Abstract In this thesis, dielectric and thermal behaviours due to the application of an electric field were studied in relaxor ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) systems of great technological importance. Special attention was given to the behaviour of the electric-field-induced phase transitions and electrocaloric effect, which are closely related to the existing and potential applications. Reactive sintering or columbite methods were used to fabricate polycrystalline PMN-PT ceramics with various compositions (x=0−0.3). In addition, commercial PMN-PT single crystals with composition close to the morphotropic phase boundary region were used. A studied PZN-PT crystal composition was grown by solution gradient cooling technique. Materials were mainly studied by means of dielectric and direct temperature measurements. The electrocaloric effect observed in a ceramic PMN-PT system was found to show distinct maximum values close to the thermal depolarization temperatures with low electric fields. The temperature range and magnitude of the electrocaloric effect was significantly expanded to high temperatures with increasing electric fields due to the contribution of polar nanoregions. The maximum electrocaloric temperature change was in the range of 0.77−1.55 °C under an electric field of 50 kV/cm. In addition, temperature change measurements on depoled PMN-0.13PT ceramics demonstrated that the electrocaloric effect is accompanied with an irreversible part below its depolarization temperature due to hysteresis loss and a possible phase transition type response related to the evolution of the macroscopic polarization. An electric field application to the <001> and <011> directions in PMN-PT crystals was found to cause distinct anomalies in the dielectric and temperature change responses. These anomalies were attributed to the complex polarization rotation routes and different phase stability regions in the electric-field-temperature phase diagrams of PMN-PT. Furthermore, measurements on PMN-PT crystals provided the first direct indications of a temporarily reversed electrocaloric effect with an increasing electric field. In addition, the measured electrocaloric trends in PZN-PT crystal were reproduced by a simple lattice model and mean-field approximation around the transition temperature. This demonstrated that the electrocaloric effect is driven mainly by the dipolar entropy lowering. / Tiivistelmä Tässä työssä tutkittiin dielektristen ominaisuuksien ja lämpötilan käyttäytymistä teknologisesti merkittävissä (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) ja (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-PT) ferrosähköisissä relaksorimateriaaleissa sähkökentän vaikutuksen alaisena. Tutkimuksen erityishuomion kohteena olivat sähköisesti indusoidut faasimuutokset sekä sähkökalorinen ilmiö, jotka liittyvät läheisesti nykyisiin sekä tulevaisuuden sovellutuksiin. Monikiteisiä PMN-PT keraamikoostumuksia (x=0−0,3) valmistettiin sekä reaktiivisella sintrauksella että kolumbiittimenetelmällä. Lisäksi tutkimuksessa käytettiin kaupallisia PMN-PT erilliskiteitä, joiden koostumus on lähellä morfotrooppista faasirajaa. Työssä käytetty PZN-PT erilliskide kasvatettiin jäähdyttämällä korkean lämpötilan liuoksesta. Materiaaleja tutkittiin pääosin lämpötilan ja dielektristen ominaisuuksien mittauksilla. Kun PMN-PT keraamisysteemiin kohdistettiin alhainen sähkökenttä, sähkökalorisen ilmiön selkeä maksimiarvo havaittiin lähellä materiaalin termistä depolarisaatiolämpötilaa. Suuremmilla sähkökentän arvoilla sähkökalorinen ilmiö voimistui ja sen lämpötila-alue laajeni korkeampiin lämpötiloihin polaaristen nanoalueiden kytkeytymisen vuoksi. Sähkökalorisen lämpötilamuutoksen maksimi vaihteli välillä 0,77−1,55 °C sähkökentän arvolla 50 kV/cm. Lisäksi lämpötilamittaukset depoolatulle PMN-0,13PT koostumukselle osoittivat, että sähkökalorisen ilmiön ohella materiaalissa esiintyy makroskooppisen polarisaation muodostumiseen liittyvä palautumaton lämpöenergia depolarisaatiolämpötilaa pienemmissä lämpötiloissa hystereesihäviön ja mahdollisen faasimuutoksen vaikutuksesta. PMN-PT erilliskiteiden dielektrisyys- ja lämpötilavasteessa havaittiin selkeitä muutoksia sähkökentän vaikuttaessa <001> ja <011> kidesuuntiin. Nämä muutokset ovat selitettävissä PMN-PT:n polarisaation kompleksisten rotaatiosuuntien ja erityyppisten sähkökenttä-lämpötila -faasidiagrammien stabiilisuusalueiden avulla. PMN-PT kiteiden mittauksissa havaittiin myös ensimmäinen suora osoitus väliaikaisesti käänteisestä sähkökalorisesta ilmiöstä sähkökentän kasvaessa. Lisäksi mitatut PZN-PT erilliskiteen sähkökaloriset ominaisuudet transitiolämpötilan läheisyydessä pystyttiin pääpiirteittäin mallintamaan käyttämällä yksinkertaista hilamallia ja keskimääräisen kentän approksimaatiota. Mallinnuksen mukaan sähkökalorinen ilmiö aiheutuu pääasiassa sähköisesti indusoidusta dipolientropian alenemisesta.

