Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity

Conventional superconductivity follows Bardeen-Cooper-Schrieffer(BCS) theory of electrons-pairing in momentum-space, while superfluidity is the Bose-Einstein condensation(BEC) of atoms paired in real-space. These properties of solid metals and ultra-cold gases, respectively, are connected by the BCS-BEC crossover. Here we investigate the band dispersions in FeTe(0.6)Se(0.4)(T(c) = 14.5 K ~ 1.2 meV) in an accessible range below and above the Fermi level(E(F)) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV (~8 K) above E(F) at the Γ-point, which shows a sharp superconducting coherence peak with gap formation below T(c). The estimated superconducting gap Δ and Fermi energy [Image: see text] indicate composite superconductivity in an iron-based superconductor, consisting of strong-coupling BEC in the electron band and weak-coupling BCS-like superconductivity in the hole band. The study identifies the possible route to BCS-BEC superconductivity.