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标题：Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity
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作者：K. Okazaki ; Y. Ito ; Y. Ota 等
期刊名称：Scientific Reports
电子版ISSN：2045-2322
出版年度：2014
卷号：4
DOI：10.1038/srep04109
出版社：Springer Nature
摘要：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_0.6Se_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 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.