
Dissertations 

1


JONATHAN PESSOA CORREIA

Size distribution of human DNA encoding via information theory

Advisor : RAIMUNDO SILVA JUNIOR

COMMITTEE MEMBERS :

RAIMUNDO SILVA JUNIOR

DORY HELIO AIRES DE LIMA ANSELMO

ANTONIO DE MACEDO FILHO

Data: Feb 12, 2021


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We analyze the coding sequence for the Homo Sapiens via a model that naturally embraces correlations among the bases in DNA sequences of living organisms. The model is based on the Shannon entropy's optimization, which is the core of all statistical arguments. As a result, we propose the doubleexponential law distribution function of the length of DNA measured in base pairs (bp). The results show that the ShortRangeCorrelations (SRC), always present in coding DNA sequences, are appropriately captured through the doubleexponential distribution and adequately describes the cumulative length distribution of DNA bases. Based on this model, we use an Empirical cumulative distribution function and the database of proteins compiled by the Ensembl Project to show consistency with the data.


2


LUCAS RODRIGUES DELGADO DE FREITAS

Gapless excitations in Kitaev materials with defects

Advisor : RODRIGO GONCALVES PEREIRA

COMMITTEE MEMBERS :

EDUARDO MIRANDA

LEONARDO DANTAS MACHADO

RODRIGO GONCALVES PEREIRA

Data: Mar 26, 2021


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A quantum spin liquid is a magnetic phase of matter remarkable for its ground state longrange entanglement and fractional excitations. A quantum spin liquid appears in the wellknown Kitaev honeycomb spinhalf 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 powerlaw dependence of the spinlattice 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 meanfield 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 lowenergy theory to calculate the spinlattice relaxation rate and found a behavior that agrees with the experimental result.


3


LEONARDO DE QUEIROZ DANTAS

Magnetic Polaritons in Gyromagnetic Medium with Graphene Layer

Advisor : DORY HELIO AIRES DE LIMA ANSELMO

COMMITTEE MEMBERS :

DORY HELIO AIRES DE LIMA ANSELMO

MANOEL SILVA DE VASCONCELOS

NILSON SENA DE ALMEIDA

Data: Apr 9, 2021


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In this work we present the theoretical study on magnetic polaritons (magnonpolaritons) in a system formed by a semiinfinite gyromagnetic medium (ferromagnetic or antiferromagnetic), with a layer of graphene at the interface between this medium and the vacuum. Thus, the analytical and numerical calculations were reviewed for cases in which a static magnetic field is applied parallel to the interface, and we propose a new case, in which the magnetic field is applied perpendicularly to the interface between the media. The results found show that, for the case of perpendicular magnetic field, there was the formation of magnetic surface polariton modes, characterizing the photonmagnon coupling. In this case, the numerical results showed that these modes propagate reciprocally, that is, independent of the direction of the wave vector for both magnetic media (ferromagnetic and antiferromagnetic). In contrast, bulk modes were not found, characterizing an independent behavior between photon and magnon in the gyromagnetic bulk. The inclusion of the graphene layer also made it possible to control the surface modes by varying the parameters existing in the optical conductivity in graphene, such as the chemical potential and the scattering rate.


4


GABRIEL MORENO CUNHA

Ephaptic Coupling in Hybrid Neuronal Model

Advisor : GUSTAVO ZAMPIER DOS SANTOS LIMA

COMMITTEE MEMBERS :

GILBERTO CORSO

GUSTAVO ZAMPIER DOS SANTOS LIMA

MADRAS VISWANATHAN GANDHI MOHAN

MARCELO DE MEIRA SANTOS LIMA

SERGIO R. LOPES

Data: Jul 26, 2021


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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 IntegrateandFire 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 IntegrateeFire 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.


5


CAMILA PEREIRA RAMOS

Abundances from the Boltzmann Equation

Advisor : FARINALDO DA SILVA QUEIROZ

COMMITTEE MEMBERS :

AION VIANA

ALEX GOMES DIAS

CLARISSA MARTINS SIQUEIRA

FARINALDO DA SILVA QUEIROZ

RODRIGO FERNANDES LIRA DE HOLANDA

Data: Jul 30, 2021


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Observational data that support the existence of dark matter (DM) has been largely documented, pointing to the presence of a nonbaryonic matter that is responsible for around 25% of the energy budget of the Universe. However, such type of matter has not been detected so far, and the standard model of particle physics (SM) does not have an explanation for DM. In this work, the weakly interacting massive particles (WIMPs), the main candidates for DM, are presented. It is then the main goal to show that this DM candidate generates the abundance required to obtain the estimated amount of DM in the Universe. This result is obtained by solving the Boltzmann equation, in the early Universe scenario.


6


HELCIO FELIPPE JUNIOR

Pearson matrices as density operators: A test of the entropic brain hypothesis using the von Neumann entropy

Advisor : MADRAS VISWANATHAN GANDHI MOHAN

COMMITTEE MEMBERS :

ALINE AMABILE VIOL BARBOSA

DRAULIO BARROS DE ARAUJO

ERNESTO CARNEIRO PESSOA RAPOSO

FERNANDA PALHANO XAVIER DE FONTES

MADRAS VISWANATHAN GANDHI MOHAN

Data: Jul 30, 2021


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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.


