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preprint1995arXiv

Electronic structure of intentionally disordered AlAs/GaAs superlattices

We use realistic pseudopotentials and a plane-wave basis to study the electronic structure of non-periodic, three-dimensional, 2000-atom (AlAs)_n/(GaAs)_m (001) superlattices, where the individual layer thicknesses n,m = {1,2,3} are randomly selected. We find that while the band gap of the equivalent (n = m = 2) ordered superlattice is indirect, random fluctuations in layer thicknesses lead to a direct gap in the planar Brillouin zone, strong wavefunction localization along the growth direction, short radiative lifetimes, and a significant band-gap reduction, in agreement with experiments on such intentionally grown disordered superlattices.

preprint1995arXiv

Density-Polarization Functional Theory of the response of a periodic insulating solid to an electric field.

The response of an infinite, periodic, insulating, solid to an infinitesimally small electric field is investigated in the framework of Density Functional Theory. We find that the applied perturbing potential is not a unique functional of the periodic density change~: it depends also on the change in the macroscopic {\em polarization}. Moreover, the dependence of the exchange-correlation energy on polarization induces an exchange-correlation electric field. These effects are exhibited for a model semiconductor. We also show that the scissor-operator technique is an approximate way of bypassing this polarization dependence.

preprint1994arXiv

Total energy global optimizations using non orthogonal localized orbitals

An energy functional for orbital based $O(N)$ calculations is proposed, which depends on a number of non orthogonal, localized orbitals larger than the number of occupied states in the system, and on a parameter, the electronic chemical potential, determining the number of electrons. We show that the minimization of the functional with respect to overlapping localized orbitals can be performed so as to attain directly the ground state energy, without being trapped at local minima. The present approach overcomes the multiple minima problem present within the original formulation of orbital based $O(N)$ methods; it therefore makes it possible to perform $O(N)$ calculations for an arbitrary system, without including any information about the system bonding properties in the construction of the input wavefunctions. Furthermore, while retaining the same computational cost as the original approach, our formulation allows one to improve the variational estimate of the ground state energy, and the energy conservation during a molecular dynamics run. Several numerical examples for surfaces, bulk systems and clusters are presented and discussed.

preprint1995arXiv

Static displacements and chemical correlations in alloys

Recent experiments in metallic solid solutions have revealed interesting correlations between static pair-displacements and the ordering behavior of these alloys. This paper discusses a simple theoretical model which successfully explains these observations and which provides a natural framework for analyzing experimental measurements of pair-displacements and chemical correlations in solid solutions. The utility and scope of this model is demonstrated by analyzing results of experiments on $Ni-Fe$ and $Cr-Fe$ alloys and results of simulations of $Cu-Au$ and $Cu-Ag$ alloys.

preprint1995arXiv

First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3

We carry out a completely first-principles study of the ferroelectric phase transitions in BaTiO $_3$ . Our approach takes advantage of two features of these transitions: the structural changes are small, and only low-energy distortions are important. Based on these observations, we make systematically improvable approximations which enable the parameterization of the complicated energy surface. The parameters are determined from first-principles total-energy calculations using ultra-soft pseudopotentials and a preconditioned conjugate-gradient scheme. The resulting effective Hamiltonian is then solved by Monte Carlo simulation. The calculated phase sequence, transition temperatures, latent heats, and spontaneous polarizations are all in good agreement with experiment. We find the transitions to be intermediate between order-disorder and displacive character. We find all three phase transitions to be of first order. The roles of different interactions are discussed.

preprint1996arXiv

The equilibrium shape of InAs quantum dots grown on a GaAs(001) substrate

The equilibrium shape of strained InAs quantum dots grown epitaxially on a GaAs(001) substrate is derived as a function of volume. InAs surface energies are calculated within density-functional theory, and a continuum approach is applied for the elastic relaxation energies.

preprint1995arXiv

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.

preprint1995arXiv

Application of a new Tight-Binding method for transition metals: Manganese

A new tight-binding total energy method, which has been shown to accurately predict ground state properties of transition and noble metals, is applied to Manganese, the element with the most complex ground state structure among the $d$ metals. We show that the tight-binding method correctly predicts the ground state structure of Mn, and offers some insight into the magnetic properties of this state.

preprint1996arXiv

Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)

