Does Fulleride Superconductivity Crossover Contain a Feshbach Resonance?

TL;DR

This paper investigates whether the crossover in fulleride superconductivity involves a Feshbach resonance, linking it to cold atom Fermion-Boson crossover theory and discussing its implications for understanding superconducting phenomena.

Contribution

It establishes that the resonance in fulleride superconductors results from crossing of Fermion and Boson bands, providing a new perspective on the superconducting crossover and related phenomena.

Findings

Resonance arises from Fermion and Boson band crossing.

Ground and excited states remain continuous across the resonance.

Collective modes transform the nature of superconductivity.

Abstract

The pioneering works by Eagles, Leggett, and Nozieres/Schmitt-Rink (reviewed and augmented by Randeria) emphasize that in the limits of the models studied both at T = 0 and, the cross-over from a BCS-type to a BEC-type superconductor is continuous. The BCS and BEC end points seem to be well-established. However, in the intermediate region, home to fulleride and high temperature superconductors, considerable extrapolation of the models must be done as there still is no exact theory. Yet, considerable current literature is devoted to what appears to be more singular-type phenomena such as quantum critical points, stripe formation, insulator to superconductor phase transitions, loss of validity of the Fermi liquid theory, etc. Using a connection we have made with cold atom Fermion-Boson crossover theory, we can establish that the resonance previously discussed is a result of the crossing of the Fermion band by the Boson band. While the ground and singly excited states appear to remain continuous, the collective modes due to the resonance transform the nature of the superconductivity. We discuss features of the resonance and the experimentally observed pre-formed BEC Cooper pair formation in fullerides, essential to the Boson-Fermion resonance theory. In addition some of the various singular phenomena discussed above can be put into perspective. PACS numbers: 61.48.+c, 74.20.-z, 81.05Tp