LOCALIZATION EFFECTS IN CO-DOPED AND NI-DOPED BI2SR2CACU2O8+GAMMA

We have studied the influence of Co and Ni doping on the electronic ba nd structure and the transport properties of high-temperature supercon ductors. The doping has dramatic effects on both the normal and the su perconducting state. Upon doping the residual resistivity increases st rongly. For sufficiently high Co concentration the temperature depende nce rho(T) turns from a simple linear dependence to one displaying a m inimum around T-min = 190 K. This minimum is followed by an upturn (d rho/dT<0) and by a transition to a superconducting state at even lower temperature (T-c = 66 K. These changes in the resistivity are accompa nied by an almost complete disappearance of the dispersing bandlike st ates in angle-resolved photoemission. We show that spatial localizatio n of the carrier states through the doping-induced disorder provides a consistent explanation of the experimental results. However, none of the standard scattering mechanisms can explain the observed localizati on. Because the increase in the residual resistivity is higher than th e unitary limit, the localization has to be through a cooperative effe ct. This rules out standard Abrikosov-Gor'kov or Kondo effects. We dis cuss the observed coexistence of localization and superconductivity in terms of the relevant length scales and compare it to theoretical pre dictions.