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The self-consistent one-electron wave functions and energy bands obtained by the LMTO-ASA method within the local density approximation (LDA) are used to calculate the wave vector and frequency dependent non-interacting spin susceptibility of paramagnetic La2CuO4 in the body-centred tetragonal (bct) structure. We show that the tendency towards the antiferromagnetic instability is strongly dependent on the effects of the matrix elements which lead to a substantial depression of the susceptibility, especially near the X-point. The Fermi surface nesting properties, although important for the susceptibility, are by far not sufficient for the instability and the interband transitions turn out to be of great significance. Our results indicate that the susceptibility is at least 3 times too small to drive this system through a transition to the antiferromagnetic state, and we discuss possible reasons for this failure.
For bulk transition metals, a first-principles generalized pseudopotential theory (GPT) of interatomic potentials has been developed in which the cohesive-energy unctional takes the form of a volume term plus sums over widely transferable two-, three-, and four-ion potentials. The GPT has been further extended to surfaces by making an internal transformation of this functional to an embedded-atom-like format in which the embedding function is identified as the bulk volume term and the atomic volume is replaced by an average electron density. Applications of the bulk and surface GPT to the calculation of structural, vacancy-formation, and surface energies in Cu and Mo, and to the investigation of surface relaxation and reconstruction in Mo are discussed.
A study of the zero temperature phase transitions in hydrogen under megabar pressures using a first-principles total-energy method is presented. An anisotropic primitive hexagonal phase is found to be particularly stable relative to other monatomic phases for pressures between 4 and 8 megabars. Calculations of the vibrational frequencies show that this phase is unstable with respect to a distortion tripling the unit cell along the c-axis. Results for this distorted hexagonal phase will be presented, including a calculation of its superconducting transition temperature Tc.
We report results from a first-principles local spin density quantum mechanical study of the energetics and elastic properties of a series of magnesium-oxygen clusters of various morphologies. The role of quantum effects, e.g. covalency, in the bonding character of diatomic MgO is determined by comparison of classical and quantum restoring force curves. The dependence of binding properties on geometry and metal to oxygen ratio is determined by comparison of binding energy curves for a series of clusters. Results show that while gross features of the binding curves may be represented by simple interatomic potentials, details require the many body corrections of a full quantum treatment.
Multiple scattering theory (MST) provides an efficient technique for solving the wave equation for the special case of muffin-tin potentials. Here MST is extended to treat space filling non- muffin tin potentials and its validity, accuracy and efficiency are tested by application of the two dimensional empty lattice test. For this test it is found that the traditional formulation of MST does not converge as the number of partial waves is increased. A simple modification of MST, however, allows this problem to be solved exactly and efficiently.
We have performed a number of first principles electronic structure calculations for YBa 2Cu 3O7_y with different oxygen orderings and concentrations. The resulting total energies have been used to assess the applicability of some of the proposed models for oxygen ordering in this system. We find that the results are consistent with an Ising-like model with asymmetric next-neighbor interactions between oxygen sites. We determine effective interaction parameters from the first principles calculations and use them to compute the phase diagram for the system, which is in excellent agreement with experiment for the tetragonal-orthorhombic I transition.
Tight-binding total energy computations are used to examine the chemical bonding and electronic structure for two new minimum-energy surface atomic structures for p(lxl) overlayers of Sb on III-V(110) surfaces. The bonding in each of these structures is unique, having no analog in either the bulk or small molecule coordination chemistry of these materials, and is a phenomenon uniquely associated with the constrained epitaxical growth of the Sb overlayer.
A detailed picture of dissociation and adsorption of oxygen on the Si(100) reconstructed surface is presented on the basis of the total-energy band structure and force calculations within the local density approximation with use of the normconserving nonlocal pseudopotentials. Dissociation of an oxygen molecule occurs at any site on the Si(100) surface. The resulting oxygen atom is adsorbed on several (meta)stable sites depending on which site the preceding molecule dissociates. Peculiar relaxation of the top-layer Si atoms is found upon oxygen adsorption. Calculated vibrational energy and valence density of states in the most stable geometry are reasonably consistent with the experimental data available, i.e., HREELS and UPS. Finally, we have found that an oxygen molecule penetrates through the oxygen-covered Si(100) surface, on which the Si dangling bonds are terminated, and then dissociates in the vicinity of the Si bond center site.
