Orientador: Silvio A. S. Vitiello / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T05:46:23Z (GMT). No. of bitstreams: 1 Reis_MarceloAugustodos_M.pdf: 11400954 bytes, checksum: da0c111de1e988997b785a1a91546049 (MD5) Previous issue date: 2008 / Resumo: Este trabalho propõe duas funções de onda variacionais baseadas no conceito de partículas sombra e que descrevem a fase sólida dos sistemas formados por átomos de 4He. Uma delas, denominada Função de Onda Sombra Livre e Periódica (PFS) fornece bons resultados para esta fase aferidos pelas suas propriedades, entre elas as energias do sistema e a distribuição radial de pares que se mostraram em bom acordo com outros resultados da literatura. A outra, denominada Função de Onda Sombra Periódica (PS) além de fornecer bons resultados para as propriedades de interesse, se destaca por conseguir descrever a fase supersólida e desse modo predizer uma Condensação de Bose-Einstein na fase sólida. Todos os resultados que obtivemos foram decorrentes de nossas próprias implementações numéricas através do método Monte Carlo Variacional / Abstract: This work proposes two variational wave functions based on the concept of shadow particles that describes the solid phase of systems formed by atoms of 4He. One of them, called Periodic Free Shadow Wave Function (PFS) provides good results for this phase like the energies of the system and the Radial Distribution Function that proved to be in good agreement with other results of the literature. The other, called the Periodic Shadow Wave Function (PS) in addition to providing good results for these properties, describes the supersolid phase and thus predicts a Bose-Einstein Condensation of the solid phase. All the results that have been obtained are due to our own numerical implementations of the Variational Monte Carlo method / Mestrado / Física da Matéria Condensada / Mestre em Física

Orientador: Silvio Antonio Sachetto Vitiello / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-19T15:54:09Z (GMT). No. of bitstreams: 1 RugelesVargas_ElkinJezzid_M.pdf: 2457986 bytes, checksum: a05f1eaea9e1009565c9c9e1adb72150 (MD5) Previous issue date: 2011 / Resumo: Neste trabalho estudamos o comportamento das energias do 4He sólido a baixas temperaturas para diferentes densidades, utilizando-se os métodos complementares de Difusão Monte Carlo (DMC) para a temperatura de 0 K e Integrais de Trajetória Pelo Método de Monte Carlo (PIMC) para as temperaturas de 1 e 2 K. Como os operadores das energias cinética ^T e potencial Û não comutam, utilizou-se o teorema de Hellman-Feymann no método DMC e propriedades termodinâmicas da energia no método PIMC, para estimar-se a energia cinética e potencial do sistema. Obtivemos resultados com um acordo muito bom com dados experimentais da literatura. O estudo da energia cinética é de interesse em sistemas que obedecem a simetria de Bose-Einstein, onde um decréscimo da energia cinética, abaixo de uma temperatura crítica Tc, pode ser considerado como uma evidência da condensação / Abstract: In this work we have studied the behavior of the energies in solid 4He at low temperatures for different densities, using two complementary methods, of Diffusion Monte Carlo (DMC) method for a temperature of 0 K and Path Integrals by Monte Carlo (PIMC) method at temperatures of 1 and 2 K. Due to the fact that the kinetic ^ T and potential ^U operators do not commute, the Helman-Feyman theorem was used in the DMC method, and thermodynamic properties of the energy in PIMC method, to estimate the kinetic and potential energy of the system. We obtained results in a good agreement with experimental data. The study of the kinetic energy is interesting in systems that obey the Bose-Einstein symmetry, where a decrease in its value, below a critical temperature Tc, can be considered as an evidence of the existence of a condensate / Mestrado / Física / Mestre em Física

Several microscopic theories point out that Bose-Einstein condensation (BEC), i.e., a macroscopic occupation of the lowest energy single particle state in many-boson systems, may appear also in quantum fluids and solids and that it is at the origin of the phenomenon of superfluidity. Nevertheless, the connection between BEC and superfluidity is still matter of debate, since the experimental evidences indicating a non zero condensate fraction in superfluid helium are only indirect. In the theoretical study of BEC in quantum fluids and solids, perturbative approaches are useless because of the strong correlations between the atoms, arising both from the interatomic potential and from the quantum nature of the system. Microscopic Quantum Monte Carlo simulations provide a reliable description of these systems. In particular, the Path Integral Monte Carlo (PIMC) method is very suitable for this purpose. This method is able to provide exact results for the properties of the quantum system, both at zero and finite temperature, only with the definition of the Hamiltonian and of the symmetry properties of the system, giving an easy picture for superfluidity and BEC in many-boson systems. In this thesis, we apply PIMC methods to the study of several quantum fluids and solids. We describe in detail all the features of PIMC, from the sampling methods to the estimators of the physical properties. We present also the most recent techniques, such as the high-order approximations for the thermal density matrix and the worm algorithm, used in PIMC to provide reliable simulations. We study the liquid phase of condensed 4He, providing unbiased estimations of the one-body density matrix g1(r). We analyze the model for g1(r) used to fit the experimental data, highlighting its merits and its faults. In particular we see that, even if it presents some difficulties in the description of the overall behavior of g1(r), it can provide an accurate estimation of the kinetic energy K and of the condensate fraction n0 of the system. Furthermore, we show that our results for n0 as a function of the pressure are in a good agreement with the most recent experimental results. The study of the solid phase of 4He is the most significant part of this thesis. The recent observation of non classical rotational inertia (NCRI) effects in solid helium has generated big interest in the study of an eventual supersolid phase, characterized at the same time by crystalline order and superfluidity. Nevertheless, until now it has been impossible to give a theoretical model able to describe all the experimental evidences. In this work, we perform PIMC simulations of 4He at high densities, according to different microscopic configurations of the atoms. In commensurate crystals we see that BEC does not appear, our model being able to reproduce the momentum distribution obtained form neutron scattering experiments. In a crystal with vacancies, we have been able to see a transition to a superfluid phase at temperatures in agreement with experimental results if the vacancy concentration is low enough. In amorphous solids, superfluid effects are enhanced but appear at temperatures higher than the experimental estimation for the transition temperature. Finally, we study also metastable disordered configurations in molecular para-hydrogen at low temperature. The aim of this study is to investigate if a Bose liquid other than helium can display superfluidity. Choosing accurately a ¿quantum liquid¿ initial configuration and the dimensions of the simulation box, we have been able to frustrate the formation of the crystal and to calculate the temperature dependence of the superfluid density, showing a transition to a superfluid phase at temperatures close to 1 K.

