BCS-BEC crossover in quantum confined superconductors

TL;DR

This paper investigates how quantum confinement in ultranarrow superconductors enables tunable BCS-BEC crossover through multiband effects, revealing coexistence of large and small Cooper pairs and implications for high-temperature superconductivity.

Contribution

It provides a detailed analysis of the multiband BCS-BEC crossover in quantum confined superconductors, highlighting the tunability via chemical potential and interaction strength.

Findings

Crossover regime extends with increased pairing strength and interaction energy.

Coexistence of large and small Cooper pairs in the crossover regime.

Optimal conditions for high-temperature superconductivity identified.

Abstract

Ultranarrow superconductors are in the strong quantum confinement regime with formation of multiple coherent condensates associated with the many subbands of the electronic structure. Here we analyze the multiband BCS-BEC crossover induced by the chemical potential tuned close to a subband bottom, in correspondence of a superconducting shape resonance. The evolution of the condensate fraction and of the pair correlation length in the ground state as functions of the chemical potential demonstrates the tunability of the BCS-BEC crossover for the condensate component of the selected subband. The extension of the crossover regime increases when the pairing strength and/or the characteristic energy of the interaction get larger. Our results indicate the coexistence of large and small Cooper pairs in the crossover regime, leading to the optimal parameter configuration for high transition temperature superconductivity.