${\mathrm{FeSe}}_{0.45}{\mathrm{Te}}_{0.55}$ (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as a function of photon energies ranging from 15–100 eV. We find that although the top of the bulk valence band shows essentially no $k_z$ dispersion, its normalized intensity exhibits a periodic variation with $k_z$. We show, using ARPES selection rules, that the intensity oscillation is a signature of band inversion indicating a change in the parity going from $\mathrm{\Gamma }$ to $Z$. We also present a simple realistic tight-binding model which gives insight into ARPES observations. Thus we provide direct evidence for a topologically nontrivial bulk band structure that supports protected surface states.

• Received 4 December 2019
• Accepted 27 May 2020

DOI:https://doi.org/10.1103/PhysRevB.101.245146