Dans cette thèse, nous étudions l’effet du désordre magnétique sur les propriétés de transport électronique du graphène et des isolants topologiques 2D de type HgTe. Le graphène et les isolants topologiques sont des matériaux dont les excitations électroniques sont assimilées à des fermions de Dirac sans masse. L’influence des impuretés magnétiques sur les propriétés de transport du graphène est étudiée dans le régime de forts champs électriques. En conséquence de la production de paires électron-trou, la réponse devient non linéaire et dépend de la polarisation magnétique. Nous étudions une transition entre un isolant topologique bi-dimensionnel conducteur, caractérisé par une conductance G = 2 (en quantum de conductance) et un isolant de Chern avec G = 1, induite par des impuretés magnétiques polarisées. / In this thesis, we study the effect of a magnetic disorder on the electronic transport properties of graphene and HgTe-type 2D topological insulators. Graphene and topological insulators are materials whose electronic excitations are treated as massless Dirac fermions.The influence of magnetic impurities on the transport properties of graphene is investigated in the regime of strong applied electric fields. As a result of electron-hole pair creation, the response becomes nonlinear and dependent on the magnetic polarization.We investigate a transition between a two-dimensional topological insulator conduction state, characterized by a conductance G = 2 (in conductance quantum) and a Chern insulator with G = 1, induced by polarized magnetic impurities.

Il a été observé que l’altération des fonctions des granules de stress, entités cytoplasmiques non-membranaires composées d’ARN et de protéines liant l’ARN (RBPs), peut conduire au développement de maladies telles que la sclérose latérale amyotrophique, la démence fronto-temporale, la myopathie à inclusions et la maladie de Paget des os. Ces pathologies sont caractérisées par un dépôt cytoplasmique d’inclusions solides enrichies en RBPs et comprenant des fibrilles. Une connexion génétique a été suggérée entre la persistance des granules de stress et l’accumulation de ces inclusions pathologiques dans le cytoplasme des patients. Dans mon manuscrit de thèse, il est mis en évidence le fait que la protéine hnRNPA1, dont les mutations entrainent les maladies mentionnées plus haut, subit une séparation de phases entre deux liquides connue également sous l’appellation « Séparation de Phases Liquide-Liquide » (LLPS) dans des gouttelettes enrichies en protéines. Bien que le domaine composé d’une séquence à faible complexité (Low Complexity sequence Domains ou LCD) soit suffisant pour obtenir cette séparation de phases, les domaines de liaison à l’ARN y contribuent également en présence d’ARN. Cela a permis d’envisager l’existence de plusieurs mécanismes intervenant dans la régulation de l’assemblage de ces granules. Un autre résultat a mis en exergue le fait que la formation de fibrilles n’est pas une obligation pour permettre la séparation de phases mais que les gouttelettes, enrichies en protéines, entrainent, par ailleurs, une augmentation de la formation de ces fibrilles. La séparation de phases liquide-liquide induite par le domaine composé d’une séquence à faible complexité semble contribuer à l’assemblage des granules de stress et à leurs propriétés liquides. Finalement, cette étude propose d’établir une réelle corrélation entre la formation des granules de stress qui deviennent persistants et l’accumulation d’inclusions pathologiques dans le cytoplasme des patients. / Stress granules are membrane-less organelles composed of RNA-binding proteins (RBPs) and RNA. Functional impairment of stress granules has been implicated in amyotrophic lateral sclerosis, inclusion body myopathy, Paget’s disease of bone and frontotemporal dementia; these diseases are characterized by solid, fibrillar, cytoplasmic inclusions that are rich in RNA binding proteins (RBPs). Genetic evidence suggests a link between persistent stress granules and the accumulation of pathological inclusions. In this thesis manuscript, I demonstrate that the disease-related RBP hnRNPA1 undergoes liquid-liquid phase separation (LLPS) into protein-rich droplets mediated by a low complexity sequence domain (LCD). While the LCD of hnRNPA1 is sufficient to mediate LLPS, the folded RNA recognition motifs contribute to LLPS in the presence of RNA, potentially giving rise to several mechanisms for regulating assembly of stress granules. Importantly, while not required for LLPS, fibrillization is enhanced in protein-rich droplets. I suggest that LCD-mediated LLPS contributes to the assembly of stress granules and their liquid properties, and provides a mechanistic link between persistent stress granules and fibrillar protein pathology in disease.

