LOCALIZATION EFFECTS IN TRANSITION-METAL DOPED BI2SR2CACU2O8-TEMPERATURE SUPERCONDUCTORS(Y HIGH)

Doping Bi2Sr2Ca1Cu2O8+y with Co causes a superconductor-insulator tran sition. We study correlations between changes in the electrical resist ivity rho(ab)(T) and the electronic bandstructure using identical sing le crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle reso lved photoemission these samples show dispersing band-like states. Co- doping decreases Te and causes and increase in the residual resistivit y. Above a threshold Co-concentration the resistivity is metallic (d r ho(ab)/dT > 0) at room temperature, turns insulating below a character istic temperature T-min and becomes superconducting at even lower temp erature. These changes in the resistivity correlate with the disappear ance of the dispersing band-like states in angle resolved photoemissio n. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experi mental features. The coexistance of insulating (d rho(ab)/dT < 0) norm al state behavior and superconductivity indicates that the superconduc ting ground state is formed out of spatially almost localized carriers .