Neste trabalho realizo uma adaptação do método de Ma, Dasgupta e Hu para o estudo e caracterização das transições de fase quânticas, induzidas por um campo transverso, em cadeias XY de spins 1/2, unidimensionais e aperiódicas, no espírito da adaptação correspondente para cadeias XXZ. O presente trabalho determina de forma analítica uma série de expoentes críticos associados às transições ferro-paramagnéticas do sistema, e dá pistas quanto à natureza das estruturas presentes no estado fundamental. Os resultados são então testados pelo emprego da técnica de férmions livres, da análise de nite size scaling e, no limite de Ising, de resultados extraídos do mapeamento do problema em uma caminhada aleatória. / We employ an adaptation of the Ma, Dasgupta, Hu method in order to analyze the quantum phase transition, induced by a transversal magnetic eld, at spin-1/2 aperiodic XY chains, in analogy to the corresponding adaptation for XXZ chains. We derive analytical expressions for some cri tical exponents related with the ferro-paramagnetic transitions, and shed light onto the nature of the ground state structures. The main results obtained by this approach were tested by the free-fermion method, nite-size scaling analyses and, at the Ising limit of the model, by using results derived from a mapping to a random-walk problem.

Transições de fase termodinâmicas são usualmente estudadas por meio do ensemble canônico e estão associadas a sistemas macroscópicos. Entretanto, tem-se tornado cada vez mais frequente e importante o estudo de sistemas físicos pequenos, cujos alcances característicos das interações equivalem aos tamanhos dos sistemas. Nestes casos pode haver inequivalência entre grandezas físicas obtidas nos ensembles canônico e microcanônico. Encontramos inúmeros exemplos destes sistemas em diversas áreas da Física. Na área de matéria condensada temos, por exemplo, o modelo Blume-Capel com interações de alcance infinito. Neste modelo as soluções exatas canônica e microcanônica são inequivalentes. Este modelo é investigado nesta tese por meio de um ensemble interpolante, conhecido como gaussiano estendido, como uma aplicação teórica preliminar. Adicionalmente, empregamos o arcabouço mecânico-estatístico no estudo de certas biomoléculas com ampla importância biológica: proteínas. Atualmente o estudo do comportamento termodinâmico destas moléculas tem ficado restrito quase que exclusivamente a abordagem via ensemble canônico. Neste trabalho analisamos os resultados da etapa microcanônica de simulações multicanônicas procurando obter aspectos físicos de biomoléculas como os domínios Src SH3 (pdb: 1NLO) e as Príons humanas (pdb: 1HJM). Caracterizamos com esta abordagem as transições de fase de enovelamento e de agregação destes sistemas. Os resultados obtidos são interpretados à luz da termoestatística microcanônica, oferecendo um ponto de vista fenomenológico alternativo à abordagem usual. / Thermodynamic phase transitions are usualy studied by the canonical ensemble and they are associated to macroscopic systems. However, it is becoming more frequent and important the study of small physical systems: whose characteristic interaction-lengths are equivalent to system sizes. In these cases there can happen inequivalences among quantities computed im the canonical and microcanonical ensembles. There are inumerous examples of that systems in various areas of physics. In the field of condensed matter there is for instance the Blume-Capel model with infinite-range interactions. The canonical and microcanonical exact solutions of this model are inequivalents. That model is investigated on this thesis through an interpolating ensemble, known as the extended gaussian, as a preliminar theoretical application. Additionally, we have employed the statistic-mechanical framework to study some biomolecules of large biological interest: proteins. Nowadays the study of the thermodynamic behavior of that molecules has been restricted almost only to the canonical approach. However, in this work we have analysed by the microcanonical step of multicanonical simulations the physical aspects of biomolecules as the domain Src SH3 (pdb: 1NLO) and the human Prions (pdb: 1HJM). Thus, we characterize with this approach the phase transitions of folding and aggregation of that systems. The results obtained are interpretated under the light of the microcanonical thermostatistics, offering an alternative phenomenological viewpoint.

