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Electrodynamics of Topological Insulators and Semimetals

Speaker:Prof. N. Peter Armitage
Affiliation:Johns Hopkins University
Date:June 21, 2019 (Friday)
Time:4:00 p.m.
Venue:Room 522, 5/F, Chong Yuet Ming Physics Building, HKU


This talk will review my group’s recent work on the electrodynamics of topological materials. In topological insulators I will discuss my group’s recent measurements of the quantized “axion” magnetoelectric effect. This is the 3D response function that is the analog of the quantized plateaus in the usual quantum Hall effect. Among other aspects, the time structure used in these measurements allows a direct measure of the fine structure constant based on a topological invariant of a solid-state system. I will also discuss various recent experiments that my group has performed on topological semimetals. These include quadratic band touching systems like Pr2Ir2O7 that are generically strongly interacting, due to the quadratic touching. We have performed terahertz spectroscopy and observed a dielectric constant as large as ε~180 at low temperatures. In such systems the dielectric constant is a measure of the relative scale of interactions, which are therefore in our material almost two orders of magnitude larger than the kinetic energy. Despite this, the scattering rate exhibits a T^2 dependence, which shows that for finite doping a Fermi liquid state survives, however with a scattering rate close to the maximal value allowed. I will also discuss our recent measurements of the symmetry protected Dirac Semimetal Cd3As2.  Here with applied magnetic field, we find evidence for both circularly polarized phonons and finite frequency signatures of the chiral anomaly, which is a unique transport mechanism that is enabled in these materials.

Coffee and tea will be served 20 minutes prior to the seminar.

Anyone interested is welcome to attend.