PPGFIS - Doutorado em Física

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Análise da variabilidade fotométrica em estrelas tipo B observadas pela missão espacial TESS
(Universidade Federal do Rio Grande do Norte, 2025-12-15) Campelo, Josafary Pereira da Silva; Martins, Bruno Leonardo Canto; https://orcid.org/0000-0001-5578-7400; http://lattes.cnpq.br/2673267660389058; ttp://lattes.cnpq.br/7759856358728107; Lopes, Carlos Eduardo Ferreira; https://orcid.org/0000-0002-8525-7977; http://lattes.cnpq.br/4656875806985353; Freitas, Daniel Brito de; http://lattes.cnpq.br/2709876077692695; Leão, Izan de Castro; http://lattes.cnpq.br/2153938352979031; Medeiros, José Renan de; http://lattes.cnpq.br/9151590034650501
Massive stars are essential for the evolution and chemical enrichment of the universe; however, their structure and evolution remain poorly understood. This work presents a detailed analysis of photometric variability in B-type stars observed by the TESS space mission, in order to expand the number of stars with known rotation periods. The analysis covers 373 B-type stars, observed in 2-minute cadences by the TESS space mission. Three integrated temporal analysis techniques were employed: FFT and Lomb-Scargle periodograms, and wavelet transform, to characterize the variability signatures in the light curves. The classification revealed different types of variability, including 14 new B-type stars with rotational modulation in the TESS data, and 16 previously studied targets had their periods confirmed based on the literature. Among the remaining stars, pulsation signatures, binary systems, and candidates for hot subdwarfs were also identified. It was observed that a significant fraction of the sample exhibits noisy or ambiguous signals, possibly due to observational limitations or complex physical phenomena. The results obtained contribute to the understanding of stellar variability and highlight the importance of combining different analytical methods for identifying patterns in stellar photometry.
Detection of non-classicality in quantum theory: insights from quantum networks and stabilizer states
(Universidade Federal do Rio Grande do Norte, 2026-03-19) Buen Abad, Fernando Santiago Zamora; Araújo, Rafael Chaves Souto; https://orcid.org/0000-0001-8493-4019; http://lattes.cnpq.br/1509277905143351; http://lattes.cnpq.br/6002874120429092; Melnikov, Dmitry; http://lattes.cnpq.br/1078982414439134; Maziero, Jonas; https://orcid.org/0000-0002-2872-986X; http://lattes.cnpq.br/1270437648097538; Cunha, Marcelo de Oliveira Terra; http://lattes.cnpq.br/4739118307577897; Pereira, Rodrigo Gonçalves; http://lattes.cnpq.br/9293110312400359
Non-classicality is a fundamental feature of quantum mechanics, manifesting in diverse forms such as non-local correlations in quantum networks and the computational advantage provided by magic states. In quantum networks, non-classicality enables the realization of tasks beyond classical capabilities, playing a crucial role in secure communication and cryptography. Concurrently, in the domain of quantum computation, non-classicality emerges through magic states, which provide the essential resources for universal quantum computation that cannot be achieved by stabilizer operations alone. This thesis presents results on the characterization and detection of nonclassicality within these different scenarios. The work is organized along two primary research lines. First, we explore the use of interventions - a tool from causal inference - to detect quantum correlations in quantum networks. We construct an interventional-observational polytope for the instrumental scenario and show stronger non-classicality certification. Subsequently, we generalize the concept of interventions from observable variables to latent variables, specifically in the quantum case where one has partial access to quantum states. We apply this generalization to demonstrate non-classicality in the challenging triangle scenario. Second, addressing the resource theory of magic, we introduce a semidevice-independent framework to detect magic in both Bell and prepareand-measure scenarios. We derive inequalities that act as witnesses of nonstabilizerness (or magic), such that a violation of these inequalities certifies the presence of magic in the underlying states without relying on full characterization of the devices. Through these contributions, we aim to establish a new research direction where tools from causal inference, Bell non-locality, and quantum networks are synergized to enable the device-independent detection of various quantum resources, including magic states.
