Evidence for topological band inversion of the phase change material Ge₂Sb₂Te₅

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 $Γ¯$ being 0.3 eV below the Fermi level E_F and a circular Fermi contour around $Γ¯$ with a dispersing diameter of 0.27–0.36 Å⁻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 Kooi-De Hosson stacking shows a valence band maximum at Γ in line with all previous calculations of the hexagonal stable phase exhibiting the valence band maximum at Γ for a trivial $Z2$ topological invariant $ν0$ and away from Γ for non-trivial $ν0$⁠. Scanning tunneling spectroscopy exhibits a band gap of 0.4 eV around E_F.