Antiferromagnetismo e ordem oculta em um modelo de Anderson

Detalhes bibliográficos
Autor(a) principal: Lausmann, Ana Claudia
Data de Publicação: 2018
Tipo de documento: Tese
Idioma: por
Título da fonte: Manancial - Repositório Digital da UFSM
Texto Completo: http://repositorio.ufsm.br/handle/1/24657
Resumo: In the absence of an external magnetic field, the heavy-fermion compound URu2Si2 exhibits a second-order phase transition at 17.5 K. This transition is marked by a mean-field-like anomaly in the specific heat, by the existence of anisotropic magnetic susceptibility, and by a huge loss of entropy. Measurements on URu2Si2 show that this transition produces a gap with magnitude between 7.5 and 5 meV which develops over 40% of the Fermi-surface. Despite intensive theoretical and experimental research, even after almost thirty years, the nature of the hidden order phase, as it is called, remains unraveled. Initially, it was thought that this transition was a kind of antiferromagnetic transition. However, pressure measurements showed that a transition to an antiferromagnetic state occurs only above 0,5 GPa. Recently, a theory based in the underscreened Anderson Lattice Model has been proposed to describe the order parameter of the hidden order phase (RISEBOROUGH; COQBLIN; MAGALHAES, 2012) of URu2Si2. In the present work, we show that the proposed model exhibits a delicate competition between the “Hidden Order” and a Neel antiferromagnetic phase, and that it might also describe the first-order transition in URu2Si2 found by applying pressure. We assume that the pressure has the effect of increasing both the tight-binding hopping integrals for the conduction and the 5f bands. We observe that the “Hidden Ordered” state only appears for a very limited range of occupation numbers. The analysis of the Helmholtz Free Energy shows a Neel antiferromagnetic state followed by a “Hidden Order ” state and then a second Neel antiferromagnetic state as the band widths are increased. The bondaries of the paramagnetic state are obtained via second-order instabilities. The Neel antiferromagnetic state also was investigated.The results show a region of first order transitions between two antiferromagnetic phases, ending at a critical point. Besides that, there is a tricritical point at the low temperature and high value of band width region. Also, it was verified changes on Fermi surface topology in the Neel antiferromagnetic state and a parcial reconstruction on Fermi surface. This reconstruction follows the line of first order transition between the antiferromagnetic phases and it extends beyond the critical point, suggesting that there may be a Lifshitz transition in this region of parameters.
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spelling Antiferromagnetismo e ordem oculta em um modelo de AndersonAntiferromagnetism and hidden order in a Anderson modelModelo de AndersonAproximação de campo médioFérmions-pesadosFunções de GreenAnderson modelMean field approximationHeavy-fermionsGreen’s functionsCNPQ::CIENCIAS EXATAS E DA TERRA::FISICAIn the absence of an external magnetic field, the heavy-fermion compound URu2Si2 exhibits a second-order phase transition at 17.5 K. This transition is marked by a mean-field-like anomaly in the specific heat, by the existence of anisotropic magnetic susceptibility, and by a huge loss of entropy. Measurements on URu2Si2 show that this transition produces a gap with magnitude between 7.5 and 5 meV which develops over 40% of the Fermi-surface. Despite intensive theoretical and experimental research, even after almost thirty years, the nature of the hidden order phase, as it is called, remains unraveled. Initially, it was thought that this transition was a kind of antiferromagnetic transition. However, pressure measurements showed that a transition to an antiferromagnetic state occurs only above 0,5 GPa. Recently, a theory based in the underscreened Anderson Lattice Model has been proposed to describe the order parameter of the hidden order phase (RISEBOROUGH; COQBLIN; MAGALHAES, 2012) of URu2Si2. In the present work, we show that the proposed model exhibits a delicate competition between the “Hidden Order” and a Neel antiferromagnetic phase, and that it might also describe the first-order transition in URu2Si2 found by applying pressure. We assume that the pressure has the effect of increasing both the tight-binding hopping integrals for the conduction and the 5f bands. We observe that the “Hidden Ordered” state only appears for a very limited range of occupation numbers. The analysis of the Helmholtz Free Energy shows a Neel antiferromagnetic state followed by a “Hidden Order ” state and then a second Neel antiferromagnetic state as the band widths are increased. The bondaries of the paramagnetic state are obtained via second-order instabilities. The Neel antiferromagnetic state also was investigated.The results show a region of first order transitions between two antiferromagnetic phases, ending at a critical point. Besides that, there is a tricritical point at the low temperature and high value of band width region. Also, it was verified changes on Fermi surface topology in the Neel antiferromagnetic state and a parcial reconstruction on Fermi surface. This reconstruction follows the line of first order transition between the antiferromagnetic phases and it extends beyond the critical point, suggesting that there may be a Lifshitz transition in this region of parameters.