Structural, optical, electronic and mechanical properties of two-dimensional BxCyNz based materials
(Universidade Federal do Rio Grande do Norte, 2026-02-06) Teixeira, Mizraim Bessa; Machado, Leonardo Dantas; https://orcid.org/0000-0003-1221-4228; http://lattes.cnpq.br/9253069541351708; https://orcid.org/0000-0001-6142-7276; http://lattes.cnpq.br/9999968497948626; Bezerra, Claudionor Gomes; https://orcid.org/0000-0001-9660-2142; http://lattes.cnpq.br/5085046065890176; Vasconcelos, Manoel Silva de; https://orcid.org/0000-0002-5118-364X; http://lattes.cnpq.br/6345339220512423; Pereira Júnior, Marcelo Lopes; https://orcid.org/0000-0001-9058-510X; http://lattes.cnpq.br/4063777837176068; Santos, Ricardo Paupitz Barbosa dos; https://orcid.org/0000-0003-1254-6353; http://lattes.cnpq.br/5313696806283523; Azevedo, Sérgio André Fontes; https://orcid.org/0000-0002-7603-5141; http://lattes.cnpq.br/2195090548621158
Developments in nanotechnology since the synthesis of graphene have enabled the investigation of other two-dimensional (2D) materials, with interesting optical, electrical, and mechanical properties. Given the great diversity of potential applications, these new materials have been intensively studied in recent years. Still, the absence of a bandgap (or a very minute one) may limit their application in digital nanoelectronics. However, there are ways to regulate the energy between the valence and conduction bands of a material, thus enabling a new range of applications in electronic circuits. For example, this can be accomplished by applying strain, doping a pure carbon allotrope with atoms like boron and nitrogen, or yet using other materials as Hexagonal Boron Nitride (hBN). For other applications, such as wearable electronic device systems, the behavior of the structure under deformation is vital, since in such systems, the material may be subjected to a degree of deformation that might compromise its functionality. This work proposes to investigate, through Density Functional Theory (DFT) simulations, novel carbon allotropes and BCN based 2D materials, as well as the electronic, optical and structural properties of either pure or hybrid structures, both relaxed and under strain. Stress-strain curves were obtained from the calculations and the mechanical properties of the chosen materials characterized. Furthermore, through band structure and Density of States (DOS) calculations, we investigate how the gap energy varies with controlled deformation. Our results show that for 2D fullerene-based materials, minor changes in composition can lead to considerable bandgap variation, but it does not significantly affect optical properties. They may be used to protect against high-energy ultraviolet or low-energy infrared photons.
How quorum sensing shapes clustering in active matter
(Universidade Federal do Rio Grande do Norte, 2025-11-28) Souza, Lucas Gabriel Bezerra de; Mohan, Madras Viswanathan Gandhi; http://lattes.cnpq.br/1995273890709490; https://orcid.org/0000-0003-0941-3568; http://lattes.cnpq.br/3013972827067502; Coelho, Rodrigo Carlos Viana; https://orcid.org/0000-0001-8904-0778; http://lattes.cnpq.br/9799841061508537; Bezerra, Claudionor Gomes; https://orcid.org/0000-0001-9660-2142; http://lattes.cnpq.br/5085046065890176; Silva, Clécio Clemente de Souza; https://orcid.org/0000-0002-0910-2543; http://lattes.cnpq.br/8076432608646837; Lima, Gustavo Zampier dos Santos; Brunnet, Leonardo Gregory; Melo, Pablo Souza de Castro
Active matter refers to nonequilibrium systems composed of a large assembly of selfpropelled entities, such as bacteria, tissue cells, animal flocks, and colloids. A common trait among them is collective behavior, arising from interactions between individuals. One communication strategy between microorganisms is chemically sensing others’ presence, a mechanism known as quorum sensing. This mechanism induces a response in which the organisms regulate their movement to achieve ecologically favorable spatial distributions. However, motion can be limited by excluded-volume interactions. When self-propulsion directions fluctuate slowly, the combination of persistent motion and excluded volume can lead to particle clustering. Quorum-sensing regulation may then alter this scenario, generating diverse spatiotemporal patterns. In this thesis, we show that active particles with steric repulsion and quorum-sensing motility reduction display reentrant clustering. As control parameters are varied, clustering disappears and then reappears. One of the phase behaviors that emerges from this interplay is a previously unexplored class of active gels induced by quorum sensing. While quorum sensing can mimic attraction, it also produces strong effects in dilute regions, leading to distinct phase behavior. Remarkably, quorum sensing leads to kinetically-arrested transient states with long memory of the system’s initial condition. We then present a quorum-sensing kinetic theory that captures these phenomena. These results link phenomena in synthetic and biological systems and show how quorum sensing, often overlooked in modeling, redefines self-organization, opening paths in active matter physics.
