Source wavelet estimation from seismic data is an important task in the subsurface imaging effort. In this study, we intend to test a Machine Learning (ML) approach to obtain the wavelet parameters from seismograms. For the sake of simplicity, it is usually assumed that the Ricker wavelet is a good approximation for the seismic source waveform. However, the attenuation caused by propagation in heterogeneous media and specific water phenomena distort the frequency content of the wavelet, an effect not adequately represented by Ricker. To correct this phenomenon, we work with the Generalized Seismic Wavelet (GSW), which is a recently proposed generalization of the Ricker wavelet. The Ricker has just one parameter, the fundamental frequency. On the other hand, the GSW has two parameters: the same fundamental frequency and the order of the fractional time derivative of the Gaussian function; the latter controls the wavelet’s shape and asymmetry in time domain. For a specific value of this additional parameter, the GSW becomes the Ricker wavelet. Using the seismogram as input, machine learning models are trained to output the generalized wavelet parameters that best explain the data. The source wavelet is then estimated as the WSG whose parameters are those provided by the model. The proposed methodology was tested and validated with synthetic seismic data, in which a marine acquisition in deep waters was modeled, using a simplified velocity model obtained from Gato do Mato, an oil field located in the Santos basin, Brazil. We propagate an acoustic wave using the following acquisition geometry: one source placed at the surface and 50 receivers placed at 2100 m in depth, spaced 100 m apart from one another in the horizontal direction. We performed one thousand numerical propagations using the GSW with different parameterizations as the source waveform. Furthermore, we randomly selected 70% of the data for training, and the other 30% for testing. Three ML algorithms have been considered for regression: Extreme Gradient Boosting (XGBoost), Bayesian Ridge  and the Random Forest. The results were satisfactory, achieving good accuracy in estimating the GSW parameters.

The distinction between cause and effect is a major challenge in physics, especially when dealing with correlated events. In quantum mechanics, this complexity is further accentuated due to phenomena such as superposition and quantum entanglement, which result in "instantaneous" correlations between distant systems, known as quantum non-locality. This work addresses the detection efficiency in quantum violations of causal limits, which are inequalities that must be respected by any local classical theory. Determining the minimum conditions necessary for detection efficiency to allow the observation of such a violation is crucial from a physical standpoint, as it is a determining factor for understanding how quantum causality works. Using theoretical models, we identify the critical detection efficiency parameters for different measurement configurations, employ instrumental variables to generate do-inequalities, and find the minimum efficiencies necessary for the violation of these inequalities.

In this study, we investigate the propagation of acoustic waves in one-dimensional phononic crystals with quasiperiodic
arrangement. These structures were built using two distinct materials, lead and epoxy, arranged in a specific sequence and
bounded by two semi-infinite layers of aluminum. During our research, we explored four notable substitution sequences:
Fibonacci, Thue-Morse, Double Period and Kolakoski. The quasiperiodic characteristic of these structures allows the
formation of regions known as "band gaps". Which prevent wave propagation for certain frequencies. This phenomenon offers
various technological applications, particularly in acoustic filters.
In the scope of our study, we examined both longitudinal and transverse propagations. To analyze the transmission spectrum,
we employed the transfer matrix methodology. Additionally, we varied the angle of incidence and introduced modifications in
the symmetry of the sequences, thus expanding our understanding of the properties of these quasiperiodic phononic crystals.

This dissertation explores updated constraints on long-lived particles originating from dark sectors by leveraging the well-established framework ofBig Bang Nucleosynthesis (BBN). It builds on the sensitivity ofBBN to late electromagnetic injections in the primordial plasma, which can alter

the abundance of light elements. The study assumes that heavy mother particles decay, emitting energetic photons that induce electromagnetic cascades, potentially disrupting nuclei formation. By focusing on the abundances ofHelium-4 and Deuterium, the research aims to provide refined limits on the injected energy and the lifetime ofthe particle. The thesis integrates theoretical insights from particle physics and cosmology to address phenomena beyond the Standard Model, such as dark matter, ultimately contributing to our understanding of the early Universe.

The AdS/CFT conjecture states an equivalence between a theory of quantum gravtivy and a theory without gravity.
It relates complementary regimes of the coupling constant and has been exhaustively investigated since its birth.
A key tool, but not the only one, is symmetry. As we will see, there is a match between the symmetries of the two
sides. We can say that this thesis is the union of two projects with the same goals, but working in different
geometries. Both projects are focused on the spin-2 field perturbation of either type IIA or IIB families of 10-
dimensional supergravity solutions. The perturbation of the metric is particularly interesting because of its
universality. In the first project, the metric has an AdS3 factor. In the second one, we have five families and each
of them has an AdS factor: AdS2, AdS4, AdS5, AdS6, and AdS7.
The “first project”, the one on AdS3\times M7, is interesting from the point of view of AdS/CFT correspondence
because it is dual to a d=2 CFT, which, in turn, possesses an infinite number of symmetry generators. In the last
few years, new important examples of such correspondence were constructed in the papers \
cite{Eberhardt:2017pty}, \cite{Eberhardt:2018ouy} and \cite{Eberhardt:2019qcl}. In this thesis, we will
investigate a less supersymmetric solutions. We will study the Kaluza-Klein spectrum of the spin two fluctuation
which is dual to the stress-energy tensor according to the conjecture.
In the “second project" we investigate five families at once and show that all of them admit a certain combination
of the warp factors as a solution. Such combination together with the respective spectra we call universal minimal
solution.

The quest to enhance magnetic properties through core@shell nanoparticles and the investigation of the heating effect
of magnetic nanoparticles using an alternating magnetic field have sparked intense scientific and technological interest.
Significant advancements in this field have been driven by progress in the production techniques of nanoscale magnetic
materials, enabling diverse applications such as permanent magnets, biomedical applications, and magnetic hyperthermia. This
thesis addresses two distinct nanostructured systems. The first focuses on the synthesis of 𝐶𝑜𝐹𝑒2𝑂4@𝐶𝑜𝐹𝑒2 nanocomposites
with core@shell structure using a method that does not involve special reagents or gases. The process involved the preparation
of glutaraldehyde-crosslinked chitosan beads containing 𝐶𝑜𝐹𝑒2𝑂4 nanoparticles, followed by high-temperature and vacuum
thermal treatment. The CO gas released during this process facilitated the reduction of 𝐹𝑒3+and 𝐶𝑜2+ ions to their zero-valent
states. After the synthesis, the structural, morphological, and magnetic properties of the samples were investigated. X-ray
diffraction analysis, transmission electron microscopy (TEM), and Mössbauer spectroscopy revealed the presence of the desired
magnetic phases. TEM images confirmed the core@shell structure. Magnetic characterization indicated exchange-coupling
between the phases under certain synthesis conditions, resulting in a maximum energy product (𝐵𝐻)𝑚𝑎𝑥= 0.67 MGOe. The
thickness of the CoFe$_2$ phase (≈ 9.0 nm) aligns with the theoretical limit expected by the Kneller-Hawig theory, which is
10.2 nm, for exchange coupling at the interface. The second system involves the synthesis and study of the magnetic properties
of Manganese Ferrite 𝑀𝑛𝐹𝑒2𝑂4 nanoparticles. In this part of the work, 𝑀𝑛𝐹𝑒2𝑂4 nanoparticles were synthesized using the
same method as the previous system but with conventional thermal treatment in the presence of air. Samples with various sizes
were obtained by thermal treatments at different temperatures, resulting in particle sizes ranging from 7.8 to 13.3 nm. At 300 K,
the saturation magnetization of the nanoparticles varied from 16.2 to 35.8 emu/g. Low-temperature Mössbauer spectroscopy
showed the presence of monophasic 𝑀𝑛𝐹𝑒2𝑂4. For all samples, the Mössbauer results at 300 K suggested superparamagnetic
behavior. AC and DC susceptibility measurements indicated that below the blocking temperature, the system behaves as a
superspin glass. Specific Loss Power (SLP) measurements were conducted at a frequency of 74 kHz and AC field amplitude of
247 Oe. The sample thermally treated at 600 °C, with a dispersion prepared at a concentration of 1.0 mg/mL, exhibited the
highest SLP of 129.1 W/g.

Half a century ago, Anderson’s resonating valence bond theory initiated a compelling research subject in condensed matter
physics: the exploration of the nature of quantum spin liquids. Today, the understanding of these phases of matter is deeply
connected to the concept of quantum entanglement. In a broad context, quantum spin liquids refer to systems wherein the
effective degrees of freedom are described by local magnetic moments residing on the lattice, exhibiting an absence of long-
range order even at very low temperatures. In addition, the ground-state wave function displays significant entanglement, and
excitations manifest fractionalization. In particular, this thesis is focused on the study of the chiral spin liquids that can be
found in Mott insulators that break time-reversal and parity symmetry. Starting from junctions composed of critical spin-1
chains, we construct a honeycomb network hosting a chiral spin liquid with gapless spectrum. The low-energy modes are
characterized by spin-1 Majorana fermions, which give rise to a legitimate two-dimensional state that harbors a Fermi
surface when the interactions at the junctions are adjusted close to chiral fixed points with staggered chirality. We explore the
physical properties and stability of the chiral spin liquid phase against boundary perturbations, employing well-controlled
analytical methods within the effective field theory of the network. Additionally, we establish evident connections with the
excitation spectrum derived from parton constructions on the kagome lattice.

Two-dimensional nanostructures are of growing interest in science due to their exceptional mechanical,
electronic, and thermal properties. As theoretical and experimental studies have advanced, analyzing how such
properties can be manipulated, especially by applying strain, has become necessary.
Our work is focused on analyzing how deformation, whether uniaxial or biaxial, affects some properties of
nanostructured materials. We studied materials composed only of carbon, such as graphene, α-graphyne, β-graphyne,
and ɣ-graphyne, and materials composed of two or more types of atoms, such as boron nitride and sheets composed of
carbon, boron, and nitrogen. These analyses are done through computational physics simulations. In the simulations
focused on mechanical properties, we use both classical methods (molecular dynamics) and quantum methods (Density
Functional Theory). In the study of the electronic properties, we used Density Functional Theory (DFT). When
studying the adsorption of gases, we used Monte Carlo simulations in the Grand canonical ensemble.
In this document, we present the results obtained, as well as perspectives for future calculations.

The soundscape, or acoustic landscape, is a concept that has received a great attention from ecologists, animal behaviorists and urban planners. The understanding and modeling of soundscape measurements in physics is still incipient. The duration of silence times in a soundscape is defined using a microphone and a typical sound intensity threshold; above the threshold, sound is perceived, below the threshold, silence is defined. In many soundscapes it was observed that the statistics of silence durations does not depend on the used threshold and follows a power law. In this dissertation, we build a null model for the statistics of silences: a microphone is placed in the center of a square matrix and sound emitters are randomly drawn in the matrix. Given a threshold, the sound is heard or not, depending on the sound intensity reaching the microphone. Assuming a squared decay of intensity and having all emitters the same power, only emitters within a critical radius are detected at the microphone. Under these conditions, the proposed model resembles a Bernoulli experiment, in which success consists of sound detection, and failure consists of inaudible sound at the microphone. The ratio between the area of the circle and the matrix defines the success probability of Bernoulli's experiment. Furthermore, the statistic between two successes follows a geometric distribution. Computer simulations confirm the theoretically established geometric distribution. Alternative tests with non-constant power have been performed, but the observed distribution in simulations is still a exponential type, and not a power law type.

Stellar flares originate from the release of a large amount of energy resulting from variations in the magnetic field of a
star. These phenomena can be used as important observational markers of the magnetic activity of low-mass stars and also
influence the atmospheric and biological conditions of planets in their surroundings. In hot subdwarf stars (sdBs), such
phenomena are presumed to be caused by events external to the star due to its interaction with the magnetic field of a binary
pair. This Master's Dissertation aims at the detection and basic characterization of eruption signatures in light curves collected
by the Transiting Exoplanet Survey Satellite (TESS) mission. Starting from an initial sample of 924 stars belonging to the TESS
Objects of Interest (TOIs) catalog, observed in sectors 31 to 40, we identified potential eruption events in 52 stars. We
additionally considered two sdB stars previously studied in the literature, for which we identified, from the TESS light curves,
eruption events in the form of giant magnetic pulses. A statistical analysis based on the color-magnitude diagram of our
working sample reveals a tendency for more flares to occur and to have higher energy in main sequence stars compared to those
out from the main sequence, as well as for cool stars compared to hot stars. These results agree with the fact that main sequence
stars and cool stars have more developed convective zones, and therefore higher magnetic activity, than evolved or hot stars. In
general, this dissertation provides new constraints for further studies on stellar magnetic activity and eruptions.

The speed of light plays a fundamental role throughout the theoretical physics. Any variation in this quantity can  lead to deep modifications on  our understanding of nature. In this dissertation, we performed a test for  temporal constancy of  speed of light by using   measurements from the Planck satellite (more precisely, the values of $\Omega_M$ and $\Omega_b$), the galaxy clusters gas mass fraction and SNe data. However, cosmological analyzes via galaxy clusters strongly depend on the mass measurements  of these structures. In our analyses, we considered three different values for the mass calibration factor ($K$) found in the literature, one of them coming from the Planck observations. We found that when the value of $K$ used was not that of the Planck satellite, a possible temporal variation of the speed of light was verified. This result shows a possible tension between the constancy of the speed of light, the values of $\Omega_M$ and $\Omega_b$ (from Satellite Planck) and the values of $K$ obtained independently from that of the Planck satellite.

The proposition of the dark matter theory was a milestone in understanding the structure and evolution of the Universe. Throughout history, astronomical observations have revealed that the amount of visible matter is not enough to explain the dynamics of galaxies and the rotation curves of spiral galaxies. This has led to the emergence of different theories of gravitation that seek to provide an explanation for this phenomenon. One of the most prominent theories is the Modified Newtonian Dynamics (MOND), which proposes a modification of Newton's law of gravitation in the low acceleration regimes ). MOND has been widely studied and tested as an alternative to the dark matter hypothesis, raising questions about its validity and limitations. One way to test theories of gravitation, including MOND, is through the study of resolved spectroscopic and astrometric binary systems, known as wide binaries. These systems consist of two stars that are sufficiently distant from each other, allowing their orbits and gravitational interactions to be observed and analyzed. In this dissertation, a detailed study of wide binaries was conducted as a test for theories of gravitation, with a focus on MOND. To obtain a study with the best data available so far, data obtained by the Gaia mission and the HARPS spectrograph were used, providing precise information about angular separation, spatial velocity, radial velocity, among other properties of these binary systems. Through the analysis of this data, the implications of gravitation theories, especially MOND, for the dynamics of these systems were investigated. Careful analyses of the properties of binary systems, combined with Gaia and HARPS data, allowed for a rigorous assessment of gravitation theories and their performance in explaining observations. The results obtained, although preliminary due to the still limited size of the sample, provided valuable information about the validity of MOND, the dark matter theory, and their ability to explain the dynamics of wide binaries, as well as the analogous problem of rotation curves of spiral galaxies.

The Standard Model of Particle Physics (SM) constitutes a successful theoretical framework that gathers all the current knowledge about the fundamental interactions of nature (except gravity) and the matter that surrounds us. Its success is due to numerous theoretical predictions that were subsequently experimentally confirmed. This dissertation thoroughly examines each of the essential elements for its understanding, as well as the need to expand its boundaries to solve phenomena still unexplained, particularly the enigma of dark matter, the nature of which is unknown.

Tides are direct consequences of the gravitational interaction between celestial bodies. In the Earth context, the tidal interaction in the Earth-Moon system is directly
observed in both increasing or decreasing of the oceanic level, once our planet’s surface is mostly covered by water. However, the discovery of gas giants exopla-
nets in close-in orbits around their host stars (called hot jupiters), made us question what would be the consequences of such extreme tides. In this master’s thesis,
we use the tidal evolution equations (both lonely and together with the stellar magnetic braking equations) to simulate how the presence of hot jupiters around
main sequence stars with masses in the range 0.5 M ⊙ < M ⋆ < 1.1 M ⊙ , affects both planetary orbits and host star’s rotation.

