Monday, July 14, 2014

My Nerd Nite talk - video

I mentioned back in February that I'd had the chance to speak at Nerd Nite Houston (facebook link - it's updated more frequently than the website).  It was a blast, and I encourage people in the area to check it out on the last Thursday of each month, location announced on the page, though so far they've all been at Notsuoh

Thanks to the fantastic videographic efforts of Jon Martensen, the video of my talk is now available on youtube here.  The talk is about 20 minutes and the rest is the audience Q&A.   All in all, a very fun experience - thanks again to Amado Guloy and the rest of the Nerd Nite folks for giving me the opportunity.

Sunday, July 13, 2014

Interesting links: peer review, falsifiability

Slow blogging - I've got the usual papers plus working on finishing a really big writing project (more about that soon), combined w/ summer travel.  The posting rate will pick up again in another week and a half.  In the meantime, here are a few interesting links from the last couple of weeks.
  • A thoroughly dishonest scientist (and I guess a couple of other people) were exposed as running an awful peer review scam.  More about this here.  The scam involved creating fake email addresses and identities to mask people essentially reviewing their own and friends' papers.  The worst thing about this whole mess is that it gives ammunition to the anti-science crowd who are convinced that scientific research is a corrupt enterprise - people like the person I wrote about here.
  • Peter Woit has written an interesting review of a book about string theory and whether the scientific method needs to be revised to deal with "post-emprical" theory verification, whatever that means.  I haven't read the book, but the idea of post-empiricism is pretty sketchy to me.
  • Natalie Wolchover has written an article about some fluid droplet experiments that show quantum-like behavior of droplets (e.g., interference-fringe-like distributions, for example).  The physics here is that the droplets are interacting with associated surface waves of an underlying fluid, and the mechanics of those waves self-consistently guides the droplets.  This is similar in spirit to Bohm's ideas about pilot waves as a way of thinking about quantum mechanics.  The authors of the fluid paper are clearly high on this idea.  These are clearly very cool experiments, but it's a huge stretch to say that they should motivate re-thinking our interpretations of quantum mechanics. 

Friday, July 04, 2014

An expression of concern about an expression of concern

There has been a big kerfluffle about Facebook conducting a mass social psychology experiment.  At heart is the issue of informed consent.  By clicking "ok" on a vaguely worded license agreement, did users really give true informed consent to participate in experiments designed to manipulate them?  The study was published in the Proceedings of the National Academy of Sciences here.  Now, in hindsight, PNAS has published an "Expression of Concern" here about whether the study was in compliance with the Common Rule regarding informed consent by human subjects.  The PNAS editors point out that as a privately funded, for-profit corporation not taking federal funding for this work, Facebook isn't technically bound by this constraint.

This is technically correct (the best kind of correct), but doesn't this have frightening implications?  Does this mean that private companies are free to perform experiments on human subjects without asking for informed consent, so long as they don't violate obvious laws like killing people?  Seems like there must be some statutes out there about human experimentation, right?  Perhaps one of my readers knows this issue....

Monday, June 30, 2014

What are universal conductance fluctuations?

Another realization I had at the Gordon Conference:  there are plenty of younger people in condensed matter physics who have never heard about some mesoscopic physics topics.   Presumably those topics are now in that awkward purgatory of being so established that they're "boring" from the research standpoint, but they are beyond what is taught in standard solid state physics classes (i.e., they're not in Ashcroft and Mermin or Kittel).  Here is my attempt to talk at a reasonably popular level about one of these, so-called "Universal Conductance Fluctuations" (UCF).

In physics parlance, sometimes it can be very useful to think about electrons in solids as semiclassical, a kind of middle ground between picturing them as little classical specks whizzing around and visualizing them as fuzzy, entirely wavelike quantum states.  In the semiclassical picture, you can think of the electrons as following particular trajectories, and still keep in mind their wavelike aspect by saying that the particles rack up phase as they propagate along.  In a typical metal like gold or copper, the effective wavelength of the electrons is the Fermi wavelength, \( \lambda_{\mathrm{F}} \sim 0.1~\)nm.  That means that an electron propagating 0.1 nm changes its quantum phase by about \(2 \pi\).  In a relatively "clean" metal, electrons propagate along over long distances, many Fermi wavelengths, before scattering.  At low temperatures, that scattering is mostly from disorder (grain boundaries, vacancies, impurities).

The point of keeping track of the quantum phase \(\phi\) is that this is how we find probabilities for quantum processes.  In quantum mechanics, if there are two paths to do something, with (complex) amplitudes \(A_{1}\) and \(A_{2}\), the probability of that something is \(|A_{1} + A_{2}|^{2}\), which is different than just adding the probabilities of each path, \(|A_{1}|^{2}\) and \(|A_{2}|^{2}\).  For an electron propagating, for each trajectory we can figure out an amplitude that includes the phase.  We add up all the (complex) amplitudes for all the possible trajectories, and then take the (magnitude) square of the sum.  The cross terms are what give quantum interference effects, such as the wavy diffraction pattern in the famous two-slit experiment.  This is how Feynman describes interference in his great little book, QED

Electronic conduction in a disordered metal then becomes a quantum interference experiment.  An electron can bounce off various impurities or defects in different sequences, with each trajectory having some phase.  The exact phases are set by the details of the disorder, so while they differ from sample to sample, they are the same within a given sample as long as the disorder doesn't change.  The conduction of the electrons is then something like a speckle pattern.  The typical scale of that speckle is a change in the conductance \(G\) of something like \(\delta G \sim e^{2}/h\).  Note that inelastic processes can change the electronic wavelength (by altering the electron energy and hence the magnitude of its momentum) and also randomize the phase - these "dephasing" effects mean that on length scales large compared to some coherence length \(L_{\phi}\), it doesn't make sense to worry about quantum interference.

