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Dive into the research topics where R. A. Nogueira is active.

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Featured researches published by R. A. Nogueira.


Materials Science and Engineering B-advanced Functional Solid-state Materials | 2002

Theoretical study of the AlxGa1−xN alloys

R. de Paiva; J. L. A. Alves; R. A. Nogueira; C. de Oliveira; H.W.L. Alves; L. M. R. Scolfaro; J. R. Leite

Abstract In this work we use a first-principles method based on the density functional theory, the full-potential linear augmented plane-wave method (FPLAPW), in order to calculate the electronic structures of the Al x Ga 1− x N alloys in the cubic modification. We adopt a model which allows the simulation of the composition x =0.0, 0.25, 0.50, 0.75 and 1.0. We obtain the equilibrium lattice parameters, the bulk moduli, the formation energies, the miscibility curves and the effective masses of the conduction and valence bands in the [100], [111] and [110] directions. The results can be used in the parameterization of theories based on effective hamiltonians. To our knowledge, this is the first time such a systematic ab initio study of effective masses of these semiconductor alloys is accomplished.


Brazilian Journal of Physics | 2006

Calculations of the atomic and the electronic structures of 4d-transition-metal nitrides

R. de Paiva; R. A. Nogueira; J.L.A. Alves

A systematic study of the zinc-blende-type 4d-transition-metal nitrides, which still need to be synthesized, is carried out by means of spin-polarized first-principles full-potential-linearized-augmented-plane- wave calculations using the local spin density approximation. In particular, lattice constants, bulk moduli, band structures and density of states are reported, and trends are discussed.


Journal of Applied Physics | 2004

Electronic structure and magnetic properties of Al1−xMnxN alloys

R. de Paiva; R. A. Nogueira; J.L.A. Alves

We apply a first-principles method based on the density functional theory within the local spin-density approximation, and the full-potential linear augmented plane-wave method, to calculate the electronic structure and magnetic properties of dilute magnetic Al1−xMnxN alloys in the zinc-blende phase. The analyses of the band structures, density of states, total energy, exchange interactions, and magnetic moments reveal that Al1−xMnxN alloys may exhibit half-metallic ferromagnetism, that the valence band is ferromagnetically coupled to the Mn atoms, and that the total magnetization of the cell is 4.0μB, which does not change with Mn concentration.


Journal of Physics: Condensed Matter | 2006

Theoretical study of dilute GaN–4d transition metal alloys

R. de Paiva; R. A. Nogueira; J.L.A. Alves

Electronic calculations were carried out for the dilute ordered alloys Ga(0.94) (TM)(0.06)N (TM = Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag) in the zinc-blende structure. The theoretical framework used was the density functional theory, using the local spin density approximation, as implemented in the full-potential linearized augmented plane wave method. We examine energy band structures, densities of states, charge distributions, and local magnetic moments and anticipate the properties of these promising systems for applications in spin electronic devices.


Brazilian Journal of Physics | 2002

First-principles calculations of the effective mass parameters of Al xGa1-xN and Zn xCd1-xTe alloys

R. de Paiva; R. A. Nogueira; C. de Oliveira; H. W. Leite Alves; J. L. A. Alves; L.M.R. Scolfaro; J. R. Leite

First-principles calculations of electronic band structures of the ordered cubic alloys AlxGa1-xN and CdxZn1-xTe are carried out. The band structures are used to provide e ective masses and Luttinger parameters which are useful in the parametrization of theories based on e ective hamiltonians.


Brazilian Journal of Physics | 1999

Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfaces

H. W. Leite Alves; J. L. A. Alves; R. A. Nogueira; J. R. Leite

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.


Brazilian Journal of Physics | 2004

First-Principles Materials Study for Spintronics: MnAs and MnN

R. de Paiva; J.L.A. Alves; R. A. Nogueira; J. R. Leite; L. M. R. Scolfaro

We report ab-initio all electrons density- functional calculations for the electronic structure of the compounds MnAs and MnN, in the zinc-blende phase. They are potential materials for use in fabrication of new functional semiconductors taking advantage of the spin degree of freedom. The aim is the establishing of the semiconductor spin electronics (spintronics) as a practical technology [H. Ohno, Semiconductor Science and Technology 17, 4 (2002).]. We compare results obtained using the theoretical approaches LDA (Local Density Approximation) and GGA (Generalized Gradient Approximation). The calculations are spin-polarized and we follow the evolution of the band structures as a function of lattice parameter. We compare also the evolution of the density of states of the majority-spins and of the minority-spins. We conclude that, depending on the lattice parameter, both materials may be half-metallic, therefore showing conduction by charge carriers of one spin direction exclusively: the majority- spin band is continuous, while the minority-spin has a gap. Both materials reach a total magnetization of the order of 4 mB. MnN changes from paramagnetic to ferromagnetic with the increase of the lattice parameter.


