Evolution of Higgs and Leggett modes with pressure in NbSe₂

The coexistence of the charge density wave and superconducting phases has been a subject of much recent interest. 2H-NbSe₂ is a model superconductor with a charge density wave. By using Raman scattering technique, we show how the competition between the charge density wave and superconductivity in 2H-NbSe₂ can be tuned by temperature and pressure. We also report the experimental observation of a Higgs mode together with an additional Leggett mode and their evolution with pressure. The implication of these collective excitation modes to the understanding of unconventional superconductivity is discussed.