Direct observation of Higgs mode oscillations in the pump-probe   photoemission spectra of electron-phonon mediated superconductors
  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. 
A.F. Kemper, M.A. Sentef, B. Moritz, J.K. Freericks, T.P. Devereaux
cond-mat.supr-con