Synthesis of Cd substituted (Cu0.5Tl0.5)Ba2Can-1(Cun-yCdy)O2n+4-δ (n=3, 4) samples and study of their superconducting properties

Abstract

(Cu0.5Tl0.5)Ba2(Can-1-xMgx)(Cun-yCdy)O2n+4-δ (n=3, 4) high Tc superconductors have been synthesized by solid state reaction method. The samples were characterized by dc- electrical resistivity, ac-magnetic susceptibility, XRD, FTIR, and dielectric measurements. The main objective of these experiments was to investigate the possible existence of electron-phonon interactions in the mechanism of high Tc superconductivity. Owing to our previous studies in which we have achieved enhanced superconductivity by doping Zn at the Cu-sites in (Cu0.5Tl0.5)Ba2(Can-1-xMgx)(Cun-yZny)O2n+4-δ superconductors, we have doped heavier Cd atoms at the Cu sites in (Cu0.5Tl0.5)Ba2(Can-1- xMgx)(Cun-yCdy)O2n+4-δ superconductors. Since Cd (4d10) and Zn (3d10) have similar ground state electronic configurations but the atomic mass of Cd is twice as that of Zn; it was expected that similar enhancement of superconductivity will be observed in Cd- doped samples if electron-phonon-interactions were not essential for mechanism of superconductivity in these compounds. The XRD of (Cu0.5Tl0.5)Ba2(Can-1-xMgx)(Cun- yCdy)O2n+4-δ samples revealed orthorhombic structure following PMMM space group along with a decrease in the c-axis length with the enhanced doping of Cd and Mg. From the resistivity measurements, a sequential suppression of the Tc(R=0) was observed with the increasing doping concentration. The ac-susceptibility measurements have shown suppression of the transition temperature from normal to superconducting state and an overall decrease in the magnitude of diamagnetism with the increasing x and y-contents. The Tc(R=0) and magnitude of diamagnetism in (Cu0.5Tl0.5)Ba2Ca3(Cu4-yCdy)O12-δ samples were improved after annealing in oxygen atmosphere. This improvement most likely arises due to the incorporation of oxygen in the charge reservoir layer which optimizes the carriers’ density in the superconducting CuO2 planes. In the FTIR absorption spectra, the apical oxygen modes are systematically changed with the increasing concentration of both the Cd and Mg which clearly indicate the incorporation of Cd and Mg in the unit cell. Fluctuation induced conductivity (FIC) analyses of the Mg- free (Cu0.5Tl0.5)Ba2Ca3(Cu4-yCdy)O12-δ samples were carried out in the neighborhood of Tc and above by applying the Aslamazov-Larkin (A-L) and the Lawrence-Doniach (L-D) models. Coherence length along the c-axis (ξc(0)), phase relaxation time (τφ), and the vii Fermi velocity of the carriers (VF), were calculated from these analyses. The 3D to 2D cross-over temperature (T3D-2D), and the temperature at which the resistivity curve starts deviation from linear behavior (T*), are shifted to lower temperatures while width of the transition region (∆Tc) is shrunken with the enhanced Cd-concentration. It is most likely that variation in the apical oxygen modes of vibration are associated with the damped harmonic oscillations of the heavier Cd-atoms (ions) in the CuO2 planes, which suppress the population of the desired phonons (required for electron-phonon interactions) from its optimum level. This reduced population of the desired phonons causes suppression in the density of the Cooper pairs which ultimately result into reducing the magnitude of superconductivity in the final compound. In the FIC analyses results, the suppression of T3D-2D, T*, and the narrowing width of the superconducting transition with the enhanced Cd-doping strongly support this argument. Further, the temperature and frequency dependent measurements of the capacitance (C) and conductance (G) were carried out in the frequency range of 10 KHz–10 MHz at various temperatures between 80 and 300K for Mg-free samples, from which the dielectric constants (ε′, ε′′) and ac-conductivity (σac) were calculated. Negative capacitance (NC) was observed for all the samples of the series. Large values of NC were observed at lower temperature and frequencies which may be attributed to the reduced thermal vibrations and higher polarizability of the material. The polarization of the samples most likely arises due to the displacement of mobile charges in the CuO2/CdO2 planes by the applied electric field relative to the static charges at Ba2+, Tl3+, and Cu2+ sites in the charge reservoir layer. But with the increasing concentration of Cd, the NC, dielectric constant and the ac-conductivity are systematically decreased at low temperatures and frequencies. The decrease in the values of ε′ and σac are most likely related to the induced anharmonic oscillations of the heavier Cd atoms in CuO2/CdO2 planes reducing the polarizability of the final compound and eventually suppresses the dielectric properties. All these studies strongly suggest that the role of electron-phonon interactions is essential for the mechanism of high Tc superconductivity in oxide superconductors.