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

• Published in: Scientific reports Vol. 4; no. 1; p. 4109
• Main Authors: Okazaki, K., Ito, Y., Ota, Y., Kotani, Y., Shimojima, T., Kiss, T., Watanabe, S., Chen, C.-T., Niitaka, S., Hanaguri, T., Takagi, H., Chainani, A., Shin, S.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 28.02.2014; Nature Publishing Group
• Subjects: 140/125; 639/301/119/1003; 639/301/119/2791; 639/301/119/995; Energy; Gases; Heavy metals; Humanities and Social Sciences; Iron; multidisciplinary; Physics; Science; Spectroscopy; Spectrum analysis; Superfluidity
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep04109

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 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.