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At least 109 records · Page 6Linked to original sources
arXiv1995

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.

mtrl-th↗
arXiv1995

Real-Space Adaptive-Coordinate Electronic Structure Calculations

We present a real-space adaptive-coordinate method, which combines the advantages of the finite-difference approach with the accuracy and flexibility of the adaptive coordinate method. The discretized Kohn-Sham equations are written in generalized curvilinear coordinates and solved self-consistently by means of an iterative approach. The Poisson equation is solved in real space using the Multigrid algorithm. We implemented the method on a massively parallel computer, and applied it to the calculation of the equilibrium geometry and harmonic vibrational frequencies of the CO_2, CO, N_2 and F_2 molecules, yielding excellent agreement with the results of accurate quantum chemistry and Local Density Functional calculations.

mtrl-th↗
arXiv1995

Band structure analysis of the conduction-band mass anisotropy in 6H and 4H SiC

The band structures of 6H and 4H SiC calculated by means of the FP-LMTO method are used to determine the effective mass tensors for their conduction-band minima. The results are shown to be consistent with recent optically detected cyclotron resonance measurements and predict an unusual band filling dependence for 6H-SiC.

mtrl-th↗
arXiv1995

Large Scale Electronic Structure Calculations with Multigrid Acceleration

We have developed a set of techniques for performing large scale ab initio calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods permit efficient calculations on ill-conditioned systems with long length scales or high energy cutoffs. The technique has been applied to systems containing up to 100 atoms, including a highly elongated diamond cell, an isolated C$_{60}$ molecule, and a 32-atom cell of GaN with the Ga d-states in valence. The method is well suited for implementation on both vector and massively parallel architectures.

mtrl-th↗
arXiv1995

Polymorphism and metastability in NbN: Structural predictions from first principles

We use ab initio pseudopotential total energy calculations with a plane wave basis set to investigate the structural energetics of various phases of polymorphic NbN. Particular attention is given to its recently discovered superconducting phase with a $T_c$ of 16.4 K, reported to have the NbO structure type. Results of total energy calculations show that it is in fact energetically unfavorable for NbN to form in this cubic structure, and its predicted theoretical lattice constant is significantly smaller than the experimental value. Various approaches to the identification of an alternative structure are discussed. In preliminary investigations, we have found two new structures that are energetically more favorable than the NbO structure.

mtrl-th↗
arXiv1995

Metastability of NbN in the ordered vacancy NbO phase

A metastable phase of NbN with superconducting Tc=16.4 K was reported recently by Treece and collaborators. The reported structure of the thin film sample deviates from the rocksalt (B1) NbN structure with 25% ordered vacancies on each sublattice (space group Pm3m) and a lattice constant of 4.442 Angstroms. Using full potential electronic structure methods, we contrast the electronic structure with that of B1 NbN. The calculated energy, 1.00 eV/molecule higher than B1 NbN, and calculated lattice constant of 4.214 Angstroms indicate that the new phase must be something other than the ordered stoichiometric Pm3m phase.

mtrl-th↗
arXiv1995

Insulator-metal transition in solid hydrogen: Implication of electronic structure calculations for recent experiments

Electronic structure calculations for compressed molecular hydrogen are performed to provide more insight into the diversity of phenomena recently observed experimentally. We perform full-potential LAPW calculations and analyze them in terms of a molecular tight-binding model. We show that $σ_g$ and $σ_u$ bands overlap occurs at rather low pressure, while an insulating state persists at specific orientations up to 200 GPa, in accord with previous work, and is due to opening of hybridization gaps at the Fermi level. We also present calculations of electronic properties such as plasma frequencies and electron-vibron coupling.

mtrl-th↗
arXiv1995

Adlayer core-level shifts of admetal monolayers on transition metal substrates and their relation to the surface chemical reactivity

Using density-functional-theory we study the electronic and structural properties of a monolayer of Cu on the fcc (100) and (111) surfaces of the late 4d transition metals, as well as a monolayer of Pd on Mo bcc(110). We calculate the ground states of these systems, as well as the difference of the ionization energies of an adlayer core electron and a core electron of the clean surface of the adlayer metal. The theoretical results are compared to available experimental data and discussed in a simple physical picture; it is shown why and how adlayer core-level binding energy shifts can be used to deduce information on the adlayer's chemical reactivity.

mtrl-th↗
arXiv1996

Semiconductor effective charges from tight-binding theory

We calculate the transverse effective charges of zincblende compound semiconductors using Harrison's tight-binding model to describe the electronic structure. Our results, which are essentially exact within the model, are found to be in much better agreement with experiment than previous perturbation-theory estimates. Efforts to improve the results by using more sophisticated variants of the tight-binding model were actually less successful. The results underline the importance of including quantities that are sensitive to the electronic wavefunctions, such as the effective charges, in the fitting of tight-binding models.

mtrl-th↗
arXiv1995

Wetting of the Au(110) substrate: Homoepitaxial islands and layers

The morphology of epitaxial structures formed in solidification of liquid Au films on the Au(110) surface was studied by molecular dynamics simulation based on a many--body interatomic potential. The (1x2) reconstructed and smooth phase at the temperature T=500 K, as well as a deconstructed and rough phase of the Au(110) substrate at T=900 K, were investigated. The three--dimensional islands with the (111) oriented facets were formed in solidification of thinner liquid films. At the same time, the substrate below these islands underwent a weak faceting process. Conversely, the solidification of thicker liquid films resulted in the flat solid films. The two surface phases of the Au(110) substrate induced different structure of these epitaxial solid films. The films were studied via construction of the Voronoi polyhedra, density profiles, and surface stress calculations.

