The special role of the first Matsubara frequency for superconductivity near a quantum-critical point -- the non-linear gap equation below $T_c$ and spectral properties in real frequencies

Abstract

Near a quantum-critical point in a metal a strong fermion-fermion interaction, mediated by a soft boson, destroys fermionic coherence and also gives rise to superconductivity. In a class of large $N$ models, one would naively expect an incoherent (non-Fermi liquid) normal state behavior to persist down to $T=0$. However, this is not the case for quantum-critical systems described by Eliashberg theory, where non-Fermi liquid part of the self-energy $Σ(ω_m)$ is large for a generic Matsubara frequency $ω_m = πT(2m+1)$, but vanishes at $ω_m = \pm πT$, while the pairing interaction between fermions with these two frequencies remains strong. This peculiarity gives rise to a non-zero $T_c$ even at large $N$[Y. Wang et al PRL 117, 157001 (2016)]. We consider the system behavior below $T_c$ and contrast the two cases when $ω_m = \pm πT$ are either special or not. We obtain the solution of the non-linear gap equations in real frequency axis and obtain the spectral function $A(ω)$ and the density of states $N(ω)$. In a conventional BCS-type superconductor $A(ω)$ and $N(ω)$ are peaked at the gap value $Δ(T)$, and the peak position shifts to a smaller $ω$ as temperature increases, i.e. the gap "closes in". However when superconductivity is induced by fermions with $ω_m = \pm πT$, the peak in $N(ω)$ remains at a finite frequency even at $T =T_c-0$, the gap just "fills in". The spectral function $A(ω)$ either shows almost the same "gap filling" behavior, or its peak position shifts to zero frequency already at a finite $Δ$ ("emergent Fermi arc" behavior), depending on the strength of the thermal contribution. We compare our results with the data for the cuprates and argue that "gap filling" behavior holds in the antinodal region, while the "emergent Fermi arc" behavior holds in the nodal region.