Variable-range hopping conductivity in La1-x Cax Mn1-y Fey O3: evidence of a complex gap in density of states near the Fermi level

The resistivity, ρ, of ceramic La1-xCaxMn1-yFeyO3 with x 0.3 and y=0.0-0.09 is found to obey, between a temperature Tv∼310-330 K and the ferromagnetic-to-paramagnetic transition temperature, TC 259-119 K (decreasing with y), the Shklovskii-Efros-type variable-range hopping conductivity law, ρ(T)=ρ0 (T) exp [(T0 /T)1/2 ]. This behaviour is governed by generation of a soft Coulomb gap Δ∼ 0.42 eV in the density of localized states and a rigid gap δ(T)∼ (Tv)(T/Tv)1/2 with δ(Tv)∼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 δ(T), decreasing when y is increased, are observed as T→TC. The prefactor of the resistivity follows the law ρ0(T)∼Tm, where m 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. (author)