Ces travaux de thèse portent sur l'étude des transitions de phases photo-induites dans différents matériaux métalliques et organométalliques. Les études présentées ont été guidées par la problématique liée à l'enregistrement et au stockage d'informations numériques. Tout d’abord, nous nous sommes intéressés à la spectroscopie THz et Raman de composés à transitions d'état de spin à base de Fe (II). Les spectres obtenus à l’aide de ces deux techniques sont confrontés aux résultats issus de simulations numériques (DFT). Nous avons également étudié la transition d’état de spin induite par des lasers CO2 dans des composés à transition de spin polymériques. Un dispositif expérimental simple permettant l'enregistrement d'information dans ce type de matériaux est proposé. Par la suite, nous avons étudié la transition réversible de la phase métallique (λ métastable à semiconductricer (β stable) dans des nanoparticules d'un oxyde de Titane (Ti3O5), soumis à des impulsions laser nanosecondes. Nous avons étudié plus particulièrement les aspects cinétiques de cette transition. Il apparaît alors que la transition λ→β se produit sur une échelle de plusieurs centaines de nanosecondes, tandis que la transition β→λ se fait en quelques dizaines de nanosecondes. Les mécanismes photoythermiques à l’origine de cette transition permettent de rendre compte de ces observations expérimentales. Enfin, nous avons étudié un complexe bimétallique analogue du bleu de Prusse Rb0.94Mn[Fe (CN)6]0.98.0.3H2O, bistable à température ambiante. Par génération de second et de troisième harmonique, nous avons mesuré les non-linéarités effectives d’ordre deux et trois de ce composé dans les phases basse et haute température. Nous montrons que l’on peut faire photo-commuter les propriétés optiques linéaires et non linéaires de ce composé. La spectroscopie et la génération d’onde THz dans ce matériau sont également présentées et discutées. / This thesis is dedicated to the study of laser-induced phase transitions in metallic and metaly organic materials. The studies presented here are guided by the recording and storage of digital information. Firstly, we focused on THz and Raman spectroscopy of two different iron Fe (II) spin crossover complexes. The spectra we recorded using these two methods are compared to numerical simulations obtained with the DFT. We have also studied the spin state transition induced byCO2 lasers in polymeric spin crossover materials. A simple experimental setup allowing data recording in this type of materials is presented. Secondly, we studied the reversible metal (λ metastable to semiconductor (β stable) phasetransition in nanoparticles of a Titanium oxide (Ti3O5), excited with nanosecond laser pulses.More specifically, we studied the kinetic aspects of this transition. It appears that the λ→β transition occurs in hundreds of nanoseconds while the β→λ transition is achieved in a few tens of nanoseconds. The photothermal mechanisms leading to this transition accounts very well to these experimental observations. Finally, we present the study of Rb0.94Mn[Fe (CN)6]0.98.0.3H2O, a Prussian blue analogue which is bistable at room temperature. Using secondy and thirdyharmonic generation, we measured the effective secondy and thirdyorder nonlinearities of this compound in low and high temperature phases. We show that we can photo-switch the linear and nonlinear optical properties of this material. THz spectroscopy and generation in this material are also presented.

