Nonadiabatic dynamics and coherent control of nonequilibrium superconductors

Motivated by recent THz pump-THz probe experiments on NbN films [1], we theoretically study the pump-probe response of nonequilibrium superconductors coupled to optical phonons. For ultrashort pump pulses a nonadiabatic regime emerges, which is characterized by oscillations of the superconducting gap [2] and by the generation of coherent phonons [3]. Using density-matrix theory, we compute the pump-probe response of the superconductor in the nonadiabatic regime and determine the signatures of the order parameter and of the phonon oscillations in the pump-probe conductivity. We find that the nonadiabatic dynamics of the superconductor reflects itself in oscillations of the pump-probe response as a function of delay time between pump and probe pulses [4]. We argue that from the analysis of this oscillatory behavior both frequency and decay time of the algebraically decaying order-parameter oscillations can be inferred.\\[4pt] [1] R. Matsunaga \textit{et. al.}, Phys. Rev. Lett. \textbf{111}, 057002 (2013). \newline [2] E. A. Yuzbashyan \textit {et. al.}, Phys. Rev. Lett. \textbf{96}, 097005 (2006). \newline [3] A. P. Schnyder \textit {et. al.}, Phys. Rev. B \textbf{84}, 214513 (2011). \newline [4] H. Krull \textit {et. al.}, arXiv:1309.7318 (submitted).