Tuning across the BCS-BEC crossover in superconducting $\mbox{Fe}_{1+y}\mbox{Se}_{x}\mbox{Te}_{1-x}$: An angle-resolved photoemission study

The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) was never realized in quantum materials. It is difficult to realize because, unlike in ultra cold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal $\mbox{Fe}_{1+y}\mbox{Se}_{x}\mbox{Te}_{1-x}$ by tuning the Fermi energy $\epsilon_{F}$ via chemical doping, which permits us to systematically change $\Delta/\epsilon_{F}$ from $0.16$ to $0.50$, where $\Delta$ 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 multi-band superconductors which go beyond those addressed in the context of cold atoms.