Metal-insulator transition in perovskite oxides: Tunneling experiments

In this paper we have investigated the composition-driven metal-insulator (MI) transitions in two AB${\mathrm{O}}_3$ classes of perovskite oxides (${\mathrm{LaNi}}_{\mathit{x}}$ ${\mathrm{Co}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{O}}_3$ and ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{Ta}}_{\mathit{y}}$ ${\mathrm{W}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{O}}_3$) 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 KTG(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‖${\mathit{k}}_{\mathit{B}}$T/e. $G(V)— has been found to follow a power law of the type G(V)=${\mathit{G}}_0$(1+{‖V‖/${\mathit{V}}^{\mathrm{*}}$ ${\mathrm{\}}}^{\mathit{n}}$) with ${\mathit{V}}^{\mathrm{*}}$=const and with n=0.5 for samples in the weak-localization region. However, as the critical region of the MI transition is approached ${\mathit{G}}_0$→0 and n→1. We also find that for samples lying in the weak-localization region Δ=${\mathit{eV}}^{\mathrm{*}}$ has a well-defined dependence on ${\mathrm{\sigma }}_0$, 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.