Wednesday, January 27, 2021

Zero bias peaks - an example of the challenge of experimental (condensed matter) physics

The puzzle-solving aspect of experimental physics is one reason why it can fun, but also why it can be very challenging.  In condensed matter, for example, we have limited experimental tools and can only measure certain quantities (e.g., voltages, currents, frequencies)  in the lab, and we can only tune certain experimental conditions (e.g., temperature, applied magnetic field, voltages on electrodes).  Getting from there to an unambiguous determination of underlying physics can be very difficult.

For example, when measuring electronic conduction in nanostructures, often we care about the differential conductance, \(dI/dV\), as a function of the bias voltage \(V\) applied across the system between a source and a drain electrode.  In an ideal resistor, \(dI/dV\) is just a constant as a function of the bias.  "Zero bias" \( (V = 0) \) is a special situation, when the electronic chemical potential (the Fermi level, at \(T = 0\)) of the source and drain electrodes are the aligned.  In a surprisingly large number of systems, there is some feature in \(dI/dV\) that occurs at \(V= 0\).  The zero-bias conductance \( (dI/dV)(V=0)\) can be suppressed, or it can be enhanced, relative to the high bias limit.  These features are often called "zero bias anomalies", and there are many physical mechanisms that can produce them.  

For example, In conduction through a quantum dot containing an odd number of electrons, at sufficiently low temperatures there can be a zero-bias peak in the conductance due to the Kondo Effect, where magnetic processes lead to forward-scattering of electrons through the dot when the Fermi levels are aligned.  This Kondo resonance peak in \(dI/dV\) has a maximum possible height of \(2e^2/h\), and it splits into two peaks in a particular way as a magnetic field is applied.  In superconducting systems, Andreev processes can lead to zero bias peaks that have very different underlying physics, and different systematic dependences on magnetic field and voltage.

Zero bias anomalies have taken on a new significance in recent years because they are one signature that is predicted for solid-state implementations of Majorana fermions involving superconductors connected to semiconductor nanowires.   These exotic quasiparticles have topological properties that make them appealing as a possible platform for quantum computingObservations of zero bias anomalies in these structures have attracted enormous attention for this reason.  

The tricky bit is, it has become increasingly clear that it is extremely difficult to distinguish conclusively between "Majorana zero modes" and cousins of the Andreev features that I mentioned above.  As I mentioned in my last post, there is a whole session at the upcoming APS meeting about this, recent papers, and now a retraction of a major claim in light of new interpretation.  It's a fascinating challenge that shows just how tricky these experiments and their analysis can be!  This stuff is just hard.

(Posting will likely continue to be slow - this is the maximally busy time of the year as department chair....)

Monday, January 18, 2021

Brief items, new year edition

 It's been a busy time, but here are a few items for news and discussion:

  • President-Elect Biden named key members of his science team, and for the first time ever has elevated the role of Presidential Science Advisor (and head of the White House Office of Science and Technology Policy) to a cabinet-level position.  
  • The President-Elect has also written a letter to the science advisor, outlining key questions that he wants to be considered.  
  • There is talk of a "Science New Deal", unsurprisingly directed a lot toward the pandemic, climate change, and American technological competitiveness.
  • The webcomic SMBC has decided to address controversy head on, reporting "Congressman Johnson comes out against Pauli Exclusion."  This would have rather negative unintended consequences, like destabilizing all matter more complex than elementary particles....
  • This session promises to be an interesting one at the March APS meeting, as it goes right to the heart of how difficult it is to distinguish Majorana fermion signatures in superconductor/semiconductor hybrid structures from spurious electrical features.  I may try to write more about this soon.
  • This paper (arxiv version) is very striking.  Looking in the middle of a sheet of WTe2 (that is, away from where the topological edge states live), the authors see quantum oscillations of the resistance as a function of magnetic field that look a lot like Landau quantization, even though the bulk of the material is (at zero field) quite insulating.  I need to think more carefully about the claim that this argues in favor of some kind of emergent neutral fermions.
  • Being on twitter for four months has made me realize how reality-warping that medium is.  Reading about science on twitter can be incredibly wearing - it feels like seemingly everyone else out there is publishing in glossy journals, winning major prizes, and landing huge grants.  This is, of course, a selection effect, but I don't think it's healthy.
  • I do think twitter has driven blog traffic up a bit, but I actually wonder if occasionally posting blog links to /r/physics on reddit would be far more effective in terms of outreach.  When one of my posts ends up there, it gets literally 50x the page views than normal.  Still, I have an old-internet-user aversion to astroturfing.  

