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Godoy, Maurício. "Comportamento dinâmico de modelos de spins mistos." Florianópolis, SC, 2004. http://repositorio.ufsc.br/xmlui/handle/123456789/86636.
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Tese (doutorado) - Universidade Federal de Santa Catarina. Centro de Ciências Físicas e Matemáticas. Programa de Pós-Graduação em Física.Made available in DSpace on 2012-10-21T09:02:36Z (GMT). No. of bitstreams: 1 201220.pdf: 1117800 bytes, checksum: 3e9aa6c23e2d84fc294945607454ec46 (MD5)
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Harris, Martin Guy. "Random surfaces : multiple Ising spins coupled to 2d quantum gravity." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260140.
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Mari, Pierre-Olivier. "Etude numérique de verres de spins Ising en dimension trois." Paris 11, 2001. http://www.theses.fr/2001PA112364.
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Les verres de spins sont des matériaux magnétiques qui présentent à basse température un ensemble de propriétés inédites dont certaines sont caractéristiques de la combinaison du désordre et de contraintes conflictuelles. La modélisation aisée de tels systèmes magnétiques en vue d'un traitement analytique ou numérique a fait du verre de spins l'archétype des systèmes désordonnés. Toutefois, la complexité et l'étendue du savoir accumulé sur ces systèmes et les matériaux vitreux en général cachent une réalité frustrante: la compréhension de la physique qui les gouverne et la connaissance de la nature de leurs transitions de phase demeurent trop souvent incomplètes. Cette thèse repose sur l'étude numérique par simulation Monte-Carlo du comportement dynamique de modèles de verres de spins Ising et de leur transition de phase. L'analyse que nous faisons de l'ensemble de nos résultats et de ceux déjà existants dans la littérature se subdivise en trois parties en relation avec les objectifs que nous nous sommes fixés. Tout d'abord, explorer la validité de deux approches alternatives (le Damage spreadinq et la méthode des exposants effectifs) permettant l'étude du comportement critique des verres de spins et, ce faisant, faire apparaître la nécessité de prendre en compte les corrections aux lois d'échelles en taille finie à travers leurs effets sur les estimations des exposants critiques. Ensuite, utiliser cette panoplie de méthodes d'analyses alternatives pour poursuivre la construction d'un ensemble de preuves numériques en faveur d'une violation des règles d'Universalité pour les verres de spins Ising. Enfin, nous avons comparé les propriétés dynamiques de plusieurs modèles numériques avec des mesures expérimentales effectuées sur un ensemble de verres de polymères. L'existence d'une dynamique similaire pour ces systèmes très différents est interprétée en utilisant un scénario unique décrivant, dans l'espace des phases, la transition vitreuse comme une marche aléatoire sur un hypercube diluéSpin glasses are magnetic materials that exhibit at low enough temperatures a number of unique properties, some of which arise from the combination of disorder and conflicting constraints. The ease with which one can modelize such systems for an analytic or numerical investigation has promoted spin glasses to the rank of archetype for disordered systems. However, the complexity and the extent of what is known on glassy materials hides a frustrating fact: the understanding of the physics that governs their behaviour and of the nature of glassy phase transitions often remains incomplete. This thesis is a study of the dynamical properties and phase transition of Ising spin glass models by means of Monte-Carlo simulations. The analysis of our results and of those taken from the literature falls into three parts and follows our main goals. First of all, to explore the validity of two alternative approaches (the Damage spreading and the effective exponent method) for the study of the critical behaviour of spin glass models and, in doing so, to point out the need to allow for corrections to finite size scaling by showing their effect on the determination of critical exponents. Secondly, using this array of alternative methods, to contribute to the ensemble of numerical evidences that indicate a violation of standard Universality rules in Ising spin glasses. Finally, to compare the dynamical properties of several Ising spin glasse models with experimental data on the dynamics of a range of glass forming polymers. The similarities observed in the dynamical behaviour of these very different systems is interpreted by using a unique scenario that describes glass-like transitions in phase space by means of a random walk on a diluted hypercube
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Brito, Rejane Alves de. "Método variacional de Bogoliubov Aplicado a modelos de Spins: Ising e Blume-Capel." Universidade Federal da Paraíba, 2017. http://tede.biblioteca.ufpb.br:8080/handle/tede/9830.
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Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq
The spin-1=2 and spin-1 Ising models, as well as the spin-1 Blume-Capel model have been studied in one-, two-, and three-dimensional lattices. A variational method based on Bogoliubov inequality for the free-energy has been employed. The trial Hamiltonians consist of clusters of free spins, pairs of spins, and a combination of free spins and pairs of spins. For the three approximations, the thermodynamic quantities of interest have been calculated, together with the critical transition temperature and the behavior close to the transition, in the latter case in order to compute the corresponding critical exponents. The results have been compared to each other as well with exact results, when available, or results coming from more reliable approximate methods. It has been noted that as more interactions are taken into account in the trial Hamiltonian, better results are obtained for the transition temperature, although the critical exponents are always the mean eld like ones.
Os modelos de Ising de spin-1=2 e spin-1, e o de Blume-Capel de spin-1 foram estudados em redes de uma, duas e três dimensões. Foi empregado o método variacional baseado na desigualdade de Bogoliubov para a energia livre. Os hamiltonianos tentativa utilizados consistem em blocos de spins livres, de pares de spins, e da combinacão de spins livres mais pares de spins. Para as três aproximacões, foram obtidas as quantidades termodinâmicas de interesse, bem como a temperatura crítica e o comportamento perto da transição, neste ultimo caso para se obter os respectivos expoentes críticos. Os resultados foram comparados entre si, bem como com os resultados exatos, quando dispon veis, ou provenientes de outras aproxima c~oes mais elaboradas. Veri ca-se que a medida que se incorpora mais intera c~oes nos hamiltonianos tentativa, melhores resultados s~ao obtidos para a temperatura de transi c~ao, embora os expoentes cr ticos continuem sempre sendo os mesmo de campo médio usual. Palavras-Chave: Modelo de Ising,Blume-Capel,desigualdade de Bogoliubov, campo médio.
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Godoy, Maurício. "Dinâmica do modelo de Ising ferromagnetico com spins mistos o = 1/2 e S= 1 /." Florianópolis, SC, 1999. http://repositorio.ufsc.br/xmlui/handle/123456789/80472.
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Dissertação (Mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas.Made available in DSpace on 2012-10-18T14:58:08Z (GMT). No. of bitstreams: 0Bitstream added on 2016-01-09T03:54:45Z : No. of bitstreams: 1 144132.pdf: 1639773 bytes, checksum: a5105f67864d1062154a319632c45e80 (MD5)
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Mirza, Behrouz. "On the transition between crystalline and gravitational phases in two dimensional theories with matter fields." Thesis, University of Oxford, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318930.
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Souza, Estefano Alves de. "Simulação perfeita e aproximações de alcance finito em sistemas de spins com interações de longo alcance." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/45/45133/tde-27052013-192003/.
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Nosso objeto de estudo são os sistemas de spins com interações de longo alcance; em particular, estamos interessados em sistemas cuja probabilidade invariante é o modelo de Ising em A^S, onde A = {-1, 1} é o espaço de spins e S = Z^d é o espaço de sítios. Apresentamos dois resultados originais que são consequências da aplicação de algoritmos de simulação perfeita e de acoplamento no contexto da construção deste tipo de sistemas e de suas respectivas probabilidades invariantes.Our object of interest are spin systems with long-range interactions. As a special case, we are interested in systems whose invariant measure is the Ising model on A^S, where A = {-1, 1} is the space of spins and S = Z^d is the space of sites. We present two original results that are byproducts of the application of Perfect Simulation and Coupling algorithms in the context of the construction of these spin systems and their respective invariant measures.
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Gabriel, Roberto Vila. "Representações gráficas para sistemas de spins com presença de campo externo : algumas relações em teoria de probabilidades." reponame:Repositório Institucional da UnB, 2016. http://repositorio.unb.br/handle/10482/22471.
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Tese (doutorado)—Universidade de Brasília, Instituto de Ciências Exatas, Departamento de Matemática, 2016.Submitted by Albânia Cézar de Melo (albania@bce.unb.br) on 2016-12-23T12:19:10Z No. of bitstreams: 1 2016_RobertoVilaGabriel.pdf: 9781683 bytes, checksum: 3fe0f89ca8df223ebdacbaf2a11fb635 (MD5)
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Durante o desenvolvimento deste trabalho o autor recebeu auxílio financeiro da agência de fomento à pesquisa, CNPq.
Este trabalho é concernente à teoria básica de percolação e representações gráficas para o modelo de Ising/Potts sobre látices com presença de campos externos invariantes e não invariantes por translação. Descrevemos, explicitamente, em termos do modelo de aglomerados aleatórios, a função distribuição e, em consequência, o valor esperado de um único spin para os modelos de Ising e de Potts de q-estados com campos externos gerais. Consideramos, também, os estados de Gibbs para a representação do modelo de Potts com campo magnético invariante e não invariante por translação, e provamos uma versão da desigualdade FKG para o chamado modelo de aglomerados aleatórios geral (modelo GRC), com condição de fronteira livre e conectada no caso invariante e não invariante translacional. Adicionando a hipótese de amenabilidade sobre o látice, obtemos a unicidade do aglomerado infinito e a quase-localidade (quase certa) das medidas de Gibbs para o modelo GRC com tais campos magnéticos. Como uma aplicação da teoria desenvolvida, mostramos a unicidade das medidas de Gibbs para o modelo de Ising ferromagnético, com campo magnético decaindo segundo uma lei de potência com potência suficientemente pequena, como conjecturado em [23]. Finalmente, apresentamos uma pequena introdução e alguns resultados sobre distância Mallows, dando algumas relações com as medidas de Gibbs apresentadas previamente.
This work is concerned with the basic theory of percolation and graphical representation for the Ising and Potts models over general lattices with invariant and non-translation invariant external field. We explicitly describe in terms of the random-cluster representation the distribution function and, consequently, the expected value of a single spin for the Ising and q-state Potts models with general external fields. We also consider the Gibbs states for the Edwards-Sokal representation of the Potts model with invariant and non-translation invariant magnetic field and prove a version of the FKG inequality for the so called general random-cluster model (GRC model) with free and wired boundary conditions in the invariant and non-translation invariant case. Adding the amenability hypothesis on the lattice, we obtain the uniqueness of the infinite connected component and the almost sure quasilocality of the Gibbs measures for the GRC model with such general magnetic fields. As a application of the theory developed, we show the uniqueness of the Gibbs measures for the ferromagnetic Ising model with a positive power-law decay magnetic field with small enough power, as conjectured in [23]. Finally we present a brief introduction and some results on Mallows distance giving some relationship with Gibbs measures presented previously.
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BOUBCHEUR, EL HASSANE. "Transition de phase dans des systemes de spins : effets de la frustration et du couplage magnetoelastique." Cergy-Pontoise, 1998. http://www.theses.fr/1998CERG0053.
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La these traite les transitions de phase dans des systemes de spins avec interaction en competition par la simulation monte carlo (mc) avec la technique de l'histogramme. Les exposants critiques ont ete calcules a partir des lois d'echelle en taille finie. La premiere partie est consacree aux effets de la frustration dans le modele xy. L'etude des diagrammes de phase et la nature des transitions ont ete menees successivement sur le reseau triangulaire antiferromagnetique (3d), le reseau en damier (3d) et le reseau carre (2d). Parmi les resultats obtenus, on note, suivant le parametre de frustration, la mise en evidence des phases reentrantes paramagnetiques et des phases partiellement ordonnees pour le reseau en damier. Pour le modele xy completement frustre a 2d, la nature de la transition de phase a fait l'objet de plusieurs etudes analytiques et numeriques. Les resultats que nous avons obtenus montrent qu'il existe une seule transition contrairement au scenario qui prevoit deux transitions distinctes discute dans la litterature. De plus, les valeurs des exposants critiques, associes a la symetrie z 2, suggerent que la transition est dans le modele couple d'ising-xy. La deuxieme partie concerne un modele a spins d'ising sur des atomes en interaction elastique et magnetique. L'interet de ce modele est l'etude de l'effet du couplage magnetoelastique sur le comportement critique du systeme en fonction de l'intensite relative des forces magnetique et elastique. A forte interaction elastique, on retrouve la classe d'universalite d'ising, mais a faible interaction elastique, les calculs des histogrammes mc montrent que la transition est associee a des spins xy. Quand les interactions sont comparables, une nouvelle phase est obtenue, elle est due essentiellement a des contractions des paires de spins antiferromagnetiques.
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Sakly, Nahed. "Investigations structurale et physique du système d'oxydes à chaînes de spins Ising (Sr, Ca)1+xCoxMn1-xO3." Thesis, Normandie, 2020. http://www.theses.fr/2020NORMC232.
