Band-edge BCS-BEC crossover in a two-band superconductor: physical properties and detection parameters

TL;DR

This paper models the BCS-BEC crossover in two-band superconductors, identifying key physical parameters and proposing experimental detection methods, with implications for understanding high-temperature superconductivity in materials like iron-based compounds.

Contribution

It introduces a theoretical model for the BCS-BEC crossover in two-band superconductors, analyzing physical properties and detection parameters relevant to experimental identification.

Findings

Superconductivity near the BCS-BEC crossover regime when the gap is comparable to the chemical potential.

Coexistence of small and large Cooper pairs enhances high-Tc superconductivity.

The gap-to-Fermi energy ratio is an effective experimental indicator for the crossover regime.

Abstract

Superconductivity in iron-based, magnesium diborides, and other novel superconducting materials has a strong multi-band and multi-gap character. Recent experiments support the possibillity for a BCS-BEC crossover induced by strong-coupling and proximity of the chemical potential to the band edge of one of the bands. Here we study the simplest theoretical model which accounts for the BCS-BEC crossover in a two-band superconductor, considering tunable interactions and tunable energy separations between the bands. Mean-field results for condensate fraction, correlation length, and superconducting gap are reported in typical crossover diagrams to locate the boundaries of the BCS, crossover, and BEC regimes. When the superconducting gap is of the order of the local chemical potential, superconductivity is in the crossover regime of the BCS-BEC crossover and the Fermi surface of the small band is smeared by the gap opening. In this situation, small and large Cooper pairs coexist in the total condensate, which is the optimal condition for high-Tc superconductivity. The ratio between the gap and the Fermi energy in a given band results to be the best detection parameter for experiments to locate the system in the BCS-BEC crossover. Using available experimental data, our analysis shows that iron-based superconductors have the partial condensate of the small Fermi surface in the crossover regime of the BCS-BEC crossover, supporting the recent ARPES findings.