Metal-insulator transition in perovskite oxides: tunneling experiments

Abstract

In this paper we have investigated the composition-driven metal-insulator (MI) transitions in two ABO₃ classes of perovskite oxides (LaNi_xCo_1−xO₃ and Na_xTa_yW_1−yO₃) in the composition range close to the critical region by using the tunneling technique. Two types of junctions (point-contact and planar) have been used for the investigation covering the temperature range 0.4 K<T<4.2 K. We find that in both classes of materials the junction conductance G(V) [=dI/dV] decreases near the zero-bias region as the MI transition is approached. However, there is a fairly strong thermal-smearing effect near the zero-bias region for ||V||<10k_BT/e. $G(V)- has been found to follow a power law of the type G(V)=G₀(1+{||V||/V^∗}ⁿ) with V^∗=const and with n=0.5 for samples in the weak-localization region. However, as the critical region of the MI transition is approached G₀→0 and n→1. We also find that for samples lying in the weak-localization region Δ=eV^∗ has a well-defined dependence on σ₀, the zero-temperature conductivity. The observed behavior can be explained either as a manifestation of depletion of density of states at the Fermi level as the MI transition is approached or as a manifestation of strong inelastic scattering in the junction region.