Mechanisms of hopping conductivity in weakly doped La1-xBaxMnO3

The resistivity, ρ, of ceramic La1-xBaxMnO3 with x = 0.02-0.10 corresponding to the concentrations of holes c∼0.15-0.17 displays an activated behaviour both above and below the paramagnetic to ferromagnetic transition temperature TC = 175-209 K, obtained from measurements of the magnetization. Above T∼310-390 K ρ(T,x) is determined by nearest-neighbour hopping of small polarons with activation energy Ea 0.20-0.22 eV. Below the onset temperature Tv = 250-280 K, depending on x, a Shklovskii-Efros-like variable-range hopping conductivity mechanism, governed by a soft temperature independent Coulomb gap, Δ∼0.44-0.46 eV, and a rigid gap, δ(T), is found. For the range T∼50-120 K, δ(T) is connected to the formation of small lattice polarons in conditions of strong electron-phonon interaction and lattice disorder. The rigid gap obeys a law δ(T)∝T1/2 within two temperature intervals above and below TC, exhibits an inflection at TC and reaches at Tv a value of δv∼0.14-0.18 eV. Such behaviour suggests a spin dependent contribution to δ(T). The localization radius of the charge carriers, a, has different constant values within the temperature intervals where δ(T)∼T1/2. With further decrease of T, a increases according to the law expected for small lattice polarons