Neste trabalho usamos um modelo hidrodinâmico com condições iniciais flutuantes evento a evento para estudar alguns observáveis em colisões de íons pesados relativísticos. Mostramos que a introdução de flutuações, caracterizadas por tubos longitudinais de alta densidade de energia, torna o cálculo dos observáveis mais próximo dos dados, em comparação à abordagem tradicional, na qual as condições iniciais são parametrizadas através de funções suaves e simétricas. No caso da distribuição de partículas em função do momento transversal, observamos um aumento do número de partículas na região onde p_t>2.0GeV, como consequência da expansão rápida dos tubos de alta densidade de energia, principalmente os tubos próximos à superfície da matéria, que chamamos de tubos periféricos. No caso de v_2 em função do momento transversal, observamos uma redução do fluxo elíptico na mesma região, devido à expansão isotrópica dos tubos periféricos. No caso de v_2 em função da pseudo-rapidez, observamos, com o uso das condições iniciais flutuantes, uma curva com um perfil triangular muito próximo dos dados. No caso da função de correlação de duas partículas, observamos que os tubos periféricos dão origem às chamadas estruturas de Ridge. Estas estruturas representam correlações de longo alcance, na direção longitudinal, entre as partículas trigger e as partículas associadas. Obtivemos uma função de correlação com três Ridges, respectivamente, nas posições: \\Delta\\phi=0, \\Delta\\phi=2 e \\Delta\\phi=-2. Mostramos, também, que o modelo reproduz o chamado efeito in-plane/out-of-plane, que está relacionado com a dependência da função de correlação de duas partículas em relação ao ângulo azimutal do momento da partícula trigger. De modo geral, observamos que, com o ajuste correto dos parâmetros do modelo, como por exemplo, a distribuição de densidade de energia inicial média e a temperatura de desacoplamento, todos os observáveis discutidos nesta tese apresentam um razoável acordo com os dados. / In this work we apply a hydrodynamic model with fluctuating initial conditions in a event by event basis to study some observables in relativistic heavy ion collisions. We show that the introduction of fluctuations, characterized by longitudinal high energy density tubes, turn the computation of the observables closer to data, in comparison to the traditional approach, in which the initial conditions are parametrized by using smooth and symmetrical functions. In the case of the transverse momentum distribution, we observe an enhancement of the number of particles in the region where p_t>2.0GeV, as a consequence of the fast expansion of the high energy density tubes, mainly the tubes that are close to the surface of the matter, called peripheral tubes. In the case of v_2 as a function of the transverse momentum, we observe a reduction of the elliptic flow at the same region, due to the isotropic expansion of the peripheral tubes. In the case of v_2 as a function of the pseudorapidity, we observe, by using the fluctuating initial conditions, a curve with a triangular shape very close to data. In the case of the two particle correlation function, we observed that the peripheral tubes give rise to the Ridge structures. These structures represent a long range correlation, in the longitudinal direction, between the trigger particles and the associated particles. We got a two particle correlation function with three Ridges, respectively, at the positions: \\Delta\\phi=0, \\Delta\\phi=2 and \\Delta\\phi=-2. In addition, we show that the model reproduces the in-plane/out-of-plane effect, which is related with the two particle correlation function dependence on the azimuthal angle of the momentum of the trigger particle. Generally, we observed that, with the proper adjustment of the parameters of the model, as, for example, the average initial energy density and the freeze-out temperature, all observables discussed in this thesis show a reasonable agreement with data.

Neste trabalho foi realizado um estudo de colisões de íons pesados relativísticos usando um modelo hidrodinâmico com simetria cilíndrica e invariância de boost longitudinal. Foram utilizadas duas equações de estado, uma com píons livres e nucleons interagentes com massas modificadas pelo meio e outra com píons e nucleons livres e exclusão de volume. Para contemplar a possível formação do Plasma de Quarks e Glúons numa colisão nuclear relativística, em ambos os modelos foi incluída uma transição para uma fase de quarks e glúons livres. Foram calculadas distribuições de momento transversal para prótons e píons produzidos em colisões no AGS, no SPS e no RHIC. Os resultados obtidos com os dois modelos foram comparados entre si e com os dados experimentais, e concluímos que a modificação das massas das partículas devido ao efeito das interações na matéria altamente densa e quente como a que é formada numa colisão nuclear é uma possibilidade que não pode ser descartada. / In this work a study on relativistic heavy ion collisions was performed using a hydrodinamical model with cylindrical symmetry and longitudinal boost invariance. Two equations of state were used, one with free pions and interacting nucleons with medium modified masses and another with free pions and nucleons and volume exclusion. To implement the possible formation of the Quark-Gluon Plasma in a relativistic nuclear collision, in both models a transition to a phase of free quarks and gluons was included. Distributions for transverse momentum were computed for protons and pions produced in collisions at the AGS, SPS and RHIC. The results obtained with the two models were compared with each other and with the experimental data, and we concluded that the modification in the masses of the particles due to the interaction effects in the highly dense and hot matter, as the one formed in a nuclear collsion, is a possibility that cannot be dismissed.

