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Master's Dissertation
DOI
10.11606/D.43.1982.tde-15072013-095812
Document
Author
Full name
Tania Tome Martins de Castro
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1982
Supervisor
Committee
Salinas, Silvio Roberto de Azevedo (President)
Becerra, Carlos Castilla
Oliveira, Mario Jose de
Title in Portuguese
Aproximações de campo médio para o modelo de Ising com interações axialmente competitivas.
Keywords in Portuguese
TERMODINÂMICA
Abstract in Portuguese
Estudamos o comportamento termodinâmico de um modelo de Ising numa rede cübica simples com interações competitivas entre primeiros e segundos vizinhos ao longo de uma das direções (modelo ANNNI). Para isto desenvolvemos uma nova aproximação de campo médio, incluindo flutuações de spins nos planos normais ao eixo de anisotropia do sistema. Construímos um diagrama de fases exibindo uma serie de fases moduladas e um ponto de Lifshitz. Concluímos que, em comparação com os resultados de campo media simples, o diagrama de fases obtido na nossa aproximação está mais perto do previsto por amostragens de Monte Carla e análises de séries de altas temperaturas.
Title in English
Mean-field approximations for the Ising model with competitive interactions axially.
Keywords in English
THERMODYNAMICS
Abstract in English
We study the thermodynamic behavior of a layered Ising model in a simple cubic lattice with competing interaction between first and second neighbours along a single direction (ANNNI model). We develop a new mean field approximation which includes spin fluctuations in the planes perpendicular to the axis of anisotropy. We construct a phase-diagram which displays a succession of modulated phases and a Lifshitz point. We conclude that, in comparison with the simple mean-field results, the phase diagram obtained in the new approximation is closer to the predictions of the Monte Carlo simulations and the analysis of high temperature series expansions.
 
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42157CASTRO.pdf (2.65 Mbytes)
Publishing Date
2013-07-15
 
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