No description available.

No description available.

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.

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published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy