We performed an extensive angle resolved photoemission (ARPES) study on FeSe 0.45 Te 0.55 (FeSeTe)
which 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. Our study reveals that although the top of 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 Γ to Z. Thus we provide the first direct evidence for a topologically non-trivial bulk band
structure that supports protected surface states.