VITOR CAVALCANTI GONÇALVES

VITOR CAVALCANTI GONÇALVES

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DFT-based Calculations of the Magnetic Hyperfine Interactions at Cd Sites in RECd Compounds with the FP-LAPW ELK Code
2020 - SCALISE, LEVY; BURIMOVA, ANASTASIA; PEREIRA, LUCIANO F.; FERREIRA, WANDERSON; SALES, TATIANE; GONÇALVES, VITOR; CABRERA-PASCA, GABRIEL; SAXENA, RAJENDRA N.; CARBONARI, ARTUR W.
In this work we tested the methodology to map magnetic hyperfine interactions in strongly correlated materials using a free open-source all-electron FP-LAPW code ELK. The RECd (RE = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) series was chosen as a laboratory system, since an almost complete set of experimental data on the hyperfine parameters at Cd sites in these compounds was acquired previously with an extremely sensitive TDPAC technique [1]. Moreover, these TDPAC results were complemented with WIEN2k DFT calculations that allow a qualitative comparison of the two codes. The ELK calculations were performed with PBE96 GGA-type exchange-correlation functional, the cases of scalar relativistic approximation and fully relativistic relaxation of the states were treated separately. A dense k-mesh and adequate R(MT)K(max) were chosen in both cases to comply with the sensitivity of magnetic hyperfine parameters. We emphasize that the exploited version of ELK accounted for the contact field only. Yet, as it is the only contribution expected for Cd site in RECd, the values of B(hf), albeit generally overestimated, have shown reasonable agreement with the experiment (Fig. 1). Spin-orbit coupling taken into account led to a decrease in deviation from experimental data. An addition of the Hubbard-like term was essential to reach a tolerable result for CeCd. This behavior may be associated with a weaker localization of the 4f electron of Ce. Concerning the potential sources of discrepancy between our ELK results and those previously obtained with WIEN2k, (i) a different way of accounting for relativistic corrections, (ii) the use of GGA (our ELK study) versus LDA (previous WIEN2k study) and other details are discussed.
Locally symmetric oxygen vacancy around Cd impurities in CeO2
2021 - FERREIRA, W.L.; PEREIRA, L.F.D.; LEITE NETO, O.F.S.; MACIEL, L.S.; GONÇALVES, V.C.; SAXENA, R.N.; CARBONARI, A.W.; COSTA, M.S.; CABRERA-PASCA, G.A.
A detailed investigation of the electronic structure in the neighborhood of Cd impurities in CeO2 has been performed by ab initio calculations to elucidate the interplay between the oxygen vacancies and electric quadrupole interactions. The quadrupole frequency related to the major component of the electric-field gradient (EFG) at impurities sites from its neighboring charge density as well as its symmetry were calculated by simulating oxygen vacancies at oxygen nearest neighbor of Cd. Results show a very good agreement with experimental hyperfine interactions measurements at the 111Cd nucleus replacing Ce at CeO2. A systematic mapping of oxygen vacancies in CeO2 supercells was proposed within the framework of density-functional theory using the WIEN2K code focusing on the electronic distribution in the vicinity of Cd impurities. Results show that the calculated values of EFG crucially depend on impurity-vacancy complex position and the striking axial symmetry observed when an oxygen monovancy is at the nearest neighborhood of Cd is explained by a rearrangement of its p orbitals.
DFT-based calculations of the magnetic hyperfine interactions at Cd sites in RCd (R = rare earth) compounds with the FP-LAPW ELK code
2021 - MACIEL, L.S.; BURIMOVA, A.; PEREIRA, L.F.D.; FERREIRA, W.L.; SALES, T.S.N.; GONÇALVES, V.C.; CABRERA-PASCA, G.A.; SAXENA, R.N.; CARBONARI, A.W.
In the work here reported, we have calculated magnetic hyperfine interactions in rare-earth (R) intermetallic compounds by using the free open-source all-electron ELK code. The RCd (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) series was chosen as a test system because an almost complete set of experimental data on the hyperfine parameters at Cd sites was acquired through the time differential perturbed angular correlation (TDPAC) spectroscopy as previously reported. Moreover, results on magnetic hyperfine field (Bhf ) from WIEN2k code were also reported allowing a qualitative comparison analysis. We emphasize that the utilized version of ELK accounted for the contact field only. Yet, as it is the only contribution expected for Cd site in RCd compounds, the calculated Bhf values are in reasonable agreement with the experimental results. The Spin-orbit coupling when taken into account led to a decrease in deviation from experimental data. Addition, the Hubbard-like term was revealed crucial in order to make Bhf predictions for CeCd, suggesting that this behavior may be associated with a weaker 4f electron localization in Ce.