mtrl-th↗
arXiv1995

Spin-dependent resonant tunneling through semimetallic ErAs quantum wells

Resonant tunneling through semimetallic ErAs quantum wells embedded in GaAs structures with AlAs barriers was recently found to exhibit an intriguing behavior in magnetic fields which is explained in terms of tunneling selection rules and the spin-polarized band structure including spin-orbit coupling.

mtrl-th↗
arXiv1995

Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes

Using first-principles total-energy pseudopotential calculations, we have studied the properties of chains of potassium and aluminum in nanotubes. For BN tubes, there is little interaction between the metal chains and the tubes, and the conductivity of these tubes is through carriers located at the inner part of the tube. In contrast, for small radius carbon nanotubes, there are two types of interactions: charge-transfer (dominant for alkali atoms) leading to strong ionic cohesion, and hybridization (for multivalent metal atoms) resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show that both effects contribute. New electronic properties related to these confined atomic chains of metal are analyzed.

mtrl-th↗
arXiv1995

Ab initio study of the volume dependence of dynamical and thermodynamical properties of silicon

Motivated by the negative thermal expansion observed for silicon between 20 K and 120 K, we present first an ab initio study of the volume dependence of interatomic force constants, phonon frequencies of TA(X) and TA(L) modes, and of the associated mode Gruneisen parameters. The influence of successive nearest neighbors shells is analysed. Analytical formulas, taking into account interactions up to second nearest neighbors, are developped for phonon frequencies of TA(X) and TA(L) modes and the corresponding mode Gruneisen parameters. We also analyze the volume and pressure dependence of various thermodynamic properties (specific heat, bulk modulus, thermal expansion), and point out the effect of the negative mode Gruneisen parameters of the acoustic branches on these properties. Finally, we present the evolution of the mean square atomic displacement and of the atomic temperature factor with the temperature for different volumes, for which the anomalous effects are even greater.

mtrl-th↗
arXiv1995

Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3

Using path-integral Monte Carol simulations and an ab initio effective Hamiltonian, we study the effects of quantum fluctuations on structural phase transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find quantum fluctuations affect ferroelectric (FE) transitions more strongly than antiferrodistortive (AFD) ones, even though the effective mass of a single FE local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress the FE transition completely, and reduce the AFD transition temperature from 130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the transition, but do reduce the transition temperature by 35-50 K. The implications of the calculations are discussed.

mtrl-th↗
arXiv1995

Microscopic Modeling of the Growth of Order in an Alloy: Nucleated and Continuous Ordering

We study the early-stages of ordering in $Cu_3 Au$ using a model Hamiltonian derived from the effective medium theory of cohesion in metals: an approach providing a microscopic description of interatomic interactions in alloys. Our simulations show a crossover from a nucleated growth regime to a region where the ordering does not follow any simple growth laws. This mirrors the experimental observations in $Cu_3 Au$. The kinetics of growth, obtained from the simulations, is in semi-quantitative agreement with experiments. The real-space structures observed in our simulations offer some insight into the nature of early-stage kinetics

cond-mat↗
arXiv1995

Structural and Magnetic Properties of Trigonal Iron

First principles calculations of the electronic structure of trigonal iron were performed using density function theory. The results are used to predict lattice spacings, magnetic moments and elastic properties; these are in good agreement with experiment for both the bcc and fcc structures. We find however, that in extracting these quantities great care must be taken in interpreting numerical fits to the calculated total energies. In addition, the results for bulk iron give insight into the properties of thin iron films. Thin films grown on substrates with mismatched lattice constants often have non-cubic symmetry. If they are thicker than a few monolayers their electronic structure is similar to a bulk material with an appropriately distorted geometry, as in our trigonal calculations. We recast our bulk results in terms of an iron film grown on the (111) surface of an fcc substrate, and find the predicted strain energies and moments accurately reflect the trends for iron growth on a variety of substrates.

mtrl-th↗
arXiv1995

Pressure Dependence of Born Effective Charges, Dielectric Constant and Lattice Dynamics in SiC

The pressure dependence of the Born effective charge, dielectric constant and zone-center LO and TO phonons have been determined for $3C$-SiC by a linear response method based on the linearized augmented plane wave calculations within the local density approximation. The Born effective charges are found to increase nearly linearly with decreasing volume down to the smallest volume studied, $V/V_0=0.78$, corresponding to a pressure of about 0.8 Mbar. This seems to be in contradiction with the conclusion of the turnover behavior recently reported by Liu and Vohra [Phys.\ Rev.\ Lett.\ {\bf 72}, 4105 (1994)] for $6H$-SiC. Reanalyzing their procedure to extract the pressure dependence of the Born effective charges, we suggest that the turnover behavior they obtained is due to approximations in the assumed pressure dependence of the dielectric constant $\varepsilon_\infty$, the use of a singular set of experimental data for the equation of state, and the uncertainty in measured phonon frequencies, especially at high pressure.

mtrl-th↗