Mestrado em Física

URI Permanente para esta coleção

http://repositorio.ufes.br/handle/10/17494

Nível: Mestrado Acadêmico
Ano de início: 1992
Conceito atual na CAPES: 4
Ato normativo: Parecer CES/CNE nº 487/2018, homologado pela Port. MEC 609, publicado no DOU em 18/03/2019.
Periodicidade de seleção: Semestral
Área(s) de concentração: Física
Url do curso: https://fisica.ufes.br/pt-br/pos-graduacao/PPGFis/detalhes-do-curso?id=1508

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Agora exibindo 1 - 12 de 12

Análise quantitativa dos teores de lignina e celulose em materiais de biomassa por RMN de 13C no estado sólido
(Universidade Federal do Espírito Santo, 2015-06-19) Cipriano, Daniel Fernandes; Freitas, Jair Carlos Checon de; Azevedo, Eduardo Ribeiro de; Alfonso, Jorge Luis Gonzalez; Schettino Junior, Miguel Ângelo
This work is dedicated to the study and development of an experimental method that allows the use of solid state 13C nuclear magnetic resonance (NMR) spectroscopy, with cross polarization (CP) and magic angle spinning (MAS), to determine the lignin and cellulose contents in lignocellulosic materials. The validity of the method was tested on di erent biomass materials by comparison to the corresponding contents obtained by the Klason method. The method was also used in a comparative study of sugarcane bagasse samples subjected to acid treatments for di erent times. For the development of the method, standard samples of cellulose and lignin as well as mixtures of both materials in known proportions were analyzed by 13C CP/MAS NMR. After acquiring the NMR spectra, graphs were constructed relating the intensity of selected spectral regions to the mass of the component responsible for that intensity. It was observed that the lignin contents of most of the analyzed lignocellulosic materials agreed well with the contents determined directly by chemical analysis. On the other hand, the cellulose contents were not correctly predicted, possibly due to the overlap of hemicellulose and cellulose signals in the NMR spectra. Regarding the acid-treated sugarcane bagasse samples, an increase in the lignin contents of the samples subjected to longer acid treatments was observed, concomitantly with the increase in the fraction of structurally ordered cellulose.
Cálculos de Propriedades Eletrônicas e Magnéticas de ligas tipo Heusler Ni2MnGa1-xSnx (0 ≤ x ≤ 1)
(Universidade Federal do Espírito Santo, 2012-10-30) Marcelo, Cesar Abraham Castañe da; Caetano, Edson Passamani; Gomes, Angelo Marcio de Souza; Caetano, Edson Passamani; Alfonso, Jorge Luis Gonzalez; Scopel, Wanderlã Luis
Using Density Funcional Theory method, we performed a systematic study on structural, electronic and magnetic properties of the Ni2MnGa Heusler alloy when Gaatoms are gradually substituted by Sn. As main results, we observed that in the Ni2MnGa1-xSnx Heusler-type alloys (0 = x = 1), the Sn-substitution leads to a linear increase of the lattice parameter of the L21- type cell, which reduces its internal stress. The Sn-substitution also favors, on the one hand, an increasing of Mn-magnetic moments, but, on the other hand, it provokes a reduction of Ni-moments. These magnetic moment behaviors lead to a gradual reduction of the total magnetic moments of the Ni2MnGa1-xSnx Heusler-type alloys, in agreement with experimental observations that suggest a decreasing of the magnetization of the L21-phase with the increase of the Sn-content. Taking into account chemical disorders among Mn and Ga (or Sn) atoms in pure Ni2MnZ (Z = Ga or Sn) Heusler alloys, we determined the most favorable energetic spin configurations, when antiferromagnetic interactions are present in these alloys. In addition, we demonstrated that in the Ni2MnGa Heusler alloy, antiferromagnetic states are more energetically favorable than that in the Ni2MnSn compound; a result that may explain the existence of martensitic phase transformation in pure Ga- compound since these interactions are responsible for the structural transformation.
Computação Quântica e Teoria Quântica de Correção de Erros.