The formation of chemisorbed O-phases on Ru(0001) by exposure to O_2 at low pressures is apparently limited to coverages Theta <= 0.5. Using low-energy electron diffraction and density functional theory we show that this restriction is caused by kinetic hindering and that a dense O overlayer (Theta = 1) can be formed with a (1x1) periodicity. The structural and energetic properties of this new adsorbate phase are analyzed and discussed in view of attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It is argued that the identified system actuates the unusually high rate of oxidizing reactions at Ru surfaces under high oxygen pressure conditions.

preprint1994arXiv

First-Principles Determination of Chain-Structure Instability in KNbO $_3$

A complete mapping in the Brillouin zone of the structural instability associated with the ferroelectric phase transitions of KNbO $_3$ has been obtained by first-principles calculations using an LAPW linear response approach. The wavevector dependence of the instability reveals pronounced two-dimensional character, which corresponds to chains oriented along $\left<100\right>$ \ directions of displaced Nb atoms. The results are discussed in relation to models of the ferroelectric phase transitions.

preprint1995arXiv

Phase Coexistence of a Stockmayer Fluid in an Applied Field

We examine two aspects of Stockmayer fluids which consists of point dipoles that additionally interact via an attractive Lennard-Jones potential. We perform Monte Carlo simulations to examine the effect of an applied field on the liquid-gas phase coexistence and show that a magnetic fluid phase does exist in the absence of an applied field. As part of the search for the magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase coexistence curves at large dipole moments, $μ$ . The critical temperature is found to depend linearly on $μ^2$ for intermediate values of $μ$ beyond the initial nonlinear behavior near $μ=0$ and less than the $μ$ where no liquid-gas phase coexistence has been found. For phase coexistence in an applied field, the critical temperatures as a function of the applied field for two different $μ$ are mapped onto a single curve. The critical densities hardly change as a function of applied field. We also verify that in an applied field the liquid droplets within the two phase coexistence region become elongated in the direction of the field.

preprint1995arXiv

Localized Excitons and Breaking of Chemical Bonds at III-V (110) Surfaces

Electron-hole excitations in the surface bands of GaAs(110) are analyzed using constrained density-functional theory calculations. The results show that Frenkel-type autolocalized excitons are formed. The excitons induce a local surface unrelaxation which results in a strong exciton-exciton attraction and makes complexes of two or three electron-hole pairs more favorable than separate excitons. In such microscopic exciton "droplets" the electron density is mainly concentrated in the dangling orbital of a surface Ga atom whereas the holes are distributed over the bonds of this atom to its As neighbors thus weakening the bonding to the substrate. This finding suggests the microscopic mechanism of a laser-induced emission of neutral Ga atoms from GaAs and GaP (110) surfaces.

preprint1995arXiv

Frustrated H-induced Instability of Mo(110)

The large interest in the properties of transition metal surfaces has been recently fostered by inelastic helium atom scattering experiments carried out by Hulpke and Luedecke. Sharp and giant anomalies in the surface phonon dispersion curves along Gamma-H and Gamma-S have been detected on W(110) and Mo(110) at a coverage of one monolayer of hydrogen. At the same critical wave vectors a smaller second indentation is present in the experimental phonon branches. Recently we have proposed a possible interpretation which is able to explain this and other experiments. In this paper we discuss results of our recent ab initio calculations of the atomic and electronic properties of the clean and H-covered Mo(110) surface in more details. For the full monolayer coverage the calculated Fermi-surface contours are characterized by strong nesting features which originate the observed anomalies.

preprint1995arXiv

Theory of alkali metal adsorption on close-packed metal surfaces

Results of recent density functional theory calculations for alkali metal adsorbates on close-packed metal surfaces are discussed. Single adatoms on the (111) surface of Al and Cu are studied with the self-consistent surface Green-function method by which the pure adsorbate-substrate interaction may be analyzed. Higher coverage ordered adlayers of K on Al(111), Na on Al(111), and Na on Al(001) are treated using the <em>ab-initio</em> pseudopotential plane wave method which affords the prediction of coverage dependent stable and metastable adsorbate geometries and phase transitions of the adsorbate layers. Together, these studies give insight and understanding into current key issues in alkali metal adsorption, namely, the nature of the adsorbate-substrate bond at low coverage and the occurrence of hitherto unanticipated adsorbate geometries, and the associated electronic properties.

preprint1995arXiv

The Static, Dynamic and Electronic Properties of Liquid Gallium Studied by First-Principles Simulation

