Topological bands in Fe(Se,Te)

Bulk FeSe_0.45Te_0.55 has recently emerged as a promising candidate to host topological superconductivity on its surface, with experimental signatures for a Dirac surface state and Majorana bound states in vortex cores. However, ARPES measurements of the bulk band structure show essentially no k_z dispersion, in apparent contradiction with DFT predictions for the significant k_z dispersion that drives the band inversion.
We reconcile the observed lack of dispersion with the predicted band inversion using a tight binding model with strongly renormalized inter-layer hopping and reasonable values of spin-orbit coupling. We use symmetry arguments to identify a sharp signature of bulk band inversion in the photon energy dependence of the ARPES matrix elements. We test our prediction for the change in orbital character of the band using ARPES data with a wide range of photon energies covering several Brillouin zones along k_z. We thus provide direct evidence for the non-trivial topology of the bulk bands in Fe(Se,Te), where the band inversion occurs in a nearly flat band due to an interplay of strong correlations and topology.