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Condensed Matter > Superconductivity

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

Abstract: In superconductivity, a surge of interests in enhancing $T_{\rm c}$ is ever mounting, where a recent focus is toward multi-band superconductivity. In $T_{\rm c}$ 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 $T_{\rm c}$ significantly, this in fact competes with the enhanced pairing arising from the heavy band, with the trade-off leading to a peaked structure in $T_{\rm c}$ 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.
Comments: 12 pages, 9 figures, Accepted version for Physical Review B
Subjects: Superconductivity (cond-mat.supr-con); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Atomic Physics (physics.atom-ph)
Journal reference: Phys. Rev. B 109, L140504 (2024)
DOI: 10.1103/PhysRevB.109.L140504
Cite as: arXiv:2402.06454 [cond-mat.supr-con]
  (or arXiv:2402.06454v2 [cond-mat.supr-con] for this version)

Submission history

From: Hiroyuki Tajima [view email]
[v1] Fri, 9 Feb 2024 15:00:50 GMT (2647kb)
[v2] Thu, 28 Mar 2024 11:52:02 GMT (2696kb)

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