769 research outputs found
Strain-Dependence of Surface Diffusion: Ag on Ag(111) and Pt(111)
Using density-functional theory with the local-density approximation and the
generalized gradient approximation we compute the energy barriers for surface
diffusion for Ag on Pt(111), Ag on one monolayer of Ag on Pt(111), and Ag on
Ag(111). The diffusion barrier for Ag on Ag(111) is found to increase linearly
with increasing lattice constant. We also discuss the reconstruction that has
been found experimentally when two Ag layers are deposited on Pt(111). Our
calculations explain why this strain driven reconstruction occurs only after
two Ag layers have been deposited.Comment: 4 pages, 3 figures, Phys. Rev. B 55 (1997), in pres
Theoretical study of O adlayers on Ru(0001)
Recent experiments performed at high pressures indicate that ruthenium can
support unusually high concentrations of oxygen at the surface. To investigate
the structure and stability of high coverage oxygen structures, we performed
density functional theory calculations, within the generalized gradient
approximation, for O adlayers on Ru(0001) from low coverage up to a full
monolayer. We achieve quantitative agreement with previous low energy electron
diffraction intensity analyses for the (2x2) and (2x1) phases and predict that
an O adlayer with a (1x1) periodicity and coverage of 1 monolayer can form on
Ru(0001), where the O adatoms occupy hcp-hollow sites.Comment: RevTeX, 6 pages, 4 figure
Anisotropy of Growth of the Close-Packed Surfaces of Silver
The growth morphology of clean silver exhibits a profound anisotropy: The
growing surface of Ag(111) is typically very rough while that of Ag(100) is
smooth and flat. This serious and important difference is unexpected, not
understood, and hitherto not observed for any other metal. Using density
functional theory calculations of self-diffusion on flat and stepped Ag(100) we
find, for example, that at flat regions a hopping mechanism is favored, while
across step edges diffusion proceeds by an exchange process. The calculated
microscopic parameters explain the experimentally reported growth properties.Comment: RevTeX, 4 pages, 3 figures in uufiles form, to appear in Phys. Rev.
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The long-wavelength behaviour of the exchange-correlation kernel in the Kohn-Sham theory of periodic systems
The polarization-dependence of the exchange-correlation (XC) energy functional of periodic insulators within Kohn-Sham (KS) density-functional theory requires a divergence in the XC kernel for small vectors q. This behaviour, exemplified for a one-dimensional model semiconductor, is also observed when an insulator happens to be described as a KS metal, or vice-versa. Although it can occur in the exchange-only kernel, it is not found in the usual local, semi-local or even non-local approximations to KS theory. We also show that the test-charge and electronic definitions of the macroscopic dielectric constant differ from one another in exact KS theory, but are equivalent in the above-mentioned approximations
Surface relaxation and ferromagnetism of Rh(001)
The significant discrepancy between first-principles calculations and
experimental analyses for the relaxation of the (001) surface of rhodium has
been a puzzle for some years. In this paper we present density functional
theory calculations using the local-density approximation and the generalized
gradient approximation of the exchange-correlation functional. We investigate
the thermal expansion of the surface and the possibility of surface magnetism.
The results throw light on several, hitherto overlooked, aspects of metal
surfaces. We find, that, when the free energy is considered, density-functional
theory provides results in good agreement with experiments.Comment: 6 pages, 4 figures, submitted to Phys. Rev. Lett. (April 28, 1996
The ZEUS Forward Plug Calorimeter with Lead-Scintillator Plates and WLS Fiber Readout
A Forward Plug Calorimeter (FPC) for the ZEUS detector at HERA has been built
as a shashlik lead-scintillator calorimeter with wave length shifter fiber
readout. Before installation it was tested and calibrated using the X5 test
beam facility of the SPS accelerator at CERN. Electron, muon and pion beams in
the momentum range of 10 to 100 GeV/c were used. Results of these measurements
are presented as well as a calibration monitoring system based on a Co
source.Comment: 38 pages (Latex); 26 figures (ps
Ab initio study of step formation and self-diffusion on Ag(100)
Using the plane wave pseudopotential method we performed density functional
theory calculations on the stability of steps and self-diffusion processes on
Ag(100). Our calculated step formation energies show that the {111}-faceted
step is more stable than the {110}-faceted step. In accordance with
experimental observations we find that the equilibrium island shape should be
octagonal very close to a square with predominately {111}-faceted steps. For
the (100) surface of fcc metals atomic migration proceeds by a hopping or an
exchange process. For Ag(100) we find that adatoms diffuse across flat surfaces
preferentially by hopping. Adatoms approaching the close-packed {111}-faceted
step edges descend from the upper terrace to the lower level by an atomic
exchange with an energy barrier almost identical to the diffusion barrier on
flat surface regions. Thus, within our numerical accuracy (approx +- 0.05 eV)
there is no additional step-edge barrier to descent. This provides a natural
explanation for the experimental observations of the smooth two-dimensional
growth in homoepitaxy of Ag(100). Inspection of experimental results of other
fcc crystal surfaces indicates that our result holds quite generally.Comment: 10 pages, 9 figures. Submitted to Phys. Rev B (October 31, 1996
Lattice Dynamics and the High Pressure Equation of State of Au
Elastic constants and zone-boundary phonon frequencies of gold are calculated
by total energy electronic structure methods to twofold compression. A
generalized force constant model is used to interpolate throughout the
Brillouin zone and evaluate moments of the phonon distribution. The moments are
used to calculate the volume dependence of the Gruneisen parameter in the fcc
solid. Using these results with ultrasonic and shock data, we formulate the
complete free energy for solid Au. This free energy is given as a set of closed
form expressions, which are valid to compressions of at least V/V_0 = 0.65 and
temperatures up to melting. Beyond this density, the Hugoniot enters the
solid-liquid mixed phase region. Effects of shock melting on the Hugoniot are
discussed within an approximate model. We compare with proposed standards for
the equation of state to pressures of ~200 GPa. Our result for the room
temperature isotherm is in very good agreement with an earlier standard of
Heinz and Jeanloz.Comment: 13 pages, 8 figures. Accepted by Phys. Rev.
Ab initio calculations for bromine adlayers on the Ag(100) and Au(100) surfaces: the c(2x2) structure
Ab initio total-energy density-functional methods with supercell models have
been employed to calculate the c(2x2) structure of the Br-adsorbed Ag(100) and
Au(100) surfaces. The atomic geometries of the surfaces and the preferred
bonding sites of the bromine have been determined. The bonding character of
bromine with the substrates has also been studied by analyzing the electronic
density of states and the charge transfer. The calculations show that while the
four-fold hollow-site configuration is more stable than the two-fold
bridge-site topology on the Ag(100) surface, bromine prefers the bridge site on
the Au(100) surface. The one-fold on-top configuration is the least stable
configuration on both surfaces. It is also observed that the second layer of
the Ag substrate undergoes a small buckling as a consequence of the adsorption
of Br. Our results provide a theoretical explanation for the experimental
observations that the adsorption of bromine on the Ag(100) and Au(100) surfaces
results in different bonding configurations.Comment: 10 pages, 4 figure, 5 tables, Phys. Rev. B, in pres
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