Interplay between superconductivity and non-Fermi liquid at a quantum-critical point in a metal. IV: The $\gamma$ model and its phase diagram at $1<\gamma <2$

Abstract

In this paper we continue our analysis of the interplay between the pairing and the non-Fermi liquid behavior in a metal for a set of quantum-critical (QC) systems with an effective dynamical electron-electron interaction $V(\Omega_m) \propto 1/|\Omega_m|^\gamma$ (the $\gamma$-model). In previous papers we studied the cases $0<\gamma <1$ and $\gamma \approx 1$. We argued that the pairing by a gapless boson is fundamentally different from BCS/Eliashberg pairing by a massive boson as for the former there exists an infinite number of topologically distinct solutions for the gap function $\Delta_n (\omega_m)$ at $T=0$ ($n=0,1,2...$), each with its own condensation energy $E_{c,n}$. Here we extend the analysis to larger $1< \gamma <2$. We argue that the discrete set of solutions survives, and the spectrum of $E_{c,n}$ gets progressively denser as $\gamma$ approaches $2$ and eventually becomes continuous at $\gamma \to 2$. This increases the strength of "longitudinal" gap fluctuations, which tend to reduce the actual superconducting $T_c$ and give rise to a pseudogap region of preformed pairs. We also detect two features on the real axis for $\gamma >1$ which become critical at $\gamma\to 2$. First, the density of states evolves towards a set of discrete $\delta-$functions. Second, an array of dynamical vortices emerges in the upper frequency half-plane. These two features come about because on a real axis, the real part of the interaction, $V' (\Omega) \propto \cos(\pi \gamma/2)/|\Omega|^\gamma$, becomes repulsive for $\gamma >1$, and the imaginary $V^{''} (\Omega) \propto \sin(\pi \gamma/2)/|\Omega|^\gamma$, gets progressively smaller at $\gamma \to 2$. The features on the real axis are consistent with the development of a continuum spectrum of $E_{c,n}$ obtained using $\Delta_n (\omega_m)$ on the Matsubara axis. We consider the case $\gamma =2$ separately in the next paper.