Polaron dynamics and coherent acoustic phonons in La_0.45Ca_0.55MnO₃ thin films studied by ultrafast pump-probe spectroscopy

Pure phase, charge ordering (CO) (001)-oriented La _0.45Ca _0.55MnO ₃ thin films of thicknesses 250-600 nm are prepared by pulsed laser deposition. The ultrafast dynamics in these films is probed by using the femtosecond optical pump-optical probe (OPOP) spectroscopy. The temporal evolution in transient reflectivity change ΔR/R has been systematically measured at various temperatures. Generally, the ΔR/R curve exhibits two relaxing components and two oscillating components. Similar to the case in La _0.7Ca _0.3MnO ₃ films, the amplitude of the fast relaxing component exhibits a similar temperature dependence with that of the resistivity, and the neutron scattering intensity due to nanoscale correlated polarons. Therefore, it corresponds to the polaron dynamics in the CO sample. Both oscillating components are ascribed to the coherent longitudinal acoustic phonons and are generated by the strain pulses. The origin of these oscillations is discussed and the sound velocity can be derived from the period of these oscillations. It is revealed that the sound velocity changed drastically near the CO transition temperature, which indicates that the pump-probe measurements can be employed as a sensitive probe to detect the CO phase transition in these samples.