Coupling of Higgs and Leggett modes in nonequilibrium superconductors

TL;DR

This paper demonstrates through numerical simulations that optical pump-probe experiments can excite and reveal coupled Higgs and Leggett modes in two-gap superconductors, providing a new way to detect these collective excitations.

Contribution

It introduces the concept of non-adiabatic excitation of coupled Higgs and Leggett modes in nonequilibrium superconductors and predicts their spectral signatures.

Findings

Optical pump-probe experiments excite both Higgs and Leggett modes.

Coupling between modes is strong in nonequilibrium conditions.

Oscillations at mode frequencies are visible in absorption spectra.

Abstract

Collective excitation modes are a characteristic feature of symmetry-broken phases of matter. For example, superconductors exhibit an amplitude Higgs mode and a phase mode, which are the radial and angular excitations in the Mexican-hat potential of the free energy. In two-band superconductors there exists in addition a Leggett phase mode, which corresponds to collective fluctuations of the interband phase difference. In equilibrium systems amplitude and phase modes are decoupled, since they are mutually orthogonal excitations. The direct detection of Higgs and Leggett modes by linear-response measurements is challenging, because they are often overdamped and do not couple directly to the electromagnetic field. In this work, using numerical exact simulations we show for the case of two-gap superconductors, that optical pump-probe experiments excite both Higgs and Leggett modes out of equilibrium. We find that this non-adiabatic excitation process introduces a strong interaction between the collective modes. Moreover, we predict that the coupled Higgs and Leggett modes are clearly visible in the pump-probe absorption spectra as oscillations at their respective frequencies.