7


LUÍS FELIPE MARTINS DE ALMEIDA MORENA REIS

Emission of Gravitational Waves in Quadratic Theories of Gravity

Advisor : LEO GOUVEA MEDEIROS

COMMITTEE MEMBERS :

LEO GOUVEA MEDEIROS

RICCARDO STURANI

RODRIGO ROCHA CUZINATTO

Data: Sep 23, 2021


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Despite the unquestionable success of General Relativity (GR) in being able to describe and predict several gravitational phenomena, this theory still has some internal issues of interest for cosmology and high energy physics, e.g. singularities, dark energy, cosmology of the early universe, quantum gravity, etc. Extended theories of gravity, which consist of modifications in GR, have been shown to be a possible solution to these problems. Therefore, in this work we will study a quadratic theory of gravity, where the terms 𝛼𝑅 2 and 𝛾𝑅 𝜈𝜉 𝑅 𝜈𝜉 are added to the EinsteinHilbert action with 𝛼 and 𝛾 being dimensionless coupling constants. This theory is usually present\ in several works being linked to the renormalization issues of gravity. In this work we will study the main aspects of this quadratic theory, focusing mainly on the properties of new fields that manifest themselves in the theory through the solution of linearized field equations. Thus, we will analyze the role that these fields play in the orbital evolution of a point mass system in a circular orbit due to the emission of gravitational waves.



Thesis 

1


SERGIO LUIZ EDUARDO FERREIRA DA SILVA

FullWaveform Inversion Based on Generalized Statistical Mechanics

Advisor : GILBERTO CORSO

COMMITTEE MEMBERS :

GILBERTO CORSO

JOAO MEDEIROS DE ARAUJO

RAIMUNDO SILVA JUNIOR

JORGE LUIS LOPEZ

WAGNER MOREIRA LUPINACCI

Data: Jan 29, 2021


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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 BoltzmannGibbs statistical mechanics. In this approach, errors are assumed to be distributed according to a Gaussian distribution. However, in practice, especially in nonlinear 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 nonGaussian 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 nonGaussian statistics. In particular, the generalized statistics in the sense of Tsallis and Kaniadakis are considered to solve a challenging problem of seismic inversion, called, FullWaveform 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 presalt 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.


2


ASNAKEW BEWKETU BELETE

The variability properties of active galactic nuclei and their intractions with the host galaxies

Advisor : JOSE RENAN DE MEDEIROS

COMMITTEE MEMBERS :

JOSE RENAN DE MEDEIROS

BRUNO LEONARDO CANTO MARTINS

IZAN DE CASTRO LEÃO

JAILSON SOUZA DE ALCANIZ

ZULEMA ABRAHAM

Data: Jan 29, 2021


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Active galactic nuclei (AGNs), a compact regions that consist of a supermassive black hole surrounded by an accretion disk in the center of massive galaxies, have long been known in their unique observational properties. Strong variability has been identified as one of those properties that can be used to probe the interior of the system. Also, most astrophysical objects, like AGNs, are possibly associated with continuous nonlinear stochastic systems due to their complexity in nature. It has been indicated that quasars, a subclass of AGN, are among complex systems that have nonlinear time series characterized by fractal behavior and also by sudden bursts of very large amplitude, implying that the dynamical evolution of quasars is nonlinear (i.e., described by nonlinear stochastic differential equations). Addi tionally, there is a suggestion that extragalactic radio sources are intermittent on timescales of ∼ 10 4  10 5 yr. Despite the complex nature of AGNs, astronomers have been trying to investigate the variability properties of AGNs using linear tech niques, such as structure function, periodogram, Fourier analysis, etc. However, most valuable information in a complex system signal is contained by its irregu lar structures and transient phenomena called singularities. Thus, particularly in physics, it is important to analyze irregular structures in a signal to deduce proper ties about the underlying physical phenomena. Moreover, among other evidences, the recently discovered tight correlation between supermassive black hole mass and the host galaxy bulge properties revealed the fact that AGNs are not isolated objects they interact and coevolve with their host galaxies. The interaction mechanisms are thought to be through AGN feedback and feeding, and have been known to af fect both the observational properties of AGNs, and the properties and kinematics of the gas in the surrounding environment, thereby regulating AGN host galaxy coevolution. Thus, understanding the types of variabilities along with their driv ing physical mechanisms and the physics behind the AGNhost galaxy interaction mechanisms(AGNfeedbackandfeeding)providesapotentiallysignificantinforma tion to constrain the nature of AGNs and get a full picture to construct a universal model to study the coevolution of the two physically connected systems. The AGN host galaxy interaction can easily be studied by modelling the kinematics of gas (in all form) in the nuclear/circumnuclear regions of the host galaxy. In this thesis I studied the nonlinear (multifractal) signature printed in the long term light curves of different classes of AGNs and its correlation with different AGN and other observational parameters, and whether multifractality is an intrinsic be haviour of AGNs and it can be affected by extrinsic variations (e.g., gravitational lensing). I applied the onedimensional multifractal detrended moving average and the wavelet transform modulus maxima approachs to characterize the nonlinearity presented in the light curves. In addition, I also present the analysis of the kinemat ics of CO molecular gas in the nuclear and circumnuclear regions of three nearby Seyfert galaxies, NGC 4968, NGC 4845, and MCG063015, by using their bright CO(21) emission line as a tracer. I used two different softwares, the 3DBased Anal ysis of Rotating Object via Line Observations (3 D BAROLO) and DiskFit, to model the kinematics of cold CO molecular gas in these three galaxies. I found that AGNs are strongly nonlinear and intermitent systems, and nonlinearity is an intrinsic behaviour of AGNs (i.e., the degree of nonlinearity does not change with cosmological redshift), indicating that AGNs are really complex systems, and the information contained in their light curves cannot be fully captured by linear analysis techniques. The nonlinear behaviour of AGNs is found to be affected by extrinsic variation (gravitational lensing). Moreover, the degree of nonlinearity is found to be inversly correlated with wavelength, the shorter the wavelength the vii stronger the nonlinearity, and strongly correlated with fractaional variability ampli tude. I did not find any significant correlation between the degree of nonlinearity and AGN parameters such as black hole mass, the size of the BroadLine Region (BLR), and continuum luminostiy. However, an important correlation is observed between the degree of nonlinearity and the Eddington ratio. I also found that re processing in the BLR introduces additional nonlinearity, i.e., the reprocessing in the BLR increases the nonlinearity of the ionizing continuum. Circular motion is found to be the dominant kinematics of the CO molecular gas in the inner molecular disk of the three nearby Seyfert galaxies, NGC 4968, NGC 4845 and MCG063015, mainly in NGC 4845 and MCG063015. However, there is a clear evidence for the presence of noncircular motions in the central regions of NGC 4968 and NGC 4845. The strongest noncircular motion is detected in the nu clear region of NGC 4968, mainly in the minor kinematic axis, with velocity ∼ 115 km s − 1 . For NGC 4968, the Ω b pattern speed for the bar put corotation at 3.5 kpc and inner Lindblad resonance (ILR) ring at R = 300pc, which corresponds to the CO ring. Moreover, of all DiskFit models, bisymmetric model is found to be the best fit model, indicating that the observed noncircular motions, mainly in NGC 4968, could be due the molecular gas in the disk of the galaxy being perturbed by the nu clear barred structure. Considering the COtoH 2 conversion factor α in the interval [0.8, 3.2], the molecular mass in the molecular disk of each galaxy is estimated to be ∼ 2.8 × 10 9 (α= 0.8) and ∼ 11 × 10 9 (α=3.2) for NGC 4968, ∼ 4 × 10 9 (α= 0.8) and ∼ 16 × 10 9 (α= 3.2) for NGC 4845, and ∼ 0.3 × 10 9 (α= 0.8) and ∼ 1.2 × 10 9 (α= 3.2) for MCG063015. For the galaxy NGC 4968, the computation of the torques exerted by the stellar bar on the gas are positive inside the molecular ring, and negative inside, revealing that the gas is accumulating in the inner Lindblad resonance.