Sobre a rotação estelar e as arquiteturas orbitais de sistemas estrela-planeta
(Universidade Federal do Rio Grande do Norte, 2025-05-08) Messias, Yuri da Silveira; Medeiros, José Renan de; http://lattes.cnpq.br/9151590034650501; https://orcid.org/0000-0002-2425-801X; http://lattes.cnpq.br/4436778152966220; Santos, Simone Daflon dos; https://orcid.org/0000-0001-9205-2307; http://lattes.cnpq.br/3395056316088979; Valio, Adriana Benetti Marques; https://orcid.org/0000-0002-1671-8370; http://lattes.cnpq.br/1041565102315246; Martins, Bruno Leonardo Canto; http://lattes.cnpq.br/2673267660389058; Leão, Izan de Castro; http://lattes.cnpq.br/2153938352979031
Since the discovery of the first extrasolar planet orbiting a main sequence star by Mayor & Queloz (1995) in 1995, more than 5,800 exoplanets have been confirmed, raising questions about how the presence of these bodies affects the activity and evolution of their host stars. It is now understood that star-planet interactions may occur through both magnetic and gravitational processes, influencing the orbital configuration of the system and favoring synchronization between stellar rotation and orbital periods, with direct implications for habitability. In this thesis, we investigate the relationship between stellar rotation and the orbital configuration of star-planet systems through two scientific cases. The first concerns the reported scarcity of close-in planets around rapidly rotating stars, a hypothesis proposed just over a decade ago, according to which only stars with Prot > 5 − 10 days would host planets with Porb < 3 days. To test this, we use an updated sample of 1,013 main-sequence stars from the Kepler and TESS missions, with measured rotation periods and confirmed or candidate planets, considering the innermost planet of each system. Our results show that the alleged scarcity disappears when the sample is expanded, suggesting that previous findings were affected by statistical limitations. Anderson-Darling and Kolmogorov-Smirnov tests reinforce this conclusion, indicating that the distribution of close-in planets around rapid rotators becomes indistinguishable from that around slow rotators. The second case investigates synchronization regimes through the behavior of the ratio Porb Prot as a function of orbital period, using a sample of 1,055 stars, composed of the initial 1,013 plus 42 additional TOIs. We define synchronous systems as those with Porb Prot = 1 ± 0.1, subsynchronous as Porb Prot < 0.9, and supersynchronous as Porb Prot > 1.1. Three distinct regions are identified: (i) Porb ≤ 2 days, dominated by subsynchronous and synchronous systems; (ii) 2 < Porb ≤ 40 days, with all three regimes but still dominated by subsynchronous systems; and (iii) Porb > 40 days, where supersynchronous systems prevail. This scenario contrasts with that of eclipsing binaries, which are predominantly synchronous. Finally, by combining our sample with a global climate model (Leconte et al., 2015), we find that planets in asynchronous rotation states, particularly supersynchronous ones located in the habitable zone, offer the most favorable conditions for habitability.The results of this work led to the publication of two papers: A Dearth of Close-in Planets around Rapidly Rotating Stars or a Dearth of Data?(Messias et al. (2022), published on Astrophysical Journal Letters) and On the behaviour of spin-orbit connection of exoplanets (Canto Martins et al. (2023), published on Nature Astronomy).
Redes neurais para propagação de ondas
(Universidade Federal do Rio Grande do Norte, 2025-08-18) Duarte, Diogo Henrique Gonçalves; Araújo, João Medeiros de; https://orcid.org/0000-0001-8462-4280; http://lattes.cnpq.br/3061734732654188; http://lattes.cnpq.br/2074393637373968; Lopes, Ricardo Júnior Campos; https://orcid.org/0000-0003-0559-9211; http://lattes.cnpq.br/2455809421513287; Corso, Gilberto; https://orcid.org/0000-0003-1748-4040; http://lattes.cnpq.br/0274040885278760; Fonseca, Jakson Miranda; https://orcid.org/0000-0001-9441-026X; http://lattes.cnpq.br/3915676505538726; Machado, Leonardo Dantas; https://orcid.org/0000-0003-1221-4228; http://lattes.cnpq.br/9253069541351708; Ferreira, Mauro Santos; Lima, Paulo Douglas Santos de
The study of wave propagation lies at the core of numerous applications across various branches of physics, ranging from quantum theory to reservoir monitoring in geophysics. Although extensively investigated and applied over the past decades, certain aspects—such as the computational cost associated with simulating wave propagation in non-homogeneous media, its application to irregular domains, and the solution of problems involving corrupted data—remain challenging and continue to be subjects of active research in several scientific fields. In light of this, recent advances in machine learning, and more specifically in neural networks, have emerged as promising tools to assist in addressing such complex tasks. Through the use of physics-informed neural networks and neural operators, we explore distinct applications of wave propagation, incorporating prior knowledge about the properties of these solutions with the aim of enhancing the networks’ learning process.