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPESNa ausência de um campo magnético externo, o composto URu2Si2 exibe uma transição de fase de segunda ordem à 17,5 K. Essa transição é marcada por uma anomalia no calor específico, pela existência de uma susceptibilidade magnética anisotrópica e por uma grande perda de entropia. Resultados experimentais mostram que essa transição produz um gap de intensidade entre 5 e 7,5 meV que se desenvolve sobre 40% da superfície de Fermi. Após mais de 30 anos de intensa pesquisa teórica e experimental, a natureza da fase "Ordem Oculta", como é conhecida, permanece uma incógnita. Inicialmente, pensava-se que essa transição era um tipo de transição antiferromagnética. Contudo, à pressão acima de 0,5 GPa o sistema evolui para um estado antiferromagnético via transição de fase de primeira ordem. Recentemente, uma teoria baseada no modelo Underscreened Anderson Lattice propôs um parâmetro de ordem para descrever a fase "Ordem Oculta" (RISEBOROUGH; COQBLIN; MAGALHAES, 2012) do URu2Si2. No presente trabalho, verifica-se que o modelo proposto exibe uma delicada competição entre as fases "Ordem Oculta" e de Neel, e que o mesmo também é capaz de descrever a transição de primeira ordem da fase "Ordem Oculta" para a fase de Neel devido ao aumento da pressão. Os resultados foram obtidos assumindo que a pressão tem como efeito o aumento do hopping das bandas d e 5f. Também observou-se que a "Ordem Oculta" aparece somente para alguns números de ocupação. A análise da energia livre de Helmholtz mostra que conforme a largura das bandas aumenta o sistema passa de um estado Neel para "Ordem Oculta", e depois novamente para um estado de Neel. Os limites da fase paramagnética foram obtidos através de instabilidades de segunda ordem. A fase de Neel também foi investigada. Os resultados mostram uma região de transições de primeira ordem entre duas fases antiferromagnéticas, que termina em um ponto crítico. Além disso, há um ponto tri-crítico localizado na região de baixas temperaturas e altos valores de largura de banda. Também verificou-se mudanças na topologia da superfície de Fermi no estado de Neel e a reconstrução de parte da superfície de Fermi. Essa reconstrução acompanha a linha de transição de primeira ordem entre as fases antiferromagnéticas e se estende além do ponto crítico sugerindo que pode existir uma transição de Lifshitz nessa região de parâmetros.Universidade Federal de Santa MariaBrasilFísicaUFSMPrograma de Pós-Graduação em FísicaCentro de Ciências Naturais e ExatasCalegari, Eleonir Joãohttp://lattes.cnpq.br/6030572534512173Morais Júnior, Carlos Alberto Vaz deMetz, Fernando LucasSilva, Leandro Barros daCarara, Marcos AndreLausmann, Ana Claudia2022-06-07T20:17:41Z2022-06-07T20:17:41Z2018-12-14info:eu-repo/semantics/publishedVersioninfo:eu-repo/semantics/doctoralThesisapplication/pdfhttp://repositorio.ufsm.br/handle/1/24657porAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-nd/4.0/info:eu-repo/semantics/openAccessreponame:Manancial - Repositório Digital da UFSMinstname:Universidade Federal de Santa Maria (UFSM)instacron:UFSM2022-06-07T20:17:41Zoai:repositorio.ufsm.br:1/24657Biblioteca Digital de Teses e Dissertaçõeshttps://repositorio.ufsm.br/ONGhttps://repositorio.ufsm.br/oai/requestatendimento.sib@ufsm.br||tedebc@gmail.comopendoar:2022-06-07T20:17:41Manancial - Repositório Digital da UFSM - Universidade Federal de Santa Maria (UFSM)false
dc.title.none.fl_str_mv Antiferromagnetismo e ordem oculta em um modelo de Anderson
Antiferromagnetism and hidden order in a Anderson model
title Antiferromagnetismo e ordem oculta em um modelo de Anderson
spellingShingle Antiferromagnetismo e ordem oculta em um modelo de Anderson
Lausmann, Ana Claudia
Modelo de Anderson
Aproximação de campo médio
Férmions-pesados
Funções de Green
Anderson model
Mean field approximation
Heavy-fermions
Green’s functions
CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA
title_short Antiferromagnetismo e ordem oculta em um modelo de Anderson
title_full Antiferromagnetismo e ordem oculta em um modelo de Anderson
title_fullStr Antiferromagnetismo e ordem oculta em um modelo de Anderson
title_full_unstemmed Antiferromagnetismo e ordem oculta em um modelo de Anderson
title_sort Antiferromagnetismo e ordem oculta em um modelo de Anderson
