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arXiv1996

Role of defects and impurities in doping of GaN

We have calculated formation energies and position of the defect levels for all native defects and for a variety of donor and acceptor impurities employing first-principles total-energy calculations. An analysis of the numerical results gives direct insight into defect concentrations and impurity solubility with respect to growth parameters (temperature, chemical potentials) and into the mechanisms limiting the doping levels in GaN. We show how compensation and passivation by native defects or impurities, solubility issues, and incorporation of dopants on other sites influence the acceptor doping levels.

mtrl-th↗

arXiv1996

Steering and isotope effects in the dissociative adsorption of H_2/Pd(100)

The interaction of hydrogen with many transition metal surfaces is characterized by a coexistence of activated with non-activated paths to adsorption with a broad distribution of barrier heights. By performing six-dimensional quantum dynamical and classical molecular dynamics calculations using the same potential energy surface derived from ``ab initio'' calculations for the system H_2/Pd(100) we show that these features of the potential energy surface lead to strong steering effects in the dissociative adsorption dynamics. The adsorption dynamics shows only a small isotope effect which is purely due to the quantum nature of hydrogen.

mtrl-th↗

arXiv1996

Diffusivity of Ga and Al adatoms on GaAs(001)

The diffusivity of Ga and Al adatoms on the (2x4) reconstructed GaAs(001) surface are evaluated using detailed ab initio total energy calculations of the potential energy surface together with transition state theory. A strong diffusion anisotropy is found, with the direction of fastest diffusion being parallel to the surface As-dimer orientation. In contrast to previous calculations we identify a short--bridge position between the two As atoms of a surface dimer as the adsorption site for Al and Ga adatoms.

mtrl-th↗

arXiv1996

A Model for the Thermal Expansion of Ag(111) and other Metal Surfaces

We develop a model to study the thermal expansion of surfaces, wherein phonon frequencies are obtained from ab initio total energy calculations. Anharmonic effects are treated exactly in the direction normal to the surface, and within a quasiharmonic approximation in the plane of the surface. We apply this model to the Ag(111) and Al(111) surfaces, and find that our calculations reproduce the experimental observation of a large and anomalous increase in the surface thermal expansion of Ag(111) at high temperatures [P. Statiris, H.C. Lu and T. Gustafsson, Phys. Rev. Lett. 72, 3574 (1994)]. Surprisingly, we find that this increase can be attributed to a rapid softening of the in-plane phonon frequencies, rather than due to the anharmonicity of the out-of-plane surface phonon modes. This provides evidence for a new mechanism for the enhancement of surface anharmonicity. A comparison with Al(111) shows that the two surfaces behave quite differently, with no evidence for such anomalous behavior on Al(111).

mtrl-th↗

arXiv1995

Ab initio studies of structural instabilities in magnesium silicate perovskite

Density-functional simulations are used to calculate structural properties and high-symmetry phonons of the hypothetical cubic phase, the stable orthorhombic phase and an intermediate tetragonal phase of magnesium silicate perovskite. We show that the structure of the stable phase is well described by freezing in a small number of unstable phonons into the cubic phase. We use the frequencies of these unstable modes to estimate transition temperatures for cubic--tetragonal and tetragonal--orthorhombic phase transitions. These are investigated further to find that the coupling with the strain suggests that phonons give a better representation than rigid unit modes. The phonons of an intermediate tetragonal phase were found to be stable except for two rotational modes. The eigenvectors of the most unstable mode of each of the cubic and tetragonal phases account for all the positional parameters of the orthorhombic phase. The phase boundary for the orthorhombic--tetragonal transition intersects possible mantle geotherms, suggesting that the tetragonal phase may be present in the lower mantle.

mtrl-th↗

arXiv1994

Density-Functional-Based Determination of the CH3-CH4 Hydrogen Exchange Reaction Barrier

Due to the overbinding that is inherent in existing {\em local} approximations to the density-functional formalism, certain reaction energies have not been accessible. Since the generalized gradient approximation significantly decreases the overbinding, prospects for density-functional-based reaction dynamics are promising. Results on the generalized-gradient based determination of the CH3-CH4 hydrogen exchange reaction are presented. Including all Born-Oppenheimer effects an energy barrier of 9.5 kcal/Mole is found which is a very significant improvement over the local-density approximation.

