XPS studies of Cu1−xTlxBa2Ca2Cu3O10−y superconductor thin films

X-ray photoemission spectroscopy (XPS) of Cu1−xTlxBa2Ca2Cu3O10−δ (Cu1−xTlx-1223) superconductors thin films is studied. These films were prepared by amorphous phase epitaxy method on the SrTiO3 substrate. This compound has Cu1−xTlxBa2O4−δ charge reservoir layer and three CuO2 planes. The atoms in the CuO2 planes have strong covalent bonding, while the some of the atoms in Cu1−xTlxBa2O4−δ charge reservoir, such as oxygen (Oδ) have predominantly secondary bonding and could be removed from the unit cell. The deliberate or inadvertent removal of the oxygen from the unit cell may change the charge state of the constituent atoms in the charge reservoir layer. The charge state of the atoms in the unit cell, ultimately determine the carriers concentration, the shape of the Fermi surface and hence control the mechanism of superconductivity. The doping via charge reservoir layer of Cu1−xTlx-1223 films had been carried out by post-annealing of the samples in the N2 atmosphere at 650 °C. It was observed that the critical temperature of thin films increases with the post-annealing at this temperature. In the present studies we have carried out XPS experiments to under stand the mechanism to the microscopic level. It is observed from these studies that before annealing the thallium in Cu1−xTlx-1223 superconductor thin films was in Tl3+ oxidation state, which changes its oxidation state from Tl3+ to Tl2+ and Tl1+. The changed oxidation state of the thallium increases the density of the carriers in the CuO2 planes, which increases their Fermi-vector KF and hence their Fermi-velocity vF. These parameters in turn increase the critical temperature of the material. In the present studies the Tc(R = 0) has increased from below 77 K to 92 K with post-annealing in nitrogen atmosphere.