(Universidade Federal do Espírito Santo, 2021-11-22) Scursulim, José Victor Soares; Sotkov, Galen Mihaylov; https://orcid.org/0000-0002-7276-016X; http://lattes.cnpq.br/6372536076880230; https://orcid.org/0000-0001-9964-5973; http://lattes.cnpq.br/0358054067584137; Brozeguini, Jardel da Costa; https://orcid.org/0000-0002-1157-5278; http://lattes.cnpq.br/6906807344681806; Roditi, Itzhak; https://orcid.org/0000-0003-2363-5626; http://lattes.cnpq.br/5770042763694895; Alfonso, Jorge Luis Gonzalez; https://orcid.org/0000-0001-5841-8731; http://lattes.cnpq.br/9030024304752445
The quantum computing must be the initial mark of the second technological revolution caused by the deepening knowledge of the humankind about the peculiar features of the quantum mechanics. If we want to unlock all the potential of this new technology, a development of a fault-tolerant quantum computing will be needed. In order to reach this stage, it is necessary an implementation of quantum error correcting codes that guarantee an exponential suppression of the errors at each step of the execution of an algorithm. Besides that, we need to learn how to isolate our qubits aiming a suppression of the decoherence effects. Quantum mechanics imposes various constraints to the process of construction of the error correction codes, one of them is the impossibility of using copies as redundancy mechanism, once no-cloning theorem does not allow the creation of an universal copy machine of quantum states. Another issue to be noted within this context, is that the measurement process causes a collapse in the quantum state, thus leading to a corruption of the information encoded in the state. Fortunately, there are some ways to bypass the problems pointed out through the entanglement and the stabilizer formalism, the first one allows us to recover the redundancy through the delocalization of the information in the Hilbert spaces of the parts that share quantum correlations, while the second one provides a process of gathering information about the errors that can occur in the quantum state without measuring it. The surface codes are one of the quantum codes that use the features highlighted to correct the errors, this type of code is one of the main candidates to be experimentally implemented on a large scale and it presents promising results in the preliminary studies.
Entropia de Tsallis aplicada ao clima da terra.
(Universidade Federal do Espírito Santo, 2018-10-30) Aviz, Sara Carolina Carrera de; Alfonso, Jorge Luis Gonzalez; Marra, Valerio; Constantinidis, Clisthenis Ponce; Morgado, Welles Antonio Martinez
Since the generalization of nonextensive statistical mechanics, it’s the concepts are being applied in several areas of science, such as the Earth’s climate. The weather of our planet is a very complex natural system with many variables that lead to properties that are not easy to compare with theorical models in the literature. Over a long period of time, it has been found that the earth has suffered changes due to climate transitions between glacial interglacials periods, which have often happened abruptly. In this job a study of the time series of deuterium corresponding to the time period which preceded the interglacial state in which we currently live. The work analyzes fluctuations in the concentration of deuterium to try to understand changes in the level of complexity of the earth’s climate over the period of time studied. Deuterium fluctuations are analyzed in light of the theory of non-extensive systems, in order to clarify the degree of extensiveness (or not) of the system. Time series of entropy for different parameters of non-extensivity are calculated. Finally, our data suggest that caculus of entropy can be an indicator of the emergence of critical transitions, at least in the studied system. The results are compared with others arising from the theory of critical slowing down, which represents an attempt to predict the approximation of the critical point. In summary, our study shows evidence that the earth’s climate can be considered a non-extensive system, and that in this system the entropy can be a interesting parameter for detecting critical climate changes.
Estudo da estrutura porosa de empacotamento compacto aleatório de esferas rígidas.
(Universidade Federal do Espírito Santo, 2017-09-29) Santana, David Wilson Oliveira; Alfonso, Jorge Luis Gonzalez; Correia, Maury Duarte; Freitas, Jair Carlos Checon de; Scopel, Wanderlã Luis
Around the 16th century, the study of the close packing of bodies had the simple function of optimizing physical space. The pioneer studies obtained great results at packing of bodies with simple geometries, provided the use of this study to model the nascent atomic structure. The studies were advanced to the point of being used to model complex structures, such as cell clusters and porous heterogeneous materials, being this last focus of this work. For this objective, we constructed random close packing of spheres hard frictionless, generated by computational algorithms commonly reported in the literature. Our studies characterized from a statistical point of view the porous medium formed by the empty regions of the random packing the spheres. Its statistical descriptors, the two-point statistical autocorrelation function ??2(??) and pore-size distribution were calculated and analyzed. Particular emphasis was given to the calculation of the entropy of the pore-size distribution and its interpretation in terms of the proximity of the system to the maximum density limit. Finally, we performed the reconstruction of the autocorrelation function calculated the natural rock the Lagoa-Salgada formation, based on statistical measurements performed on a random close packing of interpenetrable spheres. This last point is relevant in that the study showed that it is possible to reconstruct, even partially, the porous structure the natural porous material. In general, the results found in this work reproduced results presented in the literature about the morphological characterization of heterogenous media, as well as providing new ideas about the possibility of using statistical tools to reproduce the porous morphology observed in natural rocks.
