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.)

Thursday, June 29, 2017

Condensed matter/nano resources for science writers and journalists

I've been thinking about and planning to put together some resources about condensed matter physics and nanoscience that would be helpful for science writers and journalists.  Part of the motivation here is rather similar to that of doing outreach work with teachers - you can get a multiplicative effect compared to working with individual students, since each teacher interacts with many students.  Along those lines, helping science writers, journalists, and editors might have an impact on a greater pool than just those who directly read my own (by necessity, limited) writing.  I've had good exchanges of emails with some practitioners about this, and that has been very helpful, but I'd like more input from my readers.

In answer to a few points that have come up in my email discussions:

  • Why do this?  Because I'd like to see improved writing out there.  I'd like the science-interested public to understand that there is amazing, often deep physics around them all the time - that there are deep ideas at work deep down in your iphone or your morning cup of coffee, and that those are physics, too.  I know that high energy ("Building blocks of the universe!") and astro ("Origins of everything!  Alien worlds!  Black holes!") are very marketable.  I'd be happy to guide little more of the bandwidth toward condensed matter/materials/real nano (not sci-fi) popularization.  I think the perception that high energy = all of physics goes a long way toward explaining why so many people (incl politicians) think that basic research is pie-in-the-sky-useless, and everything else is engineering that should be funded by companies.  I do think online magazines like Quanta and sites like Inside Science are great and headed in a direction I like.  I wish IFLS was more careful, but I admire their reach.
  • What is the long-range audience and who are the stakeholders?  I'd like CMP and nano to reach a broad audience.  There are serious technically trained people (faculty, researchers, some policy makers) who already know a lot of what I'd write about, though some of them still enjoy reading prose that is well written.  I am more thinking about the educated lay-public - the people who watch Nova or Scientific American Frontiers or Mythbusters or Through The Wormhole (bleah) or Cosmos, or who read Popular Science or Discovery or Scientific American or National Geographic.  Those are people who want to know more about science, or at least aren't opposed to the idea.  I guess the stakeholders would be the part of the physics  and engineering community that work on solid state and nano things, but don't have the time or inclination to do serious popular communication themselves.  I think that community is often disserved by (1) the popular portrayal that high energy = all of physics and crazy speculative stuff = actual tested science; (2) hype-saturated press releases that claim breakthroughs or feel the need to promise "1000x faster computers" when real, fundamental results are often downplayed; and (3) a focus in the field that only looks at applications rather than properly explaining the context of basic research.
  • You know that journalists usually have to cover many topics and have very little time, right?  Yes.  I also know that just because I make something doesn't mean anyone would necessarily use it.  Hence, why I'm looking for input.   Maybe something like a CM/nano FAQ would be helpful.
  • You know that long-form non-fiction writers love to do their own topical research, right?  Yes, and if there was something I could do to help those folks save time and avoid subject matter pitfalls, I'd feel like I'd accomplished something.
  • You could do more writing yourself, or give regular tips/summaries to journalists and editors via twitter, your blog, etc.  That's true, and I plan to try to do more, but as I said at the top, the point is not for me to become a professional journalist (in the sense of providing breaking news tidbits) or writer, but to do what I can to help those people who have already chosen that vocation. 
  • You know there are already pros who worry about quality of science writing and journalism, right?  Yes, and they have some nice reading material.  For example, this and this from the Berkeley Science Review; this from the Guardian; this from the National Association of Science Writers.
So, writers and editors that might read this:  What would actually be helpful to you along these lines, if anything?  Some primer material on some topics more accessible and concise than wikipedia?


Tuesday, June 20, 2017

About grants: What are "indirect costs"?

Before blogging further about science, I wanted to explain something about the way research grants work in the US.  Consider this part of my series of posts intended to educate students (and perhaps the public) about careers in academic research.

When you write a proposal to a would-be source of research funding, you have to include a budget.  As anyone would expect, that budget will list direct costs - these are items that are clear research expenses.  Examples would include, say, $30K/yr for a graduate student's stipend, and $7K for a piece of laboratory electronics essential to the work, and $2K/yr to support travel of the student and the principal investigator (PI) to conferences.   However, budgets also include indirect costs, sometimes called overhead.  The idea is that research involves certain costs that aren't easy to account for directly, like the electricity to run the lights and air conditioning in the lab, or the costs to keep the laboratory building maintained so that the research can get done, or the (meta)costs for the university to administer the grant.  

So, how does the university to figure out how much to tack on for indirect costs?  For US federal grants, the magic (ahem) is all hidden away in OMB Circular A21 (wiki about it, pdf of the actual doc).  Universities periodically go through an elaborate negotiation process with the federal government (see here for a description of this regarding MIT), and determine an indirect cost rate for that university.  The idea is you take the a version of the direct costs ("modified total direct costs" - for example, a piece of equipment that costs more than $5K is considered a capital expense and not subject to indirect costs) and multiply by a negotiated factor (in the case of Rice right now, 56.5%) to arrive at the indirect costs.  The cost rates are lower for research done off campus (like at CERN), with the argument that this should be cheaper for the university.  (Effective indirect cost rates at US national labs tend to be much higher.)

Foundations and industry negotiate different rates with universities.  Foundations usually limit their indirect cost payments, arguing that they just can't afford to pay at the federal level.  The Bill and Melinda Gates Foundation, for example, only allows (pdf) 10% for indirect costs.   The effective indirect rate for a university, averaged over the whole research portfolio, is always quite a bit lower than the nominal A21 negotiated rate.  Vice provosts/presidents/chancellors for research at major US universities would be happy to explain at length that indirect cost recovery doesn't come close to covering the actual costs associated with doing university-based research.  

Indirect cost rates in the US are fraught with controversy, particularly now.  The current system is definitely complicated, and reasonable people can ask whether it makes sense (and adds administrative costs) to have every university negotiate its own rate with the feds.   It remains to be seen whether there are changes in the offing.

Saturday, June 17, 2017

Interesting reading material

Summer travel and other activities have slowed blogging, but I'll pick back up again soon.  In the meantime, here are a couple of interesting things to read:

  • Ignition!  An Informal History of Liquid Rocket Propellants (pdf) is fascinating, if rather chemistry-heavy.  Come for discussions of subtle side reactions involved in red fuming nitric acid slowly eating its storage containers and suggested (then rejected) propellants like dimethyl mercury (!!), and stay for writing like, "Miraculously, nobody was killed, but there was one casualty — the man who had been steadying the cylinder when it split. He was found some five hundred feet away, where he had reached Mach 2 and was still picking up speed when he was stopped by a heart attack."  This is basically the story from start to finish (in practical terms) of the development of liquid propellants for rockets.   That book also led me to stumbling onto this library of works, most of which are waaaaay too chemistry-oriented for me.  Update:  for a directly relevant short story, see here.
  • Optogenetics is the idea of using light to control and trigger the activation/inactivation of genes.  More recently, there is a big upswing in the idea of magnetogenetics, using magnetic fields to somehow do similar things.  One question at play is, what is the physical mechanism whereby magnetic fields can really do much at room temperature, since magnetic effects tend to be weak.  (Crudely speaking, the energy scale of visible photons is eV, much larger than the thermal energy scale of \(k_{\mathrm{B}}T \sim ~\)26 meV, and readily able to excite vibrations or drive electronic excitations.  However, one electron spin in a reasonably accessible magnetic field of 1 Tesla is \(g \mu_{\mathrm{B}}B \sim ~\) 0.1 meV.)  Here is a nice survey article about the constraints on how magnetogenetics could operate.
  • For a tutorial in how not to handle academic promotion cases, see here.