Nature and Raman signatures of the Higgs amplitude mode in the coexisting superconducting and charge-density-wave state

TL;DR

This paper studies the Higgs amplitude mode in a superconductor coexisting with a charge-density wave, revealing a sharp divergence and strong Raman signatures, with implications for interpreting experimental spectra in NbSe2.

Contribution

It demonstrates the transformation of Higgs mode behavior in coexisting states and links theoretical predictions to experimental Raman spectra in NbSe2.

Findings

Higgs mode exhibits a sharp power-law divergence in coexisting states.

Strong Raman resonance observed for both electronic and phononic mechanisms.

Results help interpret experimental Raman spectra in NbSe2.

Abstract

We investigate the behavior of the Higgs (amplitude) mode when superconductivity emerges on a preexisting charge-density-wave state. We show that the weak overdamped square-root singularity of the amplitude fluctuations in a standard BCS superconductor is converted in a sharp, undamped power-law divergence in the coexisting state, reminiscent of the Higgs behavior in Lorentz-invariant theories. This effect reflects in a strong superconducting resonance in the Raman spectra, both for an electronic and a phononic mechanism leading to the Raman visibility of the Higgs. In the latter case, our results are relevant to the interpretation of the Raman spectra measured experimentally in NbSe2.