Estudo da Nucleação Supercondutora em Estruturas Híbridas Supercondutor/Ferromagnético
(Universidade Federal do Espírito Santo, 2015-05-21) Martins, Thiago Bispo; Alfonso, Jorge Luis Gonzalez; Mello, Evandro Vidor Lins de; Silva, Ulysses Camara da; Scopel, Wanderlã Luis
The relation between magnetism and superconductivity always has been a challenge for condensed matter physics, and at the same time, essential for the understanding of the superconductivity. In principle, the Meissner effect assumes that both phenomena are excluding. However, recent advances in thin films technology have allowed the development of hybrid systems where superconducting and magnetic systems coexist. Furthermore, the physical properties of both systems are mutually modifies. The principal objective of this work is to perform a study, from theoretical point of view, of bilayers created by magnetic and superconducting layers. In essence, it is studied how the magnetic structure of a ferromagnetic system affects the superconducting order parameter inside the superconductor. The dependence of the critical temperature with the magnetic field is studied using the Ginzburg - Landau theory. Also is studied how the superconducting order parameter localizes in superconducting samples which have dimensions of the same order as, for example, the superconducting coherence length. Bilayers where the magnetic layer structure introduces a homogeneous uniaxial magnetization were studied. Also it was considered the possible influence of magnetic domains which are presented in the magnetic systems. The results show that depending of the geometry of the superconductor system the superconducting nucleation will arise at the edges of the films, or inside the domain wall between magnetics domains. The results reproduce experimental results found in the literature and provide new insights about the nucleation of the superconducting order parameter in mesoscopics systems.
Estudo teórico da estrutura eletrônica do CeCoIn5
(Universidade Federal do Espírito Santo, 2015-12-09) Claudino, Gustavo Medeiros; Alfonso, Jorge Luis Gonzalez; Freitas, Jair Carlos Checon de; Fontes, Magda Bittencourt; Scopel, Wanderlã Luis
Heavy fermion are complex systems with unconventional physical properties. Among these physical properties are the unconventional superconductivity, phase transitions at quantum critical points, non-Fermi liquid behavior (NFL), besides many others. In this thesis, it is studied the electronic structure of CeCoIn5 which is the heavy fermion superconductor with the highest critical temperature. As a first step it is explored how the the electronic structure of the CeCoIn5 compound is modified by the electronic correlations at P = 0 GPa. Latter, is simulated the effects of hydrostatic pressure applied onto the system, in particular, is addressed how the pressure modifies the electronic and magnetic properties of the CeCoIn5. It should be point out that the effects of the pressure on the electronic correlations are taken into account during the calculations. This study was conducted by using the (DFT + U) method, which allows to take into account electronic correlations at specific sites inside the compound. The lattice parameters obtained from our calculations matched the experimental ones reported in the literature for this compound. Also the calculated Bulk modulus was in concordance with the experimental value found in some reports. These results were obtained by including on site repulsive Coulomb terms in the Ce-f, Co-d and Ind orbitals. Regarding the pressure effects, it was demonstrated that the magnetic fluctuations decrease as the pressure increases. This last effect was obtained by taking into account a linear increase of the electronic correlations with the pressure. Finally, the band structure showed no significant changes with the pressure, at least in the pressure range studied here. Nevertheless, the hybridization between the Ce-f band and the conduction bands increase with the applied pressure. This result shows that the dimensionality of the system changes as pressure is applied onto the system. The results obtained in this work are compared with the ones observed in experimental T×P phase diagrams measured in CeCoIn5 single crystals.
Estudo teórico de dopagem, adsorção e intercalação de vanádio em monocamada e bicamada de disseleneto de molibdênio - MoSe2
(Universidade Federal do Espírito Santo, 2022-03-18) Piero, João Vitor Bastos Del; Scopel, Wanderlã Luis; https://orcid.org/0000000220918121; http://lattes.cnpq.br/1465127043013658; https://orcid.org/0000000150083949; Amorim, Rodrigo Garcia; Paz, Wendel Silva; http://lattes.cnpq.br/8332147920469110; Alfonso, Jorge Luis Gonzalez; http://lattes.cnpq.br/9030024304752445
In this dissertation, the results of computer simulations of two-dimensional structures using the density functional theory (DFT) will be presented. Doping, adsorption and intercalation studies of vanadium atoms in monolayers and bilayers of molybdenum dise lenide, MoSe2 will be carried out. The results obtained show that it is possible to control the electronic, structural and magnetic properties of the two-dimensional structures from the vanadium concentration to be considered in each of the proposed configurations. Among the observed changes, there is a reduction in the energy gap of the monolayer when doping with vanadium is performed, while in the bilayer there is the appearance of non-zero total magnetic moment magnetization and the variation of its final thickness when the doping and intercalation processes are carried out. For the bilayer it was found that both doping and intercalation are tools to control the magnetic properties of the material. In the other hand, for some structures it is possible to have the coexistence between the ferromagnetic and antiferromagnetic phases at room temperature, since the energy di↵erence between the phases did not exceed 20meV . Someof theseresults in dicate the possible application of such structures in electronics and spintronics devices.
Estudo teórico de monocamada e bicamada de grafeno em superfície de óxido de háfnio amorfo
(Universidade Federal do Espírito Santo, 2015-04-30) Vitória Júnior, Moacir Cézar da; Scopel, Wanderlã Luis; Arantes Junior, Jeverson Teodoro; Scopel, Wanderlã Luis; Cateano, Edson Passamani; Alfonso, Jorge Luis Gonzalez
In this work, we have used the first principle calculations based on Density Funcional Theory (DFT) to investigate the interfaces between graphene monolayer (G) and graphene bilayer (BLG) on amorphous hafnium oxide (a-HfO2) surface. We have also considered carbon and oxygen vacancies in graphene sheets and amorphous hafnium oxide, respectively. In particular, we have studied different interfaces: (i) G(VC)/a-HfO2, (ii) G(VC)/a-HfO2(VO) and (iii) BLG(VC)/a-HfO2. From point of view energetic stability, our finds showed that the interfaces formation are an exothermic process, in the case (i) and (iii) ruled by van der Waals interactions. However, for interface (ii) the formation energy is two times higher than the others and presence of the chemical bonds was also observed. Moreover, from point of view structural, the presence of the substrate induce a corrugation on graphene sheets and the interfacial equilibrium distance between them is about 3,0 Å. Additionaly, our results showed that there is a total magnetic moment for interfaces (i) and (iii). Thus, our finds reveal that the structural, electronic and magnetic properties of the graphene sheets are very sensitive to presence of the substrate
Interação de troca magnética em tricamadas do tipo Fe/Mn/Co crescidas por feixe molecular : caracterização estrutural e magnética
(Universidade Federal do Espírito Santo, 2013-03-12) Freitas, Tales Costa de; Caetano, Edson Passamani; Petrilli, Helena Maria; Larica, Carlos; Alfonso, Jorge Luis Gonzalez
MgO(001)/Cr/Ag/Fe/Mn(t)/Co/Si type films were epitaxially grown by MBE to study the coupling angle behavior of the bcc-Fe-and bcc-Co layers separated by the Mnbcc metastable phase (bcc – body centered cubic). It was investigated the influence of the epitaxial growth of the layers and their interfacial roughnesses (structural and / or chemical), induced by change Mn thickness, on the exchange coupling angle using reflection high energy electron diffraction, X-ray reflectivity (diffraction on low angles - 2? < 10 °) and Mössbauer spectroscopy of conversion electrons (1nm thick layer of 57Fe deposited at the interface Fe/Mn). Volumetric magnetic properties, which can give us the coupling angle behavior, were studied by vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR). For the set of studied samples, it was observed that all layers were epitaxially grown, except in case of Co where we could not get the results. It was particularly found that Mn layer can grow epitaxially for thicknesses smaller than 1.16 nm. For greater Mn thicknesses, it may be continued epitaxially or a phase transition occurs (transition from Mn-ccc to a –Mn phase). Furthermore, we noted that there is an enhancement of the Mn roughness with the increase of its thickness (a result seen by a reduction of RHEED signal intensity and from the fittings of the reflectivity curves). Mössbauer spectroscopy has mainly shown a presence of two fractions: one contribution which is not affected by the interface (with a-Fe properties) and other one, which is directly affected by the Fe/Mn interface and measured by a reduction in Fe coordination numbers and/or Mn neighborhood. For both fractions, the Fe magnetic moments are in the film plane, as measured by the ratio of the spectra I23 intensity, which reach a value close to 4. From FMR and VSM data, it was found that Fe-bcc and Co-bcc layers have a non collinear coupling for all samples; an effect that is associated with changes on Mn thickness as well on interfacial roughness. It was also determined the magnetic anisotropies of the Fe-bcc and Co-bcc layers; Co anisotropy is 10% bigger than the Fe one in the studied trilayers. Overall, our results indicate that the roughness on the Fe/Mn interface is nearly constant, affecting 0.4 nm of the 1nm thick 57Fe layer deposited at this interface, but now specific results were obtained on the Mn/Co interfaces. Therefore, the observed coupling angle fluctuations still need further investigations, since it cannot be purely attributed to the magnetic spring effect from the Mn antiferromagnetic phase.
Otimização das propriedades magnetocalóricas das ligas de Heusler Ni50Mn34,5In15,5 pela adição de Ga
(Universidade Federal do Espírito Santo, 2011-12-16) Guimarães, Carlos Eduardo Alves; Takeuchi, Armando Yoshihaki; Nascimento, Valberto Pedruzzi; Alfonso, Jorge Luis Gonzalez; Reis Junior, Mario de Souza
In this work, the magnetic entropy change was observed experimentally in the nonstoichiometric Heusler alloys Ni50Mn34,5(In1-xGax)15,5 with x = (0; 0,015; 0,03; 0,05; 0,075). The structural characterization of these alloys was carried by X-ray diffractometry. The magnetocaloric properties have been quantified by magnetization measurements as a function of the temperature and applied magnetic field. Magnetization measurement has shown a complex behavior due to the existence of the magneto-structural transitions in all Ga concentrations studied. The increasing of Ga concentrations in these samples resulted in an increase of the martensitic temperature, however, with no significant changes in the Curie temperature of austenite phase. The magnetization measurements as a function of the magnetic field have indicated the presence of metamagnetic transition.The inverse magnetocaloric effect was observed in all samples with structural transition of martensiticaustenitic type. Large magnetic entropy values observed for x=0,03 (36 J/kg.K) in an relatively low magnetic applied field (30 kOe) make the Ga-doped alloys good candidates for technological application as a solid refrigerant
Produção de filme espesso supercondutor SmBa2Cu3O7-t usando plasma spray.
(Universidade Federal do Espírito Santo, 2008-02-29) Piumbini, Cleiton Kenup; Cunha, Alfredo Gonçalves; Nunes Filho, Evaristo; Freitas, Jair Carlos Checon de; Alfonso, Jorge Luis Gonzalez; Cunha, Alfredo Gonçalves
Since the discovery of the critical high-temperature superconductorsin 1986, by Muller and Bednorz, the utilization of these materials, by practical ways, it is being a challengeto the researchers of the area. One of the attempts of practical application of these superconductors is to productthick and sharp layers to the transmitionof electric current. In the literature is possible to find countlessworks using the YBCO as precursors. The idea of change the yttrium by the samarium it was due his greater capacity to carry current. In this work will be presented results obtained from the productionof thick layers produced starting from the SmBa2Cu3O7-precursor. After to produce and characterize the superconductor sample of SmBa2Cu3O7-, it was ground and sieved to be sprayed. To produce the layers it was used a plasma torch of non-transferred arch, with dc current and nitrogen as workinggas dedicated to the spray deposition process. To the deposition process, the stainless steel substrates was jetted with sand, mounted in a support with internal heaters e later heated by the plasma jetuntil the temperature of 900ºC. Soon after, the deposition was made. The layers characterization was made by x-ray diffraction analysis, scanning electron microscope and electric resistivity. It was observed that the influence of the distancebetween the torch and the substrate is fundamental for the obtainingof good quality layers. The closest layers to the torch shown a little adherence to the substrate or was decompounded by the excessive heat. The farther layers shown the superconductor phase, after de oxygenation they keep added to the substrate. It was possible to observe with SEM images that happens chemical connectionsbetween the splats,thesefactis essential to the eletric current establishment. In general it was possible produce tick layer superconductorof SmBa2Cu3O7-by plasmaspray technique.

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