La capacité de photo-commuter les propriétés physico-chimiques des matériaux fonctionnels grâce à des transitions de phase induites par la lumière, ouvre des perspectives fascinantes pour diriger un matériau vers un nouvel état hors équilibre thermique. Cependant, il est fondamental de comprendre tous les phénomènes élémentaires, habituellement cachés dans une moyenne statistique lors des transformations à l'équilibre. Les études résolues en temps représentent une approche unique pour accéder à l'évolution des différents degrés de liberté du système et connaître les processus élémentaires mis en jeu lors de la commutation macroscopique. Les matériaux à transition de spin (SCO) sont d'un intérêt particulier car ce sont des systèmes photo-réversibles. Ces matériaux sont aussi des prototypes photomagnétiques et photochromiques qui commutent entre deux états de différente multiplicité de spin, nommés bas spin (LS) et haut spin (HS). Dans ce travail de thèse, nous étudions les dynamique ultrarapides électroniques et structurales de cette classe de solides moléculaires, en soulignant l'importance d'utiliser des sondes complémentaires sensibles à différents degrés de liberté. Les commutation photo-induite entre états de spin est ultra-rapide et initialement localisée à l'échelle moléculaire, où le couplage électron-phonon active des vibrations cohérentes intramoléculaires. Un transfert d'énergie ultra-rapide de la molécule au réseau, via un couplage phonon-phonon, permet de piéger efficacement le système dans le nouvel état photo-induit. Cependant, dans les solides moléculaires, l'excès d'énergie libérée de la molécule excitée résulte dans un aspect complexe multi-échelle impliquant plusieurs degrés de liberté à des échelles de temps différentes. Dans ce travail de thèse, nous avons étudié la dynamique multi-étape hors équilibre d'un système SCO présentant une brisure de symétrie entre la phase HS et la phase intermédiaire (IP) où une mise en ordre à longue distance des états HS et LS des molécules résulte en la formation d'une onde de concentration de spin (SSCW). La diffraction des rayons X résolue en temps combinée avec des études de spectroscopie optique donnent une description complète de la dynamique hors-équilibre de la SSCW hors-équilibre en mesurant l'évolution temporelle des deux paramètres d'ordre décrivant le système. / The ability to photo-switch physical/chemical properties of functional materials through photo induced phase transition opens fascinating perspectives for driving the material towards new state out of thermal equilibrium. However, it is fundamental to disentangle and understand all the dynamical phenomena, otherwise hidden in statistically averaged macroscopic transformations. Arguably, time-resolved studies are unique approach to access the necessary information on the multiple degrees of freedom and elementary processes involved during the macroscopic switching. As photo-reversible molecular switches, spin crossover (SCO) materials are of particular interest. These photomagnetic and photochromic prototype materials undergo metastable photoinduced phase transition between two states of different spin multiplicity, namely low-spin (LS) and high-spin (HS). In this PhD work it will be presented the ultrafast electronic and structural dynamics of SCO molecular solids emphasizing the importance of using complementary probes sensitive to different degrees of freedom. The photoinduced spin state switching concerns initially only an ultrafast, but localized, molecular response which through strong electron-phonon coupling activates coherent intra-molecular vibrations. An ultrafast energy transfer from the molecule to the lattice, via phonon-phonon coupling, allows an efficient trapping of the system in the new photoinduced state. However in molecular solids, the excess of energy released from the absorber molecule results in a complex multi-scale aspect involving several degrees of freedom at different time scales. In this contest, we investigated the multi-step out-of equilibrium dynamics of a SCO system undergoing symmetry breaking between the HS phase and the intermediate (IP) phase where a long range ordering of HS and LS molecules results in a spin state concentration wave (SSCW), analogous to charge or spin density waves. Combined time-resolved X-ray diffraction and optical spectroscopy studies provide a complete overview of the out-of-equilibrium thermodynamics of the SSCW, investigating how the two order parameters describing the system evolve in time.