Investigation of local magnetic properties of CeCd as a function of the unit cell volume by means of first principles calculations
2019 - PEREIRA, L.F.; SCALISE, L.M.; MESTNIK-FILHO, J.; FERREIRA, W.L.; GONÇALVES, V.C.; CARBONARI, A.W.
In the work here reported magnetic hyperfine field (Bhf) at Cd site as well as the cerium magnetic moment (m-Ce) were calculated from simulation of the CeCd compound in many different unit cell volumes. Results are compared with the experimental values of Bhf (5.10 T) obtained with Perturbed Angular Correlation spectroscopy measurements [1] and m-Ce from magnetization measurements. The electronic structure calculations of the CeCd cubic cells (under normal conditions the space group is Pm-3m) were performed using density functional theory implemented on ELK and WIEN2k codes with “full-potential linearized augmented plane wave” method and the local density or generalized gradient approximations to estimate exchange-correlation energy. Three methodologies were carried out in our simulations: just spin polarized (SP); spin-orbit coupling (SO); and, SO plus ad hoc U parameter to treat the strong correlation in the cerium 4f shell. The cell energy (CE), m-Ce and Fermi-contact contribution (BFc) of Cd-Bhf from electrons (labelled core and valence) inside the atomic spheres and others quantities were calculated for volume fractions varying from -14% to 5% of the experimental unit cell volume, in steps of 1%. The smallest energy and bestest Cd-Bhf agreement were reached when the volume is compressed by 12%. Results are discussed in terms of Ce-4f electron (de)localization and its strong correlation as well as hybridization effects between Cd and Ce observed through density of states plots to understand the dependence of Cd-BFc with the compression and relaxation of the cell. A brief comparison of Cd-BFc for 3 the methodologies and m-Ce and Cd-BFc versus volume (SP calculation) from WIEN2k are presented.
Investigation on the role of oxygen vacancies in the Cd hyperfine properties in (Co, Mn)-doped CeO2
2019 - FERREIRA, WANDERSON L.; PEREIRA, LUCIANO F.D.; COSTA, MESSIAS de S.; SCALISE, LEVY; GONÇALVES, VITOR C.; SAXENA, RAJENDRA N.; CARBONARI, ARTUR W.
Ceria is known to have intrinsic oxygen vacancies that affect specific properties for semiconductor applications. Recently, incorporation of impurities to ceria for vacancy formation control as well as the role of dopants in the physical and chemical ceria properties have gained much attention, especially when doped with transition metals. Local disorder originated from oxygen vacancies and doping is the aim of several studies in which hyperfine interactions techniques have showed a great tool to investigate it. In particular, perturbed angular correlation spectroscopy (PAC) measurements with 111Cd for pure and TM (Co, Mn)-doped CeO2 have exhibited a range of quadrupole frequencies [1, 2]. In order to elucidate the several electric field gradients (efg) in these systems, we have carried out band structure calculations by means of density functional theory (DFT) in the ceria doped with Cd and TM (Co, Mn) to study the local disorder from the hyperfine interactions point of view. We performed spin-polarized calculation using the full-potential augmented plane-waves plus local-orbitals (FP-LAPW) method and Perdew–Burke–Ernzerhof generalized gradient approximation embodied into WIEN2k code [3]. We simulated several charge states in pure and TM-doped CeO2 supercells with oxygen vacancies focusing on electronic distribution in the vicinity of Cd. Our efg results showed a good agreement with 111Cd quadrupole frequency range obtained from PAC experiments. Thus, we improved the understanding about the role of Cd and TM in the ceria oxygen vacancy formation.
Insights into anomalous magnetic hyperfine field at Ce impurity in LaMn2X2 (X=Si, Ge)
2019 - PEREIRA, LUCIANO F.D.; MESTNIK-FILHO, JOSE; CABRERA-PASCA, GABRIEL A.; BOSCH-SANTOS, BRIANNA; FERREIRA, WANDERSON L.; GONÇALVES, VITOR C.; SCALISE, LEVY; GENEZINI, FREDERICO A.; SAXENA, RAJENDRA N.; CARBONARI, ARTUR W.
The LaMn2X2 (X=Si, Ge) intermetallic compounds doped with cerium exhibit the interesting physical effects that are typical of the REMn2X2 (RE=rare earth) series (such as magnetocaloric, superconductivy) or are originated from cerium in specific electronic environment (e.g. Kondo, intermediate valence, strongly correlated electron). Recently, the magnetic hyperfine field (mhf) at 140Ce-doped LaMn2(Si(1-x)Gex)2 was determined by perturbed gamma-gamma angular correlation spectroscopy. The 140Ce mhf follow the host magnetization in LaMn2Si2, while in LaMn2(Si(1-x)Gex)2 (with x = 0.2 up to 1), it has anomalous behaviour, which was associated with 4f cerium mhf contribution. In this work, first principles band structure simulations were used to improve the understanding of the distance role (generated by interchange of Si and Ge) on mhf cerium contribution.
