16,286 research outputs found
Optical Theorem and the Inversion of Cross Section Data for Atom Scattering from Defects on Surfaces
The information content and properties of the cross section for atom
scattering from a defect on a flat surface are investigated. Using the Sudden
approximation, a simple expression is obtained that relates the cross section
to the underlying atom/defect interaction potential. An approximate inversion
formula is given, that determines the shape function of the defect from the
scattering data. Another inversion formula approximately determines the
potential due to a weak corrugation in the case of substitutional disorder. An
Optical Theorem, derived in the framework of the Sudden approximation, plays a
central role in deriving the equations that conveniently relate the interaction
potential to the cross section. Also essential for the result is the
equivalence of the operational definition for the cross section for scattering
by a defect, given by Poelsema and Comsa, and the formal definition from
quantum scattering theory. This equivalence is established here. The inversion
result is applied to determine the shape function of an Ag atom on Pt(111) from
scattering data.Comment: 29 pages, 9 Postscript figures, more info available at
http://www.fh.huji.ac.il/~dan
He Scattering from Random Adsorbates, Disordered Compact Islands and Fractal Submonolayers: Intensity Manifestations of Surface Disorder
A theoretical study is made on He scattering from three fundamental classes
of disordered ad-layers: (a) Translationally random adsorbates, (b) disordered
compact islands and (c) fractal submonolayers. The implications of the results
to experimental studies of He scattering from disordered surfaces are
discussed, and a combined experimental-theoretical study is made for Ag
submonolayers on Pt(111). Some of the main theoretical findings are: (1)
Structural aspects of the calculated intensities from translationally random
clusters were found to be strongly correlated with those of individual
clusters. (2) Low intensity Bragg interference peaks appear even for scattering
from very small ad-islands, and contain information on the ad-island local
electron structure. (3) For fractal islands, just as for islands with a
different structure, the off-specular intensity depends on the parameters of
the He/Ag interaction, and does not follow a universal power law as previously
proposed in the literature. In the experimental-theoretical study of Ag on
Pt(111), we use first experimental He scattering data from low-coverage (single
adsorbate) systems to determine an empirical He/Ag-Pt potential of good
quality. Then, we carry out He scattering calculations for high coverage and
compare with experiments. The conclusions are that the actual experimental
phase corresponds to small compact Ag clusters of narrow size distribution,
translationally disordered on the surface.Comment: 36 double-spaced pages, 10 figures; accepted by J. Chem. Phys.,
scheduled to appear March 8. More info available at
http://www.fh.huji.ac.il/~dani
Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T
We studied the tunneling density of states in YBCO films under strong
currents flowing along node directions. The currents were induced by fields of
up to 32.4T parallel to the film surface and perpendicular to the
planes. We observed a remarkable change in the tunneling conductance at high
fields where the gap-like feature shifts discontinuously from 15meV to a lower
bias of 11meV, becoming more pronounced as the field increases. The effect
takes place in increasing fields around 9T and the transition back to the
initial state occurs around 5T in decreasing fields. We argue that this
transition is driven by surface currents induced by the applied magnetic field.Comment: 4 pages, 7 figure
Structure Determination of Disordered Metallic Sub-Monolayers by Helium Scattering: A Theoretical and Experimental Study
An approach based on He scattering is used to develop an atomic-level
structural model for an epitaxially grown disordered sub-monolayer of Ag on
Pt(111) at 38K. Quantum scattering calculations are used to fit structural
models to the measured angular intensity distribution of He atoms scattered
from this system. The structure obtained corresponds to narrowly size-dispersed
compact clusters with modest translational disorder, and not to fractals which
might be expected due to the low surface temperature. The clusters have up to
two layers in height, the lower one having few defects only. The relations
between specific features of the angular scattering distribution, and
properties such as the cluster sizes and shapes, the inter-cluster distance
distribution etc., are discussed. The results demonstrate the usefulness of He
scattering as a tool for unraveling new complex surface phases.Comment: 5 pages, 3 figures, to appear in Surf. Sci. Lett. Related papers
available at http://neon.cchem.berkeley.edu/~dani/He-papers.htm
Patterns of Voting on Ballot Propositions: A Mixture Model of Voter Types
