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

K Okazaki; Y Ito; Y Ota; Y Kotani; T Shimojima; T Kiss; S Watanabe; C-T Chen; S Niitaka; T Hanaguri; H Takagi; A Chainani; S Shin

doi:10.1038/srep04109

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

Sci Rep. 2014 Feb 28:4:4109. doi: 10.1038/srep04109.

Authors

K Okazaki¹, Y Ito², Y Ota², Y Kotani¹, T Shimojima¹, T Kiss¹, S Watanabe³, C-T Chen⁴, S Niitaka⁵, T Hanaguri⁵, H Takagi⁵, A Chainani⁶, S Shin⁷

Affiliations

¹ 1] Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan [2].
² Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
³ Research Institute for Science and Technology, Tokyo University of Science, Chiba 278-8510, Japan.
⁴ Beijing Center for Crystal R&D, Chinese Academy of Science (CAS), Zhongguancun, Beijing 100190, China.
⁵ 1] RIKEN Advanced Science Institute, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan [2] TRIP, JST, Chiyoda-ku, Tokyo 102-0075, Japan.
⁶ 1] RIKEN SPring-8 Center, Sayo-gun, Hyogo 679-5148, Japan [2] Department of Physics, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578 Japan.
⁷ 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.

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.