The unreasonable effectiveness of Eliashberg theory for pairing of non-Fermi liquids

TL;DR

This paper reviews how Eliashberg theory, originally for conventional metals, effectively describes superconductivity in strongly-correlated non-Fermi liquids, showing its surprising broad applicability beyond traditional regimes.

Contribution

It demonstrates that Eliashberg equations are asymptotically exact in certain strongly-coupled non-Fermi liquid models, extending its relevance beyond conventional metals.

Findings

Eliashberg theory remains valid in non-Fermi liquids with gapless bosonic fluctuations.

Superconductivity can emerge without coherent quasiparticles.

Models show Eliashberg equations are asymptotically exact in strong coupling regimes.

Abstract

The paradigmatic Migdal-Eliashberg theory of the electron-phonon problem is central to the understanding of superconductivity in conventional metals. This powerful framework is justified by the smallness of the Debye frequency relative to the Fermi energy, and allows an enormous simplification of the full many-body problem. However, superconductivity is found also in many families of strongly-correlated materials, in which there is no {\it a priori} justification for the applicability of Eliashberg theory. In these systems, superconductivity emerges out of an anomalous metallic state, calling for a new theoretical framework to describe pairing out of a non-Fermi liquid. In this article, we review two model systems in which such behavior is found: a Fermi sea coupled to gapless bosonic fluctuations, and a system of fermions with local, strongly frustrated interactions. In both models, there is a well-defined limit in which the Eliashberg equations are asymptotically exact even in the strongly coupled regime. These models thus provide tractable examples of how superconductivity can emerge in the absence of coherent electronic quasiparticles; they also demonstrate the surprisingly wide applicability of the Eliashberg formalism, well beyond the conventional regime for which it was originally designed.