Wednesday, March 07, 2018

APS March Meeting 2018, day 2

Day 2 of the meeting was even more scattered than usual for me, because several of my students were giving talks, all in different sessions spread around.  That meant I didn't have a chance to stay too long on any one topic.   A few highlights:

Jeff Urban from LBL gave an interesting talk about different aspects of the connection between electronic transport and thermal transport.  The Wiedemann-Franz relationship is a remarkably general expression based on a simple idea - when charge carriers move, they transport some (thermal) energy as well as charge, so thermal conductivity and electrical conductivity should be proportional to each other.  There are a bunch of assumptions that go into the serious derivation, though, and you could imagine scenarios when you'd expect large deviations from W-F response, particularly if scattering rates of carriers have some complicated energy dependence.  Urban spoke about hybrid materials (e.g., mixtures of inorganic components and conducting polymers).  He then pointed out a paper I'd somehow missed last year about apparent W-F violation in the metallic state of vanadium dioxide.  VO2 is a "bad metal", with an anomalously low electrical conductivity.  Makes me wonder how W-F fairs in other badly metallic systems.

Ali Hussain of the Abbamonte group at Illinois gave a nice talk about (charge) density fluctuations in the strange metal phase (and through the superconducting transition) of the copper oxide superconductor BSSCO.  The paper is here.  They use a particular technique (momentum-resolved electron energy loss spectroscopy) and find that it is peculiarly easy to create particle-hole excitations over a certain low energy range in the material, almost regardless of the momentum of those excitations.  There are also systematics with how this works as a function of doping (carrier concentration in the material), with optimally doped material having particularly temperature-independent response. 

Albert Fert spoke about spin-Hall physics, and the conversion of spin currents in to charge currents and vice versa.  One approach is the inverse Edelstein effect (IEE).  You have a stack of materials, where a ferromagnetic layer is on the top.  Driving ferromagnetic layer into FMR, you can pump a spin current vertically downward (say) into the stack.  Then, because of Rashba spin-orbit coupling, that vertical spin current can drive a lateral charge current (leading to the buildup of a lateral voltage) in a two-dimensional electron gas living at an interface in the stack.  One can use the interface between Bi and Ag (see here).  One can get better results if there is some insulating spacer to keep free conduction electrons not at the interface from interfering, as in LAO/STO structures.  Neat stuff, and it helped clarify for me the differences between the inverse spin Hall effect (3d charge current from 3d spin current) and the IEE (2d charge current from 3d spin current). 

Alexander Govorov of Ohio also gave a nice presentation about the generation of "hot" electrons from excitation of plasmons.  Non-thermally distributed electrons and holes can be extremely useful for a variety of processes (energy harvesting, photocatalysis, etc.). At issue is, what does the electronic distribution really look like.  Relevant papers are here and here.  There was a nice short talk similar in spirit by Yonatan Dubi earlier in the day.

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