Showing papers in "Physical Review B in 1990"

Soft self-consistent pseudopotentials in a generalized eigenvalue formalism.

Abstract: A new approach to the construction of first-principles pseudopotentials is described. The method allows transferability to be improved systematically while holding the cutoff radius fixed, even for large cutoff radii. Novel features are that the pseudopotential itself becomes charge-state dependent, the usual norm-conservation constraint does not apply, and a generalized eigenproblem is introduced. The potentials have a separable form well suited for plane-wave solid-state calculations, and show promise for application to first-row and transition-metal systems.

Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films

Abstract: An empirical many-body potential-energy expression is developed for hydrocarbons that can model intramolecular chemical bonding in a variety of small hydrocarbon molecules as well as graphite and diamond lattices. The potential function is based on Tersoff's covalent-bonding formalism with additional terms that correct for an inherent overbinding of radicals and that include nonlocal effects. Atomization energies for a wide range of hydrocarbon molecules predicted by the potential compare well to experimental values. The potential correctly predicts that the \ensuremath{\pi}-bonded chain reconstruction is the most stable reconstruction on the diamond {111} surface, and that hydrogen adsorption on a bulk-terminated surface is more stable than the reconstruction. Predicted energetics for the dimer reconstructed diamond {100} surface as well as hydrogen abstraction and chemisorption of small molecules on the diamond {111} surface are also given. The potential function is short ranged and quickly evaluated so it should be very useful for large-scale molecular-dynamics simulations of reacting hydrocarbon molecules.

Superconducting transport properties of grain boundaries in YBa2Cu3O7 bicrystals.

Abstract: Previous work on the superconducting transport properties of individual grain boundaries in thin-film bicrystals of YBa{sub 2}Cu{sub 3}O{sub 7} has been extended to provide a more comprehensive picture of their weak-link characteristics. Grain boundaries with three different geometries have been studied; the transport properties of all three types of boundaries are essentially identical, which implies that the poor superconducting coupling between grains is a result of the intrinsic structural disorder at the boundary. The grain-boundary critical current densities in bicrystal films prepared by evaporation and postannealing and by laser ablation are also in good agreement; this result demonstrates that the transport properties are insensitive to preparation technique and, thus, are not dominated by the diffusion of substrate impurities into the boundary region. High grain-boundary resistivities and low {ital I}{sub {ital c}R{ital n}} products imply that the boundaries act as strong barriers to current flow with locally depressed order parameters. Strong magnetic hysteresis, associated with trapped intragranular flux, is observed; this hysteretic behavior is also responsible for an increase in the grain boundary {ital J}{sub {ital c}} for {ital H}{sub {ital a}{ital p}{ital p}}{gt}300--500 Oe.

Scanning tunneling microscopy observations on the reconstructed Au(111) surface: Atomic structure, long-range superstructure, rotational domains, and surface defects

Abstract: Keywords: Surface Electronic and Atomic Structure ; Spectroscopy ; Spin-Polarized STM Reference LNS-ARTICLE-1990-002doi:10.1103/PhysRevB.42.9307 Record created on 2009-04-14, modified on 2017-05-12

Structural properties of oxygen-deficient YBa2Cu3O7- delta.

Abstract: The structural properties of oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ have been determined by neutron powder diffraction for 0.07l\ensuremath{\delta}l0.91. The samples were produced by quenching into liquid nitrogen from controlled oxygen partial pressures at 520 \ifmmode^\circ\else\textdegree\fi{}C, and they exhibit a clearly defined ``plateau'' behavior of ${T}_{c}$ versus \ensuremath{\delta}. Superconductivity disappears at the orthorhombic-to-tetragonal transition that occurs near \ensuremath{\delta}=0.65. Structural parameters, including the copper-oxygen bond lengths, vary smoothly with \ensuremath{\delta} within each phase but exhibit different behavior in the superconducting and nonsuperconducting phases. These observations are consistent with a model in which superconducting behavior is controlled by charge transfer between the conducting two-dimensional ${\mathrm{CuO}}_{2}$ planes and the ${\mathrm{CuO}}_{\mathrm{x}}$ chains, which act as reservoirs of charge.

