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Search Results: 1 - 10 of 221007 matches for " H. W. Leite; "
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Ab initio Calculations of the Vibrational Modes of MnAs and Ga1-xMnxAs
H. W. Leite Alves
Physics , 2006, DOI: 10.1063/1.2729814
Abstract: In this work, we present our theoretical results for the equation of state and the phonon dispersions of MnAs, as well as the Mn concentration dependence of both the lattice parameter and the phonon frequencies of the cubic GaMnAs alloys. The results are in good agreement with the experimental results whenever this comparison is possible. Based on the obtained results, the lattice constants and the phonon frequencies of the alloys do not obey the Vegard rule.
Kinetic Monte Carlo simulation of the nitridation of the GaAs (100) surfaces
Castro, A. P.;Alves, H. W. Leite;
Brazilian Journal of Physics , 2006, DOI: 10.1590/S0103-97332006000300019
Abstract: we present, our preliminary results of a systematic theoretical study of the adsorption of n over as-terminated gaas (100) (2×1) surfaces. we analyzed the changes in the bond-lengths, bond-angles and the energetics involved before and after deposition. our results show that the n-atoms will prefer the unoccupied sites of the surface, close to the as dimer. the presence of n pushes the as dimer out of the surface, leading to the anion exchange between n and as atoms. based on our results, we discussed the kinetics of the n islands formation during epitaxial growth of the iii-nitrides.
Structural and electronic properties of the SiC (100) surfaces
Soares, J. S.;Alves, H. W. Leite;
Brazilian Journal of Physics , 2006, DOI: 10.1590/S0103-97332006000300017
Abstract: in this work, we present our preliminary ab initio results for the structural and electronic properties of both si- and c-terminated sic (100) surfaces in (2×1) and c(2×2) reconstruction patterns. based on our results, we found that the si-terminated surfaces are dominated by weak bonded si-dimers, which are stabilized only at si-rich conditions, leading to (3×2) or more complex reconstruction patterns, as verified experimentally. also, our results show that the c-terminated surfaces is characterized by strong triply-bonded c-dimers, in a c(2×2) reconstruction pattern, which consists of c2 pairs over si bridge sites, in agreement with experimental results.
Ab initio determination of the (100) surfaces phonon dispersions of the SiC
Soares, J. S.;Alves, H. W. Leite;
Brazilian Journal of Physics , 2006, DOI: 10.1590/S0103-97332006000300018
Abstract: in this work, we presented our preliminary ab initio results for the vibrational modes and the phonon frequencies of the sic (100) surfaces. our results are in good agreement with the available experimental data whenever this comparison is possible. for the accepted models of the c-terminated surfaces in the c(2×2) reconstruction, while in the bridge-dimer model there is an acetylene-like vibrational a1 mode at 2031 cm-1, which is infrared active, in the staggered-dimer model, there is a füchs-kliewer (fk) mode at 1328 cm-1, which is experimentally detected. for si-terminated surfaces in the p(2×1) reconstruction, instead, no fk was obtained, in contradiction with the hreels experimental results for the si-terminated surfaces, but they are in consonance with the fact that this surface should be described by a (3×2) or more complex models.
Kinetic Monte Carlo simulation of the nitridation of the GaAs (100) surfaces
A. P. Castro,H. W. Leite Alves
Physics , 2006, DOI: 10.1590/S0103-97332006000300019
Abstract: We present, in this work, our preliminary results of a systematic theoretical study of the adsorption of N over As-terminated GaAs (100) (2$\times$1) surfaces. We analyzed the changes in the bond-lenghts, bond-angles and the energetics involved before and after deposition. Our results show that the N-atoms will prefer the unoccupied sites of the surface, close to the As dimer. The presence of the N pushes the As dimer out of the surface, leading to the anion exchange between the N and As atoms. Based on our results, we discussed about the kinetics of the N islands formation during epitaxial growth of the III-Nitrides.
Efeito de micronutrientes na produ??o e no tipo de tubérculos de batata, em cultura efetuada em solos de várzea do Vale do Paraíba
Gargantini, H.;Leite, N.;Hungria, L. S.;Venturini, W. R.;
Bragantia , 1970, DOI: 10.1590/S0006-87051970000100001
Abstract: the influence of the micronutrients iron, copper, zinc, molybdenium, manganese and boron added to the basic fertilizer on yield and quality of potato (solatium tuberosum l.), growing on alluvial soils was studied. an increase in the yield of 40% in soils of avareí series and 50% of paraiba series, was associated with application of boron and molybdenium to the soil. the effect of boron was much more pronounced than that of molybdenium. application of iron, copper, zinc and manganese have no significant effect on potato yield.