A dependência com a temperatura dos tempos de relaxação nuclear e da forma da linha de ressonância dos núcleos F0, nos antiferromagnetos diluidos Fex Zn1-x F2 e Mnx Zn1-x F2, foram estudados na faixa TN&#8804;T&#8804;300K. Os tempos de relaxação spin, rede (1/T1) dos núcleos F0, os quais não estão acoplados aos spins Fe (ou Mn) através da interação hiperfina transferida, foram medidos e calculados teoricamente em função da concentração x. Os resultados teóricos apresentam uma boa concordância com os dados experimentais para concentrações na faixa 0.1&#8804;x&#8804;0.8. A dependência com a temperatura de 1/T1, para TN<T&#8804;300K foi entendido qualitativamente em ambos sistemas. Os resultados obtidos para 1/T1, em temperaturas próximas de TN foram utilizados para estudar os \"Efeitos de Campo Aleatório\" no comportamento crítico do Mn.65Zn.35 F2 com o campo aplicado paralelamente e perperdicularmente ao eixo fácil (C). Encontrou-se neste caso, que a temperatura de transição TN decresce substancialmente com o aumento da intensidade do campo somente quando H0 || C. Os resultados experimentais obtidos estão de acordo com as teorias do Efeito de Campos Aleatórios em antiferromagnetos anisotrópicos diluídos. A divergência crítica do segundo momento da linha de ressonância não homogeneamente alargada do F0 foi estudada acima de TN. Os resultados experimentais concordam com os cálculos de Heller para o alargamento não homogêneo causado por Efeitos de Campos Aleatórios. Observou-se que a forma da linha se altera na região crítica. Longe de TN ela é Gaussiana e para t&#8804;10-2 ela mostra uma tendência a se tornar Lorentziana. Abaixo de TN a sua meia largura aumenta, seguindo qualitativamente o aumento da magnetização das sub-redes. / The temperature dependence of the nuclear relaxation rates and line shapes of the F0 resonance in the diluted antiferromagnet Fex Zn1-x F2 and Mnx Zn1-x F2 are studied over a large temperature range TN&#8804;T&#8804;300K. The high (room) temperature spinlattice relaxation rates (1/ T1) of the F0 nuclei, which are not transfer hiperfine coupled to the Fe (or Mn) spins, have been measured and calculated as a function of the concentration x. Good agreement with experiment is found for the theoretical results, which have been obtained in the range 0.1&#8804;x&#8804;0.8. The temperature dependence of TN<T&#8804;300K was qualitatively understood \"Random Field Effects\" on the critical behavior of Mn.65Zn.35 F2, for fields applied parallel and perpendicular to the easy (C) axis. It was found that the transition temperature TN depressed substantially with field only for H0 || C. The experimental results are in general accord with the theory for Random Field Effects in desordered, anisotropic antiferromagnets. The critical divergence of the in homogeneously broadened F0 NMR was studied in Fe.6Zn.4F2 above TN. The experimental results agree with Heller\'s calculation of the NMR line broadening by Random Field Effects. With H0 || C the line shape changes from Gaussian towards Lorentzian for t&#8804;10-2 and below TN its line width increase qualitatively following increase in the sublattice magnetization.

