Magnetooptical determination of a topological index

TL;DR

This paper presents a method to determine the topological index of Dirac fermion systems through magnetooptical measurements of bulk velocity, demonstrated experimentally on PbSnSe and PbSnTe, applicable to BHZ-like materials.

Contribution

It introduces an experimental approach to determine topological indices via magnetooptical spectroscopy, linking velocity measurements to topological phases.

Findings

Topological index correlates with effective velocity of Dirac fermions.

Experimental validation on PbSnSe and PbSnTe shows the method's effectiveness.

Approach is applicable to all materials described by a BHZ-like model.

Abstract

When a Dirac fermion system acquires an energy-gap, it is said to have either trivial (positive energy-gap) or non-trivial (negative energy-gap) topology, depending on the parity ordering of its conduction and valence bands. The non-trivial regime is identified by the presence of topological surface or edge-state dispersing in the energy gap of the bulk and is attributed a non-zero topological index. In this work, we show that such topological indices can be determined experimentally via an accurate measurement of the effective velocity of bulk massive Dirac fermions. We demonstrate this analytically starting from the Bernevig-Hughes-Zhang Hamiltonian (BHZ) to show how the topological index depends on this velocity. We then experimentally extract the topological index in Pb1-xSnxSe and Pb1-xSnxTe using infrared magnetooptical Landau level spectroscopy. This approach is argued to be universal to all material classes that can be described by a BHZ-like model and that host a topological phase transition.