3


LESSANDRO JORGE DE LIMA

Computational Models for Thermoelectric Effects Studies in Thin Films and the Medium Range Potts Model (q = 3)

Advisor : JOAO MEDEIROS DE ARAUJO

COMMITTEE MEMBERS :

FELIPE BOHN

JOAO MEDEIROS DE ARAUJO

LÚCIO STRAZZABOSCO DORNELES

MADRAS VISWANATHAN GANDHI MOHAN

MARCIO ASSOLIN CORREA

SERGIO ANDRÉ FONTES AZEVEDO

Data: Feb 25, 2021


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In this thesis we present the results of two works in computational physics on the thermoelectric effects in thin films and the mediumrange 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 StonerWohlfarth model by the association and experimental parameters used in the longitudinal spin Seebeck effect experiment. In the second, we seek to find the socalled X and Y exponents, for the mediumrange 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 mediumrange 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).


4


LEANDRO DE ALMEIDA

Using Gravitational Microlensing to search lowmass exoplanets

Advisor : JOSE DIAS DO NASCIMENTO JUNIOR

COMMITTEE MEMBERS :

EDER MARTIOLI

FRANCISCO JOSÉ JABLONSKI

JEFFERSON SOARES DA COSTA

JOAO RODRIGO SOUZA LEAO

JOSE DIAS DO NASCIMENTO JUNIOR

Data: Mar 18, 2021


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Gravitational lenses were proposed by Albert Einstein in the course of developing the theory of general relativity. Einstein observed that massive objects curve spacetime and thus the path of light that passes close to these objects undergo deflection effects and the light is then deflected in these vicinities. The gravitational lens regime can be divided into three categories: strong lenses, weak lenses, and microlensing. This thesis is focused on the latter, the gravitational microlensing. Gravitational Microlensing (GM) events happen when, from the observer's perspective, there is an alignment between two stars, one in the background (source) and another in the foreground (lens). This alignment causes the light from the source to deviate from its original path. This deviation of light generates a magnification of the brightness of the background star and if the two stars have relative movements, a characteristic light curve is then produced. If the lens star has a planet, we can infer its presence by carefully analyzing this light curve and determining the mass fractions of the system and also the apparent major semiaxis. This technique differs from the others because it is the only one that can detect small planets beyond the snow line of its stars. These planets are essential for filling the census of exoplanets that we have in our galaxy. In this work, we focus on finding lowmass planets in solartype stars. For this, we use the semianalytical method of solving the lens equation and we propose the parameterization of the shortest distance between the path of the source star and the main lens (impact parameter $\mu_0 $) and the angle that the source path makes with the relative axis of the system (angle of impact $\alpha$). This parameterization forces the source path to pass through the region of interest of the event in which it would be possible to detect low mass planets, increasing the probability of detecting planets with the mass of the Earth. We applied our parameterization in GM events available in the literature and also in light curves of events observed at the Pico dos Dias Observatory (OPD) in the observational campaigns of 2018, 2019, and 2020, which were part of this doctorate together with the MicroFun collaboration from "Ohio State University ". In this thesis, we present a detailed discussion of planetary caustic in GM events and the effects on the formation of light curves in an event involving a planet with a mass similar to that of Earth. We present in detail the methodology for photometric observation of GM events monitored at the OPD. We also present a possible detection of 3 candidates for exoplanets, one of them using data observed in the OPD during this doctorate. In this work, we analyze the topology of GM events that produce light curves in which the detection of planets with a mass similar to that of Earth is possible.


