# Higgs-mode radiance and charge-density-wave order in 2H-NbSe$_2$

## TL;DR

This study investigates the interplay between charge-density-wave and superconductivity in 2H-NbSe$_2$ using pressure-dependent Raman spectroscopy, revealing a Higgs-type superconducting mode and mutual electronic involvement of both orders.

## Contribution

It demonstrates the pressure evolution of intertwined charge-density-wave and superconducting modes and supports a Higgs-like nature of the superconducting mode in 2H-NbSe$_2$.

## Key findings

- Collapse of charge-density-wave and superconducting modes under pressure
- Support for Higgs-type superconducting mode
- Evidence of mutual electronic degrees of freedom in both orders

## Abstract

Despite being usually considered two competing phenomena, charge-density-wave and superconductivity coexist in few systems, the most emblematic one being the transition metal dichalcogenide 2H-NbSe$_2$. This unusual condition is responsible for specific Raman signatures across the two phase transitions in this compound. While the appearance of a soft phonon mode is a well-established fingerprint of the charge-density-wave order, the nature of the sharp sub-gap mode emerging below the superconducting temperature is still under debate. In this work we use the external pressure as a knob to unveil the delicate interplay between the two orders, and consequently the nature of the superconducting mode. Thanks to an advanced extreme-conditions Raman technique we are able to follow the pressure evolution and the simultaneous collapse of the two intertwined charge density wave and superconducting modes. The comparison with microscopic calculations in a model system supports the Higgs-type nature of the superconducting mode and suggests that charge-density-wave and superconductivity in 2H-NbSe$_2$ involve mutual electronic degrees of freedom. These findings fill knowledge gap on the electronic mechanisms at play in transition metal dichalcogenides, a crucial step to fully exploit their properties in few-layers systems optimized for devices applications.