Coherent optical and acoustic phonon generation correlated with the charge-ordering phase transition in La_1−xCa_xMnO₃

We have observed coherent optical and acoustic phonon generation, which are strongly coupled to the charge-ordering (CO) transition in La_1−xCa_xMnO₃ (x=0.5, 0.58) using femtosecond optical pump-probe spectroscopy. Coherent optical phonons, observed at low temperatures, disappear above the charge-ordering temperature T_CO, while coherent acoustic phonons display the opposite behavior, disappearing gradually below T_CO. Coherent optical phonons are generated by the displacive excitation mechanism where their coupling to the photoexcited charge carriers is enhanced by the structural change corresponding to the CO phase transition. The oscillation frequency for the coherent acoustic phonon depends on the probe wavelength, which is consistent with the propagating strain pulse mechanism. The dramatic change of lattice constants across the charge-ordering transition explains the overall temperature dependence of the coherent acoustic phonon amplitude.