Evidence for photoinduced sliding of the charge-order condensate in

We use femtosecond resonant soft x-ray scattering to measure the ultrafast optical melting of charge-order correlations in La 1.875 Ba 0.125 CuO 4. By analyzing both the energy-resolved and the energy-integrated order-parameter dynamics, we find evidence of a short-lived nonequilibrium state, the features of which are compatible with a sliding charge-density wave coherently set in motion by the pump. This transient state exhibits shifts in both the quasielastic line energy and its wave vector, as expected from a classical Doppler effect. The wave-vector change is indeed found to directly follow the pump propagation direction. These results demonstrate the existence of sliding charge-order behavior in an unconventional charge-density wave system and underscore the power of ultrafast optical excitation as a tool to coherently manipulate electronic condensates.