Do neurônio ao tecido nervoso: as escalas de impacto da efaticidade neuronal
(Universidade Federal do Rio Grande do Norte, 2025-08-15) Cunha, Gabriel Moreno; Lima, Gustavo Zampier dos Santos; https://orcid.org/0000-0002-4462-0522; http://lattes.cnpq.br/6484225572798302; https://orcid.org/0000-0002-3231-9564; http://lattes.cnpq.br/3949069933473689; Corso, Gilberto; https://orcid.org/0000-0003-1748-4040; http://lattes.cnpq.br/0274040885278760; Mohan, Madras Viswanathan Gandhi; http://lattes.cnpq.br/1995273890709490; Miranda, José Garcia Vivas; https://orcid.org/0000-0002-7752-8319; http://lattes.cnpq.br/1608472474770322; Lima, Marcelo de Meira Santos; https://orcid.org/0000-0001-9602-4880; http://lattes.cnpq.br/5011624798550816
Neuronal communications are a set of biological phenomena of common interest in studies in the fields of neuroscience and physics. One kind of neuronal communication still poorly explored by the sciences is that produced by ephaptic coupling, which is characterised by occurring exclusively via local electric fields. This work will present three experiments aimed at broadening our understanding of the dynamic and physiological characteristics of ephapticity in neural networks, via the quadratic-integral-and-fire-ephatic (QIF-E) neuronal model. Therefore, different computational experiments were performed in order to comprise different spatial resolutions. In the first experimental setup, a neuron with cell degeneration properties was performed, and phase coupling of the ephaptic entrainment was calculated via Spike Field Coherence. In the second experiment, two neurons communicated via synapses and ephapticity, and the Largest Lyapunov Exponent was estimated using Rosenstein’s method. Furthermore, small-world networks with QIF-E neurons were simulated, and the average potential complexity was obtained using the Multiscale Entropy technique. The results showed that at all levels of spatial resolution studied, ephapticity promotes changes in neuronal behaviour when compared to the case without ephapticity. Thus, with this doctoral study, we show that mathematical modeling becomes an auxiliary component in the understanding of this brain communication. Finally, this study proposes a physiological function of the ephaptic communication and the possible correlations between ephapticity and neurodegenerative diseases.
Inverse problems across scales
(Universidade Federal do Rio Grande do Norte, 2024-11-29) Lima, Paulo Douglas Santos de; Araújo, João Medeiros de; https://orcid.org/0000-0001-8462-4280; http://lattes.cnpq.br/3061734732654188; http://lattes.cnpq.br/5168410652715777; Costa, Jessé Carvalho; https://orcid.org/0000-0002-2906-3588; http://lattes.cnpq.br/7294174204296739; Schleicher, Joerg Dietrich Wilhelm; https://orcid.org/0000-0002-6123-317X; http://lattes.cnpq.br/0373061112091020; Machado, Leonardo Dantas; https://orcid.org/0000-0003-1221-4228; http://lattes.cnpq.br/9253069541351708; Mohan, Madras Viswanathan Gandhi; http://lattes.cnpq.br/1995273890709490; Ferreira, Mauro Santos
Predicting the shape of the shadow generated by a 3D object is a straightforward task. However, it can be challenging to solve the inverse problem of finding the object’s shape solely from its projected shadow. Analogous situations appear in many areas of Physics, either to detect gas reservoirs or to design a new quantum material. In this regard, inverse problems typically involve extracting physical properties from a noisy and limited information scenario. The typical length scale and the interactions between physical parameters introduce distinct challenges for each inverse problem. This motivated us to cross scales in this thesis by analyzing four problems and their inverse counterparts. Guided by a cross-fertilization scheme and following a multiscale approach, we started studying what mass density a hanging cable must possess to assume a specific shape. Then, we solve the Bayesian full waveform inversion problem for single and multiple timedependent seismic surveys. To address this inverse problem, we employed the Hamiltonian Monte Carlo method with a proposed mass matrix design. After that, we migrate to the forward problem of determining self-assembled magnetic particles’ ordered structures and their collapse conditions. Finally, we investigate the quantum inverse problem of finding the spatial position of randomly distributed impurities in a one-dimensional electronic device. These problems provide valuable insights into solving inverse problems and have applications in various fields of Physics.