Using the embedded atom method (EAM), we have calculated the stable configurations of Pt, Pd, and Ni clusters, containing up to nine adatoms, on the Pt (001) surface. For Pt, we predict that the stable configurations are linear chains oriented along the <110> directions for three and five adatoms, and close-packed islands otherwise. These results, for clusters containing up to seven adatoms, were reported previously in reference 4. For Pd, the results are the same except that the stable configuration predicted for five adatoms is a close-packed island. For Ni, we predict that linear chains are the stable configurations for all numbers of adatoms. In determining these stable configurations, we allowed substrate relaxations. To assess the importance of substrate relaxations, we also performed calculations on an unrelaxed substrate. We found that substrate relaxations are important with regard to the stable configurations, and that allowing relaxation lowers the energies of chains with respect to islands.
We have begun to investigate theoretically the electronic properties of several ideal epitaxial interfaces of diamond with Ni and Cu, both of which enjoy a close lattice match. Of particular interest is the mechanism responsible for formation of the Schottky barrier, which is not yet fully understood at the microscopic level. We find that both the barrier height and the chemical bonding at the interface are strongly dependent on interface orientation (i.e., the relative positioning of the two surfaces). For orientations near the minimum total energy geometry, the calculated Fermi level is apparently pinned around 1.7 and 2.1 eV for the (111) and (001) interfaces, respectively, relative to the valence band maximum. For orientations not near the total energy minimum, the calculated barrier height is zero. A tentative explanation for this difference is proposed.
Pairwise interactions lead to close-packed structures which can be easily described by classical potentials. However, covalent solids such as silicon or silica form open structures which cannot be described by such interactions. We have developed new interatomic force fields which describe the phase stability of silicon and small silicon clusters. We consider three body forces which are adjusted to describe “covalent” →, “metallic” phase transitions instead of small amplitude atomic vibrations. These forces result in a highly accurate phase diagram for crystalline polytypes. Our force filed can be easily modified to describe energies and structures of Sin vapor phase clusters. A key aspect of the cluster problem is the transfer of bond strength from dangling bonds to back bonds. We show as a function of this back bond strength that a covalent →. metallic “first order” structural phase transition occurs in the cluster structures. We also examine the role of three body forces in silica in light of recent pressure measurements. We find that the recently developed pairwise forces are not sufficient to predict accurately the pressure dependence of the structural parameters.
Using the linear muffin-tin orbital method in the atomic sphere approximation (LMTO-ASA), we studied the electronic structure of the Si(111) interface for four different materials: CaF2, NiSi2, CoSi2, and YSi2. We examined how the interface states and Schottky barrier height depend on the interface atomic structure.
We have derived the embedding energy functional and two body potential of the Embedded Atom Method (EAM) using decreasing exponentials for both the electron density and the two body potential. The embedding function was obtained from the equation of state given by Rose et al. Because of the form of the embedding function, the equilibrium lattice constant, cohesive energy, and bulk modulus are automatically satisfied. The two parameters Φe and γ of the two body potential were determined by fitting to shear modulus and the single vacancy formation energy. Contributions of up to the third nearest neighbors were included in the evaluation of the charge density ρ and the two body potential Φ. The stability and anisotropy of each structure were estimated and compared with the available experimental data.
A detailed study of the pressure-induced phase transitions at zero temperature in InSb up to 40 GPa using a first-principles pseudopotential total-energy method is presented. In addition to InSb(I) (cubic) and (II) (polar β-Sn), we identify InSb(III) as a hexagonal phase (found earlier for GaSb) and (VI) as a polar bcc phase in agreement with recent experiments. New structural models, orthorhombic polar β-Sn and body-centered orthorhombic, are proposed as candidates for the InSb(IV) and (V) phases based on total-energy minimizations. These findings are compared with recent results for GaAs to illustrate the trends in transition paths among III-V compounds.