Submitted by Biblioteca do Instituto de Física (bif@ndc.uff.br) on 2017-06-05T18:45:59Z No. of bitstreams: 1 Dissertação de mestrado - Rafael Mynssem Brum.pdf: 1829207 bytes, checksum: 8b52407e238edb901004b2e8de1dbdaf (MD5) / Made available in DSpace on 2017-06-05T18:45:59Z (GMT). No. of bitstreams: 1 Dissertação de mestrado - Rafael Mynssem Brum.pdf: 1829207 bytes, checksum: 8b52407e238edb901004b2e8de1dbdaf (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O presente trabalho tem por finalidade descrever as propriedades termodinâmicas de um polímero em uma rede de Bethe de número de coordenação arbitrário q limitado por uma superfície na qual está fixado o monômero inicial da cadeia polimérica. Para isso, consideramos o problema de um ponto de vista combinatório de maneira a obter sua função de partição canônica e através dela (como método clássico da mecânica estatística) encontrarmos as grandezas termodinâmicas de interesse tais como a energia livre de Helmholtz, entropia, energia interna, pressão e etc. Realizamos nosso estudo inicialmente no caso atérmico e posteriormente consideramos uma interação atrativa entre os monômeros e a superfície. Também analisamos, no final de cada caso estudado (específico para q = 4 e geral para qualquer q), a transição de fase associada à adsorção do polímero na parede e seus efeitos na pressão. Notamos que a pressão, em geral, apresenta um decaimento exponencial com a distância em que o polímero está fixado. / This work describes the thermodynamic properties of a polymer on a Bethe lattice of arbitrary coordination number q which is limited by a surface on which the initial monomer of the polymer is xed. For this purpose, we consider the problem from a combinatoria point of view in order to obtain the canonical partiton function and, using clássical methods of statistical mechanics, the thermodynamic quantities of interest, such as the Helmholtz free energy, entropy internal energy, pressure, and so on. Initially we study the athermal case, and afterwards we include attractive interactions between the surface and the chain. We also, at the end of each case studied (q = 4 and general value of q), analize the adsorption transition of the polymer to the surface and its efects on the pressure. We notice that the pressure, in general, shows an exponential decay with the distance to the grafting point.

To understand the mechanism of a pressure-induced structural phase transition, it is important to know which bonding changes lead to the stabilization of the new structure. A useful approach in this regard is the quantum chemical topology, which provides a large variety of indicators for the characterization of interatomic interactions. In this work, a number of topological indicators are used to analyze the bonding changes during the pressure-induced phase transition from the cubic diamond (cd) to the β-Sn-type structure of the elements of the 14th group of the periodic table. The ability of these indicators to reflect the presence of the cd → β-Sn transition in experiment for Si, Ge and Sn and its absence for carbon is investigated. Furthermore, the effect of pressure on the interatomic interactions in the cd- and β-Sn-type structures is examined. It is observed that the energy change along the cd → β-Sn transformation pathway correlates with the evolution of certain parameters of the electron density and the electron localizability indicator (ELI-D). Accordingly, criteria of structural stability were formulated based on characteristics of interatomic interactions. These results can serve as guidelines for the investigation of other solid-state phase transformations by the topological methods.

La frustration – c'est-à-dire la présence d'interactions de nature compétitive – donne lieu à des effets de grande complexité en physique. La glace – aussi bien la phase bien connue de l'eau, que ses analogues magnétiques, dites « glaces de spin » – en offre un exemple remarquable. Pour des interactions à courte portée et des degrés de liberté classiques, son état fondamental est infiniment dégénéré, et comporte en outre des corrélations à longue portée induites par une contrainte locale, caractéristiques de la phase dite de Coulomb. Ses excitations élémentaires correspondent au retournement d'un dipôle qui se « fractionnalise » en deux monopôles. Dans cette thèse nous nous intéressons à la stabilité de cette phase de Coulomb dans la glace bidimensionnelle – réalisée aussi bien comme glace de protons dans des composés organiques, que comme glace de spin dans des systèmes nanomagnétiques. Dans le cas classique, les interactions dipolaires – présentes dans les systèmes expérimentaux – déstabilisent la phase de Coulomb dans son état fondamental. Cependant, une déformation de la simple géométrie planaire permet de récupérer cette phase dans un régime où différents états ordonnés entrent en compétition. Dans le cas quantique, les fluctuations dues à un champ magnétique transverse induisent une brisure de symétrie dans l'état fondamental qui, à basse température, cède la place à une phase de Coulomb quantique, réalisant un liquide de spin quantique avec excitations fractionnalisées. Nos résultats sont obtenus à l'aide de méthodes analytiques (analyse harmonique classique et quantique et théorie de perturbations) aussi bien que numériques (Monte Carlo) fondées sur des algorithmes originaux. / Frustration – namely the presence of competing interactions – gives rise to highly complex effects in physics. Ice – be it the well known water ice or its magnetic equivalent, the so-called spin-ice – offers a remarkable example in this context. For short-range interactions and classical degrees of freedom, its ground state is infinitely degenerate, and exhibits long-range correlations induced by a local constraint, characterizing the so-called Coulomb phase. Its elementary excitations correspond to the flip of a dipole “fractionalizing” into two monopoles. In this thesis we are interested in the stability of this Coulomb phase in the two-dimensional ice – realized in the form of a proton ice in organic compounds as well as of spin ice in nanomagnetic systems. In the classical case, dipolar interactions – present in the experimental systems – destabilize the Coulomb phase in the ground state. However, a slight deformation of the simple planar geometry allows to recover this phase in a regime where different ordered states compete with each other. In the quantum case, fluctuations due to a transverse magnetic field induce a symmetry breaking in the ground state, that melts at low temperature into a quantum Coulomb phase, realizing a quantum spin liquid with fractionalized excitations. Our results were obtained with analytical (classical and quantum normal-mode analysis and perturbation theory) as well as numerical techniques (Monte Carlo) based on original algorithms.

