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Showing papers in "Physical Review B in 1986"


Journal ArticleDOI
John P. Perdew1
TL;DR: Numerical results for atoms, positive ions, and surfaces are close to the exact correlation energies, with major improvements over the original LM approximation for the ions and surfaces.
Abstract: Langreth and Mehl (LM) and co-workers have developed a useful spin-density functional for the correlation energy of an electronic system. Here the LM functional is improved in two ways: (1) The natural separation between exchange and correlation is made, so that the density-gradient expansion of each is recovered in the slowly varying limit. (2) Uniform-gas and inhomogeneity effects beyond the randomphase approximation are built in. Numerical results for atoms, positive ions, and surfaces are close to the exact correlation energies, with major improvements over the original LM approximation for the ions and surfaces.

16,378 citations


Journal ArticleDOI
TL;DR: A consistent set of embedding functions and pair interactions for use with the embedded-atom method was determined empirically by fitting to the sublimation energy, equilibrium lattice constant, elastic constants, and vacancy-formation energies of the pure metals and the heats of solution of the binary alloys as discussed by the authors.
Abstract: A consistent set of embedding functions and pair interactions for use with the embedded-atom method [M.S. Daw and M. I. Baskes, Phys. Rev. B 29, 6443 (1984)] have been determined empirically to describe the fcc metals Cu, Ag, Au, Ni, Pd, and Pt as well as alloys containing these metals. The functions are determined empirically by fitting to the sublimation energy, equilibrium lattice constant, elastic constants, and vacancy-formation energies of the pure metals and the heats of solution of the binary alloys. The validity of the functions is tested by computing a wide range of properties: the formation volume and migration energy of vacancies, the formation energy, formation volume, and migration energy of divacancies and self-interstitials, the surface energy and geometries of the low-index surfaces of the pure metals, and the segregation energy of substitutional impurities to (100) surfaces.

3,734 citations


Journal ArticleDOI
TL;DR: Improvements over other simple functionals are also found in the exchange contributions to the valence-shell removal energy of an atom and to the surface energy of jellium within the infinite barrier model.
Abstract: The electronic exchange energy as a functional of the density may be approximated as ${E}_{x}[n]={A}_{x}\ensuremath{\int}{d}^{3}r{n}^{\frac{4}{3}}F(s)$, where $s=\frac{|\ensuremath{ abla}n|}{2{k}_{F}n}$, ${k}_{F}={(3{\ensuremath{\pi}}^{2}n)}^{\frac{1}{3}}$, and $F(s)={(1+1.296{s}^{2}+14{s}^{4}+0.2{s}^{6})}^{\frac{1}{15}}$. The basis for this approximation is the gradient expansion of the exchange hole, with real-space cutoffs chosen to guarantee that the hole is negative everywhere and represents a deficit of one electron. Unlike the previously publsihed version of it, this functional is simple enough to be applied routinely in self-consistent calculations for atoms, molecules, and solids. Calculated exchange energies for atoms fall within 1% of Hartree-Fock values. Significant improvements over other simple functionals are also found in the exchange contributions to the valence-shell removal energy of an atom and to the surface energy of jellium within the infinite barrier model.