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Ce manuscrit présente l’étude expérimentale des propriétés structurales et physiques d’une série de composés à chaînes de spins Sr4−xCaxCoMn2O9, appartenant à la famille d’oxydes A1+X(A’XB1−X)O3. Dans cette série, les chaînes 1D sont constituées des octaèdres MnO6 et prismes trigonaux CoO6 reliés par les faces, et réparties sur un réseau triangulaire avec un couplage antiferromagnétique entre elles. Ces chaînes présentent une forte anisotropie magnétique de type Ising provenant du cation Co2+ (HS, 3d7, S = 3/2). D’abord, l'étude structurale de ces chaînes révèle qu’elles peuvent être incommensurables, à cause d’un changement de degré d'oxydation du cobalt en fonction des conditions de synthèse. Ensuite, nous nous sommes intéressés à l'étude des deux composés Sr4CoMn2O9 (x=0) et Sr2Ca2CoMn2O9 (x=2). Le composé x=0 commensurable a montré l'absence de mise en ordre magnétique à longue distance (LRO) et des réponses dynamiques de relaxation de spins, typiques des Single-Ion Magnet (SIM) et Single-Chain Magnet (SCM) dont l'amplitude de leurs pics caractéristiques dépendde la (in)commensurabilité. En revanche, dans x=2, seule la réponse SIM a été observée à basse température, et qui coexiste avec le LRO à TN ~ 28 K. Les données de diffraction des neutrons montrent que cet LRO est compatible avec un état antiferromagnétique partiellement désordonné (PDA). Un régime pré-transitionnel particulier a été aussi observé entre TN et T* (~ 32.5 K), qui a été considéré comme un effet précurseur de LRO. D'un autre côté, un couplage magnéto-électrique a été également mis en évidence au sein de ce composé, dont l'origine a été interprétée par un phénomène de striction d’échange. Finalement, nous avons étudié l'anisotropie magnétique dans des échantillons orientés, dont la morphologie des grains a été optimisée par différents traitements thermiquesThis manuscript presents the experimental study of structural and physical properties of a spin chain compounds Sr4-xCaxCoMn2O9, belonging to the large oxides family A1+XA'XB1-XO3. In this series, the 1D chains are made up of the octahedra MnO6 (Mn4+) and trigonal prisms CoO6 (Co2+) connected by the faces, and distributed over a triangular lattice with an antiferromagnetic coupling between them. These chains exhibit a strong Ising-type magnetic anisotropy, originating from the cation Co2+ (HS, 3d7, S = 3/2). First of all, the structural study of these chains reveals that they can be immcommensurate, due to a change in the degree of oxidation of cobalt depending on the synthesis conditions. Then, we were interested in the study of the two compounds Sr4CoMn2O9 (x=0) and Sr2Ca2CoMn2O9 (x=2). The x=0 compound showed the absence of long-range magnetic ordering (LRO) and dynamic spin relaxation responses, typical of Single-Ion Magnet (SIM) and Single-Chain Magnet (SCM), of which the amplitude of their characteristic peaks depends on the (in)commensurability. On the other hand, in x = 2, only the SIM response was observed at low temperature, and which coexists with the LRO at TN ~ 28 K. The neutron diffraction data show that this LRO is compatible with a partially disordered antiferromagnetic state (PDA). A particular pre-transitional regime was also observed between TN and T* (~ 32.5 K), which was considered to be a precursor effect of LRO. Furthermore, a magneto-electric (ME) coupling has also been demonstrated within this compound. The mechanism of this ME coupling has been discussed as a result of exchange-striction phenomenon. Finally, we studied the magnetic anisotropy in oriented samples, whose grain morphology was optimized by different heat treatments
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Faure, Quentin. "Transitions de phases quantiques dans le composé quasi-1D antiferromagnétique de type Ising BaCo2V2O8." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAY063/document.
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Ce manuscrit présente l’étude de transitions de phase quantiques dans l’oxyde BaCo2V2O8, un système antiferromagnétique quasi-unidimensionnel constitué de chaînes d’ions cobalt portant un spin effectif S = 1/2 caractérisé par une forte anisotropie de type Ising. Lors de ce travail, nous avons étudié les propriétés statiques et dynamiques de BaCo2V2O8 sous l’effet de différents paramètres physiques.Notre première étude a porté sur l’effet d’un champ magnétique transverse, i.e. appliqué perpendiculairement à l’axe Ising. Il a été proposé que lors de l’application d’un tel champ, un champ magnétique alterné effectif est induit perpendiculairement à l’axe d’anisotropie et au champ uniforme appliqué. La comparaison d'expériences de diffusion (élastique et inélastique) de neutrons et de calculs numériques nous a permis de montrer que ce champ alterné entre en compétition avec l’anisotropie. Ceci aboutit à une transition de phase originale, dite topologique, que l'on peut modéliser par une théorie quantique des champs nommée « modèle de double sine-Gordon » qui décrit la compétition entre deux excitations topologiques duales. Nous avons pu montrer que BaCo2V2O8 sous champ magnétique transverse était la première réalisation d'une telle théorie.La seconde étude était consacrée à BaCo2V2O8 sous champ magnétique longitudinal, i.e. un champ appliqué parallèlement à l’axe Ising. La dynamique de spins a été sondée grâce à la diffusion inélastique de neutrons et nous avons montré qu’au-dessus d’un champ critique de 4 T, celle-ci semble en accord avec le spectre des fluctuations de spin attendu pour un liquide de Tomonaga Luttinger (TLL). De plus, les calculs numériques ont confirmé que, du fait de l’anisotropie de type Ising dans ce système, la majorité du poids spectral du spectre en énergie est porté par les fluctuations de spins de type longitudinales. Ce résultat est la signature d'un comportement quantique sans analogue classique avec des fluctuations de basses énergies essentiellement longitudinales pilotant la physique du système. Enfin, c’est la première fois que la dynamique de spin dans des chaînes de type Ising a pu être sondée dans cette phase TLL.Les deux dernières études sont préliminaires. Le diagramme de phase de BaCo2V2O8 a été sondé par des mesures calorimétriques sous l’application d’une pression hydrostatique et d’un champ magnétique longitudinal. Afin d’obtenir des pressions allant jusqu’à 10 GPa, nous avons utilisé une cellule à enclumes de diamant. Nous avons effectué des mesures de chaleur spécifique qui nous ont permis de sonder l'effet de la pression sur le Hamiltonien de BaCo2V2O8 au travers de son diagramme de phase $(H, P, T)$. Enfin, nous avons étudié l’effet de la substitution des ions magnétiques Co2+ par des impuretés non-magnétiques Mg2+. Les expériences de diffraction neutronique sous champ longitudinal ont montré que la température et le champ critiques diminuent proportionnellement à la concentration en impuretés. La dynamique de spins à champ magnétique nul a aussi été sondée et révèle l’apparition de modes non-dispersifs, provenant possiblement de l’effet de segmentation des chaînes par les impuretés.En conclusion, nos études expérimentales couplées à des calculs numériques nous ont permis de dévoiler une physique extrêmement riche dans ce composé modèle pour l'étude du magnétisme quantique et des transitions de phase quantiquesThis manuscript is devoted to the study of quantum phase transitions in the BaCo2V2O8 oxide, a quasi-one dimensional antiferromagnet consisting of spin chains of cobalt magnetic ions carrying an effective spin S = 1/2 showing a strong Ising-like anisotropy. To achieve this, we have studied BaCo2V2O8 under the effect of different physical parameters.Our first study concerned the effect of a transverse magnetic field, i.e. applied perpendicularly to the Ising axis. It has been shown that when BaCo2V2O8 is subjected to such a field, an effective staggered magnetic field is induced perpendicularly to both the Ising-axis and the uniform applied field. Using neutron scattering experiments (both elastic and inelastic) compared to numerical calculations, we have proved that this staggered field competes with the Ising-like anisotropy. This leads to a very original quantum phase transition. Our system can actually be mapped onto a quantum field theory called “double sine-Gordon model”, describing the competition between two dual topological excitations. We have thus shown that BaCo2V2O8 under a transverse magnetic field is the first experimental realization of such a theory.The second study was devoted to the effect of a longitudinal magnetic field, i.e. a field applied parallel to the Ising-axis. The spin-dynamics have been investigated by means of inelastic neutron scattering experiments and it has been shown that above a critical field of 4 T, it corresponds to the one expected for a Tomonaga Luttinger liquid phase (TLL). Moreover, numerical calculations have shown that, because of the Ising-like anisotropy in this system, the majority of the spectral weight in the energy spectrum is carried by longitudinal spin fluctuations. This result is the signature of a quantum behavior without classical analogous with low energy longitudinal fluctuations driving the physics of the system. Finally, this is the first time that the dispersion spectrum for an Ising-like spin 1/2 chain could be probed in this TLL phase.The last two studies are preliminary work. The phase diagram of BaCo2V2O8 has been probed by calorimetric measurements under pressure and under a longitudinal magnetic field. Pressures up to 10 GPa have been obtained using a diamond anvil cell. We have then performed specific heat measurements allowing us to investigate the effect of pressure on the Hamiltonian of BaCo2V2O8 through its (H, P, T) phase diagram. Finally, we have also started to study the effect of the substitution of magnetic ions Co2+ by non-magnetic impurities Mg2+. The neutron diffraction experiments under a longitudinal magnetic field have shown that the critical temperature and critical field decrease proportionally to the concentration of impurities. The spin-dynamics at zero-field has also been investigated and reveals the appearance of non-dispersive magnetic modes, which possibly comes from the finite size effect of the spin chains segmented by the non-magnetic impurities.In conclusion, our experimental studies associated to numerical calculations allowed us to unveil a very rich physics in this model compound for the study of quantum magnetism and quantum phase transitions
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Chen, Linxiao. "Cartes planaires aléatoires couplées aux systèmes de spins." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS096/document.
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Cette thèse vise à améliorer notre compréhension des cartes planaires aléatoires décorées par les modèles de physique statistique. On examine trois modèles particuliers à l'aide des outils provenant de l'analyse, de la combinatoire et des probabilités. Dans une perspective géométrique, on se concentre sur les propriétés des interfaces et les limites locales des cartes aléatoires décorées. Le premier modèle consiste en une famille de quadrangulations aléatoires du disque décorées par un modèle de boucles O(n). Après avoir complété la preuve de son diagramme de phase initiée par [BBG12c] (chap. II), on étudie les longueurs et la structure d'imbrication des boucles dans la phase critique non-générique (chap. III). On montre que ces statistiques, décrites par un arbre étiqueté, convergent en loi vers une cascade multiplicative explicite lorsque le périmètre du disque tend vers l'infini. Le deuxième modèle (chap. IV) consiste en une carte planaire aléatoire décorée par la percolation de Fortuin-Kasteleyn. On complète la preuve de la convergence du modèle esquissée dans [She16b] et établit un certain nombre de propriétés de la limite. Le troisième modèle (chap. V) est celui des triangulations aléatoires du disque décorées par le modèle d'Ising. Il est étroitement lié au modèle des quadrangulations décorées par un modèle O(n) quand n=1. On calcule explicitement la fonction de partition du modèle muni des conditions au bord de Dobrushin au point critique, sous une forme exploitable pour les asymptotiques. À l'aide de ces asymptotiques, on étudie le processus d'épluchage le long de l'interface d'Ising dans la limite où le périmètre du disque tend vers l'infini. Mots clés. Carte planaire aléatoire, modèle de boucles O(n), percolation de Fortuin-Kasteleyn, modèle d'Ising, limite locale, géométrie d'interfacesThe aim of this thesis is to improve our understanding of random planar maps decorated by statistical physics models. We examine three particular models using tools coming from analysis, combinatorics and probability. From a geometric perspective, we focus on the interface properties and the local limits of the decorated random maps. The first model defines a family of random quadrangulations of the disk decorated by an O(n)-loop model. After completing the proof of its phase diagram initiated in [BBG12c] (Chap. II), we look into the lengths and the nesting structure of the loops in the non-generic critical phase (Chap. III). We show that these statistics, described as a labeled tree, converge in distribution to an explicit multiplicative cascade when the perimeter of the disk tends to infinity. The second model (Chap. IV) consists of random planar maps decorated by the Fortuin-Kasteleyn percolation. We complete the proof of its local convergence sketched in [She16b] and establish a number of properties of the limit. The third model (Chap. V) is that of random triangulations of the disk decorated by the Ising model. It is closely related to the O(n)-decorated quadrangulation when n=1. We compute explicitly the partition function of the model with Dobrushin boundary conditions at its critical point, in a form ameneable to asymptotics. Using these asymptotics, we study the peeling process along the Ising interface in the limit where the perimeter of the disk tends to infinity.Key words. Random planar map, O(n) loop model, Fortuin-Kasteleyn percolation, Ising model, local limit, interface geometry
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Litaiff, Fabian Cardoso. "Modelos de spins geometricamente frustrados: transição de fase e estruturas de platores." Universidade Federal de São Carlos, 2013. https://repositorio.ufscar.br/handle/ufscar/4963.
