Emergent Fano-Feshbach resonance in two-band superconductors with an incipient quasi-flat band: Enhanced critical temperature evading particle-hole fluctuations
Physical Review B(2024)
摘要
In superconductivity, a surge of interests in enhancing T_ c is ever
mounting, where a recent focus is toward multi-band superconductivity. In
T_ c enhancements specific to two-band cases, especially around the
Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) 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., quasi-flat) band. We find that, while the GMB
corrections usually suppress T_ c significantly, this in fact competes
with the enhanced pairing arising from the heavy band, with the trade-off
leading to a peaked structure in T_ c 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.
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