METAL-INSULATOR-TRANSITION IN PEROVSKITE OXIDES - A LOW-TEMPERATURE PERSPECTIVE

A review is presented of recent experimental results of low temperatur e studies of composition driven metal-insulator transition in perovski te oxides of the ABO(3) class. The evolution of physical properties li ke 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 interes ting interplay of disorder and correlation effects. Special attention has been paid to the critical region which is marked by very low condu ctivity and d sigma/dT>0. In this region the following important obser vations emerge. (i) It is possible to have a metallic state [sigma(T=0 )=sigma(0) not equal 0] with sigma(0)/sigma(Mott)much less than 1 and d sigma/dT>0. (2) At T<2 K the conductivity follows a power law sigma similar to T-v, where the exponent can be related to the finite freque ncy response of a zero temperature phase transition. (3) The Coulomb i nteraction 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 metalli c side. (4) The magnetoconductivity is relatively smaller in the metal lic and the weakly localized region (except the hole-doped LaMnO3 and related systems) but becomes very large at the critical region.