2,643 research outputs found
Dimensional effects in photoelectron spectra of Ag deposits on GaAs(110) surfaces
It is shown that the peak structure observed in angle-resolved photoelectron
spectra of metallic deposits can only be unambiguously associated to single
electronic states if the deposit has a two dimensional character (finite along
one spatial direction). In one and zero dimensions the density of states shows
peaks related to bunches of single electron states (the finer structure
associated to the latter may not always be experimentally resolved). The
characteristics of the peak structure strongly depend on the band dispersion in
the energy region where they appear. Results for the density of states and
photoemission yield for Ag crystallites on GaAs(110) are presented and compared
with experimental photoelectron spectra.Comment: Uuencoded gz-compressed postcript file including text and three
figures; Send comments to [email protected]
Averaging the intensity of many-layered structures for accurate stacking-fault analysis using Rietveld refinement
Many technologically important synthetic and natural materials display stacking faults which lead to complex peak broadenings, asymmetries and shifts in their powder diffraction patterns. The patterns can be described using an enlarged unit cell (called a supercell) containing an explicit description of the layers. Since the supercell can contain hundreds of thousands of atoms with hundreds of thousands of hkl reflections, a Rietveld approach has been too computationally demanding for all but the simplest systems. This article describes the implementation of the speed-ups necessary to allow Rietveld refinement in the computer program TOPAS Version 6 (Bruker AXS, Karlsruhe, Germany). Techniques implemented include: a peaks buffer that allows hundreds of thousands of hkl-dependent peak shapes to be automatically approximated by a few hundred peaks; an averaging process for hundreds of large supercells with minimum impact on computational time; a smoothing technique that allows for the use of small supercells which approximate supercells ten to 20 times larger; and efficient algorithms for stacking sequence generation. The result is Rietveld refinement of supercells operating at speeds several thousand times faster than traditional Rietveld refinements. This allows quantitative and simultaneous analysis of structure and microstructure in complex stacking-faulted samples
The inverse problem in microlensing: from the optical depth to the galaxy models parameters
We present in this paper a simple method to obtain informations on galaxy
models parameters using the measured value of the microlensing optical depth.
Assuming a 100 percent MACHO's dark halo, we ask the predicted optical depth
for a given model to be the same as the observed one, in a given direction.
Writing the optical depth in terms of the given halo model parameters and
inverting this relation with respect to one of them, it is possible to get
information on it, fixing under reasonable hypothesis the other parameters.
This is what we call the "inverse problem in microlensing. We apply this
technique to the class of power-law models with flat rotation curves,
determining the range for the core radius R_c compatible with the constraints
on the halo flattening q and the measures of the optical depth towards LMC.
Next, we apply the same method to a simple triaxial model, evaluating the axial
ratios.Comment: 23 LaTex Pages, including 5 Postscript Figures, uses astron.sty,
submitted to A&
Statistical mechanics of a colloidal suspension in contact with a fluctuating membrane
Surface effects are generally prevailing in confined colloidal systems. Here
we report on dispersed nanoparticles close to a fluid membrane. Exact results
regarding the static organization are derived for a dilute solution of
non-adhesive colloids. It is shown that thermal fluctuations of the membrane
broaden the density profile, but on average colloids are neither accumulated
nor depleted near the surface. The radial correlation function is also
evaluated, from which we obtain the effective pair-potential between colloids.
