Breakdown of the Migdal-Eliashberg theory and a theory of lattice-fermionic superfluidity

Abstract

Superconductors become unstable at strong renormalized electron-phonon coupling which breaks the crystalline symmetry and gives rise to a new phase. Smoking-gun evidence for this instability is negative specific heat. The authors also predict an upper bound on the superconducting critical temperature and explain why and how a spurious lattice instability as a function of the bare electron-phonon coupling has been frequently mistaken for a genuine breakdown of the Migdal-Eliashberg theory.

Topics & Concepts