Shallow pockets and very strong coupling superconductivity in FeSe_xTe_1-x

TL;DR

This study reveals that FeSe_xTe_1-x exhibits extremely strong coupling superconductivity with a large gap-to-Fermi energy ratio, challenging conventional theories and showing similarities to cold Fermi-gas behavior.

Contribution

The paper provides the first detailed electronic-structure measurements of FeSe_xTe_1-x across the superconducting transition, highlighting its unusual strong coupling regime.

Findings

Superconducting gap Delta comparable to Fermi energy E_F

Large Delta/E_F ratio (~0.5) indicating strong coupling

Anomalous dispersion of coherence peak consistent with BCS-BEC crossover

Abstract

We measured the electronic-structure of FeSe_xTe_1-x above and below Tc. In the normal state we find multiple bands with remarkably small values for the Fermi energy, E_F. Yet,below Tc we find a superconducting gap, Delta, that is comparable in size to E_F, leading to a ratio Delta/E_F~0.5 that is much larger than found in any previously studied superconductor. We also observe an anomalous dispersion of the coherence peak which is very similar to the dispersion found in cold Fermi-gas experiments and which is consistent with the predictions of the BCS-BEC crossover theory.