3,500 citations


Journal ArticleDOI
TL;DR: In this article, a first-principles theory of the quasiparticle energies in semiconductors and insulators described in terms of the electron self-energy operator is presented.
Abstract: We present a first-principles theory of the quasiparticle energies in semiconductors and insulators described in terms of the electron self-energy operator. The full dielectric matrix is used to evaluate the self-energy operator in the GW approximation: the first term in an expansion of the self-energy operator in terms of the dynamically screened Coulomb interaction (W) and the dressed Green's function (G). Quasiparticle energies are calculated for the homopolar materials diamond, Si, and Ge as well as for the ionic compound LiCl. The results are in excellent agreement with available experimental data. In particular, the indirect band gap is calculated as 5.5, 1.29, and 0.75 eV as compared with experimental gaps of 5.48, 1.17, and 0.744 eV for diamond, Si, and Ge, respectively. The Ge results include relativistic effects. The calculated direct gap for LiCl is within 5% of experiment. Viewed as a correction to the density-functional eigenvalues calculated with the local-density approximation, the present results show a correction dominated by a large jump at the gap. It is found that because of the charge inhomogeneity, the full dielectric screening matrix must be included, i.e., local-field effects are essential. The dynamical effects are also found to be crucial. The required dielectric matrices are obtained within the density-functional approach for the static case and extended to finite frequency with use of a generalized plasmon-pole model based on sum rules. The model reproduces the \ensuremath{\omega} and ${\ensuremath{\omega}}^{\mathrm{\ensuremath{-}}1}$ moments of the exact many-body response function. The qualitative features of the electron self-energy operator are discussed. Using the static Coulomb-hole--screened-exchange approximation for illustration, the role of local fields in the self-energy operator are explained. The role of dynamical renormalization is illustrated. The same qualitative features are observed in both the homopolar and ionic materials.

2,717 citations



Journal ArticleDOI
TL;DR: A theory of the generation process is presented, and how the spatial shape of the stress pulse is related to the optical, electronic, and acoustical properties of the material is described.
Abstract: We report experiments in which picosecond light pulses are used to generate and detect very short stress pulses (coherent longitudinal phonons). We present a theory of the generation process, and describe how the spatial shape of the stress pulse is related to the optical, electronic, and acoustical properties of the material. The stress pulses are detected through a measurement of the changes they induce in the optical reflectivity of the sample surface. We describe the theory of this effect. We present experimental results we have obtained for a-${\mathrm{As}}_{2}$${\mathrm{Te}}_{3}$, a-Ge, a-${\mathrm{As}}_{2}$${\mathrm{Se}}_{3}$, and Ni.

1,347 citations


Journal ArticleDOI
TL;DR: A theoretical study of the structural and electronic properties of pseudomorphic Si/Ge interfaces, in which the layers are strained such that the lattice spacing parallel to the interface is equal on both sides.
Abstract: We present a theoretical study of the structural and electronic properties of pseudomorphic Si/Ge interfaces, in which the layers are strained such that the lattice spacing parallel to the interface is equal on both sides. The self-consistent calculations, based on the local density functional and ab initio pseudopotentials, determine the atomic structures and strains of minimum energy, and the lineup of the Si and Ge band structures. The presence of the strains causes significant shifts and splittings of the bulk bands. We derive values for the band discontinuities for (001), (111), and (110) interfaces under different strain conditions, and discuss the validity of the density-functional methods for the analysis of the interface problem. Spin-orbit splitting effects in the valence bands are included a posteriori. We express our results in terms of discontinuities in the valence bands, and deformation potentials for the bulk bands, and compare them with recent experiments on Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ heterostructures.

1,108 citations


Journal ArticleDOI
TL;DR: On montre que les concepts de percolation conduisent a la definition d'une longueur caracteristique pour la permeabilite dans des milieux poreux aleatoires.
Abstract: We show that percolation concepts lead to the definition of a characteristic length for the permeability in random porous media. Application of the model to sandstone and carbonate rocks yields quantitative agreement between theory and experiment with no adjustable parameters.

1,012 citations


Journal ArticleDOI
TL;DR: The dispersion relations are solved for waves guided by a thin, lossy metal film surrounded by media of dielectric constant and both radiative waves can be interpreted as spatial transients, which could have physical significance near a transverse plane.
Abstract: The dispersion relations are solved for waves guided by a thin, lossy metal film surrounded by media of dielectric constant ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{3}$. For symmetric structures (${\ensuremath{\epsilon}}_{1}$=${\ensuremath{\epsilon}}_{3}$), there are the usual two Fano modes whose velocity and attenuation vary with film thickness. For very thin films, one of these modes can attain multicentimeter propagation distances when \ensuremath{\lambda}g1 \ensuremath{\mu}m. In addition, there are two leaky waves which correspond to waves localized at the ${\ensuremath{\epsilon}}_{1}$ (or ${\ensuremath{\epsilon}}_{3}$) dielectric-metal interface whose fields decay exponentially across the metal film and radiate an angular spectrum of plane waves into ${\ensuremath{\epsilon}}_{3}$ (or ${\ensuremath{\epsilon}}_{1}$, respectively). Both radiative waves can be interpreted as spatial transients, which could have physical significance near a transverse plane. When ${\ensuremath{\epsilon}}_{1}$\ensuremath{ e}${\ensuremath{\epsilon}}_{3}$, there are still four distinct solutions for a given film thickness, two radiative and two nonradiative. For lossy films, there are always two nonradiative solutions for thick enough films. As the thickness goes to infinity, the four solutions reduce to two waves, each radiative and nonradiative pair becoming degenerate. The physical interpretation of these solutions and their dependence on dielectric constant and wavelength are discussed.

