We developed an experimental system for measuring temperature-modulated optical reflectivity with a frequency up to 20 kHz to study the dynamics of inhomogeneity in a crystal. By this system, we studied doped perovskite manganites, in which a ferromagnetic metallic phase and a (short-ranged) chargeordered phase coexist near the Curie temperature (TC). We found an appreciable frequency dependence of the temperature-modulated reflectivity around TC. This result can be explained by the dynamics of the domains in the two coexisting phases, i.e., by the relaxation time to reach a new equilibrium state of the two-phase coexistence when the temperature is modulated.
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