X-ray photo-emission studies of Cu1−xTlxBa2Ca3Cu4O12−y superconductor thin films

X-ray photo-emission spectroscopy (XPS) studies of Cu1−xTlxBa2Ca3Cu4O12−y superconductor thin films have been carried out for understanding the mechanism of superconductivity and to find out the reasons for the increase of zero resistivity critical temperature Tc(R = 0) with post-annealing in a nitrogen atmosphere. It is observed from these studies that reduction of charge state of thallium is a source of doping of carriers to the CuO2 planes. The reduced charge state of thallium (i.e. Tl1+) promotes lower oxygen concentration in the charge reservoir layer, which possibly results in movement of electrons to the conducting CuO2 planes. The higher density of electrons in the CuO2 planes optimizes the hole concentration “np” in these planes. The reduced charge state of thallium in the Cu1−xTlxBa2O4−δ charge reservoir layer is also supported by a shift of the Ba 3d5/2 and Ba 3d3/2 XPS lines to lower binding energies with post-annealing in nitrogen atmosphere. Moreover, the movement of the valance band spectrum to lower binding energies suggested that the electronic density of states changes in the valance band with the post-annealing in nitrogen, which possibly becomes a source of doping of carriers to the CuO2 planes. The increased doping of electrons to the CuO2 planes optimizes the Fermi-vector KF and Fermi-velocity VF of the carriers and increases the Tc(R = 0) of final compound.