Is the Migdal-Eliashberg Theory for 2+1D Critical Fermi Surface Stable?

TL;DR

This paper investigates the stability of the Migdal-Eliashberg theory in 2+1D Fermi surfaces coupled to gapless bosons, revealing instabilities in shape deformations near quantum critical points and challenging the framework's applicability.

Contribution

It introduces a diagonalization scheme for the Bethe-Salpeter ladder and uncovers soft modes leading to shape fluctuation instabilities in the theory.

Findings

Large number of soft shape fluctuation modes identified

Odd-parity deformations become unstable near the non-Fermi liquid regime

Implications for transport properties discussed in companion paper

Abstract

We diagnose the stability of the Migdal-Eliashberg theory for a Fermi surface coupled to a gapless boson in 2+1 dimensions. We provide a scheme for diagonalizing the Bethe-Salpeter ladder when small-angle scattering mediated by the boson plays a dominant role. We found a large number of soft modes which correspond to shape fluctuations of the Fermi surface, and these shape deformations follow a diffusion-like dynamics on the Fermi surface. Surprisingly, the odd-parity deformations of a convex Fermi surface becomes unstable near the non-Fermi liquid regime of the Ising-Nematic quantum critical point and our finding calls for revisit of the Migdal-Eliashberg framework. The implication of the Bethe-Salpeter eigenvalues in transport will be discussed in the companion paper [H.Guo,arXiv:2311.03458].