310 research outputs found
Neutron electric form factor at large momentum transfer
Based on the recent, high precision data for elastic electron scattering from
protons and deuterons, at relatively large momentum transfer , we
determine the neutron electric form factor up to GeV. The values
obtained from the data (in the framework of the nonrelativistic impulse
approximation) are larger than commonly assumed and are in good agreement with
the Gari-Kr\"umpelmann parametrization of the nucleon electromagnetic form
factors.Comment: 11 pages 2 figure
P-A Measurements in the 48-Ca(p,n)48-Sc Reaction at 135 MeV
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
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LEEM Investigation of the Faceting of the Pt Covered W (111) Surface
A low energy electron microscope (LEEM) has been used to investigate the faceting of W(111) as induced by Pt. The atomically rough W(111) surface, when fully covered with a monolayer film of Pt and annealed to temperatures higher than {approximately} 750 K, experiences a significant morphological restructuring: the initially planar surface undergoes a faceting transition and forms three-sided pyramids with {211} faces. The experiments demonstrate the capability of LEEM for imaging both the fully and partially faceted surface. In addition, we have observed the formation of the facets in real time, when Pt is dosed onto the heated surface. We find that the transition from planar surface, to partially faceted surface, and to fully faceted surface proceeds through the nucleation and growth of spatially separated faceted regions
Oxidation of graphene on metals
We use low-energy electron microscopy to investigate how graphene is removed
from Ru(0001) and Ir(111) by reaction with oxygen. We find two mechanisms on
Ru(0001). At short times, oxygen reacts with carbon monomers on the surrounding
Ru surface, decreasing their concentration below the equilibrium value. This
undersaturation causes a flux of carbon from graphene to the monomer gas. In
this initial mechanism, graphene is etched at a rate that is given precisely by
the same non-linear dependence on carbon monomer concentration that governs
growth. Thus, during both growth and etching, carbon attaches and detaches to
graphene as clusters of several carbon atoms. At later times, etching
accelerates. We present evidence that this process involves intercalated
oxygen, which destabilizes graphene. On Ir, this mechanism creates observable
holes. It also occurs mostly quickly near wrinkles in the graphene islands,
depends on the orientation of the graphene with respect to the Ir substrate,
and, in contrast to the first mechanism, can increase the density of carbon
monomers. We also observe that both layers of bilayer graphene islands on Ir
etch together, not sequentially.Comment: 15 pages, 10 figures. Manuscript revised to improve discussion,
following referee comments. Accepted for publication in Journal of Physical
Chemistry C, Feb. 11, 201
Missing and Quenched Gamow Teller Strength
Gamow-Teller strength functions in full spaces are calculated with
sufficient accuracy to ensure that all the states in the resonance region have
been populated. Many of the resulting peaks are weak enough to become
unobservable. The quenching factor necessary to bring into agreement the low
lying observed states with shell model predictions is shown to be due to
nuclear correlations. To within experimental uncertainties it is the same that
is found in one particle transfer and (e,e') reactions. Perfect consistency
between the observed peaks and the calculation is
achieved by assuming an observation threshold of 0.75\% of the total strength,
a value that seems typical in several experimentsComment: 11 pages, 6 figures avalaible upon request, RevTeX, FTUAM-94/0
Shell Model Study of the Double Beta Decays of Ge, Se and Xe
The lifetimes for the double beta decays of Ge, Se and
Xe are calculated using very large shell model spaces. The two neutrino
matrix elements obtained are in good agreement with the present experimental
data. For eV we predict the following upper bounds to the
half-lives for the neutrinoless mode: , and . These results are the first from a new generation of Shell
Model calculations reaching O(10) dimensions
SMMC method for two-neutrino double beta decay
Shell Model Monte Carlo (SMMC) techniques are used to calculate two-neutrino
double beta decay matrix elements. We validate the approach against direct
diagonalization for Ca in the complete -shell using the KB3
interaction. The method is then applied to the decay of Ge in the
model space using a newly calculated realistic
interaction. Our result for the matrix element is MeV, in
agreement with the experimental value.Comment: 10 pages, 3 figures available at
http://www.krl.caltech.edu/preprints/MAP.htm
NUCLEATION AND GROWTH DURING FACETING OF THE PLATINUM-COVERED W(111) SURFACE
Low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM) have been used to investigate the faceting of W(111) as induced by Pt. The atomically rough W(111) surface, when fully covered with a monolayer film of Pt and annealed to temperatures higher than {approximately}750 K, experiences a significant morphological restructuring: the initially planar surface undergoes a faceting transition and forms three-sided pyramids with {l_brace}211{r_brace} faces. When Pt is dosed onto the heated surface, the transition from planar to faceted structure proceeds through the nucleation and growth of spatially separated faceted regions, as shown by LEEM. STM reveals the atomic structure of the partially faceted surface, with large planar regions, dotted by clusters of pyramids of various sizes
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