Abstract
(Tl 0.5Pb 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 Å and c=12.0748–12.1558 Å. In (Tl 0.5Pb 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 0.5Pb 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.
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