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Previous issue date: 2013-04-08Universidade Federal de Minas Gerais
This thesis presents a study of the magnetization plateau and the phenomenon of geometrical frustration in spin systems applied to lattices with triangular structure, as well as the results achieved by applying the differential operator technique of the Ising and Heisenberg models with external magnetic field applied to the easy magnetization axis z, their phase diagrams, behavior plateaus observed and analyzed according to the Haldane conjecture and appearance of plateaus presented by Oshikawa, Yamanaka and Affleck, and also to study the behavior of the magnetic susceptibility in order to verify the behavior of frustrated systems using the frustration factor f= θWC/TN to verify frustration encountered at various stages of the study models
Este trabalho apresenta um estudo da magnetização de platô e do fenômeno de frustração geométrica em sistemas de spins, aplicados às redes com estrutura triangular, bem como os resultados alcançados aplicando-se a técnica do operador diferencial a modelos de Ising e Heisenberg com campo magnético externo aplicado sobre o eixo fácil de magnetização z, seus diagramas de fases e comportamento de platôs observados e analisados segundo a conjectura de Haldane e a condição de aparecimento de platôs apresentada por Oshikawa, Yamanaka e Affleck, e ainda, o estudo do comportamento da susceptibilidade magnética com o objetivo de verificar o comportamento dos sistemas frustrados utilizando-se o fator de frustração f= θWC/TN para verificar a frustração nas diversas fases encontradas ao longo do estudo dos modelos.
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Amazonas, Márcio Andrei Sousa. "Desenvolvimento de um algoritmo numérico na técnica do operador diferencial : aplicações em modelos de spins." Universidade Federal de São Carlos, 2012. https://repositorio.ufscar.br/handle/ufscar/8922.
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Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM)
In this work, we present the results obtained for Ising models and Heisenberg spin 1/2, where two and three-dimensional, with disorder or frustration. We apply effective field theory associated with the Operator Differential Technique - TOD. A new formulation of this technique has enabled the development of a numerical implementation where the coefficients are now constructed fully automatically. This allowed growing up the number N of spins of the cluster and thus observe the behavior of the system when it tends to the real case (N→∞), which is bounded by the computational time needed to carry out all operations. We apply this methodology to study the Ising model with random field - RFIM, where we use three probability distributions for the field: bimodal, gaussian and gaussian double-peaked. The phase-diagrams were obtained in t - h plane for the cases Ferromagnetic-F and Antiferromagnetic-AF with the aid of Maxwell's construction procedure (equality of the free energies at line phase transition) identifying the tricritical point - PTC in each case. We present two proposals for obtaining the free energy, and in one of them it was possible to study the behavior of the thermodynamic properties in the regions of 1st and 2nd order. For a second application of numerical implementation, we use the quantum model of anisotropic Heisenberg spin (1/2) (with anisotropy parameter Δ), which lies in the particular cases that are important: one-dimensional Ising (Δ=1) and isotropic Heisenberg (Δ=0), being applied in the study of magnetic thin films formed by monolayers where the presence of free surfaces substantially alters the system behavior. We simulate this case, the spin frustration of considering interactions between the first (J₁) and second (J₂) interactions with neighboring F and AF respectively, being related by the parameter α=J₁/J₂. We studied the influence of increasing the dimensionality of the system, made by increasing the number of layers (L) of the film, the behavior of the phase diagram α - t. Finally, we apply the relations of the Renormalization Group in the Heisenberg Hamiltonian for a thin film to study the behavior of critical exponents as a function of parameters such as temperature and number of layers.
Apresentamos nesta tese os resultados obtidos para os modelos de Ising e Heisenberg de spin 1/2, nos casos bi e tridimensional, com desordem ou frustração. Aplicamos a teoria de campo efetivo associada à Técnica do Operador Diferencial - TOD. Uma nova formulação desta técnica permitiu o desenvolvimento de um algoritmo onde os coeficientes são agora construídos de forma totalmente automática. Isso possibilitou crescermos o número N de spins do aglomerado e assim observar o comportamento do sistema quando tende para o caso real (N→∞), tendo como limite o tempo computacional necessário para efetivar todas as operações. Aplicamos esta metodologia no estudo do modelo de Ising com campo aleatório - RFIM, onde utilizamos três distribuições de probabilidade para o campo: bimodal, gaussiana e gaussiana duplo-pico. Os diagramas de fase no plano t - h foram obtidos para os casos Ferromagnético-F e Antiferromagnético-AF com auxílio do procedimento da construção de Maxwell (igualdade das energias livres na linha de transição de fase), identificando o ponto tricrítico - PTC em cada caso. Apresentamos duas propostas para obtenção da energia livre, sendo que em uma delas foi possível o estudo do comportamento das propriedades termodinâmicas nas regiões de 1ª e 2ª ordem. Para uma segunda aplicação dessa implementação numérica, utilizamos o modelo de Heisenberg quântico de spin 1/2 anisotrópico (com parâmetro de anisotropia Δ), que recai nos casos particulares importantes que são: Ising unidimensional (Δ=1) e Heisenberg isotrópico (Δ=0), sendo aplicado no estudo de filmes finos magnéticos formados por monocamadas onde a presença de superfícies livres altera consideravelmente o comportamento do sistema. Simulamos nesse caso, a frustração dos spins considerando interações entre primeiros (J₁) e segundos (J₂) vizinhos com interações F e AF respectivamente, estando relacionados através do parâmetro α=J₁/J₂. Estudamos a influência do aumento da dimensionalidade do sistema, feito através do acréscimo no número de camadas (L) do filme, no comportamento do diagrama de fases t - α. Para finalizar, aplicamos as relações do Grupo de Renormalização no Hamiltoniano Heisenberg para um filme fino para o estudo do comportamento dos expoentes críticos em função de parâmetros como a temperatura e número de camadas.
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Vieira, Selma Rozane. "Efeitos de campos aleat?rios e de anisotropias em vidros de spins." Universidade Federal do Rio Grande do Norte, 1999. http://repositorio.ufrn.br:8080/jspui/handle/123456789/16583.
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Previous issue date: 1999-06-18Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
Ising and m-vector spin-glass models are studied, in the limit of infinite-range in-teractions, through the replica method. First, the m-vector spin glass, in the presence of an external uniform magnetic field, as well as of uniaxial anisotropy fields, is consi-dered. The effects of the anisotropics on the phase diagrams, and in particular, on the Gabay-Toulouse line, which signals the transverse spin-glass ordering, are investigated. The changes in the Gabay-Toulouse line, due to the presence of anisotropy fields which favor spin orientations along the Cartesian axes (m = 2: planar anisotropy; m = 3: cubic anisotropy), are also studied. The antiferromagnetic Ising spin glass, in the presence of uniform and Gaussian random magnetic fields, is investigated through a two-sublattice generalization of the Sherrington-Kirpaktrick model. The effects of the magnetic-field randomness on the phase diagrams of the model are analysed. Some confrontations of the present results with experimental observations available in the literature are discussed
Modelos de vidros de spins, dos tipos Ising e m-vetorial, com intera??es de al-cance infinito, s?o estudados atrav?s do m?todo das r?plicas. Inicialmente, o vidro de spins TO-vetorial, na presen?a de um campo magn?tico externo uniforme e de campos de anisotropias uniaxiais, ? considerado. Os efeitos das anisotropias nos diagramas de fases e, em particular, na linha de Gabay-Toulouse, que sinaliza o ordenamento dos graus de liberdade de vidro de spins transversos, s?o investigados. As modifica??es na linha de Gabay-Toulouse, devido ? presen?a de campos de anisotropia que privilegiam orienta??es segundo os eixos cartesianos (m = 2: anisotropia planar; m 3: anisotropia c?bica), tamb?m s?o estudadas. O vidro de spins de Ising com favorecimento antiferromagn?tico, na presen?a de campos magn?ticos uniforme e aleat?rio obedecendo uma distribui??o de probabilidades gaussiana, ? investigado atrav?s de uma generaliza??o do modelo Sherrington-Kirpaktrick em duas subredes. Os efeitos da aleatoriedade no campo magn?tico sobre os diagramas de fases do modelo s?o analisados. Algumas confronta??es entre resultados deste trabalho e medidas experimentais existentes na literatura s?o discutidas
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Homrighausen, Ingo [Verfasser]. "Quantum quench dynamics in the transverse field Ising model of fully connected spins : An entanglement based study beyond mean field / Ingo Homrighausen." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2021. http://d-nb.info/123154208X/34.
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Maria, Marco Aurélio Euflauzino [UNESP]. "Propriedades magnéticas do modelo de Ising bidimensional em uma rede bipartida de spins 1/2 e 1: um estudo de ferrimagnetismo com vacâncias." Universidade Estadual Paulista (UNESP), 2009. http://hdl.handle.net/11449/88503.
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maria_mae_me_bauru.pdf: 534060 bytes, checksum: 9cc6f4499e6abb250f5041a9db4d905a (MD5)O composto 'La IND. 2'Cu'O IND. 4' é um isolante antiferromagnético que tem sido amplamente estudado [1]. Por outro lado, a manganita LaMa'O IND. 3'[2] é o composto que dá origem a uma grande família de sistemas magnéticos como os materiais que apresentam a magnetorresistência colossal e o ordenamento de carga [3]. A substituição de íons multivalentes como, por exemplo, nos sítios do cobre no sistema ferrimagnético com fórmula 'La IND. 2''Cu IND. 1-x''Mn IND. x''O IND. 4-δ', apresenta a dificuldade da compreensão das interações magnéticas nos planos de Cu-O (onde estão presentes os átomos de cobre e de manganês). O objetivo desta dissertação é estudar um modelo teórico para o comportamento magnético de sistemas ferrimagnéticos que, assim como o sistema acima citado, estejam relacionados à interação entre duas sub-redes de átomos, através de simulações computacionais, almejando inclusive simular a transição de fase observada experimentalmente, e possivelmente prever o tipo de ordenamento do sistema através do diagrama de fases. Para tal, utilizou-se o modelo do tipo Ising clássico, que é bastante utilizado para simulação de fenômenos que envolvem transição de fase. Nosso modelo consiste de duas sub-redes magnéticas interpenetradas, uma contendo spins meio e a outra spins um. Os spins em cada sub-rede interagem ferromagneticamente entre si e há também uma interação antiferromagnética entre as sub-redes, de modo que o modelo possui três parâmetros independentes. Além disso, certa porcentagem de vacâncias pode ser introduzida no sistema. A técnica empregada é o Monte Carlo, a qual permite a obtenção de valores médios de quantidades físicas tais como energia e magnetização. Da análise dos resultados obtidos até agora podemos concluir que é possível fazer com que as sub-redes transicionem juntas (uma única temperatura de transição)...
The antiferromagnet insulator 'La IND. 2'Cu'O IND. 4' has recently been widely studied [1}. On the other hand, the manganite compound LaMa'O IND. 3'[2] gives origin to a large family of magnetic systems such as colossal magnetoresistence materials and thos presenting charge ordering [3]. The substitution of multivalent ions as, for instance, on the cooper sistes in the ferrimagnet 'La IND. 2''Cu IND. 1-x''Mn IND. x''O IND. 4-δ', presents the difficulty of understanding the magnetic interactions inside the Cu-O planes (which contain the Cu and Mn atoms.). The aim of this master's thesis is to study a theoretical model for de magnetic behavior of the ferrimagnetic systems that, like de system above mentioned, are related to the interaction between two atoms sublattices through computational simulations, including the simulation of the experimentally observed phase transition and also possibly predicting what kind of magnetic ordering appears in phase diagram. The model studied is a classical Ising-like model. The Ising model is largely used in simulations of phase transitions. Our model consist of two interpenetrated magnetic sublattices, one containing spins one-half and the other spins one. The spins in each sublattice interact one with another ferromagneticaly and there is also an antiferromagnetic interaction between the sublattices. Besides that, a certain amount of site vacancies can be introduced in the system. Therefore, the model has three independent parameters. The Monte Carlo technique is usde, yielding the mean values of physical quantities such as energy and magnetization. By analyzing the results we obtained until now we concluded that it is possible to have the sublattices going through a phase transition together (a single transition temperature) or separately (two transition temperatures). Such control can be achieved in two different ways: by adjusting the magnitude... (Complete abstract click electronic access below)
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Maria, Marco Aurélio Euflauzino. "Propriedades magnéticas do modelo de Ising bidimensional em uma rede bipartida de spins 1/2 e 1 : um estudo de ferrimagnetismo com vacâncias /." Bauru : [s.n.], 2009. http://hdl.handle.net/11449/88503.
