Thursday, June 16, 2016

Frontiers in Quantum Materials and Devices 2016 - day 2

Continuing with my very brief (and necessarily incomplete) summary of the FQMD 2016 meeting at RIKEN at the beginning of this week:

  • Eric Heller of Harvard gave a very interesting and provocative talk about two topics, Raman scattering in graphene and then the onset of optical absorption in semiconductors.  Regarding the former (see here), he makes a strong case that the "double resonance" theoretical treatment of Raman scattering in graphene that has been highly cited since 2000 is not the right way to think about the problem.  Rather, one should use the Kramers-Heisenberg-Dirac theory of Raman scattering c. 1925-27, and keep in mind the important role played by (crystal) momentum conservation, as explained in the paper linked above.   Regarding the latter topic, he went on to argue (persuasively, in my view) that the textbook approach (literally - I described it in my own book) to the onset of optical absorption in direct-gap semiconductors as the photon energy exceeds the band gap is incomplete and gets the functional dependence on frequency wrong.  This work isn't published yet, and it wouldn't be appropriate for me to present his argument before he does, but I will definitely be keeping an eye out for this.
  • Denis Maryenko of RIKEN spoke about measurements of the anomalous Hall effect in the 2d electron gas that is present at the interface between ZnO and MnZnO.  This system is pretty impressive, with disorder so small that it supports very clean fractional quantum Hall effect, but with larger Coulomb and Zeeman energies than the more traditional GaAs/AlGaAs interface because of the different dielectric functions and g factors, respectively of the ZnO system.   Interesting (not yet published) magnetic physics appears to be taking place at the interface due apparently to point defects that support unpaired spins.
  • Pertti Hakonen from Aalto presented a nice talk about the quantum Hall effect in suspended graphene.  They have (not yet published) measurements in suspended structures made in the Corbino geometry, where there is an electrode in the center of a disk, and a second contact around the disk's perimeter.  As you might imagine, making a structure like that where the graphene disk is suspended in space, yet there is a nice contact to the central electrode without disrupting the disk, is quite a fabrication tour de force, based on an approach from here.
  • Vincent Bouchiat from CNRS, Grenoble talked about using tin-decorated graphene as a system to explore the nature of the superconductor-insulator transition.  It's a flexible material system, in that you can control the coverage of the tin (the size and distribution of tin islands), the disorder in the graphene via damage, and the carrier density in the graphene via electrostatic gating.   An earlier paper is here, and a more recent one is here.
  • Steven Richardson of Howard University spoke about the challenges of trying to make germanene, the germanium analog to graphene.  One approach that has been used in graphene growth has been to start with small, polycyclic carbon ring molecules as seeds.  Doing this in germanium has proven difficult, and Prof. Richardson's group does quantum chemistry calculations with DFT to establish the relative energetic stability and properties of candidate molecules.  From his talk I learned something I had not appreciated, that treating dispersion forces (van der Waals interactions) in DFT is really nontrivial.  
  • James Analytis of Berkeley gave a very nice talk about Weyl fermions, where I actually felt like I had a grasp of this for a few minutes.  Up to now, most of the experiments on materials that are supposed to support Weyl-like band structure have been based on photoemission, rather than actual transport.  Prof. Analytis showed particular transport signatures (quantum oscillations of resistance as a function of magnetic field) that are consistent with what one would expect from electrons actually tracing out Weyl-expected trajectories (in both real space and reciprocal space).  This work relies on impressive nanofabrication, where a focused ion beam is used to carve Cd3As2 into nanostructures + leads without killing the material quality.
  • Yoshinori Tokura from RIKEN surveyed his group's results looking at the interplay of magnetism, the quantum Hall effect, and the quantum anomalous Hall effect, built on high quality epitaxial structures based on a topological insulator (Bi1-xSbx)2Te3 and its Cr-doped relative.  Relevant papers are here, here, and here.   This is a great example of how much scientific activity can spring forth when it becomes possible to grow a new material system with very high quality.
  • Jagadeesh Moodera from MIT presented work that is similar in spirit, involving Cr doping of Bi2Se3, and then V doping of Sb2Te3.  In systems like this it is possible to see robust, ballistic transport via chiral edge states over millimeters.  Again, excellent material quality + interesting choices of materials = impressive science.
  • Joe Checkelsky of MIT spoke about exploring electronic materials with magnetically frustrated lattices.  Many systems with magnetic frustration (where magnetic moments at different lattice sites have competing interactions so that it's not possible to satisfy all of them) are insulators.  In conducting versions of these systems, there can be really funky effects where the magnetic states interact with the electrons through mechanisms like Berry curvature.  This work is in press right now and I will come back and update this once it's available online.
  • Hajime Okamoto from NTT gave a neat talk about optomechanical effects (see here for a review) - where photogenerated carriers in an AlGaAs/GaAs cantilever can couple (via the piezoelectric properties of the material) to the mechanical oscillations of the cantilever.  This makes it possible to do an interesting kind of optical driving and optical cooling of such structures.   See here and here, for example.
Whew.  Overall, a fun, interesting, and dense two days!

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