Le couplage fort entre le transport électronique dans une boîte quantique à un seul niveau et un oscillateur nano-mécanique couplé capacitivement peut conduire à une transition vers un état mécaniquement bistable et bloqué en courant. Son observation est à portée de main dans les expériences de pointe menées sur les nanotubes de carbone. Nous étudions donc la réponse mécanique du système et plus précisément la fonction spectrale de déplacement, la réponse linéaire à une solicitation externe et le comportement pendant le retour à l'équilibre. Nous montrons qu'il existe une relation étroite entre les grandeurs électriques (telles le courant électrique et la fonction spectrale des fluctuations du courant) et mécaniques. Nous constatons qu'en augmentant le couplage électromécanique, les deux fonctions spectrales présentent un pic qui s'élargit et se déplace vers les basses fréquences alors que le temps de déphasage de l'oscillateur se raccourcit. Ces effets sont maximaux à la transition où les non-linéarités dominent la dynamique et sont robustes vis-à-vis de l'effet des fluctuations extérieures et de la dissipation. Ces caractéristiques fortes ouvrent la voie à la détection de la transition vers l'état de blocage du courant dans des dispositifs actuellement étudiées par plusieurs groupes. / The strong coupling between electronic transport in a single-level quantum dot and a capacitively coupled nano-mechanical oscillator may lead to a transition towards a mechanically-bistable and blocked-current state. Its observation is at reach in carbonnanotube state-of-art experiments. Therefore, we investigate the mechanical response of the system, namely the displacement spectral function, the linear response to a driving, and the ring-down behavior, and the electric response, namely the electric current and current spectral function. We show that a close relation between electric and mechanical quantities exists. We find that, by increasing the lectromechanical coupling, the peak in both spectral functions broadens and shifts at low frequencies while the oscillator dephasing time shortens. These effects are maximum at the transition where nonlinearities dominate the dynamics, and are robust towards the effect of external uctuations and dissipation. These strong signatures open the way to detect the blockade transition in devices currently studied by several groups.

Cette thèse aborde la question du Changement Catégoriel dans la Perception et propose une approche Dynamique qui s’appuie sur la Théorie des Catastrophes et le modèle de la FRONCE. Dans nos travaux, nous assimilons les catégories perceptives à des attracteurs d’un système dynamique non linéaire. Si notre postulat est juste, alors, un ensemble de phénomènes non linéaires théoriquement associés doivent pouvoir être observé conjointement dans un changement catégoriel qui pourrait alors être considéré comme une transition de phase. Nous avons porté notre attention sur le phénomène d’Hystérèse, déjà abondamment exploré en perception, ainsi que sur les phénomènes de divergence et de ralentissement critique. Au travers d’une série de sept expérimentations, nous confrontons l’ensemble de ces hypothèses. Nous avons confirmé la présence de ces trois phénomènes lors d’un changement catégoriel. Les résultats montrent que l’hystérèse, la divergence et le ralentissement critique sont des phénomènes observés conjointement lors d’un changement de catégorie, mais qu’ils sont aussi quantitativement reliés, comme permettait de le prédire le modèle de la fronce issue de la théorie des catastrophes. D’une manière générale, nous avons conclu que les catégories perceptives étaient fortement similaires à des d’attracteurs et que l’étude et la modélisation des changements dans la perception devraient présenter un aspect nécessairement dynamique. / This thesis gets into the main question of Categorial Change in Perception and proposes a Dynamical approach that relies on the Catastrophes Theory and the model of the CUSP. In this work, we claim that perceptive categories are attractors in a non linear dynamics system. If we state right, we must find out several specific phenomenon theoretically linked to a phase transition in the moment of a categorial change. We bear witness for Hysteresis, which is yet widely documented in perception. Besides and in addition, we also look for divergence and critical slowing down phenomena. Along seven experimentations, we put those hypotheses to the test and achieve to attest for the occurrence of the three predicted phenomena over the categorial change. Not only results show that hysteresis, divergence and critical slowing down occur together during a categorial change but also they are quantitatively related as predicted by the catastrophe model of the cusp. More widely, we conclude, that perceptual categories might be regarded as strongly related to attractors and that survey and modeling of changes in perception shall include inevitable dynamics considerations henceforth.

