PHOTOINDUCED CHARGE-CARRIERS IN INSULATING CUPRATES - FERMI GLASS INSULATOR, METAL-INSULATOR-TRANSITION AND SUPERCONDUCTIVITY

The results of photoexcitation experiments in partially doped, insulating cuprates are reviewed, and the physical implications of these ''photodoping'' experiments are discussed. The existence of a Fermi-glass insulating state at intermediate doping levels is thoroughly explored. The localized electronic states near the Fermi energy (E(F)) are characterized through transport, steady-state photoconductivity (including spectral response) and time resolved transient photodoping experiments (sub-nanosecond through microsecond). The experimental results indicate an Anderson-type metal-insulator transition in the doped cuprates; i.e., the transition from metal to insulator is dominated by disorder-induced localization. The Fermi energy can be shifted across the mobility edge either by increasing the doping level or by transient photoexcitation at high pump intensities, thereby causing the metal-insulator transition. The high-T(c) superconductors, therefore, can be characterized as disordered metals with the Fermi energy relatively close to the mobility edge (E(c)). Although the photo-generated carriers are generated homogeneously, the data indicate electronic phase separation into metallic ''droplets''. The temperature dependence of the photoinduced conductivity implies that these droplets become superconducting below the intrinsic transition temperature observed in the heavily doped metallic regime.