Superconductivity Near a Quantum Critical Point: Bounds on the Transition Temperature in the $γ$-Model

Abstract

Near a quantum critical point (QCP) in a metal, strong Fermion-Fermion interactions mediated by soft collective bosons give rise to two competing phenomena: non-Fermi liquid behavior and superconductivity that deviates from conventional BCS and Migdal-Eliashberg theories. We consider the problem of obtaining closed-form analytical lower and upper bounds on transition temperatures for such systems. We focus mainly on a class of models known as the gamma-model, a variation of the Eliashberg theory of superconductivity where the effective interaction potential scales as V(Omega) proportional to 1/|Omega|^gamma. Building on a recent reformulation of Migdal-Eliashberg theory expressed as a classical infinite spin chain with nonlocal interactions [1,2], and employing a linear algebra analysis of the Hessian matrix obtained from the free energy functional, we derive rigorous, closed-form expressions for upper and lower bounds on the superconducting transition temperature for any gamma > 0. The main result of the paper is to establish an analytical upper bound on the transition temperature in closed form. Our upper bound is significantly tighter than those currently available in the literature and demonstrates rapid convergence toward results from prior numerical studies. Also, by applying the singularity condition directly to the unbounded Hessian matrix, our independently performed calculations confirm the lower bounds previously established in the literature [3].