H. W. Leite Alves

Structural, electronic, and optical properties of ZrO2 from ab initio calculations

Structural, electronic, and optical properties for the cubic, tetragonal, and monoclinic crystalline phases of ZrO2, as derived from ab initio full-relativistic calculations, are presented. The electronic structure calculations were carried out by means of the all-electron full-potential linear augmented plane wave method, within the framework of the density functional theory and the local density approximation. The calculated carrier effective masses are shown to be highly anisotropic. The results obtained for the real and imaginary parts of the dielectric function, the reflectivity, and the refraction index show good agreement with the available experimental results. In order to obtain the static dielectric constant of ZrO2, we added to the electronic part the optical phonon contribution, which leads to values of ϵ1(0)≃29.5,26.2,21.9, respectively, along the xx, yy, and zz directions, for the monoclinic phase, in excellent accordance with experiment. Relativistic effects, including the spin-orbit intera...

Molecular Signature in the Photoluminescence of α‐Glycine, L‐Alanine and L‐Asparagine Crystals: Detection, ab initio Calculations, and Bio‐sensor Applications

We present the photoluminescence spectra of α‐glycine, L‐alanine, and L‐asparagine crystals. They are broad and structured, comprising green to ultraviolet emission in the 1.75–3.60 eV range. Absorption measurements show that the band gap energies of the crystals are of the order of 5.0 eV. Ab initio calculations of their electronic structures allow for the assignment of the observed peaks in the visible region to lattice‐related processes of exciton nature associated with polaron levels. The very thin photoluminescence peaks in the ultraviolet region are assigned to intramolecular transitions, being a signature of the weakly interacting amino acid molecules in the crystals.

First principles studies of point defects and impurities in cubic boron nitride

The full-potential linear augmented-plane-wave (FLAPW) method within the frameworks of the local density approximation and the large unit cell approach was used to carry out self-consistent calculations of vacancies, substitutional and interstitial Be impurity in cubic boron nitride (c-BN). It has been found that the Be substitutional impurity replacing B leads to a shallow acceptor level just above the top of the valence band. This result makes Be a typical p-type dopant in c-BN. Be replacing N leads to occupied and empty energy levels at the middle gap region, indicating an amphoteric character for this center. Be at tetrahedral interstitial sites induce deep levels at the upper part of the band gap comprising two electrons and four holes for each center. The role played by these findings on the interpretation of experimental data taken on Be-doped c-BN is discussed.

Lattice dynamics of boron phosphide

Using the density-functional theory, norm-conserving pseudopotentials and plane-wave expansions the authors have calculated ab initio the equation of state and the principal phonon modes in boron phosphide, including their pressure dependence and the amplitude of the eigendisplacements. Good agreement with experiment is obtained, whenever a comparison is possible: in fact, most of the results are predictions. A ten-parameter valence overlap shell model is then constructed from the available experimental data, which are completed by the data obtained in the first-principle calculations: frozen-phonon frequencies and eigenvectors. The previous speculations about the anomalous behaviour of the effective charges are discussed in the context of the present results.

Structural, electronic, vibrational and dielectric properties of selected high-shape K semiconductor oxides

The semiconductor oxides SnO2, HfO2, ZrO2, TiO2 and SrTiO3 are interesting materials for applications as high-K dielectric gate materials in silicon-based devices and spintronics, among others. Here we review our theoretical work about the structural, electronic and vibrational properties of these oxides in their most stable structural phases, including dielectric properties as derived from the electronic structure taking into account the lattice contribution. Finally, we address the recent role played by the presence of transition metal atoms in semiconductor oxides, considering in particular SnO2 as an example in forming diluted magnetic alloys.

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.

Ab initio calculation of the (100) and (110) surface phonon dispersion of GaAs and GaN

1) reconstruction presentsinteresting features which is closely related to the reduced mass of the compound. All the studied cases show thepresence of a Rayleigh mode. The calculated properties for the (110) surfaces agree very well with the availableexperimental data from HEELS and inelastic He-atom scattering as well as with other theoretical calculations.

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

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.

Hydrogen, oxygen and chlorine adsorption on ag(110) surface: a cluster calculation

Abstract The adsorption of atoms on Ag(110) surfaces has been widely investigated both theoretically and experimentally. The importance of the (110) face results from the much better catalytic properties of the single crystal Ag(110) compared with polycrystalline samples. The aim of this work is to study the systems Ag(110): H, O, Cl by means of rigorous ab initio quantum-chemical calculations. We have investigated several possible binding sites, geometries, elastic constants, binding energies and charge distributions for H, O and Cl on Ag(110) surfaces simulated by clusters Ag n ( n = 3,10).

Ab initio calculation of the dynamical properties of PPP and PPV

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.