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Orientador: André Luiz MalvezziBanca: José Varalda
Banca: José Humberto Dias da Silva
O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp
Resumo: O composto 'La IND. 2'Cu'O IND. 4' é um isolante antiferromagnético que tem sido amplamente estudado [1]. Por outro lado, a manganita LaMa'O IND. 3'[2] é o composto que dá origem a uma grande família de sistemas magnéticos como os materiais que apresentam a magnetorresistência colossal e o ordenamento de carga [3]. A substituição de íons multivalentes como, por exemplo, nos sítios do cobre no sistema ferrimagnético com fórmula 'La IND. 2''Cu IND. 1-x''Mn IND. x''O IND. 4-δ', apresenta a dificuldade da compreensão das interações magnéticas nos planos de Cu-O (onde estão presentes os átomos de cobre e de manganês). O objetivo desta dissertação é estudar um modelo teórico para o comportamento magnético de sistemas ferrimagnéticos que, assim como o sistema acima citado, estejam relacionados à interação entre duas sub-redes de átomos, através de simulações computacionais, almejando inclusive simular a transição de fase observada experimentalmente, e possivelmente prever o tipo de ordenamento do sistema através do diagrama de fases. Para tal, utilizou-se o modelo do tipo Ising clássico, que é bastante utilizado para simulação de fenômenos que envolvem transição de fase. Nosso modelo consiste de duas sub-redes magnéticas interpenetradas, uma contendo spins meio e a outra spins um. Os spins em cada sub-rede interagem ferromagneticamente entre si e há também uma interação antiferromagnética entre as sub-redes, de modo que o modelo possui três parâmetros independentes. Além disso, certa porcentagem de vacâncias pode ser introduzida no sistema. A técnica empregada é o Monte Carlo, a qual permite a obtenção de valores médios de quantidades físicas tais como energia e magnetização. Da análise dos resultados obtidos até agora podemos concluir que é possível fazer com que as sub-redes transicionem juntas (uma única temperatura de transição)... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The antiferromagnet insulator 'La IND. 2'Cu'O IND. 4' has recently been widely studied [1}. On the other hand, the manganite compound LaMa'O IND. 3'[2] gives origin to a large family of magnetic systems such as colossal magnetoresistence materials and thos presenting charge ordering [3]. The substitution of multivalent ions as, for instance, on the cooper sistes in the ferrimagnet 'La IND. 2''Cu IND. 1-x''Mn IND. x''O IND. 4-δ', presents the difficulty of understanding the magnetic interactions inside the Cu-O planes (which contain the Cu and Mn atoms.). The aim of this master's thesis is to study a theoretical model for de magnetic behavior of the ferrimagnetic systems that, like de system above mentioned, are related to the interaction between two atoms sublattices through computational simulations, including the simulation of the experimentally observed phase transition and also possibly predicting what kind of magnetic ordering appears in phase diagram. The model studied is a classical Ising-like model. The Ising model is largely used in simulations of phase transitions. Our model consist of two interpenetrated magnetic sublattices, one containing spins one-half and the other spins one. The spins in each sublattice interact one with another ferromagneticaly and there is also an antiferromagnetic interaction between the sublattices. Besides that, a certain amount of site vacancies can be introduced in the system. Therefore, the model has three independent parameters. The Monte Carlo technique is usde, yielding the mean values of physical quantities such as energy and magnetization. By analyzing the results we obtained until now we concluded that it is possible to have the sublattices going through a phase transition together (a single transition temperature) or separately (two transition temperatures). Such control can be achieved in two different ways: by adjusting the magnitude... (Complete abstract click electronic access below)
Mestre
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Platini, Thierry. "Chaînes de spins quantiques hors de l'équilibre." Phd thesis, Université Henri Poincaré - Nancy I, 2008. http://tel.archives-ouvertes.fr/tel-00439896.
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Les travaux exposés dans ce manuscrit sont consacrés à l'étude de la dynamique hors équilibre de chaînes quantiques décrites par le modèle XY. Nous commençons par considérer la dynamique unitaire obtenue par la mise en contact de sous-systèmes voisins thermalisés à des températures différentes. L'état initial de la chaîne est alors inhomogène et la dynamique tend à l'homogénéisation. Lorsque le système est initialement divisé en deux sous-systèmes semi-infini préparés aux températures $T_b=\infty$ et $T_s$ nous obtenons analytiquement la fonction de Green associée à la dynamique du courant et du profil d'aimantation. Les résultats sont généralisés pour les températures $T_b$ finies permettant l'étude de l'état stationnaire. Dans le cas particulier où $T_s=T_b=0$, nous étudions le comportement de l'entropie d'intrication entre sous-systèmes. Cette quantité présente un accroissement "rapide", prédit par la théorie conforme (dans le cas d'un système critique), suivi d'une relaxation algébrique vers la valeur d'équilibre. Dans la dernière partie la dynamique du système est obtenue par l'interaction avec l'environnement, décrite par le processus d'interactions répétées. Nous examinons la structure de la matrice densité réduite du système et donnons une équation d'évolution de l'ensemble des corrélateurs à deux points. Finalement, nous étudions l'évolution temporelle du modèle $XX$ en contact avec un ou deux bains aux températures $T_1$ et $T_2$. Lorsque $T_1=T_2$, l'étude du comportement du système, pour les temps courts, dévoile l'état stationnaire. Dans la situation $T_1\ne T_2$, nous vérifions numériquement que le profil d'aimantation est plat et proposons l'introduction d'un désordre dynamique qui permet l'installation d'un gradient d'aimantation.
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Freitas, Augusto dos Santos. "Estudo de sistemas magnéticos desordenados via modelos clássicos de spins por meio de técnicas analíticas." Universidade Federal de Sergipe, 2014. https://ri.ufs.br/handle/riufs/5250.
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In this work, we study the magnetic properties of classical spin models, namely spin-2 Ising
model with site dilution and mixed-bond spin 1/2 Ising model by means of the Effective Field
Theory (EFT), with applications to describe of the magnetic properties of Fe-Al, Fe-Mn and
Fe-Mn-Al alloys. In here, we obtain the van der Waerden identity for a generic spin value S,
for example, to be used in the description of spin-2 Ising model and expressions were used to
phenomenological description of the dependence of exchange interaction on the concentration
of aluminum and manganese atoms. Behavior of magnetization versus temperature, critical
temperature as a function of the concentration of aluminum atoms and exchange interaction as
a function of the concentration of aluminum atoms to the Fe-Al alloys were studied. Furthermore,
this model allows for a more accurate determination of the critical values qc and Tc for
q = 0. For the Fe-Mn alloys were described the M(T), zero field susceptibility as a function of
temperature, critical temperature versus manganese concentration and average hyperfine field
as a function of the manganese concentration. For the Fe-Mn-Al alloys, the magnetization as
a function of temperature, magnetization as a function of the manganese concentration, critical
temperature versus iron concentration and average hyperfine field as a function of the aluminum
concentration were studied. It is shown that the EFT technique is not only a robust technique for
the description of the thermodynamic properties of classical spin models and can also be widely
applied to obtain phase diagrams of real magnetic systems, with the advantage of reduced computational
cost compared to the other techniques. All phase diagrams described in this work
were obtained through the numerical solution of the equations arising from the approximations
made by the EFT approach. Finally, prospects of use of the other models are described, as well
as other analytical techniques to the description of frustrated magnetic systems.Neste trabalho, são estudadas as propriedades magnéticas de modelos clássicos de spins, a saber os modelos de Ising de spin 2 com diluição por sítios e de spin 1/2 com interações mistas, por meio da Técnica do Operador Diferencial (DOT), com aplicações à descrição das propriedades magnéticas de ligas Fe-Al, Fe-Mn e Fe-Mn-Al. Para tanto, foi descrito um método simples para obtenção da identidade de van der Waerden para um valor genérico de spin $S$, por exemplo, para ser utilizada na descrição do modelo de Ising de spin 2, e foram utilizadas expressões fenomenológicas para a descrição da dependência da interação de troca relativamente à concentração de átomos de alumínio e manganês para o estudo das propriedades das ligas consideradas. Os resultados obtidos indicam que os modelos clássicos utilizados, aliados à Técnica do Operador Diferencial, são alternativas viáveis para a descrição física de sistemas magnéticos reais. Diagramas de magnetização {it versus} temperatura, temperatura crítica como função da concentração de átomos de alumínio e interação de troca como função da concentração de átomos de alumínio, para as ligas Fe-Al, foram estudados. No caso do diagrama de magnetização como função da temperatura, os resultados para o modelo de spin 2 com diluição por sítios são qualitativamente idênticos aos do modelo de Ising de spin 1/2, com a diferença de que os valores obtidos para a magnetização por sítio no estado fundamental diferem daqueles obtidos para o modelo de Ising de dois estados. Além disso, tal modelo permite uma determinação mais precisa dos valores da concentração crítica de átomos de alumínio, $q_c$, acima da qual a magnetização espontânea vai a zero em $T>0$, e da temperatura crítica $T_c$ para $q=0$. Para as ligas Fe-Mn, foram descritos os diagramas magnetização {it versus} temperatura, susceptibilidade a campo nulo como função da temperatura, temperatura crítica {it versus} concentração de átomos de manganês e campo hiperfino médio como função da concentração de átomos de manganês. A comparação entre os resultados teóricos e experimentais demonstra boa concordância com o modelo utilizado. No caso das ligas Fe-Mn-Al, foram estudados os diagramas de magnetização como função da temperatura, magnetização {it versus} concentração de átomos de alumínio, magnetização como função da concentração de átomos de manganês, temperatura crítica {it versus} concentração de átomos de ferro e campo hiperfino médio como função da concentração de átomos de alumínio. A concordância teoria-experimento é excelente e demonstra a viabilidade dos modelos utilizados para a descrição das propriedades magnéticas de tais ligas. Neste trabalho, mostra-se que a Técnica do Operador Diferencial não é somente uma técnica robusta para a descrição das propriedades termodinâmicas de modelos clássicos de spins como também pode ser amplamente aplicada para obtenção de diagramas de fase de sistemas magnéticos reais, com grande vantagem de custo computacional em comparação com outras técnicas. Tal técnica também pode ser, como discutido neste trabalho, interpretada como caso particular de outra mais geral: o Método Variacional. Todos os diagramas de fase aqui descritos foram obtidos por meio da resolução numérica das equações oriundas das aproximações feitas por meio da Técnica do Operador Diferencial. A utilização do Método Variacional em sua aproximação de campo médio, aplicada ao modelo XY clássico em duas dimensões, no estudo de sistemas magnéticos frustrados, tais como as jarositas, surge como perspectiva de trabalhos futuros.
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Jassionnesse, Lionel. "Contrôle optimal et métriques de Clairaut-Liouville avec applications." Thesis, Dijon, 2014. http://www.theses.fr/2014DIJOS047/document.
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Le travail de cette thèse porte sur l'étude des lieux conjugué et de coupure de métriques riemanniennes ou pseudo-riemanniennes en dimension 2. On se place du point de vue du contrôle optimal pour appliquer le principe du maximum de Pontryagin afin de caractériser les extrémales des problèmes considérés.On va utiliser des méthodes géométriques, numériques et d'intégrabilité pour étudier des métriques de Clairaut-Liouville ou de Liouville sur la sphère. Dans le cas dégénéré de révolution, l'étude de l'ellipsoïde utilise des méthodes géométriques pour déterminer le lieu de coupure et la nature du lieu conjugué dans les cas oblat et prolat. Dans le cas général, les extrémales auront deux types de comportements distincts qui se rapportent à ceux observés dans le cas de révolution, et sont séparés par celles passant par des points ombilicaux. Les méthodes numériques sont utilisées pour retrouver rapidement la dernière conjecture géométrique de Jacobi : le lieu de coupure est un segment et le lieu conjugué contient quatre points de rebroussement.L'étude d'une métrique pseudo-riemannienne vient d'un problème de contrôle quantique où le but est de transférer en temps minimal l'état d'un spin à travers une chaîne de trois spins couplés par des interactions de type Ising. Après réduction, la métrique obtenue possède une intégrale première supplémentaire et on peut donc la mettre sous forme de Liouville, ce qui nous donne les équations des géodésiques. En dehors du cas particulier de Grushin, dont la caustique est décrite, on utilise les méthodes numériques pour étudier le lieu conjugué et le lieu de coupure dans le cas généralThe work of this thesis is about the study of the conjugate and cut loci of 2D riemannian or almost-riemannian metrics. We take the point of view of optimal control to apply the Pontryagin Maximum Principle in the purpose of characterize the extremals of the problem considered.We use geometric, numerical and integrability methods to study some Liouville and Clairaut-Liouville metrics on the sphere. In the degenerate case of revolution, the study of the ellipsoid uses geometric methods to fix the cut locus and the nature of the conjugate locus in the oblate and prolate cases. In the general case, extremals will have two distinct type of comportment which correspond to those observed in the revolution case, and are separated by those which pass by umbilical points. The numerical methods are used to find quickly the Jacobi's Last Geometric Statement : the cut locus is a segment and the conjugate locus has exactly four cusps.The study of an almost-riemannian metric comes from a quantum control problem in which the aim is to transfer in a minimal time the state of one spin through an Ising chain of three spins. After reduction, we obtain a metric with a second first integral so it can be written in the Liouville normal form, which leads us to the equations of geodesics. Outside the particular case of Grushin, of which the caustic is described, we use numerical methods to study the conjugate locus and the cut locus in the general case
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Atas, Yasar Yilmaz. "Quelques aspects du chaos quantique dans les systèmes de N-corps en interaction : chaînes de spins quantiques et matrices aléatoires." Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112221/document.
