Variable-range hopping conductivity in La_1-xCa_xMnO₃

In ceramic La_1-xCa_xMnO₃ samples with 0⩽x⩽0.15 and the concentration of holes c = 0.18-0.22 between T = 77 and 340 K the resistivity, ρ, is shown to have an activated character both above and below the ferromagnetic Curie temperature, T_C. The temperature dependence of the local activation energy, E_loc (T) = dln ρ(T)/d(kT)^-1 gives evidence for a variable-range hopping conductivity over states of the Coulomb gap. The width of the gap Δ≈0.24 eV, the effective dielectric permeability κ≈3.6 and the carrier localization radius ξ≈5.1 Å in the paramagnetic phase are determined. The value of ξ suggests hopping of small lattice polarons at T>T_C. With decreasing temperature two successive transitions, at temperatures T_C' and T_C'', associated with divergence of the correlation length, are observed in the vicinity of T_C. They can be interpreted in terms of critical increase (as T approaches T_C' from higher temperatures) and decrease (when T shifts from T_C'' towards lower temperatures) of the radius of magnetic polarons, generated by a small fraction of the lattice polarons as T→T_C. The difference between T_C and the temperature of the first divergence of ξ, T_C'<T_C, gives evidence of percolative character of the FM-PM transition.