Il est important de bien connaitre le comportement des combustibles nucléaires dans des conditions extrêmes afin d’assurer la sureté des réacteurs et de prévoir les conséquences d’un éventuel accident. L’objectif principal de cette thèse est l’étude des transitions de phase à très haute température de matériaux envisagés pour les combustibles nucléaires de quatrième génération. Dans ce but, une méthode a été développée à l’institut européen des transuraniens (ITU) pour étudier ces matériaux à des températures excédant 2500K. La technique utilisée consiste à chauffer l’échantillon à l’aide d’un laser de haute puissance et à mesurer sa température par pyrométrie. Le signal d’un second laser réfléchi par la surface de l’échantillon est aussi étudié afin de mieux caractériser les transitions de phase. Les avantages de cette technique résident dans la rapidité des expériences (de quelques dizaines de ms à quelques secondes), et dans le contrôle de l’atmosphère, ce qui permet de limiter les effets d’évaporation ou d’oxydation/réduction de l’échantillon. Il convient de signaler que seule la partie centrale de l’échantillon est fondue, la phase liquide sondée est ainsi confinée au sein de l’échantillon lui-même, ce qui évite toute interaction avec le système de fixation. Nos résultats sur les carbures d’uranium sont en accord avec ceux de la littérature, et ont permis d’affiner le calcul des diagrammes de phase pour ces matériaux stables à haute température. La technique que nous avons mise au point a été utilisée, pour la première fois, pour étudier des matériaux de haute activité. Des résultats originaux ont été obtenus sur les systèmes PuO2, NpO2, UO2-PuO2 et Pu-C. / Understanding the behaviour of nuclear materials in extreme conditions is of prime importance for the analysis of the operation limits of nuclear fuels, and prediction of possible nuclear reactor accidents, relevant to the general objectives of nuclear safety research. The main purpose of this thesis is the study of high temperature phase transitions in nuclear materials, with special attention to the candidate fuel materials for the reactors of the 4th Generation. In this framework, material properties need to be investigated at temperatures higher than 2500K, where equilibrium conditions are difficult to obtain. Laser heating combined with fast pyrometer is the method used at the European Institute for Transuranium Elements (JRC – ITU). It is associated to a novel process used to determine phase transitions, based on the detection, via a suited low-power (mW) probe laser, of changes in surface reflectivity that may accompany solid/liquid phase transitions. Fast thermal cycles, from a few ms up to the second, under almost container-free conditions and control atmosphere narrow the problem of vaporisation and sample interactions usually meet with traditional method. This new experimental approach has led to very interesting results. It confirmed earlier research for material systems known to be stable at high temperature (such as U-C) and allowed a refinement of the corresponding phase diagrams. But it was also feasible to apply this method to materials highly reactive, thus original results are presented on PuO2, NpO2, UO2-PuO2 and Pu-C systems.

A função partição do modelo SOS é analisada, mostrando heuristicamente que acima da transição de rugosidade, TR, este modelo se torna essencialmente um problema unidimensional. Os resultados para a rugosidade superficial e calor específico são comparados com uma simulação Monte Carlo de uma rede mxm (m=20, 40, 80, 100). Um novo modelo SOS unidimensional é introduzido o qual mostra um bom acordo para regiões de altas e baixas temperaturas com o modelo SOS bidimensional. Foi dedicada atenção especial na estimativa da temperatura de transição pelo método Monte Carlo, a qual foi estabelecida como sendo TR= 0.80. Através de um aprimoramento no método Monte Carlo, aplicado a um sistema contendo 224 discos duros, obteve-se o loop de van der Waals, indicando assim, uma coexistência de fases para densidades entre 1.29 &#60 &#964 &#60 1.36. Este resultado até então, só tinha sido obtido através da técnica dinâmica molecular para 870 discos / The partition function of the SOS model is analysed and it is shown heuristically that above the roughening transition temperature, TR, this model becomes essentially na one-dimensional problem. The results for the surface roughness and surface specific heat are compared with a Monte Carlo simulation of a mxm lattice (m= 20, 40, 80, 100). A new one-dimensional SOS model is introduced wich shows a good agreement with the ordinary two dimensional SOS model for low and high temperatures. Especial attention was paid for the Monte Carlo estimation of the roughening transition temperature which was stablished as being TR= 0.80. Through na improvement in the choice of the most representative configurations in the Monte Carlo procedure, applied to a system containing 224 hard disks, it was possible to obtain the van der Waals-type loop curve, showing then, a fase coexistence in a density range 1.29 &#60 &#964 &#60 1.36. This result has so far only been obtained by molecular dynamic procedure for 870 disks

