Emergent Fano-Feshbach resonance in two-band superconductors with an incipient quasiflat band: Enhanced critical temperature evading particle-hole fluctuations

Abstract

In superconductivity, a surge of interests in enhancing Tc is ever mounting, where a recent focus is toward multiband superconductivity. In Tc enhancements specific to two-band cases, especially around the Bardeen-Cooper-Schrieffer to Bose-Einstein condensate crossover considered here, we have to be careful about how quantum fluctuations affect the many-body states, i.e., particle-hole fluctuations suppressing the pairing for attractive interactions. Here, we explore how to circumvent the suppression by examining multichannel pairing interactions in two-band systems. With the Gor'kov-Melik-Barkhudarov (GMB) formalism for particle-hole fluctuations in a continuous space, we look into the case of a deep dispersive band accompanied by an incipient heavy-mass (i.e., quasiflat) band. We find that, while the GMB corrections usually suppress Tc significantly, this in fact competes with the enhanced pairing arising from the heavy band, with the trade-off leading to a peaked structure in Tc against the band-mass ratio when the heavy band is incipient. The system then plunges into a strong-coupling regime with the GMB screening vastly suppressed. This occurs prominently when the chemical potential approaches the bound state lurking just below the heavy band, which can be viewed as a Fano-Feshbach resonance, with its width governed by the pair-exchange interaction. The diagrammatic structure comprising particle-particle and particle-hole channels is heavily entangled, so that the emergent Fano-Feshbach resonance dominates all the channels, suggesting a universal feature in multiband superconductivity and superfluidity.

Source-reported materials — not catalogue approval

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