Effect of Pb Doping on the Superconducting Properties of (Cu0.5−x Pb x Tl0.5)Ba2Ca2Cu3O10−δ

Abstract

The effect of Pb doping on the superconducting properties of (Cu0.5−xPbxTl0.5)Ba2Ca2Cu3O10−δ (x = 0.0, 0.15, 0.25, 0.35) samples has been investigated. Lead is doped in Cu0.5Tl0.5Ba2O4−δ charge reservoir layer and at the CuO2 planar sites. A multiphase material is achieved with the doping of Pb at the CuO2 planar sites; however, a predominant single-phase (Cu0.5−xPbxTl0.5)Ba2Ca2Cu3 O10−δ (x = 0.0, 0.15, 0.25, 0.35) material is synthesized with the doping of Pb at the charge reservoir layers. For- mation of multiphase material with the doping of lead at the planar sites showed that its substitution at the planar site is not possible and the formation of PbO2 planes is less likely. In the samples doped at the charge reservoir layer, the zero critical temperature (Tc (R = 0)) is systematically depressed with the increased concentration of lead. The Tc (R = 0) and magnitude of the diamagnetism are enhanced after post- annealing the samples in oxygen atmosphere. An apical oxy- gen mode is observed around 438 cm −1 in undoped samples, which is shifted to 457-461 cm −1 in the Pb-doped sam- ples. This shift in the peak position is most likely associated with the connectivity of apical oxygen atoms with Pb atoms of (Cu0.5−xPbxTl0.5)Ba2O4−δ (x = 0.0, 0.15, 0.25, 0.35) charge reservoir layers. The presence of Pb in the charge reservoir layer and its increased concentration, somehow, stops the flow of mobile carriers to the conducting CuO2 planes. The decreased density of mobile carriers diminishes the critical temperature and magnitude of diamagnetism in the final compound. The increased oxygen diffusion in the unit cell achieved by post-annealing in oxygen replenishes the concentration of carriers in conducting CuO2 planes, which increases the Tc (R = 0) and the magnitude of dia- magnetism. These experiments have shown that the density of mobile carriers plays a vital role in the mechanism of su- perconductivity and their depressed density suppresses the superconductivity parameters.