Doctor of Philosophy / Department of Chemistry / Mark D. Hollingsworth / Urea inclusion compounds (UICs) are classic examples of nanoporous, host:guest materials in which the linear channels of the honeycomb structure of the urea host can include various types of long-chain compounds (the guests). By using synchrotron X-ray radiation sources, a deeper understanding of these materials is made possible through detailed structural studies. In particular, this dissertation describes a series of structural phase transitions that occur upon cooling two related UICs containing alkanedione guest molecules. UICs may be classified as either commensurate or incommensurate structures, depending on whether the repeat lengths of the host (c[subscript h]) and guest (c[subscript g]) along the channel axis are related by a small whole number ratio. Crystals of 2,8-nonanedione/urea and 2,11-dodecanedione/urea, which are incommensurate structures at room temperature, undergo "lock-in" phase transitions below room temperature to generate commensurate structures in which the guest repeat lengths are elongated. Upon nucleation and growth of these elongated, commensurate phases, other molecules in the same channels are compressed to give successively shorter guest repeat lengths. Further lock-in phase transitions give a multitude of commensurate and incommensurate phases during cooling. The crystal structures of two of these commensurate phases have been determined using synchrotron sources. The "self-compression" observed in these 1-D crystals serves as a paradigm for understanding solid-state reactions in three-dimensional crystals.

In this dissertation, I study critical phenomena and phase transitions in systems with long-range interactions, in particular, the ferromagnetic Ising model with quenched site dilution and the asset exchange model with growth. In the site-diluted Ising model, I focus on the effects of quenched disorder on both critical phenomena and nucleation. For critical phenomena, I generalize the Harris criterion for the mean-field critical point and the spinodal, and find that they are not affected by dilution, whereas pseudospinodals are smeared out. For nucleation, I find that dilution reduces the lifetime of the metastable state. I also investigate the structure of nucleating droplets in both nearest-neighbor and long-range Ising models. In both cases, nucleating droplets are more likely to occur in spatially more dilute regions. I also modify the asset exchange model to include different types of economic growth, such as constant growth and geometric growth. For constant growth, one agent eventually gets almost all the wealth regardless of the growth rate. For geometric growth, the wealth distribution depends on the way that the growth is distributed among agents, which is represented by the parameter 𝛾. For the evenly distributed growth, 𝛾=0, and as 𝛾 increases, the growth in the total wealth is distributed preferentially to richer agents. For 𝛾=1, the wealth of every agent grows at a rate that is linearly proportional to his/her wealth. I find a phase transition at 𝛾=1. For 𝛾<1, there is an rescaled steady state wealth distribution and the system is effectively ergodic. In this state, the wealth at all ranks grows exponentially in time and inequality stays constant. For 𝛾>1, one agent eventually obtains almost all the wealth, and the system is not ergodic. For 𝛾=1$, the dynamics of the poor agents' wealth is similar to that of a geometric random walk. In addition, I elucidate the effects of unfair trading, inhomogeneity in agents, modified growth which only depends on richest $1% agents' wealth, and a finite range of wealth exchange.