La connaissance des étoiles et des planètes denses nécessite une détermination fine du comportement thermodynamique de la matière dans ces objets. L'une des approches les plus fécondes aujourd'hui est celle des simulations ab initio, utilisant le formalisme de la physique statistique et la théorie de la fonctionnelle de la densité. Cette approche a notamment montré ses performances en reproduisant avec succès un grand nombre de résultats expérimentaux.Dans la première partie de ce travail de thèse, ces méthodes sont appliquées à l'étude des« glaces », impliquées dans des planètes telles que Uranus ou Neptune. Nous avons dans un premier temps confirmé le travail pré-existant sur le cas de l'eau (équations d'état et existence d'une phase superionique), puis nos avons étendu ces résultats aux cas de planètes plus denses, telles que les exoplanètes appelées« super-Jupiters ». Nous atteignons des limites de pression auxquelles le comportement est analytiquement connu, nous permettant de proposer un ajustement numérique pour l'eau dans une gamme de pression et température extrêmement large. Les autres glaces(méthane et ammoniac) ont seulement été étudiés dans les conditions des planètes de notre système solaire.Nous nous sommes ensuite intéressés au cas des naines blanches et à leur dynamique de refroidissement ~; il s'agit des restes d'étoiles les plus courants, et ils peuvent ainsi être utilisés comme moyen de datation de la galaxie. En particulier, la composition de ces objets conduit à des transitions de phase binaires ayant de lourdes répercussions sur leur temps de refroidissement. Nous avons investigué ce diagramme binaire à l'aide d'une méthode ab initio, et nous proposons de nouvelles stratégies numériques ainsi que des résultats confirmant partiellement les récents travaux dans le domaine. / A correct knowledge of dense stars and planets need an accurate determination of the thermodynamic behavior of matter in these objects. One of the most efficient approaches nowadays is to perform ab initio simulations, using both the statistical physics formalism and the density functionnal theory. This approach has shown its capabilities by reproducing many experimental data.In the first part of this thesis project, these methods are used to study planetary``ices'', found in planets such as Uranus or Neptun. We first confirmed the existing literature on water (equations of state and existence of a superionic phase); we then extended these results to denser planets, such as the so-called ``super-Jupiter''exoplanets. We reach very high pressures, until where the behavior is analytically established; this permitted us to construct a numerical fit for water in a very large temperature and pressure range. The other planetary ices (methane and ammonia) were thenstudied in the conditions of our solar system.We then considered white dwarves, and their cooling dynamics: they are the most usualstar remnants, so that they can be used as cosmochronometers. The composition of these objects lead to binary phase transitions, which can have important consequences on their cooling time. We used ab inition methods to investigate this binary diagram, and wesuggest new numerical strategies, leading to new results which partially confirm theprevious literature.