mtrl-th↗

arXiv1996

On The Low-Frequency Vibrational Modes of C$_{60}$

The vibrational spectrum of C$_{60}$ is compared to the spectrum of a classical isotropic elastic spherical shell. We show correlations between the low frequency modes of C$_{60}$ and those of the spherical shell. We find the spherical model gives the approximate frequency ordering for the low frequency modes. We estimate a Poisson ratio of $σ\approx 0.30$ and a transverse speed of sound of $v_s\approx 1800$ m/s for the equivalent elastic shell. We also find that $ω({\rm M_1})/ω({\rm M_0})=\sqrt{3\over 2}$ for the shell modes ${\rm M_0}$ and ${\rm M_1}$, independent of elastic constants. We find that this ratio compares favorably with an experimental value of 1.17.

mtrl-th↗

arXiv1996

Motional diminishing of optical activity: a novel method for studying molecular dynamics in liquids and plastic crystals

Molecular dynamics calculations and optical spectroscopy measurements of weakly active infrared modes are reported. The results are qualitatively understood in terms of the "motional diminishing" of IR lines, a process analogous to the motional narrowing of a nuclear magnetic resonance (NMR) signal. In molecular solids or liquids where the appropriate intramolecular resonances are observable, motional diminishing can be used to study the fluctuations of the intermolecular interactions having time scales of 1psec to 100psec.

chem-ph↗

arXiv1996

Influence of molecular vibrations on dissociative adsorption

The influence of molecular vibrations on dissociative adsorption is studied by six-dimensional quantum dynamical calculations. For the system H_2 at Pd(100), which possesses non-activated pathways, it is shown that large vibrational effects exist and that they are not due to a strongly curved reaction path and a late dissociation-hindering minimum barrier, as was previously assumed. Instead, they are caused by the lowering of the H-H vibrational frequency during the dissociation and the multi-dimensionality of the potential energy surface. Still there are quantitative discrepancies between theory and experiment identified.

mtrl-th↗

arXiv1995

Force calculation and atomic-structure optimization for the full-potential linearized augmented plane-wave code WIEN

Following the approach of Yu, Singh, and Krakauer [Phys. Rev. B 43 (1991) 6411] we extended the linearized augmented plane wave code WIEN of Blaha, Schwarz, and coworkers by the evaluation of forces. In this paper we describe the approach, demonstrate the high accuracy of the force calculation, and use them for an efficient geometry optimization of poly-atomic systems.

mtrl-th↗

arXiv1995

Periodic Slab LAPW Computations for Ferroelectric BaTiO$_3$

Linearized augmented plane wave (LAPW) calculations are performed for periodic (001) and (111) slabs of BaTiO$_3$ to understand the effects of surfaces on ferroelectric BaTiO$_3$. The (111) slab is found to be much less stable than the (001) slab. The average surface energies are respectively 3700 erg/cm$^2$ and 1600 erg/cm$^2$. The depolarization field is sufficiently large in the ideal unrelaxated slab to completely inhibit the ferroelectric instability. No mid-gap surface states are evident, but there are surface states in the upper gap in the unrelaxed slab and at the top of the valence band. The dangling surface Ti bonds self-heal making the Ti-O surface very reactive and an excellent epitaxial substrate. The charge density on atoms only one unit cell away from the surface are almost identical to the bulk. Keywords: ferroelectric, surface, slab, electronic structure, depolarization, BaTiO$_3$, thin film

mtrl-th↗

arXiv1994

Real Space Approach to Electronic-Structure Calculations

We have applied the Finite Element Method to the self-consistent electronic structure calculations of molecules and solids for the first time. In this approach all the calculations are performed in "real space" and the use of non-uniform mesh is made possible, thus enabling us to deal with localized systems with ease. To illustrate the utility of this method, we perform an all-electron calculation of hydrogen molecule in a supercell with LDA approximation. Our method is also applicable to mesoscopic systems.

mtrl-th↗

arXiv1995

Magnetocrystalline Anisotropy Energy of a Transition Metal Monolayer: A Non-perturbative Theory

The magnetocrystalline anisotropy energy $E_{anis}$ for a monolayer of Fe and Ni is determined using a fully convergent tight-binding calculation including $s$-$d$ hybridization. The spin-orbit interaction $λ_{so}$ is treated non-perturbatively. Remarkably, we find $E_{anis}\proptoλ_{so}^2$ and important contributions to $E_{anis}$ due to the lifting of degeneracies near the Fermi-level. This is supported by the calculated decrease of the anisotropy energy with increasing temperature on a scale of several hundred K. Our results clarify the present debate on the origin of $E_{anis}$.