The study of the rotational evolution of the Sun and solar-type stars is undoubtedly one of the fields of stellar
astrophysics in major evidence today. There are numerous issues that are still far from being fully understood, which makes
research in this field intense and dynamic. Among these issues are the understanding of the phenomena that influence the
angular momentum of stars during their formation, still in the Pre-Main Sequence phase; the understanding of the
mechanisms that act in the stellar interiors throughout evolution, which end up determining, for example, the rotation profile
of a solid body observed on the Sun; or which transport mechanisms act in evolved stars, which undergo profound changes in
their structure, and manifest peculiar phenomena such as lithium enrichment and alterations in many chemical abundances. In
all cases, we know that stellar magnetism plays a prominent role, however, it is still far from being fully understood. In this
thesis we use models computed with the Toulouse-Geneva Stellar Evolution Code (TGEC) to study the rotational evolution of
four samples of solar-type stars, in addition to revisiting studies with analogous stars from open clusters. In our analysis, we
used two different magnetic brake prescriptions, which simulate the loss of angular momentum due to stellar winds.
Additionally, we also studied a sample of giant solar-type stars that are rich in lithium, investigating the possible relationship
with the magnetic character of some of these stars. As far as main sequence stars are concerned, we found some convergences
and some discrepancies between our models and the stellar samples studied. We found that the sample with seismological
data, composed of stars of intermediate and older ages, is not well restricted for a study on the rotation and magnetic
evolution of the Sun. The sample of low-activity stars appears to be affected by a decrease in magnetic braking despite
differences in metallicity, although targets with higher metallicity seem to better follow our evolutionary paths. Finally, wefound a mismatch between our rotation evolution tracks and the position of the youngest stars. As for lithium-rich giant stars,
we found that stars previously classified as red giants (RGB) may be in a different evolutionary state. Furthermore, we found
that most stars in our sample with surface magnetic field detection show at least moderate rotation speeds, but even then, we
were unable to detect a magnetic field in two rapidly rotating stars. Because of our small sample of magnetic giants, it is
difficult to determine whether the presence of a surface magnetic field and the Li-enrichment phenomena in giants could be
somehow linked.

In this thesis we use a network of chiral junctions to realize non-Abelian topological phases in two dimensions. Our
construction is based on boundary fixed points of low-energy field theories describing Y junctions of critical spin-chain
models. To set the stage, we first study a single junction of gapless spin-1 chains belonging to the SU(2)_2 universality class,
whose spectrum includes fractional excitations such as Majorana fermions. We find that chiral fixed points appear as special
points on a transition line that separates two regimes described by open boundary conditions. Remarkably, along this transition
the junction behaves as a tunable spin circulator as the spin conductance varies continuously with the coupling constant of a
marginal boundary operator. We then construct a family of topological chiral spin liquids departing from a honeycomb
network made of critical spin-S chains. The chiral spin liquid phase harbors SU(2)_{k=2S} anyons, which stem from the
underlying Wess-Zumino-Witten models that describe the constituent spin chains of the network. The network exhibits
quantized spin and thermal Hall conductances. We illustrate our construction by inspecting the topological properties of the
SU(2)_2 model. We find that this model has emergent Ising anyons, with spinons acting as vortex excitations that bind
Majorana zero modes. We also show that the ground state of this network is threefold degenerate on the torus, asserting its
non-Abelian character. Our work provides a controllable analytical framework to study non-Abelian topological phases,
sheding new light on the stability of such phases in artificial quantum materials.

In this work, we investigate the impact of extensions of gravity in cosmic inflationary scenarios, in which a scalar
field, dubbed inflaton is considered as responsible for the accelerated expansion of the primordial Universe. In particular, we
will see how such changes affect the cosmic microwave background radiation temperature power spectrum, determined with
great precision by the most recent experiments that investigate it. Initially, we consider that the inflaton is non-minimally
coupled to gravity, and we will check the predictions for two specific models. The first one is characterized by the β -
exponential potential, which generalizes the well-known power-law inflation, and it is motivated by brane dynamics; the next
model arises from a supergravity realization, usually called the Witten-O’Raifeartaigh inflation. Through a statistical analysis,
we will establish constraints on cosmological parameters of the specific models, determining if there is any preference from
data for the presence of a non-minimal coupling. The connection of such models with the reheating period is investigated next,
in a way that we seek to establish the relation between the duration and temperature of reheating with the properties of the
model. By choosing the Witten-O’Raifeartaigh one, we use the results of the previous analysis to numerically model the post-
inflationary era, in a way that non-linear effects on the growth of perturbations of the field are considered, resulting in an
amplification of said fluctuations through a preheating process. These fluctuations can be responsible for the formations of
long-lived structures dubbed oscillons, argued as a possible source of primordial gravitational waves. Finally, we check the
possibility of application of the β-exponential scenario to the warm inflation picture, in which the energy dissipation of the
field into radiation is realized at the same time that inflation proceeds, with the degree of dissipation impacting greatly the
predictions of a given model. We will see that for the β-exponential model, it is possible to make the strong dissipation regime
concordant with the swampland conditions, recently investigated in the literature in the context of warm inflation.

The recent detections of gravitational waves from coalescing binaries by the LIGO and Virgo interferometers not
only opened a new window of observation into the Universe but also revived interest in the relativistic two-body problem. In
this thesis, after presenting the general theory of gravitational waves, we review the effective field theory approach within the
so-called Non-Relativistic General Relativity. In this framework, the different scales involved in the problem define different
dynamical regions, with short scales describing the conservative interaction between the two bodies and long-distance scales
accounting for gravitational-wave processes. At higher orders, however, radiation modes start to affect the conservative
dynamics of the system through the so-called tail effect.
We then study emission amplitudes for the class of nonlinear processes of tails, which are processes of order G_N^2, and
represent the effect of scattering gravitational radiation off the static background curvature, including not only mass tails but
also the “failed” tails due to the angular momentum of the source. As originally shown in our work, some of these amplitudes
present anomalies, which are then properly understood and corrected with field theory tools which have straightforward
counterpart in traditional methods. We also investigate the relation between emission and self-energy diagrams and, in
particular, show that a correction to the anomalous self-energy diagrams is necessary to correctly account for conservative
terms that arise from radiative processes. From this, we obtain the correct contribution to the conservative 5PN stemming from
the electric quadrupole angular momentum failed tail, correcting previous results of the literature. Besides this, we also
compute for the first time the conservative contributions from the angular momentum failed tail, for arbitrary multipole
moments.
Subsequently, we explore the natural higher-order extension (of order G_N^3) of the mass tails, called tails of tails, for generic
electric and magnetic multipoles. As we will see, both long- and short-distance divergences are encountered, which are then
properly understood and dealt with in terms of standard renormalization techniques. In this case, we are able to resum the
logarithmic contributions through the integration of the renormalization group equation.
Thus, in this thesis, we have extended the post-Newtonian perturbative expansion in the particularly thorny region of processes
involving the mixture of radiative and potential modes.

The effect of heating magnetic nanoparticles from the application of an alternating magnetic field has aroused great interest in the scientific community. In the biomedical field, for example, this effect is used in cancer therapy through magnetic hyperthermia. However, the factors that can influence the heating of these materials are not yet fully understood and have generated several discussions, with discrepancies between the results presented in the literature. Although superparamagnetic systems have been widely explored for applications in magnetic hyperthermia, in most cases the effects of magnetic interactions between nanoparticles on the heating efficiency of these materials are completely neglected. In this work, we aim experimental investigation on the existence of magnetic interactions between superparamagnetic nanoparticles and their influence on magnetic hyperthermia. From the production and structural, morphological, magnetic, and calorimetric characterization of nanoparticles MgFe₂O₄,  γ-Fe₂O₃, ZnFe₂O₄, and CuFe₂O₄ it was made a systematic investigation of the magnetic properties of these samples, which led to the belief that they are associated with the existence of exchange interactions in the system. For this study, it was first confirmed that the samples consist of pure ferrite nanoparticles, without secondary phases and superparamagnetic behavior at room temperature. Nevertheless, it was found that the magnetic response of these samples is not well described by the well-known Langevin function, even taking into account the size distribution of the nanoparticles. To address the reasons for such a deviation of non-interacting behavior of a superparamagnetic system, from equilibrium and dynamic magnetization measurements, it was considered a theoretical approach based on modified Weiss mean field approximation, in which it was used to describe the nature of the interactions presente in the samples. From the results, positive values were found for the parameter  θ associated with the mean field and which is directly related to the interactions within the system. This result suggests the existence of magnetizing effects due to interactions in the system, which, according to the Weiss mean field theory, are fingerprints of the presence of exchange forces between nanoparticles. In addition, it was shown that the equilibrium susceptibility initial and the relaxation time are strongly affected by such interactions, directly influencing the Specific Loss Power (SLP), a key parameter in the context of magnetic hyperthermia.

Nowadays, it is increasingly necessary to develop clean energy sources as well as energy-efficient electronic devices. In this
context, the Anomalous Nernst Effect (ANE) has the potential for energy-efficient and sustainable integrated systems
applications. This work presents a proposal for the applicability of the thermal sensorial properties of the alloy Titanium-
Copper (TiCu) integrated into nanostructured ferromagnetic systems for the study of the Anomalous Nernst Effect. For this
purpose, we produced and investigated the ability of Ti1−xCux thin films to act as temperature sensors based on the variation of
electrical resistance (RTD sensors). We used the magnetron sputtering technique to deposit Ti1−xCux films in three
compositions: Ti0.74Cu0.26, Ti0.70Cu0.30, and Ti0.54Cu0.46. We tested the performance of Ti1−xCux films produced as a potential
temperature sensor through temperature variation protocols in the range between 35 and 110 °C. Among the compositions
produced, the film Ti0.70Cu0.30 showed the most promising results. The Ti0.70Cu0.30 film showed linearity and stability in the
thermoresistive response, as well as excellent thermal sensitivity, quantified by the TCR coefficient α. The α obtained for the
film Ti0.70Cu0.30 was −1.99 × 10−3°C−1. We integrated the Ti0.70Cu0.30 film into a TiCu/Insulator/NiFe multilayer ferromagnetic
nanostructure and compared it with a TiCu/NiFe nanostructure to study the influence of the insulating layer, especially on the
magnetic behavior. As an insulating material, we use Kapton. Both samples showed soft magnetic behavior, characterized by
low values of the coercive field (Hc) and saturation field (Hs). The presence of the insulating layer influenced a slight
difference in the coercivity of the multilayer structures, for TiCu/Isolate/NiFe Hc ≈ 5 Oe and for TiCu/NiFe Hc ≈ 9 Oe, due to
the flexibility of the surface of the Kapton better accommodated the particles during the deposition process. However, in
practice, the magnetic behavior of the two structures is similar. We investigated the Anomalous Nernst Effect through the
voltage VANE, using three values for the applied temperature gradient ∇T. The voltage VANE of the samples showed smooth
behavior as a function of the magnetic field, with low coercivity and saturation field, as well as the magnetic behavior. The
TiCu sensor was able to read the temperature variations during all stages of the voltage measurement process VANE in both
samples for the three values of ∇T. We calculated the Anomalous Nernst Coefficient (SN) for the TiCu/Insulator/NiFe sample,

where we obtained SN = 0.60 μV/K, and for the TiCu/NiFe sample, SN = 0.46 μV/K. These values are in agreement with SN
values reported in the literature for NiFe monolayer structures at room temperature. Therefore, Ti0.70Cu0.30 film is an excellent
candidate to act as an RTD-type temperature sensor, with potential applicability in different systems.

The ΛCDM model, also known as the concordance model of Cosmology , is the
best to describe the different phases of evolution of the universe and agrees with most
observational data. However, in recent years, several problems have emerged that have
challenged this model. In order to explain some of these problems, many alternative models
have been proposed, such as the addition of new dynamic fluids modeled by different fields,
modifications of Einstein’s general relativity and deeper modifications to standard physics,
such as the possibility of varying fundamental constants. In this thesis, using the most
recent observational data, we test possible deviations from the theory of general relativity,
as well as possible temporal variations in the speed of light. As results, we did not obtain
departures from the standard physics, however, more restrictive constraints should be
performed in the future with better observational data.

Full Waveform Inversion (FWI) is a high-resolution technique used to estimate an ensemble of subsurface models
that describe the physical structure of Earth's interior considering the entire seismic wavefield. Hamiltonian Monte Carlo
(HMC) method has acquired great attention in solving seismic inversion problems due to its capacity of sampling on high-
dimensional model spaces by avoiding the random walk behavior. In this work, we verify the feasibility of using the HMC
method on the FWI problem under the acoustic wave approximation. We consider as laboratory two synthetic velocity models
under different data variance scenarios. The results suggest that the inversion process is strongly dependent on data variance
and on the choice of mass matrix, implying that more efforts are necessary to real data applications.

The success of General Relativity in describing gravitational phenomena is unquestionable. Despite this, there are conceptual problems in General Relativity, such as the presence of singularities in solutions to the Einstein equation, energy and dark matter, in addition to the fact that the theory is not renormalizable, which makes it impossible to quantize it in a conventional way. One way to try to circumvent these problems is to use the so-called theories of modified gravitation. In the scope of this work we will focus our study on the quadratic gravitation model that has been receiving attention due to its application in the context of quantum gravity as it is a renormalizable theory. After a brief review of the theory of gravitational waves in General Relativity, the proposed gravitation is explored. The quadratic gravity field equations are deduced, linearized, and then solutions of these equations are studied. In addition, a detailed study of the degrees of freedom in this theory of gravitation is made. An analysis of the emission of gravitational waves by binary point black hole systems is also made and, in particular, we study the orbital dynamics and the waveform that would be observed by a detector. Finally, we compare the results with General Relativity and obtain constraints in the parameters of the theory.

Right-handed neutrinos appear in several extensions beyond the Standard Model, specially in connection to neutrino masses.
Motivated by this, we present a model of right-handed neutrino dark matter that interacts with Standard Model particles
through a new gauge symmetry as well as via mass mixing between the new vector field and the Z boson, and investigate
different production mechanisms. We derive the dark matter relic density in the Standard Cosmological Model, when the
Hubble rate is faster than usual, when dark matter decouples in a matter domination epoch, and when it decouples in a
radiation domination regime, which is then followed by a matter domination era. The direct detection rate features a
spin-independent but velocity suppressed operators, as well as a spin-dependent operator when the mass mixing is correctly
accounted for. We put all these results into perspective with existing flavor physics, atomic parity violation, and collider
bounds. Lastly, we outline the region of parameter space in which a weak scale right-handed neutrino dark matter stands as a
viable dark matter candidate.

The world energy demand grows year after year and because of the environmental impact of fossil fuels consumption, it is
urgent the search for a sustainable energy source. Hydrogen is a great candidate for this post, but his large-scale production still
faces some adversities. The cleanest way to produce this fuel is from the water electrolysis, which occurs through two half-
reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The last one has slower kinetics,
which makes the total process not efficient. Thus, it is important to chose the right catalyst, however, nowadays the frequently
used cayalyst are based on rare and consequently more expensive materials, such as Pt and Ru. In this context, it is necessary
to search for catalysts of cheaper materials. Therefore, this work is dedicated to the synthesis and analysis of the physical
and electrochemical properties of Fe-W and Fe-Mo nanocomposites for application as OER catalyst. The nanocomposites were
produced by the sol-gel method, their structural characterizations were performed through X-ray difraction (XRD), their
magnetic properties studied by vibrating-sample magnetometry (VSM) and by Mössbauer spectroscopy (MS), and their
electrochemical specicities were based on linear scan voltammetry (LSV), electrochemically active area (ECSA) and
electrochemical impedance spectroscopy (EIS) measurements. Thus, for the system Fe-W, a nanocomposite containing two
distinct phases was obtained, Fe and Fe6W6C, and for the system Fe-Mo a composite that has the metals Fe, Mo and possibly
a non-homogeneous alloy of these two metals. The magnetic characterizations pointed to soft ferromagnetic materials, with
low remanent and coercive elds. On the other hand, the electrochemical measurements showed to be promising because both
composites presented overpotential values classied as ideal and very close to RuO2 in electrolysis in alkaline medium.

Abelian anyons are many-body states of electrons characterized by a permutation phase between 0 and π. The non-abelian
generalization of this concept was proposed by Kitaev to make fault-tolerant quantum computers. For non-abelian anyons,
the phased permutation operators give rise to matrix representations of the permutation group, among them the braid
groups. Knot Theory and Braid Group have all the tools that are needed to build quantum braid circuits. Taking advantage of
existing technologies of study of transport properties of interacting systems, we use the T-matrix formalism to analyze
random T-matrix models and random R-matrix models as circuits of random braids. A special attention is given to Chalker-
Coddington’s percolation model and renormalization approaches that rely on solving systems of linear equations or
performing tensor contractions. As a simple example we introduce a quasi-1D random braid model in polynomial
representation and analyze its transmission probability distribution.

There are several studies related to the presence of debris disk with metallicity, rotation and age of stars. However, little is
known in the literature about the relationship between the presence of a debris disk and the circumstellar habitability zone and
how the range of the habitability zone affects the disk. To carry out this work, we selected the main catalog of (Cotten & Song
2016), this catalog contains 507 stars showing potential excess infrared, without ambiguities, from observations made with the
WISE satellite.
For a better understanding of the profile of the stars present in the catalog, the Cor-Magnitude GAIA was built, based on the
methodologies of (Messias et al. 2022) and (Gordon et al. 2021), for the exhibition of the work. We can see in this diagram that
only 171 stars belong to the Main Sequence, this selection was made due to the evolutionary stage of the stars, it is susceptible
to important biases, which can influence the results obtained. With the sample of Main Sequence stars, the (Kopparapu et al.
2014) model was applied to calculate the Circumstellar Habitability Zones. As this model has a temperature limit of 2700K to
7200K, only 83 stars fall within the approached limit. By calculating the distance from the habitability zone, it was possible to
separate the sample into 3 small groups: stars that have a debris disk radius greater than the outer radius of the conservative
ZH; stars that have the radius of the debris disk inside the conservative ZH; stars that have a debris disk radius smaller than the
inner radius of the conservative ZH. Finally, we compare our results with the mean radius of the asteroid belt in order to
observe systems with architecture similar to the solar one.

