Metal-insulator transition in perovskite oxides: a low-temperature perspective

Abstract

A review is presented of recent experimental results of low temperature studies of composition driven metal-insulator transition in perovskite oxides of the ABO₃ class. The evolution of physical properties like conductivity, tunnelling, density of states and magnetoconductivity has been studied at low temperatures (T<10 K) because composition is varied so that the sample goes from the metallic state to the critical region through a weakly localized region. The results show an interesting interplay of disorder and correlation effects. Special attention has been paid to the critical region which is marked by very low conductivity and dσ/dT>0. In this region the following important observations emerge. (1) It is possible to have a metallic state [σ(T=0)=σ₀≠0] with σ₀/σ_Mott<<1 and dσ/dT>0. (2) At T<2 K the conductivity follows a power law σ∼T^ν, where the exponent can be related to the finite frequency response of a zero temperature phase transition. (3) The Coulomb interaction plays a major role and evidence from tunnelling experiments suggests that a gap in the density of states at the Fermi level opens up continuously as the critical region is approached from the metallic side. (4) The magnetoconductivity is relatively smaller in the metallic and the weakly localized region (except the hole-doped LaMnO₃ and related systems) but becomes very large at the critical region.