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The crossover from the superconductivity of Bardeen-Cooper-Shrieffer (BCS) regime to Bose-Einstein condensation (BEC) regime holds the key to understanding the nature of pairing and condensation of fermions [1]. In electron systems in solids, however, it is generally difficult to control the paring strength of the electrons, and complete evidence of the BCS-BEC crossover has been elusive so far. Here, we provide the first example of complete evidence for the BCS-BEC crossover in an iron-based superconductor FeSe_1-xS_x from laser-excited angle-resolved photoemission spectroscopy, and propose a multiband mechanism for BCS-BEC crossover in this system.

[1] M. Randeria et al., Annu. Rev. Cond. Matt. Phys. 5, 209-232 (2014).