We propose an efficient methodology for the complete version of the wave oriented to atarget area through
the Patched Green’s Function-(PGF) method. The method consists of separating the velocity model into
two regions of interest, “external” and “target” regions. When trying the verylarge system it can be
considered as projected arrays. In addition, we address a situation in whichthe observed data is
contaminated with noise. In this scenario, we formulate a FWI rubased on themaximization of the
Reney statistic together with the PGF method, we build a robust objective function in this scenario scenario.
Results relevant to the results obtained with the results-based version of the FWI, based on the results
obtained with the PGF methodology, related to the results obtained with the results-based method of the
FWI based on the results statistics based on the FWI, especially not -Gaussians. The main advantage of the
PGF method is that it is able to optimize the approximate methods of thecontinuous method in each ideal
method, it is worth emphasizing that mathematically the PGF methodis mathematically exact and does not
involve the PGF method is thus a computationally approximated cost to precision, robust of the FWI based
in the Reney statistic to data with non-Gaussian noise, it maybe related to the entropic index, especially for
the value ofα→2/3.

Nanoscale magnetic systems, such as ferrites, perovskites and core@shell structures, are a group of promising materials, due to their vast technological potential and the possibility of advancing the understanding of fundamental physics related to nanoscale materials. Taking this into account, we synthesize CoFe₂O₄@BiFeO₃ core/shell nanoparticles by coprecipitation and systematically investigate the structural, morphological, and magnetic properties in such nanostructured system. Through structural and morphological characterization, we demonstrate the obtainment of core@shell nanostructure with pure phases. By performing a broad experimental magnetic analysis, we evaluate the magnetic response for the precursor phases, as well as for the core@shell nanostructure. For this latter, we identify fingerprints of the wasp-waisted behavior and exchange bias effect, disclosing the exchange coupling between the phases taking place at the interface. Further, we reveal the exchange coupling between core and shell is strongly affected by a spin glass behavior arisen from the spin disorder in the CoFe₂O₄@BiFeO₃ core/shell nanoparticles. After all, our findings allow us to place the used procedures taking into account the processes of adapted coprecipitation and calcination as a feasible route to the production of high-quality core@shell nanostructures. However, in order not to be bound by a single method, core@shell samples CoFe₂O₄@BiFeO₃ with three core sizes and their precursors were synthesized through hydrothermal synthesis. This second set of samples also showed promising results, as the material obtained showed pure phases for all samples and strong indications of magnetic coupling between the phases that make up the core and the shell, indicating great efficiency of the hydrothermal method.

Stellar clusters provide key information for studying from stellar origins to the Galactic structure, and even for fundamental
physics tests. In particular, Messier 67 (M67) is an important open cluster because it has similar metallicity and age to the Sun.
We present an observational study of different phenomena that can affect radial velocity (RV) measurements of stars
belonging to M67, for solving information about the cluster kinematics, as well as for a unique test of General Relativity.
Based on a recent technique, the difference between two independent RV measurements, spectroscopic and astrometric, of
stars belonging to a given cluster, allows empirically detecting, for example, the cluster’s global movement and the
gravitational redshift predicted by General Relativity. Spectroscopic and astrometric RV measurements were obtained from
public archives of the HARPS spectrometer and the Gaia satellite, respectively. Two initial samples, of 1278 and 144 stars,
were carefully selected, resulting in a final sample of 74 stars. We detected quantitatively the gravitational redshift (GR) in
M67, thus adding a new support to the General Theory of Relativity. We also detected for the first time an apparent rotational
(or shear) motion of M67, with a rotation gradient of ~ 66 m/s/pc. With these results, a new database with RV values
representing unbiased measurements are provided as a by-product of this Thesis. Finally, we calculated from newer data a
virial mass of 1980 +/- 100 M and a tidal radius of 14.2 +/- 0.3 pc for M67, in agreement with previous estimates. Overall, the
empirical determination of the cluster rotation is very useful for kinematic studies, either of the cluster itself or on its relation
with the Galactic structure. In addition, the present study contributes to fulfilling a gap among the tests of General Relativity,
since this theory has been little tested in stellar environments.

The study of nanostructured materials, more specifically, magnetic nanoparticles systems, is a topic challenging the scientific community. Such systems, besides presenting very interesting properties from the point of view of fundamental physics, such as superparamagnetism, superferromagnetism and spin glass, are of multidisciplinar interest due to their huge potential in nanotechnology and biotechnology applications. Specifically, biomedical applications, such as magnetic hyperthermia therapies, require the use of nanoparticles with reduced dimensions under the action of low amplitude alternating magnetic fields. In this context, the so-called Linear Response Theory (LRT) receives enormous attention, since it corresponds to an excellent candidate for the description of processes in the low fields regime. Furthermore, fundamental parameters of magnetic systems that are extremely important for maximizing the efficiency of nanoparticles in medical and technological applications, such as magnetic susceptibility, are obtained within the validity limits of this theory. In this thesis we investigate the existence of fundamental limits and inequalities in magnetic processes in which nanoparticle systems are under the action of low amplitude magnetic fields, that is, in processes in which the Linear Response Theory proves to be valid. First, considering magnetic processes in thermodynamic equilibrium, we find inequalities associated to the equilibrium magnetic susceptibility, saturation magnetization and effective anisotropy constant in blocked magnetic nanoparticles systems. By means of an experimental evaluation, using a sample of barium hexaferrite nanoparticles (BaFe12O19), we verify that such inequalities, in fact, are not violated. Still in the context of equilibrium processes, we show that the inequality found for the susceptibility in blocked nanoparticles must also be valid for non-interacting superparamagnetic systems. Through the evaluation of the experimental results obtained in superparamagnetic systems of magnetite (Fe3O4), magnesium ferrite (MgFe2O4) and zinc ferrite (MgFe2O4), we verify that this inequality is violated in all cases, a fact that we believe to be a signature of the existence of correlations mediated by exchange forces between nanoparticles within the systems. Next, considering dynamic processes, we also obtain inequalities associated to the real and imaginary components of the dynamic susceptibility in addition to the power limit that can be generated through the interaction of alternating magnetic fields with superparamagnetic nanoparticles. In order to evaluate the validity of our theoretical approach also for magnetodynamic processes, we carry out calorimetric procedures under alternating magnetic field effect using the same superparamagnetic samples considered in the equilibrium processes and verify the total agreement of the experimental results with our theoretical expectations. In summary, in this thesis we unequivocally demonstrate the existence of fundamental inequalities that limit the behavior of magnetically blocked and superparamagnetic non-interacting nanoparticle systems in addition to the implications of their violations. Such results are promising tools for maximizing the efficiency of magnetic nanoparticles in biotechnological and biomedical applications since they elucidate the real impacts of effects that are still poorly understood in the literature, such as the effect of inter-particle interactions in magnetic processes.

The M-type barium hexaferrite (BaFe₁₂O₁₉), an iron oxide with hexagonal geometry, is a material that play an important technological role in applications such as permanente magnets, microwave devices and magnetic recording. In this work, we study the influence of two experimental approaches used to improve the maximum energy product, (BH)_max, of barium hexaferrite: doping with lanthanum (La) and cobalt (Co), and the coupling of magnetic phases. In the first one, we produced samples of barium hexaferrite, using the Ionic Coordination Reaction (ICR) method, doped with a fixed amount of La and a variable amount of cobalt, Ba_0.7La_0.3Fe_12-xCo_xO₁₉, x = (0.0, 0.25, 0.50, 0.75 and 1.0). In the second, through the polyol process, we coated samples of pure and impurity-free barium hexaferrite (also obtained by the ICR method) with superparamagnetic nanoparticles of cobalt ferrite (CoFe₂O₄), obtaining coupled nanocomposites of BaFe₁₂O₁₉/CoFe₂O₄ with

different proportions. Then, these samples were submitted to a reductive process in a hydrogen atmosphere, where there was a partial reduction of cobalt ferrite, giving rise to a new nanocomposite of the BaFe₁₂O₁₉/CoFe₂O₄@CoFe₂ type. These samples were characterized by the use of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), as a function of magnetic field and temperature and Mössbauer spectroscopy. In the first approach, XRD measurements showed the presence of a small amount of α-Fe₂O₃ in the doped samples. The SEM images revealed a morphology like nanorod, smaller than the critical limit for single-domain particles. Optimized magnetic properties were achieved for the sample with concentration of Co x = 0.25. Furthermore, this sample displayed the highest value of (BH)_max of 6.98 kJ/m³ (0.877 MGOe), which corresponds to a significant improvement of 8% over the pure sample. For the second approach, XRD analyzis revealed the presence of two phases, BaFe₁₂O₁₉ and CoFe₂O₄, for the as prepared nanocomposites, and three phases, BaFe₁₂O₁₉, CoFe₂O₄ and CoFe₂, for the nanocomposites after reduction. The TEM micrographs showed that the different phases are structurally coupled. Magnetization (M_r) and remanent demagnetization (M_d) measurements were performed at room temperature. Through M_r and M_d it was possible to make the Henkel and δm plots of the nanocomposites that provided information about the nature of the interactions between the phases and about the magnetic behavior. We detected a coupling of the exchange-spring type between the phases of the nanocomposites.

The discovery of graphene and its exceptional properties has motivated research on two-dimensional (2D) materials for over a decade. Since then, several other 2D materials with diverse properties have been discovered, such as hexagonal boron nitride (h-BN). More recently, two-dimensional materials have been combined into hybrid structures, leading to the creation of solids with adjustable properties. In this document, the mechanical properties of hybrid nanostructures composed of graphene and h-BN were investigated using classical molecular dynamics. Two types of arrangements were considered: (i) square sheets of graphene containing circular and hexagonal h-BN domains and (ii) square sheets of h-BN containing circular, hexagonal, triangular and double triangular graphene domains. The results obtained here show that, for both types of structures, the Young's modulus of the hybrid structure depends essentially on the fraction of h-BN and graphene in the structure, with Young's modulus values decreasing linearly as the concentration of h-BN increases in (i) and increasing with increasing graphene in (ii). We also analyze the temporal evolution of the fracture patters and of the stress concentration during the simulations. From this analysis we conclude that the fracture always starts in the interface region between graphene and h-BN, because it was found that the B-C and C-N bonds are weaker than the C-C and B-N bonds. Finally, we observe for both types of arrangements that the mechanical properties of the hybrid structures are not altered when the sheet length and the domain diameter are increased proportionally. This indicates that our results may be valid for much larger structures than those tested here, such as those that are synthesized experimentally.

Different space missions are revolutionizing our understanding of stellar variability, revealing a new view of dynamic phenomena associated with stars in different regions of the HR diagram. Furthermore, environments with planet-hosting stars have proved to be complex and primordial laboratories for the understanding of star-planet relationships, where much of the information about this interaction is revealed by the photometric behavior of the star including rotation, pulsation and transient events. Rotation is a parameter that can be characterized by the detection of spots on the surface of the star and plays, in fact, an important role in controlling stellar evolution and in a potential scenario of planetary habitability. In this sense, the study of periodicities in stars with planets, from photometric observations, such as those associated with the KEPLER and TESS missions, opens new perspectives for the study of different stellar properties, including rotation and magnetic activity. In this work, we carried out a search for signatures of photometric variability in the light curves of 2215 TOI's (Tess Objects of Interest) stars with planets discovered by the TESS telescope, via planetary transit. For objects with clear photometric modulation, we determined their periodicities by applying the BLS, FFT, Lomb-Scargle and Wavelet techniques. The results of this analysis are described in Canto et al. (2020), where, from the total of 2215 stars, we found 289 TOI's with rotation signature, 13 TOI's with pulsation signature subdivided into 5 different classes, 316 TOI's with ambiguous variability, 1586 TOI's with low signal-to-noise ratio and 11 eclipsing binary TOI's. We studied the P_orb / P_rot × P_orb distribution of stars with rotation signature and identified a possible synchronicity between the rotational period and the orbit of the innermost planet for some of them. In addition, in this subsample we found 30 stars that present eruption events, a phenomenon of great importance in the study of planetary habitability, as they are events that can impact the development and continuity of biological activity. Likewise, recent studies on pulsating stars have shown that carbon, one of the fundamental elements for the existence of life, may be being released into the interstellar medium through these variables, which makes our subsample of 13 TOI's more interesting as they are targets with potential planets. Finally, we made a solid characterization of the evolutionary state of the analyzed stellar sample, from a Color-Magnitude Diagram with data from GAIA, and we identified 1511 stars as probable members of the Main Sequence (MS) and 546 evolved stars, which reveals us that the distribution of our sample is predominantly from MS, including stars with rotation signature.

Despite being the most abundant stars in the Galaxy, M dwarfs are one of the least known objects in terms of their physical properties, especially regarding its magnetic activity and the underlying dynamo mechanism. Thus, a broad study on different activity diagnostics and their relationship with stellar rotation is necessary. In this work we performed a spectroscopic survey of 122 M dwarfs present in the Southern Continuous Viewing Zone (CVZ) of the TESS space mission, based on

observations with the Gemini and SOAR telescopes. We obtained at least one optical spectrum for each star, with R ≈ 2000, from which we measured the chromospheric emission in the Hα line. Our sample has 21 stars with measured rotation periods (������������ ), of which 12 were determined in this work from TESS light curves. The active fraction of the stars in our sample is consistent with that expected for field M dwarfs, confirming that late spectral-type stars generally remain active longer than early-type stars. Two stars presented enough rotation to be considered magnetically saturated, although they are at activity levels below the saturation region in the rotation-activity diagram, indicating the possibility that M dwarfs with short periods also present inactivity in Hα. We also analyzed a sample of 1788 field M dwarfs from Kepler and K2 missions, for which we investigated the distributions of ������������ and the photometric activity index ������ℎ, associated with the standard deviation of the light curve, using color-magnitude diagrams with accurate distance and magnitude data from Gaia mission. We note that the stars with the highest values of ������ℎ in the sample (≳ 10000 ppm) are in general very young, with isochronal ages ≤ 50 Myr. We

compared the photometric index with canonical chromospheric and coronal activity indicators in order to validate it as a proxy

for magnetic activity in low-mass stars. Based on this index, we also studied the rotation-activity diagram, taking into account the transition to fully convective stars (≈ 0.35 ���⊙), where we noticed the same distribution between these stars and those partially convective in both saturated and unsaturated regimes, suggesting a solar-like dynamo behavior, even for stars without

the presence of a tacocline

A quantum spin liquid is a magnetic phase of matter remarkable for its ground state long-range entanglement and fractional excitations. A quantum spin liquid appears in the well-known Kitaev honeycomb spin-half model. In a magnetic field, there is a phase transition to a topological phase with an energy gap in the bulk and chiral Majorana fermions at the edge However, recent nuclear magnetic resonance experiments suggest excitations without a gap in this phase, signalled by a power-law dependence of the spin-lattice relaxation rate at low temperatures. In this work, I propose a mechanism to account for this experimental result. I use the localization of chiral Majorana modes in defects to explain the presence of gapless modes in the bulk. Treating the weak interaction between the chiral modes in the defects within a mean-field approximation, I find that these modes remain gapless when the interaction is below a critical value that depends on the magnetic field strength. I used the effective low-energy theory to calculate the spin-lattice relaxation rate and found a behavior that agrees with the experimental result.

There is growing interest in the impact of electrical fields generated in the brain. Transmembrane ionic currents originate electric fields in the extracellular space and are capable of affecting nearby neurons, a phenomenon called ephatic communication. In the present work, the Quadratic Integrate-and-Fire model was adapted to include the ephatic coupling behavior and its results were compared to the empirical results. Therefore, the analysis tools were divided according to the neuronal activity regime. For the subthreshold regime, circular statistics were used to describe the phase differences between the stimulus signal and the modeled membrane response; In the suprathreshold regime, the Population Vector and Spike Field Coherence were used to estimate phase preferences and the coupling intensity between the stimulus and the spikes of the model. The subthreshold phase difference was sensitive to the characteristic membrane response time, as well as the frequency of the stimulus given to the model. On the other hand, the intensity of the coupling between spikes and stimulus was sensitive to the intensity of noise added to the stimulus signal and also to the stimulus frequency. The preferential phase of spikes are sensitive, according to the model, only to the stimulus frequency. Such results are consistent with the results observed in empirical experiments on ephatic neuronal coupling. It was observed that the Quadratic Integrate-e-Fire model with ephatic coupling is able to successfully model this neuronal communication. Thus, the model makes it possible to pursue further studies on the physiological importance of ephatic coupling in the brain, including significant implications for our understanding of brain processing for neuroscience.

