Thermal effects on collective modes in disordered $s$-wave superconductors

TL;DR

This paper studies how thermal fluctuations affect collective modes in disordered two-dimensional s-wave superconductors, revealing mode softening, persistent amplitude modes, and a momentum-dependent incoherent background.

Contribution

It demonstrates that amplitude modes remain finite at finite temperature and describes the momentum-dependent spectral background caused by thermal quasiparticles.

Findings

Phase mode softens with increasing temperature.

Finite-energy amplitude modes survive thermal fluctuations.

Thermally excited quasiparticles create a momentum-dependent spectral halo.

Abstract

We investigate the effect of thermal fluctuations on the two-particle spectral function for a disordered $s$-wave superconductor in two dimensions, focusing on the evolution of the collective amplitude and phase modes. We find three main effects of thermal fluctuations: (a) the phase mode is softened with increasing temperature reflecting the decrease of superfluid stiffness; (b) remarkably, the non-dispersive collective amplitude modes at finite energy near ${\bf q}=[0,0]$ and ${\bf q}=[\pi,\pi]$ survive even in presence of thermal fluctuations in the disordered superconductor; and (c) the scattering of the thermally excited fermionic quasiparticles leads to low energy incoherent spectral weight that forms a strongly momentum-dependent background halo around the phase and amplitude collective modes and broadens them. Due to momentum and energy conservation constraints, this halo has a boundary which disperses linearly at low momenta and shows a strong dip near the $[\pi,\pi]$ point in the Brillouin zone.