BCS-BEC Crossover and Pairing Fluctuations in a Two Band Superfluid/Superconductor: A T Matrix Approach

TL;DR

This paper studies pairing fluctuations in a two-band fermionic system across the BCS-BEC crossover using a T matrix approach, revealing enhanced critical temperatures and generalized Tan's contact in multiband superfluids.

Contribution

It introduces a diagrammatic T matrix method for two-band systems, extending Tan's contact concept and comparing results with the Nozières–Schmitt-Rink approximation.

Findings

Critical temperature is significantly enhanced by multiband effects.

Generalized Tan's contact is calculated for different pair-exchange couplings.

Thermodynamic quantities show crossover behavior from BCS to BEC regimes.

Abstract

We investigate pairing fluctuation effects in a two band fermionic system, where a shallow band in the Bardeen--Cooper--Schrieffer--Bose--Einstein condensation (BCS-BEC) crossover regime is coupled with a weakly interacting deep band. Within a diagrammatic $T$ matrix approach, we~report how thermodynamic quantities such as the critical temperature, chemical potential, {and~momentum distributions} undergo the crossover from the BCS to BEC regime by tuning the intraband coupling in the shallow band. We also generalize the definition of Tan's contact to a two band system and report the two contacts for different pair-exchange couplings. The present results are compared with those obtained by the simpler Nozi\`eres--Schmitt--Rink approximation. We confirm a pronounced enhancement of the critical temperature due to the multiband configuration, as well as to the pair-exchange coupling.