Le sujet principal de cette thèse est l'utilisation de la Théorie Fonctionnelle de la Densité dans sa variante à liaisons fortes (DFTB) pour l'étude de l'effet de la pression sur des nanotubes de carbone. Nous commençons par étudier l'effondrement radial sous l'effet de la pression de nanotubes de carbone (CNTs) individualisés, soit dans leur forme originale (vides), soit remplis avec des molécules d'eau ou de dioxyde de carbone. Nous étudions à continuation le processus d'effondrement radial de fagots de nanotubes de carbone a faible nombre de parois (double ou triple-parois) en fonction de la pression combinant modélisations et études expérimentales. Finalement nous présentons une étude sur les propriétés électroniques et magnétiques d'une monocouche de MoS2 déposée sur une surface de Ni(111) dans le cadre de laThéorie Fonctionnelle de la Densité incluant des interactions de van der Waals. Le manuscrit est structuré en 7 chapitres. Le chapitre 1 est notre introduction à cette thèse, incluant les motivations, les connaissances préalables sur nos sujets que nous intéressent ici, ainsi que notre contribution et principaux résultats. Le chapitre 2 présente les principaux éléments et définitions sur les CNTs. Nous décrivons ensuite les propriétés électroniques des CNTs et celles du graphène qui constitue un système de référence. Le chapitre 3 contient les éléments théoriques de notre étude. D'abord nous faisons une courte introduction à la Théorie Fonctionnelle de la Densité (DFT). Ensuite nous présentons deux des fonctions d'échange-corrélation les plus utilisées, suivi d'une revue sur les fonctions de van der Waals dont la DFT ordinaire ne peut rendre compte. Finalement, nous faisons une brève introduction à la méthode DFTB que nous utilisons dans nos modélisations des CNTs. Dans le chapitre 4 nous présentons nos modélisations sur l'effondrement radial sous pression hydrostatique de nanotubes de carbone contenant soit de l'eau doit du dioxyde de carbone. Nous montrons que la présence de ces molécules à l'intérieur des tubes modifie la dynamique du processus d'effondrement radial, donnant lieu soit à un support mécanique et repoussant la pression d'effondrement radial soitréduisant leur stabilité mécanique. Pour les CNTs vides, l'effondrement radial est très peu affecté par la nature du milieu transmetteur de pression, mais déterminé par le diamètre des nanotubes de carbone. Nos modélisations avec la méthode DFTB sont en excellent accord avec les modèles de milieux continues surla dépendance de la pression d'effondrement radial avec le diamètre du tube, d, mais montrent égalementune déviation de ce modèle pour les petites valeurs de d, ce qui est dû au moins en partie à la nature atomistique des nanotubes de carbone. Dans le chapitre 5, nous présentons une étude théorique de l'effondrement radial en fonction de la pression pour des nanotubes de carbone à parois simple, double et triple. Nos modélisations sont réalisées par DFTB pour des diamètres internes allant de 0.6 à 3.3 nm. Quand les parois sont séparées par la distance graphitique, nous montrons que la pression d'effondrement radial, Pc, est déterminé par le diamètre du tube interne, din, mais avec un important écart par rapport à une loi à la Lévy-Carrier, Pcdin-3. Nous proposons une expression modifiée, Pcdin3= (1- 2/din2) où  et  sont des paramètres numériques. Dans le chapitre 6 nous étudions par DFT les propriétés électroniques et magnétiques d'une monocouche de MoS2 déposée sur une surface de Ni(111). La prise en compte des interactions de van der Waals s'est avérée essentielle afin de stabiliser la monocouche de MoS2. L'interface est métallique en raisonde la présence sur le niveau de Fermi d'états d du Mo. Elle présente une barrière Schottky de 0.3 eV et une probabilité tunnel pour les électrons élevée. Enfin le dernier chapitre constitue une synthèse des derniers résultats et la présentation de quelques perspectives / In this thesis we mainly use the density functional tight-binding method (DFTB) to investigate the effect of high pressure on carbon nanotubes (CNTs). We start by investigating the collapse behavior of individualized CNTs, either empty or filled with water and carbon dioxide molecules. Then we study the collapse process of bundled few-wall (double, triple wall) carbon nanotubes as the function of pressure combining theoretical and experimental studies. Afterwards, we investigate the electronic and magnetic properties of a monolayer MoS2 on the Ni(111) surface with accounting for van der Waals interactions