The new metal-insulator transitions and nanoscale phase separation in doped cuprates

We use the theory of extrinsic and intrinsic polarons to examine the new mechanisms of metal-insulating transitions (MITs) and nanoscale phase separation in hole-doped cuprates and propose a unified description of these interrelated phenomena. We argue that the relevant charge carriers in these polar materials are extrinsic (impurity-bound) and intrinsic large polarons. We show that the strong carrier-defect-phonon and carrier-phonon interactions together with the charge inhomogeneities are responsible for the carrier localization, the ordering of polaronic carriers and formation of their superlattices, the new MITs and nanoscale phase separation, which are accompanied by the stripe formation in the lightly to the slightly underdoped cuprates. We demonstrate that in doped cuprates La2-xSrxCuO4 and YBa2Cu3O7-δ the static (insulating) and dynamic (metallic) stripes coexist in the doping range x≃0.03-0.13. Our results are in quantitative agreement with experimental findings in these high-Tc materials.