When the continuous symmetry of a physical system is spontaneously broken, two types of collective modes typically emerge: the amplitude and the phase modes of the order-parameter fluctuation. For superconductors, the amplitude mode is referred to most recently as the Higgs mode as it is a condensed-matter analog 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 for 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 progress in the study of the Higgs mode in superconductors.

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