La littérature des dernières décennies regorge de références concernant les bienfaits des acides gras oméga 3, tels que l’EPA (C20:5 n-3) et le DHA (C22:6 n-3) qui jouent un rôle essentiel dans la prévention de nombreuses pathologies comme les maladies neurodégénératives (type Alzheimer). Les sources majeures d’EPA et DHA sont celles d’origine marine. C’est dans ce contexte que cette étude a choisi de s’intéresser aux phospholipides provenant de têtes de saumon. L’objectif était de les extraire, de purifier la phosphatidylcholine (PC) issue du mélange de lipides et d’en déterminer ses propriétés d’auto-assemblage en bicouches. Une approche expérimentale par la caractérisation physico-chimique de ce PC a été complétée par une étude théorique du même composé en utilisant les techniques de simulation par dynamique moléculaire qui a permis une caractérisation à l’échelle moléculaire des bicouches lipidiques. La caractérisation a permis d’aboutir au profil détaillé de la composition du mélange PC saumon, d’élaborer le diagramme de phase PC –eau, de déterminer les propriétés d’empaquetage d’hydratation de ce lipide. Les paramètres choisis pour l’étude en dynamique moléculaire ont permis de reproduire de manière fidèle les résultats expérimentaux, validant ainsi le modèle et les conditions de simulations déterminés au préalable. La caractérisation des propriétés structurales de la PC de saumon sous forme de multicouches a permis d’approfondir la compréhension des mécanismes d’interactions à l’échelle moléculaire existant entre les lipides insaturés eux-mêmes. / The literature of recent decades is replete with references regarding the benefits of omega 3 fatty acids such as EPA (C20:5 n-3) and DHA (C22:6 n-3) which play an essential role in preventing many diseases such as neurodegenerative diseases (Alzheimer's type). The major sources of EPA and DHA are those of marine origin. It is within this context that this study chose to deal with phospholipids from salmon heads. The objective was to extract, purify phosphatidylcholine (PC) derived from the mixture of lipids and determine its properties of self-assembly into bilayers. An experimental approach by the physicochemical characterization of this PC was supplemented by a theoretical study of the same compound using the techniques of molecular dynamics simulation that allowed a molecular-scale characterization of lipid bilayers. The characterization resulted in detailed profile of the mixture composition of salmon PC, to draw up the phase diagram of PC-water, to determine the packing and hydration properties of this lipid. The parameters chosen for the study of molecular dynamics have faithfully reproduced the experimental results, thus validating the model and simulation conditions determined in advance. The characterization of structural properties of the PC as a multilayer salmon has deepened the understanding of interaction mechanisms at the molecular level between unsaturated lipids themselves.

Orientadores: Alex Antonelli, Maurice de Koning / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-18T18:03:40Z (GMT). No. of bitstreams: 1 CajahuaringaMacollunco_OscarSamuel_M.pdf: 6673580 bytes, checksum: a344f89389b318a79fb73ef903615a6b (MD5) Previous issue date: 2011 / Resumo: A física dos líquidos super-resfriados é um dos problemas para o qual ainda não existe uma única teoria que tenha capturado com sucesso todas as características destes materiais, principalmente a origem da dinâmica complexa, e também a possível ocorrência de transições líquido-líquido nesse regime. Nosso trabalho está focado nas propriedades dinâmicas do gálio, que é evidenciada pelas funções de correlação temporais antes e depois da transição de fase líquido-líquido. Primeiro, foram feitas simulações atomísticas no gálio através de dinâmica molecular usando o modied embedded atom model (MEAM) e condições periódicas de contorno em uma super-célula contendo 1152 átomos, com o propósito de mostrar a transição de fase líquido-líquido obtida em recentes pesquisas teóricas. Para estudar a dinâmica do sistema como um todo, calculamos o deslocamento quadrático médio, que revela o platô em tempos intermediários, o qual se torna mais notório após a transição líquido-líquido. Esse comportamento pode ser originado por uma dinâmica espacialmente homogênea ou uma dinâmica espacialmente heterogênea. Para saber qual das duas hipóteses é mais relevante foi medido o parâmetro não-gaussiano de ordem 2, porque este nos fornece informação do grau de heterogeneidade dinâmica do sistema, e que mostrou que nosso sistema possui uma dinâmica heterogênea. Posteriormente, foi calculada a auto-função intermediária de espalhamento usando o método da transformada rápida de Fourier que é mas eciente para tempos de correlação longos. Esta função fornece a correlação na densidade de partículas no espaço recíproco, que mostra também um platô em tempos intermediários. Com o fim de analisar