Two-Photon Absorption by Impurity Scattering and Amplitude Mode in Conventional Superconductors

TL;DR

This paper theoretically analyzes two-photon absorption in conventional superconductors, revealing a divergence in absorption edge in dirty limits as a probe for the amplitude mode, independent of impurity scattering.

Contribution

It demonstrates that the amplitude mode's behavior in two-photon absorption is unaffected by impurity scattering, providing a new method to probe this mode in superconductors.

Findings

Diverging absorption edge in dirty limit

Amplitude mode is impurity scattering independent

Two-photon absorption spectrum reveals superconducting properties

Abstract

We theoretically investigate the two-photon absorption spectrum of conventional s-wave superconductors. The calculation is carried out in the local limit at absolute zero. The direct excitation of quasiparticles is excluded under the condition that the frequency of the external field is lower than twice the magnitude of the superconducting gap. We calculate vertex corrections with the conserving approximation, and show that the amplitude mode is independent of the impurity scattering in the local limit. Because of this property, a diverging absorption edge appears in the dirty limit. We propose this behavior as a probe of the amplitude mode.