Structural properties and electronic structure of low-compressibility materials: β- Si 3 N 4 and hypothetical β- C 3 N 4

Abstract: We present a first-principles pseudopotential study of the structural and electronic properties of \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and the hypothetical compound \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$, which is ${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ in the \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ structure, with C substituted for Si, is used as a prototype for investigating the properties of possible covalent C-N solids. The calculated lattice constant, bulk modulus, and electronic band structure of \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ are in good agreement with experimental results. This gives support for the predicted properties of \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. The bulk modulus of \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ is found to be comparable to diamond, and its moderately large cohesive energy suggests that the prototype structure may be metastable. Although the crystal structure and the valencies of the constituent atoms are similar in \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$, the electronic bonding properties in these two solids are found to differ. The large core size and repulsive p pseudopotential of the second-row element, Si, results in a more ionic Si-N bond compared with a covalent C-N bond.

Pulsed-laser evaporation technique for deposition of thin films: Physics and theoretical model.

Abstract: We have studied in detail the physical phenomena involved in the interaction of high-powered nanosecond excimer-laser pulses with bulk targets resulting in evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating these laser-plasma--solid interactions has been developed. In this model, the laser-generated plasma is treated as an ideal gas at high pressure and temperature, which is initially confined in small dimensions, and is suddenly allowed to expand in vacuum. The three-dimensional expansion of this plasma gives rise to the characteristic spatial thickness and compositional variations observed in laser-deposited thin films of multicomponent systems. The forward-directed nature of the laser evaporation process has been found to result from anisotropic expansion velocities of the atomic species which are controlled by the dimensions of the expanding plasma.Based on the nature of interaction of the laser beam with the target and the evaporated material, the pulsed-laser evaporation (PLE) process can be classified into three separate regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation, heating, and initial three-dimensional isothermal expansion, and (iii) adiabatic expansion and deposition of thin films. The first two processes occur during the time interval of the laser pulse, while the last process initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The equations of compressible gas dynamics are set up to simulate the expansion of the plasma in the last two regimes. The solution of the gas-dynamics equations shows that the expansion velocities of the plasma are related to its initial dimensions and temperature, and the atomic weight of the species. Detailed simulations analyzing the salient features of the laser-deposition process have been carried out. The effects of various beam and substrate parameters including pulse energy density, substrate-target distance, irradiated spot size, and atomic mass of the species have been theoretically analyzed. This model predicts most of the characteristic experimental features of the laser evaporation and deposition of thin films. These characteristic features include (a) the effect of pulse energy density on atomic velocities, (b) the forward-directed nature of the deposit and its dependence on energy density, (c) spatial compositional variations in multicomponent thin films as a function of energy density, (d) dependence of the atomic velocities with atomic weights of various species in multicomponent films, (e) athermal non-Maxwellian-type velocity distribution of the atomic and molecular species, and (f) thickness and compositional variations as a function of substrate-target distance and irradiated spot size.

Phonon scattering and energy relaxation in two-, one-, and zero-dimensional electron gases

Abstract: We report on calculations of intrasubband and intersubband phonon scattering in quantum-confined electron gases based on lattice-matched ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP quantum wells. Dimensionality effects on the emission of acoustic phonons are studied comparing the scattering times of two-, one-, and zero-dimensional electron gases as a function of the lateral confinement. Optical phonon scattering in quantum wells and wires is discussed using a phenomenological broadening of the one-dimensional density of states. The energy relaxation rates of heated electron gases due to phonon emission and absorption have been calculated for lattice temperatures ${\mathit{T}}_{\mathit{l}}$ between 0.3 and 20 K. For a given heating power per electron, the electron temperature ${\mathit{T}}_{\mathit{e}}$ in a quantum wire can be greater or smaller than that in the corresponding quantum well, depending on the electron density ${\mathit{n}}_{\mathit{s}}$, while the energy relaxation in quantum dots with significant quantization energies is always slower than in the corresponding wells and wires.

