Absence of BCS-BEC Crossover in FeSe0.45Te0 55 Superconductor

TL;DR

This study uses high-resolution ARPES to investigate the electronic structure of FeSe0.45Te0.55, concluding that the observed flat band-like feature arises from band combination and that there is no evidence of BCS-BEC crossover in this superconductor.

Contribution

The paper provides direct experimental evidence that the flat band-like feature is due to band combination, ruling out BCS-BEC crossover in FeSe0.45Te0.55.

Findings

Flat band-like feature results from band combination, not BCS-BEC crossover.

The dyz band follows BCS-like dispersion in the superconducting state.

No evidence of BCS-BEC crossover in FeSe0.45Te0.55 superconductor.

Abstract

In iron-based superconductor Fe(Se,Te), a flat band-like feature near the Fermi level was observed around the Brillouin zone center in the superconducting state. It is under debate whether this is the evidence on the presence of the BCS-BEC crossover in the superconductor. High-resolution laser-based angle-resolved photoemission measurements are carried out on high quality single crystals of FeSe0.45Te0.55 superconductor to address the issue. By employing different polarization geometries, we have resolved and isolated the dyz band and the topological surface band, making it possible to study their superconducting behaviors separately. The dyz band alone does not form a flat band-like feature in the superconducting state and the measured dispersion can be well described by the BCS picture. We find that the flat band-like feature is formed from the combination of the dyz band and the topological surface state band in the superconducting state. These results reveal the origin of the flat band-like feature and rule out the presence of BCS-BEC crossover in Fe(Se,Te) superconductor.