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Mon travail de thèse est consacré à l’étude de quelques aspects de la physique quantique des systèmes quantiques à N corps en interaction. Il est orienté vers l’étude des chaînes de spins quantiques. Je me suis intéressé à plusieurs questions relatives aux chaînes de spins quantiques, du point de vue numérique et analytique à la fois. J'aborde en particulier les questions relatives à la structure des fonctions d'onde, la forme de la densité d'états et les propriétés spectrales des Hamiltoniens de chaînes de spins. Dans un premier temps, je présenterais très rapidement les techniques numériques de base pour le calcul des vecteurs et valeurs propres des Hamiltonien de chaînes de spins. Les densités d’états des modèles quantiques constituent des quantités importantes et très simples qui permettent de caractériser les propriétés spectrales des systèmes avec un grand nombre de degrés de liberté. Alors que dans la limite thermodynamique, les densités d'états de la plupart des modèles intégrables sont bien décrites par une loi gaussienne, dans certaines limites de couplage de la chaîne de spins au champ magnétique et pour un nombre de spins N fini sur la chaîne, on observe l’apparition de pics dans la densité d’états. Je montrerais que la connaissance des deux premiers moments du Hamiltonien dans le sous-espace dégénéré associé à chaque pics donne une bonne approximation de la densité d’états. Dans un deuxième temps je m'intéresserais aux propriétés spectrales des Hamiltoniens de chaînes de spins quantiques. L’un des principal résultats sur la statistique spectrale des systèmes quantiques concerne le comportement universel des fluctuations des mesures telles que l’espacement entre valeurs propres consécutives. Ces fluctuations sont bien décrites par la théorie des matrices aléatoires mais la comparaison avec les prédictions de cette théorie nécessite généralement une opération sur le spectre du Hamiltonien appelée unfolding. Dans les problèmes quantiques de N corps, la taille de l’espace de Hilbert croît généralement exponentiellement avec le nombre de particules, entraînant un manque de données pour pouvoir faire une statistique. Ces limitations ont amené l’introduction d’une nouvelle mesure se passant de la procédure d’unfolding basée sur le rapport d’espacements successifs plutôt que les espacements. En suivant l’idée du “surmise” de Wigner pour le calcul de la distribution de l’espacement, je montre comment calculer une approximation de la distribution du rapport d’espacements dans les trois ensembles gaussiens invariants en faisant le calcul pour des matrices 3x3. Les résultats obtenus pour les différents ensembles de matrices aléatoires se sont révélés être en excellent accord avec les résultats numériques. Enfin je m’intéresserais à la structure des fonctions d’ondes fondamentales des modèles de chaînes de spins quantiques. Les fonctions d’onde constituent, avec le spectre en énergie, les objets fondamentaux des systèmes quantiques : leur structure est assez compliquée et n’est pas très bien comprise pour la plupart des systèmes à N corps. En raison de la croissance exponentielle de la taille de l’espace de Hilbert avec le nombre de particules, l’étude des vecteurs propres est une tâche très difficile, non seulement du point de vue analytique mais aussi du point de vue numérique. Je démontrerais en particulier que l’état fondamental de tous les modèles que nous avons étudiés est multifractal avec en général une dimension fractale non trivialeMy thesis is devoted to the study of some aspects of many body quantum interacting systems. In particular we focus on quantum spin chains. I have studied several aspects of quantum spin chains, from both numerical and analytical perspectives. I addressed especially questions related to the structure of eigenfunctions, the level densities and the spectral properties of spin chain Hamiltonians. In this thesis, I first present the basic numerical techniques used for the computation of eigenvalues and eigenvectors of spin chain Hamiltonians. Level densities of quantum models are important and simple quantities that allow to characterize spectral properties of systems with large number of degrees of freedom. It is well known that the level densities of most integrable models tend to the Gaussian in the thermodynamic limit. However, it appears that in certain limits of coupling of the spin chain to the magnetic field and for finite number of spins on the chain, one observes peaks in the level density. I will show that the knowledge of the first two moments of the Hamiltonian in the degenerate subspace associated with each peak give a good approximation to the level density. Next, I study the statistical properties of the eigenvalues of spin chain Hamiltonians. One of the main achievements in the study of the spectral statistics of quantum complex systems concerns the universal behaviour of the fluctuation of measure such as the distribution of spacing between two consecutive eigenvalues. These fluctuations are very well described by the theory of random matrices but the comparison with the theoretical prediction generally requires a transformation of the spectrum of the Hamiltonian called the unfolding procedure. For many-body quantum systems, the size of the Hilbert space generally grows exponentially with the number of particles leading to a lack of data to make a proper statistical study. These constraints have led to the introduction of a new measure free of the unfolding procedure and based on the ratio of consecutive level spacings rather than the spacings themselves. This measure is independant of the local level density. By following the Wigner surmise for the computation of the level spacing distribution, I obtained approximation for the distribution of the ratio of consecutive level spacings by analyzing random 3x3 matrices for the three canonical ensembles. The prediction are compared with numerical results showing excellent agreement. Finally, I investigate eigenfunction statistics of some canonical spin-chain Hamiltonians. Eigenfunctions together with the energy spectrum are the fundamental objects of quantum systems: their structure is quite complicated and not well understood. Due to the exponential growth of the size of the Hilbert space, the study of eigenfunctions is a very difficult task from both analytical and numerical points of view. I demonstrate that the groundstate eigenfunctions of all canonical models of spin chain are multifractal, by computing numerically the Rényi entropy and extrapolating it to obtain the multifractal dimensions
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Sarvezuk, Paulo Willian Carvalho. "Estudo do sistema quase unidimensional AxA'1-xNb2O6 (A e A'=Ni, Fe e Co) : preparação e caracterização das propriedades e estruturas magnéticas." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2011. http://hdl.handle.net/10183/49345.
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Cette étude expérimentale est consacrée à la structure cristalline et aux propriétés magnétiques des phases orthorhombiques ANb2O6 (A = métaux magnétiques) qui ont retenues notre attention en tant que système Ising modèle quasi 1D. Ce comportement magnétique original de basse dimension résulte à la fois de la force des interactions magnétiques le long des chaînes d'atomes magnétiques quasi-unidimensionnelles, et à la faiblesse des interactions entre les chaînes qui sont de nature antiferromagnétique. Lorsque ces composés sont ordonnés l’ensemble de ces interactions inter et intra chaîne conduit à un ordre antiferromagnétique. Notre investigation s’appuie sur une caractérisation systématique de la série de composés AxA’1-xNb2O6 (A et A’ = Ni, Fe et Co), par des mesures variées et complémentaires, notamment: diffraction des rayons X à température ambiante, diffraction de neutrons au dessus et en dessous de la température d'ordre magnétique, mesures magnétiques : évolution thermique de courbes d’aimantation isochamp et mesures d’aimantation isotherme. De plus, des mesures de la chaleur spécifique et de spectroscopie Mössbauer ont été réalisées sur certains échantillons sélectionnés. Nous avons mis à jour, selon la concentration, des comportement très différents dans les systèmes pseudo binaires Fe-Co, Ni-Fe ou Co-Ni qui peuvent soit présenter un état ordonné soit conserver un état paramagnétique jusqu'à de très basses températures. Nos mesures démontrent que la température d’ordre magnétique et les vecteurs de propagations diffèrent sensiblement selon la composition car ces systèmes sont caractérisés par une compétition entre les interactions magnétiques mises en jeu dans un réseau triangulaire interchaines. Cette étude montre que le désordre cationique Fe/Co induit une réduction substantielle des interactions tant inter que intra chaînes, ce qui traduit la tendance à défavoriser l’établissement d’un ordre magnétique à longue portée. De manière similaire, le système NixFe1-xNb2O6 ne présente pas d’ordre magnétique pour x=0,2 et ceci jusqu’à 400 mk au moins.Este trabalho é dedicado à investigação das propriedades estruturais e magnéticas destes compostos de estrutura ortorrômbica (A = metais magnéticos) ANb2O6 que chamaram nossa atenção como um sistema de Ising quase-1D. Esse comportamento magnético original de baixa dimensionalidade é resultado das interações magnéticas ao longo das cadeias de átomos magnéticos quase uni-dimensional, e ainda pelas fracas interações antiferromagnética entre as cadeias. Quando esses compostos são ordenadas todas essas interações dentro e entre as cadeias leva a uma ordem global antiferromagnético. Nossa investigação é baseada em uma caracterização sistemática da série de compostos AxA’1-xNb2O6 (A et A’ = Ni, Fe e Co), por diferentes tipos de medidas complementares, tais como: difração de raios X à temperatura ambiente, difração de nêutrons acima e abaixo da temperatura de ordenamento magnético, medidas magnéticas: evolução térmica das curvas de magnetização e isoterma de magnetização sob variação de campo aplicado. Além disso, medidas de calor específico e espectroscopia Mössbauer foram realizadas em algumas amostras selecionadas. Nós encontramos interessantes resultados, para diferentes concentrações, com diferentes comportamentos nos sistemas pseudobinários Fe-Co, Ni-Fe ou Co-Ni, que podem permanecer no estado paramagnético até temperaturas muito baixas ou ordenarse dependendo da série analisada. Nossas medidas mostram que a temperatura de ordenamento magnético e vetores de propagação diferem substancialmente, dependendo da composição, por esses sistemas serem caracterizados por uma competição entre as interações magnéticas que estão envolvidas em uma rede triangular intermoleculares. Este estudo mostra que a desordem catiônicos Fe/Co induz uma redução substancial em ambas as interações dentro das; e entre as; cadeias, refletindo a tendência de se opor à criação de uma ordem de longo alcance magnético. Da mesma forma, o sistema NixFe1-xNb2O6 não mostra ordenamento magnético nem em medidas de até 400 mk realizadas para x = 0,2.
This work is dedicated to the investigation of structural and magnetic properties of these compounds with orthorhombic structure (A = magnetic metals) ANb2O6 that caught our attention as a system of quasi-1D Ising. This unique magnetic behavior is the result of low-dimensional magnetic interaction along the chains of magnetic atoms almost uni-dimensional, and also by weak antiferromagnetic interactions between the chains. When these compounds are ordered all these interactions between and within the chain leads to a global antiferromagnetic order. Our research is based on a systematic characterization of the series of compounds AxA'1-xNb2O6 (A et A' = Ni, Fe and Co) by differents types of complementary measures, such as X-ray diffraction at room temperature, neutron diffraction above and below the magnetic ordering temperature, magnetic measurements: thermal evolution of magnetization curves and isotherm magnetization variation in applied field. In addition, specific heat measurements and Mössbauer spectroscopy were performed on some selected samples. We found interesting results depending on the concentration, different behaviors on these pseudo binary systems Ni-Fe, Fe-Co, or Co-Ni, which can remain in the paramagnetic state up to very low temperatures or to order depending on the series analyzed. Our measurements show that the magnetic ordering temperature and propagation vectors differ substantially depending on the composition, for these systems are characterized by a competition between magnetic interactions that are involved in a triangular lattice intermolecular. This study shows that disorder cationic Fe / Co induced a substantial reduction in both the interactions within and between them; chains, reflecting the tendency to oppose the creation of a long-range magnetic order. Likewise, the system NixFe1-xNb2O6 dont shows magnetic ordering even at measures taken up to 400 mk for x = 0.2.
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Hystad, Grethe. "Periodic Ising Correlations." Diss., The University of Arizona, 2009. http://hdl.handle.net/10150/196130.