5


GABRIEL CAMPELO DE MELO FERRAZ

Spin Chain Network Construction of Chiral Spin Liquids

Advisor : RODRIGO GONCALVES PEREIRA

COMMITTEE MEMBERS :

AMIR ORDACGI CALDEIRA

MARIANA MALARD SALES

RAFAEL CHAVES SOUTO ARAUJO

RODRIGO GONCALVES PEREIRA

TOMMASO MACRI

Data: Mar 26, 2021


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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 KalmeyerLaughlin 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 tightbinding 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.


6


MÁRCIO ASSUNÇÃO TEIXEIRA

SIMUTOOLS: A specter simulation tool for NIRPS

Advisor : BRUNO LEONARDO CANTO MARTINS

COMMITTEE MEMBERS :

BRUNO LEONARDO CANTO MARTINS

EDUARDO JANOT PACHECO

HELOISA MARIA BOECHATROBERTY

IZAN DE CASTRO LEÃO

JOSE RENAN DE MEDEIROS

Data: Apr 7, 2021


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With the possibility of using the variation in radial velocity to find exoplanets, that has begun with the discovery of 51 Pegasi b, the exoplanet scene has been a target of great interest. From this pioneer discovery up to now, various spectrometers were and have been built with the purpose of searching for exoplanets. In this context, NIRPS (Near InfraRed Planet Searcher) is included, a spectrometer which will be mounted jointly to HARPS (High Accuracy Radial velocity Planet Searcher), in Chile, and will operate in the near infrared range, with the goal of searching for explanets orbiting low mass stars. In this work, we present a program developed for NIRPS, SIMUTOOLS, which will simulate the specter that reachs the detector (H4RG). These simulations expect to: anticipate any particularity that may happen to the instrument, or the detector, so that it may be corrected beforehand, prior to NIRPS’ functioning; and also test and calibrate the data reduction software (DRS). This code was successful in its simulations, presenting the particularities of each type of observation, showing intrinsic effects due to the instrument, the difference between high accuracy and high efficiency modes and, lastly, the simulation of specters from different M stars, in which we may also realize the influence from atmospheric absorption in the observation.


7


DESIANE MAIARA GOMES DOS SANTOS

Scalefree behavior in hailstone sequences generated by the Collatz map

Advisor : MADRAS VISWANATHAN GANDHI MOHAN

COMMITTEE MEMBERS :

DORY HELIO AIRES DE LIMA ANSELMO

ERNESTO CARNEIRO PESSOA RAPOSO

MADRAS VISWANATHAN GANDHI MOHAN

MARCIO ASSOLIN CORREA

MARCOS GOMES ELEUTERIO DA LUZ

RICARDO VIANA

Data: Apr 23, 2021


Show Abstract

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 scaleinvariant 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 scalefree power law for the Collatz map. We analytically derive the scaling exponents, displaying excellent agreement with the numerical estimations. The scalefree 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 $log[n]$.


8


MARCUS VINÍCIUS PINHEIRO LOPES

Thermoelectric effects in Co2FeAl/W bilayers with uniaxial and cubic anisotropies

Advisor : MARCIO ASSOLIN CORREA

COMMITTEE MEMBERS :

THIAGO JOSÉ DE ALMEIDA MORI

FELIPE BOHN

MARCIO ASSOLIN CORREA

MAURICIO ROBERTO BOMIO DELMONTE

MÁRCIO MEDEIROS SOARES

Data: Apr 26, 2021


Show Abstract

The conversion of temperature gradients applied to magnetic materials has been the subject of study for its importance in the possibility of building devices that take advantage of it. The application of these temperature variations generate electrical responses due to the production of spin and charge currents that gave rise to an area of spintronics, the spin caloritronics. From these currents it is possible to measure the thermoelectric voltage in magnetic materials, produced by the direct conversion of the temperature gradient into electrical voltage. In this work, this thermovoltage is mainly associated with two phenomena, the Seebeck spin (SSE) and anomalous Nernst (ANE) effects. Both are investigated in samples of Co2FeAl/W (CFA/W) grown on silicon (Si) and gallium arsenide (GaAs) substrates. The samples were prepared by magnetron sputtering with the substrate under to distinct temperature conditions during deposition, from 300 K up to 773 K. Here, the bilayers were deposited onto oriented substrates in order to induce magnetocrystalline anisotropy, thus allowing the obtainment of systems with an overlap of anisotropies such as uniaxial and cubic. The samples were characterized from the structural and magnetic point of views. Specifically, the structural characterization was performed by Xray diffractometry (XRD), while the quasistatic magnetic behavior was obtained through magnetization curves measured using a Vibrating Sample Magnetometer (VSM). Thermoelectric response was explored via longitudinal Spin Seebeck effect (LSSE). In this configuration, the temperature gradient is normal to the plane of the film, and the external magnetic field lies in the plane. Then, the thermoelectric voltage is measured with electrical contacts fixed in the film plane. Computer simulations were also carried out using the adapted StonerWohlfarth model, which together with the experimental data show that the thermoelectric response of the samples is the result of the interaction between the uniaxial and cubic contributions of magnetocrystalline anisotropy. The results place CFA grown onto GaAs substrate as a promising candidate for active elements in spintronic devices.


