Shallow pockets and very strong coupling superconductivity in FeSe _xTe _1-x

The celebrated Bardeen-Cooper-Schrieffer (BCS) theory has been successful in explaining metallic superconductors, yet many believe that it must be modified to deal with the newer high-temperature superconductors. A possible extension is provided by the BCS-Bose-Einstein condensate (BEC) theory, describing a smooth evolution from a system of weakly interacting pairs to a BEC of molecules of strongly bound fermions. Despite its appeal, spectroscopic evidence for the BCS-BEC crossover has never been observed in solids. Here we report electronic structure measurements in FeSe _xTe _1-x showing that these materials are in the BCS-BEC crossover regime. Above the superconducting transition temperature, T _c, we find multiple bands with remarkably small values for the Fermi energy ε _F. Yet, in the superconducting state, the gap Δ is comparable to ε _F. The ratio Δ/ε F ≈0.5 is much larger than found in any previously studied superconductor, resulting in an anomalous dispersion of the coherence peak very similar to that found in cold Fermi gas experiments, in agreement with the predictions of the BCS-BEC crossover theory.