2p x-ray absorption of 3d transition-metal compounds - an atomic multiplet description including the crystal-field

Abstract: The metal 2p x-ray-absorption spectra (or ${\mathit{L}}_{2,3}$ edges) of 3d transition-metal compounds are calculated, using atomic multiplet theory with inclusion of the cubic crystal field. A general overview of the effect of the cubic crystal field on the shape of the 3${\mathit{d}}^{\mathit{N}}$ to 2${\mathit{p}}^{5}$3${\mathit{d}}^{\mathit{N}+1}$ excitation spectrum is given for 14 common valencies of 3d transition-metal ions. Comparison to some high-resolution 2p x-ray-absorption spectra shows excellent agreement, which confirms the validity of the approach. Possible refinements of the theory, including lower-symmetry calculations and the inclusion of configuration interaction, are discussed.

Electronic properties of random alloys : special quasirandom structures

Abstract: Structural models needed in calculations of properties of substitutionally random ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$ alloys are usually constructed by randomly occupying each of the N sites of a periodic cell by A or B. We show that it is possible to design ``special quasirandom structures'' (SQS's) that mimic for small N (even N=8) the first few, physically most relevant radial correlation functions of an infinite, perfectly random structure far better than the standard technique does. These SQS's are shown to be short-period superlattices of 4--16 atoms/cell whose layers are stacked in rather nonstandard orientations (e.g., [113], [331], and [115]). Since these SQS's mimic well the local atomic structure of the random alloy, their electronic properties, calculable via first-principles techniques, provide a representation of the electronic structure of the alloy. We demonstrate the usefulness of these SQS's by applying them to semiconductor alloys. We calculate their electronic structure, total energy, and equilibrium geometry, and compare the results to experimental data.

Ground-state degeneracy of the fractional quantum Hall states in the presence of a random potential and on high-genus Riemann surfaces

Abstract: The fractional quantum Hall (FQH) states are shown to have q\ifmmode \tilde{}\else \~{}\fi{} $^{\mathit{g}}\mathrm{fold}$ ground-state degeneracy on a Riemann surface of genus g, where q\ifmmode \tilde{}\else \~{}\fi{} is the ground-state degeneracy in a torus topology. The ground-state degeneracies are directly related to the statistics of the quasiparticles given by \ensuremath{\theta}=p\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\pi}/q\ifmmode \tilde{}\else \~{}\fi{}. The ground-state degeneracy is shown to be invariant against weak but otherwise arbitrary perturbations. Therefore the ground-state degeneracy provides a new quantum number, in addition to the Hall conductance, characterizing different phases of the FQH systems. The phases with different ground-state degeneracies are considered to have different topological orders. For a finite system of size L, the ground-state degeneracy is lifted. The energy splitting is shown to be at most of order ${\mathit{e}}^{\mathrm{\ensuremath{-}}\mathit{L}/\ensuremath{\xi}}$. We also show that the Ginzburg-Landau theory of the FQH states (in the low-energy limit) is a dual theory of the U(1) Chern-Simons topological theory.

Phenomenological model of nuclear relaxation in the normal state of YBa2Cu3O7.

Abstract: A phenomenological model of a system of antiferromagnetically correlated spins is shown to give a good quantitative description of NMR, nuclear-quadrupole-resonance, and Knight-shift measurements on yttrium, planar copper, and planar oxygen sites in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The antiferromagnetic correlation length is estimated to be \ensuremath{\sim}2.5 lattice constants at T=100 K. The temperature dependence of the correlation length ceases at ${\mathit{T}}_{\mathit{x}}$\ensuremath{\simeq}100 K. The enhancement of the observed relaxation rates over what is expected for weakly interacting electrons is calculated and shown to be large. Extension of the calculation to other cuprate superconductors is discussed.

