Resonant pair-exchange scattering and BCS-BEC crossover in a system composed of dispersive and heavy incipient bands: a Feshbach analogy

TL;DR

This paper theoretically demonstrates that a two-band system with very different masses exhibits resonant pair scattering, leading to a BCS-BEC crossover and enhanced superfluidity, analogous to Feshbach resonance phenomena.

Contribution

It introduces the concept of resonant pair scattering in a two-band system with heavy incipient bands, revealing a novel mechanism for pairing enhancement.

Findings

Resonant pair scattering induces effective intraband attraction.

The mechanism enhances superfluidity and superconductivity.

Analogy established between pair scattering and Feshbach resonance.

Abstract

We theoretically show that a two-band system with very different masses harbors a resonant pair scattering that leads to novel pairing properties, as highlighted by the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover. Most importantly, the interband pair-exchange coupling induces an effective intraband attraction in each band, enhancing the superfluidity/superconductivity. The effect, a kind of Suhl-Kondo mechanism, is specifically enhanced when the second band has a heavy mass and is incipient (lying close to, but just above, the chemical potential, $\mu$), which we call a resonant pair scattering. By elucidating the dependence of the effective interactions and gap functions on $\mu$, we can draw an analogy between the resonant pair scattering and the Feshbach resonance.