by the density functional theory (DFT). The manuscript is structured in 7 chapters and the following paragraphs summarize the content by chapter of this document.Chapter 1 is our introduction of this thesis, including the motivation and background of our topic as well as our important findings and results. Chapter 2 introduces the main concepts and definitions of CNTs. Then we describe the electronic properties of CNTs as well graphene as a comparison. Chapter 3 consists of the theoretical framework used for our study. Firstly, a short introduction of Density Functional Theory (DFT) is presented. Next we list two mainly used exchange-correlation functions in DFT, then followed by an overview of van der Waals functions which normal DFT cannot account for. Finally, we briefly introduces the Density Functional Tight-Binding method (DFTB) which we use for our CNTs modeling simulation.In chapter 4, we present simulations of the collapse under hydrostatic pressure of carbon nanotubes containing either water or carbon dioxide. We show that the molecules inside the tube alter the dynamics of the collapse process, providing either mechanical support and increasing the collapse pressure, or reducing mechanical stability. At the same time the nanotube acts as a nanoanvil, and the confinement leads to the nanostructuring of the molecules inside the collapsed tube. In this way, depending on the pressure and on the concentration of water or carbon dioxide inside the nanotube, we observe the formation of 1D molecular chains, 2D nanoribbons, and even molecular single and multi-wall nanotubes. For the perfect empty CNTs, collapse behavior theoretically is barely affected by the PTM environment under high pressure but only mainly is determined by the CNTs diameter. Our simulation using DFTB method gives good agreement both for the d dependence predicted by continuum mechanics models and for the deviation at small d which is at least partly due to the atomistic nature of the carbon nanotubes. In chapter 5, we present a theoretical study of the collapse process of single-, double and triple-wall CNTs as a function of pressure. Our theoretical simulations were performed using DFTB for inner tube diameters ranging from 0.6 nm to 3.3 nm. When the walls are separated by the graphitic distance, we show that the radial collapse pressure, Pc, is mainly determined by the diameter of the innermost tube, din and its value significantly deviates from the usual Pcdin-3 Lévy-Carrier law. A modified expression, Pcdin3= (1- 2/din2) with  and  numerical parameters is proposed. In chapter 6 we investigate the electronic and magnetic properties of a monolayer of MoS2 deposited on a Ni(111) surface using DFT method. Accounting for van der Waals interactions is found to be essential to stabilize the chemisorbed MoS2 monolayer. The interface is metallic due to Mo d states positioned at the Fermi energy, with a Schottky barrier of 0.3 eV and a high tunneling probability for electrons. Small magnetic moments are induced on Mo and S atoms, while we measure a significant demagnetization of the Ni layer at the interface. Finally the last chapter synthesizes the main results of this work presenting also some perspectives

Cette thèse porte sur l'étude des gaz tridimensionnels de bosons de spin 1 avec interaction ferromagnétique. Nous montrons que la transition superfluide à température finie peut être étudiée par une théorie des champs classique de symétrie O(3)*O(2). Ce type de modèle est utilisé pour décrire le magnétisme frustré dans de nombreux matériaux, en particulier les antiferroaimants sur réseau triangulaire. La nature de la transition de phase dans le modèle O(3)*O(2) (premier ou second ordre) fait l'objet d'une controverse d'ordre à la fois expérimental, numérique et théorique. Notre approche théorique est basée sur le groupe de renormalisation non-perturbatif et prédit une transition faiblement du premier ordre avec un comportement "pseudo-critique". Nous relions ce dernier au comportement critique observé dans le modèle O(N)*O(2) lorsque N est supérieur ou égal à 5.3. Dans les gaz de bosons de spin 1 (87Rb, 41K et 7Li), la longueur de corrélation à la transition est grande devant la taille typique des gaz dans les expériences actuelles. Néanmoins les comportements en loi de puissance observés près de la transition permettent de définir des exposants "pseudo-critiques" non-universels. La valeur de ces exposants varie d'un gaz à l'autre. Nous discutons dans quelle mesure la détermination expérimentale de