Sign problem in the numerical simulation of many-electron systems

Abstract: We discuss the problems that arise in the numerical simulation of many-electron systems when the measure of the functional integrals is not positive definite. We present theoretical arguments and numerical data which indicate that the expectation value of the sign of the measure decreases exponentially as the inverse temperature \ensuremath{\beta} increases, unless the measure is forced to be positive by an explicit symmetry. We therefore conclude that a recent proposal for dealing with the sign problem due to Sorella et al. Leads to an uncontrolled approximation. In the cases we have studied it is a good approximation for the ground-state energy, and we present a method for calculating the correction needed to make it exact. However, for some physical quantities, such as the d-wave pair field susceptibility, the neglect of signs can yield misleading results.

Optical properties due to electronic transitions in two-dimensional semiconductors (CnH2n+1NH3)2PbI4.

Abstract: Optical spectra in the visible and uv regions are investigated in layer-type perovskite compounds (${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{2\mathit{n}+1}$${\mathrm{NH}}_{3}$)${\mathrm{PbI}}_{4}$ with n=4, 6, 8, 9, 10, and 12. The spacing between the ${\mathrm{PbI}}_{4}$ layers changes from 15.17 \AA{} for n=4 to 24.51 \AA{} for n=12. In spite of these different spacings, the optical spectra are almost the same for these compounds, which means that the interaction between the layers is weak. The lowest exciton is located at 2.56 eV at 1.6 K, and its oscillator strength and binding energy are 0.7 per formula unit and 320 meV, respectively. These values are very large compared with those in a three-dimensional analog ${\mathrm{PbI}}_{2}$. The large oscillator strength and binding energy can be explained by the small dielectric constant of the alkylammonium ``barrier layer,'' which strengthens the Coulomb interaction between an electron and a hole.

Dissipative flux motion in high-temperature superconductors

Abstract: The dissipation below ${\mathit{T}}_{\mathit{c}}$ has been studied for representatives of all classes of cuprate high-temperature superconductors, including ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, and Bi and Tl compounds. The results are parametrized in the framework of flux creep, with the largest activation energies found in ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{7}$. It is argued that the magnitude of dissipative flux motion is more related to the electronic anisotropy of the material than the actual defect structure. The thermally activated flux creep model, whose parameters are extracted from dc measurements, consistently describes also dynamic measurements, including the irreversibility line and the melting transition. Finally, the similarities in dissipative behavior are emphasized between high-${\mathit{T}}_{\mathit{c}}$ materials, very thin films, and layered low-${\mathit{T}}_{\mathit{c}}$ superconductors.

Chiral Luttinger liquid and the edge excitations in the fractional quantum Hall states.

Abstract: The low-energy effective theory of the edge excitations in the fractional quantum Hall (FQH) states is derived. The edge excitations are shown to form a new kind of state which is called the chiral Luttinger liquid (\ensuremath{\chi}LL). The effective theory is exactly soluble. This enables us to easily calculate all the low-energy properties of the edge excitations. We calculate the electron propagator and the spectral function, which clearly demonstrate the non-Fermi-liquid behaviors of the \ensuremath{\chi}LL. We also calculate the interference effects between excitations on different edges. We demonstrate that the properties of the edge excitations are closely related to the properties of the FQH states on compacted spaces. Thus the properties of the edge excitations can be used to characterize the topological orders in the FQH states. We also show that the FQH states with filling fractions \ensuremath{ u}\ensuremath{ e}1/l must have at least two branches of edge excitations.

Analysis of the composite structures in diamond thin films by Raman spectroscopy.

Abstract: Diamond and diamondlike thin films produced by various chemical-vapor-deposition processes have been examined using Raman spectroscopy. These films exhibit features in the Raman spectra, suggesting that they are composites of crystalline and amorphous diamond and graphitic structures. The components of this composite structure that contribute to the Raman scattering are discussed in terms of ${\mathrm{sp}}^{2}$- and ${\mathrm{sp}}^{3}$-bonded structures. The use of Raman spectroscopy as a technique for estimating the ${\mathrm{sp}}^{2}$-to-${\mathrm{sp}}^{3}$ bonding ratio is considered. Powder composites of BN-diamond and graphite-diamond have been studied as a means of modeling the films, and a simple theoretical model of the Raman scattering from these samples is proposed. From these results it is shown that it is necessary to make assumptions about the domain size of the graphitic ${\mathrm{sp}}^{2}$ regions. It is found that the Raman scattering associated with ${\mathrm{sp}}^{2}$ bonding in the films is much stronger than that from single-crystalline or microcrystalline graphite structures. Shifts of the vibrational modes are also observed. The optical and vibrational properties of the ${\mathrm{sp}}^{2}$ component in the films implies a different atomic microstructure. A model of the ${\mathrm{sp}}^{2}$-bonding configurations in the films is proposed which may account for the observed features in the Raman spectra.

