4,404 research outputs found
High-dimensional quantum dynamics of adsorption and desorption of H at Cu(111)
We performed high-dimensional quantum dynamical calculations of the
dissociative adsorption and associative desorption of hydrogen on Cu(111). The
potential energy surface (PES) is obtained from density functional theory
calculations. Two regimes of dynamics are found, at low energies sticking is
determined by the minimum energy barrier, at high energies by the distribution
of barrier heights. Experimental results are well-reproduced qualitatively, but
some quantitative discrepancies are identified as well.Comment: 4 two column pages, revtex, 4 figures, to appear in Phys. Rev. Let
Thickness-dependent thermal properties of amorphous insulating thin films measured by photoreflectance microscopy
In this work, we report on the measurement of the thermal conductivity of thin insulating films of SiO2 obtained by thermal oxidation, and Al2O3 grown by atomic layer deposition (ALD), both on Si wafers. We used photoreflectance microscopy to determine the thermal properties of the films as a function of thickness in the 2 nm to 1000 nm range. The effective thermal conductivity of the Al2O3 layer is shown to decrease with thickness down to 70% for the thinnest layers. The data were analyzed upon considering that the change in the effective thermal conductivity corresponds to an intrinsic thermal conductivity associated to an additional interfacial thermal resistance. The intrinsic conductivity and interfacial thermal resistance of SiO2 were found to be equal to 0.95 W/m·K and 5.1 × 10− 9 m2K/W respectively; those of Al2O3 were found to be 1.56 W/m·K and 4.3 × 10− 9 m2K/W
A New H I Survey of Active Galaxies
We have conducted a new Arecibo survey for H I emission for 113 galaxies with
broad-line (type 1) active galactic nuclei (AGNs) out to recession velocities
as high as 35,000 km/s. The primary aim of the study is to obtain sensitive H I
spectra for a well-defined, uniformly selected sample of active galaxies that
have estimates of their black hole masses in order to investigate correlations
between H I properties and the characteristics of the AGNs. H I emission was
detected in 66 out of the 101 (65%) objects with spectra uncorrupted by radio
frequency interference, among which 45 (68%) have line profiles with adequate
signal-to-noise ratio and sufficiently reliable inclination corrections to
yield robust deprojected rotational velocities. This paper presents the basic
survey products, including an atlas of H I spectra, measurements of H I flux,
line width, profile asymmetry, optical images, optical spectroscopic
parameters, as well as a summary of a number of derived properties pertaining
to the host galaxies. To enlarge our primary sample, we also assemble all
previously published H I measurements of type 1 AGNs for which can can estimate
black hole masses, which total an additional 53 objects. The final
comprehensive compilation of 154 broad-line active galaxies, by far the largest
sample ever studied, forms the basis of our companion paper, which uses the H I
database to explore a number of properties of the AGN host galaxies.Comment: To appear in ApJS; 31 pages. Preprint will full-resolution figures
can be downloaded from http://www.ociw.edu/~lho/preprints/ms1.pd
Rotation of hydrogen molecules during the dissociative adsorption on the Mg(0001) surface: A first-principles study
Using first-principles calculations, we systematically study the potential
energy surfaces and dissociation processes of the hydrogen molecule on the
Mg(0001) surface. It is found that during the dissociative adsorption process
with the minimum energy barrier, the hydrogen molecule firstly orients
perpendicular, and then rotates to be parallel to the surface. It is also found
that the orientation of the hydrogen molecule at the transition state is
neither perpendicular nor parallel to the surface. Most importantly, we find
that the rotation causes a reduction of the calculated dissociation energy
barrier for the hydrogen molecule. The underlying electronic reasons for the
rotation of the hydrogen molecule is also discussed in our paper.Comment: 14 pages, 4 figure
Sidechain control of porosity closure in multiple peptide-based porous materials by cooperative folding
Porous materials find application in separation, storage and catalysis. We report a crystalline porous solid formed by coordination of metal centres with a glycylserine dipeptide. We prove experimentally that the structure evolves from a solvated porous into a non-porous state as result of ordered displacive and conformational changes of the peptide that suppress the void space in response to environmental pressure. This cooperative closure, which recalls the folding of proteins, retains order in three-dimensions and is driven by the hydroxyl groups acting as H-bond donors in the peptide sequence through the serine residue. This ordered closure is also displayed by multipeptide solid solutions in which the combination of different sequences of amino acids controls their guest response in a non-linear way. This functional control can be compared to the effect of single point mutations in proteins, where the exchange of single amino acids can radically alter structure and functio
Aharonov-Bohm-like effect for light propagating in nematics with disclinations
Using a geometric approach for the propagation of light in anisotropic media,
we investigate what effect the director field of disclinations may have on the
polarization state of light. Parallel transport around the defect, of the
spinor describing the polarization, indicates the acquisition of a topological
phase, in analogy with the Aharonov-Bohm effect.Comment: 6 pages, to appear in Europhysics Letter
Properties of Active Galaxies Deduced from H I Observations
We completed a new survey for H I emission for a large, well-defined sample
of 154 nearby (z < 0.1) galaxies with type 1 AGNs. We make use of the extensive
database presented in a companion paper to perform a comprehensive appraisal of
the cold gas content in active galaxies and to seek new strategies to
investigate the global properties of the host galaxies and their relationship
to their central black holes (BHs). We show that the BH mass obeys a strong,
roughly linear relation with the host galaxy's dynamical mass. BH mass follows
a looser, though still highly significant, correlation with the maximum
rotation velocity of the galaxy, as expected from the known scaling between
rotation velocity and central velocity dispersion. Neither of these H I-based
correlations is as tight as the more familiar relations between BH mass and
bulge luminosity or velocity dispersion, but they offer the advantage of being
insensitive to the glare of the nucleus and therefore are promising new tools
for probing the host galaxies of both nearby and distant AGNs. We present
evidence for substantial ongoing BH growth in the most actively accreting AGNs.
In these nearby systems, BH growth appears to be delayed with respect to the
assembly of the host galaxy but otherwise has left no detectable perturbation
to its mass-to-light ratio or its global gas content. The host galaxies of type
1 AGNs, including those luminous enough to qualify as quasars, are generally
gas-rich systems, possessing a cold interstellar medium reservoir at least as
abundant as that in inactive galaxies of the same morphological type. This
calls into question current implementations of AGN feedback in models of galaxy
formation that predict strong cold gas depletion in unobscured AGNs. (Abridged)Comment: To appear in ApJ; 14 page
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