Effect of Pb doping on the superconducting properties of (Cu _0.5-x Pb _x Tl _0.5)Ba ₂Ca ₂Cu ₃O _10-δ

The effect of Pb doping on the superconducting properties of (Cu _0.5-x Pb _x Tl _0.5)Ba ₂Ca ₂Cu ₃O _10-δ (x=0.0, 0.15, 0.25, 0.35) samples has been investigated. Lead is doped in Cu _0.5Tl _0.5Ba ₂O _4-δ charge reservoir layer and at the CuO ₂ planar sites. A multiphase material is achieved with the doping of Pb at the CuO ₂ planar sites; however, a predominant single-phase (Cu _0.5-x Pb _x Tl _0.5)Ba ₂Ca ₂Cu ₃O _10-δ (x=0.0, 0.15, 0.25, 0.35) material is synthesized with the doping of Pb at the charge reservoir layers. Formation 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 PbO ₂ planes is less likely. In the samples doped at the charge reservoir layer, the zero critical temperature [T _c (R=0)] is systematically depressed with the increased concentration of lead. The T _c (R=0) and magnitude of the diamagnetism are enhanced after post-annealing the samples in oxygen atmosphere. An apical oxygen mode is observed around 438 cm ^-1 in undoped samples, which is shifted to 457-461 cm ^-1 in the Pb-doped samples. This shift in the peak position is most likely associated with the connectivity of apical oxygen atoms with Pb atoms of (Cu _0.5-x Pb _x Tl _0.5)Ba ₂O _4-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 CuO ₂ 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 CuO ₂ planes, which increases the T _c (R=0) and the magnitude of diamagnetism. These experiments have shown that the density of mobile carriers plays a vital role in the mechanism of superconductivity and their depressed density suppresses the superconductivity parameters.