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${}_{0.4}$($T_c$ = 14.5 K $\sim$ 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 ($\sim$ 8 K) above $E_F$ at the $\Gamma$-point, which shows a sharp superconducting coherence peak with gap formation below $T_c$. The estimated superconducting gap $\Delta$ and Fermi energy ${\epsilon }_F$ 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.

26 pages, 16 figures, to be published in Scientific Reports

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