Soft-x-ray magnetic circular dichroism at the l2,3 edges of nickel

Abstract: Magnetic circular dichroism (MCD) has been observed at the ${\mathit{L}}_{2,3}$ absorption edges of ferromagnetic nickel by use of circularly-polarized soft-x-ray synchrotron radiation. The MCD intensity ratio between the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ edges is found to differ appreciably from that predicted by a simple exchange-split-valence-band model. Fine MCD features, imperceptible in the absorption spectra, are also observed, and a tentative interpretation is given. This work, demonstrating the feasibility of MCD measurements in the soft-x-ray region, provides a new approach to study 3d and 4f ferromagnetic systems with their respective dipole-permitted 2p\ensuremath{\rightarrow}3d and 3d\ensuremath{\rightarrow}4f transitions.

Subplantation Model for Film Growth from Hyperthermal Species

Abstract: A model describing film growth from hyperthermal (\ensuremath{\sim}1--${10}^{3}$ eV) species impinging on substrates is presented. The model involves a shallow subsurface implantation process called ``subplantation,'' energy loss, preferential displacement of atoms with low displacement energy ${\mathit{E}}_{\mathit{d}}$, leaving the high-${\mathit{E}}_{\mathit{d}}$ atoms intact, sputtering of substrate material, and inclusion of a new phase due to incorporation of a high density of interstitials in a host matrix. Epitaxial or preferred orientation may result from the angular dependence of the ${\mathit{E}}_{\mathit{d}}$ and the boundary conditions imposed by the host matrix, i.e., the ``mold'' effect. The discussion focuses on deposition of carbon diamondlike films, but examples of other systems, such as Si, Ge, and Ag, are provided as well. The model is supported by classical-ion-trajectory calculations and experimental data. The calculations probe the role of ion range, local concentration, backscattering coefficient, sputtering yield, and ion-induced damage in film evolution. The experimental data emphasize in situ surface-analysis studies of film evolution. The physical parameters of the deposition process that are treated are as follows: (i) nature of bombarding species (${\mathrm{C}}^{+}$ versus ${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$ versus ${\mathrm{C}}_{2}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{m}}^{+}$, ${\mathrm{Ar}}^{+}$, and ${\mathrm{H}}^{+}$), (ii) ion energy, (iii) type of substrate, and (iv) substrate temperature during deposition.

L2,3 x-ray-absorption edges of d0 compounds: K+, Ca2+, Sc3+, and Ti4+ in Oh (octahedral) symmetry.

Abstract: The ${L}_{2}$,3 x-ray-absorption edges of 3${d}^{0}$ compounds are calculated with use of an atomic description of the 2${p}^{6}$3${d}^{0}$ to 2${p}^{5}$3${d}^{1}$ excitation, with the inclusion of the crystal field. For reasons of clarity, we confine ourselves to ${d}^{0}$ compounds in octahedral symmetry, but the same approach is applicable to all other ${d}^{N}$ compounds in any point-group symmetry. The experimental spectra of ${\mathrm{FeTiO}}_{3}$, ${\mathrm{Sc}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{ScF}}_{3}$, ${\mathrm{CaF}}_{2}$, and the potassium halides are well reproduced by the present calculations, including the previously misinterpreted small leading peaks. The splitting between the two main peaks in both the ${L}_{3}$ and ${L}_{2}$ edge are related, though not equal, to the crystal-field splitting. Comparison to experiment showed that the broadening of the main multiplet lines is different. This can be related to Coster-Kronig Auger processes for the ${L}_{2}$ edge and to a solid-state broadening which is a combination of vibrational (phononic) and dispersional broadenings. With the full treatment of the atomic multiplets, the atomic effects can be separated from solid-state effects, which offers a better description of the latter. This includes vibrational broadenings, the covalent screening of the intra-atomic Coulomb and exchange interactions, via the position of small leading peaks, and surface effects. The same general framework can be used to discuss crystal-field effects in both lower symmetries, with the possibility of polarization-dependent spectra (e.g., ${\mathrm{TiO}}_{2}$), and partly filled d bands.

