Optimization of carriers by self-doping in Cu0.5Tl0.5Ba2Ca2Cu3−yMyO10−δ superconductor

The elements of fourth group (IV-A) of the periodic table have been doped in Cu0.5Tl0.5Ba2Ca2Cu3−yMyO10−δ (M=Si, Ge, Sn, and y=0, 1) samples to explore any possible electron-phonon interaction mechanism in oxide high temperature superconductors. We have observed that doped atoms with masses significantly different from that of Cu atom suppress the superconducting properties of the final compound. The postannealing experiments in flowing oxygen were also carried out to optimize the carriers density in the unit cell. After postannealing in oxygen the superconducting properties of all the samples are improved, which is most likely due to the optimization of carriers density in the material. It is observed from Fourier transform infrared absorption measurements that the CuO2 planar mode is hardened with the doping of Si, whereas, it is softened with the incorporation of Ge and Sn in the unit cell. The apical oxygen modes are softened with Si doping but these modes are hardened with the incorporation of Ge and Sn. It is most likely that the density of phonons is significantly altered due to increased anharmonic oscillations produced by heavier and lighter atoms in MO2/CuO2 (i.e., M=Sn,Si) planes, which suppress the phonon population from a desired level and hence the superconductivity. The shift in oxygen phonon modes after postannealing is most probably linked to the change in oxidation state of thallium.