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 _0.42Ca _0.58MnO ₃ (LCMO) thin film and a La _0.25Pr _0.375Ca _0.375MnO 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 T _CO/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.