936 citations


Journal ArticleDOI
Ian K. Robinson1
TL;DR: In this paper, the authors present x-ray-diffraction profiles from a variety of different crystals which are characteristically diffuse in the direction perpendicular to the surface through which the incident and diffracted beams pass, but sharp in both parallel directions.
Abstract: We present x-ray-diffraction profiles from a variety of different crystals which are characteristically diffuse in the direction perpendicular to the surface through which the incident and diffracted beams pass, but sharp in both parallel directions. We show that these effects arise from truncation of the crystal lattice at the surface. To explain the precise form of the momentum-transfer dependence of the intensity across the reciprocal-space zone, it is necessary to include the effects of surface roughness on an atomic scale. Such measurements therefore allow highly sensitive roughness determinations to be made. Understanding the origin of these streaks of intensity will have significant impact on the practice of x-ray crystallographic determinations of surface structure.

754 citations


Journal ArticleDOI
TL;DR: Single-photon- and multiphoton-induced luminescence spectra were obtained from clean samples of silver, copper, and gold with both smooth and rough surfaces and reveal new features which are correlated with interband transitions at selected symmetry points in the Brillouin zone.
Abstract: Single-photon- and multiphoton-induced luminescence spectra were obtained from clean samples of silver, copper, and gold with both smooth and rough surfaces. The spectra reveal new features which are correlated with interband transitions at selected symmetry points in the Brillouin zone. Calculating luminescence spectra based on simplified models of the band structures of the noble metals and taking into account the Fresnel local-field corrections, we find qualitative agreement with the observed spectra from smooth samples. The agreement between theory and experiment is less satisfactory for rough samples. The influence of surface roughness on the luminescence is largely attributable to local-field enhancement in the rough surface protrusions.

Journal ArticleDOI
TL;DR: The real part, n, of the complex index of refraction is determined to be n(E)=n(\ensuremath{\infty})+ (${E}^{2}$-BE+C) using Kramers-Kronig analysis, where n is a constant greater than unity.
Abstract: An expression for the imaginary part, k, of the complex index of refraction, N=n-ik, for amorphous materials is derived as a function of photon energy E: k(E)=A(E-${E}_{g}$${)}^{2}$/(${E}^{2}$-BE+C) where A, B, and C are positive nonzero constants characteristic of the medium such that 4C-${B}^{2}$g0. ${E}_{g}$ represents the optical energy band gap. The real part, n, of the complex index of refraction is then determined to be n(E)=n(\ensuremath{\infty})+(${B}_{0}$E+${C}_{0}$)/ (${E}^{2}$-BE+C) using Kramers-Kronig analysis, where ${B}_{0}$ and ${C}_{0}$ are constants that depend on A, B, C, and ${E}_{g}$, and n(\ensuremath{\infty}) is a constant greater than unity. Excellent agreement was found between these formulas and experimentally measured and published values of n and k of amorphous silicon, hydrogenated amorphous silicon, amorphous silicon nitride, and titanium dioxide.