9


REBECCA DE MOURA BRAZ DINIZ

Continuous Phase Transition in a Diluted Block Voter Model with Sociophysical and Biophysical Applications

Advisor : JOAO MEDEIROS DE ARAUJO

COMMITTEE MEMBERS :

ALEXANDRE DA SILVA ROSAS

CLAUDIONOR GOMES BEZERRA

JOAO MEDEIROS DE ARAUJO

MADRAS VISWANATHAN GANDHI MOHAN

RAIMUNDO NOGUEIRA DA COSTA FILHO

Data: May 26, 2021


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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 meanfield 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.


10


ACÁCIO SILVEIRA DE MELO

Anomalous Nernst Effect in magnetostrictive films on flexible substrate

Advisor : MARCIO ASSOLIN CORREA

COMMITTEE MEMBERS :

CLAUDIONOR GOMES BEZERRA

FELIPE BOHN

JOAO PAULO SINNECKER

JULIAN PENKOV GESHEV

MARCIO ASSOLIN CORREA

MATHEUS GAMINO GOMES

Data: Jun 4, 2021


Show Abstract

The anomalous Nersnt effect (ANE) in nanostructured magnetic materials is a key phenomenon for optimizing control and using internal energy in electronic devices. In this work, we will explore the magnetic properties and ANE in a flexible magnetostrictive film with uniaxial anisotropy induced and subjected to mechanical stress, in addition, we will promote an integration between the ANE effect and the magnetostrictive phenomena, which in turn, in_uences the effective anisotropy of Films. Therefore, a theoretical study of the behavior of magnetization in this system will be presented, so that we can understand how it will influence the thermoelectric voltage response. At the same time, we will integrate this study with the experimental data obtained, in this way we can manipulate the ANE through the magnetic field and the applied mechanical stress. Theoretically, we calculate the magnetization behavior and the thermoelectric voltage response from the theoretical approach to planar geometry, with magnetic free energy density that takes into account the induced contributions of uniaxial and magnetoelastic anisotropy. Experimentally, we verified changes in the effective magnetic anisotropy, changing the applied stress, and explored the anomalous Nernst effect, as a powerful tool to investigate the magnetic properties of magnetostrictive materials. We found agreement between the experiment and the numerical calculations, thus elucidating the magnetic behavior and the thermoelectric voltage response. Finally, evidence was provided to confirm the validity of the theoretical approach to describe the magnetic properties and the anomalous Nernst effect in ferromagnetic magnetostrictive films with uniaxial magnetic anisotropy and subjected to external stresses.


11


ANA CAROLINA MATTIUCI FIGUEIREDO

A study of the binary systems rotation and their relationship with stellar and orbital parameters

Advisor : JOSE DIAS DO NASCIMENTO JUNIOR

COMMITTEE MEMBERS :

GABRIEL ARMANDO PELLEGATTI FRANCO

JOSE DIAS DO NASCIMENTO JUNIOR

MARCELO EMÍLIO

MATTHIEU SEBASTIEN CASTRO

SILVIA HELENA PAIXAO ALENCAR

Data: Jun 10, 2021


Show Abstract

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 TaurusAuriga 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.


12


RAFAEL RAMON FERREIRA

SPECTROSCOPIC STUDY OF CHROMOSPHERIC ACTIVITY IN SOLAR TYPE STARS: The search for characterizing analogs stars in Maunder minimum

Advisor : MATTHIEU SEBASTIEN CASTRO

COMMITTEE MEMBERS :

ADRIANA BENETTI MARQUES VALIO

GUSTAVO FREDERICO PORTO DE MELLO

JOSE DIAS DO NASCIMENTO JUNIOR

MATTHIEU SEBASTIEN CASTRO

SILVIA HELENA PAIXAO ALENCAR

Data: Jun 11, 2021


Show Abstract

Since the first observations of sunspots made by Thomas Harriot and subsequently by Galileo and several other scientists, astronomers looking for methods to better understand the magnetic phenomena in the Sun and in solartype stars. It is well established that the first indicators used to characterize magnetic activity were sunspots. It is also worth highlighting one of the most intriguing phenomena of the evolution of solar magnetism known as Maunder's minimum, which occurred specifically between 1645 and 1715. What makes this minimum of activity is enigmatic is the registration of very few sunspots and practically no variability in the area. over 70 years, remaining to this day without a consolidated explanation. In the twentieth century, with the advancement of observational techniques related to spectroscopy, astronomers began to make observations of solar/stellar activity also through the spectra. The solar/stellar chromospheric activity, has been the subject of exhaustive treatment over the last six decades in the literature. The main spectroscopic indicators come from the Ca II H\&K resonant lines, particularly the indicators S_{MW} and the index with photosphere correction R_{HK} are the main ones. Spectroscopic measurements of the activity of solartype stars have been carried out since 1960, in an attempt to answer the question of how unique the magnetic evolution of the Sun is in relation to other solartype stars. All this background has allowed to determine that stars of the solar type have activity cycles, in the case of the Sun a primary cycle of 11 years is well established and a possible secondary cycle of 88 years. In our work we obtained as main results: 1) The characterization and determination of the activity cycle of the solar analog HD43587, as well as estimates of its rotational period and characterization of the magnetic field; 2) Development of theoretical tools to estimate the observations of stellar spots globally in other stars of the solar type; 3) Analysis and determination of the activity cycle of 30 stars of the solar type indicated in the literature as in the state of Maunder Minimum; 4) Calibration of activity indexes based on the Ca II H&K lines using the Coudé spectrograph located at the Pico dos Dias Observatory.