The entropic brain hypothesis states that key functional parameters should exhibit increased entropy during psychedelic
induced altered brain states. This hypothesis has gained significant support over the years, particularly via thresholding Pearson
correlation matrices of functional connectivity networks. However, the thresholding procedure is known to have drawbacks,
mainly its arbitrariness in the threshold value selection. In this work, we propose an entirely objective, threshold independent
method of entropy estimation. Let R be a generic N×N Pearson correlation matrix. We define ρ = R/N and prove that ρ
satisfies the necessary conditions for a density operator. Therefore, the von Neumann entropy S = –tr(ρlnρ ) can be directly
calculated from the Pearson matrix. We then calculate the entropy of functional correlations of the human brain. Consistent
with the entropic brain hypothesis, we find that entropy increases during the acute effects of the psychedelic indigenous
beverage ayahuasca.

The subsurface imaging is a central procedure in seismic exploration and, consequently, a topic of great economic interest. In general, the seismic data inversion process is employed to estimate the physical parameters of the subsurface. In geophysical applications, seismic inversion is usually formulated as an optimization problem that aims to minimize the difference between the modeled data and the observed data through the Gauss' law of error, which is closely linked to the Boltzmann-Gibbs statistical mechanics. In this approach, errors are assumed to be distributed according to a Gaussian distribution. However, in practice, especially in non-linear problems, errors are seldom Gaussian and, therefore, this approach may fail to reconstruct physical models, especially when there are outliers in the data set. Thus, the error laws determined by  non-Gaussian statistics are essential for a robust seismic inversion. In this way, we present in this work new methodologies for the execution of seismic inversions based on non-Gaussian statistics. In particular, the generalized statistics in the sense of Tsallis and Kaniadakis are considered to solve a challenging problem of seismic inversion, called, Full-Waveform Inversion. In this work, we present the foundation and formalism of the FWI based on generalized statistical mechanics, as well as the results of several numerical tests carried out on two different subsurface models: (i) we consider a very dense seismic acquisition geometry in a Marmousi case study; and then, (ii) we employ an OBN acquisition in a case study with a representative model of the Brazilian pre-salt field. At the end, we compare the conventional FWI with the FWI based on Tsallis and Kaniadakis statistics. The results suggest that the FWI based on generalized statistical mechanics is a powerful
methodology, especially in noisy environments. In addition to the proposed methodology to provide better reconstruction of  the subsurface models, no computational cost is added compared to the conventional approach.

In this thesis we present the results of two works in computational physics on the thermoelectric effects in thin films and the medium-range Potts model (q = 3). In the first, we seek to obtain the magnetic behavior and thermoelectric response of Co2FeAl/W films from numerical calculations, we compare experiment and theory, we explore the possibility of obtaining evolution of the magnetic response and the thermoelectric voltage with the magnitude and direction of the magnetic field. We show that the thermoelectric voltage curves can be obtained from a change in the Stoner-Wohlfarth model by the association and experimental parameters used in the longitudinal spin Seebeck effect experiment. In the second, we seek to find the so-called X and Y exponents, for the medium-range Potts model, which describes the transition from one universality class to another as the range of interactions increases. Such an effect is called a crossover phenomenon. We present the results obtained so far for the medium-range Potts model. We use the Metropolis algorithm to perform numerical Monte Carlo simulations of the Potts model of the state q = 3. With this algorithm we simulate systems with network sizes, L, equal to 32, 46, 64, 92, 128, 182 and 256. We found that as the range of interactions, Rm increases, the critical exponents α/ν, β/ν and γ/ν vary. Therefore, in this work we try to use two algorithms in two applications involving thin films and the Potts model (q = 3).

In this thesis work, it was sought to present a new construction method of a chiral spin quantum liquid formed from Heisenberg spin chains. The spin chains, initially assembled in trios, were coupled in the form of Y junctions. In each Y junction, a chiral interaction was added between the three spins at the edges of each chain, in addition to the exchange interaction between neighbouring spins in the bulk of the chains. Through the previous study of an isolated Y junction its extension was investigated here by establishing the set of these junctions in the form of a hexagonal network of spin chains. The theoretical analysis was based on low energies using the bosonization technique and applying invariant boundary conditions to the bosonic modes or the chains' spin currents. It was concluded, with the established chiral spin liquid, that the CSL is of the Kalmeyer-Laughlin type, which its elementary excitations correspond to spinons, fractional magnetic excitations of semonic statistics. The chiral spin liquid found is also accompanied by a quantized spin Hall conductance and edge modes at the network boundaries. The (gapped)spectrum of the spinons and the stability of the chiral liquid were studied through the backscattering perturbation between the chiral bosonic fields of neighboring hexagons and an effective tight-binding model for spinons. The theory was able to predict a phase transition when the spinon gap closes, resulting in the condensation of the spinons or the formation of a stable CSL. As a perspective, the method developed in this work can be extended to other topological phases of matter and eventually be useful in the application of topological quantum computing.

The Collatz conjecture, perhaps the most elementary unsolved problem in mathematics, claims that for all positive integers $n$, the map $n\mapsto n/2$ for even $n$ and $n \mapsto 3n+1$ for odd $n$ reaches 1 after a finite number of iterations. We examine the Collatz map's orbits, known as hailstone sequences, and ask whether or not they exhibit scale-invariant behavior, in analogy with certain processes observed in real physical systems. We develop an efficient way to generate orbits for extremely large $n$'s (e.g., higher than $n \approx 10^{3,000}$), allowing to statistically analyze very long sequences. We find strong evidence of a scale-free power law for the Collatz map. We analytically derive the scaling exponents, displaying excellent agreement with the numerical estimations. The scale-free sequences seen in the Collatz dynamics are consistent with geometric Brownian motion with drift, which is compatible with the validity of the Collatz conjecture. Our results lead to another conjecture (conceivably testable through direct, nonetheless very time consuming, numerical simulations): given an initial $n$, the average number of iterations needed to reach $1$ is proportional, to lowest order, to $log[n]$.

The Diluted Block Voter Model gives a biophysical interpretation of a continuous phase transition in a magnetic network system, we also give some insights into the sociophysical context of elections. We simulated a regular square lattice of N spins variable with two states σi = ±1, and edge L = √N . Each spin can receive the outflow influence of the persuasive cluster spins (N_PCS), a randomly chosen square block. This chosen adjacente spin have q resistance to accept the outflow influence. So, it rejects the PCS influence with probability q and agrees with the majority of PCS with probability (1 - q). The dilution is due to the fraction f of sites with no resistance q = 0 while the complementary 1 - f sites effective have q resistance, named q_c its critical value when phase transition occurs. Our parameters of interests are q_c, f and N_PCS. We calculate the magnetization, susceptibility and Binder’s cumulant of the system to map the phase transition showing that some configuration of parameters lead us to a null magnetization region. We compare the results of large groups of influence with the mean-field theory and we use the finite scale size theory. In biophysical terms, in some configurations of vaccine efficiency q, contagion group N_PCS, and campaign absences f, we have an ordered phase (magnetization > 0) Where there is an epidemic propagation. In social terms, for some configuration of social temperature q, Social pressure N_PCS and social neglect f, we have a disordered phase (null magnetization)where there is no consensus.

Stellar rotation is an important parameter to be considered in the study of stellar evolution and structure, as
it relates to several equally relevant parameters such as mass and temperature and influences the chemical composition of the
star. In binary systems the study of rotation allows us to understand the tidal effects that a system can suffer and the process
that makes the system achieve synchronization and circularization. To understand how rotation behaves and relates to other
stellar parameters in binary systems, we use a method of analyzing light curves and measuring the rotation period. We apply
this method to light curves of the CoRoT and TESS space missions in stars belonging to eclipsing and spectroscopic binary
systems. In addition, we analyzed the relationships between other parameters such as orbital period and eccentricity, projected
rotational velocity and effective temperature, among others. We finished this study with correlation diagrams to have an
overview between the different groups of binaries. We obtained excellent results in determining the rotation period for 70
eclipsing binary systems, 26 spectroscopic binary systems and 15 systems belonging to the Taurus-Auriga region. We reaffirm
the values of the eccentricities for 18 eclipsing binary systems and found 38 synchronized systems of which 12 are
synchronized and circularized. For the spectroscopic systems, we found that for the stars of the main sequence, the shorter the
orbital period, the greater the probability of the system being synchronized, however, it is possible to find red giants of long
period synchronized. We also found that the correlations between rotation, orbital parameters, mass and temperature change
according to the different types of binary systems.

The presence of circumstellar material is seen as a powerful indicator for the formation of

planetary systems similar to our Solar System. It is known that nearby binary stars have a more extensive

habitability zone when compared to individual stars, making them the target of several researches,

however, studies on debris discs in systems of this nature are still incipient. Faced with this con-

juncture, it is sought through this thesis, debris discs in spectroscopic binary systems. The sample

was obtained from the WEBDA database, initially containing 25 open agglomerates with the lar-

gest populations of spectroscopic binary stars available, totaling 1033 candidates. The search for

excess in the infrared occurred in the 3.4 μm, 4.6 μm, 12 μm and 22 μm bands of WISE and was

performed through a comparison from the SEDs (Spectral Energy Distribution) obtained by the

photometric data from the GAIA missions, 2MASS and WISE and the synthesized curve genera-

ted by the BT-Settl model, available on the virtual platform VOSA (Observatory Sed Analyzer).

The stars with excess in the infrared have passed through visual inspection and had their galactic

latitudes checked, resulting in a final sample with 6 confirmed spectroscopic binary systems with

disc presence. After these procedures, the main parameters that characterize the debris discs were

calculated and, among other results, it was possible to verify that non-evolved binary spectroscopic

systems, belonging and open agglomerates, can house stable circumbinary debris discs.

Recent detections of gravitational waves, together with the increasing precision of electromagnetic observations of neutron
stars and black holes, have expanded the possibility of testing the theory of General Relativity. In this work, we parameterize
extensions of General Relativity (GR), adding terms of high curvature, built from the fourth powers of the Riemann tensor,
therefore, departing from GR in the ultraviolet (UV). The presence of these operators modifies the gravitational potential
between compact objects, as well as their multipole moments. Through an Effective Field Theory approach, we calculate the
corrections to the gravitational potential and radiative multipole within the post-Newtonian approximation to GR and obtain
also the equations of motion for the spin and these quantities are directly comparable with astronomical observations.

The synthesis of superparamagnetic magnetite nanoparticles has been a prominent topic in the field of condensed matter research due to its applicability in biomedicine and other technologies. In this work, we describe the synthesis of nanoparticles of pure magnetite, magnetite coated with polyvinylpyrrolidone polymer (PVP), and PVP-coated magnetite doped with neodymium and gadolinium. We studied the structural and magnetic properties by X-ray diffraction (XRD), magnetization measurements (VSM), transmission electron microscopy (MET), and Mössbauer spectroscopy. The nanoparticles showed superparamagnetic behavior at room temperature. The values obtained for saturation magnetization, coercivity, and remanence indicated the superparamagnetic nature of the samples obtained. The presented approach provides an easy route to prepare PVP-coated magnetite nanoparticles, as well as ones doped with neodymium and gadolinium.

Full waveform inversion (FWI) is one of the most studied techniques today to recover subsurface parameters that affect
wave propagation. One of the main difficulties in applying FWI in 3D seismic surveys is its high computational cost. This
problem has been solved with the advancement of computational technology, but there are still difficulties to apply a FWI to real
3D data. In this work are studied implemented to optimize a FWI, as which are based on the physical properties of waves. To
this end, this work was done in two parts. In the first part, the encoded simultaneous sources technique was used, in which the
FWI gradient is calculated by simultaneously propagating information from several sources grouped in one or several subgroups,
taking advantage of the superposition principle of the wave equation. In order to reduce the crosstalk noise that appears in the
gradient calculation using simultaneous sources, two encoding schemes have been proposed, by phase rotation and by static
limited to a period of the dominant frequency. In phase rotation encoding a phase rotation is applied to each shot where the
rotation angle is chosen randomly for each shot. In static coding the traces of each shot are temporally shifted and the value of
this shift is chosen randomly for each shot having a maximum value of one period of the dominant frequency of the data. With
the help of numerical tests on synthetic 2D data to demonstrate that the proposed schemes have a faster convergence than
encodings by polarization (multiply by +1 or -1 randomly each shot) and by static without being limited. In the second part, a
workflow was developed to apply a FWI to real data acquired in the Brazilian pre-salt region. This data was obtained with an
Ocean Bottom Node (OBN) 3D acquisition, where the receivers are spread at the ocean bottom. In the developed workflow,
several existing techniques are combined to optimize the use of memory and processing, which take advantage of wave
propagation properties. On the OBN data for inversions made using sources and receivers decimation strategies to reduce
computational cost. The encoded source schemes studied in the first part were also tested. These testicles also use various
functions with the objective of an intention of melody and a resolution of inversions. It was verified that the inversions recovered
information related to the pre-salt reservoir and that both decimation and source coding strategies reduced the computational cost
of FWI without compromising the quality of the inversion.

Circumstellar disks are components of the formation of planetary systems and, at the same time, a product of these processes.
On the other hand, debris disks are vestiges of the planetary formation process, indicating the formation of planetary systems.
Note that the most important stages of circumstellar disks are: the protoplanetary disk and the secondary disk (or debris disk),
which can be associated with stellar ages. The chromospheric activity index, logR’HK, is used as a star age tracer. Thus, it is
reasonable to seek a relationship between chromospheric activity index and the presence of a circumstellar disc, however,
studies of this nature are still rare. In this scenario, we look for circumstellar disks in a sample of 2845 field stars with known
chromospheric activities. We searched for infrared excess, using WISE data in the bands 3.4 μm, 4.6 μm, 12 μm and 22 μm.
These data, in turn, were combined with data from the 2MASS and GAIA missions to generate a Spectral Energy Distribution
(SED) on the VOSA virtual platform and the SED curves were synthesized by the BT-Settl model. The infrared excess was
characterized by the excess significance and the stars that were candidates to host circumstellar disks underwent visual
inspection of the images, as well as their galactic latitudes were verified. At the end of this process, four stars were confirmed
with circumstellar disks, classified as debris disks. The parameters that characterize the debris disks (temperature, luminosity
fraction, radius and mass) were calculated and compared with the results in the literature. We observe that the luminosity
fraction and mass of debris disks decrease with the age of the star systems. Finally, we found that the debris disks in our
sample were found around more chromospherically active stars. In comparison with the results in the literature, there seems to
be an anti-correlation between the luminosity fraction of the debris disks and the chromospheric activity index.

The conformal field theory has been show as a valuable asset in theoretical physics over the course of the last decades. In this work such theory was used as a way to evaluate four-point boundary connectivities in a two-dimensional model, where bonds are dictated by the Q-Potts model. For this model Q has integer values and 1 ≤ Q ≤ 4, that define unitary statistical models. In this paper we use a formal extension that Q  have real values and  0 ≤ Q ≤ 4$, range were still possible to evaluate connectivities as solutions of a differential equation  in conformal field theory. Such computations were made in order to obtain the universal ratio and inspect the consistency of results for $Q$ enclosed by two integers. Thereby, we were able to evaluate the universal ratio for diversified statistical models and achieve relevant results, in the sense that we showed the existence of a consistency or uniformity between the universal ratio and Q.

Quantum spin liquids are among the most studied phases of matter which depart from the paradigm of symmetry
breaking order and Fermi-liquid theory. In this work, we investigated a spin-1/2 Heisenberg model on the kagome lattice with
exchange interaction Jd between spins connected by the diagonals of the hexagons and a staggered chiral interaction Jχ that
couples the three spins in the triangles. Applying parton construction, we show that the model can exhibit a gapless chiral
quantum spin liquid phase in the regime of dominant Jχ . The gapless phase is equivalent to the one found in other mean-field
solutions based on Majorana fermions with symmetry protected line Fermi surfaces. Our goal is to study the changes in the
spectrum of the spinons as a function of the ratio Jd/Jχ for two different cases of a specific ansatz in the mean-field theory. We
expect that in the regime of relevant Jd , a non-coplanar ordered magnetic state appears, the cuboc-2 state. The apparent phase
transition is analyzed in terms of the mean-field orders parameters and the search for instabilities in generalized susceptibility.
This work provides a unique example of a phase transition between a chiral spin liquid and a non-trivial ordered magnetic
state.