Neste trabalho foi realizado um estudo de colisões de íons pesados relativísticos usando um modelo hidrodinâmico com simetria cilíndrica e invariância de boost longitudinal. Foram utilizadas duas equações de estado, uma com píons livres e nucleons interagentes com massas modificadas pelo meio e outra com píons e nucleons livres e exclusão de volume. Para contemplar a possível formação do Plasma de Quarks e Glúons numa colisão nuclear relativística, em ambos os modelos foi incluída uma transição para uma fase de quarks e glúons livres. Foram calculadas distribuições de momento transversal para prótons e píons produzidos em colisões no AGS, no SPS e no RHIC. Os resultados obtidos com os dois modelos foram comparados entre si e com os dados experimentais, e concluímos que a modificação das massas das partículas devido ao efeito das interações na matéria altamente densa e quente como a que é formada numa colisão nuclear é uma possibilidade que não pode ser descartada. / In this work a study on relativistic heavy ion collisions was performed using a hydrodinamical model with cylindrical symmetry and longitudinal boost invariance. Two equations of state were used, one with free pions and interacting nucleons with medium modified masses and another with free pions and nucleons and volume exclusion. To implement the possible formation of the Quark-Gluon Plasma in a relativistic nuclear collision, in both models a transition to a phase of free quarks and gluons was included. Distributions for transverse momentum were computed for protons and pions produced in collisions at the AGS, SPS and RHIC. The results obtained with the two models were compared with each other and with the experimental data, and we concluded that the modification in the masses of the particles due to the interaction effects in the highly dense and hot matter, as the one formed in a nuclear collsion, is a possibility that cannot be dismissed.

Neste trabalho usamos um modelo hidrodinâmico com condições iniciais flutuantes evento a evento para estudar alguns observáveis em colisões de íons pesados relativísticos. Mostramos que a introdução de flutuações, caracterizadas por tubos longitudinais de alta densidade de energia, torna o cálculo dos observáveis mais próximo dos dados, em comparação à abordagem tradicional, na qual as condições iniciais são parametrizadas através de funções suaves e simétricas. No caso da distribuição de partículas em função do momento transversal, observamos um aumento do número de partículas na região onde p_t>2.0GeV, como consequência da expansão rápida dos tubos de alta densidade de energia, principalmente os tubos próximos à superfície da matéria, que chamamos de tubos periféricos. No caso de v_2 em função do momento transversal, observamos uma redução do fluxo elíptico na mesma região, devido à expansão isotrópica dos tubos periféricos. No caso de v_2 em função da pseudo-rapidez, observamos, com o uso das condições iniciais flutuantes, uma curva com um perfil triangular muito próximo dos dados. No caso da função de correlação de duas partículas, observamos que os tubos periféricos dão origem às chamadas estruturas de Ridge. Estas estruturas representam correlações de longo alcance, na direção longitudinal, entre as partículas trigger e as partículas associadas. Obtivemos uma função de correlação com três Ridges, respectivamente, nas posições: \\Delta\\phi=0, \\Delta\\phi=2 e \\Delta\\phi=-2. Mostramos, também, que o modelo reproduz o chamado efeito in-plane/out-of-plane, que está relacionado com a dependência da função de correlação de duas partículas em relação ao ângulo azimutal do momento da partícula trigger. De modo geral, observamos que, com o ajuste correto dos parâmetros do modelo, como por exemplo, a distribuição de densidade de energia inicial média e a temperatura de desacoplamento, todos os observáveis discutidos nesta tese apresentam um razoável acordo com os dados. / In this work we apply a hydrodynamic model with fluctuating initial conditions in a event by event basis to study some observables in relativistic heavy ion collisions. We show that the introduction of fluctuations, characterized by longitudinal high energy density tubes, turn the computation of the observables closer to data, in comparison to the traditional approach, in which the initial conditions are parametrized by using smooth and symmetrical functions. In the case of the transverse momentum distribution, we observe an enhancement of the number of particles in the region where p_t>2.0GeV, as a consequence of the fast expansion of the high energy density tubes, mainly the tubes that are close to the surface of the matter, called peripheral tubes. In the case of v_2 as a function of the transverse momentum, we observe a reduction of the elliptic flow at the same region, due to the isotropic expansion of the peripheral tubes. In the case of v_2 as a function of the pseudorapidity, we observe, by using the fluctuating initial conditions, a curve with a triangular shape very close to data. In the case of the two particle correlation function, we observed that the peripheral tubes give rise to the Ridge structures. These structures represent a long range correlation, in the longitudinal direction, between the trigger particles and the associated particles. We got a two particle correlation function with three Ridges, respectively, at the positions: \\Delta\\phi=0, \\Delta\\phi=2 and \\Delta\\phi=-2. In addition, we show that the model reproduces the in-plane/out-of-plane effect, which is related with the two particle correlation function dependence on the azimuthal angle of the momentum of the trigger particle. Generally, we observed that, with the proper adjustment of the parameters of the model, as, for example, the average initial energy density and the freeze-out temperature, all observables discussed in this thesis show a reasonable agreement with data.

