A time-resolved optical study of the paramagnetic dielectric-ferromagnetic metal transition in La_0.7Ca_0.3MnO₃

The relaxation dynamics of charge carriers in an epitaxial La0.7Ca0.3MnO3 film was studied by the light reflection method with a femtosecond time resolution in a wide temperature range. The relaxation of time-resolved photoinduced reflections was shown to be two-component in the whole temperature range including the paramagnetic dielectric-ferromagnetic metal transition at Tapproximate to150-160 K. The fast relaxation component had a maximum lifetime (tausimilar to500 fs) in the transition region, and its contribution to relaxation increased as temperature decreased. The lifetime of the slow component was minimum (tausimilar to15 ps) in the transition region. In addition, fast oscillations assigned to coherent phonons appeared in photoinduced responses at T<200 K. The dephasing time of these oscillations increased as temperature decreased, whereas their frequency changed insignificantly.