title: Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity.
abstract: 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)(Tc = 14.5 K ~ 1.2 meV) in an accessible range below and above the Fermi level(EF) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV (~8 K) above EF at the Γ-point, which shows a sharp superconducting coherence peak with gap formation below Tc. The estimated superconducting gap Δ and Fermi energy [Symbol: see text]F 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.
authors.affiliation: 1] Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan [2].
authors.forename: K
authors.initials: K
authors.lastname: Okazaki
authors.affiliation: Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Ito
authors.affiliation: Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Ota
authors.affiliation: 1] Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan [2].
authors.forename: Y
authors.initials: Y
authors.lastname: Kotani
authors.affiliation: 1] Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan [2].
authors.forename: T
authors.initials: T
authors.lastname: Shimojima
authors.affiliation: 1] Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan [2].
authors.forename: T
authors.initials: T
authors.lastname: Kiss
authors.affiliation: Research Institute for Science and Technology, Tokyo University of Science, Chiba 278-8510, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Watanabe
authors.affiliation: Beijing Center for Crystal R&D, Chinese Academy of Science (CAS), Zhongguancun, Beijing 100190, China.
authors.forename: C-T
authors.initials: CT
authors.lastname: Chen
authors.affiliation: 1] RIKEN Advanced Science Institute, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan [2] TRIP, JST, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Niitaka
authors.affiliation: 1] RIKEN Advanced Science Institute, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan [2] TRIP, JST, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Hanaguri
authors.affiliation: 1] RIKEN Advanced Science Institute, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan [2] TRIP, JST, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Takagi
authors.affiliation: 1] RIKEN SPring-8 Center, Sayo-gun, Hyogo 679-5148, Japan [2] Department of Physics, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578 Japan.
authors.forename: A
authors.initials: A
authors.lastname: Chainani
authors.affiliation: 1] Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan [2] TRIP, JST, Chiyoda-ku, Tokyo 102-0075, Japan [3] RIKEN SPring-8 Center, Sayo-gun, Hyogo 679-5148, Japan [4] CREST, JST, Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Shin