Femtosecond dynamics of the structural transition in mixed valence manganites

We investigate the structural response of charge and orbitally ordered (CO/OO) manganites to ultrafast optical excitation using optical reflectivity and x-ray diffraction as a probe. We study a La sub(0.42) Ca sub(0.58) MnO sub(3) (LCMO) thin film and a La sub(0.25) Pr sub(0.375) Ca sub(0.375) MnO sub(3) (LPCMO) single crystal. For both materials we observe oscillations in the optical responses that are assigned to a coherent optical phonon generated via displacive excitation. The coherent phonon disappears either when increasing the temperature above TCO / OO or when raising the excitation fluence above a certain threshold. At low excitation fluences the amplitude and lifetime of this phonon behave similarly to the order parameter of the structural phase transition.