Thesis (MScIng)--University of Stellenbosch, 2006. / Bibliography.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1987. / Vita. Abstract. Includes bibliographical references (leaves 48-49). Also available via the Internet.

No description available.

High TC superconductivity is regarded as one of the most challenging problems in the physical sciences of the 20th century. The ultimate goal is to create room temperature superconductors that have the potential to revolutionize many aspects of modern technology. The La-214 family of the cuprate high TC superconductors is comprised of single layers of CuO2 planes, and the original high TC superconductor was from this family. As such these copper oxides have been of great topical interest for 35 years since their discovery. This family displays a complex phase diagram with magnetism, charge order, an enigmatic pseudogap phase all intertwined with superconductivity. Neutron scattering famously first observed so-called stripe order phases by observing incommensurate elastic peaks centered around (π, π) positions in reciprocal space in the Nd-LSCO member of this family in 1995. Since then, many related studies of stripe ordered phases have been carried out on different branches of the La-214 family, including LBCO, LSCO, and Nd-LSCO. This thesis presents a comprehensive study of a series of La1.6−xNd0.4SrxCuO4 (Nd-LSCO) crystals, from their synthesis and single crystal growth, to their advanced characterization using elastic and inelastic neutron scattering techniques. The thesis begins with a detailed description of the methods of materials synthesis and single crystal growth and the related characterization of the materials synthesized. It then moves on to explore the evolution of magnetic order and low energy excitations at various Sr doping levels from x = 0.12 to 0.26 in Nd-LSCO single crystals. These data show that spin stripe magnetic order persists beyond the putative quantum critical point associated with the pseudogap phase, and extends at least as far as the end of superconductivity. The behaviour of the associated dynamic spin stripes in these materials is also investigated with modern neutron spectroscopic methods, and these are also found to be pervasive across this range of doping. The implications of these data and their analysis are discussed and a new phase diagram is proposed which shows strong correlations between superconductivity and parallel spin stripe order and fluctuations. / Thesis / Doctor of Philosophy (PhD)

No description available.

<p>This thesis presents results from numerical simulations of the Nernst effect dueto phase fluctuations in models of two-dimensional granular superconductors. Inaddition other transport properties, such as thermal conductivity and electrical re-sistivity are calculated. The models are based on a phase only description withLangevin or resistively and capacitively shunted Josephson junction (RCSJ) dy-namics, generalized to be valid for any type of two-dimensional lattice structure.All transport coefficients are evaluated from equilibrium correlation functions usingKubo formulas.</p><p>In Paper I, anomalous sign reversals of the Nernst signal eN , corresponding tovortex motion from colder to hotter regions, are observed. These are attributedto geometric frustration effects close to magnetic fields commensurate with theunderlying lattice structure. The effect is seen also in systems with moderategeometric disorder, and should thus be possible to observe in real two-dimensionalgranular superconductors or Josephson junction arrays.</p><p>Paper II presents two different derivations of an expression for the heat current inLangevin and RCSJ dynamics. The resulting expression is through our simulationsseen to obey the required Onsager relation, as well as giving consistent resultswhen calculating κ and eN via Kubo formulas and through the responses to anapplied temperature gradient. In zero magnetic field and at low-temperatures, thecontribution to the thermal conductivity κ in RCSJ dynamics is calculated usinga spin-wave approximation, and is shown to be independent of temperature anddiverge logarithmically with system size. At higher temperatures, κ shows a non-monotonic temperature dependence. In zero magnetic field κ has a anomalouslogarithmic size dependence also in this regime. The off-diagonal component ofthe thermoelectric tensor αxy is calculated and displays the very same ∼1/T dependence at low temperatures predicted from calculations based on Gaussiansuperconducting fluctuations.</p>

No description available.

No description available.

No description available.

No description available.