como relaxa cada partícula, baseados na ideia da dinâmica espacialmente heterogênea, foi possível separar as partículas pela sua dinâmica em dois grupos, um grupo que possui a dinâmica mais difusiva e outro que possui a dinâmica menos difusiva. Finalmente, foram caracterizados estruturalmente esses dois grupos e, comparando-os com as funções estruturais do sistema todo, concluímos que as duas fases presentes no líquido depois da transição, o líquido de alta densidade possui uma dinâmica mais difusiva e o líquido de baixa densidade possui uma dinâmica menos difusiva. Foi possível com estes resultados identicar espacialmente as duas fases líquidas e observar claramente os seus próprios domínios e que estes não estão misturados / Abstract: The physics of supercooled liquids still lacks a single theory which can successfully capture all features of these materials, mainly the cause for their complex dynamics and the possibility of liquid-liquid phase transitions in this regime. Our work is focused on the dynamics of liquid gallium, which was investigated through the correlation functions before and after the liquid-liquid phase transition. First, atomistic simulations were performed using the modied embedded atom model (MEAM) and periodical boundary conditions in a 1152-atom supercell, in order to obtain the liquid-liquid phase transition observed in recent previous simulations. To study the dynamics of the whole system, we calculate the mean square displacement, which shows the plateau for intermediate times that becomes much more noticeable after the liquidliquid transition. This behavior can be caused by either a spatially homogeneous dynamics or a spatially heterogeneous dynamics. In order to find out which hypothesis is more relevant for our case, the second order non-Gaussian parameter was determined, since it provides the degree of heterogeneity of the dynamics of the system, showing that system exhibits a heterogeneous dynamics. Later, the self-intermediate scattering function was calculated using the fast Fourier transform method, which is more ecient for long correlation times. This function gives the density particle correlation in reciprocal space, showing a plateau in intermediate times as well. In order to analyze how each particle relaxes, based on the idea of a spatially heterogeneous dynamics, it was possible to gather the particles according to their mobility in two groups, one which has a more diffuse dynamics and another which has a less diffuse dynamics. Finally, these two groups were structurally characterized and by comparing them with the structural functions of the whole system, it was possible to determine that the liquid of higher density has a more diffusive dynamics, whereas the lower density liquid has less diffusive dynamics. From these results we were able to spatially identify the two liquid phases, which clearly display their own domains that do not mix with each other / Mestrado / Física da Matéria Condensada / Mestre em Física

Orientador: Alex Antonelli / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-29T05:01:21Z (GMT). No. of bitstreams: 1 CajahuaringaMacollunco_OscarSamuel_D.pdf: 15985103 bytes, checksum: f54d17d6335170c62196c9e329f257d9 (MD5) Previous issue date: 2015 / Resumo: Apesar de serem substâncias muito diferentes, silício e gálio, na fase líquida, compartilham muitas anomalias nas suas propriedades termodinâmicas. Evidências teóricas e experimentais sugerem que esses líquidos podem sofrer a chamada transição de fase líquido-líquido (LLPT). Especula-se que este tipo de transição é acompanhada por uma transição dinâmica entre um líquido frágil e um líquido forte, com base nas hipóteses de que o surgimento de um "dip" na função auto-intermediária de espalhamento logo após o regime balístico e o aparecimento do excesso de modos vibracionais em baixas frequências após a LLPT estariam relacionados com o comportamento de líquidos fortes. Foi realizado um estudo da dinâmica desses sistemas através das funções de correlação nas vizinhanças da LLPT usando simulações clássicas. Observamos apenas no caso do silício o aparecimento do "dip" na função auto-intermediária de espalhamento. A densidade de estados vibracionais reduzida de ambos os líquidos apresenta picos em baixas frequências, sugerindo que ambos os líquidos seriam fortes. Desta forma, nos dois casos estudados, as duas hipóteses não são conclusivas. Visando um melhor entendimento, determinamos a viscosidade de cisalhamento de ambos os líquidos em um amplo intervalo de temperatura. A apresentação desses resultados no chamado gráfico de Angell, indica que, em ambos os casos, a LLPT é acompanhada de uma transição de um líquido frágil para um líquido menos frágil, o que não dá suporte às especulações de uma transição frágil-forte. Além de investigar a LLPT em gálio "bulk", foi também estudado o gálio líquido confinado em nanofendas, ou seja, um sistema quase-bidimensional. Foi observada