No description available.

No description available.

No description available.

Solid solutions of general formula La1.5-xBa1.5+x-yAyCu3Oz (A=Ca, Sr) have been studied. These form over the complete composition range 0x0.5, 0y0.5, excluding compositions centred on the La-123 composition, LaBa2Cu3Oz. The solid solutions have properties which range from semiconducting (x=y=0) to superconducting, with Tc (max.) = 78K for the composition LaBa1.5Ca0.5Cu3O7.01, and were found to be tetragonal across their entire range. Rietveld refinement of X-ray powder data has shown that whereas Sr occupies the Ba site, Ca occupies the La site with concomitant displacement of La onto the Ba site. The magnetic behaviour and oxygen contents of these solid solutions are discussed. The variation of Tc with oxygen content has been studied for the composition LaBa1.5Ca0.5Cu3Oz; for quenched samples, Tc varies linearly with oxygen content, but slow cooled samples show a "Double Plateau" behaviour, and thus Tc depends critically upon the cooling regime. Analogues RE1.5-xBa1.5+x-yCayCu3Oz (RE = Nd, Pr, Sm, Eu, Gd and Dy) have also been studied; the extent of the solid solution decreases with decreasing size of rare earth. For Dy, no appreciable solid solubility was observed. For Nd and Pr, the solid solutions centre on the 336 composition La1.5Ba1.5Cu3Oz; for the smaller rare earths this composition cannot be prepared in air, and the solid solutions are centred on the orthorhombic, REBa2Cu3Oz phase. All were found to be superconducting except the Pr-based solid solutions; results suggest that these contain Pr4+ as well as Pr3+. In the solid solutions RE1+xBa2-xCu3Oz (x0, RE = Sm, Eu and Gd), critical temperatures exhibit a marked dependence on the size of the A-site cations.

Graduation date: 1998

Graduation date: 1996

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

The properties of superconducting YBa₂Cu₃O₇₋(δ) thin films have been studied. Films have been prepared by multilayer deposition followed by ex situ furnace annealing. Deposition consists of a combination of dc triode sputtering from metallic targets of Y and Cu and thermal evaporation from a BaF₂ source. Superconducting and structural properties of the films strongly depend on the annealing conditions. Several heat treatment cycles were investigated, as well as different compositions. Best results were obtained for films deposited on (100) SrTiO₃ substrates, exhibiting T(c)(onset) as high as 92 K and zero resistance by 85 K. The second part of this dissertation examines the properties of ceramic Nd₂₋ₓCeₓCuO₄₋(δ) and Nd₁ͺ₈₅Ce₀ͺ₁₅(Cu₁₋(y)Zn(y))O₄₋(δ) bulk samples. Superconducting properties are examined as a function of x and y. Accurate (± 0.001 Å) lattice parameter calculations are performed from experimental x-ray diffraction data. Comparisons with previous zinc-doping studies in the hole superconducting material La₁ͺ₈₅Sr₀ͺ₁₅CuO₄₋(δ) are made. Theoretical implications and the question of electron-hole symmetry in the copper oxide superconductors are also discussed.

In this thesis, we study the effects of doping in two-dimensional quantum antiferromagnets. We consider cases where the undoped parent compound is a Mott insulator with long-range antiferromagnetic order and focus on the low-doping situations. The limit of localized impurities is studied in a system consisting of a host magnet and two additional weakly coupled spins. We derive the effective Hamiltonian describing the interaction between these impurities as a function of their distance and show that it exhibits xyz anisotropy, leading to NMR and EPR line broadening. We calculate the magnetization disturbance in the host magnet induced by a single impurity and find that it always enhances Neel order. Relaxing the localization constraint, we investigate the single-hole dynamics of the t-J model on the honeycomb lattice. Using exact diagonalizations, series expansion and the self-consistent Born approximation, we calculate the quasi-particle dispersion, bandwidth and residues and compare our findings with the well-established results for the square lattice. Similar to the latter case, we find an almost flat band along the edges of the magnetic Brillouin zone and well-defined hole pockets around the corners. The most important part of this thesis is devoted to the magnetic properties of lightly doped La2-xSrxCuO4, the simplest and by far most studied cuprate superconductor. Starting from the undoped parent compound, we calculate the spin-wave spectrum and the spin-flop transitions in a uniform magnetic field at zero temperature. We then consider the low-doping regime and derive the effective field theory describing the spin dynamics in insulating La2-xSrxCuO4, x ≤ 0.055, at low temperature. The spin structure resulting from the spiral solution of the extended t-J model, obtained by taking into account the Coulomb trapping of holes by Sr ions, is confined in the copper-oxide planes. Our solution explains why the incommensurate structure is directed along the orthorhombic b axis and allows us to calculate the positions and shapes of the neutron scattering peaks numerically. These results are in perfect agreement with experimental data. We also show that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the spin-glass ground state.

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.