LOCALIZATION AND INTERACTION EFFECTS DURING SUPERCONDUCTOR-INSULATOR TRANSITION OF BI2SR2CA1-XGDXCU2O8+D

An extensive study has been made of the resistivity of superconducting and semiconducting samples of the Bi2Sr2Ca(1-x)Gd(x)Cu2O8+d system. The effect of changing the Gd concentration and the annealing conditions is found to be a gradual change in the normal-state resistivity measured at 280 K (rho-n). With the increase in rho-n, T(c) is depressed. The form of the T(c) depression is found to be consistent with a theory of localization and interaction effects on the superconductivity. In the insulator regime, however, the resistivity is due to variable-range hopping (VRH), the dimensionality of which changes from two to three as the rho-n increases away from the superconductor-insulator boundary. The observation of the two-dimensional VRH behavior in juxtaposition with the superconductivity is in qualitative agreement with a theoretical model that considers the competition between superconductivity and localization in a disordered system. When rho-n > 1 OMEGA cm, the resistivity variation is found to be dominated by multiphonon-assisted hopping.