No description available.

In this thesis we have developed electronic and spin model Hamiltonians to understand magnetism in molecule based magnets like photomagnets, high-nuclearity transition metal complexes and single molecule magnets. In chapter 1, we provide an overview of molecular magnets. Here, we present a survey on the literature available on molecule based magnets. The chapter throws light on various phenomena found in molecular magnetic systems that range in dimensions from 3D down to molecular dimension. This is followed by a brief introduction to high-nuclearity transition metal complexes and single molecule magnets (SMMs). In the last two sections of this chapter, we discuss Light Induced Excited Spin State Trapping (LIESST) and photomagnetism in some molecular systems. Chapter 2 discusses various theoretical models that have been developed for magnetism. We begin with an introduction to the spin Hamiltonian and the origin of direct and kinetic exchange in simple systems and extend it to larger systems. Then we introduce the concept of superexchange proposed by Goodenough and Kanamori, followed by introduction to anisotropic Dzyalashinskii-Moria (DM) exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. We also discuss molecular magnetic anisotropy, long-range magnetic interactions and higher order exchange interactions. These are effective model Hamiltonians that do not provide microscopic origin of magnetism, hence electronic model Hamiltonians need to be invoked. We introduce electronic model Hamiltonians like Huckel, Hubbard and Pariser-Parr-Popple (PPP) models and then present numerical techniques like valencebond (VB) and constant MS techniques that are used to exactly solve these model Hamiltonians. We present a many-body electronic model involving the active orbitals on the transition metal ions for photomagnetism in MoCu6 cluster, in chapter 3. The model is exactly solved using a valence bond approach. The ground state solution of the model is highly degenerate and is spanned by five S=0 states, nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in all these states correspond to six Cu(II) ions and one diamagnetic Mo(IV ) ion. The optically excited chargetransfer (CT) state in each spin sector occurs at nearly the same excitation energy of 2.993 eV for physically reasonable parameter values. We find that the excitation cross sections in different spin manifolds are similar in magnitude. The lifetime of the S=3 excited states is expected to be the largest as the number of states below that energy is very sparse in this spin sector compared to other spin sectors. This shows that photomagnetism is not due to preferential excitation to the S = 3 state. The inputs from the electronic model allows us to develop a kinetic model. In this model, photomagnetism is attributed to a long lived S=3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model photomagnetism by including internal conversions and intersystem crossings. The key feature of the model is the assumption of existence of two kinds of S=3 states; one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the lowenergy states. The trapped S=3 state can decay via a thermally activated barrier to the other S = 3 state. The experimental XMT vs. T variation for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model. Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and M (M = MoV ,WV , NbIV ). Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in chapter 4 we develop a microscopic model for Ni(II) - M systems and solve it exactly using a valence bond approach. We identify direct exchange coupling, splitting of the magnetic orbitals and interorbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II) - M system. We present quantum phase diagrams which delineate the high-spin and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The electronic model for A − B systems has been employed to reproduce the experimental magnetic data of the { NiW }2 system. In chapter 5, we present a theoretical approach to calculate the molecular magnetic anisotropy parameters, DM and EM for single molecule magnets in any eigenstate of the exchange Hamiltonian, treating the anisotropy Hamiltonian as a perturbation. Neglecting inter-site dipolar interactions, we calculate molecular magnetic anisotropy in a given total spin state from the known single-ion anisotropies of the transition metal centers. The method is applied to Mn12Ac and Fe8 in their ground and first few excited eigenstates, as an illustration. We have also studied the effect of orientation of local anisotropies on the molecular anisotropy in various eigenstates of the exchange Hamiltonian. We find that, in case of Mn12Ac, the molecular anisotropy depends strongly on the orientation of the local anisotropies and the spin of the state. The DM value of Mn12Ac is almost independent of the orientation of the local anisotropy of the core Mn(IV ) ions. In the case of Fe8, the dependence of molecular anisotropy on the spin of the state in question is weaker. We have also calculated the anisotropy constants for several sets of exchange parameters and find that in Mn12Ac the anisotropy increases with spin excitation gap while in Fe8, the anisotropy is almost independent of the gap. We have modeled the magnetic property of Nb6Ni12 cluster using a spin Hamiltonian in chapter 6. From Goodenough-Kanamori rules we should expect a ferromagnetic exchange between Nb and Ni ions. However, the magnetic studies indicate that the interaction is antiferromagnetic. We give reasons for the anomaly and fit the XMT data using an antiferromagnetic Heisenberg model. The observed XMT value at 2 K however does not correspond to ferrimagnetic ground state of Stot=9 and we invoke intermolecular interaction to explain this feature.