The goal of this thesis is to construct an effective description of the dynamics of curves in three dimensions applicable in the context of protein physics. The model is intended to reproduce a variety of geometric characteristics observed in real proteins, assuming that proteins can be approximated by long continuous curves. The degrees of freedom in the effective description are parameterized by the curvature and torsion functions. The minimal necessary model is constructed in terms of an energy functional depending on four phenomenological parameters. We discuss static configurations of the effective model, which appear in two different classes: minima of the effective potential and soliton-like configurations interpolating between pairs of minima. In proteins such classes correspond to most common secondary structures (alpha-helices and beta-strands) and motifs connecting those structures (loops, turns, hairpins etc.) In order to obtain the values for the parameters, the solutions of the model are compared with real protein structures extracted from the Protein Data Bank. A very specific relation between the curvature and torsion predicted by the model allows us to universally fix three of the parameters of the model. We show that the fourth parameter is not universal. It controls the characteristic size of the motifs connecting alpha-helices and beta-strands and may be used to explain the absence of beta-strands in some proteins and their abundance in the others. In the most appealing scenario the model features ``stable'' alpha-helices and ``metastable'' beta-strands. We study the classical and quantum stability of the metastable configurations. We derive estimates for the temperatures of classical phase transitions and show that quantum transitions are suppressed. We also discuss how realistic discrete proteins “stabilize” beta strands and hairpin-like motifs.

Ferromagnetic structures in confined geometries have attracted great interest, as geometric confinement opens new routes for manipulating fundamental magnetic properties required by major applications such as logic devices, magnetic sensors, nano-oscillators and magnetic memories. We report a theoretical study of the impact of dipolar interaction on the magnetic phases of the core@shell (Fe@Py) rectangular nanocylinders.Our results indicate that the dipolar interaction between the core and the shell is capable of causing significant changes in the magnetic phases of the isolated Fe cylinder and the Py ring. We show that the geometric parameters of flat Fe@Py core@shell cylinders can be chosen in such a way to control the nucleation of domain walls in the Py shell. It is also possible to fine-tuning the domain wall position and width by using only magnetic energies. On the other hand, bimagnetic nanoparticles combining different functionalities of two magnetic materials opens new perspectives for key applications such as permanent magnets, recording media, and magnetic hyperthermia. A theoretical analysis of the impact of the composition of FeP t@CoFe2 and FeP t@Fe bimagnetic nanocylinders on the maximum energy product (BH)max was performed. (BH)max is the determining parameter of the permanent magnet quality. The best composition is determined by the competing trends imposed by the dipolar energy and a ferromagnetic core@shell interface exchange energy. It was observed that the dipolar interaction has a negative impact on the intensity of (BH)max for shell thicknesses above a theresehold value, which depends on the material. The results show that the best shell material is the one with highest exchange stiffness.

Great discoveries in fundamental research very often lead to a new technology era. This is about to happen in nanomagnetism. Currently, there are promising perspectives of micro-wave nanoantenna’s design, based on the use of systems consisting of soft ferromagnetic nanoelements suitably designed to hold interacting magnetic vortices. Besides, there are demands for controlling the vortices chiralities. Experimental reports indicate that the vortex pair excitation spectra and magnetoresistance depend on the vortices chiralities. Furthermore, the synchronization of the vortex’s dynamics, owing to vortices interaction, leads to high-quality microwave spectra, with narrow emission lines, as desired for nanoantennas. The vortex magnetic profile results from a competition between the trends imposed by the exchange and the dipolar energies. Nanostructuring brings new elements and promising perspectives of tailoring the vortex magnetic profile of soft ferromagnetic nanoelements to suit device application demands. Placing two nanoelements together, if the nanoelements distance is comparable to its own geometrical dimensions, one may modify the intrinsic vortex profile of the soft material, because the dipolar field in each nanoelement may have a considerable contribution emanating from the other nanoelement. We have investigated the magnetic structure at remanence of pairs of vortices of a pair of identical and coaxial 21 nm height Fe circular nanocylinders with the diameter ranging from 81 nm to 129 nm, and the uniaxial anisotropy easy axis in the plane of the circular faces. In addition, we have investigated similar Py systems for comparison purposes. We have shown that by choosing appropriate values of the nanocylinders distance, one may control the dipolar interaction strength, allowing to set the relative chirality of the vortices in pairs of circular Fe nanoelements, as required for device applications. Furthermore, we have shown that the vortices chiralities and locations in the nanocylinders may be controlled by using external fields of moderate strength. For circular nanocylinders pairs, we have used preparations routes with the external field-aligned either in the x-axis direction, parallel the easy axis of the Fe uniaxial anisotropy, or in the y-axis direction, perpendicular to the easy axis of the Fe uniaxial anisotropy. We have shown that owing to the impact of the uniaxial anisotropy in the sequence of magnetic phases along either the x-axis or the y-axis preparation route, and the remnant state may be either a same chirality vortex pair or an opposite chirality vortex pair. In the case of Py nanocylinders, there is no uniaxial anisotropy. Our results show that for Py nanocylinder pairs, the magnetic pattern at remanence does not depend on the direction of the in-plane external field used in the preparation route. We notice that one may have surprising features from the dipolar interaction between the Fe nanoelements. For instance, we have found that isolated 21 nm height and 129 nm diameter Fe nanocylinders hold a vortex at remanence. However, contrary to intuition, coaxial pairs of these nanocylinders, placed one on top of the other at short distances, may display, instead, an almost uniform state at remanence. This might be valuable information for designing nano-oscillators based on pair of vortices. We have also investigated the remnant state of pairs of identical elliptical 25nm height Fe nanocylinders with face-to-face distance ranging from 20nm to 60nm. The Fe nanocylinders uniaxial anisotropy easy axis is in the plane of the elliptical faces pointing along the minor axis direction. To induce vortex nucleation, we have used the preparation route with the external field along the minor axis direction. We have shown that controlling the dipolar interaction strength by changing the elliptical nanoelements distance, one may design the magnetic structure of four interacting vortices at remanence. For instance, we investigated pairs of Fe (195 nm x 305 nm x 25 nm) nanocylinders, each with a 195 nm and 305nm long minor and major axes, respectively. The isolated Fe nanocylinders hold a single vortex at the center at remanence. A pair of these nanocylinders, with a 35 nm face-to-face distance, instead, shows a pair of opposite chirality vortices in each nanocylinder. We have shown that there are relevant changes in the strength of the dipolar and local fields, owing to the change of the magnetic structure that results from vortices motion. We suggest that this may affect the restoring torques for vortices oscillations around the equilibrium positions.

Rare earth (RE) intermetallic compounds and transition metals (TM) are widely used in the production of permanent magnets and have, in general, ferromagnetic behavior. Most of these compounds are produced in an arc furnace or in ball mills, physical methods that do not allow controlling many variables in the process. Currently, much of the research in this area is related to the production of these compounds through chemical methods. In this work, we propose a process for the synthesis of magnetic nanoparticles of the NdCo type via the polyol method, with the use of PVP, for five samples with variation in the molar proportions of the compounds. PVP was used to protect the particles against aggregation and oxidation. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements were performed for structural and morphological analysis, respectively. DC magnetization measurements, vibrant sample magnetometry at 5 K and 300 K and magnetic thermogravimetry (MTG), were performed for magnetic characterization. The XRD measurements showed the formation of the NdCo₂, Nd₂Co₇, Nd₅Co₁₉ and Nd₂Co₁₇ phases, with predominance of the Nd₂Co₁₇ phase, for four of the five samples, for four of the five samples. The diffractogram network parameters were obtained through the rietveld refinement and showed that the average crystallite sizes ranged from 6.5 nm to 20.7 nm. Through TEM it was possible to observe ellipsoidal and spherical nanoparticles, distributed in a PVP matrix, with size distributions ranging from 6.1 nm to 12.4 nm, and which crystallographic planes and their respective phases contributed to the formation of the images. The DC magnetization measurements as a function of temperature indicated the Curie temperatures (Tc) of the phases and are in accordance with the literature and XRD measurements, the magnetization values showed elevations in the vicinity of Tc which are related to the PVP phase shift to high temperatures, and the consequent formation of metastable phases, rich in Cobalt, reported in the literature. The ferromagnetic behavior was observed in all samples at both 5 K and 300 K in the measurements of magnetometry of vibrant sample. With the TGM magnetic thermogravimetry measurements, the transition temperatures for the phases reported in the previous measurements were obtained, including for the metastable phases, through the simple interpolation method.

In this work, we present empirical results obtained through the analysis of time series of three different complex systems: barkhausen noise signals in magnetic systems, accelerometer signals that recorded motor activity in rodents and acoustic signals that recorded background noise in some ecosystems ( soundscapes). In particular, we found that the time series of these three systems have a common feature: intermittency, all with some time interval distributed according to a power law. More precisely, in the case of Barkhausen noise, we obtained an exponential distribution for waiting times, in contrast to the already well-known power law for avalanche durations. For the accelerometer signals obtained in mice, we show that there is an alternation between a distribution of the
power law form for the durations of the avalanches of motor activity and an exponential distribution for the quiet intervals. In the case of acoustic time series, we show that there is an alternation between a lognormal distribution for the intervals of acoustic activity (the sound times) and a power law for the intervals of quiet for all analyzed ecosystems.

In recent years, with the advent of increasingly robust and accurate astronomical observations, further investigations of the role of fundamental constants of nature has become a fascinating field of research, because any space-time variation of these constants would indicate the presence of a new physics and would put in check the relativistic cosmology itself, based on the Cosmological Principle.In this thesis, by using galaxy clusters observations, type Ia Supernovae measurements, and Strong Gravitational Lensing Systems we propose three methods to probe a possible time variation of the fine structure constant (). Such possible variation is investigated in the specific context of the Runaway Dilaton model, which was originated from string theories. We conclude that, although the observational bounds obtained on a possible variation of  () are not competitive with those from quasar spectroscopy (), our proposals provide completely independent bounds with different redshift intervals. Finally, a test to probe a possible deviation of the cosmic distance duality relation (CDDR) is also proposed, , by using scaling-relation measurements from galaxy clusters properties plus type Ia supernova. The cosmological bounds obtained on possible deviations from the relation are compatible with the limits present in the literature and point to the validity of the duality relation.

Using the coprecipitation method, samples (precursors) of the CoFe₂O₄-Ag nanocomposite were synthesized, varying the concentration of CoFe₂O₄. These samples were subjected to thermal reduction, under H₂ atmosphere at 350 °C, for different time intervals in order to obtain a core-shell nanostructure of CoFe₂O₄@CoFe₂ dispersed in the Ag matrix. Through X-ray diffractometry, it was observed the phases CoFe₂O₄, CoFe₂ and Ag,, where the peak of CoFe₂ (shell) becomes more evident for samples reduced for a longer time. Rietveld refinements provide values for the size of CoFe₂O₄ nanoparticles (core) which decrease size with the increase in the reduction time, from 30 to 90 minutes. This decrease was also observed in Transmission Electron Microscopy images, and is around 0.5 nm. Meanwhile, the shell thickness (δ_CF) changes from 2.5 nm to 3 nm between samples with a lower concentration of CoFe₂O₄ reduced by 30 and 90 minutes. The nanoparticles of Ag, in the precursor samples, have diameters of 11 nm, but this value increases to ≈ 15 - 20 nm after the heat treatment. Magnetization measurements, at 300 K, reveal a decrease in the coercive field (H_c) as δ_CF increases, and the behavior of the curves suggests an exchange spring coupling. A phenomenological model was developed to describe the behavior of magnetic interactions in relation to the distance between the core shell nanoparticles, D_nc, based on the analysis of  Henkel and δm plots which are obtained through the relationship between the remaining reverse and direct magnetizations. For small D_nc distances, there is a greater contribution of dipolar interactions. When D_nc increases, the exchange interactions are more pronounced, as the contribution due to coupling at the core-shell interface is evidenced. Magnetization measurements as a function of the field in 4 K by zero field cooling and field cooling under a 1 T cooling field exhibit a horizontal shift in the curves, which can be attributed to freezing spins at the core-shell interface. It was found that the field of exchange bias (H_EB) in samples with lesser δ_CF is approximately double that presented in samples with thicker δ_CF. This increase in H_EB is related to the ratio of the surface area on one disordered layer to the volume of the ferromagnetic layer.

In this work we examine an alternative effective formulation for the $t$-$J$ model which is believed
to capture the essential physics behind the underdoped regime of the high-$T_c$ cuprates. After recasting
the original problem in this new representation, we check the consistency of our procedure and discuss
some physical consequences of such a representation including, the crossover between the regimes of small
and large Fermi surfaces which is observed experimentally. We then explore the possiblity of constructing a
path integral representation for the $t$-$J$ quantum partition function departing from the proposed
formulation. Finally, we use the latter formalism to derive a continuum effective field theory in $(2+1)$
spacetime dimensions which features small hole-like pockets near the nodal regions of the Brillouin zone
along with an unconventional mechanism for superconductivity.

In this work we will focus in statistical physics applied to problems of Bose-Einstien condensation at finite temperatures. We begin by a brief introduction to the classical methods, already well established by fundamental courses. Afterwards, starting from a kinetic theory, we will obtain the equations of motion for the atoms in the condensate state, as well as for the Bogoliubov quasiparticles of the thermal cloud of excited atoms. Through this kinetic description, we will also obtain, from quantum principles of quantum mechanics, the Born-Bogoliubov-Green-Kirkwood-Ygon (BBGKY) hierarchy, which can be truncated to obtain the Boltzmann-Vlasov Equation. Finally, we will use the same microscopic principles to study the damping of collective excitations of the Bose-Einstein condensate in the thermal regime.

The increased demand for low-power and high-performance electronic systems has been driving the discovery of new materials and the study of their physical and chemical properties. In this sense, the integration of different techniques allows us to develop multifunctional materials with specific properties and characteristics for certain technological applications. In this scenario, this work presents a proposal for the production of hybrid materials through the integration between tape casting and magnetron sputtering techniques. To this end, the magnetron sputtering technique was used to grow trilayered thin films nanostructures of Ni₈₁Fe₁₉/Cr/Ni₈₁Fe₁₉, using as substrates Al₂O₃ and ZrO₂ ceramic flexible sheet produced by the tape casting technique. As a comparison standard, similar films were grown onto amorphous rigid substrate (glass), which has been widely studied in the literature. The slurries, suspensions of ceramic materials, presented the pseudoplastic behavior, which is recommended for the tape casting technique. The films were produced with a fixed total thickness of 300 nm, but varying the thickness of the Ni₈₁Fe₁₉ layers between 75 and 142.5 nm and the Cr layer thickness between 15 and 150 nm. The structural characterization revealed that the change of the used substrate did not modify the structural properties of Ni₈₁Fe₁₉ films. The magnetic measurements reveal that films produced onto flexible sheet show an evolution of the contribution of out-of-plane anisotropy with the increase of Ni₈₁Fe₁₉ layer thickness, as well as the dependence of magnetic properties with the Ni₈₁Fe₁₉ layer thickness, results that mirroring the magnetic properties found in Ni₈₁Fe₁₉ thin films on the glass in the literature. Based on these results, we were able to keep the magnetic properties of the Ni₈₁Fe₁₉ alloy, even when the flexible sheet of Al₂O₃ and ZrO₂ ceramic tapes were employed. Thus we conclude that the flexible sheets are excellent candidates for moldable device applications as ceramic-magnetic coating.

Pré-defesa Doutorado

From current cosmological observations, dark matter and dark energy are the energetic components that dominate the evolution and dynamics of the present universe, as baryonic matter and radiation have a share of less than 5% of total cosmic energy. Although necessary to describe some observational data the nature of the dark sector remains a mystery to cosmology. In this thesis, we study a cosmological models scenarios with bulk viscosity. Endowed with an alternative interpretation of the bulk viscosity and the dynamics of the Universe from the microscopic effects based in the nonadditive Statistical Mechanics we propose a extended ΛCDM model. This scenario take account nonadditive effects about the energy equipartition theorem as well as an interpretation of the viscous dark matter through nonextensive/dissipative correspondence (NexDC). In order to impose observational constraints on model parameters and compare models, we implement a Bayesian Analysis considering data from the type Ia supernovae, baryon acoustic oscillations and cosmic background radiation. The results obtained were that the non-extensive viscosity effect is discarded at 1σ confidence, but at 2σ we have a scenario with viscosity. From a model comparison point of view, the proposed models have unfavorable inconclusive evidence regarding the ΛCDM. In the second part of the thesis, we consider a description of viscous dark energy in the scope of universe dynamics modified by nonadditive theory as well as the standard. In order to impose observational limits, we performed a Bayesian Analysis using data from cosmic chronometers, type Ia supernova, baryonic acoustic oscillations, and cosmic background radiation. As a results of the analysis, we obtain that a bulk viscosity is discarded in 1σ in the dark energy context. In the comparison models, we can conclude that the viscous dark energy models are also discarded compared to the ΛCDM model.