Journal ArticleDOI
TL;DR: A new phenomenon involving the dynamic localization of the moving particle is shown to result in the case of a sinusoidally varying field: the particle is generally delocalized except for the cases when the ratio of the field magnitude and the field frequency is a root of the ordinary Bessel function of order 0.
Abstract: The motion of a charged particle on a discrete lattice under the action of an electric field is studied with the help of explicit calculations of probability propagators and mean-square displacements. Exact results are presented for arbitrary time dependence of the electric field on a one-dimensional lattice. Existing results for the limiting cases of zero frequency and zero field are recovered. A new phenomenon involving the dynamic localization of the moving particle is shown to result in the case of a sinusoidally varying field: The particle is generally delocalized except for the cases when the ratio of the field magnitude and the field frequency is a root of the ordinary Bessel function of order 0. For these special cases it is found to be localized. This localization could be used, in principle, for inducing anisotropy in the transport properties of an ordinarily isotropic material.

Journal ArticleDOI
TL;DR: In this article, it was shown that paramagnon exchange near a spin-density-wave instability gives rise to a strong singlet-$d$-wave pairing interaction.
Abstract: We investigate the three-dimensional Hubbard model and show that paramagnon exchange near a spin-density-wave instability gives rise to a strong singlet $d$-wave pairing interaction. For a cubic band the singlet (${d}_{{x}^{2}\ensuremath{-}{y}^{2}} \mathrm{and} {d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$) channels are enhanced while the singlet (${d}_{\mathrm{xy}},{d}_{\mathrm{xz}},{d}_{\mathrm{yz}}$) and triplet $p$-wave channels are suppressed. A unique feature of this pairing mechanism is its sensitivity to band structure and band filling.

Journal ArticleDOI
TL;DR: In this article, the Si(001) surface has been examined with use of scanning tunneling microscopy (STM) images, revealing a dimer-type reconstruction and inconsistent with chain and vacancy models.
Abstract: The atomic structure of the Si(001) surface has been examined with use of scanning tunneling microscopy (STM). The STM images reveal a dimer-type reconstruction and are inconsistent with chain and vacancy models. Both buckled and nonbuckled dimers are observed, giving rise to regions of (2×1), c(4×2), and p (2×2) symmetry. The surface has a high density of vacancy-type defects, which appear to induce or stabilize buckling of the dimers at room temperature. The STM images also reveal the atomic structure at steps and defects. At high annealing temperature the step density increases dramatically, eventually leading to faceting.

Journal ArticleDOI
TL;DR: This work has shown that not only the intensity of the response of the immune system to carbon dioxide but also its ability to reprogram theresponse of the nervous system to accommodate high levels of carbon dioxide.
Abstract: Note: Tagantsev, Ak Af Ioffe Engn Phys Inst,Leningrad 194021,UssrPart 2E4504Times Cited:19Cited References Count:16 Reference LC-ARTICLE-1986-010doi:10.1103/PhysRevB.34.5883 Record created on 2006-08-21, modified on 2017-11-27

Journal ArticleDOI
TL;DR: In this article, the authors derived the tight-binding (TB) orbitals by exact transformation of the conventional set of linear muffin-tin orbitals (LMTO's) for crystalline silicon.
Abstract: Plots of the tight-binding (TB) orbitals recently derived by exact transformation of the conventional set of linear muffin-tin orbitals (LMTO's) are presented for crystalline silicon. The TB-LMTO's are found to be extremely compact. As a simple application we show how non-spherically-averaged charge densities may be obtained from standard LMTO calculations. For silicon this charge density is found to be in excellent agreement with the one obtained from a linear augmented plane-wave full-potential calculation. This is true even when the LMTO calculation employs the atomic-sphere approximation for the one-electron potential. A self-contained account of the TB-MTO formalism is presented and a simple way of including the quadratic energy dependence of the MTO's is derived.