13


ANDRÉ CHAVES DE BRITO

About Medium Infrared Excess in Binary Spectroscopic Systems

Advisor : BRUNO LEONARDO CANTO MARTINS

COMMITTEE MEMBERS :

BRUNO LEONARDO CANTO MARTINS

EDUARDO JANOT PACHECO

IZAN DE CASTRO LEÃO

JOSE RENAN DE MEDEIROS

MARCOS PEREZ DIAZ

Data: Jun 30, 2021


Show Abstract

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 BTSettl 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 nonevolved binary spectroscopic
systems, belonging and open agglomerates, can house stable circumbinary debris discs.


14


EVERSON FRAZÃO DA SILVA

Absorption in Photonic Quasicrystals

Advisor : MANOEL SILVA DE VASCONCELOS

COMMITTEE MEMBERS :

DORY HELIO AIRES DE LIMA ANSELMO

EUDENILSON LINS DE ALBUQUERQUE

JOAO MILTON PEREIRA JUNIOR

MANOEL SILVA DE VASCONCELOS

RAIMUNDO NOGUEIRA DA COSTA FILHO

Data: Jul 27, 2021


Show Abstract

After Yablonovitch's pioneering work (in 1987) on photonic crystals, there was a growing interest in studies on the propagation of electromagnetic waves in periodic multilayers, mainly because of the possibility of guiding, allowing or prohibiting the propagation of light in certain frequency bands with the use of band gaps. On the other hand, after Kohmoto's pioneering work (also in 1987), the study of photonic quasicrystals has been receiving a lot of attention. Photonic quasicrystals exhibit long range order, nontranslational symmetry and a characteristic in the autosimilarity optical spectra, a very interesting property in fractal studies. In this doctoral thesis, we have study absorption and optical transmission spectra in multilayers, by using the theory of photonic quasicrystals. The technique that we use for the calculation of the optical spectrum is the technique of the transfer matrix. The photonic quasicrystals we studied are formed by the sequences of Octonacci and Dodecanacci. Also, Graphenes are materials that have received a lot of attention for their optoelectronic properties, even in the absence of a band gap. In this thesis we use graphene at the interfaces between the layers to obtain the spectra and compare the results with the case without Graphenes. We also investigated the effect of the chemical potential for the control of band gaps in these structures, as well as the influence of the oblique incidence. The study of coherent perfect absorption has been quite highlighted also in recent years, mainly for several technological applications, such as modulators, Bragg reflectors, plasmon structures, among others. On the other hand, Hermitian systems, previously present only in quantum mechanics, had their studies expanded to acoustic and electronic systems. NonHermitian systems had their equivalent in optical systems where what we call paritytime symmetry (PT symmetry) occurs, where the refractive indices of the material satisfy the condition n (x) = n * ( x). In this thesis, we also calculated the transmission spectrum in a photonic crystal with PT symmetry, which are formed by quasiperiodic sequences well known in the literature, namely: Fibonacci, Thue Morse and Double Period. In our results, we have shown the existence of high transmission peaks as a result of the definition of PT symmetry in photonic quasi crystals.


15


ALINE NASCIMENTO LINS

Short distance tests of General Relativity From Astronomy

Advisor : RICCARDO STURANI

COMMITTEE MEMBERS :

DMITRY MELNIKOV

FRANCISCO DE ASSIS DE BRITO

GEORGE EMANUEL AVRAAM MATSAS

RAISSA FERNANDES PESSOA MENDES

RICCARDO STURANI

Data: Aug 25, 2021


Show Abstract

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 postNewtonian approximation to GR and obtain also the equations of motion for the spin and these quantities are directly comparable with astronomical observations.


16


HEBERTT LEANDRO SILVA

Gravitational Waves Fundamental: Applications in Astrophysics and Cosmology

Advisor : RICCARDO STURANI

COMMITTEE MEMBERS :

RICCARDO STURANI

LEO GOUVEA MEDEIROS

RAIMUNDO SILVA JUNIOR

JAILSON SOUZA DE ALCANIZ

VALERIO MARRA

Data: Sep 16, 2021


Show Abstract

The detection of gravitational waves, predicted by General Relativity, opened up possibilities to study physics previously unexplored. Based on affirmation, the present work is divided into two parts. First, we analyze an effect derived from geodesic deviation equation, called memory effects. We propose a model, representing a simplified description of astrophysical source emitting radiation and analyse its effect on detector a far away from the source after the radiation has passed. The particular memory effect we investigate is due to the change in longitudinal modes, rather than the wellstudy memory effect in radiative modes. The toy model proposed assumes spherical symmetry in which the entire source initially at rest is converted into radiation. Both longitudinal and transverse(radiative) modes contribute to the Riemann tensor and interact with the detector and the possibility of detecting time varying longitudinal modes is studied. In the second part of the thesis, we propose a way to measure the Hubble constant,which enters the relationship between the redshift z and luminosity distance $d_L$, and it is a crucial parameter in Cosmology that provides a measure of the Universe expansion rate. The application consists of measuring $ H_0 $ using the gravitational wave detections from binary black hole coalescences whose amplitude depends inversely on the luminosity distance $d_L$ without electromagnetic counterparts enabling a measure of the {\it{redshift}}. The method consists of breaking the degeneracy between $H_0$ and the redshift by using the statistical information on the astrophysical binary black hole population using the source distribution as a function of redshift.


