Evolution of coherent collective modes through consecutive charge-density-wave transitions in the (PO_2)_4(WO_3)_(12) monophosphate tungsten bronze

摘要

All-optical femtosecond relaxation dynamics in a single crystal of monophosphate tungsten bronze (PO_2)_4(WO_3)_2m with alternate stacking m = 6 of WO3 layers was studied through the three consequent charge-density-wave (CDW) transitions. Several transient coherent collective modes associated with the different CDW transitions were observed and analyzed in the framework of the time-dependent Ginzburg-Landau theory. Remarkably, the interference of the modes leads to an apparent rectification effect in the transient reflectivity response. A saturation of the coherent-mode amplitudes with increasing pump fluence well below the CDWs destruction threshold fluence indicates a decoupling of the electronic and lattice parts of the order parameter on the femtosecond timescale.