Microwave emission by magnetic materials at well-localized frequencies is of great interest for future nanotechnology applications. Theoretical models were developed in this work to study the excitation spectra of nanostructured magnetic systems. We use micromagnetic simulations, the Landau-Lifshitz resolvent matrix and a generalized dynamic susceptibility tensor. The first part of this work is devoted to setting the Landau-Lifshitz equations of motion for a nanostructured system composed, according to the micromagnetic theory, and the elements of the Landau-Lifshitz equations resolvent matrix, as well as the elements of the dynamic susceptibility tensor. The theoretical models are then applied to describe the excitation spectra of systems such as uniformly magnetized ferromagnetic nanostripes, domain walls of ferromagnetic nanostripes coupled to antiferromagnetic vicinal substrates and magnetic vortices of circular nanodisks. The resolvent matrix allows obtaining the spectral density of states, and is applied to the study the spectra of domain walls, allowing the characterization of some modes of walls oscillations in a frequency range below the magnetic domain spectral band. Using the dynamic susceptibility tensor, the spatial distribution of resonance modes along the entire nanostructure could be identified. Thus, the similarities and differences between the domain, domain wall and vortex magnetic excitations can be investigated using the theoretical models developed in this study. Thus, we suggest, it possible to predict microwave field values to excite each of the observed oscillation modes.

Photometric variability can provide important information about white dwarfs, whether related to rotation or the presence of companions - planetary, substellar or stellar-remnant. The rotation can provide clues to the physics of white dwarf formation, once it is a remnant of their angular momentum, which went through several stages of stellar evolution. The presence of substellar companions or stellar-remnant provides clues about the final stages of stellar evolution and plays an important role in that evolution. The high photometric precision and the large amount of data provided by space missions, such as Kepler and K2, combined with the correct tools, make it possible to analyze the photometric variability of white dwarf stars. In order to detect the relative periodicities, present in the light curve obtained by these space missions, several mathematical transformations have been applied, such as the Fourier transform, one of the most used in astrophysics, and the Wavelet, which has been applied in a wide range of areas, including Statistics, Geophysics, Image Coding and Compression, Turbulence and Astrophysics. The Wavelet transform is a powerful tool, since it has functions located in both frequency and time, allowing the detection of the temporal evolution of various phenomena, which makes it ideal for analyzing non-stationary signals. In this context, we use these tools in order to study the photometric variability of 25 white dwarf stars observed by the Kepler mission. From the light curves of these stars, we analyze the Lomb-Scargle periodograms, determine the possible physical mechanisms responsible for the detected modulations and perform local and global wavelet spectra. The first one is interpreted as the signal energy distribution, while the second one is the temporal integration of the local map. Finally, we discuss the physical meaning of the results by establishing a comparative study between the periodicities determined in this work and those found in literature.

Studies about circumstellar material in different evolutionary stages has been increasingly developing due to the emergence of high photometric resolution space missions, covering an even larger spectrum of infrared wavelength. Among them, Spitzer, Herschel and WISE, for instance, present a particular aspect by offering observations in wavelength compatible to the temperature of the Solar System's Asteroid Belt and Zodiacal Cloud. In this present thesis, a search for debris disk is performed around 34030 objects, defined in the literature as being in the Main Sequence, observed by the Kepler mission and based on the WISE mission photometric information. At first, a diagnosis is performed on the presence of infrared excess on these stars, this detection is potentially associated to the presence of circumstellar material. Then, a study is made about the evolutionary stage of the sample, including their location on the HR diagram. This analysis was done based on data from the GAIA mission and showed that only about 16500 stars are actually Main Sequence, with around 17000 presenting characteristics of Subgiants or binary systems, according to their location. By utilizing data treatment tools, including VOSA, this work has shown the presence of debris disk around 35 Main Sequence stars, 13 Subgiants and one anomalous star, with well defined locations in the HR diagram, raising questions about the physical characteristics of these disks in relation to the stellar age. The Main Sequence stars possess debris disks that are more massive and more luminous than the Solar System's Asteroid Belt and Zodiacal Cloud, bringing up the possibility that the disk was refueled with additional material, produced by violent stochastic events, including the disintegration of large bodies such as planets or massive asteroids. In Subgiants stars, on average, the mass and luminosity of the disks are in the same order as the values seen in the Main Sequence stars, this is incompatible with the notion that these disks are first generation remnants and have survived the first evolutionary stages. Therefore, such results seem to point to the presence of dynamical physical processes happening in the circumstellar environment during the Subgiant stage, refueling the disks with material produced in this stellar evolutionary stage. The Subgiants' disks presented in this work can not be explained solely by the evolutionary context or under the consideration that they were preserved during the passage from the Main Sequence to the Subgiant stage and along the evolution outside the Main Sequence.

The phenomena of raising the temperature of magnetic nanoparticles under an alternating magnetic field, known as magnetic hyperthermia, is an outstanding field, which gives rise to challenges in the context of fundamental physics and providing new roads to applications, such as in the cancer treatment and in the control of thermally activated drug delivery. Thus, the complete understanding of the behavior of these systems with reduced dimensions becomes a key point and the optimization of the production processes and the properties of these materials, a challenging task. In this work, we perform a theoretical and experimental investigation of the magnetic hyperthermia in MgO.Fe2O3 and FeO.Fe2O3 superparamagnetic nanoparticles. Specifically, we aim to fully understand the influence of the composition, nanoparticle size, as well as amplitude and frequency of the field on the specific absorption rate of the samples. Here, we propose a theoretical model to describe the thermal behavior of magnetic nanoparticles, providing further insights on well-known parameters found in literature. To test the robustness of the approach, we apply the theoretical model to describe the magnetic hyperthermia curves obtained experimentally. To obtain the magnetic hyperthermia curves, an experimental system is developed, making possible to generate magnetic fields with frequency up to 100 kHz and amplitude up to 200 Oe. The excellent agreement between theoretical and experimental results provides support to confirm the validity of our approach to describe the thermal behavior of magnetic nanoparticles.

The effective theories of curves described by gauge theories are analyzed with the intent to apply them to modeling physics of extended quasi-one dimensional systems, such as protein molecules, elastic cords, or even vortices in superconductivity. In this study one perceives how the geometry of curves, apparently simple, can be related to subjects of paramount importance for Physics or even Biology. In this work we start with Frenet equations that model curves and consequently introduce gauge symmetries, the spontaneous symmetry breaking, as well as solitary waves. With the aid of the idea of effective gauge theories, the mathematical framework is elaborated to produce an energy functional, through which one can arrive at the equations that govern the dynamics of curves, being prototypes of long molecules. It was shown graphically that the most simple theoretical model can adapt the basic secondary structures of proteins, called $\alpha$-helix and $\beta$-strand. The properties of the potential of the theory have been analyzed together with  characteristics of the structure of curves such as folds, which reflect the supersecondary structures of proteins (structural motifs). The classical and quantum stability properties have been studied using the standard field theory methods. It was shown that the motifs, realized in the model as static solitons, are unstable configurations in the limit of infinite smooth curves, which preserve translational symmetry.

The study of new solutions in General Relativity motivated investigations of Cauchy problems for alternative gravitational regimes, which led to the need to elaborate techniques to split the spacetime into three plus one dimensions. The 3+1 formalism arises as a mathematical tool for decomposing the components of the metric and the curvature. The central mechanism found in the literature is known as the ADM formalism, developed initially as an attempt to construct a theory of canonical quantum gravity, which later its formulation was applied to evolve the Einstein equation numerically. As a subfield of Gravitation, the Numerical Relativity investigates phenomena in strong field systems and other scenarios which are not possible to solve analytically. In this dissertation, we will present an introduction to the 3+1 formalism, as well as gauge and initial conditions to prepare some gravitational systems. To consolidate this theoretical approach, we will show techniques of Numerical Relativity to evolve the spacetime from of given initial configurations.

In this dissertation, we investigated the non-coding DNA of the 24 human chromosomes, through the generalized statistics. This study, carried out with the recent generalizations of the Standard Theory of Statistical Mechanics, has been more important and analyzed as the statistical properties of the genetic sequences, in order to obtain information about the size distributions. In works already carried out on non-coding human DNA the use of q-exponential distributions of Tsallis generalized formalism through the maximization of non-extensive entropy is reported to study the size distribution of these sequences. In this dissertation, we broaden these studies through the use of k-statistics originating from Kaniadakis generalized formalism. Namely, a brief comparison between the two analyzes is interesting. We present the values of the deformation parameters k, in Kaniadakis formalism, and the entropic index q in the Tsallis formalism, which describe the size distributions for all the chromosomes of the non-coding human DNA.

In the present work we do the theoretical study of the propagation of electromagnetic waves in a multilayer system, whose refractive index of each layer is modulated by the function that describes the potential of the one-dimensional Harper model. We apply the transfer matrix method to obtain the transmittance spectra as a function of the reduced frequency w/w₀. In order to identify possible edge states or topological states, we calculate the transmittance spectrum as a function of w/w₀ and the parameter f for three cases: first, when layer thicknesses are given by the optical-length ratio I_jd_j=l₀/4, where I_j is the refractive index of the layer j; the second, when all thicknesses of the layers are equal, d_j=d; and finally when the thicknesses of the layers are related by  d_2j=2d_2j+1. In addition, we obtained the transmittance spectrum as a function of the periodicity control parameter b. At the critical point l=0.5, we reproduce the photon equivalent of the Hofstadter butterfly, which corresponds to a critical state in the metal-insulating transition, that is, for l >0.5, the system is equivalent to a state and when l >0.5, the system is equivalent to an insulating state.

Theoretical models for predicting the properties of quasicrystalline materials have been of considerable interest to the scientific community recently. However, they are mainly related to the optical and electronic characteristics of the system, and a study of the elementary oscillations, such as phonons, of one-dimensional quasicrystalline lattices is still necessary. Recently published works have shown that the localization properties of the Harper model can be modeled in a quasicrystal through the Hamiltonian of Aubry-André, considering the immeasurable potential with the lattice parameter. This model proved to present itself as a topological insulator, exhibiting border states and nontrivial phases for the electronic case. Motivated by these results, in this work, we present a study on the vibrational properties of one-dimensional quasi-crystals, highlighting the topological states of edge. For this, we model a one-dimensional quasicristal through the Aubry-André model with the potential parameter defined by the golden ratio (b = (1+) / 2). We performed the numerical calculations from the exact numerical diagonalization of the Hamiltonian. In our results, we find the multifractal frequency spectrum known as the "Hofstadter's butterfly", which emerges as a critical state of a transition from metal-insulating type states to the value of modulation of the dimensionless spring constant of 1.0. We also show by calculating the wavelength, that there exist certain states that cross the largest gaps of the spectrum (as a function of the phi phase) are edge states in the system, where there are state locations in them.

According to the standard theory of stellar evolution, low-mass stars (spectral type K and G) must reach the beginning of the main sequence (ZAMS) with Li abundance near the meteorological value, which is ∼3.3 dex, and hold a high abundance until they reach the first dredge-up zone in the Red Giant Branch (RGB). After completion of the dilution processes, in the red clump, such stars should present a relatively low Li abundance. However, approximately 1-2% of all observed K and G giant stars have an abnormally high Li abundance (≥ 1.5dex). It is possible to find in the literature many attempts to reconcile theory and observation, but none of them is capable of explaining all the scenarios in which these chemical anomalies occur.

Our work aims to present a new study of lithium-rich G and K spectral type stars and analyze the possible existence of particular characteristics to the lithium-rich stars that present detected magnetic field. We assemble a sample of 20 giant stars — taken from Charbonnel e Balachandran (2000), Kumar, Reddy e Lambert (2011), and Lèbre et al. (2009) — and calculate their fundamental parameters and the longitudinal magnetic field for a sub-sample of stars with observed high-resolution spectra available at PolarBase (Petit et al., 2014). Concerning the evolutionary states, we have used parallaxes recently provided by ESA’s Gaia spacecraft. To do so, we use the spectral analysis tool iSpec (Blanco-Cuaresma et al.,2014) and the Least-Squares Deconvolution (LSD) technique (Donati et al., 1997). We obtained results for fundamental parameters and Li abundance using the same procedure for all stars. Thus, we have the reliability in comparing stars from different samples that possibly had their spectra treated in different ways. We obtained relations between Li abundance, rotation velocity, and presence of a magnetic field according to what is predicted in the literature. We conclude that each star needs to be analyzed individually and with greater spectroscopic detail so that the real nature of its Li abundance is unraveled. The ¹²C/¹³C isotopic ratio, and the C/N elemental ratio, need to be investigated so that we can determine precisely the position of some stars (from our sample) on the H-R diagram.

In this thesis we present a theoretical study of the propagation of magnons in the

magnetostatic regime and of electromagnetic waves in aperiodic superlattices of rare earths formed by dysprosium (Dy) and holmium (Ho). The superlattices studied are the quasiperiodic Fibonacci and the deterministic aperiodic Thue-Morse and Double-Period, all of which are constructed through the periodic stacking of unit cells, which are formed by a two-layer substitution rule of type A/B, where this rule depends on the type of sequence considered for the construction of the superlattice. As a consequence of the periodic of the unit cells, it is possible to apply Bloch’s theorem, which together with the matrix-transfer method, allows us to find the bulk and surface magnetostatic modes. These modes present a self-similar behavior of the volume bands, so, at the limit of the generation number , these spectra become a Cantor-type fractal set. It has also been verified scaling and localization properties of these modes, which are different for Dy and Ho. In the propagation of electromagnetic waves we analyze the optical spectrum of reflection (or reflectance) for the same rare earth superlattices. We use here the Drude model to determine the dielectric function of rare earths, which are metals, so we have that reflection is dominant for these systems. We also use the Maxwell equations and the matrix-transfer method, which allows us to obtain the reflection coefficients. Therefore, we analyzed in these structures the spectra for the normal incidence of s-polarized (TE waves) and p-polarized waves (TM waves), where it was observed that they present an autosimilar behavior and a mirror symmetry in their spectra in the near infrared region, with reflectance for both polarizations. In the oblique incidence it was observed that the reflectance is sensitive to the variation of the angle, also varying with the type of superlattice and its generation, where the spectra are different for Dy and Ho. In the far infrared region we have seen that reflectance is dominant in rare earths, as is expected for metals.

In this work, three samples of $La_2MnFeO_6$  (LFMO) were prepared by solid state reaction, heat treated at 1200 $^{\circ}C$ at treatment times equivalent to 24, 48 and 96 $hrs$, each identified by name LFMO24, LFMO48 and LFMO96, respectively. The obtained structures are double-$perovskite$, monoclinic with spatial group $ P2_1 / n $ and lattice parameters given by $a$ = 5.52 Å, $b$ = 5.51 Å and $c$ = 7.81 Å. The sample crystallite sizes are equivalent to 361.0 $nm$ (LFMO24), 348.9 $nm$ (LFMO48) and 370.83 $nm$ (LFMO96). According to the specific heat (HC) measurements, the LFMO's have a ferrimagnetic phase transition of approximately 10 $K$, according to data adjustment using the mean field theory. Phase transitions due to charge location were observed, occurring at different temperatures depending on the degree of ordering of each system, in which the most disorganized state tends to have this transition tending to 140 $K$. An excess in HC is identified and modulated according to the sum of the defects of Frenkel and Schottky, where LFMO24 and LFMO48 presented smaller and greater amount of defects, respectively. The contribution of the crystalline lattice was stipulated using the Thirring Model, with Debye temperatures of the order of the expected for this type of material. DC, AC and magnetic hysteresis magnetization measurements confirm the existence of weak ferromagnetism behavior at room temperature in all LFMOs, in samples LFMO48 and LFMO96 it is assigned directly to the atoms of $Mn$, but in LFMO24, sample that presented higher value of magnetization at this temperature, there is an increase provided to the $Fe$ ions, in spite of the $M$ spectroscopy for the $^{57}Fe$, to show a doublet. The real part of the AC susceptibility, $\chi_{AC}^{'}$, complemented by different theoretical models, confirms a cluster-glass type phase, generated by agglomerates in a certain temperature range in the LFMO's, by about 30 $K$, where $\chi_ AC^{'}$ shows peaks that vary with frequency $f$. The strong presence of the Griffiths phase in the $1/\ chi_{AC}^{'} curves of LFMO48 and LFMO96 suggest that this phase is initiated at 65 $K$, occurring differently at LFMO24.

