Saturday, February 22, 2020

Brief items

As we head out of a very intense week here and toward the March APS meeting, a few brief items:

  • Speaking of the March Meeting, I hear (unofficially) that travel restrictions due to the coronavirus have made a big dent - over 500 talks may be vacant, and the program committee is working hard to explore options for remote presentation.  (For the record, I fully endorse the suggestion that all vacant talks be delivered in the form of interpretive dance by Greg Boebinger.)
  • There will be many talks about twisted bilayers of various 2D materials at the meeting, and on that note, this PRL (arxiv version here) shows evidence of "strange metallicity" in magic-angle bilayer graphene at temperatures above the correlated insulator state(s).
  • Following indirectly on my post about condensed matter and Christmas lights, I want to point out another example of how condensed matter physics (in the form of semiconductor physics and the light emitting diode) has changed the world for the better in ways that could never have been anticipated.  This video shows and this article discusses the new film-making technique pioneered in the making of The Mandalorian.  Thanks to the development of organic LED displays, infrared LEDs for motion tracking, and lots of processing power, it is possible to create a floor-to-ceiling wraparound high definition electronic backdrop.  It's reconfigurable in real time, produces realistic lighting on the actors and props, and will make a lot of green screen compositing obsolete.  Condensed matter:  This is The Way.
  • Superconducting gravimeters have been used to check to see if there are compact objects (e.g., hunks of dark matter, or perhaps microscopic black holes) orbiting inside the earth.  I remember reading about this issue while in college.  Wild creative idea of the day:  Maybe we should use helioseismology to try to infer whether there are any such objects orbiting inside the sun....


Anonymous said...

Regarding TBLG bad metal behavior, isn't it possible that you just have a really low Debye temperature in twisted systems because of the Moire superstructure? This would give linear in temperature resistivity in a trivial way.

Douglas Natelson said...

Anon, interesting question. At first blush, I don't know how to properly think about the effective Debye temperature of the twisted systems - certainly the individual layers have high values (~ 1800 K by some estimates if I recall properly), but I don't know how to think about the coupled system. A general question that has come up over and over in my own career has been how seriously to take power law dependences when the data extend over rather limited ranges. In my grad work, for example, it's very tough to tell the difference between a logarithmic dependence on a quantity and a weak power law (say exponent = 0.3) when you only have data that extends over maybe a decade of the independent variable. I need to read the paper carefully and really think about it in any particular case.

Anonymous said...


Inferring power laws and critical phenomena, especially with multiple competing effects at play, is tricky business!

thm said...

I think the March Meeting is smaller to begin with, before any travel restrictions. I count 847 scientific sessions, of which 410 are Focus Sessions, down from 863 and 518 last year. There might be slightly fewer invited speakers, but they don't show up in the invited speakers list until the speaker uploads an abstract, so hard to tell. I found it sort of surprising because I think this is the first time the session numbers broke 70, but there are no sessions number 13, 14, or 69 for some reason. (Last year used 1--69 without skipping any.)

Matthew Foster said...

Hey Doug,

The relevant scale in monolayer is the Bloch-Grueneisen temperature, which is roughly TBG ~ 70 \sqrt{n}, with n the electron density measured in units of 10^{12} / cm^2. For T > TBG, the scattering off of acoustic phonons is quasielastic, and should give linear-in-T resistivity. This is because elastic impurity scattering due to short-range-correlated dirt is a marginal perturbation to 2D Dirac carriers; acoustic phonons work the same way, but with an effective "impurity strength" that grows with T. Because the scattering is quasielastic, it plays a less destructive role in the hydrodynamic regime than optical phonons, for example.

See e.g. this old 2010 paper from Dmitry and Philip, which indeed observed linear-in-T resistivity above TBG: