Theoretical considerations on the transport property of La_1-xCa_xMnO₃ in the paramagnetic regime

The transport properties of La_1-xCa_xMnO₃ single crystal in the paramagnetic state is investigated from both the theoretical and the experimental point of view. The temperature dependence of the resistivity calculated by using the Monte Carlo technique within the framework of the double-exchange model shows a sharp peak structure and a large magnetoresistance under the application of the magnetic field in the vicinity of the metal-insulator transition temperature, T_MI. The experimental result for the resistivity can be fitted well with both a variable-range hopping (VRH) and a small-polaron hopping (SPH) conduction in the paramagnetic state, whereas the Monte Carlo simulation works well with a VRH model but displays some deviations close to T_MI with a SPH model. This is largely due to poor knowledge of the lattice effect in the double-exchange model. On the basis of our analysis on the transport property above T_MI, we propose that both the spin-disorder scattering and the lattice distortion should be considered to explain the nature of the localization in the paramagnetic regime.