Thesis advisor: Ziqiang . Wang / The transition between many-body localized states and the delocalized thermal states is an eigenstate phase transition at finite energy density outside the scope of conventional quantum statistical mechanics. We apply support vector machine (SVM) to study the phase transition between many-body localized and thermal phases in a disordered quantum Ising chain in a transverse external field. The many-body eigenstate energy E is bounded by a bandwidth W=Eₘₐₓ-Eₘᵢₙ. The transition takes place on a phase diagram spanned by the energy density ϵ=2(Eₘₐₓ-Eₘᵢₙ)/W and the disorder strength ẟJ of the spin interaction uniformly distributed within [-ẟJ, ẟJ], formally parallel to the mobility edge in Anderson localization. In our study we use the labeled probability density of eigenstate wavefunctions belonging to the deeply localized and thermal regimes at two different energy densities (ϵ's) as the training set, i.e., providing labeled data at four corners of the phase diagram. Then we employ the trained SVM to predict the whole phase diagram. The obtained phase boundary qualitatively agrees with previous work using entanglement entropy to characterize these two phases. We further analyze the decision function of the SVM to interpret its physical meaning and find that it is analogous to the inverse participation ratio in configuration space. Our findings demonstrate the ability of the SVM to capture potential quantities that may characterize the many-body localization phase transition. To further investigate the properties of the transition, we study the behavior of the entanglement entropy of a subsystem of size L_A in a system of size L > L_A near the critical regime of the many-body localization transition. The many-body eigenstates are obtained by exact diagonalization of a disordered quantum spin chain under twisted boundary conditions to reduce the finite-size effect. We present a scaling theory based on the assumption that the transition is continuous and use the subsystem size L_A/ξ as the scaling variable, where ξ is the correlation length. We show that this scaling theory provides an effective description of the critical behavior and that the entanglement entropy follows the thermal volume law at the transition point. We extract the critical exponent governing the divergence of ξ upon approaching the transition point. We again study the participation entropy in the spin-basis of the domain wall excitations and show that the transition point and the critical exponent agree with those obtained from finite size scaling of the entanglement entropy. Our findings suggest that the many-body localization transition in this model is continuous and describable as a localization transition in the many-body configuration space. Besides the many-body localization transition driven by disorder, We also study the Coulomb repulsion and temperature driving phase transitions. We apply a finite-temperature Gutzwiller projection to two-dimensional Hubbard model by constructing a "Gutzwiller-type" density matrix operator to approximate the real interacting density matrix, which provides the upper bound of free energy of the system. We firstly investigate half filled Hubbard model without magnetism and obtain the phase diagram. The transition line is of first order at finite temperature, ending at 2 second order points, which shares qualitative agreement with dynamic mean field results. We derive the analytic form of the free energy and therefor the equation of states, which benefits the understanding of the different phases. We later extend our approach to take anti-ferromagnetic order into account. We determine the Neel temperature and explore its interesting behavior when varying the Coulomb repulsion. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第21876号 / 人博第905号 / 新制||人||216(附属図書館) / 2018||人博||905(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 田部 勢津久, 教授 吉田 鉄平 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

Classical and quantum spin systems are widely used models in both experimental and theoretical condensed matter physics. In many materials, the electronic interactions can be difficult to model exactly. However, in some insulators (Mott insulators), the magnetic (spin) interactions can be captured well with spin-only models. Several models are studied in this thesis. First, I report the solution of a long-standing issue in a classical frustrated spin model (i.e., where the quantum effects are neglected and the complexity is due to competing interactions): the nature of the thermal phase transition to a stripe state in the two dimensional (2D) J1-J2 model. Here J1 and J2 are nearest- and next-nearest-neighbor couplings. Monte Carlo simulations with single-spin updates are used for the calculations, and an extended-ensemble method, a generalization of the Wang-Landau algorithm, is also developed and tested. I focus on the study of "weak universality" behavior (continuously varying critical exponents, with one of the exponents staying fixed), for which I show a correspondence with a known class of conformal field theories with charge c = 1. Next, moving to quantum spins, to shed light on magnetic systems studied experimentally and to investigate new types of quantum states of interest in developing theories of quantum magnetism, I study the S = 1/2 Heisenberg model on the 2D square lattice with added six-spin interactions (the so-called J-Q3 model) as well as a set of 3D quantum antiferromagnets on dimerized lattices. Here I use the stochastic series expansion quantum Monte Carlo method. In the study of the J-Q3 model, I report on a similar weak-universality behavior as in the classical J1-J2 model, but with a mapping to a different known class of c = 1 conformal field theories. The critical behavior of the system again shows continuously changing exponents, in a way which corresponds to a gradual weakening of Z4 symmetry-breaking to an emergent U(1) symmetry. In the study of dimerized antiferromagnets, I report on a universal behavior of the Néel temperature TN, which can be related to ground state parameters independently of the microscopic interaction details in several different models.

