The Johns Hopkins University, USA

Abstract 

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here we lower the chemical potential of three-dimensional (3D) Bi2Se3 films to 30 meV above the Dirac point, and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time- domain terahertz polarimetry. For fields higher than 5 T, we observed quantized Faraday and Kerr rotations, whereas the DC transport is still semi-classical.A non-trivial Berry phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. 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’ll also discuss our most recent measurements on topological insulator single crystals that give evidence for a half quantized Hall effect on the TI surfaces.

Brief CV of Prof. N. Peter Armitage: 

N. Peter Armitage is a physicist whose research centers on material systems which exhibit coherent quantum effects at low temperatures, like superconductors and quantum magnetism. He is exploiting and developing recent technical breakthroughs using very low frequency microwave and THz range radiation to probe these systems at their natural frequency scales.

Prof. Armitage has been the recipient of a DARPA Young Faculty Award, an NSF Career Award, a Sloan Research Fellowship, was a three-time Kavli Frontiers Fellow, the Spicer Award from the Stanford Synchrotron Radiation Laboratory, the McMillan Award from the University of Illinois and 2016 Genzel Prize. He was also the co-chair of the 2014 Gordon Research Conference in Correlated Electron Systems.

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