Raman and optical spectroscopic studies of small-to-large polaron crossover in the perovskite manganese oxides

We present an optical reflectance and Raman-scattering study of the A(1-x)A' xMnO3 system as a function of temperature and doping (0.2 less than or equal to x less than or equal to 0.5). The metal-semiconductor transition in the A(1-x)A' xMnO3 system is characterized by a change from a diffusive electronic Raman-scattering response in the high-temperature paramagnetic phase, to a flat continuum scattering response in the low-temperature ferromagnetic phase. We interpret this change in the scattering response as a crossover from a small-polaron-dominated regime at high temperatures to a large-polaron-dominated low-temperature regime. Interestingly, we observe evidence for the coexistence of large and small polarons in the low-temperature ferromagnetic phase. We contrast these results with those obtained for EuB6, which is a low-T-c magnetic semiconductor with similar properties to the manganites, but with a substantially reduced carrier density and polaron energy.