author Lausmann, Ana Claudia
author_facet Lausmann, Ana Claudia
author_role author
dc.contributor.none.fl_str_mv Calegari, Eleonir João
http://lattes.cnpq.br/6030572534512173
Morais Júnior, Carlos Alberto Vaz de
Metz, Fernando Lucas
Silva, Leandro Barros da
Carara, Marcos Andre
dc.contributor.author.fl_str_mv Lausmann, Ana Claudia
dc.subject.por.fl_str_mv Modelo de Anderson
Aproximação de campo médio
Férmions-pesados
Funções de Green
Anderson model
Mean field approximation
Heavy-fermions
Green’s functions
CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA
topic Modelo de Anderson
Aproximação de campo médio
Férmions-pesados
Funções de Green
Anderson model
Mean field approximation
Heavy-fermions
Green’s functions
CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA
description In the absence of an external magnetic field, the heavy-fermion compound URu2Si2 exhibits a second-order phase transition at 17.5 K. This transition is marked by a mean-field-like anomaly in the specific heat, by the existence of anisotropic magnetic susceptibility, and by a huge loss of entropy. Measurements on URu2Si2 show that this transition produces a gap with magnitude between 7.5 and 5 meV which develops over 40% of the Fermi-surface. Despite intensive theoretical and experimental research, even after almost thirty years, the nature of the hidden order phase, as it is called, remains unraveled. Initially, it was thought that this transition was a kind of antiferromagnetic transition. However, pressure measurements showed that a transition to an antiferromagnetic state occurs only above 0,5 GPa. Recently, a theory based in the underscreened Anderson Lattice Model has been proposed to describe the order parameter of the hidden order phase (RISEBOROUGH; COQBLIN; MAGALHAES, 2012) of URu2Si2. In the present work, we show that the proposed model exhibits a delicate competition between the “Hidden Order” and a Neel antiferromagnetic phase, and that it might also describe the first-order transition in URu2Si2 found by applying pressure. We assume that the pressure has the effect of increasing both the tight-binding hopping integrals for the conduction and the 5f bands. We observe that the “Hidden Ordered” state only appears for a very limited range of occupation numbers. The analysis of the Helmholtz Free Energy shows a Neel antiferromagnetic state followed by a “Hidden Order ” state and then a second Neel antiferromagnetic state as the band widths are increased. The bondaries of the paramagnetic state are obtained via second-order instabilities. The Neel antiferromagnetic state also was investigated.The results show a region of first order transitions between two antiferromagnetic phases, ending at a critical point. Besides that, there is a tricritical point at the low temperature and high value of band width region. Also, it was verified changes on Fermi surface topology in the Neel antiferromagnetic state and a parcial reconstruction on Fermi surface. This reconstruction follows the line of first order transition between the antiferromagnetic phases and it extends beyond the critical point, suggesting that there may be a Lifshitz transition in this region of parameters.
publishDate 2018
dc.date.none.fl_str_mv 2018-12-14
2022-06-07T20:17:41Z
2022-06-07T20:17:41Z
dc.type.status.fl_str_mv info:eu-repo/semantics/publishedVersion
dc.type.driver.fl_str_mv info:eu-repo/semantics/doctoralThesis
format doctoralThesis
status_str publishedVersion
dc.identifier.uri.fl_str_mv http://repositorio.ufsm.br/handle/1/24657
url http://repositorio.ufsm.br/handle/1/24657
dc.language.iso.fl_str_mv por
language por
dc.rights.driver.fl_str_mv Attribution-NonCommercial-NoDerivatives 4.0 International
http://creativecommons.org/licenses/by-nc-nd/4.0/
info:eu-repo/semantics/openAccess
rights_invalid_str_mv Attribution-NonCommercial-NoDerivatives 4.0 International
http://creativecommons.org/licenses/by-nc-nd/4.0/
eu_rights_str_mv openAccess
dc.format.none.fl_str_mv application/pdf
dc.publisher.none.fl_str_mv Universidade Federal de Santa Maria
Brasil
Física
UFSM
Programa de Pós-Graduação em Física
Centro de Ciências Naturais e Exatas
publisher.none.fl_str_mv Universidade Federal de Santa Maria
Brasil
Física
UFSM
Programa de Pós-Graduação em Física
Centro de Ciências Naturais e Exatas
dc.source.none.fl_str_mv reponame:Manancial - Repositório Digital da UFSM
instname:Universidade Federal de Santa Maria (UFSM)
instacron:UFSM
instname_str Universidade Federal de Santa Maria (UFSM)
instacron_str UFSM
institution UFSM
reponame_str Manancial - Repositório Digital da UFSM
collection Manancial - Repositório Digital da UFSM
repository.name.fl_str_mv Manancial - Repositório Digital da UFSM - Universidade Federal de Santa Maria (UFSM)
repository.mail.fl_str_mv atendimento.sib@ufsm.br||tedebc@gmail.com
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