DSpace Coleção:
http://repositorio.ufla.br/jspui/handle/1/42124
Tue, 07 Dec 2021 09:28:54 GMT2021-12-07T09:28:54ZFenômenos cosmológicos por meio da gravitação f (Q)
http://repositorio.ufla.br/jspui/handle/1/46850
Título: Fenômenos cosmológicos por meio da gravitação f (Q)
Autor: Silva Junior, José Tarciso Souza
Resumo: The General Relativity (RG) theory describes the gravitational interaction quite satisfactorily even at cosmological levels, with impressive agreement with experimental observations. However, it presents some problems in the description of some gravitational phenomena, mainly with regard to the formation of structures in our universe, the presence of singularities, description and interpretation of some constituents, among others. In order to remedy these difficulties, alternative theories have been proposed in recent decades, in general minimal modifications to Einstein's theory. In particular, the description of gravitational phenomena from alternative geometries to Riemannian geometry has received great attention. Instead of formalism being constructed in terms of Riemannian geometry and curvature tensor, it is possible to develop models analogous to general relativity in terms of Weitzenböck geometry and torsion tensor, known as teleparallel theory, or in terms of Weyl geometry and the non-metricity tensor, whose model is known as symmetric teleparallel theory. These two models can be formulated based on a gauge theory, whose symmetry group is the translation group, which guarantees a big difference in the formal development of these models in different contexts. After a great interest in the analysis of models involving the T torsion, recently the alternative and analogous theory in terms of the non-metricity tensor Q has been considered in some analyzes due to the possibility of more general couplings than those of the torsion tensor. In this way, we took advantage of the possibility of exploring new gravitational phenomena in the context of the symmetrical teleparallel theory in this work. Therefore, in this work we review the main points involving the description of the universe in relation to some cosmological phenomena, and the application of the model in the description of the phenomenon of “Big Bounce”.
Descrição: Arquivo retido, a pedido do autor até julho de 2022.Thu, 12 Aug 2021 00:00:00 GMThttp://repositorio.ufla.br/jspui/handle/1/468502021-08-12T00:00:00ZAspectos de cosmologia e gravitação não local
http://repositorio.ufla.br/jspui/handle/1/46190
Título: Aspectos de cosmologia e gravitação não local
Autor: Oliveira, Dimas Jackson de
Resumo: The General Theory of Relativity has been confirmed experimentally by several results since its
proposal in 1915. However, it is known that this theory has its validity limit, not being applied
to very small length scales and initial times of the universe and does not provide an explanation
for the accelerated expansion of the universe. There is still no satisfactory interpretation for the
origin of the cosmological constant in the field equations, which describes this expansion in the
LCDM cosmological model. Although this model is the most accepted today, it is necessary to
postulate the existence of dark energy so that the experimental results are in accordance with
the predicted by General Relativity. Due to these and other open questions, several theories
have been proposed to minimally modify Einstein’s gravitational theory, such as f (R) theories,
non metricity theories and nonlocal theories. In this work we will present the fundamental
physical and mathematical concepts of gravitation and cosmology, based on General Relativity.
We will present the concept of non-locality in physical theories, through examples of quantum
mechanics, electrodynamics and a discussion of non-local gravitation. We will also present a
discussion about non-local operators and whether the principle of causality is violated by the
presence of non-local terms in an effective quantum action. We will discuss the modified gravity
model of Deser-Woodard, which minimally modifies the Einstein-Hilbert action, through nonlocal
terms inspired by quantum corrections of effective action. This model was proposed
in 2019 in order to explain the accelerated expansion of the universe, without cosmological
constant. We will analyze the perturbative solutions of this model, which allow a comparison
between theoretical and experimental data for the effective Newtons’s gravitational constant
and the growth rate of structures fs8. We will apply the Deser-Woodard model to bouncing
universe, finding exact and numerical solutions for the auxiliary fields, which contain non-local
information. The results of this analysis show a feasibility of a connection between the bouncing
universe and the LCDM model, based on non-local cosmology.
