Condensation energy in strongly coupled superconductors

TL;DR

This paper investigates the condensation energy in strongly coupled, electronically mediated superconductors, revealing feedback effects on spin excitations and fermions that differ from traditional BCS theory.

Contribution

It demonstrates how strong coupling alters the condensation energy mechanism through feedback effects on spin excitations and fermions, diverging from BCS predictions.

Findings

Condensation energy is influenced by feedback on spin excitations.

Electronic contribution to condensation energy becomes positive at strong coupling.

Kinetic energy gain occurs at high coupling strengths.

Abstract

We consider the condensation energy in superconductors where the pairing is electronic in origin and is mediated by a collective bosonic mode. We use magnetically-mediated superconductivity as an example, and show that for large spin-fermion couplings, the physics is qualitatively different from the BCS theory as the condensation energy results from the feedback on spin excitations, while the electronic contribution to the condensation energy is positive due to an ``undressing'' feedback on the fermions. The same feedback effect accounts for the gain of the kinetic energy at strong couplings.