Orientador: Maurice de Koning / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-26T09:46:10Z (GMT). No. of bitstreams: 1 LandinezBorda_EdgarJosue_D.pdf: 16266231 bytes, checksum: 7240bfb0050d5a5b6df4d1741c7e9f54 (MD5) Previous issue date: 2014 / Resumo: Esta Tese aborda o problema de determinar propriedades mecânicas fundamentais de 4 He solido na fase hcp usando simulação atomística. Tais propriedades têm sido associadas à possível existência de uma fase cristalina superfluida conhecida como super-solidez. Embora experimentos sofisticados investigando propriedades mecânicas do Hélio sólido têm sido realizados, os resultados dependem da dinâmica microscópica que nem sempre é acessível experimentalmente. Desta maneira as interpretações dos experimentos ficam sujeitos a debates. Neste contexto a modelagem atomística é uma ferramenta útil para fornecer informação qualitativa e quantitativa do comportamento atomístico das propriedades mecânicas. Através de Monte Carlo de integrais de trajetória (PIMC) calculamos a resistência ideal de cisalhamento e tensão de Peierls de uma discordância screw para este sólido quântico protótipo. A resistência ideal estabelece um limite teórico superior de comportamento elástico do material. É a máxima tensão que um cristal livre de defeitos pode suportar, sem produzir uma deformação plástica. Os reultados mostram que o limite de resistência ideal é acompanhado pela nucleação homogênea de uma falha de empilhamento. A resistência ideal é anisotrçopica no plano basal, mas esta anísotropia é bem descrita pela lei de Schmid, que é baseida em conceitos clássicos. Além disso, a comparação do valor da resistência ideal com os de um conjunto grande de cristais clássicos mostra que os resultados para 4 H e estão bem descritos pelo modelo Frenkel-Orowan modificado, que também se baiseia em conceitos puramente clássicos. Em relação às propierdades de discordância, investigamos a estrutura do caroço e a tensão de Peierls para a discordância screw no plano basal. Os resultados mostram uma forte tendência de dissociação e uma tensão de Peierls da ordem e 0.4 bar e estão totalmente consistentes com o comportamento em sólidos clássicos. Assim, os resultados da Tese mostram que, mesmo em regimes de temperatura onde a fase líquida é superfluida, o ordenamento cristalino parece suprimir os efeitos quânticos resposáveis pela super-fluidez no líquido. / Abstract: This Thesis addresses a number of issues related to the determination of the fundamental mechanical properties of hcp 4 He. These properties have been linked to the possible existence of a superfluid crystalline phase know as supersolid. While many sophisticated experiments have investigated these properties, their results often require assumptions regarding the underlying processes on the atomic scale which are not accessible experimentally. In this way, the experimental interpretations often remain subject of debate. In this context, atomistic modeling is a useful tool that allows one to obtain both qualitative as well quantitative information about these processes. Using Path-integral Monte Carlo simulations we compute the ideal shear strength (ISS), as well as the structure and mobility parameteres for the screw dislocation for this prototypical quantum solid. The ideal shear strength represents a theoretical upper limit of the elastic response of a crystal, being defined as the maximum stress a defect-free crystal can withstand before deforming plastically. Our resuls show that the ISS limit is accompanied by the homogeneous nucleation of a stacking fault. Furthermore, the ISS on the basal plane is found to be anisotropic, although the anisotropy is well described by Schmid¿s law of resolved shear stress. In addition, comparison of the intrinsic ISS value to values of a large set of classical crystals shows that the results for 4 He are well described by the modified Frenkel-Orowan model, which is also based on purely classical concepts. Considering dislocation properties, we investigate the core structure and Peierls stress for the screw dislocation in the basal plane. The results show a strong dissociation tendency of the core and a Peierls stress of the order of 0.4 bar, consistent with the behavior of classical solids. Accordingly the results of this Thesis show that, even in the temperature regime where the liquid phase is superfluid the crystalline order seems to suppress the quantum effects for the liquid superfluidity / Doutorado / Física / Doutor em Ciências

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