Les matériaux multiferroïques magnétoélectriques (ME) présentant simultanément des propriétés couplées de ferromagnétisme et de ferroélectricité suscitent beaucoup d’attention, non seulement pour leurs applications (comme la RAM magnétique de nouvelle génération), mais aussi pour la compréhension de la physique relative à ce couplage. Le but de cette thèse était de découvrir et de caractériser de nouveaux composés potentiellement multiferroïques, d’où le choix des trirutiles inverses. Ce manuscrit présente l'étude détaillée du trirutile inverse Mn2TeO6 et quelques résultats sur la série substituée au chrome Mn2-xCrxTeO6. Les composés Mn2TeO6 et substitués sont préparés par réaction à l'état solide à relativement basse température (<700°C). De nombreuses expériences de diffraction de neutrons et de rayons-X ont été réalisées pour étudier les structures cristallines et magnétiques en fonction de la température (de 700°C à 1.5K), ces données ont été analysées en lien avec les caractérisations des propriétés magnétiques et électriques. Mn2TeO6 s’est révélé un matériau complexe et riche en transitions en fonction de la température. En température décroissante, Mn2TeO6 présente d’abord une transition d’une structure tétragonale (P42/mnm) à une double maille monoclinique (P21/c) vers 400°C due à l'effet Jahn-Teller. Une seconde transition très hystérétique apparait à plus basse température, avec la coexistence de deux phases monocliniques entre 45 et 100K. Des transitions magnétiques sont également observées par des mesures de susceptibilité magnétique et de diffraction neutronique. La structure cristalline à température ambiante met en évidence un ordre orbitalaire, dû au manganèse trivalent, complexe et inédit avec une alternance d’octaèdres MnO6 allongés et aplatis décrivant des chevrons. L’impact de l’effet Jahn-Teller induit par le manganèse trivalent est confirmé dans les composés substitués au Cr (avec x ≥ 0,15) qui conservent la structure quadratique sur toute la gamme de température. / Magnetoelectric (ME) multiferroic materials, which present simultaneously two coupled properties between ferromagnetism and ferroelectricity, have attracted much attention recently, not only owing to their application perspectives, e.g., next-generation magnetic RAM, but also for the rich physics associated with the understanding of this coupling. Inverse trirutiles are of particular interest here since ME properties have been reported in this family of compounds. This manuscript presents the study of inverse trirutile Mn2TeO6 and its Cr-substitution series Mn2-xCrxTeO6. Mn2TeO6 and Cr-substituted series were prepared by solid state reaction at relatively low-temperature (< 700°C). Thanks to an extensive use of different techniques performed in a large temperature range (1.5K to 700°C), encompassing synchrotron, neutron and electron diffraction experiments combined with physical properties measurements, the very complex behaviour of Mn2TeO6 was revealed. A structural transition at 400°C from tetragonal (P42/mnm) to monoclinic (P21/c) is observed first, and related to a cooperative Jahn-Teller effect. Further cooling the sample, a hysteretic structural transition is observed spanning more than 50K, which leads to the coexistence of two monoclinic phases. A series of magnetic transitions are also observed between 48K and 22K, with magnetization, heat capacity measurement and neutron diffraction. Cr-substituted (x ≥ 0.15) samples crystallize in the tetragonal phase, implying the suppression of the cooperative Jahn-Teller effect, and involving a simpler, though short–range, magnetic order.

No description available.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18999号 / 工博第4041号 / 新制||工||1622(附属図書館) / 31950 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 作花 哲夫, 教授 安部 武志, 教授 邑瀬 邦明 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM