out a grain structure with lateral grain sizes of 15-20 nm.  Maybe the wire isn't all that homogeneous?  One standard way physicists look at the quality of metal films is to consider the electrical resistance of a square patch of film (\(R_{\square}\), the "sheet resistance" or "resistance per square"), and compare that number with the "resistance quantum", \(R_{\mathrm{q}}\equiv h/2e^2\), a combination of fundamental constants that sets a scale for resistance.  If you had two pieces of metal touching at a single atom, the resistance between them would be around the resistance quantum.  For our wire material, \(R_{\square}\) is a little under 4 \(\Omega\), so \(R_{\square} << R_{\mathrm{q}}\), implying that the grains of our material are very well-connected - that it should act like a pretty homogeneous film.  This is why the variation shown in the figure is surprising.  Annealing the wires does change the voltage pattern as well as smoothing it out.  This is a pretty good indicator that the grain boundaries really are important here.  We hope to understand this better - it's always fun when a system thought to be well understood surprises you.

Friday, July 07, 2017

Two books that look fun

Two books that look right up my alley:

  • Storm in a Teacup by Helen Czerski.  Dr. Czerski is a researcher at University College London, putting her physics credentials to work studying bubbles in physical oceanography.  She also writes the occasional "everyday physics" column in the Wall Street Journal, and it's great stuff.
  • Max the Demon vs. Entropy of Doom by Assa Auerbach and Richard Codor.   Prof. Auerbach is a serious condensed matter theorist at the Technion.  This one is a kick-starter to produce a light-hearted graphic novel that is educational without being overly mathematical.  Looks fun.  Seems like the target audience would be similar to that for Spectra.

Thursday, July 06, 2017

Science and policy-making in the US

Over twenty years ago, Congress de-funded its Office of Technology Assessment, which was meant to be a non-partisan group (somewhat analogous to the Congressional Budget Office) that was to help inform congressional decision-making on matters related to technology and public policy.  The argument at the time of the de-funding was that it was duplicative - that there are other federal agencies (e.g., DOE, NSF, NIH, EPA, NOAA) and bodies (the National Academies) that are capable of providing information and guidance to Congress.   In addition, there are think-tanks like the Rand CorporationIDA, and MITRE, though those groups need direction and a "customer" for their studies.   Throughout this period, the executive branch at least had the Office of Science and Technology Policy, headed by the Presidential Science Advisor, to help in formulating policy.  The level of influence of OSTP and the science advisor waxed and waned depending on the administration.   Science is certainly not the only component of technology-related policy, nor even the dominant one, but for the last forty years (OSTP's existence) and arguably going back to Vannevar Bush, there has been broad bipartisan agreement that science should at least factor into relevant decisions.

We are now in a new "waning" limit, where all of the key staff offices at OSTP are vacant, and there seems to be no plan or timeline to fill them.     The argument from the administration, articulated in here, is that OSTP was redundant and that its existence is not required for science to have a voice in policy-making within the executive branch.   While that is technically true, in the sense that the White House can always call up anyone they want and ask for advice, removing science's official seat at the table feels like a big step.  As I've mentioned before, some things are hard to un-do.   Wiping out OSTP for at least the next 3.5 years would send a strong message, as does gutting the science boards of agencies.   There will be long-term effects, both in actual policy-making, and in continuity of knowledge and the pipeline of scientists and engineers interested in and willing to devote time to this kind of public service.   (Note that there is a claim from an unnamed source that there will be a new OSTP director, though there is no timeline.)