Evidence for topological band inversion of the phase change material Ge2Sb2Te5

TL;DR

This study provides experimental evidence of topological band inversion in Ge2Sb2Te5, a phase change material, through angle-resolved photoemission and scanning tunneling spectroscopy, confirming theoretical predictions of topological surface states.

Contribution

First experimental observation of topological surface states in Ge2Sb2Te5, validating theoretical models of its topological properties in the cubic phase.

Findings

Observation of a topological surface state in Ge2Sb2Te5

Agreement with density functional theory calculations

Identification of a 0.4 eV band gap around E_F

Abstract

We present an angle-resolved photoemission study of a ternary phase change material, namely Ge2Sb2Te5, epitaxially grown on Si(111) in the metastable cubic phase. The observed upper bulk valence band shows a minimum at Gamma-bar being 0.3 eV below the Fermi level E_F and a circular Fermi contour around Gamma-bar with a dispersing diameter of 0.27-0.36 Anstroms^-1. This is in agreement with density functional theory calculations of the Petrov stacking sequence in the cubic phase which exhibits a topological surface state. The topologically trivial cubic KH stacking shows a valence band maximum at Gamma in line with all previous calculations of the hexagonal stable phase exhibiting the valence band maximum at Gamma for a trivial Z_2 topological invariant nu_0 and away from Gamma for non-trivial nu_0. Scanning tunneling spectroscopy exhibits a band gap of 0.4 eV around E_F.