Signatures of parity anomaly in a topological insulator

Speaker: Wouter Beugeling – University of Wurzburg

Host: Lars Fritz

Abstract:

The idea that band inversion in a narrow-gap material can lead to Dirac-type surface states was noted by Volkov and Pankratov in the 1980s. Around the same time, a connection was made to the concept of parity anomaly for Dirac fermions, known from 2+1D quantum electrodynamics. Only about two decades later, it was realized that the surface states of topological insulators are the gapless Dirac states predicted by Volkov and Pankratov. Finding signatures of parity anomaly in topological insulators has remained a challenge to this day, because the contributions from two surface states naturally cancel.

In this work, we show evidence for parity anomaly by measuring spectral asymmetry, an anomalous contribution to the Hall conductance, linked to band inversion. We have performed magneto-transport measurements on gated high-mobility heterostructures containing a 70 nm layer of tensile strained HgTe, a three-dimensional topological insulator. We observe a remarkable re-entrant quantum Hall effect in the p-type regime, which can be traced to spectral asymmetry of a single surface state. A complicating factor is the coexistence of the topological surface state states with massive ones, so that special care is required to extract the relevant signatures. We have tackled this problem by intelligent use of the top and bottom gates and by simulating the Landau level spectra using k·p theory.