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

TL;DR

This study reveals an electron band just above the Fermi level in FeTe$_{0.6}$Se$_{0.4}$ that exhibits superconductivity, suggesting a potential pathway for BCS-BEC crossover in iron-based superconductors.

Contribution

It uncovers an electron band above the Fermi level with superconducting coherence, providing evidence for a BCS-BEC crossover route in iron-based superconductors.

Findings

Electron band 0.7 meV above $E_F$ shows superconducting gap.

Coexistence of BEC-like and BCS-like superconductivity in different bands.

Identification of a new 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.