17


JOSÉ CARLOS DE SOUZA CARNEIRO

Synthesis of superparamagnetic magnetite nanoparticles: coated with PVP and doped with neodymium and gadolinium.

Advisor : JOSE HUMBERTO DE ARAUJO

COMMITTEE MEMBERS :

EDIMILSON FÉLIX DA SILVA

IGOR FROTA DE VASCONCELOS

JOSE HUMBERTO DE ARAUJO

JOÃO MARIA SOARES

SUZANA NOBREGA DE MEDEIROS

Data: Sep 24, 2021


Show Abstract

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 PVPcoated magnetite doped with neodymium and gadolinium. We studied the structural and magnetic properties by Xray 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 PVPcoated magnetite nanoparticles, as well as ones doped with neodymium and gadolinium.


18


MICHELLE QUEIROZ DA SILVA

Coprecipitação; moagem de alta energia; ilmenita; titanohematita; vidros de spin.

Advisor : FELIPE BOHN

COMMITTEE MEMBERS :

EDIMILSON FÉLIX DA SILVA

FELIPE BOHN

JOÃO MARIA SOARES

RODOLFO BEZERRA DA SILVA

SERGIO ANDRÉ FONTES AZEVEDO

SUZANA ARAÚJO BARBOSA

SUZANA NOBREGA DE MEDEIROS

Data: Sep 30, 2021


Show Abstract

Iron oxides and titanium oxides are of great scientific importance due to their magnetic, electrical, morphological, and physicalchemical properties, all of which enable various applications such as catalytic processes, semiconductors, spintronics, among others. Among the mixed binary oxides, systems of the type Fe_{2y}TiO_{3}, a series of solid solutions with compositions between hematite (αFe_{2}O_{3}) and Ilmenite (FeTiO_{3}), have attracted the interest of theoretical and experimental areas due to their magnetic characteristics (antiferromagnetic with weak ferromagnetism). In this work, we synthesized compounds of the type xTiO_{2}(1x) Fe_{3}O_{4} by means of highenergy ball milling, with x = 0.2; 0.4; 0.6; 0.8, of which we selected the concentration x = 0.2 for a more detailed analysis. For the synthesis of the composites, we used the precursor powders TiO_{2} (anatase) and Fe_{3}O_{4} (synthesized here by means of coprecipitation), and both were stoichiometrically mixed and milled in a ball mill for 24 hours. Subsequently, we submitted he resulting samples to heat treatment at temperatures of 700°C, 900°C and 1100°C for 3 h. After that, the composites were characterized through Xray diffraction, vibrant sample magnetometry, and Mössbauer spectroscopy. We used the Physical Properties Measurements System – PPMS to obtain ZFC/FC curves, magnetic susceptibility, and magnetization by different treatment temperatures. For just milled samples, the Xray diffraction analysis did not show the formation of new crystalline phases. For the milled sample submitted to heat treatment, the presence of ilmenite (FeTiO_{3}) and hematite (αFe_{2}O_{3}) was observed, with the former presenting phase concentration of 100% at the temperature of 1100°C. Magnetic analysis and Mössbauer spectroscopy revealed the presence of Fe^{2+} and Fe^{3+} ions. The combined system FeTiO_{3}  αFe_{2}O_{3} is characterized as titanohematites. The sample calcined at a temperature of 700°C also showed behavior compatible with the dynamics of spin glasses. The results found suggest to the obtainment of highquality FeTiO_{3}αFe_{2}O_{3} and FeTiO_{3} composites from syntheses and accessible thermal treatments with several applications in the field of Condensed Matter Physics.


19


EDWIN HUMBERTO FAGUA DUARTE

Optimized methods for Full Waveform Inversion in 3D

Advisor : JOAO MEDEIROS DE ARAUJO

COMMITTEE MEMBERS :

CLAUDIONOR GOMES BEZERRA

GILBERTO CORSO

JESSÉ CARVALHO COSTA

JOAO MEDEIROS DE ARAUJO

JORGE LUIS LOPEZ

REYNAM DA CRUZ PESTANA

Data: Oct 21, 2021


Show Abstract

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 presalt 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 presalt reservoir and that both decimation and source coding strategies reduced the computational cost of FWI without compromising the quality of the inversion.


20


FRANCISCO ALEXANDRE DE OLIVEIRA FEITOSA

Propagation of electromagnetic waves in Kolakoski quasicrystal with graphene

Advisor : CLAUDIONOR GOMES BEZERRA

COMMITTEE MEMBERS :