Now, anything that alters the relative phases of the different trajectories will lead to fluctuations in the conductance on that scale (within a coherent region).  A magnetic field can do this, because the amount of phase racked up by propagating electrons depends not just on their wavelength (basically their momentum), but also on the vector potential, a funny quantity discussed further here.  So, ramping a magnetic field through a (weakly disordered) metal (at low temperatures) can generate sample-specific, random-looking but reproducible, fluctuations in the conductance on the order of \(e^{2}/h\).  These are the UCF. 

By looking at the UCF (their variation with magnetic field, temperature, gate voltage in a semiconductor, etc.), one can infer \(L_{\phi}\), for example.  These kinds of experiments were all the rage in ordinary metals and semiconductors in the late 1980s and early 1990s.  They enjoyed a resurgence in the late '90s during a controversy about coherence and the fate of quasiparticles as \(T \rightarrow 0\), and are still used as a tool to examine coherence in new systems as they come along (graphene, atomically thin semiconductors, 2d electron gases in oxide heterostructures, etc.). 

Thursday, June 26, 2014

Gordon Conference thoughts

Because of travel constraints I'm missing the last day of the meeting, but here are some thoughts, non-science first:
  • These meetings remain a great format - not too big, a good mix of topics, real opportunities for students and postdocs to interact w/ lots of people, chances for older researchers to play soccer and ultimate frisbee, etc.  As travel costs rise and internet connectivity improves, there are going to be sensible reasons to have fewer in-person meetings of otherwise distant participants, but there remains no substitute for a good conversation face-to-face over a coffee or a beer.
  • College dorm rooms, while better than when I was a student, are still not high on ambiance.  Generic fitted and top sheets for bedding appear to be made from dryer lint.
  • Food options have become progressively healthier and tastier in general.
  • Mount Holyoke is a lovely campus, with very loud and happy frogs.
  • A session about cuprate superconductors correlated with the literal gathering of storm clouds in an otherwise sunny week.
  • About 30% of the audience got the reference (after about a 5 second delay) when, on a slide about magnetic interactions (\(J_{zz} S^{z}_{i} \cdot S^{z}_{j}\)), there was an unlabeled picture of Jay-Z.  
A couple of science thoughts (carefully brief to avoid violating the GRC policy about discussing conference talks and posters):
  • Cuprate superconductors remain amazingly complicated, even after years of improving sample quality and experimental techniques. 
  • Looking at driven systems is becoming very exciting.  Basically under some circumstances you can use light to flip on or off topological changes in band structure, for example.
  • It remains very challenging to figure out how to think about systems with low energy excitations that don't look like long-lived quasiparticles. 

Sunday, June 22, 2014

Gordon Conference

I am going to be at the Gordon Research Conference on correlated electrons for the next few days. Should be fun, but blogging about such meetings is generally frowned upon (don't want to discourage people from frank discussions and showing brand new, untried stuff).  There are rules about confidentiality for these meetings.  I'll write more later in the week on other topics.

Sunday, June 15, 2014

FeSe on SrTiO3: report of 100 K superconductivity

I'd heard rumors about this for a while.  I presume that the posting of this on the arxiv means that some form of this paper is in submission out there to a suitably glossy, high impact journal that requires reference citations in its abstracts.  Background:  Bulk FeSe superconducts below around 8 K at ambient pressure (see here).  Under pressure, that transition can be squeezed up beyond 35 K (see here).  The mechanism for superconductivity in this material is up for debate, as far as I know (please feel free to add a reference or two in the comments). 

These investigators have a very fancy ultrahigh vacuum system, in which they are able to grow single layer FeSe on top of SrTiO3 (with the substrate doped with niobium in this case).  This material is not stable in air, and apparently doesn't do terribly well even when coated with some protective layer.  However, these folks have a multi-probe scanning tunneling microscope system in their chamber, along with a cold stage, so that they can perform electrical measurements in situ without ever exposing their single layer to air.  They find that the electrical resistance measured in their four-point-probe configuration drops to zero below around 100 K (as high as 109 K, depending on the sample).  One subtle point that clearly worried them:  SrTiO3 is know to have a structural phase transition (the onset of ferroelasticity - see here) at around 105 K, so they wanted to be sure that what they saw wasn't somehow an artifact of that substrate effect.  (Makes me wonder what happens to superconductivity in the FeSe depending on the ferroelastic domain orientation underneath it.)  For the lay audience:  liquid nitrogen boils at ambient pressure at 77 K.  This would be the first iron-based superconductor to cross that threshold, a domain previously limited to the copper oxides.   Remember, if the bulk transition is at 8 K and the single layer case exceeds 100 K, it doesn't seem crazy to hope for some related system with an additional factor of three or four that takes us beyond room temperature.

Important caveats:  Right now, they have resistance measurements and tunneling spectroscopy measurements.  Because of the need for in situ measurement they don't have Meissner data.  It's also important to realize that the restrictions here (not air stable; only happens in single layer material when ultraclean) are not small.  At the same time, this is potentially very exciting, and hopefully it holds up well and can be the foundation for more exciting materials.