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 $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ system as a function of temperature and doping $(0.2\mathrm{}<~x<~0.5).$ The metal-semiconductor transition in the $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ 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 ${\mathrm{EuB}}_6,$ which is a low-$T_c$ magnetic semiconductor with similar properties to the manganites, but with a substantially reduced carrier density and polaron energy.