Comportamento multifractal e variabilidade em AGNs: estudo de quasares amplificados por lente gravitacional e análise espectral multibanda
(Universidade Federal do Rio Grande do Norte, 2025-02-27) Souza, Rhimon Alves de Assis; Medeiros, José Renan de; http://lattes.cnpq.br/9151590034650501; https://orcid.org/0000-0002-5955-5882; http://lattes.cnpq.br/7626012075766287; Martins, Bruno Leonardo Canto; Leão, Izan de Castro; Freitas, Daniel Brito de; Alcaniz, Jailson Souza de
The study of quasars with images distorted by gravitational lensing provides a unique opportunity to test astrophysical theories, such as the nature of dark matter and the cosmological model. The analysis of the light curves of these quasars is essential, as it yields critical information about the properties of the lensing object and the emitting regions. The aim of this research is to investigate multifractal behavior in lensed quasars, examining its relationship with the surrounding environment and the impact of microlensing on variability. In this work, we demonstrate the connection between multifractality and variability in images A and B (formed by gravitational lensing) of 14 quasars, selected based on the availability of long-term, high-cadence time series observations. Furthermore, a multi-band analysis was implemented to study variability across different structures of the Active Galactic Nucleus (AGN), providing insights into the physical processes occurring in distinct regions of the object. To this end, the second part of this study focuses on the analysis of the AGN Mrk 421, aiming to examine the multifractality of light curves across various emission bands, correlate multifractal metrics with variability, and thus establish nonlinear statistical analysis tools as a robust framework for the study of variability in AGNs.
Polaritons de magnons em estruturas giromagnéticas com grafeno
(Universidade Federal do Rio Grande do Norte, 2025-03-24) Dantas, Leonardo de Queiroz; Vasconcelos, Manoel Silva de; Anselmo, Dory Hélio Aires de Lima; https://orcid.org/0000-0003-0180-9189; http://lattes.cnpq.br/0554474279738500; https://orcid.org/0000-0002-5118-364X; http://lattes.cnpq.br/6345339220512423; https://orcid.org/0009-0001-9127-2030; http://lattes.cnpq.br/6718638150093852; Costa, Carlos Humberto Oliveira; Bezerra, Claudionor Gomes; Machado, Leonardo Dantas; Mello, Vamberto Dias de
Electromagnetic waves propagating in magnetic crystals can couple with spin waves (magnons) excited in these materials, giving rise to hybrid modes known as magnon polaritons. These modes have been extensively studied in various materials and geometries due to their potential for technological applications in devices operating from the microwave to the far-infrared ranges. Recent studies have shown that the introduction of additional layers, especially graphene, can significantly modify the response of these systems, particularly under the influence of a static magnetic field with a specific orientation. In particular, when the field is applied perpendicular to the graphene layer, it excites the Landau levels of electrons on its surface, altering its conductivity and, consequently, the dynamics of the polaritons at the interface between media. In this work, we present a theoretical study on magnon polaritons in three distinct systems: a semi-infinite medium, a thin film, and a periodic superlattice. All structures are composed of gyromagnetic materials, each containing a single graphene layer at their interfaces, under the application of a magnetic field perpendicular to the interfaces. Our results reveal the emergence of new hybrid modes that are highly sensitive to tunable parameters, such as the magnetic field strength and the Fermi energy of the electrons in graphene.
Majorana mean-field theory with Z2 vortices: Electrical tests in the extended Kitaev model
(Universidade Federal do Rio Grande do Norte, 2024-11-07) Freitas, Lucas Rodrigues Delgado de; Pereira, Rodrigo Gonçalves; http://lattes.cnpq.br/9293110312400359; https://orcid.org/0000-0002-4491-8492; http://lattes.cnpq.br/7421671978200891; Anselmo, Dory Hélio Aires de Lima; https://orcid.org/0000-0003-0180-9189; http://lattes.cnpq.br/0554474279738500; Vernek, Edson; Andrade, Eric de Castro e; Machado, Leonardo Dantas
The Kitaev honeycomb model realizes a quantum spin liquid where spin degrees of freedom are fractionalized into itinerant Majorana fermions and localized vortices. The entanglement among these particles holds promise for executing fault-tolerant topological quantum computation. While several materials have been proposed to host a quantum spin liquid phase, concrete evidence confirming its existence remains elusive. To aid in identifying this phase, we propose an electrical test using the charge profile, offering a signature for the Z2 vortices in Kitaev materials. This study explores the electric charge response in Kitaev materials, considering a generalized spin model including Heisenberg, off-diagonal exchange interactions, and magnetic Zeeman field. Majorana mean-field theory is employed to analyze spin correlations around vortices, revealing spatial anisotropy in the charge distribution, quantified by the electric quadrupole moment. This mean-field approach remains exact in the pure Kitaev limit and captures the fundamental physics across the Kitaev spin liquid phase. The quadrupole-quadrupole interaction between distant vortices is shown to be either repulsive or attractive, depending on the parameters. We predict that electrically biased scanning probe tips can help create vortices at predetermined locations. The findings of our study pave the way for electrically manipulating Ising anyons in Kitaev spin liquids, offering a promising path for technological applications of Kitaev materials.