A relevant problem in Statistical Physics and Mathematical Physics is to derive numerically precise expressions and exact analytical forms to calculate the distributions of Lévy α-stable P_α(x;β). In practice, these distributions are usually expressed in terms of the Fourier Integral of its characteristic function. In fact, known closed-form expressions are relatively scarce given the huge space of parameters: 0 < α ≤ 2 (Lévy index), -1 ≤ β ≤ 1 (asymmetry), σ > 0 (scale) and - ∞ < µ < ∞ (offset). In the formal context, important exact results rely on special functions,

such as the Meijer-G, Fox-H functions and finite sum of hypergeometric functions, with only a few exceptional cases expressed

in terms of elementary functions (Gaussian and Cauchy distributions). From a more practical point of view, methods such as series expansions, e.g., allow an estimation of the Lévy distributions with high numerical precision, but most of the approaches are restricted to a small subset of the parameters and, although sophisticated, this algorithms are time-consuming. As an additional contribution to this problem, we propose new methods to describe the symmetric stable distributions, with parameters β = 0, µ = 0, σ = 1. We obtain a description through a closed analytical form, via series of formal power making use of the Borel regularization sum procedure (for α = 2/M, M = 1, 2, 3...). Furthermore we obtain an approximate expression (for 0 < α ≤ 2) by dividing the domain of the integration variable into sub-intervals (windows), performing proper series expansion

inside each window, and then calculating the integrals term by term.

The pulsation of the stars has become, from decades to now, one of the main mechanisms to obtain information about their interiors. With the advent of the missions, the need to understand how
global parameters of the star behaved in relation to the modes is crucial to the entire study of stellar evolution, especially in the Red Giant Branch. Many computational codes use inferential statistics for gather
information, all very peculiar to evolutionary states since the main sequence
until the asymptotic giant branch. Our work was to develop a tool
which will serve as a basis for new trials and works, within the Asteroseismology of Clump stars, which obtain the values of maximum oscillation frequency and the separations between these frequencies are pillars for stellar physical understanding. Herein to this, the correct modeling associated with the inclusion of metallicity obtained
from spectroscopy, may generate changes, not yet fully understood
in the seismic scales relations for mass and radius. In this way, the study of seismology of Clump stars may be the key to a promising scenario for Archeology Galactic and for the complete understanding of processes in the stellar interior.

This work was shown in the physical and characterization of nanoparticles magnetite and ferrite of cobalt doped with Sm: Fe 2.68Sm0.32O4 and CoFe (2-x) SmxO4 (x = 0.05, 0.10, 0.15 and 0.20). As magnetite nanoparticles were produced using the polyol method, where the polymer polyvinylpyrrolidone (PVP) was used for protection as nanoparticles of the oxidation. As nanoparticles of Sm-doped cobalt ferragens, they were constructed by the ionic rope reaction (ICR) method and covered by a chitosan matrix.
A crystal structure and magnetic properties were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), Mössbauer spectroscopy and DC magnetometry.
A magnetite sample has a print size distribution having a diameter of 5.2 nm. As the bees presented under the superparamagnetic effect of 4 × 105 J / m3 at 5 K. In a constant environment, the MH presented the has a single paramagnetic component, while the 5 K is a saturation magnetization of 18.9 emu / ha MH curve , field cooling (FC) has a displacement with a bias shift field of ~ 110 Oe. The benefits are discussed in terms of spatial distribution of actions, as determined by Mössbauer spectroscopy. Immunization of magnetocrystalline anisotropy.
The cobalt silver samples are coated by the chitosan biopolymer, with a variable variation of the Sm-ion concentration dependent sizes, ranging from 9.49 nm to 5.62 nm. The coercive field values, saturation magnetization and anisotropy were subjected to structural changes with doping. A theological dose was made taking into account the form of distribution of particles, blocked and superparamagnetic. The mega-sounds spectrum was when performance was used in three sixths in each of the crystallographic groups, at room temperature and at 12 K. The results were calculated in relation to the random distribution of Smenoe A and B levels. were for applications of an AC magnetic field of 70.5 kHz and intensity of 200 Oe. The value was maximum for the sampling of a series of 12.09 W / g, which aims to use hyperthermia.

We are embedded in a networked world, and in the last few years network studies and their

 properties have expanded considerably. The main reason is that several systems can be 

modeled through so-called complex networks. Examples of systems easily modeled as networks are: 

the society, the Web, the brain, among others. To understand the behavior of these systems, several 

models in the area of Complex Networks have been proposed. Barabási and Albert proposed 

a model that included two basic mechanisms (growth and preferential bonding),

 reproducing a characteristic behavior in some real systems: distribution of 

connectivity in power law. As a consequence of the Barabási and Albert model,

 other network models emerged, considering different types of factors included i

n the preferential link mechanism.

 Models that use this mechanism satisfactorily explain the

 emergence of distributions that follow power law in real networks.

 However, the preferred connection is not the only mechanism through which a network can grow and 

generate this type of connectivity distribution. Therefore, in this work we analyze two models that use

different mechanisms of preferential linkage and that are able to generate networks without a typical scale:

 the vertex copy model and the deconstruction model of Poisson networks. We compare the results 

with the networks derived from the Barabási and

 Albert model,

 because 

we believe that studying di

In this work we investigate the infinite ranged Blume-Capel model (BC), under a random field following a Gaussian distribution. The model is exactly soluble and from the corresponding free energy density we have obtained a wealthy of phase diagrams. We initially revisit the BC model under a uniform crystal field and in zero magnetic field. This allowed us to regain the well-known mean-field results for this model. In the next step we revised the Kaufman-Kanner work on the BC model under a bimodal random magnetic field and uniform crystal field. In the case we paid special attention to crystal field versus temperature phase diagrams.

Finally, we investigated the phase diagrams for the Gaussian random field case. The obtained phase diagrams display a rich variety of critical phenomena, with the presence of tricritical points, in some cases, related to the occurrence of  both frist-oder and continuous transition lines.     

The magnetoimpedance effect (MI) is an interesting tool for a completely analysis of the magnetic nanostructures, since is possible to observe the magnetization dynamics behavior at a wide range of frequency and external magnetic field,  in both saturated and unsaturated regimes.  In the last years studies about the MI effect as a function of sample geometry has been performed. In particular in trilayer systems and meander-lines samples, separately.  Based on this, here we seeks the integration  between these two geometry (trilayer and meander-line) and the structural and magnetic properties (quasi-static and dynamics) are explored in [NiFe/Cr/NiFe] trilayered samples. The samples were produced by Magnetron Sputtering technique under glass amorphous substrate with thicknesses of 75 - 142.5 nm for the ferromagnetic layer and 15-150 nm for the Cr layer, totalizing 10 samples. However, the total thicknesses of the trilayered samples are remained constant of 300 nm, irrespectively of the produced sample. Associated with this, for each set of thicknesses three in-plane substrate geometry were explored. The first one, a rectangular geometry, traditionally studied, the last two a meander-line geometry are studied. In particular, to reach these geometry for the samples, masks were used during the deposition process.  The structural characterization, trough X-ray diffraction, shows a crystalline behavior with a [111] texture, usually found for NiFe thin films grown onto amorphous substrate. The quasi-static characterization was performed through magnetization curves, obtained from a Vibrating Sample Magnetometer (VSM), with external magnetic field applied in the film-plane for distinct direction with respect to the mains axis of the samples, to verify the anisotropy behavior. In a general point of view, the magnetization curves reveal the isotropic behavior in the film-plane and a strongly dependence with the NiFe thicknesses.  On the other hand, a weak dependence with the sample geometry (rectangular or meander-line) is observed, as expected. Finally, for the magnetization dynamics study the magnetoimpedance effect was used in a wide range of field ( 700 Oe) and frequency (0.5 up to 3.0 GHz). Besides, two field configuration between the external magnetic field and the drive current were explored, named by longitudinal and transverse measurements. The MI measurements present a double or multiple peaks structures, characteristic of the FMR contribution in the MI variations in this frequency range. With respect to the MI effect dependence,  as a function of the thicknesses ratio of the NiFe and Cr layers,  is possible to observe a strongly influence of the out-of-plane magnetic anisotropy in the peaks structures in the MI curves, mainly for the samples with thick NiFe layer.  Lastly, the MI curves as a function of the sample geometry reveal changes in the dynamics magnetization, carrying asymmetric MI curves and a complexity in the peak structure.

One and two-dimensional materials, such nanowires, nanotubes, graphene, and boron nitride, triggered enormous interest due to their potential application in novel electronic and heat management devices. In contrast to three-dimensional bulk materials, thermal transport in low-dimensions can be quite different. Graphene exhibits extraordinary physical properties, including an extremely high thermal conductivity. A novel graphene-derived two- dimensional crystal with a C2N stoichiometry and evenly distributed holes and nitrogen atoms in its basal plane, nitrogenated holey graphene (NHG), has recently been synthesized. Here we report an investigation of the thermal conductivity of NHG via non-equilibrium molecular dynamics simulations based on the Tersoff interatomic potential. We perform an analysis of finite-size effects and extrapolate our simulation results to estimate the properties of very large NHG sheets. The intrinsic thermal conductivity of NHG at room temperature was found to be 67.23 W/m-K, with an effective phonon mean free path of 16.84 nm. Both quantities are much smaller than the corresponding ones for graphene. Under uniaxial or biaxial strain we observe an increase in thermal conductivity and the corresponding effective phonon mean-free-path, although not as pronounced as in graphene. 

In this work we used the chemical polyol method for the preparation of Sm-Co nanoparticles. The synthesized samples were named in the following forms: ARS-0%; ARS-5%, ARS-7% and ARS-9%, being the percentages referring to the variation of Acetic Acid used in the synthesis. The structural characterization was performed by     X-ray diffraction and transmission electron microscopy (TEM). The structural parameters were determined by the Rietvelt method using the Maud software. The following phases were identified: SmCo7; Sm2CO7; Sm2Co17 and Sm5Co19. For the study of magnetic properties, measurements with the vibrating sample magnetometer were used to obtain magnetization curves as a function of the magnetic field applied at room temperature. The PPMS was used to measure magnetization curves as a function of temperature. The Curie temperatures obtained are in accordance with the phases obtained by the structural characterization methods. Images of the samples were obtained through the SEM and MET for morphological study and determination of the size of the nanoparticles of the samples. Our results show the feasibility of the polyol method to obtain Sm-Co nanoparticles and the influence of Acetic Acid in the synthesis process.

Iron Oxide nanoparticles have been used em several biomedical applications  due to their biocompatibility and biodegradability. Among these applications, magnetic hyperthermia of tumors has been proposed as an alternative treatment of several neoplastic diseases. In this work, we have used high energy ball milling to produce iron oxide  nanoparticles with sizes smaller than 70 nm. We have prepared two groups of samples, the first group is composed of magnetite and α-Fe, the second group is composed of magnetite, wüstite and α-Fe nanoparticles. To improve the sample´s dispersibility in aqueous medium, the samples were functionalized with oleic acid and Pluronic-F127 three-block copolymer. The structural and chemical properties of samples were studied through x-ray diffraction, transmission electron microscopy (TEM) and Mössbauer spectroscopy. The magnetic properties were studied through AC susceptibility and DC magnetization as a function of temperature and field.  From the Mössbauer studies we noticed a significant fraction of Fe2,5+ located in octahedral sites of iron oxide, this result indicates that the samples  has a an stoichiometry similar to pure magnetite. The magnetic measurements showed the Verwey transition at about 120 K. From the TEM images we verified that the wüstite phase is formed on the surface of magnetite nanoparticles. Therefore, we showed that the wüstite phase and oleic acid surfactant prevent the magnetite nanoparticles from further oxidation. The shift of hystereis loops observed in the field-cooled samples was ascribed to exchange interaction between the wüstite and magnetite phases. The AC susceptibility showed characteristic peaks of magnetite wall domains, this result indicated that a fraction of particles are multidomain. The samples were submitted to an AC magnetic field and we observed an increase in temperature of e 11º C and 53º C for samples functionalized and bare, respectively. The combination of magnetic properties, the ability to release heat in presence of an AC field, and the stability of particles in aqueous suspension suggest that these samples are good candidates for magnetic hyperthermia.

The main objective of this thesis is to investigate the competing ordered phases in the metallic heavy fermion compound URu₂Si_2,  which displays a body-centered tetragonal lattice. We first provide a study case of the competition between antiferromagnetic (AF) and spin liquid (SL) phases. The antiferromagnetic state is study with spin-wave theory. Whereas the spin liquid analysis is carry out in an algebraic spin liquid representation. In the second part, we describe an effective theory for Raman scattering experiments at these particularly phases. We provide insight about the hidden order phase displayed by the heavy fermion compound URu₂ Si₂ .

The aim of this work is to synthesize and study crystalline, non-aggregated and monodispersed nanoparticles of three systems containing Ni and/or Cu. The samples were prepared through the sol-gel method assisted by chitosan polymer. The first system is a nanocomposite with average particle size of 9-27 nm. The phases are mainly Ni, α-NiH and β-NiH. The α-NiH phase showed a significant magnetic contribution at temperatures below 50 and the hydrides were chemically stable when stored in air along several months. The second system is an ionic semiconductor of Cu(I)Cl with spherical particles and with average diameter of 9 nm. The third system consists of the solid solution of Ni and Cu atoms, and these samples are a mixing of NixCu(1-x) (0,50<x<0,92) alloys with average particle sizes in the range of 11-90 nm. The transmission electron microscopy images seems to reveal Cu rich alloy phases near to the particles surface, and Ni rich alloys located at the center. Magnetic measurements as a function of temperature in the range of 300-800 K showed evidences of magnetic transitions related to alloys with 75<x<92. For temperatures below Tc of the Ni92Cu8 alloy, it was observed a magnetic signal due to the Hopkinson phase. For T>Tc we noticed a feature related to the Griffiths phase which is due to Ni clusters dispersed in the Cu rich alloy phase. The critical parameters for this phase are in close agreement with the literature. The analysis of the hysteresis curves revealed an enhancement of the magnetocrystalline anisotropy constant (Keff) and this result was ascribed to the development of a surface magnetic anisotropy. The Griffiths phase and the Keff effect seem to be related the Cu rich alloy phase, which may be located at the nanoparticles surface. Experiments in samples subjected to AC magnetic field showed absorption specific rates with increasing values in agreement with the whole Ni content of the samples. The maximum variation of temperatures occurred for intervals of 200 s, indicating that these samples have potential for use in magneto hyperthermia in the treatment of tumors.

Iron niobate (FeNbO4) has been applied to the photodiode in solar energy converters,
gas sensors, photo detectors, and electronic devices. However, a full description of your
dielectric properties is still scarceness so. By using a high energy mill, was prepared three
kinds of samples (24, 48 and 72 hours) of precursors. After milling process, the resultant
ponders were to calcinate for precisely 4 hours and in a temperature of 1 300 celsius.
We used a hydraulic press about 1570 Kgf to produce samples. Next, samples were calcinated during a time considered the same for all of them and underwent to dielectric
characterization. In the way to characterize the samples, the formers were submitted to
X rays, scanning electronic microscopy and X rays uorescence. Results have been shown
that samples milling by 24 hours, has 94.48% of FeNbO4 and 5.52% of Fe2O3, by 48h
has 97.82% of FeNbO4 and 2,18% de Fe2O3, and by 72h 93,68% of FeNbO4 and 6.32% of
Fe2O3. Dielectric properties were analyzed through I-V characteristic curves and complex
dialectic permitivity against frequency. Characteristic curves exhibited semiconductors
electronic devices behave like thyristor class. Furthermore, actual work constitutes a fully
new route to sintering iron niobate. Results still indicated the samples have potential
application in the electronic devices.

Superparamagnetic nanoparticles clusters are currently driving considerable research attention. The interest stems from chances of designing systems with promising potential for technological applications, and from the fundamental viewpoint, tailoring new magnetic phases. The initial magnetic susceptibility and the stray field, at remanence, are key features for the optimization of magnetic systems for biomedical applications. Also, the existence of dipolar ferromagnetism, in the absence of exchange energy, has been one of the focus of magnetism for decades. We report a theoretical discussion of the impact of the dipolar interactions on the magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles, with size ranging from 9nm to 12nm, arranged with uniform density in hundreds nanometer size volumes. We show that the magnetic phases, and the initial susceptibility, are controlled by the dipolar interaction. Also, the topological nanoparticle arrangement, the nanoparticle size, and the packing density, are key features. We show that the dipolar interaction alone may stabilize classical magnetic phases, well known for systems with large content of exchange and anisotropy energies. In addition, we have found that at remanence the nanoparticles clusters magnetic phase have a unique property. The dipolar energy leads to thermal stabilization of the individual nanoparticles moments. Large nanoparticles densities may allow nearly full thermal value of the nanoparticles magnetic moments. Despite this, the nanoparticles cluster is superparamagnetic, with a rather small stray field at remanence, as required for biomedical safety. Nanoparticle clustering in large eccentricity ellipsoidal volumes are promising systems for both low field and large field biomedical applications. For low field applications, there is a large increase in the initial susceptibility, with enhancement in the efficacy of vector targeting and also for hyperthermia absorption rate. For high field applications, the enhancement of the stray is much stronger than that for spherical clusters. Our theoretical model reproduces typical properties of Fe3O4 nanoparticles spherical clusters, as well as intriguing results for Fe and Co quasi-one-dimensional systems.

