Ti-doped (Cu0.5Tl0.5)Ba2Ca2 (Cu3−xTix)O10−δ (x = 0, 0.25, 0.50, 0.75) superconductors

Abstract

Ti-doped (Cu0.5Tl0.5)Ba2Ca2 (Cu3−xTix)O10−δ (x=0, 0.25, 0.50, 0.75) samples have been synthesized at 860°C by following two step solid state reaction methods and studied their superconducting properties by X-ray diffraction, resistivity, ac-susceptibility, FTIR absorption measurements. Ti-doped samples have shown orthorhombic crystal structure and the cell parameters increase with the increase of Ti-contents. The Tc(R=0) and onset of diamagnetism increases with increase Ti-doping in the final compound. The magnitude of superconductivity, however, increases for Ti-doping of y=0.25,0.75. The numbers of carriers in the conducting planes have been optimized by carrying out post-annealing of the samples in oxygen atmosphere. After the post-annealing the Tc(R=0) is improved in un-doped samples whereas it is suppressed in all Ti-doped samples. After post annealing in oxygen atmosphere, the magnitude of superconductivity (measured in emu/gm.Os) remains unchanged in un-doped samples whereas it is suppressed in all Ti-doped samples. In FTIR absorption measurements, the CuO2/TiO2 planar oxygen modes are slightly hardened in Ti-doped samples. The most likely reason for the hardening of the planar oxygen modes is decrease in the mass of Ti (47.90amu) in comparison with Cu (63.54amu) atoms; the atoms with lighter-mass vibrate at higher frequency. It is most likely that the existence of atoms of different masses in CuO2/TiO2 planes may suppress the density of phonon population due to setting-in of an-harmonic oscillations. The phonons of harmonic oscillation with wave vector q interact with q′ of an-harmonic oscillation and produce a new phonon with wave vector Q. The phonons with wave vector Q do not contribute in the Cooper-pair formation. It is most likely that a decrease in the population of desired phonons required for higher Tc occurs that in turn suppress the density of the Cooper-pair which in turn promote a decrease in the magnitude of superconductivity. These studies indicate the essential role of electron–phonons-interactions in the mechanism of high Tc superconductivity.