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

The resistivity, ρ, of ceramic La 1−x Ca x Mn 1−y Fe y O 3 with x = 0.3 and y = 0.0–0.09 is found to obey, between a temperature T v ≈ 310–330 K and the ferromagnetic-to-paramagnetic transition temperature, T C = 259–119 K (decreasing with y ), the Shklovskii–Efros-type variable-range hopping conductivity law, ρ( T ) = ρ 0 ( T ) exp [( T 0 / 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 ) ≈ δ( T v )( T / T v ) 1/2 with δ( T v ) ≈ 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 → T C . The prefactor of the resistivity follows the law ρ 0 ( T ) ∼ T m , 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.