Direct observation of Higgs mode oscillations in the pump-probe photoemission spectra of electron-phonon mediated superconductors

TL;DR

This study uses non-equilibrium formalism to observe Higgs mode oscillations in superconductors via pump-probe photoemission spectra, revealing direct excitation and detection of the Higgs mode through nonlinear electromagnetic effects.

Contribution

It demonstrates the direct observation of Higgs mode oscillations in superconductors using time-dependent photoemission spectra under ultrafast pump fields.

Findings

Higgs mode oscillations occur at twice the superconducting gap frequency.

Pump field partially melts the superconducting order.

Higgs mode can be excited without symmetry breaking.

Abstract

Using the non-equilibrium Keldysh formalism, we solve the equations of motion for electron-phonon superconductivity, including an ultrafast pump field. We present results for time-dependent photoemission spectra out of equilibrium which probes the dynamics of the superconducting gap edge. The partial melting of the order by the pump field leads to oscillations at twice the melted gap frequency, a hallmark of the Higgs or amplitude mode. Thus the Higgs mode can be directly excited through the nonlinear effects of an electromagnetic field and detected without any additional symmetry breaking.