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

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.

28 pages, 3 figures; submitted to the special issue Eliashberg90 dedicated to Gerasim Eliashberg. Journal reference added

References in corpus (8)

• Theory of Intertwined Orders in High Temperature Superconductors
• Are non-Fermi-liquids stable to Cooper pairing?
• Instabilities near the onset of spin density wave order in metals
• A controlled expansion for certain non-Fermi liquid metals
• Spin-triplet pairing instability of the spinon Fermi surface in a U(1) spin liquid
• Neutral Fermion Excitations in the Moore-Read state at ν=5/2
• Quantum Phase Transitions and the ν=5/2 Fractional Hall State in Wide Quantum Wells
• Pairing in half-filled Landau level

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• Quantum Interference of Hydrodynamic Modes in a Dirty Marginal Fermi Liquid
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• Spectrum of a Gross-Neveu Yukawa model with flavor disorder in $d=3$
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• Fluctuation Spectrum of Critical Fermi Surfaces
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