Journal ArticleDOI
Markus Büttiker1
TL;DR: This work investigates the resistance of a series of two (or more) ob- stacles and study the transition from completely coherent transmission through the sample to completely incoherent transmission.
Abstract: Landauer's approach which yields the resistance of an obstacle in an otherwise perfect wire due to elastic scattering at the obstacle is augmented by including localized inelastic scatterers within the sample. The inelastic scatterers invoked consist of an electron reservoir coupled via a lead to the wire. The key advantage of this method is that the effect of inelastic scattering can be studied by solving an elastic scattering problem. We investigate the resistance of a series of two (or more) ob- stacles and study the transition from completely coherent transmission through the sample to completely incoherent transmission. For a sample with a small transmission probability, increasing inelastic scattering decreases the resistance. At an intermediate value of inelastic scattering, the resistance reaches a minimum to increase again when inelastic scattering processes start to dominate the resistance.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the effects at a temperature-driven first-order transition by analyzing various moments of the energy distribution and the rounding of the singularities and the shifts in the location of the specific heat maximum.
Abstract: We study the finite-size effects at a temperature-driven first-order transition by analyzing various moments of the energy distribution. The distribution function for the energy is approximated by the superposition of two weighted Gaussian functions yielding quantitative estimates for various quantities and scaling form for the specific heat. The rounding of the singularities and the shifts in the location of the specific-heat maximum are analyzed and the characteristic features of a first-order transition are identified. The predictions are tested on the ten-state Potts model in two dimensions by carrying out extensive Monte Carlo calculations. The results are found to be in good agreement with theory. Comparison is made with the second-order transitions in the two- and three-state Potts models.

Journal ArticleDOI
TL;DR: In this article, the authors discuss the details of their analysis of the mathematical and structural properties of quasicrystals and discuss the computation of the diffraction pattern of a quasilattice, using as an example the case of icosahedral orientational symmetry.
Abstract: In a recent paper, we introduced the concept of quasicrystals [Phys. Rev. Lett. 53, 2477 (1984)], a new class of ordered atomic structures. Quasicrystals have long-range quasiperiodic translational order and long-range orientational order. In the present paper and the following one, we discuss the details of our analysis of the mathematical and structural properties of quasicrystals. We begin with a general overview of our analysis. We then discuss our computation of the diffraction pattern of a quasilattice, using as an example the case of icosahedral orientational symmetry. We demonstrate that two quasilattices with the same orientational symmetry and quasiperiodicity which are not locally isomorphic will have diffraction patterns with different peak intensities. Finally, we describe some examples of computer modeling of atomic quasicrystals.

Journal ArticleDOI
TL;DR: A good guideline for superlattice formation is the following: growth at moderate supersaturations of metal pairs with comparable \ensuremath{\gamma}'s in the unique NW orientation, or, possibly, nucleation and growth along unidirectional steps.
Abstract: We discuss the physical phenomena fundamental to the understanding of the structure and growth of crystalline superlattices: (i) the growth mode as determined by surface energies, supersaturation, and lattice misfit; and (ii) the dependence of epitaxial orientation on lattice matching, atomic bonding, and film thickness, in the topical case of epitaxy at (111) fcc/(110) bcc interfaces. For uniformity monolayer-by-monolayer [Frank--van der Merwe (FM)] growth is desirable. This may be adversely affected by the formation of misfit dislocations. Continued FM growth may be achieved with alternate A and B layers at moderate supersaturation, provided that the surface energies ${\ensuremath{\gamma}}_{A}$ and ${\ensuremath{\gamma}}_{B}$ are compatible. The suggestion that it is possible otherwise at sufficiently high supersaturation is a misconception. The main epitaxial orientations in the present case---the Nishiyama-Wassermann (NW) and the Kurdjumov-Sachs (KS) orientations---have been previously predicted on the (energetically justified) basis of geometrical relationships alone. The predictive power of this model is demonstrated for hexagonal interfaces. Ideally, to predict the evolution of the structure and orientation of a growing (thickening) film atomic forces must be allowed for. We model these forces by means of crystallinity and harmonicity of film, and by a truncated--Fourier-series adsorbate-substrate interaction. Various forms of homogeneous and oscillatory film strains, affecting orientation and structure, are illustrated graphically. We conclude that a good guideline for superlattice formation is the following: (a) growth at moderate supersaturations of metal pairs with comparable \ensuremath{\gamma}'s in the unique NW orientation (0.8\ensuremath{\lesssim}b/a\ensuremath{\lesssim}1.0, a and b are nearest-neighbor distances); or, possibly, (b) nucleation and growth along unidirectional steps.