CARLOS HUMBERTO OLIVEIRA COSTA

CLAUDIONOR GOMES BEZERRA

DORY HELIO AIRES DE LIMA ANSELMO

MANOEL SILVA DE VASCONCELOS

SERGIO ANDRÉ FONTES AZEVEDO

VAMBERTO DIAS DE MELLO

Data: Oct 22, 2021


Show Abstract

In this work, we simulate the incidence of electromagnetic waves in an onedimensional photonic quasicrystal composed of multilayers following the Kolakoski sequence with graphene embedded at the interfaces. The system is formed by the juxtaposition of two blocks of different dielectric materials, A and B, with A being silicon dioxide, (SiO2) and B being titanium dioxide, (T iO2). Between the blocks A and B we insert layers of graphene. We used the Transfer Matrix Method, which simplifies the algebra involved, to perform the numerical calculation that generated the data analyzed in this work. Next, we present the objectives of this work: generate the necessary data to establish the transmission and reflection spectra, investigate the effects of the graphene monolayer and evaluate the effects of the quasiperiodicity on the physical behavior of the system. To do so, we impose variations on the following parameters of the system: angle of incidence, chemical potential of graphene, Kolakoski generation index and the type of electromagnetic propagation mode. The results obtained reveal that the presence of graphene reduces the transmissivity across the entire frequency range and also induces a band gap in the low frequency transmission. Furthermore, we observe that the band gap produced by graphene is omnidirectional and that its thickness increases with the increase of graphene’s chemical potential, which can be adjusted by the value of the electrical voltage. Furthermore, we identified frequency ranges that present a Bragg gap


21


NIZOMAR DE SOUSA GONÇALVES

Sobre a relação entre excesso no infravermelho médio e a atividade cromosférica de estrelas do campo.

Advisor : BRUNO LEONARDO CANTO MARTINS

COMMITTEE MEMBERS :

BRUNO LEONARDO CANTO MARTINS

EDUARDO JANOT PACHECO

IZAN DE CASTRO LEÃO

JOSE RENAN DE MEDEIROS

MARCELO BORGES FERNANDES

Data: Nov 22, 2021


Show Abstract

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 BTSettl 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 anticorrelation between the luminosity fraction of the debris disks and the chromospheric activity index.


22


FRANCISCO DE ASSIS CHAVES DE BRITO

Infrared Excess and Debris Discs in M67 Cluster Stars

Advisor : JOSE RENAN DE MEDEIROS

COMMITTEE MEMBERS :

HELIO JAQUES ROCHA PINTO

BRUNO LEONARDO CANTO MARTINS

HELOISA MARIA BOECHATROBERTY

IZAN DE CASTRO LEÃO

JOSE RENAN DE MEDEIROS

Data: Dec 21, 2021


Show Abstract

The search for circumstellar material orbiting different star families is one of the most studied topics in Stellar Astronomy, mainly because the presence of this material indicates the existence of exoplanets. In addition, their physical properties can tell us how much a planetary system may resemble the Solar System, indicating how this system may evolve. Current studies show the presence of circumstellar material with temperatures close to those found in the asteroid belt of the Solar System, but with widely varying dimensions and compositions. Although most studies of circumstellar dust and disks reveal that this material tends to disappear for stellar ages exceeding 300400 million years, there are research showing that debris disks can be found orbiting stars with ages on the order of the solar age, i.e., 4 to 5 billion years. The main objective of this doctoral thesis is to search for debris disks that orbit stars of the Open Cluster M67, which is close to the age of the Sun. From this point of view M67 Cluster may contain stars at different evolutionary stages and may offer unique conditions to search for debris disks. It should be noted that this aspect will offer conditions to better understand how circumstellar debris disks evolve with age. This study adopted the strategy of searching for infrared excess in M67 stars, using observations collected by the WISE Mission, which provides fluxes at wavelengths corresponding to the bands 3.3  4.7  12 and 22 μm. O diagnóstico para identificação do excesso consistiu na comparação entre distribuições espectrais de energia, m. The diagnostics to identify the excess in M67 stars compared the spectral distributions of energy, built from theoretical models, and observational fluxes obtained at different wavelengths, extending from about 0.5 to 22 μm. O diagnóstico para identificação do excesso consistiu na comparação entre distribuições espectrais de energia, m, covering observations made by the Gaia Mission, 2MASS Project and WISE Mission. From an initial sample of 1520 stars, members of M67, 1380 stars were identified, presenting observations at WISE wavelengths 3.3  4.7  12 and 22 μm. O diagnóstico para identificação do excesso consistiu na comparação entre distribuições espectrais de energia, m, defined as bands W1, W2, W3 and W4 respectively. These stars with WISE measurements also showed observations in the G, G RP and G BP bands of Gaia, as well as in the J, H and K bands of 2MASS. From the sample of 1380 stars, three objects designated J085103.24+114547.3, J085202.94+105932.7 and J085116.70+114529.6 show excesses with physical meaning (SNR > 3), absence of contamination by other sources or artifacts and circular images. These results point to a fraction of 0.2% of M67 stars with an excess at 22 μm. O diagnóstico para identificação do excesso consistiu na comparação entre distribuições espectrais de energia, m, a value far below that cited in the literature for stars on the order of or with greater ages than the solar age, which is about 4%. Considering that this excess is associated with the presence of circumstellar material, defined as debris disk, we used standard models to estimate the physical parameters associated with the disks: luminosity fraction, radius, mass and temperature. Although the temperatures of the material, between 225 and 300 K, are on the order of the temperatures known for the solar Zodiacal Cloud, the values obtained for the luminosity fraction and the mass, respectively 10 4 and 10 5 times higher than the Zodiacal Cloud values, show that the material around the stars J085103.24+114547.3, J085202.94+105932.7 and J085116.70+114529.6 is brighter and denser than what is expected at the age of M67. Such material must also be composed of secondgeneration dust and debris and no longer be composed of material from the planetary formation phase. This unusual characteristic, that is, brighter and denser material, seems to indicate the presence of one or more replenishment mechanisms in these regions, through violent collisions between asteroids, planetesimals and planets.

