Tuning across the BCS-BEC crossover in the multiband superconductor Fe_{1+ y} Se _x Te_{1- x} : An angle-resolved photoemission study

The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) is difficult to realize in quantum materials because, unlike in ultracold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal, Fe_1+y Se _x Te_1-x , by tuning the Fermi energy ε_F via chemical doping, which permits us to systematically change Δ/ε_F from 0.16 to 0.50, where Δ is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap, and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multiband superconductors, which go beyond those addressed in the context of cold atoms.