Variable-range hopping conductivity in thin film of the ladder compound [Ca1+δCu2O3

Abstract

Low-temperature charge transport in [Ca1+δCu2O3]4 films with δ=0 and 0.08 is investigated at T=1.4–290 K in magnetic fields of B=0–30 T. Between Tv≈20 and 40 K, depending on δ, and Tv′≈2–3 K three-dimensional (3D) Mott variable-range hopping (VRH) conductivity is observed. Deviations from 3D Mott VRH conductivity below Tv′ can be attributed to the onset of the Shklovskii–Efros VRH regime, accompanied by violation of the 3D hopping condition, Rh≪d (where Rh is the mean hopping length and d is the thickness of the film). The density of the localized states (DOS) at the Fermi level, g3(μ)≈4.4×1020 eV−1 cm−3, the values of the localization radius, α≈46 and 34 Å, the concentration of sites involved in hopping charge transfer, N≈9.9×1018 and 9.0×1018 cm−3, the width of the band of localized electron states, 2W≈22 and 20 meV, and the width of the Coulomb gap in the DOS spectrum, 2Δ≈1 and 1.6 meV are obtained for films with δ=0 and 0.08, respectively. These values characterize the films as being near the 3D metal–insulator transition given by the critical concentration NC≈1.3×1019 cm−3.