Sistemas antiferromagneticos unidimensionais de spin-1
Ano de defesa: | 2007 |
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Autor(a) principal: | |
Orientador(a): | |
Banca de defesa: | |
Tipo de documento: | Tese |
Tipo de acesso: | Acesso aberto |
Idioma: | por |
Instituição de defesa: |
Universidade Federal de Minas Gerais
UFMG |
Programa de Pós-Graduação: |
Não Informado pela instituição
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Departamento: |
Não Informado pela instituição
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País: |
Não Informado pela instituição
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Palavras-chave em Português: | |
Link de acesso: | http://hdl.handle.net/1843/ESCZ-7AUNFP |
Resumo: | Attending to the current tendency of research in quantum magnetism,our object of study comprises one-dimensional spin-1 antiferromagnetic(AFM) systems. They are analysed for two different physical pictures: anoise-driven 1D Nonlinear Sigma Model (NLSM) for S = 1 and the 1DAFM Heisenberg Model for S = 1 coupled to phonons. In the first work,we present a method of extracting the NLSM thermodynamics based onthe coupling between the sigma field and an external noise source. Thisapproach succeeds in capturing the thermal behavior of the spin-1 AFMchain subjected to a crystalline field enjoying certain stochastic properties. This is an effective method for the NLSM at finite temperature, moved by a phenomenological reasoning which is justified by comparison with experiment. The model excitations gap and line-width are derived from a perturbative calculation for the Greens function. The second work accounts for the interplay between vibrational and magnetic degrees of freedom, the so-called magnon-phonon coupling, which plays an important role in magnetism. The diagramatic Greens function perturbative method is used to investigate the phonon dynamics for the spin-1 1D antiferromagnet within the framework of an interaction Heisenberg model. This is treated via the Holstein-Primakoff transformations in the context of a modified spin wave theory. The so-obtained phonon relaxation function provides a measure ofthe effect of magnon-phonon coupling on phonon energy and life-time. |