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We consider the finite two-dimensional Ising model on a lattice with periodic boundaryconditions. Kaufman determined the spectrum of the transfer matrix on the finite,periodic lattice, and her derivation was a simplification of Onsager's famous result onsolving the two-dimensional Ising model. We derive and rework Kaufman's resultsby applying representation theory, which give us a more direct approach to computethe spectrum of the transfer matrix. We determine formulas for the spin correlationfunction that depend on the matrix elements of the induced rotation associated withthe spin operator. The representation of the spin matrix elements is obtained byconsidering the spin operator as an intertwining map. We wrap the lattice aroundthe cylinder taking the semi-infinite volume limit. We control the scaling limit of themulti-spin Ising correlations on the cylinder as the temperature approaches the criticaltemperature from below in terms of a Bugrij-Lisovyy conjecture for the spin matrixelements on the finite, periodic lattice. Finally, we compute the matrix representationof the spin operator for temperatures below the critical temperature in the infinite-volume limit in the pure state defined by plus boundary conditions.
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Feng, Shuangtong. "Efficient Parallelization of 2D Ising Spin Systems." Thesis, Virginia Tech, 2001. http://hdl.handle.net/10919/36263.
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The problem of efficient parallelization of 2D Ising spin systems requires realistic algorithmic design and implementation based on an understanding of issues from computer science and statistical physics. In this work, we not only consider fundamental parallel computing issues but also ensure that the major constraints and criteria of 2D Ising spin systems are incorporated into our study. This realism in both parallel computation and statistical physics has rarely been reflected in previous research for this problem.
In this thesis,we designed and implemented a variety of parallel algorithms for both sweep spin selection and random spin selection. We analyzed our parallel algorithms on a portable and general parallel machine model, namely the LogP model. We were able to obtain rigorous theoretical run-times on LogP for all the parallel algorithms. Moreover, a guiding equation was derived for choosing data layouts (blocked vs. stripped) for sweep spin selection. In regards to random spin selection, we were able to develop parallel algorithms with efficient communication schemes. We analyzed randomness of our schemes using statistical methods and provided comparisons between the different schemes. Furthermore, algorithms were implemented and performance data gathered and analyzed in order to determine further design issues and validate theoretical analysis.
Master of Science
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Canning, Andrew Magnus. "Ising spin models of partially connected neural networks." Thesis, University of Edinburgh, 1988. http://hdl.handle.net/1842/13304.
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Leite, Filho Raimundo Valmir. "Impurezas Magnéticas em Ferromagnetos de Ising com Campo Transverso." reponame:Repositório Institucional da UFC, 2005. http://www.repositorio.ufc.br/handle/riufc/12930.
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LEITE FILHO, Raimundo Valmir. Impurezas Magnéticas em Ferromagnetos de Ising com Campo Transverso. 2005. 109 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2005.Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-23T18:17:55Z No. of bitstreams: 1 2005_tese_rvleitefilho.pdf: 656815 bytes, checksum: af381150f33bff4b9142c5eeef4e117b (MD5)
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A Green function formalism is used to calculate the spectrum of excitations associated with magnetic impurities implanted in a ferromagnetic thin film described by the transverse Ising model. Using the equations of motion method, explicit expressions for the Green function are determined for a ferromagnetic without impurities. The Green’s functions for a ferromagnetic film containing impurities are obtained through Dyson equation. We consider only the “defect” modes that appear below the bulk band of the pure material. In order to assess the influence of the position of the impurities in the film on the excitations spectra, we consider three different geometrical arrangement for the impurities: impurities line perpendicular to the surface film, four impurities in a plane paralel to the surface, and four impurities in a plane perpendicular to the surfaces of the film. We obtain results for the frequencies localized modes as a function of the exchange parameter between two impurities neighborings, of the exchange parameter between the impurities and their neighborings, and the effective field parameter at the impurities.
O formalismo de funções de Green é usado para calcular o espectro de excitações associados a impurezas magnéticas implantadas em um filme ferromagnético descrito pelo modelo de Ising com campo transverso. Através do método da equação de movimento expressões explícitas para as funções de Green são determinadas para um ferromagneto sem impurezas. A função de Green para um filme ferromagnético contendo impurezas é obtida através da equação de Dyson. O espectro de ondas de spin relativo ás impurezas é obtido para freqüências abaixo do limite inferior da banda de volume para um material puro. Com o objetivo de avaliar a influência da posição das impurezas no filme no espectro de excitações, consideramos três diferentes disposições geométricas para as impurezas: linha de impurezas perpendicular á superfície do filme, quatro impurezas em um plano paralelo `as superfícies e quatro impurezas em um plano perpendicular `as superfícies do filme. Obtemos resultados para a freqüência dos modos localizados como função do parâmetro de troca entre duas impurezas vizinhas, do parâmetro de troca entre as impurezas e seus vizinhos e do parâmetro de campo efetivo nas impurezas.
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Cochran, Christopher S. "Even-number spin correlations on two-dimensional Ising lattice structures." Virtual Press, 2002. http://liblink.bsu.edu/uhtbin/catkey/1237760.
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Many physical systems can be represented by a regular arrangement of molecules in a lattice structure. Knowing how neighboring molecules in the lattice interact with one another can give great insight into a material's macroscopic behavior. A very popular and effective means of investigating these microscopic interactions is the Ising Model. This model, suggested first by Wilhelm Lenz in 1920 and later expanded by Ernst Ising in 1925, is based on the assumptions that each molecule in a lattice structure can be represented by its spin value (+l or -1) and that only nearest neighbors contribute to the total interaction energy. The Ising Model, which was initially used in the study of ferromagnetic systems, can now be used to study a variety of physical systems. Some of these include antiferromagnetic crystals, binary alloys, DNA, and lattice gasses.Department of Physics and Astronomy
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BERNARDI, LORENZO. "Dynamique et exposants critiques dans les verres de spin ising." Paris 11, 1996. http://www.theses.fr/1996PA112314.
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Le sujet de cette these porte sur les verres de spin ising, qui sont les archetypes des systemes complexes en physique des solides. Bien que la phase verre de spin presente des aspects remarquables tels que le vieillissement nous nous sommes plutot interesse au comportement des verres de spin a la transition. Dans la premiere partie de cette these les verres de spin sont introduit d'un point de vue experimental, afin de degager les concepts clefs, a savoir la frustration et le desordre. Muni de ces concepts il est alors possible d'aborder les modeles theoriques. Enfin pour conclure la partie introductive une presentation des bases de la methode monte carlo est donnee. La seconde partie est la presentation de nos resultats. Dans un premier temps nous avons etudie l'influence de la distribution des interactions sur les exposants critiques. Dans le cadre de la theorie des phenomenes critiques les exposants critiques ne doivent varier qu'avec la dimension d'espace et le nombre de composantes du parametre d'ordre ; c'est la notion de classe d'universalite. Nous apportons ici une preuve numerique de la non-universalite pour les verres de spin ising. Dans un second temps nous nous sommes interesse aux proprietes des verres de spin pour des temperatures superieures a la temperature de transition. En effet l'etude de la relaxation dans les verres de spin indique l'existence d'un regime non-exponentiel compris dans une gamme de temperatures allant de la temperature de transition a une temperature environ quatre fois superieure. Afin de determiner cette temperature nous avons utilise plusieurs techniques numeriques. Enfin nous avons etudie l'influence du desordre sur un systeme completement frustre. Ces deux dernieres etudes nous ont alors permis de donner une explication simple, basee sur l'espace des phases, de la transition verre de spin
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Mehrafarin, M. "Some topics in the theory of mixed spin ising models." Thesis, University of Liverpool, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.377142.
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Nakamura, Y., and J. W. Tucker. "Monte Carlo Study of a Mixed Spin-1 and Spin-3/2 Ising Ferromagnet." IEEE, 2002. http://hdl.handle.net/2237/7158.
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Hernandez, Hernandez Fabio 1990. "Estados de impureza no modelo de Ising quântico." [s.n.], 2016. http://repositorio.unicamp.br/jspui/handle/REPOSIP/322412.
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Orientador: Guillermo Gerardo Cabrera OyarzúnDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: A descrição da dinâmica quântica de sistemas de muitos corpos é um ingrediente chave para computação e simulações quânticas. No presente projeto, estudamos a dinâmica de cadeias de spin na presença de impurezas ou defeitos. O sistema de Ising quantico (Ising com campo transverso) com uma impureza foi solucionado de forma exata. Este sistema de spins pode ser simulado de forma analítica por partículas quânticas (transformação de Jordan-Wigner). Caracterizamos o espectro, as autofunções e a evolução temporal da magnetização para estados iniciais particulares, focando no papel desempenhado pelos estados de impureza. Finalmente observamos oscilações remanescentes na magnetização, após a relaxação do sistema, para alguns valores dos parâmetros da impureza nos quais existem dois estados ligados no espectro de energias
Abstract: The description of dynamics of quantum many-body systems is a key ingredient to perform quantum computation and/or simulations of quantum behavior. In the present proposal, we study the time evolution of quantum spin chains with impurities at one of the boundaries, in order to understand the role of defects in relaxation properties. The quantum (transverse) Ising model with an impurity has been solved in exact form, using the Jordan-Wigner transformation, where spins are mapped onto spinless fermions, thus simulating analytically a spin system with particles. We completely characterize the spectrum, with the presence of bound states depending on values of the impurity parameters. We calculate the local magnetization and observe its relaxation for particular non-homogeneous initial states. Surprisingly, remanent Rabi oscillations are observed at asymptotically long times, when the spectrum displays two bound states
Mestrado
Física
Mestre em Física
1247646/2013
CAPES
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Silva, Romero Tavares da. "ALEATORIEDADE EM MODELOS DE ISING." Universidade de São Paulo, 1993. http://www.teses.usp.br/teses/disponiveis/43/43133/tde-22052012-133450/.
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Na primeira parte deste trabalho propomos uma aproximacão de campo médio dinâmico para analisar modelos de Ising com elementos e aleatoriedade definidos por distribuicões de probabilidades discretas. Analisamos o modelo com campo aleatório (S = 1/2), com interações aleatórias (S = 1/2), com diluição de sítios (S = 1/2) e com anisotropia aleatória (S = 1), obtendo os respectivos diagramas de fases. Na segunda parte analisamos modelos de vidros de spin (S= 3/2) com anisotropia de campo cristalino. Estudamos o modelo de van Hemmen, e o modelo clássico à la Sherrington e Kirkpatrick dentro do esquema de réplicas simétricas, obtendo os diagramas de fases correspondentes.In the first part of this work we propose a dynamical mean field approximation to analyse Ising models with elements of randomnss, defined by discret probability functions. We have analysed the random field model (S = 1/2); the random bond model (S = 1/2); the site diluted model (S = 3/2) and the random crystal field model (S = 1), obtaining the respective phase diagrams. In the second part we have analysed spinglass models (S = 3/2) in the presence of a crystal field. We have studied the van Hemmen and the classic spin glass model à la Sherrington and Kirkpatrick, using replica symmetric scheme, to obtain the corresponding phase diagrams.
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Khoshbakht, Hamid [Verfasser]. "The two-dimensional Ising spin glass at zero temperature / Hamid Khoshbakht." Mainz : Universitätsbibliothek Mainz, 2019. http://d-nb.info/1179820118/34.
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Filho, Raimundo Valmir Leite. "Impurezas MagnÃticas em Ferromagnetos de Ising com Campo Transverso." Universidade Federal do CearÃ, 2005. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=2917.
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Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgicoFundaÃÃo de Amparo à Pesquisa do Estado do CearÃ
O formalismo de funÃÃes de Green à usado para calcular o espectro de excitaÃÃes associados a impurezas magnÃticas implantadas em um filme ferromagnÃtico descrito pelo modelo de Ising com campo transverso. AtravÃs do mÃtodo da equaÃÃo de movimento expressÃes explÃcitas para as funÃÃes de Green sÃo determinadas para um ferromagneto sem impurezas. A funÃÃo de Green para um filme ferromagnÃtico contendo impurezas à obtida atravÃs da equaÃÃo de Dyson. O espectro de ondas de spin relativo Ãs impurezas à obtido para freqÃÃncias abaixo do limite inferior da banda de volume para um material puro. Com o objetivo de avaliar a influÃncia da posiÃÃo das impurezas no filme no espectro de excitaÃÃes, consideramos trÃs diferentes disposiÃÃes geomÃtricas para as impurezas: linha de impurezas perpendicular à superfÃcie do filme, quatro impurezas em um plano paralelo `as superfÃcies e quatro impurezas em um plano perpendicular `as superfÃcies do filme. Obtemos resultados para a freqÃÃncia dos modos localizados como funÃÃo do parÃmetro de troca entre duas impurezas vizinhas, do parÃmetro de troca entre as impurezas e seus vizinhos e do parÃmetro de campo efetivo nas impurezas.