Surface reconstructions of GaAs(100) observed by scanning tunneling microscopy.

Abstract: We present atomic-resolution images, obtained with scanning tunneling microscopy (STM), of the various reconstructions of smooth, in situ grown GaAs(100) surfaces. The outermost layer of the c(4\ifmmode\times\else\texttimes\fi{}4) reconstruction consists of three As-As adatom dimers parallel to [011]. Cells containing two or three As-As dimers have been observed on the 2\ifmmode\times\else\texttimes\fi{}4 surface. The ``1\ifmmode\times\else\texttimes\fi{}6'' surface seen in low-energy electron diffraction has a 2\ifmmode\times\else\texttimes\fi{}6 unit cell containing two As-As dimers. Diffraction patterns implying 4\ifmmode\times\else\texttimes\fi{}6 symmetry are seen to arise from the coexistence of 2\ifmmode\times\else\texttimes\fi{}6 and 4\ifmmode\times\else\texttimes\fi{}2 units. The c(8\ifmmode\times\else\texttimes\fi{}2) surface is made up of two Ga-Ga dimers and two missing dimers per 4\ifmmode\times\else\texttimes\fi{}2 cell. Atomic models, which are consistent with both the STM images and electron-counting heuristics, are also shown.

Scattering-matrix formulation of curved-wave multiple-scattering theory: Application to x-ray-absorption fine structure.

Abstract: Curved-wave multiple-scattering contributions to XAFS (x-ray-absorption fine structure) are calculated with use of an efficient formalism similar to that based on the plane-wave approximation, but with scattering amplitudes f(\ensuremath{\theta}) replaced by distance-dependent ``scattering matrices'' ${\mathrm{F}}_{\ensuremath{\lambda},\ensuremath{\lambda}\ensuremath{'}}$(\ensuremath{\rho},\ensuremath{\rho}\ensuremath{'}). Here \ensuremath{\rho}=kR, k being the photoelectron wave number and R is a bond vector, while the matrix indices \ensuremath{\lambda}=(\ensuremath{\mu},\ensuremath{ u}) represent terms in a convergent expansion that generalizes the small-atom approximation. This approach is based on an exact, separable representation of the free propagator (or translation operator) matrix elements, ${\mathrm{G}}_{\mathrm{L},\mathrm{L}\ensuremath{'}}$(kR), in an angular momentum L=(l,m) and site basis. The method yields accurate curved-wave contributions for arbitrarily high-order multiple-scattering paths at all positive energies, including the near-edge region. Results are nearly converged when the intermediate \ensuremath{\lambda} summations are truncated at just six terms, i.e., (6\ifmmode\times\else\texttimes\fi{}6) matrices. The lowest-order (1\ifmmode\times\else\texttimes\fi{}1) matrix ${\mathrm{F}}_{00,00}$ is the effective, curved-wave scattering amplitude, f(\ensuremath{\rho},\ensuremath{\rho}\ensuremath{'},\ensuremath{\theta}), and yields a multiple-scattering expansion equivalent to the point-scattering approximation. Formulas for multiple-scattering contributions to XAFS and photoelectron diffraction are presented, and the method is illustrated with results for selected multiple-scattering paths in fcc Cu.