Hyperfine interactions in Gd2Ti2O7 pyrochlore
2019 - PEREIRA, LUCIANO F.D.; FERREIRA, WANDERSON L.; GONÇALVES, VITOR C.; SCALISE, LEVY; GENEZINI, FREDERICO A.; CORREA, EDUARDO L.; SAXENA, RAJENDRA N.; CARBONARI, ARTUR W.
The cubic pyrochlore structure of the RE2Ti2O7 (R= rare earth) form is highly geometrically frustrated antiferromagnets and Gd2Ti2O7 is particularly interesting due to the fact gadolinium has a large magnetic moment and its orbital momentum is zero. Up to nowadays, a few investigations of the local structure on Gd2Ti2O7 using hyperfine interactions techniques were made. Namely, the magnetic hyperfine interaction at Gd previously reported from Mossbauer effect spectroscopy with 155Gd [1] and perturbed angular correlation (PAC) have used to reach electric field gradient at 111mCd and 111In probe nuclei [2]. However, these studies are lacking in interpretation, so first principles calculations may led to the elucidation of the electronic structure at the specific crystalline site. Then, Gd2Ti2O7 single and doped-Cd cell simulations have performed with full potential linearized augmented plane waves methods and generalized gradient approximation from Perdew-Burke-Ernzerhorf within the WIEN2k code. Electric field gradient and magnetic hyperfine components were evaluated.
Study of the local magnetics and electrics properties at Gd2Ti2O7 by PAC spectroscopy and ab initio calculations
2019 - CORREA, E.L.; PEREIRA, L.F.; FERREIRA, W.L.; BOSCH-SANTOS, B.; SCALISE, L.; GONÇALVES, V.; SANTOS, R. dos; SCHELL, J.; CARBONARI, A.W.
Gd2Ti2O7 exhibits a complex magnetic behavior with a geometrically frustrated antiferromagnetic ordering below TN = 1.1 K and a second magnetic transition at 0.7 K. In the work here reported, perturbed angular correlation (PAC) technique was used in order to measure electric quadrupole hyperfine interactions at 111mCd probe nuclei on Ti sites. These experimental results along with those for magnetic hyperfine interactions at 155Gd previously reported from Mossbauer Effect spectroscopy were interpreted with electronic structure first-principles calculation. Single and doped-Cd cell simulations have been performed with potential linearized augmented plane waves methods and generalized gradient approximation from Perdew-Burke-Ernzerhorf embodied within the WIEN2k all-electron code. PAC measurements have exhibited 111mCd probes occupying two fraction sites characterized by well-defined quadrupole frequencies (as shown in the spin-rotation spectra in the figure) with almost the same values for the corresponding electric field gradient (efg), however, with slightly difference in the asymmetry parameters. Theoretical efg’s obtained for Cadmium atoms located at Gd or Ti sites of the pyrochlore structure indicate that both experimental hyperfine parameters corresponding to Cd at Titanium site.
Development of a novel data analysis procedure for half-life measurements
2013 - GONÇALVES, VITOR C.; ZAHN, GUILHERME S.; GENEZINI, FREDERICO A.
The precise determination of nuclear parameters for nuclides with a long half-life is of extreme relevance, as these nuclides are often used as standard radiation sources for calibration of detector systems, for instance. On the other hand, the determination of a precise value for the half-life of ling-lived nuclei can be di±cult, as it may require that the activity of a single radioactive source be followed for a long period of several months or even years. In a previous paper, the decay of 57Co and 60Co was studied using daily detector veri¯cation data from the Neutron Activation Analysis Lab of IPEN-CNEN/SP. In this analysis, the presence of many outliers in the datasets proved to be a big issue, so a robust data ¯tting procedure was developed. The aim of the present work was to enhance this data ¯tting method. The enhancement was attained using pre-¯tting ¯lters to deal with data points and uncertainties that fall too far from the perceived behavior of the data, thus reducing the possibility that these very gross outliers would interfere with the ¯tting procedure. Moreover, a software implementation of the ¯tting procedure was developed, written in the Python computing language. The aim is that this software, still in an early stage of development, will be able to ¯t experimental data (initially, decay curves) dealing properly with outliers that may exist in the original dataset.
Determinação da meia-vida do Ti-51
2013 - GONCALVES, VITOR C.; ZAHN, GUILHERME S.