Superconducting Order Parameter Fluctuation in Doped and Composite Cuprate Superconductors

Abstract

This thesis describes work in the area of high temperature superconductors. Attempts have been made for the synthesis of doped and composites of YBCO superconductors to study the electrical and magnetic properties.The general objective of this work is to look at the effect of inhomogeneities and secondary phases on Superconducting order parameter fluctuation(SCOPF)in doped and composite superconductors.The influence of inhomogeneities on the microstructural and electrical properties is investigated for the simultaneous incorporation of Ga3+ and Zn2+ to the Cu-site of YBa2Cu3O7-δ.The evolution of anisotropy has also been analyzed for the effect of both hole doping to the Y- site through Ca,and CuO-chain electronic state anisotropy by Zn doping to the Cu-site through SCOPF analysis.Conductivity fluctuation analysis performed around Tc, provides evidence that dimensionality of fluctuations depends on the doping level in the mean field regimes.The inclusion of sub-μ sized BZO particles and nano-BaTiO3 to the YBCO matrix bring about structural as well as dimensional fluctuation changes in the electrical transport properties.The characteristic scaling of the curves obtained from excess conductivity (Δσ) as a function of reduced temperature, ε for YBCO + x BaZrO3 composites (x = 0, 1, 2.5, 5 and 10 wt.%) in 8 T magnetic field showed a crossover between two to three-dimensional (TLD) behaviors in the mean-field region but the critical region is not witnessed due to smearing out of data points in high magnetic field like 8 T. Study of magneto-resistance and vortex dynamics in YBCO superconductors has been done to get
requisite information for properties such as critical current density, quantized flux trapping and activation energy. The activation energy and critical current density associated with magnetic field gives the vital inferences on inter and intra-granular behaviour. The resistive broadening in the tailing region due to application of magnetic field has been explained on the Ambegaokar–Halperin (AH) phase-slip theory. When magnetic field is applied to type-II superconductors magnetic flux can penetrate in the form of Abrikosov vortices flux lines, each carrying a quantum of magnetic flux Ф0 0 = h / 2e . As density of flux lines increases due to application of magnetic field these vortices form flux line lattice (FLL) due to interaction. In order to have a detail understanding of flux pinning mechanism, critical current density studies have also been carried out in YBCO + x BaZrO3 (BZO) composites. The flux pinning effect of submicron BZO particles in the composites enhances the current density Jc from 8 x 104 Acm-2 for pristine YBCO to 2 x 105 A-cm-2 for YBCO + 5 wt % BZO composite samples in field varying from 1–16 T at a temperature of 40 K.