PrBa₂Cu₃O₆₊_x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa₂Cu₃O₆₊_x:it is the only compound which has the same structure as YBa₂Cu₃O₆₊_x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa₂Cu₃O₆₊_x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O₂ planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa₂Cu₃O₆₊_x, I have used instead the compounds PrO₂ and Y_0.3Pr_0.7Ba₂Cu₄O₈, which contain praseodymium in a chemically similar environment. Y_0.3Pr_0.7Ba₂Cu₄O₈ shows the same broadening effects as PrBa₂Cu₃O₆₊_x; PrO₂ shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.

Thesis (Ph.D.)--McMaster University (Canada), 1995. / Source: Dissertation Abstracts International, Volume: 56-12, Section: B, page: 7001. Adviser: P. S. Nicholson.

Thesis (Ph. D.)--Northeastern University, 2008. / Title from title page (viewed April 3, 2009). Graduate School of Arts and Sciences, Dept. of Physics. Includes bibliographical references (p. 129-136).

Thesis (Ph. D.)--Florida State University, 2004. / Advisor: Dr. Jim P. Zheng, Florida State University, College of Engineering, Dept. of Electrical and Computer Engineering. Title and description from dissertation home page (viewed Jan. 13, 2005). Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xvii, 109 p.; also includes graphics (some col.). Includes bibliographical references (p. 105-109). Available online via OhioLINK's ETD Center

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 126-133).

We use a combination of Density Functional Theory and Dynamical Mean Field Theory (DFT+DMFT) to study electronic correlations in unconventional superconductors, with a focus on nickelate analogs of cuprate superconductors. We study the infinite layer nickelate superconductor NdNiO₂ in parallel with the isostructural CaCuO₂. Our results point to superconductivity in the nickelate being cupratelike, with correlations dominated by a hybrid Ni-𝑑_{𝑥²-𝑦²} and O-𝑝 band, and with the extra bands not contributing substantially to the superconducting state. We find that the infinite layer nickelate NdNiO₂ and the trilayer nickelate Pr₄Ni₃O₈ are virtually identical in terms of correlation physics when compared at the same chemical doping, despite the differences in Fermiology, indicating that the number of layers can stand in for chemical doping for some properties related to electronic correlations. We find that as opposed to in narrow window DFT+DMFT, in wide window DFT+DMFT the choice of downfolding procedure leads to very different results. This is an important ambiguity in the method that must be resolved or the method is incomplete by itself. We also study Sr₂MoO₄ in parallel with the Hund's superconductor Sr₂RuO₄, and find that Sr₂MoO₄ is a particle-hole dual of Sr₂RuO₄ but without the van Hove singularity at the Fermi level, which disentangles the influence of the van Hove singularity from Hund's physics. We show that Sr₂MoO₄ has a characteristic Hund's peak on the occupied of the spectral function, indicating that the peak should be observable by photoemission experiments.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

In the research reported here, a two-dimensional Ising-type model was used to study the tracer diffusion coefficients of oxygen ions in YBa2Cu307-x (YBCO) for various values of oxygen concentration and temperature. The first chapter provides a brief introduction to the problem considered in this thesis. This is followed by a short review of superconductivity, with particular reference to the experimental and theoretical aspects of high-temperature superconductivity and its applications. The structure of YBCO is then presented, with emphasis on its behaviour for various values of oxygen concentration and temperature. The model used in the simulations and the results for tracer diffusion coefficients and related quantities obtained by previous workers concludes the chapter. Results from other theoretical models are also included. In the second chapter a theoretical background for the Monte Carlo simulations employed in the present investigation is discussed. This chapter includes a short account on how these numerical methods evolved. The Monte Carlo approach to numerical evaluation of integrals is described. Then the idea of Markov sampling for obtaining members of the canonical ensemble is presented, and the Metropolis algorithm is described. In particular, convergence and the detailed balance condition are discussed. The consequence of starting from an arbitrary initial state when evaluating an observable of interest, and the effect on the reliability of the values of observables, are analysed. The chapter closes with a discussion of Monte Carlo methods applied to a lattice model in statistical mechanics. In chapter three results for the oxygen distribution produced by the diffusion of oxygen in YBCO are presented. These results were obtained from Monte Carlo simulations of the anti-symmetric nextto- nearest neighbour Ising (ASYNNNI) model. In addition to the uniform equilibrium distributions obtained by previous workers, it is found that this model also possesses nonuniform equilibrium oxygen distributions. The nonuniform distributions show a rich phase structure, and some preliminary results for this structure are presented. Results for the tracer diffusion coefficients in the nonuniform equilibrium distributions are given in the last chapter. Both local and global tracer diffusion coefficients for nonuniform equilibrium configurations are presented. Results obtained for uniform and nonuniform equilibrium configurations are compared. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.

The class of high Tc superconductors share one common structural aspect, the existence of planes of copper and oxygen ions. These planes are thought to be the source of the superconducting behaviour. They can be represented as a two-dimensional lattice of ions, which facilitates their study using numerical models. One such model is the t − J model. In most studies utilising numerical models, the planes have been considered isotropic. However, recent analysis of cuprate structure has illustrated that this may not be representative of the copper oxide planes. A number of cuprate structures exhibit different phases in which the planes are not isotropic, such as the low temperature orthorhombic and low temperature tetragonal phases. This work will examine the effects of introducing anisotropy into the t − J model in order to understand how these phases affect the results gained from numerical studies. / x, 89 leaves : ill. ; 29 cm.