Materials Science and Engineering B-advanced Functional Solid-state Materials | 1999

Theoretical LEED parameters for the zinc-blende GaN (110) surface

H. W. Leite Alves; J. L. A. Alves; J.L.F. da Silva; J. R. Leite; R. A. Nogueira

We present a theoretical study of the equilibrium atomic structure of the GaN (110) surface based on accurate, parameter-free, self-consistent total energy and force calculations using the density functional theory, the local-density approximation for the exchange-correlation term and the full potential linear augmented plane wave approach associated to the slab supercell model. We are concerned with the LEED parameters Δ 1 ⊥, Δ 1x , Δ 2 ⊥, d 12 ⊥, d 12x and ω for the (110) surface. We analyze the changes in the bond-lengths and in the bond-angles at the anion and cation sites. We conclude that similarly to the III-Arsenides (110) and III-Phosphides (110) surfaces the GaN (110) surface relax such that the Ga-surface atom moves inward and the N-surface atom moves outward. The driving mechanism for this atomic rearrangement is that the Ga atom tends to a planar sp 2 -like bonding situation with its three N neighbours and the N atom tends to a p-bonding with its three Ga neighbors.


Microelectronics Journal | 2003

Atomic and electronic structures of InxGa1−xN quantum dots

C. de Oliveira; J.L.A. Alves; H.W.L. Alves; R. A. Nogueira; J.R. Leite

Abstract We have calculated the atomic and electronic structures of the In x Ga 1− x N random alloys (0≤ x ≤1) based on 950 atom clusters. A valence-force-field method with a Keating-type potential is used for the strain energy calculation and, therefore, the equilibrium geometries. We analyzed the bond-length and bond-angle distribution in the alloy due to the random fluctuations of the atom positions. The change in the average Ga–N and In–N lengths is calculated as a function of the composition x and compared to recent experimental data. Once we have the equilibrium geometries we calculate the electronic structures of the alloys by means of the semi-empirical quantum method Extended Hueckel.


Radiation Effects and Defects in Solids | 2001

Model calculations of the GaN(111) surface: Relaxations and native defects

C. de Oliveira; R. de Paiva; R. A. Nogueira; José Alves; H. W. Leite Alves

Abstract The success of crystaline GaN as an electronic and optical material has motivated enormous theoretical and experimental efforts in recent years in order to understand its properties. In this work we report a quantitative theory of the GaN (111) surface relaxed according to several models and of native defects. We are concerned with the vacancies (VGa and VN) and anti-site defects (NGa and GaN) as bonded on the uppermost atomic layer. Our approach is to use total energy ab initio model calculations of planar clusters of sizes ranging from 1 × 1 to 6 × 6 Ga3N3 units. We follow the convergence of the surface electronic states, atomic charges, density of states, as a function of the clusters size. The results suggest that a Ga-terminated (111) surface is favored and should be smooth. A surface constrained to be N-terminated should be rough. VGa should not introduce defect states in the surface band gap. VN introduces occupied and unoccupied energy levels in the surface band gap. NGa and GaN should introduce energy levels close to both valence and conduction band edges.

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R. de Paiva

Universidade Federal de Minas Gerais

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J. R. Leite

University of São Paulo

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J.L.A. Alves

Universidade Federal de São João del-Rei

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C. de Oliveira

Universidade Federal de Minas Gerais

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H. W. Leite Alves

Universidade Federal de São João del-Rei

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H.W.L. Alves

Universidade Federal de São João del-Rei

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J.R. Kaschny

Universidade Federal do Rio Grande do Sul

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José Alves

Universidade Federal de Minas Gerais

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S. Azevedo

Federal University of Paraíba

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