R. de Paiva

Stability and electronic structure of BxNyCz nanotubes

We apply a first-principles method, based on the density functional theory, to calculate the structural stability and electronic properties of BxNyCz nanotubes. We follow the evolution of the electronic and structural properties as a function of the composition, atomic structure and nanotube diameter. The results indicate that nanotubes present a large variety of electronic properties, showing a remarkable dependence on these parameters. The formation energy decreases with the tube diameter, D, and has a strong dependence on the tube stoichiometry. Additionally, the results show that the strain energy of the tubes, relative to the corresponding unstrained sheet material, varies as 1/Dn. For BC2N the classical strain law (n = 2) is clearly obtained. Nevertheless, in the case of BCN, the exact value of n is a matter of discussion.

Theoretical study of the AlxGa1−xN alloys

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.

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

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.

Electronic structure and magnetic properties of Al1−xMnxN alloys

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.

Theoretical study of dilute GaN–4d transition metal alloys

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.

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

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.

Theoretical optical parameters for III-nitride semiconductors

Abstract The III-nitride compounds (GaN, AlN, BN, and InN) are semiconductor materials which are promising for application in optoelectronics. They find applications in light emitting diodes, laser diodes and luminescent alloys. In the present work we calculate by means of an ab initio method the optical response functions for these compounds in their cubic phase (zinc-blend). We obtain the absorption coefficients, α ( E ), the dielectric constant, ϵ ( E ), the reflectance, R ( E ), and the index of refraction, n ( E ) from the calculated energy band structures of the semiconductors. The values are compared to the available values in the literature.

First-Principles Materials Study for Spintronics: MnAs and MnN

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.

Cubic (BN)xC2(1-x) ordered alloys : a first-principles study of the structural, electronic, and effective mass properties

We apply a first-principles method based on the density functional theory within the generalized gradient approximation, and the full-potential linear augmented plane-wave method, to calculate the structural and electronic properties of cubic (BN)xC2(1−x) ordered alloys. We investigate the equilibrium lattice parameters, the bulk moduli, the density of states, the band-gap energies and the effective masses of the conduction and valence bands along the [111],[100] and [110] directions. The obtained results are used to provide effective-mass and Luttinger parameters, and to give an important guideline to the materials design for optoelectronic devices, we link the first-principles band calculations with effective mass theory.

Structural and electronic properties of aluminium nitride nanocones

We apply a first-principles method based on the density functional theory, within the generalized gradient approximation, and the pseudopotential method, to calculate the stability of aluminium nitride conical sheets of nanometre size. In the present work we investigate the formation energies, the density of states, bandgap energies and the ratio between the antiphase boundary and elastic energies. Our calculations show that the structural defects at the conical nanostructures play a crucial role in the determination of the electronic properties, making the AlN nanocones promising candidates for applications in light and field emission nanodevices.