Effect of 3d metal ion doping on the structure and superconductivity of (Tl_(0.5)Pb_(0.5)Sr_2CaCu_2O_7

摘要

(Tl_(0.5)Pb_(0.5))Sr_2Ca(Cu_(2-x)M_x)O_7 (M = Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single '1212' phase. They crystallize in a tetragonal unit cell with a = 3.8028-3.8040 A and c= 12.0748-12.1558 A. In (Tl_(0.5)Pb_(0.5))Sr_2Ca(Cu_(2-x)M_x)O_7 system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around -6.5 and -7.0 K/at%, respectively. For (Tl_(0.5)Pb_(0.5))Sr_2Ca (Cu_(2-x)Zn_x)O_7 system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (-2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_(x~2-y~2) state due to the full d band of Zn ions.