In this paper we analyze the patterns of behavior voters exhibit over a set of votes. We explore a set of structural estimation problems that involve analyzing several votes at one time and develop estimation techniques for identifying and analyzing patterns. Using the information in these patterns, we introduce a method for studying voter heterogeneity based on a finite mixture model. Finally, we employ data containing actual micro-level vote returns to estimate the mixture model parameters
Star Formation Across the Taffy Bridge: UGC 12914/15
We present BIMA two-field mosaic CO(1-0) images of the Taffy galaxies (UGC
12914/15), which show the distinct taffy-like radio continuum emission bridging
the two spiral disks. Large amounts of molecular gas (1.4 x 10^{10} Msun, using
the standard Galactic CO-to-H conversion applicable to Galactic disk giant
molecular clouds [GMCs]) were clearly detected throughout the taffy bridge
between the two galaxies, which, as in the more extreme case of HI, presumably
results from a head-on collision between the two galaxies. The highest CO
concentration between the two galaxies corresponds to the H_alpha source in the
taffy bridge near the intruder galaxy UGC 12915. This HII region is also
associated with the strongest source of radio continuum in the bridge, and
shows both morphological and kinematic connections to UGC 12915. The overall CO
distribution of the entire system agrees well with that of the radio continuum
emission, particularly in the taffy bridge. This argues for the star formation
origin of a significant portion of the radio continuum emission. Compared to
the HI morphology and kinematics, which are strongly distorted owing to the
high-speed collision, CO better defines the orbital geometry and impact
parameter of the interaction, as well as the disk properties (e.g., rotation,
orientation) of the progenitor galaxies. Based on the 20cm-to-CO ratio maps, we
conclude that the starburst sites are primarily located in UGC 12915 and the
H_alpha source in the bridge and show that the molecular gas in the taffy
bridge is forming into stars with star formation efficiency comparable to that
of the target galaxy UGC 12914 and similar to that in the Galactic disk.Comment: Minor typo/style corrections to match with the published version (AJ,
Nov. issue). A single .ps.gz file of the entire paper can be downloaded from
http://spider.ipac.caltech.edu/staff/gao/Taffy/all.ps.g
Inflammatory Airway Disease of Horses - Revised Consensus Statement
The purpose of this manuscript is to revise and update the previous consensus statement on inflammatory airway disease (IAD) in horses. Since 2007, a large number of scientific articles have been published on the topic and these new findings have led to a significant evolution of our understanding of IAD
Post-transcriptional gene regulation: From genome-wide studies to principles
Abstract.: Post-transcriptional regulation of gene expression plays important roles in diverse cellular processes such as development, metabolism and cancer progression. Whereas many classical studies explored the mechanistics and physiological impact on specific mRNA substrates, the recent development of genome-wide analysis tools enables the study of post-transcriptional gene regulation on a global scale. Importantly, these studies revealed distinct programs of RNA regulation, suggesting a complex and versatile post-transcriptional regulatory network. This network is controlled by specific RNA-binding proteins and/or non-coding RNAs, which bind to specific sequence or structural elements in the RNAs and thereby regulate subsets of mRNAs that partly encode functionally related proteins. It will be a future challenge to link the spectra of targets for RNA-binding proteins to post-transcriptional regulatory programs and to reveal its physiological implication
Progressive Star Bursts and High Velocities in the Infrared Luminous, Colliding Galaxy Arp 118
In this paper we demonstrate for the first time the connection between the
spatial and temporal progression of star formation and the changing locations
of the very dense regions in the gas of a massive disk galaxy (NGC 1144) in the
aftermath of its collision with a massive elliptical (NGC 1143). These two
galaxies form the combined object Arp 118, a collisional ring galaxy system.
The results of 3D, time-dependent, numerical simulations of the behavior of the
gas, stars, and dark matter of a disk galaxy and the stars and dark matter in
an elliptical during a collision are compared with multiwavelength observations
of Arp 118. The collision that took place approximately 22 Myr ago generated a
strong, non-linear density wave in the stars and gas in the disk of NGC 1144,
causing the gas to became clumped on a large scale. This wave produced a series
of superstarclusters along arcs and rings that emanate from the central point
of impact in the disk. The locations of these star forming regions match those
of the regions of increased gas density predicted the time sequence of models.
The models also predict the large velocity gradients observed across the disk
of NGC 1144. These are due to the rapid radial outflow of gas coupled to large
azimuthal velocities in the expanding ring, caused by the impact of the massive
intruder.Comment: 12 pages in document, and 8 figures (figures are separate from the
document's file); Submitted to Astrophysical Journal Letter
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