This thesis treats two different aspects of quantum spin dynamics. First, it is shown that magnetic chains with uniaxial symmetry support stable configurations that separate ferromagnetic domains of opposite magnetization. These configurations, domain walls is a quantum system, are found to remain stable even when they interact with spin waves. We found that a domain wall transmits the longitudinal component of a spin wave while the transverse component is being reflected. Our results indicate that the common, classical description using Landau-Lifshitz-Gilbert models cannot be applied to microscopic systems. We also studied the domain wall dynamics in the vicinity of the quantum critical point and showed that the domain wall in the Heisenberg-Ising model is dynamically stable. As we approach the quantum critical point, the width of the domain wall diverges with an exponent of one half. Subsequently, we studied decoherence processes of two ferro- or antiferromagnetically coupled spins that interact with an environment of spins. The conditions under which the two-spin system evolves from the up-down state to its ground state have been determined. We demonstrate that this relaxation process is effective for a fairly limited range of model parameters and that the symmetry and connectivity of the coupling to the environment significantly affects the features of the decoherence processes.
Date and time: 18 januari 2008, 14.45
PhD student: Shengjun Yuan
Dissertation: Relaxation and decoherence in quantum spin system
Promotor: prof. H.A. de Radet
Faculty: Mathematics and Natural Sciences
Location: Aula Academiegebouw, Broerstraat 5, Groningen
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