Variable-range hopping conductivity in La1-xCaxMn1-yFeyO3:: evidence of a complex gap in density of states near the Fermi level

The resistivity, rho, of ceramic La1-xCaxMn1-yFeyO3 with x = 0.3 and y = 0.0-0.09 is found to obey, between a temperature T-v approximate to 310-330 K and the ferromagnetic-to-paramagnetic transition temperature, T-C = 259119 K (decreasing with the Shklovskii-Efros-type variable-range hopping conductivity law, rho(T) = rho(0)(T)exp[(T-0/T)(1/2)], This behaviour is governed by generation of a soft Coulomb gap Delta approximate to 0.42 eV in the density of localized states and a rigid gap delta(T) approximate to delta(T-v)(T/T-v)(1/2) with delta(T-v) approximate to 0. 16, 0.13 and 0.12 eV at y = 0.03, 0.07 and 0.09, respectively. Deviations from the square root dependence of delta(T), decreasing when y is increased, are observed as T --> T-C. The prefactor of the resistivity follows the law rho(0)(T) similar to T-m, where in changes from 9/2 at y = 0 to 5/2 in the investigated samples with y = 0.03. 0.07 and 0.09, which is connected to introduction of an additional fluctuating short-range potential by doping with Fe.