T_c suppression and conduction mechanisms in Bi_2.1Sr_1.93Ca_0.97-xR_xCu₂O_8+y (R= Pr, Gd, and Er) systems

Abstract

Systematic substitutional studies in the Bi_2.1Sr_1.93Ca_0.97-xR_xCu₂ O_8+y (R=Pr,Gd,Er; 0≤x≤0.3 in steps of 0.05 and 0.4≤x≤0.97 in steps of 0.1) system were carried out in order to determine the effect 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 of Pr is limited to x=0.6 under the preparative conditions employed. Resistivity and ac susceptibility studies have shown that superconductivity persists up to x=0.4 and a metal-semiconductor transition occurs for x>0.4. The most interesting observation is that the rate of T_c suppression for the superconducting phases is found to be identical 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≤x≤0.97 exhibit the phenomenon of Mott's variable-range-hopping mechanism (VRH). The physical parameters related to VRH such as localization length (α⁻¹), hopping range (R), and activation energy (W) for conduction, have been evaluated and discussed in detail.