Interplane coupling and superconducting properties of Cu0.5Tl0.5Ba2(Ca2-xNax) Cu3O10-y (x= 0, 0.25, 0.5, 1) Samples

Abstract

I have synthesized Na doped Cu0.5Tl0.5Ba2(Ca2-xNax) Cu3O10-y (x=0, 0.25, 0.5, 1) samples for the studies of role of lower oxidation state at the inter-planar sites in the mechanism of high Tc superconductivity. The main objective of these studies is to understand the role of oxidation state of atoms at the inter- CuO2-plane sites in determining the phase coherence of the carriers and its role in the mechanism of high Tc superconductivity. The samples are synthesized by a two-step solid-state reaction method and their superconducting properties are studied by x-ray diffraction analysis, FTIR absorption measurements, resistivity measurements, Fluctuation-Induced Conductivity, and variable Range Hopping model for the semiconducting sample. These samples have shown orthorhombic crystal structure in which their c-axis length increases and the volume of unit cell suppresses with increased Na doping in the final compound. The FTIR absorption measurements of these samples have shown softening of apical oxygen mode of type Cu(1)-OA Cu (2) Tl-OA-CuO2 and CuO2 planar oxygen mode in Na-doped samples. Cu0.5Tl0.5Ba2(Ca2-xNax) Cu3O10-y (x=0, 0.25, 0.5) samples have shown metallic variation of resistivity from room temperature down to onset of superconductivity with onset of superconductivity around 102, 100, 98K and zero resistivity critical temperature at 98, 92, 90K, respectively. Cu0.5Tl0.5Ba2(Ca2-xNax) Cu3O10-y (x=1) sample have, however, shown semiconducting behavior that followed the variable range hopping conductivity with energy of activation around 4.2meV. Excess conductivity analyses of Cu0.5Tl0.5Ba2(Ca2-xNax) Cu3O10-y (x=0, 0.25, 0.5) samples have shown the values of coherence length along the c-axis ξc(0), the inter-layer coupling J, the Fermi velocity VF and the phase relaxation time τϕ of the carriers are not significantly altered with Na-doping in the final compound. Moreover, the values of Bc0(T), Bc1(T), and Jc(0) are not significantly altered in Na doped samples showing that the population of inadvertent defects acting as the pinning centers are not significantly changed in doped samples. The values of London penetration depth and the Ginzburg-Landau (GL) parameter κ=λ/ξ in Na-doped samples are, therefore, not significantly changed in Na-doped samples. These studies have demonstrated that the intrinsic superconductivity parameters remain unchanged despite variation in the X-ray diffraction spectra, the softening of phonon modes, and the suppression of the zero-resistivity critical temperature, displayed the importance of phase coherence of the carriers in various CuO2 planes in the mechanism of high Tc superconductivity. The doped Na-atoms promote the de-coupling of CuO2 planes that in turn suppress phase coherence of the carriers in the various conducting planes demonstrating importance of it in the mechanism of high Tc superconductivity.