Phase signatures in third-harmonic response of Higgs and coexisting modes in superconductors

TL;DR

This paper explores how phase analysis of third-harmonic generation signals in superconductors can reveal microscopic details and mode couplings, aiding the interpretation of complex experimental data involving multiple collective excitations.

Contribution

It introduces phase analysis of THG signals as a novel method to distinguish and understand coexisting modes and interactions in superconductors.

Findings

Phase of THG signals encodes microscopic details.

Impurities and Coulomb interactions influence phase response.

Phase analysis can help identify mode couplings in experiments.

Abstract

Third-harmonic generation (THG) experiments on superconductors can be used to investigate collective excitations like the amplitude mode of the order parameter known as Higgs mode. These modes are visible due to resonances in the THG signal if the driving frequency matches the energy of the mode. In real materials multiple modes can exist giving rise to additional THG contributions, such that it is difficult to unambiguously interpret the results. In this paper, we additionally analyze the phase of the THG signal, which contains microscopic details beyond classical resonances as well as signatures of couplings between modes which are difficult to observe in the amplitude alone. We investigate how the Higgs mode, impurities or Coulomb interaction affects the phase response and consider exemplary two systems with additional modes. We argue that extracting this phase information could be valuable in future experiments.