Date of Award
1994
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Abstract
It is now widely known that the new high-temperature superconductors are antiferromagnetic insulators in their normal state. Experimental results indicate that layered cuprates such as {dollar}La\sb{lcub}2-x{rcub}Sr\sb{lcub}x{rcub}CuO\sb4{dollar} and {dollar}YBa\sb2Cu\sb3O\sb{lcub}6+x{rcub}{dollar} go from antiferromagnetic to superconducting state as x changes in the stoichiometry with doping. This suggests a possible correlation between the properties of antiferromagnetism and superconductivity in these materials. Also, the magnetic properties in the normal state such as Neel temperature, its dependence on doping, the sublattice magnetization etc. have an unusual behaviour which is not yet fully understood. The existing theories are not able to explain these magnetic properties. The aim of this thesis, therefore, is to understand the magnetic properties of these compounds in the antiferromagnetic insulating state.;We have developed a theory which takes into account the antiferromagnetic correlations present within the CuO{dollar}\sb2{dollar} layers and also the weak but finite interlayer coupling between these CuO{dollar}\sb2{dollar} planes. Our theory is based on the Green's function approach by which we have obtained an expression for the Neel temperature, its doping dependence, the sublattice magnetization, the magnetic susceptibility and the magnetic correlation length. We have obtained self-consistent expressions for sublattice magnetization and susceptibility. We have found analytical expressions for some of these magnetic properties which would prove extremely helpful to the experimentalists. We have compared our theoretical results to the experimental result for doping dependent Neel temperature, sublattice magnetization, magnetic susceptibility and the magnetic correlation length. It is shown that the theory gives good agreement with experiments. We have also developed a new method by using the spectral function of the spin waves to calculate the magnetic susceptibility in the presence of an external magnetic field. It is shown that at high enough temperatures the results obtained are quite consistent with the experimental results. It is shown that the magnetic properties have a crossover from 3-D to quasi-2D behaviour with temperature and the ratio of inter to intraplanar couplings in these materials.;Our theory gives some very significant predictions. It has shown a logarithmic behaviour for the Neel temperature and a square root of temperature behaviour for the sublattice magnetization near the Neel temperature. It also predicts an exponential behaviour for the magnetic correlation length and its evaluation gives good agreement with other theoretical and simulation results. A comparative analysis of our results with those of spin-wave theory and other theories is also presented.
Recommended Citation
Singh, Ravi Prakash, "Self-consistent Theory For Magnetic Properties In High-temperature Superconductors" (1994). Digitized Theses. 2372.
https://ir.lib.uwo.ca/digitizedtheses/2372