Ab initio calculation of the dynamical properties of PPP and PPV
R. L. de Sousa,H. W. Leite Alves
Physics , 2006, DOI: 10.1590/S0103-97332006000300072
Abstract: In this work, we have calculated the vibrational modes and frequencies of the crystalline PPP (in both the Pbam and Pnnm symmetries) and PPV (in the P21/c symmetry). Our results are in good agreement with the available experimental data. Also, we have calculated the temperature dependence of their specific heats at constant volume, and of their vibrational entropies. Based on our results, at high temperatures, the PPP is more stable in the Pnnm structure than in the Pbam one.
Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfaces
Alves, H. W. Leite;Alves, J. L. A.;Nogueira, R. A.;Leite, J. R.;
Brazilian Journal of Physics , 1999, DOI: 10.1590/S0103-97331999000400044
Abstract: we present a systematic theoretical study of several iii-nitride (110) surfaces based on accurate, parameter-free, self-consistent total energy and force calculations using the density functional theory, the generalized gradient approximation(gga) for the exchange-correlation term, and the full potential linear augmented plane wave (fplapw) approach associated with the slab supercell model. we studied aln, bn, gan and inn and analyzed the theoretical trends for the equilibrium atomic structures and surface band structures. we used supercells built up of 7 atomic layers and a vacuum region equivalent of 5 atomic layers. for the ga and in species, the 3d and 4d electrons were treated properly as valence electrons. in connection with the atomic structures, we are concerned with the leed parameters d1^, d1x, d2^ , d12^, d12x and w for the (110) surface. we analyzed the changes in the bond-lengths and in the bond-angles at the anion and cation sites. we conclude that similarly to the iii-arsenide (110) and iii-phosphide (110) surfaces, the iii-nitride (110) surfaces relax such that the cation-surface atom moves inward and the n-surface atom moves outward. the large coulomb energy of the iii-nitrides as compared with the other iii-v compounds re ects in the smaller value of the tilt angle w and in the small value of d1^. to our knowledge, this is the rst time the fplapw method is used for such a systematic study of iii-nitrides, and we compare our results with recent results obtained with other approaches as reported in the literature.
Planar force-constant method for lattice dynamics of cubic III-nitrides
Alves H. W. Leite,Alves J. L. A.,Scolfaro L. M. R.,Leite J. R.
Brazilian Journal of Physics , 2002,
Abstract: Using the density-functional theory within the Full Potential Linear Augmented Plane-Wave (FP-LAPW) method, we have calculated, in this work, the equation of state, the effective charges and the phonon dispersion along [100] and [111] directions for the cubic III-Nitrides(c-BN, c-AlN, c-GaN and c-InN), including hydrostatic strains dependence. A good agreement with the micro-Raman scattering experiment is obtained for the phonon modes at G. Our results show that, for c-InN case, the apparent discrepancy between the known experimental results is a consequence of hydrostatic effects on the sample due to differences of the used Raman methods.
Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfaces
Alves H. W. Leite,Alves J. L. A.,Nogueira R. A.,Leite J. R.
Brazilian Journal of Physics , 1999,
Abstract: We present a systematic theoretical study of several III-nitride (110) surfaces based on accurate, parameter-free, self-consistent total energy and force calculations using the density functional theory, the generalized gradient approximation(GGA) for the exchange-correlation term, and the Full Potential Linear Augmented Plane Wave (FPLAPW) approach associated with the slab supercell model. We studied AlN, BN, GaN and InN and analyzed the theoretical trends for the equilibrium atomic structures and surface band structures. We used supercells built up of 7 atomic layers and a vacuum region equivalent of 5 atomic layers. For the Ga and In species, the 3d and 4d electrons were treated properly as valence electrons. In connection with the atomic structures, we are concerned with the LEED parameters delta1^, delta1x, delta2^ , d12^, d12x and omega for the (110) surface. We analyzed the changes in the bond-lengths and in the bond-angles at the anion and cation sites. We conclude that similarly to the III-arsenide (110) and III-phosphide (110) surfaces, the III-nitride (110) surfaces relax such that the cation-surface atom moves inward and the N-surface atom moves outward. The large Coulomb energy of the III-nitrides as compared with the other III-V compounds re ects in the smaller value of the tilt angle omega and in the small value of delta1^. To our knowledge, this is the rst time the FPLAPW method is used for such a systematic study of III-nitrides, and we compare our results with recent results obtained with other approaches as reported in the literature.
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