Metal-insulator transitions in cuprate high-temperature superconductors

Abstract

We study the possible mechanisms of carrier localization and metal–insulator transitions (MITs) in inhomogeneous hole-doped cuprates. We show that the insulating state of doped cuprates is different from the Mott insulating state of undoped cuprates characterized by the charge-transfer (CT) gap. The dopant- and carrier-driven inhomogeneities favor the charge segregation and ordering in the form of a three-dimensional (3D) network of carrier-rich and carrier-poor stripes. The specific charge ordering results in the formation of different superlattices and energy bands of localized carriers in inhomogeneous high-Tc cuprates. The energy bands associated with the charged stripes develop in the middle of the CT gap. We argue that the in-gap impurity and polaronic states (bands) develop into metallic states at some critical doping levels. We use the uncertainty relation to obtain the specific conditions for the MITs in doped cuprates. The applicability limits of these MITs in cuprates are clarified. Our results are in good agreement with the existing experiments on La-based and other cuprates.