Ja n 19 99 Coexistence of localized and itinerant carriers near T C in calcium - doped manganites

We explore the possibility that polaronic distortions in the paramagnetic phase of La 0.67 Ca 0.33 MnO 3 manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting field-and temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the two-component system, and find that a limit with low percolation treshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility.