Física

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Programa de Pós-Graduação em Física

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A energia de Casimir e a energia de transição de fase Ferromagnética em Duplas Perovskquitas
(Universidade Federal do Espírito Santo, 2017-05-26) Rocha, Jefferson Rodrigues; Orlando, Marcos Tadeu D’Azeredo; Orlando, Marcos Tadeu DAzeredo; Nogueira, José Alexandre; Batista, Antonio Brasil; Soares, Jacyra Ramos; Orlando, Marcos Tadeu D’Azeredo; Justo Filho, João Francisco
This thesis is based on Bordag’s theory of Casimir energy. This proposal relates the Casimir energy between conductor plasma planes and the thermal energy in the transition in double ferromagnetic conductive perovskites. Using this association it was possible to calculate the critical temperature of Curie ???? relating the fraction of the energy between these materials and the parameters of Sommerfeld ?? of each compound. To evaluate experimentally our proposal we considered the transition temperatures ???? and the parameters of 3 samples: ????2??????????6,????2?? ????????6 e ????2?? ????????6.We have found that for these samples the distance ?? between the conducting planes is associated with the Curie temperature Tc. Our proposal for the calculation of ???? is consistent with the values found in the literature for samples with these characteristics (ferrometal and conductive). Based on our theory we proposed the preparation of a new sample that presented a parameter ?? similar to that of the sample ????2??????????6 thus generating a Curie temperature ???? ˜ 635??,namely:????1.8??0.2??????????6. This new compound is a double perovskite with ferromagnetic metallic characteristics and octahedral geometry formed by planes containing ??????6 and presenting ?? ?? = 635??.
A Quantização da Gravitação em 3D com Constante Cosmológica Positiva
(Universidade Federal do Espírito Santo, 2012-07-31) Almaraz, Zui Agustin Oporto; Constantinidis, Clisthenis Ponce; Piguet, Olivier; Vanzella, Daniel Augusto Turolla; Ribeiro, Maria Cristina Batoni Abdalla; Alvarenga, Flávio Gimenes; Gomes, Jose Francisco; Gonçalves, Sergio Vitorino de Borba
It was shown by Bonzom and Livine that 2+1 gravity with cosmological constant contains an ambiguity that depends on a Barbero–Immirzi-like parameter ? known from 4D gravity. Based on this fact, we showed that, for the ? > 0 case, the Lorentzian theory can be partially reduced, thanks to a suitable gauge fixing, to a Chern–Simons theory with compact gauge group SU(2). Then we revised already known loop quantization of Chern–Simons theory for the case of a space with the topology of a cylinder. Finally we construct a quantum observable which, despite having a not trivial spectrum at the quantum level, it corresponds to a classical quantity identically null.
A Supercondutividade e o Formalismo de Nambu
(Universidade Federal do Espírito Santo, 2013-04-26) Eleutério, Fernando Henrique Santos; Belich Junior, Humberto; Hernaski, Carlos André; Passos, Carlos Augusto Cardoso; Orlando, Marcos Tadeu DAzeredo; Belich Junior, Humberto
In this paper we presented a study of the superconducting phase transition, which outlines the thermodynamics involved in a description of the mean field (order parameter). The Ginzburg-Landau model uses an order-parameter-dependent functional to describe the thermodynamic phase transition involving. The free energy function given in terms of this parameter provides the essential elements to extract the behavior of entropy, specific heat, and the number of superelectrons. However, this model does not explain how the pair of electrons can overcome the Coulomb repulsion energy and stabilize. Using the formalism of Nambu it is possible to expose how this occurs. On the other hand, the superconducting ceramics (eg, Hg, Re-1223, studied at the Group of Applied Physics UFES) consist of grains that form an interpenetrating set Josephson microjunction (the "weak-link"). Thus one can use the concepts in this type of join to present the microscopic physical processes involved in high-temperature superconductors. Finally, in this thesis one describes the formation of this type of joint.
Abordagem relativística para o átomo de hidrogênio em um cenário de comprimento mínimo introduzido pela álgebra Lorentz-covariante de Quesne-Tkachuk
(Universidade Federal do Espírito Santo, 2013-11-29) Francisco, Ronald Oliveira; Fabris, Júlio César; Nogueira, José Alexandre; Helayel Neto, José Abdalla; Bazeia Filho, Dionisio; Rodrigues, Davi Cabral
One of the reasons for gravity can not be included in the standard model is its non-renormalizability. Almost all the proposals for the quantization of gravity leads to existence of a minimal length which acts as a natural regulator. That effect suggests a generalized uncertaintly relations between position and momentum operators what results in a modified commutation relation between these operators, and thus changing the whole structure of quantum mechanics. In this work we calculate the magnitude of this minimum length through the energy of the ground state of the hydrogen atom in Dirac’s theory, based on the Quesne-Tkachuck commutation relation. Comparing with experimental data, we obtained the value of the minimum length is orders of magnitude less than or equal to 10-20m
Algumas considerações sobre conjuntos de bases para cálculos de propriedades elétricas
(Universidade Federal do Espírito Santo, 2009-10-13) Arruda, Priscilla Mendes; Canal Neto, Antônio; Muniz, Eduardo Perini; Franco, Mauro Lúcio; Jorge, Francisco Elias; Canal Neto, Antônio
Recently, segmented contracted basis sets of double, triple, and quadruple zeta valence quality plus polarization functions (XZP, X=D, T, and Q, respectively) for the atoms from H to Ar were reported. In this work, with the objective of having a better description of polarizabilities, the QZP set was augmented with diffuse (s and p symmetries) and polarization (p, d, f, and g symmetries) functions that were chosen to maximize the mean dipole polarizability at the UHF and UMP2 levels, respectively. At the HF and B3LYP levels of theory, electric dipole moment and static polarizability for a sample of molecules were evaluated. Comparison with experimental data and results obtained with a similar size basis set, whose diffuse functions were optimized for the ground state energy of the anion, was done
Análise perturbativa em modelos cosmológicos fantasmas
(Universidade Federal do Espírito Santo, 2010-02-19) Jardim, Deborah Faragó; Fabris, Júlio César; Tonini, Eduardo Valentino; Dito, Giuseppe; Silva, Saulo Carneiro de Souza; Gonçalves, Sergio Vitorino de Borba; Zimdahl, Winfried Ernst Wilhelm
Dark energy models have became extensively studied in recent years. Earlier proposals have emerged this century to describe the Universe by means of a fluid whose equation of state was breaking with the paradigm imposed by the standard cosmology. Fluid of this kind require to violate some energy conditions and have a very peculiar behavior, denominated for this reason "phantom fluid". This work investigates the structures formation through the study of cosmological perturbations in Universe which scenario is the phantom cosmology. They have proposed two models, the first them by solving the perturbed equations for scalar modes, using a hydrodynamic and fields representation for the fluid and then making the asymptotic behavior analysis. The second model is a composite of two fluids, one component of pressureless matter and another with negative pressure, represented by a self-interacting scalar field . In this case, the solution was obtained by numerical calculation. Results were confronted with the observational data and it showed models phantom of this nature are quite possible and its open a new window of research
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.
Análise termobarométrica aplicada à decomposição do oxalato de cálcio hidratado.
(Universidade Federal do Espírito Santo, 2017-04-10) Cevolani, Messias Bicalho; Cunha, Alfredo Gonçalves; Oliveira, José Roberto de; Alfonso, Jorge Luis Gonzales; Bueno, Thiago Eduardo Pedreira
This work is concerned with the characterization of the Thermobarometric Analysis technique applied to the decomposition of Calcium Oxalate Monohydrate (CaC2O4.H2O). This decomposition occurs in three steps with three different gaseous products. This is a technique that makes it possible to obtain direct measurements of temperature and pressure simultaneously during the heat treatment process, this was done in constant volume inside a quartz bulb. Consequently, it was possible to identify thermodynamic parameters of the phase transition and verify the energy conservation quantity such as enthalpy, entropy and Gibbs free energy through the pressure and temperature measurements. The samples were decomposed by varying systematically the initial experimental conditions such as mass, heating rate, heat treatment final temperature and initial pressure. The present results allowed the identification and description of several physical phenomena related to the compound decomposition process. It was possible to identify that an increase in pressure displaced the of the first and third steps transition temperature to higher levels. In the second transition step, the increase in pressure caused the transition temperature to move to lower levels. It has been observed a proportional relationship between mass and the pressure for each transition steps. Therefore, the mass variation also displaces the transition temperatures of the three decomposition steps. Analyzing the initial pressures of each experiment, it was possible to identify a small amounts of H2O, such as humidity, in many samples and in the quartz bulb dedicated to the process. The amount of H2O relative to this humidity is exponential in relation to the initial pressure used to close the ampoules. In this work, it also discovered some limitations of the Thermobarometric Analysis technique when applied to the decomposition of CaC2O4.H2O. By this way, the temperature range limited to 800°C was identified as the most adequate for the experiments of this work.
Anisotropia unidirecional anômala induzida em bicamadas NiFe/IrMn
(Universidade Federal do Espírito Santo, 2015-08-31) Krohling, Alisson Carlos; Nascimento, Valberto Pedruzzi; Bueno, Thiago Eduardo Pedreira; Pereira, Rodrigo Dias; Moscon, Paulo Sérgio; Caetano, Edson Passamani
In this Master dissertation, anomalous unidirectional anisotropy and Exchange Bias effect on multilayer Si(100)/Ta(3nm)/NiFe(3nm)/IrMn(7nm)/Ta(1nm), prepared at room temperature by DC Sputtering, were systematically studied by X-ray difraction and magnetization measurements. In particular, as-prepared and heat at 600 K samples were investigated by X-ray diffraction in low (reflectivity) and high (conventional) angle geometries to characterize the bulk and interface film structural properties. Magnetization measurements, performed in a broad temperature range (300 - 600 K), were done to study magnetic properties of these two samples (as-prepared and annealed). Room temperature M(H) curves have shown a horizontal loop shift effect for the as-prepared sample. This effect was here associated with an anomalous unidirectional anisotropy induced during the sample preparation procedure (intrinsic properties), i. e., an effect that is present before the field cooling process needed to induce the Exchange bias effect. This anomalous behavior, observed in as-prepared Si(100)/Ta(3nm)/NiFe(3nm)/IrMn(7nm)/Ta(1nm) film, may be explained assuming an interface interduffusion effect without structural change at the Ta/NiFe interface. This interdiffusion effect, that occurs during the sample preparation, favors a magnetically harder NiFeTa phase (when compared with the NiFe phase); consequently there will be for the as-prepared film two magnetic contributions: (i) a soft NiFe phase magnetically coupled to the IrMn layer and that will be responsible for the observed horizontal loop shift effect (anomalous unidirectional anisotropy) and (ii) a harder NiFeTa phase, which is not influenced by the anomalous unidirectional anisotropy, but that have their magnetic moments rotated incoherently with those spins of the NiFe layer. The combination of low and high temperatures magnetic and structural data indicates that it is possible to remove this anomalous anisotropy when the sample is heated around the interval 520-570 K, which is the blocking temperature range of this effect. However, the high temperature data suggest an enhancement of the interface atomic interdiffusions Ta/NiFe and NiFe/IrMn, leading to a sample degradation as well as formation of a NiFeIrMnTa phase, with a completely disappearance of the NiFe contribution.
Aprendizagem de máquina aplicada a métodos de classificação de supernovas
(Universidade Federal do Espírito Santo, 2018-09-06) Silva, Rodrigo Duarte; Casarini, Luciano; Marra, Valerio; Fabris, Júlio César; Reis, Ribamar Rondon de Rezende dos
Future observational research with investments, telescopes and technologies never before seen, are being proposed in an attempt to unravel the mysteries of the Universe. In our work, we provide an overview of this scenario, with special attention to the classification of supernovae that will be done by LSST (Large Synoptic Survey Telescope) from 2022. Initially, we introduce the physics that involve the Supernova event and its observation, with the objective of treating the problem of photometric classification of supernovae. We provide important references in the use of different machine learning and neural networks for this purpose. We include results from the use of some of the computational methods and the theory behind them, highlighting their potentialities and vunerabilities. Machine learning methods may involve supervision or not. We aim to describe the application of these powerful tools in the analysis of observational data and verify unexpected results.
As supernovas tipo Ia e a cosmologia
(Universidade Federal do Espírito Santo, 2010-08-09) Oliveira, Paulo Louzada Castro de; Fabris, Júlio César; Ricaldi, William Santiago Hipolito; Alcaniz, Jailson Souza de
This dissertation is dedicated to the investigation on the nature of the dark matter and dark energy in the Universe through the technique that uses the observations of explosions os supernovae type Ia as indicators of distances, used at the end of the last century to detect the acelerated expansion of the Universe. Since then some projects dedicated to observation of supernovae where executed and published their results, which are used here to estimate parameters and compare cosmological models. The objective of this work is to study the power of the test based on the distances to supernovae and its response to different cosmological models and free parameters. To begin with we review the standard model of Cosmology and the evidences that suport the existance of dark matter and dark energy, followed by a study on the supernovae itself and its various observational and astrophysical aspects that allow the construction of the magnitude calibration techniques and the estimatives of its distances. Before moving on to the cosmological analyses we introduce the appropriate tool which is the bayesian statistics, studied here only in its basic tools and aplications. At last we aply these tools to the supernovae data sets available in the literature know as the Gold, SNLS, Essence and Constitution, to study some cosmological models, beginning with the concordance ?CDM model and the ?CDM parametrization that tests the supposition that ?X = -1 for the dark energy. After that we test two dynamical vaccum energy ?(t) models and a special case of the quartessence Generalized Chaplygin Gas model. The results show that when used alone the supernovae data sets leeds to quite less expressive results when compared to the literature where they are used together with other evidences. And also two of the data sets including the most up to date show unexpected irregularities that point to the existance of still uncontrolled effects on the magnitude calibration techniques. Both results show that the cosmological test based on supernovae data will go through another significant improvement on its quality in the near future.
Bases gaussianas geradas com os métodos Monte Carlo Simulated Annealing e Particle Swarm Optimization.
(Universidade Federal do Espírito Santo, 2017-10-03) Reis, Thiago Mello dos; Alfonso, Jorge Luis Gonzalez; Caetano, Edson Passamani; Continentino, Mucio Amado; Nascimento, Valberto Pedruzzi; Macedo, Waldemar Augusto de Almeida
The Monte Carlo Simulated Annealing and Particle Swarm Optimization methods were used to generate adapted Gaussian basis set for the atoms from H to Ar, in the ground state. A study about the e ciency and the reliability of each method was performed. In order to check the reliability of the proposed methods, we perform an speci c study considering a training set of 15 atoms, namely: N, Mg, Al, Cl, T i, N i, Br, Sr, Ru, P d, Sb, Cs, Ir, Tl, At. First of all, the Improved Generator Coordinate Hartree-Fock Method was applied to generate adapted basis which was used as start point to generate new Gaussian basis sets. After that, the Monte Carlo Simulated Annealing and Particle Swarm Optimization methods were developed from parallel studies, however following the same procedure so as we could have the possibility to compare them. Previously applying of the developed methods we perform some calibrations in order to de ne the values of the parameters of the algorithms; we perform studies about annealing schedules (for the Monte Carlo Simulated Annealing method), the total of swarm's particle (for the Particle Swarm Optimization method), and the total of steps for each algorithm. After the calibration procedure, both methods were applied, with the variational principle, to the Hartree-Fock wave function to give us the fully optimized Gaussian basis sets. Next, the basis sets were contracted by considering the lowest total energy loss, prioritizing the contraction of the most internal exponents. The last two steps of our procedure were the addition of polarized and di use functions, respectively. These procedures were performed by using the methods which we developed in this work through calculations to the MP2 level. The basis sets that have been generated in this work were used in some atomic and molecular calculations; we compare such results with relevant results from literature. We veri ed that, if we consider the same computational e ciency for both Monte Carlo Simulated Annealing and Particle Swarm Optimization methods, there is a vi small di erence between them as regards the accuracy, so that by using the Monte Carlo Simulated Annealing method we obtain best results. When we compare the results of this work with those from literature we note similar results for the properties that were studied, however the proposed methods in this work are more e cient, and we can de ne a single total numbers of steps for the algorithms even though we are treating with di erent atomic systems. In addition, we verify that the proposed methods in this work are more accurate than other similar methods presented in the literature, in the task of nding the global minima of the uncontracted basis sets to HF level of theory. It will be necessary to perform additional studies to check the real relationship between the accuracy of the methods. We do not verify the in uence of several parameters of the Particle Swarm Optimization algorithm in this work. The fact that the developed methods in this work have been constructed through Double Zeta basis does not prevent them to be used for larger basis sets, the two methods are able to be applied to generate Gaussian basis sets in the atomic environment for Gaussian basis sets with di erent qualities
Bases gaussianas geradas em ambientes atômico e molecular
(Universidade Federal do Espírito Santo, 2014-09-19) Arruda, Priscilla Mendes; Canal Neto, Antônio; Jorge, Francisco Elias; Freitas, Jair Carlos Checon de; Poveda Calviño, Luis Argel; Campos, Cesar Turczyn
A Gaussian basis set of small size for the atoms from H through Ar was constructed from a single sequence of exponents of 19 Gaussian functions, based on the generator coordinate Hartree-Fock method. For each atom studied a segmented contraction scheme was proposed and polarization functions and di use functions were added. Applications of the new contracted and uncontracted basis sets for the the calculation of the equilibrium geometries and electric properties (electric dipole moments and polarizabilities) of various molecular systems were performed and comparisons were made between theoretical and experimental results. The universal Gaussian basis sets presented in this work (with and without contraction) are competitive when compared to adapted basis sets of similar size. The Monte Carlo simulated annealing method was adapted to optimize correlated Gaussian-type functions in non-relativistic molecular environments. Starting from an atom-centered atomic Gaussian basis set, the uncontracted functions were reoptimized in the molecular environments corresponding to the BF, CN -, CO, CO2, CS, H2O, N2 and NO+ systems. These new molecule-optimized basis sets, called hybrid-basis sets HXZP (X = D, T, Q, and 5), were used to calculate total energies, harmonic vibrational frequencies, equilibrium geometries, electric dipole moments, and polarizabilities, at correlated level of theory. The present methodology is a simple and e ective way to improve molecular correlated wave functions, without the need to enlarge the molecular basis set. This methodology was employed to generate hierarchical sequences of molecular basis sets with increasing size, which we used to obtain complete basis set limits. xv xvi Additionally, the Monte Carlo simulated annealing method was employed to generate a hierarchical sequence of molecule-optimized basis set to CN -, CO, CO2, CS, H2O, N2 and NO+ systems. In these basis sets, all exponents were optimized in the molecular environment. Contraction schemes were proposed to each studied system, resulting in a nal molecular basis set, which we called MXZP (X = D, T, and Q). We used this hierarchical sequence of basis set to calculate total energies, harmonic vibrational frequencies, equilibrium geometries, electric dipole moments, and polarizabilities, at correlated level of theory. Also, we used that hierarchical sequences of molecular basis sets to obtain complete basis set limits. We veri ed, for the studied systems, that the MXZP basis sets produced more suitable molecular properties than the corresponding HXZP basis sets
Buracos negros fantasmas.
(Universidade Federal do Espírito Santo, 2010-04-23) Rodrigues, Manuel Eleuterio; Fabris, Júlio César; Alvarenga, Flávio Gimenes; Mello, Eugênio Ramos Bezerra de; Piguet, Olivier; Sotomayor, Patrício Anibal Letelier
We obtain the general solution of the Einstein-(anti) Maxwell-(anti) Dilaton (EMD)theory for a static, spherically symmetric spacetime in four dimensions. In the phantom(anti) cases, we obtain new solutions with non-degenerate horizon and make the analysis oftheir causal structures. The causal structures of some solutions are exotics, some of themcan not be described in two-dimensional usual Penrose diagrams. We obtain also, usingthe method of Sigma model, new stationary solutions with axial symmetry, for Einstein-anti-Maxwell-Dilaton (E ¯MD) and Einstein-anti-Maxwell-Dilaton-Axion (E ¯MDA) theory.We analyse the causal structure of these new solutions.
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.
Caracterização estrutural dos compostos Ca2MnReO6 e Ca1,8Ce0,2MnReO6 através da técnica de absorção de raio X
(Universidade Federal do Espírito Santo, 2014-11-24) Rodrigues, Erika Laurina Oliveira; Orlando, Marcos Tadeu D’Azeredo; Lourenço, José André; Takeuchi, Armando Yoshihaki; Muccillo, Eliana Navarro dos Santos; Orlando, Marcos Tadeu DAzeredo
In this work will be presented a description about the synthesis methodsusing solid state reaction using the encapsulated quartz tube technique and a description about the morphogolical and strutural characteristics of the compounds Ca2MnReO6and Ca1,8Ce0,2MnReO6. The analysis of the diffraction patternusing Rietveld show us that the dopped sample doped with 5% cerium is monophasic to and it was also possible to note an expansion of the unit cell without changing the symmetry of the doped sample. The electron microscopy images showed a homogeneous size and shape of the particles in the sample. The measurements of X-ray absorption by synchrotron radiation at the edge K of Mnand the edge L3of Reshow that both elements have mixed valence in samples with and without doping , assuming a value for Re+7 in the doped , while the change was not observed value the valence of Mn.
Caracterização Estrutural, Morfológica e Magnética da Dupla Perovsquita Ca2MnReO6.
(Universidade Federal do Espírito Santo, 2009-05-26) Souza, Danilo Oliveira de; Belich Junior, Humberto; Orlando, Marcos Tadeu D’Azeredo; Nogueira, José Alexandre; Mazzocchi, Vera Lúcia; Orlando, Marcos Tadeu D’Azeredo; Belich Junior, Humberto
Compounds with structure of ordered double perovskyte, A2B’B’’O6 (A being an alkaline-earth and B being a transition-metal in alternate sites) have attracted interest of the researches because their strong correlated structural and magnetic properties. Moreover, they are suitable candidates to produce devices with great application in spin-electronic, named “spintronics”. The special group of these compounds, the Re-based ordered double perovskites, i. e., the double perovskites where the B’’ sites are occupied by Re atoms, have been studied since 1961. However, it was only at the last decade, in spintronics framework, that these compounds have received a special attention. This work describes the synthesis and characterization of Ca2MnReO6 ordered double perovskyte. It does that by marking out sintered procedures and the structural and magnetic properties, which have been investigated by synchrotron x – ray powder diffraction, x – ray absorption spectroscopy and magnetic measurements. SEM images have shown homogeneity phase composition, the morphology and size of the ceramic grains. The Rietveld refinement has revealed a monophase compound and it was used to determinate the lattice parameters. The x – ray absorption spectroscopy agrees with the diffraction results and point to a formal (or effective) valence for Re atoms as +5,5. An interpretation of this result is that there exists a mixed valence for Re atoms, balanced by mixed valence of Mn atoms. The magnetic susceptibilities measurements were performed with temperature variation and have pointed a paramagnetic phase transition in 120 K with a spin glass type state below this value. An explanation for the magnetic result may be given with assumption of low contribution magnetic moment of Re atoms, or a nonexistent one.
Caracterização por espectroscopia Raman do endocarpo de babaçu tratado termicamente
(Universidade Federal do Espírito Santo, 2014-04-23) Ocaris, Enrique Ronald Yapuchura; Emmerich, Francisco Guilherme; Cunha, Alfredo Gonçalves; Freitas, Jair Carlos Checon de; Cançado, Luiz Gustavo de Oliveira Lopes; Emmerich, Francisco Guilherme
This work deals with the characterization by Raman spectroscopy of the endocarp of babassu coconut (a native biomass in the Northeast and Central Brazil) subjected to heat-treatment temperatures (HTT) between 800 and 2200° C with intervals of 200° C. We have also samples heat treated at lower temperatures (200, 400 and 650 ºC) that allowus to observe theinitial structural evolution of the material. During the carbonization process, which involves heat treatment of the samples in an inert atmosphere, large structural changes in the studied carbon material (which is non-graphitizable) are produced. The used tool in this work to observe these changes is Raman spectroscopy. This technique has allowed determining thein-plane average size Laof the graphite-like crystallites of the materialfor graphitizable and graphitic carbons such as nanografites, and evaluatingthe crystallinity and the structural defects.Two different laser excitation energies (EL) were used: 532 nm (2,33 eV) and633 nm (1,96 eV).The Ramanspectra ofall samples of heat treated endocarp of babassu coconutpresent the bands Dand Gcharacteristics of carbonaceous materials. The dispersivebehavior of theDbandwas observed, wherethe frequency ofmaximumintensityvarieswhen changingthelaserexcitation energy.Such behavioris the oppositeof that observedin theGband, whichexhibits anon-dispersivebehavior.TheDand G bandshavebroadpeaksfor the sampleswith lowHTT. With increasingHTTthe widthsof the peaksDand Gdecrease andthe linesbecomes more prominent due tocarbonizationprocess.Thiscarbonization that occurswith asignificant growthofthe graphite-likecrystallites of the materialis evidencedby the appearanceof theG'band. TheG’ band can be seenmore clearlyin the spectraobtainedwith the532 nmlasersourcewithHTTsamplesfrom1800 °C.Withthe lasersource of633nmtheG'bandis onlyobservedin the samplewithHTTof 2200ºC.Insamples from the withHTTof2000 °Cthe presenceof theD’bandisobserved, as well as otherminor peaks(2D', D+G,and T+D).Measurements of Lavalues for the samples with HTT between 800 and 2200°C obtained with x-ray diffraction were used to verify the linear relationship (developed by Cançado et al. for nanografites) between Laand the inverse of the ratio of integrated intensities of the Raman bands Dand G(ID/IG)taking into account the laser excitation energy. It was observed that the dependence of Laon the laser excitation energy, as suggested by Cançado et al., was fundamentalin putting the Raman measurements obtained at different laser energies in the same curve. However thestraight lineof (????????)????4as a function of 1/????with the best correlation coefficient for the heat-treated endocarp of babassu coconut does not pass through the origin as in nanografites. Our results indicate that for non-graphitizable carbons such as the heat-treated endocarp of babassu coconut the expression of Cançado et al. require the addition of a negative constant term (a linear coefficient)in the straight line of (????????)????4as a function of 1/????.The new obtained expression indicated that the maximum size of the crystallite size Lafor heat treated endocarp of babassu coconut is of the order of 11 nm, which is compatible value fora non-graphitizable material. It would be important that future works with other materials could check the results here obtained, including the values of the obtained constants (angular coefficient and linear coefficient of the fitting).Raman lines due to the presence of structures containing Silicon in the materialwere also observed.In the HTT of 800°C we have regions of the sample with a line associated with the presence of SiO2, while in In the HTT of 2000°C we have a line associated with the presence of SiC.This occurs isbecause the endocarp of babassu contains SiO2in the mineral matter (ashes), which is transformed into SiCfrom HTT of 1200°C, as it is verified with other techniques. The Raman lines due to the presence of silicon (in the form of SiO2or SiC) are not observed in all spectra, because the structures containing silicon are not evenly distributed throughout the material from the microscopic point of view, as it is detailed in a mapping of the sample with HTT of 800°C. Thestructural changes involving silicon studied with Raman Spectroscopy will be reported in more detailsin future works.
Colapso esférico em presença de energia escura.
(Universidade Federal do Espírito Santo, 2017-04-18) Rocha Junior, Mario; Piattella, Oliver Fabio; Ricaldi, William Santiago Hipólito; Velten, Hermano Endlich Schneider; Casarini, Luciano; Marra, Valerio
It is a known fact that the Universe goes through of expansion where there is strong evidences that the presence of the so-called dark energy is responsible for accelerating this processe. From this point of view, this work aims to use the spherical collapse theory to study the behaviour of a structure, here know as overdensity immersed in a Friedmann-Robertson-Walker background universe. Considering its density as an ideal fluid whose state equation is p = w? we will initially apply the theory in an Einstein-de Sitter Universe and then proceed to the universe model with presence of dark energy where we will analyze the ?CDM and the wCDM models with specific values for the constant w.
Comprimento mínimo em mecânica quântica via modificação da álgebra de Heisenberg
(Universidade Federal do Espírito Santo, 2010-05-31) Dorsch, Gláuber Carvalho; Nogueira, José Alexandre; Constantinidis, Clisthenis Ponce; Helayel Neto, José Abdalla; Fabris, Júlio César
Although gravity could not be quantized yet, there are good theoretical evidences that the unification of General Relativity and Quantum Mechanics should lead to the existence of a minimal observable length, a length scale below which the very notion of length looses meaning. Such effect proves to be worth some further analysis, for it acts as a natural regulator parameter, thus avoiding the divergences that plague Quantum Field Theories. In order to investigate some of the physical consequences of the existence of this peculiar effect, we propose to include it in the framework of Quantum Mechanics by modifying the Heisenberg algebra (i.e., the commutation relation of position and momentum operators, Xˆ and Pˆ), so that a minimum non-zero value for the uncertainty ?x emerges, which, being a limitation to the localizability of particles, acts as a minimal length. The Hilbert space of the theory must be modified accordingly. As we will see, the changes are not merely quantitative. On the contrary, our main result is that the familiar concept of “position measurement” must be reformulated, as well as other concepts related to it. We present a proposal for such reformulation.

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