A Green function formalism is used to calculate the spectrum of excitations associated with magnetic impurities implanted in a ferromagnetic thin film described by the transverse Ising model. Using the equations of motion method, explicit expressions for the Green function are determined for a ferromagnetic without impurities. The Greenâs functions for a ferromagnetic film containing impurities are obtained through Dyson equation. We consider only the âdefectâ modes that appear below the bulk band of the pure material. In order to assess the influence of the position of the impurities in the film on the excitations spectra, we consider three different geometrical arrangement for the impurities: impurities line perpendicular to the surface film, four impurities in a plane paralel to the surface, and four impurities in a plane perpendicular to the surfaces of the film. We obtain results for the frequencies localized modes as a function of the exchange parameter between two impurities neighborings, of the exchange parameter between the impurities and their neighborings, and the effective field parameter at the impurities.
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Kenneway, Debra A. "An Investigation of the Two-Dimensional Ising Spin Glass Using Information Theoretic Measures." Fogler Library, University of Maine, 2005. http://www.library.umaine.edu/theses/pdf/KennewayDA2005.pdf.
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Lima, Karlla Adriana Pereira de. "Vidro de spin ising no limiar da percolação: comportamento dinâmico e histerese." Universidade Federal de Pernambuco, 2006. https://repositorio.ufpe.br/handle/123456789/6711.
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Previous issue date: 2006Conselho Nacional de Desenvolvimento Científico e Tecnológico
Neste trabalho são apresentados os resultados da dinâmica do sistema FexZn1-xF2 em sua fase vidro de spin próximo ao limiar da percolação,a x = 0.25 ? xp = 0.24, por meio do estudo da autocorrelação temporal de spin via simulação Monte Carlo. Foram realizados vários tipos de protocolos a fim de verificar a dependência da relaxação com o campo magnético, H, e o tempo de espera (tempo de aplicação do campo), tw, sendo estes: resfriamento a campo nulo (Zero Field Cooling - ZFC) nos regimes lento e quenched; e o resfriamento com campo magnético aplicado (Field Cooling - FC). Em cada caso o sistema foi estudado nos regimes de campos magnéticos baixos, intermediários e altos, com respeito à energia de troca dominante entre os spins. Além disso, reproduziu-se a curva de histerese para tal composto a T = 0.12K, na qual há o surgimento de degraus, ou platôs, bem definidos para certos intervalos de campo, sendo em seguida realizado um estudo da relaxação também para esta região. A dinâmica da função de autocorrelação de spins, C(t,tw), após o resfriamento ZFC quenched mostrou resultados em acordo com o cenário de crescimento e relaxação de domínios via ativação térmica, no regime de campos baixos e intermediários, H<5T. Em particular, no regime H ? 2T foram obtidos ótimos colapsos dos dados de C(t,tw) para tw= 10, 102, 103 e 104, a T = 5.1 K, utilizando ambas as hipóteses de escala aditiva e multiplicativa em processos de ativação sobre barreiras logarítmicas de energia, características da proximidade da percolação, x ? xp. Por outro lado, para 2T ? H ? 5T somente a hipótese aditiva é compatível com bons colapsos dos dados, enquanto que para H ? 5T nenhum tipo de colapso foi obtido, podendo sugerir, que a presença de campos magnéticos intensos afeta de forma significativa a natureza da fase magnética desordenada do sistema a x = 0.25
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Sakellariou, Jason. "Inverse inference in the asymmetric Ising model." Phd thesis, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00869738.
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Recent experimental techniques in biology made possible the acquisition of overwhelming amounts of data concerning complex biological networks, such as neural networks, gene regulation networks and protein-protein interaction networks. These techniques are able to record states of individual components of such networks (neurons, genes, proteins) for a large number of configurations. However, the most biologically relevantinformation lies in their connectivity and in the way their components interact, information that these techniques aren't able to record directly. The aim of this thesis is to study statistical methods for inferring information about the connectivity of complex networks starting from experimental data. The subject is approached from a statistical physics point of view drawing from the arsenal of methods developed in the study of spin glasses. Spin-glasses are prototypes of networks of discrete variables interacting in a complex way and are widely used to model biological networks. After an introduction of the models used and a discussion on the biological motivation of the thesis, all known methods of network inference are introduced and analysed from the point of view of their performance. Then, in the third part of the thesis, a new method is proposed which relies in the remark that the interactions in biology are not necessarily symmetric (i.e. the interaction from node A to node B is not the same as the one from B to A). It is shown that this assumption leads to methods that are both exact and efficient. This means that the interactions can be computed exactly, given a sufficient amount of data, and in a reasonable amount of time. This is an important original contribution since no other method is known to be both exact and efficient.
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Rosa, Priscila Ferrari Silveira 1988. "Dinâmica quântica de estados de impurezas em cadeias de spin." [s.n.], 2010. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277877.
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Orientador: Guillermo Gerardo Cabrera OyarzúnDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin"
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Resumo: A descrição da dinâmica quântica de sistemas de muitos corpos é um ingrediente chave para a computação quântica. No presente projeto, propomos o estudo da dinâmica em cadeias finitas de spin 1/2 na presença de impurezas ou defeitos. O modelo adotado será o de Ising quântico com campo transverso, do qual é possível obter o espectro de forma exata na presença de uma impureza. A dinâmica do sistema é movida exclusivamente por flutuações quânticas, cuja origem é o Princípio da Incerteza. Investigamos a relaxação de estados iniciais caracterizados por uma magnetização espacialmente não homogênea e que não possuam hipóteses sobre a proximidade com o estado de equilíbrio. Dessa forma, a matriz densidade inicial será dependente apenas de uma única coordenada espacial. A investigação então é realizada através da grandeza de interesse, a saber, a evolução temporal do valor médio das componentes de Fourier da magnetização, (SzQ)t. Soluções exatas, tanto analíticas quanto numéricas, são obtidas. Um dos objetivos iniciais deste trabalho consiste na busca de processos de relaxação lentos. Para os casos de solução analítica (impurezas periódica e antiperiódica) observamos relaxações oscilatórias e amortecidas por uma lei de potência no tempo do tipo (t/tQ)-vQ, onde tQ e vQ são dois parâmetros livres e Q é o número de onda associado a cada componente de Fourier. Há uma criticalidade no expoente vQ, o qual muda de 3/2 para 1/2 para certos valores de Q críticos. Por outro lado, para os casos de solução numérica (impurezas arbitrárias), os processos de relaxação são distintos daqueles citados acima. Os estados iniciais analisados são do tipo produto direto ferromagnético com um único spin virado, próximo ou distante da impureza. Neste caso, as evoluções temporais oscilam em torno deum valor médio não nulo e há uma larga faixa de valores de Q na qual os modos não se extinguem completamente
Abstract: The description of many body systems quantum dynamics is a key ingredient for quantum computation. In the present project we study finite spin-1/2 chains dynamic properties in the presence of impurities or defects. We adopt the quantum Ising model with transverse field, of which it is possible to obtain the energy spectrum by exact calculations in the presence of one impurity. The system dynamics is driven exclusively by quantum fluctuations, whose origin is the Uncertainty Principle. We investigate the relaxation of initial states characterized by spatially inhomogeneous magnetization without any hypothesis about the proximity with the equilibrium state. Thus, the initial density matrix will be dependent of only one spatial coordinate. The investigation then is realized through the temporal evolution of the magnetization's Fourier components. Exact solutions, analytical and numerical, are obtained. One of the goals of this work consist in the search of slow relaxation processes. For the analytical cases (periodic and anti-periodic impurities) we observe oscillatory relaxations with a decay given by a power law in time (t/tQ)-vQ, where tQ and vQ are two free parameters and Q is the wave number associated to a Fourier component. There is a criticality in the exponent vQ: its value changes from 3/2 to 1/2 for certain critical values of Q. On the other hand, for the numerical cases (arbitrary impurities), the relaxation processes are distinct from the cases cited above. The initial state analyzed is a ferromagnetic direct product with only one flipped spin, near or far from the impurity. In this case, the temporal evolutions oscillate around a finite mean value and there is a large interval of Q values in which the modes do not extinguish completely
Mestrado
Física da Matéria Condensada
Mestra em Física
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Chiruta, Daniel-Gabriel. "The analysis of hysteretic behavior in bistable spin transition nanomaterials and its applications towards nanoelectronics devices." Versailles-St Quentin en Yvelines, 2014. http://www.theses.fr/2014VERS0020.
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L’objectif principal de ce travail est d’appliquer des méthodes exactes (matrice de transfert) ou semi-exactes (en utilisant des simulations Monte Carlo avec l’algorithme de l’échantillonnage entropique) à l’étude du comportement des matériaux moléculaires. Nous avons utilisé le modèle type-Ising en tenant compte des interactions à courte et à longue portée afin de simuler la réponse à des effets extérieurs dans des composés à transition de spin de taille macroscopique ainsi que des tailles nanométriques. Grâce à leur bistabilité ces composés à transition de spin sont potentiellement utilisables dans la fabrication de nouveaux dispositifs (capteurs de températures et/ou de pression, stockage de l’information). Notre travail contient deux parties. La première partie, les trois premiers chapitres, est consacrée à l’état de l’art des matériaux à transition de spin (SCO) et à la description de modèles et méthodes proposés pour expliquer le phénomène de transition de spin. La deuxième partie, les 4 derniers chapitres, concerne nos études théoriques sur l’effet de la taille, la forme et l’effet des molécules en surface dans le domaine des matériaux à transition de spin. Cette thèse, dans le domaine de la Sciences des matériaux, traite à travers tous ces chapitres de deux axes. Dans un premier axe nous avons modélisé et simulé le comportement de plusieurs matériaux SCO existant en utilisant le modèle type Ising afin de comprendre le mécanisme de transition de spin. Nous avons également analysé les effets des différents facteurs extérieurs, notamment l’effet des molécules en surface, dans les composés à transition de spin avec différents types de configurations : 1D, 2D et 3D. Ayant trouvé un bon accord entre les résultats numériques et les données expérimentales, nous avons étudié de nouveaux comportements thermiques de ces matériaux à transition spin obtenus expérimentalement : transition incomplète et transition à plusieurs étapesThe main purpose of this thesis is to develop exact methods (i. E. Matrix transfer) or semi-exact methods (using Monte Carlo technique with entropic sampling algorithm) to study the behaviour of molecular materials. Using an Ising like model that takes into account both short-range and long-range interactions in Spin Crossover (SCO materials) the response resulting from the spin state switching phenomenon (from bulk materials down to nanoscale size) was simulated. SCO materials have potential applications in the fabrication of novel devices (i. E. Storing information, sensing, and display). This work contains two main parts divided in seven chapters. The first part, the first three chapters, is devoted to some overview of SCO materials and to the description of several models and methods proposed to explain the Spin Transition (ST) phenomenon while the second part, the last four chapters, is focused on some theoretical studies on size and shape effects as well as the molecules at the surface effect in the SCO area which is a new subject. This thesis, in the field of Computing Materials Science, treats two axes. In the first axe we have modeled and simulated the behaviour of several existing materials using an Ising like model in order to understand the ST mechanism and the effects of different external factors in different SCO compounds in 1D, 2D or 3D structures. From the good agreement between the numerical and the experimental data in the first part, we have studied in the second part different architectures and we have predicted some novel SCO behaviours, obtained recently experimentally, as incomplete or multi-step transition
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Liers, Frauke. "Contributions to determining exact ground states of ising spin glasses and to their physics." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=972890793.
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Gulener, Fatih. "Etude experimentale de la relaxation des moments magnetiques dans un verre de spin ising." Paris 11, 2001. https://tel.archives-ouvertes.fr/tel-00001917.
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Nous etudions principalement les proprietes dynamiques dans le verre de spin quasi-ising metallique fe xtis 2 ou les atomes de fer sont inseres par intercalation dans une structure en feuillets. Des mesures d'aimantation statique au squid et de caracterisation structurale par diffraction de rayons x (diagramme de poudre, affinement rietveld) sont presentees afin de comprendre le diagramme de phase. Nous etudions la forme de la relaxation de spin en fonction de la temperature et de la concentration d'impuretes x. L'interet de travailler avec ce type de materiau tres anisotrope est de se placer dans les conditions proches des modeles theoriques et des simulations numeriques. Ceux-ci, en dessous de la temperature de gel t g et a t = t g, montrent une relaxation de forme algebrique. Afin de sonder la dynamique au-dessus de t g, nous utilisons les techniques de spectroscopie de depolarisation de muons (sr) pour x = 2,3 et 5%) et d'echo de spin de neutrons (nse) (x = 20 et 36%). Ces deux procedes d'investigation ensemble donnent acces a une fenetre de temps qui va de la nanoseconde a la microseconde. Ils revelent un caractere quasi-statique et une dynamique lente des spins sur une plage importante de temperature au-dessus de t g. La forme de la fonction d'autocorrelation dynamique spin-spin q(t) est determinee grace a un modele de depolarisation de muons. L'analyse des spectres experimentaux s'appuie sur la comparaison avec les courbes provenant de calculs numeriques bases sur ce modele ainsi que sur les ajustements effectues. Les resultats sont coherents avec une l'existence fonction q(t) suivant la forme d'ogielski. Le comportement thermique des parametres microscopiques est en accord avec les simulations d'ogielski sur des systemes ising 3d. Nous constatons une difference qualitative de la dynamique entre ce compose quasi-ising et les composes heisenberg aufe x et agmn x. Pour ce dernier, l'observation des spectres de depolarisation indique une relaxation quasi algebrique.
