Polaron absorption in a perovskite manganite La_0.7Ca_0.3MnO₃

Optical conductivity spectra of a polycrystalline La_0.7Ca_0.3MnO₃ sample were investigated at temperatures between 20 and 300 K. The spectra at a metallic region clearly showed two types of absorption features, i.e., a sharp Drude peak and a broad mid-infrared absorption band. The phonon frequency renormalization occurred with the spectral weight transfer during the metalinsulator transition. Furthermore, the spectra at very low temperatures were quite consistent with the large polaron absorption behavior. A crossover from a small to a large polaron state is suggested to explain the observed spectral features.