Saturday, January 09, 2021

Questions that show who you are as a physicist

There are some cool new physics and nanoscience results out there, but after a frankly absurd week (in which lunatics stormed the US Capitol, the US reached 4000 covid deaths per day, and everything else), we need something light.  Stephen Colbert has started a new segment on his show called "The Colbert Questionert" (see an example here with Tom Hanks - sorry if that's region-coded to the US), in which he asks a list of fifteen questions that (jokingly) reveal the answerer's core as a human being.   These range from "What is your favorite sandwich?" to "What do you think happens when you die?".  Listening to this, I think we need some discipline-specific questions for physicists.  Here are some initial thoughts, and I'd be happy to get more suggestions in the comments.  

  • Food that you eat when traveling to a conference or talk but not at home?
    • Science fiction - yes or no?
    • What is your second-favorite subdiscipline of physics/astronomy/science?
    • Favorite graph:  linear-linear? Log-log?  Log-linear?  Double-log?  Polar?  Weird uninterpretable 3D representation that would make Edward Tufte's head explode?
    • Lagrangian or Hamiltonian?
    • Bayesian or frequentist?
    • Preferred interpretation of quantum mechanics/solution to the measurement problem?

    Friday, January 01, 2021

    Idle speculation can teach physics, vacuum energy edition

    To start the new year, a silly anecdote ending in real science.

    Back when I was in grad school, around 25 years ago, I was goofing around chatting with one of my fellow group members, explaining about my brilliant (ahem) vacuum energy extraction machine.  See, I had read this paper by Robert L. Forward, which proposed an interesting idea, that one could use the Casimir effect to somehow "extract energy from the vacuum" - see here (pdf).  
    Fig from here.

    (For those not in the know: the Casimir effect is an attractive (usually) interaction between conductors that grows rapidly at very small separations.  The non-exotic explanation for the force is that it is a relativistic generalization of the van der Waals force.  The exotic explanation for the force is that conducting boundaries interact with zero-point fluctuations of the electromagnetic field, so that "empty" space outside the region of the conductors has higher energy density.   As explained in the wiki link and my previous post on the topic, the non-exotic explanation seemingly covers everything without needing exotic physics.)

    Anyway, my (not serious) idea was, conceptually, to make a parallel plate structure where one plate is gold (e.g.) and the other is one of the high temperature superconductors.  Those systems are rather lousy conductors in the normal state.  So, the idea was, cool the system just below the superconducting transition.  The one plate becomes superconducting, leading ideally to dramatically increased Casimir attraction between the plates.  Let the plates get closer, doing work on some external load.  Then warm the plates just slightly, so that the superconductivity goes away.  The attraction should lessen, and the plate would spring back, doing less work of the opposite sign.  It's not obvious that the energy required to switch the superconductivity is larger than the energy one could extract from running such a cycle.   Of course, there has to be a catch (as Forward himself points out in the linked pdf above).  In our conversation, I realized that the interactions between the plates would very likely modify the superconducting transition, probably in just the way needed to avoid extracting net energy through this process.  

    Fast forward to last week, when I randomly came upon this article.  Researchers actually did an experiment using nanomechanical resonators to try to measure the impact of the Casimir interactions on the superconducting transition in (ultrasmooth, quench-condensed) lead films.  They were not able to resolve anything (like a change in the transition temperature) in this first attempt, but it shows that techniques now exist to probe such tiny effects, and that idly throwing ideas around can sometimes stumble upon real physics.