# Higgs Mode in Superconductors

## TL;DR

This paper reviews recent progress in observing and understanding the Higgs mode in superconductors, a collective excitation related to symmetry breaking, made accessible through advanced nonlinear terahertz spectroscopy techniques.

## Contribution

It provides a comprehensive overview of recent experimental and theoretical developments in detecting and analyzing the Higgs mode in superconductors.

## Key findings

- Higgs mode has been observed in superconductors using nonlinear terahertz spectroscopy.
- The Higgs mode is a scalar excitation that does not couple linearly to electromagnetic fields.
- Recent techniques have enabled the study of Higgs mode despite its elusive nature for decades.

## Abstract

When a continuous symmetry of a physical system is spontaneously broken, two types of collective modes typically emerge: the amplitude and phase modes of the order-parameter fluctuation. For superconductors, the amplitude mode is recently referred to as the ''Higgs mode'' as it is a condensed-matter analogue of a Higgs boson in particle physics. Higgs mode is a scalar excitation of the order parameter, distinct from charge or spin fluctuations, and thus does not couple to electromagnetic fields linearly. This is why the Higgs mode in superconductors has evaded experimental observations over a half century after the initial theoretical prediction, except for a charge-density-wave coexisting system. With the advance of nonlinear and time-resolved terahertz spectroscopy techniques, however, it has become possible to study the Higgs mode through the nonlinear light-Higgs coupling. In this review, we overview recent progresses on the study of the Higgs mode in superconductors.