Interaction potential for SiO2 : a molecular-dynamics study of structural correlations

Abstract: distance. In lattice-structure calculations with the total potential function, a-cristobalite and aquartz are found to have the lowest and almost degenerate energies, in agreement with experiments. The energies for p-cristobalite, p-quartz, and keatite are found to be higher than those for acristobalite and a-quartz. Molecular-dynamics calculations with this potential function correctly describe the short- and intermediate-range order in molten and vitreous states. In the latter, partial pair-distribution functions give Si— 0, 0— 0, and Si— Si bond lengths of 1.62, 2.65, and 3.05 A, respectively. The vitreous state consists of nearly ideal Si(Ol&2)4 tetrahedra in corner-sharing configurations. The Si— U— Si bond-angle distribution has a peak at 142' and a full width at half maximum {FWHM) of 25' in good agreement with nuclear magnetic resonance experiments. The calculated static structure factor is also in agreement with neutron-diffraction experiments. Partial static structure factors reveal that intermediate-range Si-Si, O-O, and Si-0 correlations between 4 and 8 A give rise to the first sharp diffraction peak {FSDP). The FSDP is absent in charge-charge structure factor, which indicates that charge neutrality prevails over length scales between 4 and 8 0 A. Dynamical correlations in vitreous and molten states, phonon densities of states of crystalline and vitreous Si02, infrared spectra of crystalline, vitreous and molten states, isotope effect, distribution of rings and their structure in molten and vitreous states, and structural transformations at high pressures will be discussed in subsequent papers.

Lattice dynamics and origin of ferroelectricity in BaTiO3: Linearized-augmented-plane-wave total-energy calculations.

Abstract: Phonon frequencies and eigenvectors have been computed from first principles for the three optic ${\mathit{F}}_{1\mathit{u}}$ modes in ${\mathrm{BaTiO}}_{3}$ using the full-potential linearized-augmented-plane-wave method. We find that the ferroelectric instability in ${\mathrm{BaTiO}}_{3}$ can be understood from calculations for a perfect crystal with periodic boundary conditions. The energy wells for the soft-mode distortion are deeper for rhombohedral [111] displacements relative to tetragonal [001] displacements, but they are relatively shallow and comparable to the transition temperature. The nonrigid part of the charge-density distortion is centered around the Ti ion rather than the O, and the Ti charge is closer to 2.9+ than 4+. There is significant hybridization between the Ti and O, but the Ba is quite ionic and is well described as a ${\mathrm{Ba}}^{2+}$ ion. The Ti-O hybridization is essential to the ferroelectric instability.

First-principles calculation of the magnetocrystalline anisotropy energy of iron, cobalt, and nickel

Abstract: The magnetocrystalline anisotropy energies of the elements iron, cobalt, and nickel have been calculated by means of the linear muffin-tin orbital (LMTO) method in the atomic-sphere approximation (ASA) within the framework of the local-spin-density approximation (LSDA). The so-called ``force theorem'' is used to express the total-energy difference, when spin-orbit coupling is included, as a difference in sums of Kohn-Sham single-particle eigenvalues. The results depend strongly on the location and dispersion of degenerate energy bands near the Fermi surface, and particular attention must be paid to the convergence of the Brillouin-zone integral of the single-particle eigenvalues. The calculated values of the anisotropy energy are too small by comparison with experiment, and we do not predict the correct easy axis for cobalt and nickel. We find that the variation of the anisotropy energy with changes in strain, in the magnitude of the spin-orbit coupling, for different choices of the exchange-correlation potential and for varying numbers of valence electrons are not capable of explaining these incorrect results. By comparing our calculated energy bands with those obtained by a full-potential linear augmented plane wave (FLAPW) method we conclude that the discrepancy is not attributable to terms in the potential that are neglected in the ASA.

Quantized transmission of a saddle-point constriction

Abstract: Discussion de la quantification de la conductance en l'absence et en presence d'un champ magnetique pour une constriction de porte decomposee avec un potentiel local au niveau du goulot formant une selle. Des criteres simples pour l'apparition des marches de la conductance et la precision de la quantification sont donnes en termes d'incurvation de la selle

Size dependence of electron-phonon coupling in semiconductor nanospheres: The case of CdSe.

