## Monday, March 05, 2018

### APS March Meeting 2018, day 1

As I explained yesterday, my trip to the APS is even more scattered than in past years, but I'll try to give some key points.  Because of meetings and discussions with some collaborators and old friends, I didn't really sit down and watch entire sessions, but I definitely saw and heard some interesting things.

Markus Raschke of Colorado gave a nice talk about the kinds of ultrafast and nonlinear spectroscopy you can do if you use a very sharp gold tip as a plasmonic waveguide.  The tip has a grating milled onto it a few microns away from the sharp end, so that hitting the grating with a pulsed IR laser excites a propagating surface plasmon mode that is guided down to the really sharp point.  One way to think about this:  When you use the plasmon mode to confine light down to a length scale $\ell$ comparable to the radius of curvature of the sharp tip, then you effectively probe a wavevector $k_{\mathrm{eff}} \sim \2\pi/\ell$.  If $\ell$ is a couple of nm, then you're dealing with $k_{\mathrm{eff}}$ values associated in free space with x-rays (!).  This lets you do some pretty wild optical spectroscopies.  Because the waveguiding is actually effective over a pretty broad frequency range, that means that you can get very short pulses down there, and the intense electric field can lead to electron emission, generating the shortest electron pulses in the world.

Andrea Young of UCSB gave a very pretty talk about looking at even-denominator fractional quantum Hall physics in extremely high quality bilayer graphene.  Using ordinary metal electrodes apparently limits how nice the effects can be in the bilayer, because the metal is polycrystalline and that disorder in local work function can actually matter.   By using graphite as both the bottom gate and the top gate (that is, a vertical stack of graphite/boron nitride/bilayer graphene/boron nitride/graphite), it is possible to tune both the filling fraction (ratio of carrier density to magnetic field) in the bilayer and the vertical electric field across the bilayer (which can polarize the states to sit more in one layer or the other).  Capacitance measurements (e.g., between the top gate and the bottom gate, or between either gate and the bilayer) can show extremely clean quantum hall data.

Sankar Das Sarma of Maryland spoke about the current status of trying to use Majorana fermions in semiconductor wire/superconductor electrode structures for topological quantum computing.  For a review of the topic overall, see here.   This is the approach to quantum computing that Microsoft is backing.  The talk was vintage Das Sarma, which is to say, full of amusing quotes, like "Physicists' record at predicting technological breakthroughs is dismal!" and "Just because something is obvious doesn't mean that you should not take it seriously."  The short version:  There has been great progress in the last 8 years, from the initial report of possible signatures of effective Majorana fermions in individual InSb nanowires contacted by NbTiN superconductors, to very clean looking data involving InAs nanowires with single-crystal, epitaxial Al contacts.  However, it remains very challenging to prove definitively that one has Majoranas rather than nearly-look-alike Andreev bound states.

In case you are interested in advanced (beyond-first-year) undergraduate labs and how to do them well, you should check out the University of Minnesota's site, as well as the ALPhA group from the AAPT.   There is also an analogous group working on projects to integrate computation into the undergraduate physics curriculum.

One potentially very big physics news story that I heard about during the day, but won't be here to see the relevant talk: [Update:  Hat tip to a colleague who pointed out that there is a talk tomorrow morning that will cover this!]  There are back-to-back brand new papers in Nature today by Yuan Cao et al. from the Jarillo-Herrero group at MIT.  (The URLs don't work yet for the articles, but I'll paste in what Nature has anyway.)  The first paper apparently shows that when you take two graphene layers and rotationally offset them from graphite-like stacking by 1.05 degrees (!), the resulting bilayer is alleged to be a Mott insulator.  The idea appears to be that the lateral Moire superlattice that results from the rotational offset gives you very flat minibands, so that electron-electron interactions are enough to lock the carriers into place when the number density of carriers is tuned correctly.  The second paper apparently (since I can't read it yet) shows that as the carrier density is tuned away from the Mott insulator filling, the system becomes a superconductor (!!), with a critical temperature of 1.7 K.  This isn't particularly high, but the idea of tuning carrier density away from a Mott state and getting superconductivity is basically the heart of our (incomplete) understanding of the copper oxide high temperature superconductors.  This is very exciting, as summarized in this News and Views commentary and this news report.

#### 4 comments:

Anonymous said...

It looks like you can access the Jarillo-Herrero articles in PDF form (I couldn't get the html links to work either).

Douglas Natelson said...

Thanks. Updating the links. Nature has not made this easy.

Anonymous said...

Thanks for sharing the news Doug.

For those who don't have a subscription to Nature, the graphene superconductivity paper is here:

https://arxiv.org/pdf/1803.02342.pdf

Anonymous said...

Here is the other paper:

https://arxiv.org/pdf/1802.00553.pdf