24,008 research outputs found
Pre-encounter observations of 951 Gaspra
Photometry and colorimetry of 951 Gaspra were obtained on nine nights during the 1990 opposition. A composite lightcurve constructed using data from eight of those nights yielded a synodic rotational period of 7.04346 +/- 0.00006 hours, a mean absolute V magnitude of 11.8026 +/- 0.0025, and a slope parameter of 0.285 +/- 0.005. The apparent discrepancy can be easily resolved by realizing that their determination is based primarily on data obtained after opposition. Different phase functions pre- and post-opposition are a natural consequence of a changing aspect during an opposition. If the sub-Earth latitude on Gaspra is at a less equatorial aspect after opposition than it was before opposition, then we would expect to see a shallower phase function (corresponding to a larger numerical value of the slope parameter). Adding weight to this hypothesis is the last observation of the opposition, made in May after Gaspra had passed post opposition quadrature, which is displaced toward brighter absolute magnitudes relative to the rest of our data, indicating an even more poleward sub-Earth latitude than earlier in the opposition. Because the orbits of Earth and Gaspra are nearly coplanar, a substantial change in sub-Earth latitude during the opposition would not have been possible unless the obliquity of the asteroid's rotational axis is not small
Phase transitions in diluted negative-weight percolation models
We investigate the geometric properties of loops on two-dimensional lattice
graphs, where edge weights are drawn from a distribution that allows for
positive and negative weights. We are interested in the appearance of spanning
loops of total negative weight. The resulting percolation problem is
fundamentally different from conventional percolation, as we have seen in a
previous study of this model for the undiluted case.
Here, we investigate how the percolation transition is affected by additional
dilution. We consider two types of dilution: either a certain fraction of edges
exhibit zero weight, or a fraction of edges is even absent. We study these
systems numerically using exact combinatorial optimization techniques based on
suitable transformations of the graphs and applying matching algorithms. We
perform a finite-size scaling analysis to obtain the phase diagram and
determine the critical properties of the phase boundary.
We find that the first type of dilution does not change the universality
class compared to the undiluted case whereas the second type of dilution leads
to a change of the universality class.Comment: 8 pages, 7 figure
Gravitating superconducting strings with timelike or spacelike currents
We construct gravitating superconducting string solutions of the U(1)_{local}
x U(1)_{global} model solving the coupled system of Einstein and matter field
equations numerically. We study the properties of these solutions in dependence
on the ratio between the symmetry breaking scale and the Planck mass. Using the
macroscopic stability conditions formulated by Carter, we observe that the
coupling to gravity allows for a new stable region that is not present in the
flat space-time limit. We match the asymptotic metric to the Kasner metric and
show that the relations between the Kasner coefficients and the energy per unit
length and tension suggested previously are well fulfilled for symmetry
breaking scale much smaller than the Planck mass. We also study the solutions
to the geodesic equation in this space-time. While geodesics in the exterior
space-time of standard cosmic strings are just straight lines, test particles
experience a force in a general Kasner space-time and as such bound orbits are
possible.Comment: 16 pages including 14 figure
Ferroelectric properties of charge-ordered alpha-(BEDT-TTF)2I3
A detailed investigation of the out-of-plane electrical properties of
charge-ordered alpha-(BEDT-TTF)2I3 provides clear evidence for
ferroelectricity. Similar to multiferroic alpha-(BEDT-TTF)2Cu[N(CN)2]Cl, the
polar order in this material is ascribed to the occurrence of bond- and
site-centered charge order. Dielectric response typical for relaxor
ferroelectricity is found deep in the charge-ordered state. We suggest an
explanation in terms of the existence of polar and nonpolar stacks of the
organic molecules in this material, preventing long-range ferroelectricity. The
results are discussed in relation to the formation or absence of electronic
polar order in related charge-transfer salts.Comment: 8 pages, 4 figures. Revised version as accepted for publication in
Phys. Rev.
Systematic Mapping of the Hubbard Model to the Generalized t-J Model
The generalized t-J model conserving the number of double occupancies is
constructed from the Hubbard model at and in the vicinity of half-filling at
strong coupling. The construction is realized by a self-similar continuous
unitary transformation. The flow equation is closed by a truncation scheme
based on the spatial range of processes. We analyze the conditions under which
the t-J model can be set up and we find that it can only be defined for
sufficiently large interaction. There, the parameters of the effective model
are determined.Comment: 16 pages, 13 figures included. v2: Order of sections changed.
Calculation and discussion of apparent gap in Section IV.A correcte
A BeppoSAX observation of the super-soft source CAL87
We report on a BeppoSAX Concentrator Spectrometer observation of the
super-soft source (SSS) CAL87. The X-ray emission in SSS is believed to arise
from nuclear burning of accreted material on the surface of a white dwarf (WD).
An absorbed blackbody spectral model gives a chi^2_v of 1.18 and a temperature
of 42 +/- ^13 _11 eV. However, the derived luminosity and radius are greater
than the Eddington limit and radius of a WD. Including an O viii edge at 0.871
keV gives a significantly better fit (at > 95% confidence) and results in more
realistic values of the source luminosity and radius. We also fit WD atmosphere
models to the CAL87 spectrum. These also give reasonable bolometric
luminosities and radii in the ranges 2.7-4.8 10^{36} erg/s and 8-20 10^7 cm,
respectively. These results support the view that the X-ray emission from CAL87
results from nuclear burning in the atmosphere of a WD.Comment: 4 pages. Accepted for publication in A&A (Letters
A system of relational syllogistic incorporating full Boolean reasoning
We present a system of relational syllogistic, based on classical
propositional logic, having primitives of the following form:
Some A are R-related to some B;
Some A are R-related to all B;
All A are R-related to some B;
All A are R-related to all B.
Such primitives formalize sentences from natural language like `All students
read some textbooks'. Here A and B denote arbitrary sets (of objects), and R
denotes an arbitrary binary relation between objects. The language of the logic
contains only variables denoting sets, determining the class of set terms, and
variables denoting binary relations between objects, determining the class of
relational terms. Both classes of terms are closed under the standard Boolean
operations. The set of relational terms is also closed under taking the
converse of a relation. The results of the paper are the completeness theorem
with respect to the intended semantics and the computational complexity of the
satisfiability problem.Comment: Available at
http://link.springer.com/article/10.1007/s10849-012-9165-
Reply to the Comment on `Glassy Transition in a Disordered Model for the RNA Secondary Structure'
We reply to the Comment by Hartmann (cond-mat/9908132) on our paper Phys.
Rev. Lett. 84 (2000) 2026 (also cond-mat/9907125).Comment: 1 page, no figures. Accepted for publication in Phys. Rev. Let
Coherent control of photon transmission : slowing light in coupled resonator waveguide doped with Atoms
In this paper, we propose and study a hybrid mechanism for coherent
transmission of photons in the coupled resonator optical waveguide (CROW) by
incorporating the electromagnetically induced transparency (EIT) effect into
the controllable band gap structure of the CROW. Here, the configuration setup
of system consists of a CROW with homogeneous couplings and the artificial
atoms with -type three levels doped in each cavity. The roles of three
levels are completely considered based on a mean field approach where the
collection of three-level atoms collectively behave as two-mode spin waves. We
show that the dynamics of low excitations of atomic ensemble can be effectively
described by an coupling boson model. The exactly solutions show that the light
pulses can be stopped and stored coherently by adiabatically controlling the
classical field.Comment: 10 pages, 6 figure
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