Determining a topological index from Landau level spectroscopy and magnetotransport in the quantum limit

Speaker: Badih A. Assaf, Department of Physics, Ecole Normale Supérieure, PSL, CNRS
Date & Time: Thursday June 8, 2017 at 12:00 - 1:00 PM
Location: AMPEL 311
Local Contact: Josh Folk
Intended Audience:

When a Dirac fermion system acquires an energy gap (or mass), it is said to have either trivial or non-trivial band topology depending on parity ordering of the conduction and valence band edges. A non-trivial material is generally identified via the observation of topological Dirac surface states dispersing in its bulk energy gap, and is attributed a non-zero topological index. However, no direct experimental determination of the topological index of any 3D condensed matter system has yet been attempted. In this talk, I first will show that this index can be determined experimentally via an accurate measurement of the effective Dirac velocity of bulk massive Dirac fermions. This approach is analytically proven starting from the Bernevig-Hughes-Zhang Hamiltonian, then experimentally confirmed by extracting the topological index from the Dirac velocity measured in Pb1-xSnxSe using infrared magnetooptical Landau level spectroscopy. Finally, I will discuss the observation of a negative longitudinal magnetoresistance at high magnetic fields in Pb1-xSnxSe in the topological regime and related this magnetoresistance to the anomalous behavior of the N=0 Landau level whose field dispersion might again be related to the topological index. Overall, our work establishes that non-trivial band topology can lead to highly unusual bulk transport and optical phenomena in topological insulators