SUPERCONDUCTIVITY AND RESISTIVITY STUDIES IN BI2SR2CA1-XMXCU2O8+Y (M=EU, DY, TM AND 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.6)

Electrical resistivity measurements have been performed on the Bi2Sr2Ca1-xMxCu2O8+y (M = Eu, Dy, Tm) system with various heat treatments. A gradual change in normal state resistivity (rho(n)) and T(c) has been observed with increasing dopant concentration. The results provide experimental evidence for a transition from a metallic behaviour to a variable range hopping regime at low temperatures. In the insulator regime (highly semiconducting-like) the resistivity behaviour is found to turn from 2D to 3D variable range hopping for x greater-than-or-equal-to 0.6. The results are discussed in terms of a theoretical model which determines the intrinsic competition between superconductivity and localization in an essentially localized system. T(c) data are compared with the earlier reported findings.