Journal ArticleDOI
TL;DR: It is found that magnetic moments can change discontinuously with volume and that there are ranges of coexistence for different magnetic phases.
Abstract: The different magnetic phases of the bcc and fcc forms of Fe, Co, and Ni are studied by analyzing total-energy surfaces in moment-volume parameter space obtained from energy-band calculations using a local-spin-density approximation. The surfaces, found by calculating total energies while holding both the magnetic moment and the volume fixed, offer a method for studying phases that are inaccessible to traditional self-consistent-field methods. We find that magnetic moments can change discontinuously with volume and that there are ranges of coexistence for different magnetic phases. In the multiphase ranges, these elemental magnetic systems exhibit metamagnetic behavior. Our results show that bcc Co is ferromagnetic for all volumes studied, that fcc Co can exist in either a nonmagnetic or a ferromagnetic phase, and that there is a range of volumes where the two phases can coexist. For Fe, the bcc form exhibits a stable ferromagnetic phase for all volumes considered, but the fcc form can exist in any of three phases---a nonmagnetic, a low-spin, and a high-spin phase---all of which can coexist in limited volume ranges. For Ni, the fcc form exhibits a stable ferromagnetic phase, but the bcc form can exist in both a nonmagnetic and, at expanded volumes, a ferromagnetic phase. The volume ranges for all magnetic phases are clearly identified for the bcc and fcc forms of Fe, Co, and Ni.

Journal ArticleDOI
TL;DR: A two-component density-functional theory is presented for electron-positron systems and it is demonstrated that there are cancellation effects which render the corresponding changes in observable annihilation characteristics relatively small.
Abstract: A two-component density-functional theory is presented for electron-positron systems. The phase diagram of a two-component Fermi-Coulomb system is discussed, and explicit expressions are derived for exchange-correlation functionals for use in the local-density approximation. The scheme is then applied in a fully self-consistent calculation of electron and positron densities in atomic vacancies in metals, treated in the jellium model. Comparison with conventional calculations, which do not meet true electron-positron self-consistency, reveals considerable changes in the density distributions. However, we demonstrate that there are cancellation effects which render the corresponding changes in observable annihilation characteristics relatively small.

Journal ArticleDOI
TL;DR: In this article, a theory of the collective excitation spectrum in the fractional quantum Hall effect was presented, which is closely analogous to Feynman's theory of superfluid helium, and the predicted spectrum has a large gap at k = 0 and a deep magneto-roton minimum at finite wave vector, in excellent quantitative agreement with recent numerical calculations.
Abstract: We present a theory of the collective excitation spectrum in the fractional quantum Hall effect which is closely analogous to Feynman's theory of superfluid helium. The predicted spectrum has a large gap at k=0 and a deep magneto-roton minimum at finite wave vector, in excellent quantitative agreement with recent numerical calculations. We demonstrate that the magneto-roton minimum is a precursor to the gap collapse associated with the Wigner crystal instability occurring near \ensuremath{ u}=(1/7). In addition to providing a simple physical picture of the collective excitation modes, this theory allows one to compute rather easily and accurately experimentally relevant quantities such as the susceptibility and the ac conductivity.

Journal ArticleDOI
TL;DR: Various thermoelectric linear transport coefficients are defined and calculated for two reservoirs connected with ideal multichannel leads and a segment of an arbitrary disordered system and the expression obtained is used to predict the critical behavior of the electronic thermopower near the mobility edge.
Abstract: Various thermoelectric linear transport coefficients are defined and calculated for two reservoirs connected with ideal multichannel leads and a segment of an arbitrary disordered system. The reservoirs have different temperatures and chemical potentials. All of the inelastic scattering (and, thus, the dissipation) is assumed to occur only in the reservoirs. The definitions of the chemical potentials and temperature differences across the sample itself (mostly due to elastic scattering) are presented. Subtleties of the thermoelectric effects across the sample are discussed. The associated transport coefficients display deviations from the Onsager relations and from the Cutler-Mott formula for the thermopower (although the deviations vanish for a large number of channels and/or high resistance). The expression obtained is used to predict the critical behavior of the electronic thermopower near the mobility edge. It is shown to satisfy a scaling form in the temperature and separation from the mobility edge.