Abstract: We study electron-phonon coupling in the case of Fr\"ohlich or polar interaction, with special emphasis on the size dependence of the coupling strength for semiconductor nanospheres exhibiting quantum confinement of the carriers. We first derive the expression of the vibrational LO (longitudinal optic) and SO (surface optic) eigenfunctions for a sphere in the continuum approximation. After having quantized the vibrational eigenmodes, we give the electron-phonon interaction Hamiltonian. Using a model electronic charge distribution, we then show that the coupling strength is size independent when the typical dimensions of the electron charge distribution scale as the sphere radius. These theoretical considerations are then compared with experimental results obtained using resonant Raman scattering by CdSe-doped glasses with particles of various sizes. The experiments confirm the size independence of the coupling strength and also show the existence of surface modes.

Temperature-dependent exciton linewidths in semiconductors

Abstract: The temperature-dependent linewidths of excitons in semiconductors due to the interaction of the exciton with both LO phonons and with acoustic phonons are studied with use of a Green's-function approach in which the exciton-phonon interaction is treated perturbatively. The interaction between the excitons and the LO phonons is taken to be of the Fr\"ohlich form, and the contribution to the linewidth is obtained in closed form. In this case it is found that scattering of the exciton to both bound and continuum states is important and that it is important to treat the continuum states fully as Coulomb scattering states. In describing optical-absorption processes, the fact that absorption occurs from polariton states, which are states composed of excitons coupled to light, is taken into account. The linewidths due to the exciton--LO-phonon interaction are evaluated for a series of II-VI and III-V compound semiconductors, and are shown to account for the existing experimental results for temperatures \ensuremath{\gtrsim}80 K. The contributions to the linewidth due to the interaction of excitons with acoustic phonons via both the deformation potential and the piezoelectric couplings are treated, and it is found that the deformation-potential coupling dominates for all of the materials considered. Because of the small velocity of sound, scattering to only intraband intermediate states, i.e., those in which the internal exciton quantum numbers do not change, is found to contribute to the linewidth. In the case of acoustic phonons, it is found to be important to treat optical absorption as originating from polariton states in order to evaluate properly the magnitude of this contribution to the linewidth. The acoustic-phonon contribution to the linewidths is compared with experiment for temperatures \ensuremath{\lesssim}80 K, for which it dominates the temperature dependence.

Size-dependent melting temperature of individual nanometer-sized metallic clusters

Abstract: Gold and silver clusters with diameters in the nanometer-size range were grown in an inert-gas beam and deposited on the end of a tungsten field emitter The field-emission current from an individual cluster is used to study a size-dependent change in shape of the cluster as a function of temperature Three experimental signatures indicate an abrupt change in cluster shape at a temperature below the bulk melting point This temperature is found to depend on cluster size and is in agreement with a thermodynamic model for cluster melting, provided the cluster diameter is greater than \ensuremath{\sim}2 nm

Atomistic locking and friction.

Abstract: When solid bodies contact and slide against each other, the frictional phenomenon occurs. The origin of the related frictional force is studied by assuming the existence of two clean crystal surfaces, which follows the current experimental trends. This study theoretically clarifies the atomistic origin of the frictional force intrinsically generated by the molecular interactions between the constituent atoms of solids, but not the force extrinsically generated by surface asperities, the existence of foreign atoms, etc. Furthermore, this study assumes that the constituent atoms of the two contacting surfaces interact with each other due to the interaction potential. This study found that there are two origins: atomistic locking and dynamic locking. Atomistic locking occurs when the configuration of the atoms on a contact surface continuously changes with the sliding distance and when the interatomic potentials have an arbitrary strength. In contrast, dynamic locking occurs when the configuration discontinuously changes due to the dynamic process and if the interatomic potential is stronger than a specific given value. A criterion is derived for the occurrence of dynamic locking. From studying various systems, it can be seen that dynamic locking is unlikely to occur in realistic systems. The frictional forces due to atomistic locking are calculated for \ensuremath{\alpha}-iron. One other important finding prior to the experiments is that certain unique cases exist where the frictional force exactly vanishes if completely clean solid surfaces are prepared.