298 research outputs found
Structure and Stability of Si(114)-(2x1)
We describe a recently discovered stable planar surface of silicon, Si(114).
This high-index surface, oriented 19.5 degrees away from (001) toward (111),
undergoes a 2x1 reconstruction. We propose a complete model for the
reconstructed surface based on scanning tunneling microscopy images and
first-principles total-energy calculations. The structure and stability of
Si(114)-(2x1) arises from a balance between surface dangling bond reduction and
surface stress relief, and provides a key to understanding the morphology of a
family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures
is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published
in Phys. Rev. Let
Separable Dual Space Gaussian Pseudo-potentials
We present pseudo-potential coefficients for the first two rows of the
periodic table. The pseudo potential is of a novel analytic form, that gives
optimal efficiency in numerical calculations using plane waves as basis set. At
most 7 coefficients are necessary to specify its analytic form. It is separable
and has optimal decay properties in both real and Fourier space. Because of
this property, the application of the nonlocal part of the pseudo-potential to
a wave-function can be done in an efficient way on a grid in real space. Real
space integration is much faster for large systems than ordinary multiplication
in Fourier space since it shows only quadratic scaling with respect to the size
of the system. We systematically verify the high accuracy of these
pseudo-potentials by extensive atomic and molecular test calculations.Comment: 16 pages, 4 postscript figure
Thermodynamic aspects of materials' hardness: prediction of novel superhard high-pressure phases
In the present work we have proposed the method that allows one to easily
estimate hardness and bulk modulus of known or hypothetical solid phases from
the data on Gibbs energy of atomization of the elements and corresponding
covalent radii. It has been shown that hardness and bulk moduli of compounds
strongly correlate with their thermodynamic and structural properties. The
proposed method may be used for a large number of compounds with various types
of chemical bonding and structures; moreover, the temperature dependence of
hardness may be calculated, that has been performed for diamond and cubic boron
nitride. The correctness of this approach has been shown for the recently
synthesized superhard diamond-like BC5. It has been predicted that the
hypothetical forms of B2O3, diamond-like boron, BCx and COx, which could be
synthesized at high pressures and temperatures, should have extreme hardness
Singular Structure and Enhanced Friedel Oscillations in the Two-Dimensional Electron Gas
We calculate the leading order corrections (in ) to the static
polarization , with dynamically screened interactions, for the
two-dimensional electron gas. The corresponding diagrams all exhibit singular
logarithmic behavior in their derivatives at and provide significant
enhancement to the proper polarization particularly at low densities. At a
density of , the contribution from the leading order {\em fluctuational}
diagrams exceeds both the zeroth order (Lindhard) response and the self-energy
and exchange contributions. We comment on the importance of these diagrams in
two-dimensions and make comparisons to an equivalent three-dimensional electron
gas; we also consider the impact these finding have on computed
to all orders in perturbation theory
Relativistic separable dual-space Gaussian Pseudopotentials from H to Rn
We generalize the concept of separable dual-space Gaussian pseudopotentials
to the relativistic case. This allows us to construct this type of
pseudopotential for the whole periodic table and we present a complete table of
pseudopotential parameters for all the elements from H to Rn. The relativistic
version of this pseudopotential retains all the advantages of its
nonrelativistic version. It is separable by construction, it is optimal for
integration on a real space grid, it is highly accurate and due to its analytic
form it can be specified by a very small number of parameters. The accuracy of
the pseudopotential is illustrated by an extensive series of molecular
calculations
Anomalous enhancement of tetragonality in PbTiO3 induced by negative pressure
Using a first-principles approach based on density-functional theory, we find
that a large tetragonal strain can be induced in PbTiO3 by application of a
negative hydrostatic pressure. The structural parameters and the dielectric and
dynamical properties are found to change abruptly near a crossover pressure,
displaying a ``kinky'' behavior suggestive of proximity to a phase transition.
Analogous calculations for BaTiO3 show that the same effect is also present
there, but at much higher negative pressure. We investigate this unexpected
behavior of PbTiO3 and discuss an interpretation involving a phenomenological
description in terms of a reduced set of relevant degrees of freedom.Comment: 9 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/st_pbti/index.htm
New Superhard Phases for 3D C60-based Fullerites
We have explored new possible phases of 3D C60-based fullerites using
semiempirical potentials and ab-initio density functional methods. We have
found three closely related structures - two body centered orthorhombic and one
body centered cubic - having 52, 56 and 60 tetracoordinated atoms per molecule.
These 3D polymers result in semiconductors with bulk moduli near 300 GPa, and
shear moduli around 240 GPa, which make them good candidates for new low
density superhard materials.Comment: To be published in Physical Review Letter
Ab initio Pseudopotential Plane-wave Calculations of the Electronic Structure of YBa_2Cu_3O_7
We present an ab initio pseudopotential local density functional calculation
for stoichiometric high-Tc cuprate YBa_2Cu_3O_7 using the plane-wave basis set.
We have overcome well-known difficulties in applying pseudopotential methods to
first-row elements, transition metals, and rare-earth materials by carefully
generating norm-conserving pseudopotentials with excellent transferability and
employing an extremely efficient iterative diagonalization scheme optimized for
our purpose. The self-consistent band structures, the total and site-projected
densities of states, the partial charges and their symmetry-decompositions, and
some characteristic charge densities near E_f are presented. We compare our
results with various existing (F)LAPW and (F)LMTO calculations and establish
that the ab initio pseudopotential method is competitive with other methods in
studying the electronic structure of such complicated materials as high-Tc
cuprates. [8 postscript files in uuencoded compressed form]Comment: 14 pages, RevTeX v3.0, 8 figures (appended in postscript file), SNUTP
94-8
Quantum Monte Carlo calculation of Compton profiles of solid lithium
Recent high resolution Compton scattering experiments in lithium have shown
significant discrepancies with conventional band theoretical results. We
present a pseudopotential quantum Monte Carlo study of electron-electron and
electron-ion correlation effects on the momentum distribution of lithium. We
compute the correlation correction to the valence Compton profiles obtained
within Kohn-Sham density functional theory in the local density approximation
and determine that electronic correlation does not account for the discrepancy
with the experimental results. Our calculations lead do different conclusions
than recent GW studies and indicate that other effects (thermal disorder,
core-valence separation etc.) must be invoked to explain the discrepancy with
experiments.Comment: submitted to Phys. Rev.
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.Comment: 24 pages, Revtex 3.0, 11 figures, accepted for publication in Phys.
Rev.
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