This entropically-driven interaction shares many similarities with the familiar
depletion interaction. It is shown to be always attractive with range
controlled by the membrane correlation length. The depth of the potential well
is comparable to the thermal energy, but depends only indirectly upon membrane
rigidity. Consequenses for stability of the suspension are also discussed
The Green--Schwarz Superstring in Extended Configuration Space and Infinitely Reducible First Class Constraints Problem
The Green--Schwarz superstring action is modified to include some set of
additional (on-shell trivial) variables. A complete constraints system of the
theory turns out to be reducible both in the original and in additional
variable sectors. The initial first class constraints and second
class ones are shown to be unified with first and second class
constraints from the additional variables sector, resulting with
-covariant and linearly independent constraint sets. Residual
reducibility proves to fall on second class constraints only.Comment: 14 pages, LaTe
On the Evolved Nature of CK Vul
Original paper can be found at: http://www.astrosociety.org/pubs/cs/381.html Copyright ASPCK Vul was classified as the oldest observed nova. Recent studies have proven however, that CK Vul cannot be unambiguously classified as any known kind of eruptive variable. We present the optical and radio observations of the remnants of the eruption of CK Vul in the year 1670 in order to discuss possible scenarios for this object. We have measured the proper motion which proves that the nebula is attributed to the star observed during its 1670−1672 brightening. A large bipolar nebula of 70 arcsec is discovered in a deep Hα image. Radio observations reveal a barely resolved source placed in the expansion center of the ejecta
Cluster Analysis of Extremely High Energy Cosmic Rays in the Northern Sky
The arrival directions of extremely high energy cosmic rays (EHECR) above
eV, observed by four surface array experiments in the northern
hemisphere,are examined for coincidences from similar directions in the sky.
The total number of cosmic rays is 92.A significant number of double
coincidences (doublet) and triple coincidences (triplet) are observed on the
supergalactic plane within the experimental angular resolution. The chance
probability of such multiplets from a uniform distribution is less than 1 % if
we consider a restricted region within of the supergalactic
plane. Though there is still a possibility of chance coincidence, the present
results on small angle clustering along the supergalactic plane may be
important in interpreting EHECR enigma. An independent set of data is required
to check our claims.Comment: 9 pages, 6 tables, 8 figures. submitted to Astroparticle Physic
Continuity of Optimal Control Costs and its application to Weak KAM Theory
We prove continuity of certain cost functions arising from optimal control of
affine control systems. We give sharp sufficient conditions for this
continuity. As an application, we prove a version of weak KAM theorem and
consider the Aubry-Mather problems corresponding to these systems.Comment: 23 pages, 1 figures, added explanations in the proofs of the main
theorem and the exampl
Generating asymptotically plane wave spacetimes
In an attempt to study asymptotically plane wave spacetimes which admit an
event horizon, we find solutions to vacuum Einstein's equations in arbitrary
dimension which have a globally null Killing field and rotational symmetry. We
show that while such solutions can be deformed to include ones which are
asymptotically plane wave, they do not posses a regular event horizon. If we
allow for additional matter, such as in supergravity theories, we show that it
is possible to have extremal solutions with globally null Killing field, a
regular horizon, and which, in addition, are asymptotically plane wave. In
particular, we deform the extremal M2-brane solution in 11-dimensional
supergravity so that it behaves asymptotically as a 10-dimensional vacuum plane
wave times a real line.Comment: 23 pages, 1 eps figure; harvmac; v2:refs added; v3:minor comments
adde
Mass Composition of Cosmic Rays in the Range 2 x 10^17 - 3 x 10^18 Measured with Haverah Park Array
At the Haverah Park Array a number of air shower observables were measured
that are relevant to the determination of the mass composition of cosmic rays.
In this paper we discuss measurements of the risetime of signals in large area
water-Cherenkov detectors and of the lateral distribution function of the
water-Cherenkov signal. The former are used to demonstrate that the CORSIKA
code, using the QGSJET98 model, gives an adequate description of the data with
a low sensitivity, in this energy range, to assumptions about primary mass. By
contrast the lateral distribution is sufficiently well measured that there is
mass sensitivity. We argue that in the range 0.2-1.0 EeV the data are well
represented with a bi-modal composition of 34+-2 % protons and the rest iron.
We also discuss the systematic errors induced by the choice of hadronic model.Comment: 16 pages, 13 figures. Accepted for publication in Astroparticle
Physic
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