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

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 Nozières-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.

Theoretical Possibilities for Flat Band Superconductivity

“…For the cuprates specifically, effects of hydrostatic and uniaxial pressure for multi-band models [1] or microstrains [27] have also been discussed. For attractively interacting two-band systems, the effects of the second band have been studied with Nozières-Schmitt-Rink formalism [28], BCS pairing Hamiltonian [29], and/or in the context of the BCS-BEC crossover [30] or Lifshitz transitions [31]. Thus a future problem is how these would apply to the flat-band cases.…”

A Dual-Species Bose-Einstein Condensate with Attractive Interspecies Interactions

“…Most of these phenomena are accessible thanks to the ability to control the sign and strength of interactions, opening the way to the study of unconventional matter phases. For instance, in two-component Fermi gases, by tuning the interparticle scattering length close to a Feshbach resonance, it has been possible to explore the Bose-Einstein condensate (BEC)-BCS crossover [18], and in multicomponent Fermi systems, even more exotic scenarios have been discussed which are relevant for high-Tc superconductivity [19][20][21][22][23]. On the other hand, in bosonic mixtures, Feshbach resonances have been recently exploited to produce and study novel quantum phases, arising by competing interaction forces.…”

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

“…In this work, we theoretically investigate the multi-band BCS-BEC crossover and the effects of pairing fluctuations within the framework of the many body T matrix approximation, extending our previous work [27], which was based on the Nozières-Schmitt-Rink (NSR) approximation [44]. While this extension is crucial to access dynamical quantities such as spectral functions, one has to examine also how higher fluctuation effects beyond the NSR scheme appear in thermodynamic quantities.…”

“…Combining these two aspects of unconventional superconducting/superfluid properties is a promising route toward room temperature superconductivity [25,26]. One striking feature predicted in the multi-band BCS-BEC crossover is the screening of pairing fluctuations [26,27]. Since pairing fluctuations are known to lower generally the superconducting/superfluid critical temperature, this screening effect is expected to enhance the critical temperature compared to the single band counterpart.…”