Journal ArticleDOI
TL;DR: It is shown that the anisotropic even-Parity pairings are assisted and the odd-parity as well as the isotropic even- parity pairing are impeded by antiferromagnetic spin fluctuations which are observed in heavy-fermion solids.
Abstract: It is shown that the anisotropic even-parity pairings are assisted and the odd-parity as well as the isotropic even-parity pairings are impeded by antiferromagnetic spin fluctuations which are observed in heavy-fermion solids.

Journal ArticleDOI
TL;DR: In this article, it was shown that a minimal basis set consisting of only one s and three p atom-centered LMTO's plus one s LMTO at the tetrahedral, interstitial site (that is, a total of five orbitals per atom) is sufficient for the calculation of ground-state properties.
Abstract: It is shown that in the muffin-tin orbital (MTO) method a minimal basis set suffices for the calculation of one-electron energies and wave functions, provided that the higher partial waves, ${\ensuremath{\varphi}}_{l}$(E,r), are included by means of L\"owdin downfolding. In terms of a recently proposed transformation theory the downfolding can be considered as the transformation to a set of MTO's whose tails can be continuously and differentiably augmented by the higher partial waves. Simplified linearization schemes provide Hamiltonian and overlap matrices in terms of energy-independent, linear muffin-tin orbitals (LMTO's). By application to band-structure and total-energy calculations for C, Si, and Ge it is demonstrated that a minimal basis set consisting of only one s and three p atom-centered LMTO's plus one s LMTO at the tetrahedral, interstitial site (that is, a total of five orbitals per atom) is sufficient for the calculation of ground-state properties.

Journal ArticleDOI
TL;DR: Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, ..beta..-Sn), V (simple hexagonal), VII (hexagonal close-packed), and the metastable phase III (body-centered-cubIC (BC8)) and on the coexistence of the phases.
Abstract: X-ray-diffraction data have been obtained on Si in a diamond anvil cell to pressures of \ensuremath{\sim}50 GPa. Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, \ensuremath{\beta}-Sn), V (simple hexagonal), VII (hexagonal close-packed), and the metastable phase III [body-centered-cubic (BC8)] and on the coexistence of the phases. Comparison is made between these data and the predictions of ab initio calculations for these structures and their equations of state.

Journal ArticleDOI
TL;DR: The results show that additional harmonic excitations coexist with sound waves below 1 THz, and that these excitations correspond to relative rotation of ${\mathrm{SiO}}_{4}$ tetrahedra.
Abstract: Measurements of the elastic and inelastic neutron scattering from vitreous silica in the frequency range 0.3 to 4 THz and with scattering vectors in the range 0.2 to 5.3 A${\r{}}^{\mathrm{\ensuremath{-}}1}$ are analyzed in conjunction with heat-capacity measurements on the same samples to provide a microscopic description of low-frequency vibrational modes. The results show that additional harmonic excitations coexist with sound waves below 1 THz, and that these excitations correspond to relative rotation of ${\mathrm{SiO}}_{4}$ tetrahedra.

Journal ArticleDOI
TL;DR: The approach provides, for what it is believed to be the first time, a connection between stochastic lattice-gas dynamical methods and the interatomic potential function.
Abstract: A new method is presented for describing the classical dynamics (e.g., diffusion, desorption) of adsorbed overlayers of atoms or molecules, starting from arbitrary interatomic potentials. Provided that a certain dynamical criterion is met, the method yields classically exact results, but with many orders of magnitude less computation than direct molecular dynamics. The approach provides, for what we believe to be the first time, a connection between stochastic lattice-gas dynamical methods and the interatomic potential function. As a sample application, the diffusion constants are computed for two-dimensional rhodium clusters of up to 75 atoms on the Rh(100) surface at T = 2000 K. For clusters larger than n = 15 atoms, the diffusion constant scales as n/sup -1.76//sup +- //sup 0.06/, and the dominant mechanism for the diffusion is found to be atoms running along the edges of the cluster blocks.