Modelagem em superestatística da distribuição de tamanho de sequências de DNA codificantes para Genoma Eucarionte
(Universidade Federal do Rio Grande do Norte, 2024-09-24) Costa, Marcone Oliveira da; Silva Júnior, Raimundo; https://orcid.org/0000-0001-8318-7824; http://lattes.cnpq.br/2680905746363331; https://orcid.org/0000-0002-3958-6577; http://lattes.cnpq.br/9640604659863372; Macedo Filho, Antônio de; Silva, Luciano Rodrigues da; Vasconcelos, Manoel Silva de; Silva, Sérgio Luiz Eduardo Ferreira da
This study explores the fundamental principles of superstatistical formalism and applies these concepts to the analysis of genomic data from eukaryotic organisms. A generalized Heston model was employed within the superstatistical framework to capture short-range correlations in the size distributions of coding regions, exons, of the Homo sapiens genome. For vegetables studies, a time series analysis approach was adopted to describe short-range correlations in the Cucurbitaceae family. The resulting models, the q-Gamma and q-Inverse Gamma distributions, proved effective in describing genomic data. Furthermore, the application of Bayesian analysis allowed us to assess the uncertainty associated with model parameters and to select the most appropriate model for data description. Our findings indicate that the superstatistical approach and the use of generalized distributions are valuable tools in genomic data analysis, offering deeper insights into short-range correlations in exon size distribution.
Supergravity solutions on AdS3,4,5,6,7 and the AdS/CFT correspondence
(Universidade Federal do Rio Grande do Norte, 2024-01-25) Lima, Mariana Cristina de; Melnikov, Dmitry; http://lattes.cnpq.br/1078982414439134; http://lattes.cnpq.br/8918944566846459; Orazi, Emanuele; Tomasiello, Alessandro; Trancanelli, Diego; Nastase, Horatiu Stefan; Fleury, Thiago Simonetti
The AdS/CFT conjecture states an equivalence between a theory of quantum gravity and a theory without gravity. It relates complementary regimes of the coupling constant of the dual pair 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 theories. 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 × 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 [1], [2] and [3]. In this thesis, we will investigate less supersymmetric solutions. We will study the KaluzaKlein 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.
Influência da deformação em propriedades mecânicas, eletrônicas e de adsorção em nanoestruturas
(Universidade Federal do Rio Grande do Norte, 2024-05-29) Oliveira, Raphael Benjamim de; Machado, Leonardo Dantas; https://orcid.org/0000-0003-1221-4228; http://lattes.cnpq.br/9253069541351708; https://orcid.org/0009-0008-3913-6614; http://lattes.cnpq.br/2288164965308974; Woellner, Cristiano Francisco; Galvão, Douglas Soares; Silva, Edimilson Félix da; Fulco, Umberto Laino
Two-dimensional nanostructures are the subject of growing interest in science due to their exceptional mechanical, electronic, and thermal properties, among others. As theoretical and experimental studies advance, it has become necessary to analyze how these properties can be influenced through the use of deformation. Our main objective was to analyze how deformation, whether uniaxial or biaxial, affects the various properties of nanostructured systems. Materials such as graphene, α-graphene, β-graphene, and γ-graphene, hexagonal boron nitride, as well as heterostructures composed of carbon, boron, and nitrogen. To carry out these investigations, we used computer simulations. We used methodologies with classical approximations, through molecular dynamics, and methodologies with quantum approximations, through Density Functional Theory, to estimate the physical properties of the structures presented.