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Tartas, Jean. "Computer simulation study of domain growth in the two-dimensional ferromagnetic spin-flip Ising model." Thesis, McGill University, 1988. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=64103.
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Nikoletopoulos, Theodore. "Equilibrium and non-equilibrium statistical mechanics of disordered Ising spin models with mixed range interactions." Thesis, King's College London (University of London), 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412486.
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Morais, Junior Carlos Alberto Vaz de. "Transições de primeira ordem no modelo vidro de spin ising fermiônico em um campo transverso." Universidade Federal de Santa Maria, 2006. http://repositorio.ufsm.br/handle/1/9201.
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Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorThe present work analyzes the fermionic Ising spin glass model in a transverse magnetic field. The problem is written in the Grand Canonical ensemble with the spin operators represented by bilinear combinations of fermionic fields. The transverse field is a spin "flip" mechanism and inserts quantum fluctuations, which can lead the transition temperature Tj to quantum critical point (QCP). In this case, the phase transition from spin glass to paramagnetic phases occurs in T = o. On the other hand, charge fluctuations controlled by chemical potentiall-L show the existence of a tricritical point. The replica method with replica symmetry and static approximation are used to solve this problem. However, the replica symmetry solution shows some problems, like negative entropy in low temperatures. Thus, the stability conditions are derived to test the generated solution of this IDodel. The main purpose of this work is to study the competition between spin glass and paramagnetic phases with both quantum and charge fluctuations. After the stability conditions are derived, a detailed sight into the generated solution is shown under the form of diagrams phase. The stability conditions, however, show another applications in the present problem, once they can be used, for instance, in the tricritical point derivation.
o presente trabalho analisa o modelo vidro de spin Ising com um campo transverso, descrito no ensemble grande canônico, em uma formulação fermiônica, em que os operadores de spin são escritos como uma combinação bilinear de operadores de criação e destruição. Neste modelo, o campo transverso r é um mecanismo de "fiipagem" dos spins, que insere fiutuações de natureza quântica, que podem conduzir a temperatura de transição Tf ao ponto crítico quântico (PCQ). Neste caso, a transição de fase vidro de spin-paramagnética ocorre em T = O. Por outro lado, fiutuações de carga controladas pelo potencial químico mostram a existência de um ponto tricrítico. O método das réplicas com simetria de réplicas e a aproximação estática são usados para resolver este problema. Contudo, a solução com simetria de réplicas apresenta certos problemas na fase vidro de spin como, por exemplo, entropia negativa em baixas temperaturas. Desta forma, as condições de estabilidade são derivadas para testar a solução gerada pelo modelo. O objetivo geral é observar os efeitos combinados das fiutuações quânticas e de carga na competição entre as fase paramagnética e vidro de spin. A partir das condições de estabilidade, as soluções geradas pela simetria de réplicas são vistas detalhadamente na forma de diagramas de fase. As condições de estabilidade, todavia, mostram outras aplicações no presente problema, uma vez que podem, inclusive, ser utilizadas na derivação do ponto tricrítico.
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BRITO, Janete Batista de. "Histerese e irreversibilidade em vidro de spin ising próximo ao limiar de percolação: FexZn1-xF2." Universidade Federal de Pernambuco, 2003. https://repositorio.ufpe.br/handle/123456789/6874.
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Previous issue date: 2003Conselho Nacional de Desenvolvimento Científico e Tecnológico
Na presente dissertação realizamos estudos através de simulação computacional, utilizando o método Monte Carlo (MC) do antiferromagneto (AF) diluído FexZn1−xF2, próximo da região de percolação (x = 0.24) e na presença de um campo magnético externo H. O sistema AF FexZn1−xF2 mostra-se para x = 0.25 com características típicas de um Vidro de Spins (VS s) genuíno, com uma temperatura crítica bem definida. Por meio de um modelo microscópico adequado para a descrição de AF diluídos, em que as razões entre as constantes de trocas medidas no sistema são consideradas, mostramos através da utilização da simulação MC que, para baixas temperaturas, o sistema Fe0.25Zn0.75F2 possui uma forte dependência com a história da medida ou do procedimento numérico utilizado. Os valores obtidos através das medidas da magnetização, quando resfriamos a campo magnético nulo e posteriormente aquecemos com campo magnético não-nulo (ciclo ZFC), são diferentes dos valores quando a medida é feita com resfriamento e aquecimento com campo magnético não nulo ou finito (ciclo FC). Em particular, estudamos a dependência temporal das magnetizações remanentes associadas aos dois ciclos, FC e ZFC, após a retirada do campo magnético. Essa forte dependência com a história evidenciou-se, também, nos resultados que obtivemos para as curvas de histerese e para a linha de Almeida- Thouless, com os resultados sendo comparados com os obtidos experimentalmente e por outros métodos teóricos
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Gulener, Fatih. "Etude expérimentale et théorique de la relaxation des moments magnétiques dans un verre de spin Ising." Phd thesis, Université Paris Sud - Paris XI, 2001. http://tel.archives-ouvertes.fr/tel-00001917.
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Nous étudions principalement les propriétés dynamiques dans le verre de spin quasi-Ising métallique FexTiS2 où les atomes de fer sont insérés par intercalation dans une structure en feuillets. Des mesures d'aimantation statique au SQUID et de caractérisation structurale par diffraction de rayons X (diagramme de poudre, affinement Rietveld) sont présentées afin de comprendre le diagramme de phase. Nous étudions la forme de la relaxation de spin en fonction de la température et de la concentration d'impuretés x. L'intérêt de travailler avec ce type de matériau très anisotrope est de se placer dans les conditions proches des modèles théoriques et des simulations numériques. Ceux-ci, en dessous de la température de gel Tg et à T = Tg, montrent une relaxation de forme algébrique. Afin de sonder la dynamique au-dessus de Tg, nous utilisons les techniques de spectroscopie de dépolarisation de muons (mSR) (pour x = 2, 3 et 5 %) et d'écho de spin de neutrons (NSE) (x = 20 et 36 %). Ces deux procédés d'investigation ensemble donnent accès à une fenêtre de temps qui va de la nanoseconde à la microseconde. Ils révèlent un caractère quasi-statique et une dynamique lente des spins sur une plage importante de température au-dessus de Tg. La forme de la fonction d'autocorrélation dynamique spin-spin q(t) est déterminée grâce à un modèle de dépolarisation de muons. L'analyse des spectres expérimentaux s'appuie sur la comparaison avec les courbes provenant de calculs numériques basés sur ce modèle ainsi que sur les ajustements effectués. Les résultats sont cohérents avec une l'existence fonction q(t) suivant la forme d'Ogielski. Le comportement thermique des paramètres microscopiques est en accord avec les simulations d'Ogielski sur des systèmes Ising 3D. Nous constatons une différence qualitative de la dynamique entre ce composé quasi-Ising et les composés Heisenberg AuFex et AgMnx. Pour ce dernier, l'observation des spectres de dépolarisation indique une relaxation quasi algébrique.
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Nakamura, Yasuyuki. "Existence of a compensation temperature of a mixed spin-2 and spin-5/2 Ising ferrimagnetic system on a layered honeycomb lattice." The American Physical Society, 2000. http://hdl.handle.net/2237/7150.
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LIMA, Karlla Adriana Pereira de. "Efeito do campo magnético na fase ordenada de um vidro de Spin Ising no limiar da percolação." Universidade Federal de Pernambuco, 2012. https://repositorio.ufpe.br/handle/123456789/11166.
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O efeito do campo magnético na fase vidro de spin a campo nulo de um composto antiferromagn etico Ising tridimensional dilu do, FexZn1xF2, x = 0:25, pr oximo ao limiar da concentra c~ao de percola c~ao de primeiros vizinhos, xp 0:24, e investigado via simula c~oes Monte Carlo e de campo m edio s to-a-s tio. Aqui consideramos as constantes de acoplamento de curto alcance obtidas diretamente do composto em quest~ao, ao inv es de uma distribui c~ao estoc astica de intera c~oes de troca (exchange). Estudamos as din^amicas da fun c~ao de correla c~ao, magnetiza c~oes termo-remanente e isotermo-remanente, e a termodin^amica da histerese e magnetiza c~oes nos ciclos de refriamento a campo nulo (zero eld cooling) e com campo ( eld cooling), assim como as con gura c~oes microsc opicas de spin associadas; os resultados foram obtidos para o aglomerado de spin (cluster ) percolante e para a amostra inteira. Para temperaturas muito baixas, o comportamento do sistema e dominado pela competi c~ao entre as intera c~oes antiferromagn eticas e o campo aplicado. Com o aumento da temperatura, nossas conclus~oes s~ao consistentes com um comportamento v treo ou metaest avel do tipo vidro de spin no contexto de um cen ario do modelo de droplets modi cado, com ativa c~ao t ermica dos dom nios fractais correlacionados sobre barreiras de energia logar tmicas pr oximas ao limiar da concentra c~ao de percola c~ao de primeiros vizinhos.
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Vetel, Jérôme. "Dynamique des systèmes magnétiques désordonnés de type Ising Fe0. 35Mg0. 65Br2 et Fe0. 8Mg0. 2Cl2." Toulouse, INSA, 1990. http://www.theses.fr/1990ISAT0005.
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LE MEMOIRE PRESENTE L'ETUDE DU COMPORTEMENT DYNAMIQUE DE DEUX SYSTEMES MAGNETIQUES DESORDONNES DE TYPE ISING. DANS UNE PREMIERE PARTIE, ON ANALYSE LE COMPORTEMENT CRITIQUE DYNAMIQUE DU VERRE DE SPIN 3D-ISINGFe0. 35Mg0. 65Br2. L'ETUDE DE LA DIVERGENCE DU TEMPS DE RELAXATION CARACTERISTIQUE MET EN EVIDENCE UN CROSSOVER DE DIMENSION 2D-3D QUAND LA TEMPERATURE DECROIT. ON MONTRE QUE LE COMPORTEMENT BASSE TEMPERATURE, CONFORMEMENT AUX PREDICTIONS THEORIQUES SUR LES SYSTEMES 3D ISING, EST BIEN ANALYSE PAR UNE LOI DE PUISSANCE DE T=1T#C/T ASSOCIEE A UNE TEMPERATURE DE TRANSITION FINIE T#C=1,950,05 K. L'EXPOSANT CRITIQUE ZV=9,71,3 EST TRES COMPARABLE D'UNE PART A CELUI PREDIT PAR LA THEORIE ET D'AUTRE PART A CELUI MESURE DANS D'AUTRES SYSTEMES VERRE DE SPIN. EN REVANCHE LE COMPORTEMENT HAUTE TEMPERATURE EST DECRIT PAR UNE LOI D'ARRHENIUS CARACTERISTIQUE D'UNE TRANSITION DE PHASE A T#C=0, ASSOCIEE AU COMPORTEMENT 2D-ISING. DANS LA DEUXIEME PARTIE ON ETUDIE, PAR VISUALISATION DE DOMAINES, LA FORMATION ET LA DYNAMIQUE DE DOMAINES MAGNETIQUES DANS LE SYSTEME Fe0. 8Mg0. 2Cl2, QUI SOUMIS A UN CHAMP MAGNETIQUE UNIFORME CONSTITUE UN SYSTEME EN CHAMPS ALEATOIRES. ON CONSTATE LA DISPARITION DE LA TRANSITION METAMAGNETIQUE, ET A SUFFISAMMENT BASSE TEMPERATURE, ON MET EN EVIDENCE UN NOUVEAU PROCESSUS D'AIMENTATION SE TRADUISANT PAR L'APPARITION DE DOMAINES MAGNETIQUES D'ETENDUE BIEN SUPERIEURE A CELLE TRADITIONNELLEMENT OBSERVEE. ON ETUDIE AUSSI, EN FONCTION DU CHAMP APPLIQUE ET DE LA TEMPERATURE, LA DYNAMIQUE DES DOMAINES ET ON MONTRE D'IMPORTANTS PHENOMENES DE METASTABILITE, ASSOCIES A DES TEMPS DE RELAXATION TRES GRANDS