T(C) SUPPRESSION AND CONDUCTION MECHANISMS IN BI2.1SR1.93CA0.97-XRXCU2O8+Y (R=PR, GD, AND ER) SYSTEMS

Systematic substitutional studies in the Bi2.1Sr1.93Ca0.97-xRxCu2O8+y (R = Pr,Gd,Er; 0 less-than-or-equal-to x less-than-or-equal-to 0.3 in steps of 0.05 and 0.4 less-than-or-equal-to x less-than-or-equal-to 0. 97 in steps of 0.1) system were carried out in order to determine the effects of the magnetic moment and ionic radius of the rare-earth ion on the T(c) suppression rate. X-ray-diffraction studies indicate that the solid solubility of Gd and Er exists up to x = 0.97 whereas that o f Pr is limited to x = 0.6 under the preparative conditions employed. Resistivity and ac susceptibility studies have shown that superconduct ivity persists up to x = 0.4 and a metal-semiconductor transition occu rs for x > 0.4. The most interesting observation is that the rate of T (c) suppression for the superconducting phases is found to be identica l for all rare earths. We have explained that hole filling rather than Abrikosov-Gor'kov pair breaking is responsible for the decrease in T( c). The insulating phases with 0.5 less-than-or-equal-to x less-than-o r-equal-to 0.97 exhibit the phenomenon of Mott's variable-range-hoppin g mechanism (VRH). The physical parameters related to VRH such as loca lization length (alpha-1), hopping range (R), and activation energy (W ) for conduction, have been evaluated and discussed in detail.