946 research outputs found
Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments
The study of the formation of molecular hydrogen on low temperature surfaces
is of interest both because it allows to explore elementary steps in the
heterogeneous catalysis of a simple molecule and because of the applications in
astrochemistry. Here we report results of experiments of molecular hydrogen
formation on amorphous silicate surfaces using temperature-programmed
desorption (TPD). In these experiments beams of H and D atoms are irradiated on
the surface of an amorphous silicate sample. The desorption rate of HD
molecules is monitored using a mass spectrometer during a subsequent TPD run.
The results are analyzed using rate equations and the activation energies of
the processes leading to molecular hydrogen formation are obtained from the TPD
data. We show that a model based on a single isotope provides the correct
results for the activation energies for diffusion and desorption of H atoms.
These results can thus be used to evaluate the formation rate of H_2 on dust
grains under the actual conditions present in interstellar clouds.Comment: 30 pages, 1 table, 6 figures. Published versio
Pressure Dependence of the Elastic Moduli in Aluminum Rich Al-Li Compounds
I have carried out numerical first principles calculations of the pressure
dependence of the elastic moduli for several ordered structures in the
Aluminum-Lithium system, specifically FCC Al, FCC and BCC Li, L1_2 Al_3Li, and
an ordered FCC Al_7Li supercell. The calculations were performed using the full
potential linear augmented plane wave method (LAPW) to calculate the total
energy as a function of strain, after which the data was fit to a polynomial
function of the strain to determine the modulus. A procedure for estimating the
errors in this process is also given. The predicted equilibrium lattice
parameters are slightly smaller than found experimentally, consistent with
other LDA calculations. The computed elastic moduli are within approximately
10% of the experimentally measured moduli, provided the calculations are
carried out at the experimental lattice constant. The LDA equilibrium shear
modulus C11-C12 increases from 59.3 GPa in Al, to 76.0 GPa in Al_7Li, to 106.2
GPa in Al_3Li. The modulus C_44 increases from 38.4 GPa in Al to 46.1 GPa in
Al_7Li, then falls to 40.7 GPa in Al_3Li. All of the calculated elastic moduli
increase with pressure with the exception of BCC Li, which becomes elastically
unstable at about 2 GPa, where C_11-C_12 vanishes.Comment: 17 pages (REVTEX) + 7 postscript figure
Anharmonic effects in the A15 compounds induced by sublattice distortions
We demonstrate that elastic anomalies and lattice instabilities in the the
A15 compounds are describable in terms of first-principles LDA electronic
structure calculations. We show that at T=0 V_3Si, V_3Ge, and Nb_3Sn are
intrinsically unstable against shears with elastic moduli C_11-C_12 and C_44,
and that the zone center phonons, Gamma_2 and Gamma_12, are either unstable or
extremely soft. We demonstrate that sublattice relaxation (internal strain)
effects are key to understanding the behavior of the A15 materials.Comment: 5 pages, RevTex, 3 postscript figures, Submitted to Phys. Rev. Lett.
Apr. 23, 1997 July 7, 1997: minor corrections, final accepted versio
Effects of pressure on diffusion and vacancy formation in MgO from non-empirical free-energy integrations
The free energies of vacancy pair formation and migration in MgO were
computed via molecular dynamics using free-energy integrations and a
non-empirical ionic model with no adjustable parameters. The intrinsic
diffusion constant for MgO was obtained at pressures from 0 to 140 GPa and
temperatures from 1000 to 5000 K. Excellent agreement was found with the zero
pressure diffusion data within experimental error. The homologous temperature
model which relates diffusion to the melting curve describes well our high
pressure results within our theoretical framework.Comment: 4 pages, latex, 1 figure, revtex, submitted to PR
Ab Initio Calculation of Spin Gap Behavior in CaV4O9
Second neighbor dominated exchange coupling in CaV4O9 has been obtained from
ab initio density functional (DF) calculations. A DF-based self-consistent
atomic deformation model reveals that the nearest neighbor coupling is small
due to strong cancellation among the various superexchange processes. Exact
diagonalization of the predicted Heisenberg model yields spin-gap behavior in
good agreement with experiment. The model is refined by fitting to the
experimental susceptibility. The resulting model agrees very well with the
experimental susceptibility and triplet dispersion.Comment: 4 pages; 3 ps figures included in text; Revte
Force-matched embedded-atom method potential for niobium
Large-scale simulations of plastic deformation and phase transformations in
alloys require reliable classical interatomic potentials. We construct an
embedded-atom method potential for niobium as the first step in alloy potential
development. Optimization of the potential parameters to a well-converged set
of density-functional theory (DFT) forces, energies, and stresses produces a
reliable and transferable potential for molecular dynamics simulations. The
potential accurately describes properties related to the fitting data, and also
produces excellent results for quantities outside the fitting range. Structural
and elastic properties, defect energetics, and thermal behavior compare well
with DFT results and experimental data, e.g., DFT surface energies are
reproduced with less than 4% error, generalized stacking-fault energies differ
from DFT values by less than 15%, and the melting temperature is within 2% of
the experimental value.Comment: 17 pages, 13 figures, 7 table
Sunyaev-Zel'dovich Cluster Profiles Measured with the South Pole Telescope
We present Sunyaev-Zel'dovich measurements of 15 massive X-ray selected
galaxy clusters obtained with the South Pole Telescope. The Sunyaev-Zel'dovich
(SZ) cluster signals are measured at 150 GHz, and concurrent 220 GHz data are
used to reduce astrophysical contamination. Radial profiles are computed using
a technique that takes into account the effects of the beams and filtering. In
several clusters, significant SZ decrements are detected out to a substantial
fraction of the virial radius. The profiles are fit to the beta model and to a
generalized NFW pressure profile, and are scaled and stacked to probe their
average behavior. We find model parameters that are consistent with previous
studies: beta=0.86 and r_core/r_500 = 0.20 for the beta model, and (alpha,
beta, gamma, c_500)=(1.0,5.5,0.5,1.0) for the generalized NFW model. Both
models fit the SPT data comparably well, and both are consistent with the
average SZ profile out to the virial radius. The integrated Compton-y parameter
Y_SZ is computed for each cluster using both model-dependent and
model-independent techniques, and the results are compared to X-ray estimates
of cluster parameters. We find that Y_SZ scales with Y_X and gas mass with low
scatter. Since these observables have been found to scale with total mass, our
results point to a tight mass-observable relation for the SPT cluster survey.Comment: 21 pages, 24 figures, updated to published versio
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