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Localization Effects in Bi 2 Sr 2 Ca(Cu,Co) 2 O 8+y High Temperature Superconductors

with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity ρ ab (T) and the electronic bandstructure using identical single crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle resolved photoemission these samples show dispersing band-like states. Co-doping decreases T C and causes and increase in the residual resistivity. Above a threshold Co-concentration the resistivity is metallic (d ρ ab /dT >0) at room temperature, turns insulating below a characteristic temperature T min and becomes superconducting at even lower temperature. These changes in the resistivity correlate with the disappearance of the dispersing band-like states in angle resolved photoemission. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experimental features. Therefore the T C reduction in 3d-metal doped high-temperature superconductors is not caused by Abrikosov Gor’kov pairbreaking but by spatial localization of the carriers. The observed suppression of T C indicates that the system is in the homogenous limit of the superconductor-insulator transition. The coexistance of insulating (d ρ ab /dT <0) normal state behavior and superconductivity indicates that the superconducting ground state is formed out of spatially almost localized carriers.