Descrição: Arquivo retido, a pedido do autor até abril 2022.Thu, 08 Apr 2021 00:00:00 GMThttp://repositorio.ufla.br/jspui/handle/1/461902021-04-08T00:00:00ZPropriedades magnetoelásticas de uma cadeia de diamante Ising- Heisenberg com spin-1/2 na vizinhança de um ponto triplo
http://repositorio.ufla.br/jspui/handle/1/46079
Título: Propriedades magnetoelásticas de uma cadeia de diamante Ising- Heisenberg com spin-1/2 na vizinhança de um ponto triplo
Autor: Ferreira, Nayara
Resumo: We study magnetoelastic properties of a spin-1/2 Ising-Heisenberg diamond chain, whose
elementary unit cell consist from two decorating Heisenberg spins and one nodal Ising spin. It is
assumed that each couple of the decorating atoms including the Heisenberg spins harmonically
vibrates perpendicularly to the chain axis, while the nodal atoms involving the Ising spins are
placed at rigid positions when ignoring their lattice vibrations. An effect of the magnetoelastic
coupling on a ground state and finite-temperature properties is particularly investigated close to
a triple coexistence point depending on a spring-stiffness constant ascribed to the Heisenberg
interaction. The magnetoelastic nature of the Heisenberg dimers is reflected through a non-null
plateau of the entropy emergent in a low-temperature region, whereas the specific heat displays
an anomalous peak slightly below the temperature region corresponding to the entropy plateau.
The magnetization also exhibits a plateau in the same temperature region at almost saturated
value before it gradually tends to zero upon increasing of temperature. The magnetic susceptibility
displays within the plateau region an inverse temperature dependence, which fails slightly
above this plateau, whereas an inverse temperature dependence is repeatedly recovered at high
enough temperatures.Thu, 28 Jan 2021 00:00:00 GMThttp://repositorio.ufla.br/jspui/handle/1/460792021-01-28T00:00:00ZOs efeitos de uma impureza em uma cadeia tipo diamante Ising-XXZ no emaranhamento térmico, na coerência quântica e no teletransporte quântico
http://repositorio.ufla.br/jspui/handle/1/46007
Título: Os efeitos de uma impureza em uma cadeia tipo diamante Ising-XXZ no emaranhamento térmico, na coerência quântica e no teletransporte quântico
Autor: Freitas, Marcos Paulo Ribeiro de
Resumo: Entanglement is not only a fundamental concept in Quantum Mechanics, but it is also an
extremely important tool for quantum information, being quite sensitive mainly to variations
in the magnetic field, temperature and the interaction parameters of a chain. On the other hand,
quantum coherence is also an important quantum resource, while quantum teleportation is an
impressive information process that uses entangled channels. In this work it is proposed the
study of effects of an impurity in an Ising-XXZ diamond type chain in the measurement of some
quantum correlations (entanglement), in quantum coherence and, also, the effects on quantum
information teleportation. In this model, the fundamental platelet of the Ising-XXZ spin-1/2
diamond type chain is formed by the XXZ interaction between the interstitial Heisenberg dimers
and the Ising coupling between the nodal and interstitial spins, respectively. The impurity in our
model is a tag with all the interaction parameters different from the rest of the chain. We obtain
the exact solution of the model using the transfer matrix method. We demonstrate that quantum
entanglement can be controlled and adjusted by varying the impurity parameters. Our results
show that the thermal entanglement of the model exhibits a clear improvement in performance
when we control and manipulate impurities in comparison with the original anisotropic model.
In addition, we found that the effects of impurity generate gains in quantum coherence and
greatly increase the quantum teleportation efficiencyMon, 18 Jan 2021 00:00:00 GMThttp://repositorio.ufla.br/jspui/handle/1/460072021-01-18T00:00:00Z