ces exposants permettrait d'apporter des éléments concrets en faveur d'une transition du premier ordre dans le modèle O(3)*O(2). / We study tridimensional spin-1 Bose gases with ferromagnetic interaction. The superfluid transition at finite temperature can be studied by a classical field theory with O(3)*O(2) symmetry. This kind of model has been used to describe frustrated magnetism in several materials, in particular antiferromagnets on a triangular lattice. The nature of the phase transition in the O(3)*O(2) model (first or second-order) has been the subject of experimental, numerical and theoretical controversy. Our theoretical approach is based on the nonperturbative renormalization group and predicts a weakly first-order transition with "pseudo-critical'' behavior. We relate the latter to the critical behavior observed in the O(N)*O(2) model when N is greater than or equal to 5.3. In spin-1 Bose gases (87Rb, 41K and 7Li), the correlation length at the transition is larger than the typical size of the system in current experiments. However the power-law behavior observed near the transition allows one to define nonuniversal "pseudo-critical" exponents. The value of these exponents varies from one gas to the other. We discuss to what extent the experimental determination of these exponents could support the claim of a first order transition in the O(3)*O(2) model.

Les cristaux liquides sont des matériaux organiques utilisés pour réaliser des dispositifs électroniques ; avant de les intégrer dans des applications, il est nécessaire d'étudier leurs propriétés physico-chimiques et diélectriques pour optimiser leurs performances. Ce travail de thèse est consacré aux nanocolloïdes obtenus par dispersion de nanoparticules ferroélectriques dans un cristal nématique ; il s'agit d'étudier l'influence des inclusions sur les transitions de phases et sur les propriétés diélectriques de la matrice. L'étude des transitions de phases à l'aide de l'Analyse Enthalpique Différentielle (AED) a mis en évidence l'influence des nanoparticules ferroélectriques ; ceci résulte de deux principaux effets ; la polarisation spontanée des nanoparticules et l'ancrage entre les molécules du cristal liquide et les inclusions. La caractérisation diélectrique a révélé le couplage entre la polarisation macroscopique des inclusions et le champ électrique ; ce couplage se manifeste par une augmentation des températures de transitions de phases par rapport à celles déterminées par l'AED. La compétition entre les effets de la polarisation sous champ électrique et de l'ancrage induit une modification des permittivités (parallèle et perpendiculaire) et de l'anisotropie diélectriques. L'utilisation des nanoparticules fortement polaires sélectionnées a confirmé l'importance de la polarisation macroscopique des nanoparticules pour améliorer les propriétés des nanocolloïdes étudiés ; en effet, de très faibles quantités de ces nanoparticules donnent lieu à des améliorations plus significatives que celles obtenues par les nanoparticules brutes. / Liquid crystals are organic materials used to make electronic devices ; before using this material in applications, it is necessary to study their physical-chemical and dielectric properties in order to optimize their performance. This study is devoted to the nanocolloids obtained by dispersing ferroelectric nanoparticles in a nematic liquid crystal ; it means an inclusion influences the phase transitions temperatures and the dielectric properties of the host. The phase transitions measured by using Differential scanning calorimetry (DSC) evidenced the ferroelectric nanoparticles influence ; which is attributed two effects : the nanoparticles spontaneous polarization and the anchoring effect between nanoparticles and liquid crystal. The dielectric characterisation revealed the coupling between the macroscopic polarization of the inclusions and the electric field ; this coupling is manafested by an increase of phase transition temperatures compared to those determined by DSC. The competition between the polarization effect under an electric field and the anchoring effect induces a modification of the permittivities (parallel and perpendicular) and the dielectric anisotropy. Using harvested nanoparticles, the study confirmed the importance of the nanoparticles polarization to increase the properties of the studied nanocolloids. In fact, very small quantity of the harvested nanoparticles presents more significant improvements than those obtained with the non-harvested nanoparticles.