Les imageurs CMOS représentent un outil d’avenir pour de nombreuses applications scientifiques de haut vol, tellesque l’observation spatiale ou les expériences nucléaires. En effet, ces imageurs ont vu leurs performancesdémultipliées ces dernières années grâce aux avancées incessantes de la microélectronique, et présentent aussi desavantages indéniables qui les destinent à remplacer les CCDs dans les futurs instruments spatiaux. Toutefois, enenvironnement spatial ou nucléaire, ces imageurs doivent faire face aux attaques répétées de particules pouvantrapidement dégrader leurs performances électro-optiques. En particulier, les protons, électrons et ions présents dansl’espace ou les neutrons de fusion nucléaire peuvent déplacer des atomes de silicium dans le volume du pixel et enrompre la structure cristalline. Ces effets de déplacement peuvent former des défauts stables introduisant des étatsd’énergie dans la bande interdite du silicium, et ainsi conduire à la génération thermique de paires électron-trou. Parconséquent, ces radiations non-ionisantes produisent une augmentation permanente du courant d’obscurité despixels de l’imageur et donc à une diminution de leur sensibilité et de leur dynamique. L’objectif des présents travauxest d’étendre la compréhension des effets de déplacement sur l’augmentation du courant d’obscurité dans lesimageurs CMOS. En particulier, ces travaux se concentrent sur l’étude de la forme de la distribution de courantd’obscurité en fonction du type, de l’énergie et du nombre de particules ayant traversé l’imageur, mais aussi enfonction des caractéristiques de l’imageur. Ces nombreux résultats permettent de valider physiquement etexpérimentalement un modèle empirique de prédiction de la distribution du courant d’obscurité pour une utilisationdans les domaines spatial et nucléaire. Une autre partie majeure de ces travaux consiste à utiliser pour la première foisla technique de spectroscopie de courant d’obscurité pour détecter et caractériser individuellement les défautsgénérés par les radiations non-ionisantes dans les imageurs CMOS. De nombreux types de défauts sont détectés etdeux sont identifiés, prouvant l’applicabilité de cette technique pour étudier la nature des défauts cristallins généréspar les effets de déplacement dans le silicium. Ces travaux avancent la compréhension des défauts responsables del’augmentation du courant d’obscurité en environnement radiatif, et ouvrent la voie au développement de modèles deprédiction plus précis, voire de techniques permettant d’éviter la formation de ces défauts ou de les faire disparaître. / CMOS image sensors are envisioned for an increasing number of high-end scientific imaging applications such asspace imaging or nuclear experiments. Indeed, the performance of high-end CMOS image sensors has dramaticallyincreased in the past years thanks to the unceasing improvements of microelectronics, and these image sensors havesubstantial advantages over CCDs which make them great candidates to replace CCDs in future space missions.However, in space and nuclear environments, CMOS image sensors must face harsh radiation which can rapidlydegrade their electro-optical performances. In particular, the protons, electrons and ions travelling in space or thefusion neutrons from nuclear experiments can displace silicon atoms in the pixels and break the crystalline structure.These displacement damage effects lead to the formation of stable defects and to the introduction of states in theforbidden bandgap of silicon, which can allow the thermal generation of electron-hole pairs. Consequently, nonionizingradiation leads to a permanent increase of the dark current of the pixels and thus a decrease of the imagesensor sensibility and dynamic range. The aim of the present work is to extend the understanding of the effect ofdisplacement damage on the dark current increase of CMOS image sensors. In particular, this work focuses on theshape of the dark current distribution depending on the particle type, energy and fluence but also on the imagesensor physical parameters. Thanks to the many conditions tested, an empirical model for the prediction of the darkcurrent distribution induced by displacement damage in nuclear or space environments is experimentally validatedand physically justified. Another central part of this work consists in using the dark current spectroscopy techniquefor the first time on irradiated CMOS image sensors to detect and characterize radiation-induced silicon bulk defects.Many types of defects are detected and two of them are identified, proving the applicability of this technique to studythe nature of silicon bulk defects using image sensors. In summary, this work advances the understanding of thenature of the radiation-induced defects responsible for the dark current increase in space or nuclear environments. Italso leads the way to the design of more advanced dark current prediction models, or to the development ofmitigation strategies in order to prevent the formation of the responsible defects or to allow their removal.