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 Nozières–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.
Highlights
Superconductivity and superfluidity play a central role in modern physics
The interband coupling parameter is taken at the values λ12 = 1 and λ12 = 2, while the temperature is set at the superfluid critical temperature T matrix approach (Tc), as determined by Equation (15)
We investigated the Bardeen–Cooper–Schrieffer–Bose–Einstein condensation (BCS-Bose–Einstein condensate (BEC)) crossover and the effects of pairing fluctuations for a two band system within the framework of the many body T matrix approximation
Summary
Superconductivity and superfluidity play a central role in modern physics. The first microscopic theory for superconductivity, namely the Bardeen–Cooper–Schrieffer (BCS) theory [1], has successfully been applied to various quantum many body systems. A variety of non-trivial phenomena has been proposed (e.g., topological phase soliton [7,8], odd-frequency pairing [9], multiple Leggett mode [10], hidden criticality [11], stable Sarma phase [12], and Lifshitz transitions with resonance effects and amplification of the critical temperature [13,14]) As another direction toward the unconventional properties of superconductors, the crossover from the weakly coupled BCS state to the Bose–Einstein condensate (BEC) of a tightly bound molecule with increasing the two body attractive interaction has extensively been discussed [15,16]. To realize the situation where the shallow band is close to the Lifshitz transition [46], we take ζ
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