a transição de fase de primeira ordem entre um líquido isotrópico e um líquido ordenado. Por meio de uma série de análises estruturais, tanto de ordem translacional quanto orientacional, conseguimos identificar o líquido ordenado como a fase hexática, que é prevista ocorrer em sistemas bidimensionais segundo a teoria de Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY). Como a LLPT em gálio bulk foi identificada em simulações de dinâmica clássica, realizamos um estudo visando obter evidência da LLPT em simulações de primeiros princípios, que são em princípio mais realísticas. Utilizando o chamado método-Z, que permite estimar a temperatura de fusão, determinamos que o regime super-resfriado do gálio líquido descrito por cálculos de primeiros princípios ocorre em temperaturas abaixo de 400 K. Partindo de um líquido em equilíbrio em 500 K, realizamos simulações a pressão constante igual a zero em que o líquido foi resfriado para 260 K em um intervalo de 145 ps. Não foi observada uma mudança abrupta no volume durante o resfriamento. A fim de melhorar a amostragem em nossas simulações de primeiros princípios, realizamos um estudo de dinâmica molecular a volume constante com troca de réplicas. Foram usadas um total de 10 réplicas entre 400 K e 260 K, em simulações com a duração de 100 ps. Neste caso, a pressão decresceu monotonicamente com a temperatura, o que é característico de líquidos simples que não sofrem LLPT. Cabe ressaltar que nas simulações clássicas que identificaram a LLPT em gálio, a taxa de resfriamento foi muito mais lenta, da ordem de nano-segundos, que seria inexequível para simulações de primeiros princípios / Abstract: Despite of being very different substances, silicon and gallium, in their liquid phase, share several anomalous thermodynamic properties. Theoretical and experimental evidence suggest that these liquids can undergo the so-called liquid-liquid phase transition (LLPT). There has been speculated that this transition is accompanied by a dynamic transition form a fragile liquid to a strong liquid, based on the hypothesis that the development of a dip in the self-intermediate scattering function, just after the ballistic regime, and the appearance of excess low frequency vibrational modes just after the LLPT would be related to the behavior of strong liquids. It was performed a study of the dynamics of these systems through the correlation functions in the vicinity of the LLPT using classical simulations. We observed the development of a dip in the self-intermediate scattering function only in the case o silicon. The reduced density of vibrational states of both liquids exhibits peaks in low frequencies, suggesting both liquids to be strong. Therefore, for the two cases we studied, the two hypotheses are not conclusive. In order to achieve a better understanding, we determined the shear viscosity of both liquids for a wide range of temperatures. By plotting the data in the so-called Angell plot one can see that, in both cases, the LLPT is accompanied by a transition of a fragile liquid to a less fragile liquid, which does not give support to the speculations of a fragile-to-strong transition. Aside from investigating the LLPT in bulk gallium, it was also studied liquid gallium confined in nanoslits. i.e., a quasi two-dimensional system. It was observed a first-order transition between an isotropic liquid and an ordered liquid. Through a series of structural analyses, both translational and orientational, we were able to identify the ordered liquid as the hexatic phase, which is predict to occur in two-dimensional systems according to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. Since the LLPT in gallium has been found in classical molecular dynamics simulations, we performed a study aimed to obtain evidence of the LLPT in first-principles simulations, which are, in principle, more realistic. Using the so-called Z-method, which allows one to estimate the melting temperature, we determined the supercooled regime of liquid gallium, described by first-principles, to occur below 400 K. Starting from an equilibrated liquid at 500 K, we performed simulations at zero pressure in which the liquid was cooled down to 260 K in an interval of 145 ps. No abrupt change in the volume was observed as the liquid was cooled. In order to improve sampling in our first-principles simulations, we performed a study using constant volume molecular dynamics with replica exchange. Ten replicas were used between 400 K and 260 K, in simulations 100 ps long. In this case, pressure decreased monotonically with temperature, which is the behavior of simple liquids, which do not exhibit the LLPT. It should be emphasized that in the classical simulations that identified the LLPT in gallium, the cooling rates were much slower, of the order of nanoseconds, which would be unfeasible for first-principles simulations / Doutorado / Física / Doutor em Ciências / 1000949 / CAPES