## Monday, September 28, 2015

### DOE Experimental Condensed Matter Physics PI meeting 2015 - Day 1

Things I learned at today's session of the DOE ECMP PI meeting:
• In the right not-too-thick, not-too-thin layers of the 3d topological insulator Bi1.5Sb0.5Te1.7Se1.3 (a cousin of Bi2Se3 that actually is reasonably insulating in the bulk), it is possible to use top and bottom gates to control the surface states on the upper and lower faces, independently.  See here.
• In playing with suspended structures of different stackings of a few layers of graphene, you can get some dramatic effects, like the appearance of large, sharp energy gaps.  See here.
• While carriers in graphene act in some ways like massless particles because their band energy depends linearly on their crystal momentum (like photon energy depends linearly on photon momentum in electrodynamics), they have a "dynamical" effective mass, $m^* = \hbar (\pi n_{2d})^{1/2}/v_{\mathrm{F}}$, related to how the electronic states respond to an electric bias.
• PdCoO2 is a weird layered metal that can be made amazingly clean, so that its residual resistivity can be as small as 8 n$\Omega$-cm.  That's about 200 times smaller than the room temperature resistivity of gold or copper.
• By looking at how anisotropic the electrical resistivity is as a function of direction in the plane of layered materials, and how that anisotropy can vary with applied strain, you can define a "nematic susceptibility".  That susceptibility implies the existence of fluctuations in the anisotropy of the electronic properties (nematic fluctuations).  Those fluctuations seem to diverge at the structural phase transition in the iron pnictide superconductors.  See here.   Generically, these kinds of fluctuations seem to boost the transition temperature of superconductors.
• YPtBi is a really bizarre material - nonmetallic temperature dependence, high resistivity, small carrier density, yet superconducts.
• Skyrmions (see here) can be nucleated in controlled ways in the right material systems.  Using the spin Hall effect, they can be pushed around.  They can also be moved by thermally driven spin currents, and interestingly skyrmions tend to flow from the cold side of a sample to the hot side.
• It's possible to pump angular momentum from an insulating ferromagnet, through an insulating antiferromagnet (NiO), and into a metal.  See here.
• The APS Conferences for Undergraduate Women in Physics have been a big hit, using attendance as a metric.  Extrapolating, in a couple of years it looks like nearly all of the undergraduate women majoring in physics in the US will likely be attending one of these.
• Making truly nanoscale clusters out of some materials (e.g., Co2Si, Mn5Si3) can turn them from weak ferromagnets or antiferromagnets in the bulk into strong ferromagnets in nanoparticle form.   See here.