Coherent vs incoherent pairing in 2D systems near magnetic instability

TL;DR

This paper investigates how incoherent fermions and diffusive spin excitations influence superconductivity in 2D systems near magnetic instability, revealing a distinct quantum-critical pairing regime with a saturation of the pairing temperature.

Contribution

It introduces a new understanding of pairing mechanisms involving incoherent fermions and diffusive spin excitations near magnetic criticality, differing from traditional BCS theory.

Findings

Pairing instability temperature increases with magnetic correlation length.

In the quantum-critical regime, pairing differs qualitatively from BCS.

Pairing temperature saturates as magnetic correlation length diverges.

Abstract

We study the superconductivity in 2D fermionic systems near antiferromagnetic instability, assuming that the pairing is mediated by spin fluctuations. This pairing involves fully incoherent fermions and diffusive spin excitations. We show that the competition between fermionic incoherence and strong pairing interaction yields the pairing instability temperature $T_{ins}$ which increases and saturates as the magnetic correlation length $\xi \to \infty$. We argue that in this quantum-critical regime the pairing problem is qualitatively different from the BCS one.