cond-mat↗

arXiv1995

He Scattering from Compact Clusters and from Diffusion-Limited Aggregates on Surfaces: Observable Signatures of Structure

The angular intensity distribution of He beams scattered from compact clusters and from diffusion limited aggregates, epitaxially grown on metal surfaces, is investigated theoretically. The purpose is twofold: to distinguish compact cluster structures from diffusion limited aggregates, and to find observable {\em signatures} that can characterize the compact clusters at the atomic level of detail. To simplify the collision dynamics, the study is carried out in the framework of the sudden approximation, which assumes that momentum changes perpendicular to the surface are large compared with momentum transfer due to surface corrugation. The diffusion limited aggregates on which the scattering calculations were done, were generated by kinetic Monte Carlo simulations. It is demonstrated, by focusing on the example of compact Pt Heptamers, that signatures of structure of compact clusters may indeed be extracted from the scattering distribution. These signatures enable both an experimental distinction between diffusion limited aggregates and compact clusters, and a determination of the cluster structure. The characteristics comprising the signatures are, to varying degrees, the Rainbow, Fraunhofer, specular and constructive interference peaks, all seen in the intensity distribution. It is also shown, how the distribution of adsorbate heights above the metal surface can be obtained by an analysis of the specular peak attenuation. The results contribute to establishing He scattering as a powerful tool in the investigation of surface disorder and epitaxial growth on surfaces, alongside with STM.

chem-ph↗

arXiv1995

The Structure of the Stoichiometric and Reduced SnO2 (110) Surface

First-principles calculations based on density functional theory (DFT) and the pseudopotential method have been used to study the stoichiometric and reduced SnO2 (110) surface. The ionic relaxations are found to be moderate for both the stoichiometric and reduced surfaces, and are very similar to those found in recent DFT-pseudopotential work on TiO2. Removal of neutral oxygen leaves two electrons per oxygen on the surface, which are distributed in channels passing through bridging oxygen sites. The associated electron density can be attributed to reduction of tin from Sn4+ to Sn2+, but only if the charge distribution on Sn2+ is recognized to be highly asymmetric. Reduction of the surface gives rise to a broad distribution of gap states, in qualitative agreement with spectroscopic measurements.

mtrl-th↗

arXiv1995

The Adsorption of H2O on TiO2 and SnO2(110) Studied by First-Principles Calculations

First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H$_2$O adsorption on the (110) surface of TiO$_2$ and SnO$_2$. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and the cases of full and half coverage are studied. Both molecular and dissociative (H$_2$O $\rightarrow$ OH$^-$ + H$^+$) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrical configurations. It is found that for both TiO$_2$ and SnO$_2$ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO$_2$ and SnO$_2$ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented and their relation with experimental UPS spectra is discussed.

mtrl-th↗

arXiv1996

Dynamical Quantum Processes of Molecular Beams at Surfaces: Dissociative Adsorption of Hydrogen on Metal Surfaces

Due to the improvement of computer power and the development of efficient algorithms it is now possible to combine high-dimensional quantum dynamical calculations of the dissociative adsorption of molecular beams with reliable ab-initio potential energy surfaces (PES). In this brief review two recent examples of such studies of the systems H_2/Cu(111), where adsorption is hindered by a noticeable energy barrier, and H_2/Pd(100), where activated as well as non-activated paths to adsorption exist, are presented. The effect of lateral surface corrugations on the sticking probability in the tunneling and the classical regime and the role of additional parallel momentum are discussed in the context of the H_2/Cu(111) results. For the system H_2/Pd(100) it is shown that the initial decrease of the sticking probability with increasing kinetic energy, which is usually attributed to a precursor mechanism, can be explained by dynamical steering. In addition, the influence of rotation on the adsorption and desorption dynamics is examined.

mtrl-th↗

arXiv1996

Six-Dimensional Quantum Dynamics of Adsorption and Desorption of H_2 at Pd(100): No Need for a Molecular Precursor Adsorption State

We report six-dimensional quantum dynamical calculations of dissociative adsorption and associative desorption of the system H_2/Pd(100) using an ab-initio potential energy surface. We focus on rotational effects in the steering mechanism, which is responsible for the initial decrease of the sticking probability with kinetic energy. In addition, steric effects are briefly discussed.

mtrl-th↗