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

Abstract

The resistivity, ρ,of ceramic La1−xCaxMn1−yFeyO3 with x = 0.3 and y = 0.0–0.09is found to obey, between a temperature Tv ≈ 310–330K and the ferromagnetic-to-paramagnetic transition temperature, TC = 259–119 K(decreasing with y),the Shklovskii–Efros-type variable-range hopping conductivitylaw, ρ(T) = ρ0 (T) exp [(T0 /T)1/2 ].This behaviour is governed by generation of a soft Coulomb gap Δ ≈ 0.42eV in the density of localized states and a rigid gapδ(T) ≈ δ(Tv)(T/Tv)1/2 with δ(Tv) ≈ 0.16, 0.13 and0.12 eV at y = 0.03,0.07 and 0.09, respectively. Deviations from the square root dependence of δ(T), decreasing wheny is increased, areobserved as T → TC.The prefactor of the resistivity follows the law ρ0 (T) ∼ Tm, wherem changesfrom 9/2 aty = 0 to5/2 in the investigatedsamples with y = 0.03,0.07 and 0.09, which is connected to introduction of an additional fluctuatingshort-range potential by doping with Fe.