Electron–Phonon Interactions and Superconductivity in (Cu0.5Tl0.5)Ba2Ca3(Cu4−y Ti y )O12−δ (y = 0, 0.25, 0.50, 0.75, 1.0)

Abstract

We have used two-step solid state reaction method for the synthesis of (Cu0.5Tl0.5)Ba2Ca3(Cu4−y Ti y ) O12−δ (y = 0, 0.25, 0.50, 0.75, 1.0) superconductors at 880 °C. The oxygen contents in the samples were optimized by carrying out self-doping which is accomplished by post-annealed in flowing oxygen environment at 500 °C for approximately 5 h. The superconducting properties of all the samples were improved, after the self-doping of the carriers in the conducting planes. In the x-ray diffraction scans of the samples, the a-axis length of tetragonal unit cell increases whereas the c-axis decreases with increased Ti doping in the final compound. The Fourier transform infrared spectrometer (FTIR) absorption measurements of these samples have shown that the apical oxygen mode at 548 cm−1 and the planar oxygen mode at 596 cm−1 are softened with increased Ti doping. The origin of softening of planar oxygen mode lies in increase bond lengths of apical oxygen atoms promoted by larger covalent radius of Ti (1.32 A) atoms relative to Cu (1.17 A) atoms that in turn promotes the softening of the apical oxygen modes. Doped Ti (47.90 amu) atoms at the Cu (63.54 amu) sites initiate the an-harmonic oscillations resulting into the suppression of density of phonon modes. The suppression in the values of superconductivity parameters with Ti doping at the Cu sites shows the essential role of phonon in mechanism of high Tc superconductivity and hence the electron–phonons interactions. The excess conductivity analyses (FIC) of conductivity data of oxygen-post-annealed samples have shown decrease in the mean field critical temperature, coherence length along the c-axis, interlayer coupling, and Fermi velocity with increase Ti doping. However, the values of B c, B c1, and J c(0) increase with Ti doping, showing increase in the population of the pinning centers.