Investigando as moléculas do DNA usando ferramentas de física estatística
(Universidade Federal do Rio Grande do Norte, 2024-04-30) Correia, Jonathan Pessoa; Silva Júnior, Raimundo; https://orcid.org/0000-0001-8318-7824; http://lattes.cnpq.br/2680905746363331; http://lattes.cnpq.br/8759510435086814; Corso, Gilberto; https://orcid.org/0000-0003-1748-4040; http://lattes.cnpq.br/0274040885278760; Fulco, Umberto Laino; Matias, Fernanda Selingardi; Macedo Filho, Antônio de
We investigate various statistical techniques for analyzing DNA sequences, concentrating on four key species: humans, coronaviruses, maize, and soybeans. These species were chosen because of their significance in medicine, biology, the economy, the treatment of sickness, and agriculture. We examined the structure and dynamics of the DNA sequences of these species using techniques like Chaos Game Representation, Multifractal Detrended Fluctuation Analysis, and the complexity-entropy plane. Additionally, we used models based on Tsallis statistics to characterize the distribution of base pair lengths in human DNA, and we used Bayesian statistics to determine which model best fit these distributions. The findings demonstrated a power law association in the genetic sequences under investigation, emphasizing the persistence of their composition and the existence of multifractal features. These results highlight the intricacy and challenge of genetic makeup prediction while also highlighting the potential of these statistical tools to advance our knowledge of DNA sequences and their applications to technology, agriculture, and health. The results open up new avenues for investigation, recommending the use of these approaches on a larger variety of creatures in an effort to fully explore the enormous genetic complexity that exists.
Search for dark sectors: beyond the standard model of particle physics
(Universidade Federal do Rio Grande do Norte, 2024-04-05) Jesus, Álvaro Santos de; Queiroz, Farinaldo da Silva; https://orcid.org/0000-0002-7141-5532; http://lattes.cnpq.br/9966377792236208; https://orcid.org/0000-0002-4749-9107; http://lattes.cnpq.br/6174949745385699; Vasconcelos, Manoel Silva de; Silva Júnior, Raimundo; Mendes, Tereza Cristina da Rocha; Melo, Téssio Rogério Nobrega Borja de; Gonçalves, Victor Paulo Barros
The Standard Model of Particle Physics is one of the best theoretical models in the history of physics. However, there is a growing number of pieces of evidence pointing to the fact that the model is, in fact, incomplete. In this work, we discuss the existence and properties of extensions to the Standard Model called Dark Sectors, presenting methods of detection for these new particles and possible candidates, such as WIMP dark matter, Axion-Like Particles, and the Dark Photon. Firstly, we use a famous extension to the Standard Model known as the Two Higgs Doublet Model augmented by a new U(1)Lµ−Lτ gauge symmetry to propose a solution to the muon anomalous magnetic moment tension while presenting a viable dark matter candidate simultaneously. Then, we propose an experiment for searching dark sector particles in Brazil by repurposing some subsystems of the currently inactive UVX synchrotron light generator, named “Search for Dark Sectors” (SeDS). Within the framework of SeDS, we find that, if constructed, this experiment has the potential to produce appealing results in the search for dark photons and Axion-Like Particles, being able to search for regions on the parameter space {mA′, ϵ2} for dark photons and {gaee, ma} for the Axion-Like particles that are currently unexplored by other experiments in the literature, thus being a promising experiment in the search for dark sectors.
Majorana Fermi surface state in a network of quantum spin chains
(Universidade Federal do Rio Grande do Norte, 2024-04-25) Oliviero, Fabrizio Giovanni; Pereira, Rodrigo Gonçalves; http://lattes.cnpq.br/4165894706071303; Aguiar, Fernando Iemini de Rezende; Melnikov, Dmitry; http://lattes.cnpq.br/1078982414439134; Gomes, Pedro Rogério Sérgio; Araújo, Rafael Chaves Souto
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.