First-principles molecular dynamics simulations having a duration of 8 ps have been used to study the static, dynamic and electronic properties of l-Ga at the temperatures 702 K and 982 K. The simulations use the density-functional pseudopotential method and the system is maintained on the Born-Oppenheimer surface by conjugate gradients relaxation. The static structure factor and radial distribution function of the simulated system agree very closely with experimental data, but the diffusion coefficient is noticeably lower than measured values. The long simulations allow us to calculate the dynamical structure factor $S(q,ω)$ . A sound-wave peak is clearly visible in $S(q,ω)$ at small wavevectors, and we present results for the dispersion curve and hence the sound velocity, which is close to the experimental value. The electronic density of states is very close to the free-electron form. Values of the electrical conductivity calculated from Kubo-Greenwood formula are in satisfactory accord with measured data.

preprint1995arXiv

A Self-Consistent First-Principles Technique Having Linear Scaling

An algorithm for first-principles electronic structure calculations having a computational cost which scales linearly with the system size is presented. Our method exploits the real-space localization of the density matrix, and in this respect it is related to the technique of Li, Nunes and Vanderbilt. The density matrix is expressed in terms of localized support functions, and a matrix of variational parameters, L, having a finite spatial range. The total energy is minimized with respect to both the support functions and the elements of the L matrix. The method is variational, and becomes exact as the ranges of the support functions and the L matrix are increased. We have tested the method on crystalline silicon systems containing up to 216 atoms, and we discuss some of these results.

preprint1995arXiv

Atomistic Aspects of Diffusion and Growth on the Si and Ge (111) Surfaces

The stability of interfaces and the mechanisms of thin film growth on semiconductors are issues of central importance in electronic devices. These issues can only be understood through detailed study of the relevant microscopic processes. Experimental studies are able to provide detailed, atomic scale information for model systems. Theoretical analysis of experimental results is essential in explaining certain surprising observations and in providing guidance for optimizing conditions and methods of growth. We review recent theoretical work on the diffusion of adatoms, the structure of adsorbate monolayers, and their implications for growth on the Si and Ge (111) surfaces. The theoretical analysis consists of first-principles calculations of the total-energy and entropy factors for stable, metastable and saddle-point configurations. These calculations are supplemented by Monte Carlo simulations of simple models that afford direct contact with experimental observations.

preprint1996arXiv

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).

preprint1995arXiv

Electronic-structure-induced deformations of liquid metal clusters

Ab initio molecular dynamics is used to study deformations of sodium clusters at temperatures $500\cdots 1100$ K. Open-shell Na $_{14}$ cluster has two shape isomers, prolate and oblate, in the liquid state. The deformation is stabilized by opening a gap at the Fermi level. The closed-shell Na $_8$ remains magic also at the liquid state.

preprint1995arXiv

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

preprint1995arXiv

First-principle study of excitonic self-trapping in diamond

We present a first-principles study of excitonic self-trapping in diamond. Our calculation provides evidence for self-trapping of the 1s core exciton and gives a coherent interpretation of recent experimental X-ray absorption and emission data. Self-trapping does not occur in the case of a single valence exciton. We predict, however, that self-trapping should occur in the case of a valence biexciton. This process is accompanied by a large local relaxation of the lattice which could be observed experimentally.

preprint1994arXiv

Order-N Density-Matrix Electronic-Structure Method for General Potentials

A new order-N method for calculating the electronic structure of general (non-tight-binding) potentials is presented. The method uses a combination of the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and Daw, and a representation of the density matrix based on ``travelling basis orbitals''. The method is applied to several one-dimensional examples, including the free electron gas, the ``Morse'' bound-state potential, a discontinuous potential that mimics an interface, and an oscillatory potential that mimics a semiconductor. The method is found to contain Friedel oscillations, quantization of charge in bound states, and band gap formation. Quantitatively accurate agreement with exact results is found in most cases. Possible advantages with regard to treating electron-electron interactions and arbitrary boundary conditions are discussed.

preprint1996arXiv

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.

preprint1995arXiv

Numerical Studies on the Magnetism of Fe-Ni-Mn Alloys in the Invar Region

By means of self-consistent semi-empirical LCAO calculations we study the itinerant magnetism of (Fe_{0.65}Ni_{0.35})_{1-y} Mn_y alloys for y between 0 and 0.22 at T=0 K, neglecting only the transverse spin components. We find that the magnetic behaviour is quite complicated on a local scale. In addition to ferromagnetic behaviour, also metastable spin-glass-like configurations are found. In the same approach, using a direct numerical calculation by the Kubo-Formalism without any fit parameters, we also calculate the electrical conductance in the magnetic state and find that the $y$ -dependence observed in the experiments is well reproduced by our calculations, except of an overall factor of rougly 5, by which our resistivities are too large.

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