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Doctoral Thesis
DOI
10.11606/T.43.1993.tde-12062012-140736
Document
Author
Full name
Marcelo Henrique Romano Tragtenberg
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1993
Supervisor
Committee
Yokoi, Carlos Seihiti Orii (President)
Bindilatti, Valdir
Curado, Evaldo Mendonca Fleury
Oliveira, Paulo Murilo Castro de
Wreszinski, Walter Felipe
Title in Portuguese
Estudo de modelos para sistemas modulados magnéticos e estruturais
Keywords in Portuguese
Modelo ANNNI
Modelo de Frenkel-Kontorova
Abstract in Portuguese
Estudamos o comportamento de modelos para sistemas modulados magnéticos e estruturais. A primeira parte deste trabalho e dedicada ao modelo de Ising com interações competitivas numa rede de Bethe, no limite de coordenação infinita, num campo magnético. Focalizamos nossa atenção no comportamento das fases comensuráveis na presença de campo. Obtivemos vários diagramas T H utilizando algoritmos numéricos muito mais eficientes do que a simples iteração do mapeamento associado ao modelo. Na segunda parte estudamos o modelo de FrenkelKontorova com primeiro e segundo harmônicos no potencial externo. Encontramos e investigamos transições de segunda ordem no interior das fases comensuráveis de período ímpar. Essas transições, denominadas simétricaassimétricas, estão associadas à quebra da simetria por reflexão que ocorre para potenciais suficientemente fortes.
Title in English
Study designs for systems and magnetic modulated and structurals
Keywords in English
Ising model
Magnetic field
Modulated systems
Abstract in English
We studied the behavior of models for magnetically and structurally modulated systems. The first part of this work is dedicated to the study of the Ising model with competing interactions on a Bethe lattice, in the infinite coordination limit, in a magnetic field. We focused our attention on the behavior of the commensurate phases in the presence of a field. We obtained various T H phase diagrams using numerical methods far more efficient than simple iteration of the mapping associated to the model. In the second part we studied the FrenkelKontorova model with first and second harmonics in the external potential. We found and investigated the second order transitions within the commensurate phases of odd periodicity. These transitions, called symmetricasymmetric transitions, are related to the breaking of reflection symmetry which occurs at high potentials.
 
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46067_Tragtenberg.pdf (62.10 Mbytes)
Publishing Date
2012-06-14
 
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