Using a first principles pseudopotential total energy approach with a localized basis for the electronic wave functions, we have investigated the structural and bonding properties of β-Si3N4 and β-C3N4, which is a proposed structure for carbon nitride. The latter system is used as a prototype for studying the properties of possible covalent C-N solids. For β-Si3N4, calculated structural properties such as lattice constant and bulk modulus are in excellent agreement with experimental values. This gives support for the predicted properties of β-C3N4.β/C3N4 is found to be a good candidate for a new low-compressibility solid, with compressibility comparable to diamond. Despite similarities between β-Si3N4 and β-C3N4 in terms of crystal structure and valency of constituent elements, differences are found in their electronic bonding properties. A comparison of the bonding in β-Si3N4 and β-C3N4 with other first and second row semiconductors is useful for understanding trends in the structural properties of these materials.
For the class of materials in which covalent effects are important, there is still no simple and reliable scheme, adapted to computer simulations, that can handle angle de- pendent forces. Either they are based on the introduction of three body (or higher)  interactions, or demand unphysical behavior from the many body functions used [2,3]. In the first case, computer efficiency is considerably low due to the large amounts of calculations required; in the second case a negative curvature of the embedding function must be assumed for materials in which the Cauchy pressure is negative, and this is contrary to the current interpretations of that function.
In the present work we derive a method to introduce many body shear forces, suited to computer simulations, which is free from the shortcomings mentioned above.
We make use of the existing formalism for calculating atomic forces within the local density approximation (LDA) to determine forces in all-electron, local orbital electronic structure calculations. The forces are calculated as the proper total energy derivatives, including the necessary basis-set corrections. Our technique evaluates the LDA potential exactly on a variationally determined integration mesh which allows all integrals relevant to the electronic structure problem to be computed to any desired accuracy. We demonstrate the high accuracy of forces calculated using our method with an application to the molybdenum dimer. Several issues concerning the accuracy of the forces are discussed, including self-consistency effects, and the effects of integration error. We discuss the use of the forces in dynamical routines for geometry optimizations.
A conceptually complete formalism for the quasiparticle effective masses in semiconductors is proposed. Our approach is based. on a generalized form of the theory, including the effects of the nonlocal, energy dependent electron self: energy operator Σ, which accounts for the electron-electron interaction. This introduces two -important effects on the expression of the effective mass: an explicit energy renormalization and an extra contribution to the matrix element that enters the usual . Our preliminary numerical results for prototypical GaAs show promising improvements over the results from the local density approximation for the calculated electron effective mass compared to experimental data.
Indium compounds and corresponding epitaxially grown superlattices have provided good single crystals suitable for accurate experimental measurements and have added new interest to the study of the constituent bulk semiconductors and the II-IV-V2 chalcopyrite crystal phases. This paper reports results of structural and electronic properties of narrow gap binary and ternary semiconductors determined selfconsistently from first principles using both the full potential linearized augmented plane wave (FLAPW) and norm-conserving pseudopotentials (PP) total-energy methods.
The optical response of thin dielectric films can be influenced by grain morphology and the presence and distribution of defects. In the limit of random defects and small electric field amplitudes, approximate methods exist to model the real part of the dielectric constant in terms of volume fractions and bulk dielectric constants of the film components. Explicit inclusion of nonlinear polarizabilities and details of the microstructure, such as particle phase, shape, and orientation requires a more exact approach.
We have developed a method to self-consistently determine the local internal electric field and polarization in the long wavelength limit for model films with a random distributions of defects of arbitrary phase and orientation. From this we have calculated the real part of the dielectric constant as a function of nonlinear polarizability of the components, and have shown the effect of defect phase and orientation on the dielectric constant of the film.