Efeitos da transferência radiativa em diferentes técnicas de rastreamento do campo magnético no meio interestelar
(Universidade Federal do Rio Grande do Norte, 2022-07-14) Santana, Josias Valentim; Leão, Izan de Castro; Medeiros, José Renan de; http://lattes.cnpq.br/9151590034650501; http://lattes.cnpq.br/2153938352979031; https://orcid.org/0000-0002-2880-9077; http://lattes.cnpq.br/2032268805847036; Martins, Bruno Leonardo Canto; Lopes, Carlos Eduardo Ferreira; Freitas, Daniel Brito de
Probing magnetic fields in molecular clouds is typically difficult, even with the use of polarimetry. Newtechniques have been developed to track magnetic fields in turbulent media using velocity channel maps, which can be obtained by common spectrometers. In this thesis, we study magnetohydrodynamic turbulence based on simulations of high resolution three-dimensional data cubes representing optically thick interstellar media, in order to obtain calibrations that allow mapping the magnetic field of real environments. Ten complete high-resolution data cubes, containing position-position-velocity (PPV) maps with a resolution of 7923, were produced with the ZEUS-MP/3D code. The cubes simulate emission lines of the isotopes 12CO, 13CO and C18O from synthetic molecular clouds and include the radiative transfer processes. The conditions for the medium are from single-fluid isothermal magnetohydrodynamic simulations with forced agitation. Three techniques that allow tracking the morphology of the magnetic field without polarimetry were tested: correlation function anisotropy (CFA), principal component analysis of anisotropies (PCAA), and velocity gradient technique (VGT). Previous studies have tested these techniques using low resolution numerical data for optically thin media and high resolution non-periodic subblocks for optically thick media. This thesis represents a new study whose scaling parameters applied to high-resolution full data cubes improve the accuracy of magnetic field measurement. This new analysis shows that, in the presence of absorption, the VGT and CFA techniques can successfully track the orientation of moderate and thick magnetic fields, which is in agreement with the low resolution numerical results obtained for optically thin media. The anisotropy of the turbulence vortices tends to vary both with the change of the Alfvenic Mach number and with the velocity channel. However, a preferred direction for anisotropy manifests itself when we treat the same velocity channel with non-zero modulus for 13CO and C18O, whereas when the modulus of velocity is close to zero, 12CO and C18O are the preferred direction. Similar to previous simulations of optically thin media, our results show that the PCCA technique is also well viable for the study of optically thick media. We can conclude that the technique that best explains the various cases we analyzed is VGT, in which it can better explain the magnetic field for the most diverse characteristics of a medium. However, all magnetic field tracking techniques require adjustments.
Nanoestruturas magnéticas de ferritas: análise das propriedades magnéticas e estruturais
(Universidade Federal do Rio Grande do Norte, 2024-02-23) Nunes, Marcos Santos; Araújo, José Humberto de; Torres, Marco Antonio Morales; http://lattes.cnpq.br/0091292234916055; https://orcid.org/0000-0002-3390-8600; http://lattes.cnpq.br/2455132422249405; http://lattes.cnpq.br/8380510294599672; Dantas, Ana Lucia; Júnior, Andrea Paesano; Bohn, Felipe; Araújo, José Humberto de; Soares, João Maria
The enhancement of magnetic properties in core-shell nanoparticle structures and the e ect of heating magnetic nanoparticles through the application of an alternating magnetic eld have sparked signi cant scienti c and technological interest. Advances in this eld have been driven by progress in techniques for producing magnetic materials on the nanoscale, enabling various applications such as permanent magnets, biomedical applications, and magnetic hyperthermia. This thesis addresses two distinct nanostructured systems. The rst focuses on the synthesis of CoFe2O4@CoFe2 nanocomposites with a core-shell structure, using a method that does not involve special reagents or gases. The process involved the preparation of glutaraldehyde-crosslinked chitosan spheres containing CoFe2O4 nanoparticles, followed by high-temperature and vacuum heat treatment. The CO gas released during this process facilitated the reduction of Fe3+ and Co2+ ions to their zero-valent states. Upon production of the samples, their structural, morphological, and magnetic properties were investigated. X-ray di raction analysis, transmission electron microscopy (TEM), and Mössbauer spectroscopy revealed the presence of the desired magnetic phases. TEM images con rmed the core-shell structure con guration. Magnetic characterization indicated a magnetic exchange coupling between the phases under certain synthesis conditions, resulting in a maximum energy product (BH)max = 0.67 MGOe. The thickness of the CoFe2 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 addresses 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 heat treatment in the presence of air. Particles with various sizes were obtained by heat treatments at di erent temperatures, 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, Mössbauer results at 300 K suggest the samples are in the superparamagnetic regime. AC and DC susceptibility measurements indicated that below the blocking temperature, the system behaves as a superspin glass. Speci c loss power (SLP) measurements were conducted at a frequency of 74 kHz and an AC eld amplitude of 247 Oe. Our results for both systems were promising, suggesting potential applications in permanent magnets and magnetic hyperthermia. Speci cally, we highlight the CoFe2O4@CoFe2 and MnFe2O4 systems.
Vínculos observacionais em possíveis desvios da física padrão
(Universidade Federal do Rio Grande do Norte, 2023-12-08) Mendonça, Isaac Edson Cruz Ramalho; Holanda, Rodrigo Fernandes Lira de; http://lattes.cnpq.br/2669928747407450; Leão, Izan de Castro; Silva Júnior, Raimundo; Nunes, Rafael da Costa; Pereira, Saulo Henrique
The Λ-CDM 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.

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