Many models in cosmology typically assume the standard bulk viscosity. On this thesis, we study an alternative interpretation for the origin of the bulk viscosity. Using nonadditive statistics proposed by Tsallis, we propose a bulk viscosity component that can only exist by a nonextensive effect through the nonextensive/dissipative correspondence (NexDC). We consider the LambdaCDM model for a flat universe with a dissipative nonextensive viscous dark matter component, following the Eckart theory of bulk viscosity, without any perturbative approach. In order to analyse cosmological constraints, we use one of the most recent observations of Type Ia Supernova, baryon acoustic oscillations and cosmic microwave background data. It's shown that only exists viscous effect in 2 sigma confidence level scenarios.

Grandes esforços observacionais têm sido direcionados para investigar a natureza da chamada energia escura. Nesta dissertação derivamos vínculos sobre modelos de energia escura utilizando três diferentes observáveis: medidas da taxa de expansão H(z) (compiladas por Meng et al. em 2015);  módulo de distância de 580 Supernovas do Tipo Ia  (catálogo Union Compilation 2.1, 2011); e as observações do pico de oscilação de bárions (BAO) e a radiação cósmica de fundo (CMB) utilizando a chamada razão CMB/BAO, que relaciona 6 picos de BAO  (um pico determinado através dos dados do Survey 6dFGS, dois através do SDSS e três através do WiggleZ). A análise estatística utilizada foi o método do χ² mínimo (marginalizado ou minimizado sobre h sempre que possível) para vincular os parâmetro cosmológicos: Ω_m, Ω_Λ, ω e δω₀. Esses testes foram aplicados em duas parametrizações do parâmetro ω da equação de estado da energia escura, p=ωρ (aqui, p é a pressão e ρ é a densidade de energia da componente). Numa,  ω é considerado constante e menor que -1/3, conhecido como modelo XCDM; na outra parametrização, o parâmetro da equação de estado varia com o redshift, no qual o chamamos de Modelo GS. Esta última parametrização é baseada em argumentos que surgem da teoria da inflação cosmológica. Para efeitos de comparação também foi feita a análise do modelo ΛCDM. A comparação dos modelos cosmológicos com as diferentes observações leva a diferentes melhores ajustes. Assim, para classificar a viabilidade observacional dos diferentes modelos teóricos, utilizamos dois critérios de informação, ou seja, o critério de informação bayesiana (BIC) e o critério de informação Akaike (AIC). A ferramenta matriz de Fisher foi incorporada aos nossos testes para nos fornecer a incerteza dos parâmetros de cada modelo teórico. Verificamos que a complementariedade dos testes é necessária para não termos espaços paramétricos degenerados. Fazendo o processo de minimização encontramos, dentro da região de 1σ (68%), que para o Modelo XCDM os melhores ajustes dos parâmetros são Ω_m=0,28±0,012 e ω_X=-1,01±0,052. Enquanto que para o Modelo GS os melhores ajustes são Ω_m=0,28±0,011 e δω₀=0,00±0,059. E realizando uma marginalização encontramos, dentro da região de 1σ (68%), que para o Modelo XCDM os melhores ajustes dos parâmetros são Ω_m=0,28±0,012 e ω_X=-1,01±0,052. Enquanto que para o Modelo GS os melhores ajustes são Ω_m=0,28±0,011 e δω₀=0,00±0,059.

Grande tem sido o interesse nas difusões anômalas, pois se apresentam nas mais diversas áreas do conhecimento. A introdução de perfil de memória no caminhante aleatório torna-o numa dinâmica estocástica não-markoviana, cujas correlações criam superdifusão, persistencia e log-periodicidade. Apresentamos uma revisão da literatura sobre os perfis de memória e introduzimos nosso modelo. O modelo de memória binomial pode selecionar diferentes regiões de perda de memória, desde a inicial até a recente. Dessa forma, investigamos o impacto da posição da perda de memória no comportamento superdifusivo do caminhante aleatório e unificamos muitos dos resultados da literatura. Obtivemos que memórias iniciais geram maior superdifusão medidas pelo coeficiente de Hurst, enquanto que memórias recentes tendem a diminuir a superdifusão, tornando mais caminhantes adeptos da difusão normal. Também investigamos o regime de memória curta inicial, com largura tendendo a zero.  Observamos log-periodicidade para alguns caminhantes sugerindo regimes diferentes de comportamento log-periodico, incluindo aqueles considerados de difusão normal. Uma particularidade do modelo binomial são os resutados extremamente simétricos para o diagrama Hxr.

The richness of optical and electronic properties of graphene has attracted enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and stability. Until recently, the main focus has been on fundamental physics and the physics of electronic devices. However, we believe that the full potential of graphene is in photonics and optoelectronic, where the combination of their optical and electronic properties are unique and can be fully exploited even in the absence of an electronic "band gap". In this master thesis we studied the optical transmissivity spectra in periodic dielectric multilayer (photonic crystals) and multilayers who obey quasiperiodic sequences (photonic quasicrystals) composed of graphene and compare our results with the same structures without graphene. Thus, first we calculate the transmittance spectrum photonic crystal formed by alternating layers of dielectric permittivities with  and B, for comparative purposes. In the second stage, we have introduced between the dielectric monolayers a monolayer of graphene. Then we study the photonic Fibonacci’s quasicrystals, with and without graphene entres the dielectric layers, which can be generated by a recurrence relation of the type: Sj + 1 = Sj Sj-1 where S0 = B and S1 = A. In both cases we use the transfer-matrix technique to obtain the transmittance spectra. We have also studied a generalization of the Fibonacci quasicrystal structure called “Octonacci”, where the n-th stage of these multilayer structure (Sn) is given by the recurrence rule: Sn = Sn-1 Sn-2Sn-1, with n>2 to S1=A and S2=B. Finally, for completeness, we study the further generalization of the Fibonacci sequence called Dodecanacci, which can be generated starting from the inflation rule: A-> AABAA and B-> AB. Our results show that all the optical spectra are affected and their "band gap", and slightly translated to high frequencies. We also show that the properties of fractality and self-similarity of the spectra are maintained to high frequencies. Our results show a good insight into new devices that use the quasiperiodic multilayer instead of the well known Bragg reflectors.

Among several aspects related to the Sun's activity history, the extended period of the Sun's evolution with a low chromospheric activity level and a low quantity of observed sunspots compared with other epochs, known as Maunder Minimum, remains as a puzzle for the stellar evolution theory.  In this work we study HD 43587 a solar analog star that is a seismic primary target from the CoRoT mission and presents measurements of activity index along 50 yrs by the Mount Wilson program and outher spectroscopic measurements.  Based on the similarity of HD 43587 with the Sun and by using observations collected by the CoRoT satellite as well as data from the literature, our preliminary analysis (Lithium abundance and chromosferic activity) confirms the evolutionary status of HD 43587. The CoRoT light curve indicates also a flat activity profile, that is a indicative of very low chromospheric activity. All this measurements and analysis make this star an excellent Maunder minimum candidate.

Esta Tese compreende um estudo teórico sobre a influência da anisotropia magnetocristalina nas propriedades magnéticas estáticas e dinâmicas de filmes finos: monocamadas e tricamadas acopladas através dos campos de troca bilinear e biquadrático, para situações nas quais os sistemas são crescidos em direções não usuais [hkl] de baixa simetria. Usando uma teoria baseada em um modelo fenomenológico realístico para descrever sistemas magnéticos nanométricos, consideramos a energia magnética livre total incluindo a interação Zeeman, anisotropias magnetocristalinas cúbica e uniaxial, anisotropias desmagnetizante e de superfície, bem como os termos de troca. Os cálculos numéricos são conduzidos através da minimização da energia magnética total a partir da determinação das configurações estáticas de equilíbrio. Consideramos parâmetros experimentais da literatura para ilustrar nossos resultados em sistemas tipo Fe/Cr/Fe. Em particular, um total de seis diferentes cenários magnéticos é analisado para três grupos de campos de troca e as direções de crescimento de baixa simetria [211] e [321]. Após minimizarmos numericamente a energia total, utilizamos as configurações de equilíbrio para obter curvas de magnetização e de magnetoresistência com as respectivas fases magnéticas e os campos críticos de transições de fases. Estes resultados também são usados para a determinação da fronteira de ocorrência dos estados saturados. Dentro do contexto das ondas de spin, resolvemos a equação de movimento para estes sistemas a fim de encontrarmos as respectivas relações de dispersão associadas. Os resultados mostram curvas de magnetização e magnetoresistência similares em ambos os cenários [211] e [321], com comportamento de transição magnética equivalente. No entanto, a combinação entre simetrias peculiares e a influência da energia de troca resulta em propriedades cativantes, incluindo a geração de estados magnéticos dependentes da direção de crescimento e aumento de incompatibilidade entre valores de campo de saturação da magnetização e magnetoresistência para eixos anisotrópicos cúbicos intermediários, particularmente no regime onde os campos de troca bilinear e biquadrático são comparáveis. As relações de dispersão e os resultados estáticos são coerentes, com as fases magnéticas também presentes em ambos os modos acústico e óptico. Excitações de Goldstone são observadas particularmente nos eixos anisotrópicos cúbicos intermediários, um efeito relacionado às transições de segunda ordem e à quebra espontânea de simetria imposta pela combinação entre o acoplamento biquadrático e as anisotropias cúbica e uniaxial.

Based on the third law of thermodynamics, we question 

whether generalized entropies satisfy this fundamental property. 

In general terms, the third Law states that, for systems with fundamental

 states not degenerate in equilibrium, The entropy approaches zero as the 

temperature (in absolute scale) is also Approaches zero. However, the entropy

 can disappear only with the temperature in the Absolute zero. In this context, we 

propose a direct analytical procedure to test whether Generalized entropy satisfies 

the third law, assuming only a general form of Entropy S and energy U of an arbitrary 

classical N-level system. Mathematically, The method depends on the exact calculation 

of the parameter β = dS / dU in terms of the probabilities of microstates pi. Finally, we 

determine the relation between the Entropy S → 0 (or, more generally, S → Smin) and 

the minimum temperature limit β → ∞. THE Comparison, we apply the Boltzmann-Gibbs 

entropy method (model Standard), Kaniadakis and Tsallis (generalized models). For 

the last two, we illustrate the Power of the method by calculating the intervals of the 

entropic parameters in which the entropy Satisfies the third law. The results obtained

 showed that, for the κ-entropy, the values Usually assigned to the parameter κ satisfy 

the third law (-1 <κ <1). However, For q-entropy the same does not occur. We have 

shown that q-entropy can disappear Temperatures of zero for certain values of q. As a 

concrete example, we consider the one-dimensional Ising model with interactions of first

 neighbors, which Is one of the most important models in all physics. Classically, the Ising

 model is Resolved through the canonical ensemble, 

but it can also be solved through Of generalized ensembles.

The interest in studying the objects similar to the Sun, stars labeled as solar-type stars,
analogs and solar twins, brings in its essence an attempt to find out another reference
star and, furthermore, provides an investigation of evolutionary dynamic of our star as
a function of various parameters. For this, we used three distinct samples of observable
data, 170 solar-type stars from BCool catalog and observed with spectropolarimeters
ESPaDOnS e NARVAL, 88 solar-twin stars of HARPS surveys, and 20 solar-analog stars
from Kepler.
From these data, we have investigated mainly the correlation among the rotation
period, lithium abundance and stellar age. For the BCool stars and solar-twin from
HARPS, we have used the rotation period determined through of chromospheric activity,
in the case of Kepler solar analogs, the rotation period it is derived from photometric
modulation. The lithium abundance for most of the solar-type and solar-twin stars have
been collected from literature, while for the solar analogs, the lithium abundance were
determined in the LTE regime using Kurucz atmospheric models and the MOOG code.
For stellar age, we have used the gyrochronology method, which was calibrated using the
Sun and a selection of open clusters, to redetermine them and comparing them with those
derived from standard isochronal.
Our results indicate that exist a decay law for the rotation period as a function of
lithium abundance. This correlation becomes more clear for the solar-analog and solar-
twin stars, even the rotation period being determined through distinct mechanisms for
each case. For stellar ages, measured from standard isochronal and gyrochronology, we
realized that they diverge considerably when the stars are older than the Sun. This result
v
has also been investigated by van Saders et al. (2016) and reflect our limitation about the
stellar evolution and mixing mechanisms.
Our work has resulted in five publications in indexed journals, two already in print
format, one recently submitted and other in final stage of conclusion.

After the pioneering discovery of a giant planet orbiting 51 Peg by Mayor $\&$ Queloz (1995), about two decades ago, the literature reports the discovery of more than 3434 confirmed planets (exoplanet.eu), in about 2568 planetary systems. Solar mass main sequence field stars host the vast majority of these exoplanets. The observation of these stars offers several advantages, including brightness and a large variety of stellar characteristics, such as mass, age, chemical composition and evolutionary status. However, the widely differing characteristics of field stars also represents a drawback for our capability to derive precise conclusions to very basic questions, including the role of stellar environment on planet formation. There is no clear answer for the fact that main-sequence stars hosting giant planets are metal rich (Gonzalez 1997; Santos et al. 2004), while evolved stars hosting giant planets are not (Pasquini et al. 2007). Indeed, different phenomena have been proposed to explain this discrepancy in metallicity, including stellar pollution acting on main-sequence stars (Laughlin $\&$ Adams 1997, e.g.), a planet formation mechanism favouring the birth of planets around metal rich stars (Pollack al. 1996) and the stellar environment (Haywood 2009). The observation of stars in open cluster offers the possibility to strictly control the stellar characteristics, because each cluster represents a homogeneous set of stars. Besides, open cluster stars were formed at the same time and in the same circumstances and thus are expected to have the same age, metallicity, and galactocentric distance. From the work of Mermilliod $\&$ Mayor 2008 we choose clusters harbouring giants stars to be included in our survey. We used the WEBDA cluster database (Mermilliod 1995) to get information about our sample. The main criteria we focused on were the age of the cluster (between 0.02 and a few Gyr, with TO masses $>$ 1,5 M$_{\bigodot}$) and the magnitude of its giant stars (brighter than V = 13.5). Then we rejected stars with colour index (B - V) larger than 1.4, because cool, bright giants are known to be RV unstable. The observations were performed using HARPS (Mayor et al. 2003), the planet hunter at the ESO 3.6 m telescope. In high accuracy mode (HAM), it has an aperture on the sky of one arcsecond, and a resolving power of 115000. The spectral range covered is 380 - 680 nm. Our spectral analysis is based on the MARCS models of atmospheres and Turbospectrum spectroscopic tool. We determined stellar parameters and metallicity from LTE analysis of Fe I and Fe II lines. Once we get the high resolution and high S/N spectra, we also computed Li abundances that was obtained using the line at 6707.78 {\AA}, Si I, Na I, Mg I, Al I, Ca I, Ti I, Co I, Ni I, Zr I, La II and Cr I. We presented a spectroscopic characterisation of 42 giants, in 12 open clusters, using high resolution spectroscopy. All these clusters are part of a survey for giant planets orbiting intermediate-mass giant stars and the results show that all the clusters studied have $[Fe/H]$ values close to solar, results that agree with the literature with a small dispersion. These abundances will enable us to perform a comparative analysis of the abundances of stars with and without planets, from which it will be possible to detect differences, anomalies and determine the level of planet-star interactions. The goal of this campaign is to study the formation of giant planets in OCs to understand whether a different environment might affect the planet formation process, the frequency, and the evolution of planetary systems with respect to field stars. In addition, searching for planets in OCs enables us to study the dependency of planet formation on stellar mass and to compare the chemical composition of stars with and without planets in detail.

A crucial problem in the study of anomalous diffusion and transport refers to adequate analysis of trajectory data. The analysis and inference of Lévy walk model from empirical or simulated trajectories of particles in two and three-dimensions (2D and 3D) is much more hard than in 1D because path curvature is nonexistent in 1D but pretty common in higher dimensions. Lately, a new method to detect Lévy walks, which considers 1D projections of 2D or 3D trajectory data, has been proposed by Humphries et al. The main idea of this method is to explore the fact that a 1D projection of a high-dimensional Lévy walk is itself a Lévy walk. In this work, we ask whether or not this projection method is capable enough to clearly distinguish a 2D Lévy walk with curvature from a simple Markovian correlated random walk. We focus this work in challenging case in which both 2D walks have the same probability density functions (pdf) of step sizes as well as of turning angles between succesive steps. Our approach extends the original projection the original projection method by introducing a rescaling of the projected data. After a projection and coarse graining, the renormalized pdf for the travel distances between successive turnings is seen to possess a fat tail when there is an underlying Lévy process. We exploit this effect to infer a Lévy walk process in the original high-dimensional curved trajectory. In contrast, there is no fat tail when a (Markovian) is analyzed. We show that this procedure works very well in clearly identifying a Lévy walk even when there is noise from curvature. The present protocol may be useful in realistic contexts involving ongoing debates on the presence (or not) of Lévy walks related to animal movement on land (2D) and air and oceans (3D).