# Enhanced critical temperature, pairing fluctuation effects, and BCS-BEC crossover in a two-band Fermi gas

## TL;DR

This paper investigates how two-band Fermi systems exhibit enhanced superfluid critical temperatures and complex pairing phenomena across the BCS-BEC crossover, extending the NSR approach to include interband effects.

## Contribution

It extends the NSR approach to two-band systems, revealing enhanced critical temperatures and reduced pseudogap regions compared to single-band models.

## Key findings

- Critical temperature is significantly increased in two-band systems.
- Pseudogap effects are reduced due to interband coupling.
- Two types of composite bosons become entangled in the strong-coupling BEC regime.

## Abstract

We study the superfluid critical temperature in a two-band attractive Fermi system with strong pairing fluctuations associated with both interband and intraband couplings. We focus specifically on a configuration where the intraband coupling is varied from weak to strong in a shallow band coupled to a weakly-interacting deeper band. The whole crossover from the Bardeen-Cooper-Schrieffer (BCS) condensation of largely overlapping Cooper pairs to the Bose-Einstein condensation (BEC) of tightly bound molecules is covered by our analysis, which is based on the extension of the Nozi\`{e}res-Schmitt-Rink (NSR) approach to a two-band system. In comparison with the single-band case, we find a strong enhancement of the critical temperature, a significant reduction of the preformed pair region where pseudogap effects are expected, and the entanglement of two kinds of composite bosons in the strong-coupling BEC regime.