79,782 research outputs found
Shell structure of superheavy nuclei in self-consistent mean-field models
We study the extrapolation of nuclear shell structure to the region of
superheavy nuclei in self-consistent mean-field models -- the
Skyrme-Hartree-Fock approach and the relativistic mean-field model -- using a
large number of parameterizations. Results obtained with the Folded-Yukawa
potential are shown for comparison. We focus on differences in the isospin
dependence of the spin-orbit interaction and the effective mass between the
models and their influence on single-particle spectra. While all relativistic
models give a reasonable description of spin-orbit splittings, all
non-relativistic models show a wrong trend with mass number. The spin-orbit
splitting of heavy nuclei might be overestimated by 40%-80%. Spherical
doubly-magic superheavy nuclei are found at (Z=114,N=184), (Z=120,N=172) or
(Z=126,N=184) depending on the parameterization. The Z=114 proton shell
closure, which is related to a large spin-orbit splitting of proton 2f states,
is predicted only by forces which by far overestimate the proton spin-orbit
splitting in Pb208. The Z=120 and N=172 shell closures predicted by the
relativistic models and some Skyrme interactions are found to be related to a
central depression of the nuclear density distribution. This effect cannot
appear in macroscopic-microscopic models which have a limited freedom for the
density distribution only. In summary, our findings give a strong argument for
(Z=120,N=172) to be the next spherical doubly-magic superheavy nucleus.Comment: 22 pages REVTeX, 16 eps figures, accepted for publication in Phys.
Rev.
A Laboratory Investigation of Supersonic Clumpy Flows: Experimental Design and Theoretical Analysis
We present a design for high energy density laboratory experiments studying
the interaction of hypersonic shocks with a large number of inhomogeneities.
These ``clumpy'' flows are relevant to a wide variety of astrophysical
environments including the evolution of molecular clouds, outflows from young
stars, Planetary Nebulae and Active Galactic Nuclei. The experiment consists of
a strong shock (driven by a pulsed power machine or a high intensity laser)
impinging on a region of randomly placed plastic rods. We discuss the goals of
the specific design and how they are met by specific choices of target
components. An adaptive mesh refinement hydrodynamic code is used to analyze
the design and establish a predictive baseline for the experiments. The
simulations confirm the effectiveness of the design in terms of articulating
the differences between shocks propagating through smooth and clumpy
environments. In particular, we find significant differences between the shock
propagation speeds in a clumpy medium compared to a smooth one with the same
average density. The simulation results are of general interest for foams in
both inertial confinement fusion and laboratory astrophysics studies. Our
results highlight the danger of using average properties of inhomogeneous
astrophysical environments when comparing timescales for critical processes
such as shock crossing and gravitational collapse times.Comment: 7 pages, 6 figures. Submitted to the Astrophysical Journal. For
additional information, including simulation animations and the pdf and ps
files of the paper with embedded high-quality images, see
http://pas.rochester.edu/~wm
Probing spacetime foam with extragalactic sources
Due to quantum fluctuations, spacetime is probably ``foamy'' on very small
scales. We propose to detect this texture of spacetime foam by looking for
core-halo structures in the images of distant quasars. We find that the Very
Large Telescope interferometer will be on the verge of being able to probe the
fabric of spacetime when it reaches its design performance. Our method also
allows us to use spacetime foam physics and physics of computation to infer the
existence of dark energy/matter, independent of the evidence from recent
cosmological observations.Comment: LaTeX, 11 pages, 1 figure; version submitted to PRL; several
references added; very useful comments and suggestions by Eric Perlman
incorporate
SO(10) theory of R-parity and neutrino mass
We study the Higgs sector of a SO(10) grand unified theory which predicts
exact conservation of R-parity at all scales and incorporates the see-saw
mechanism. We find possible intermediate scales and light states compatible
with the constraints coming from the running of the gauge couplings. Such a
pattern could lower the SO(10) breaking scale, allowing the d=6 proton decay
operators to be comparable in magnitude to the d=5 ones.Comment: 22 pages, 4 figure
Electronic and magnetic properties of multishell Co nanowires coated with Cu
The structural, electronic, and magnetic properties of ultrathin Cu-coated Co
nanowires have been studied by using empirical genetic algorithm simulations
and a tight-binding model Hamiltonian in the unrestricted Hartree-Hock
approximation. For some specific stoichiometric compositions, Cu atoms occupy
the surface, while Co atoms prefer to stay in the interior, forming the perfect
coated multishell structures. The outer Cu layers lead to substantial
variations of the magnetic moment of interior Co atoms, depending on the
structure and thickness of Cu layers. In particular, single Co atom row in the
center of nanowire is found to be nonmagnetic when coated with two Cu layers.
All the other Co nanowires in the coated Cu shell are still magnetic but the
magnetic moments are reduced as compared with Co nanowires without Cu coating.
The interaction between Cu and Co atoms induces nonzero magnetic moment for Cu
atoms.Comment: 7 pages, 2 fugure
Action and Energy of the Gravitational Field
We present a detailed examination of the variational principle for metric
general relativity as applied to a ``quasilocal'' spacetime region \M (that
is, a region that is both spatially and temporally bounded). Our analysis
relies on the Hamiltonian formulation of general relativity, and thereby
assumes a foliation of \M into spacelike hypersurfaces . We allow for
near complete generality in the choice of foliation. Using a field--theoretic
generalization of Hamilton--Jacobi theory, we define the quasilocal
stress-energy-momentum of the gravitational field by varying the action with
respect to the metric on the boundary \partial\M. The gravitational
stress-energy-momentum is defined for a two--surface spanned by a spacelike
hypersurface in spacetime. We examine the behavior of the gravitational
stress-energy-momentum under boosts of the spanning hypersurface. The boost
relations are derived from the geometrical and invariance properties of the
gravitational action and Hamiltonian. Finally, we present several new examples
of quasilocal energy--momentum, including a novel discussion of quasilocal
energy--momentum in the large-sphere limit towards spatial infinity.Comment: To be published in Annals of Physics. This final version includes two
new sections, one giving examples of quasilocal energy and the other
containing a discussion of energy at spatial infinity. References have been
added to papers by Bose and Dadhich, Anco and Tun
Analysis of dependence among size, rate and duration in internet flows
In this paper we examine rigorously the evidence for dependence among data
size, transfer rate and duration in Internet flows. We emphasize two
statistical approaches for studying dependence, including Pearson's correlation
coefficient and the extremal dependence analysis method. We apply these methods
to large data sets of packet traces from three networks. Our major results show
that Pearson's correlation coefficients between size and duration are much
smaller than one might expect. We also find that correlation coefficients
between size and rate are generally small and can be strongly affected by
applying thresholds to size or duration. Based on Transmission Control Protocol
connection startup mechanisms, we argue that thresholds on size should be more
useful than thresholds on duration in the analysis of correlations. Using
extremal dependence analysis, we draw a similar conclusion, finding remarkable
independence for extremal values of size and rate.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS268 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A Quantitative Non-radial Oscillation Model for the Subpulses in PSR B0943+10
In this paper, we analyze time series measurements of PSR B0943+10 and fit
them with a non-radial oscillation model. The model we apply was first
developed for total intensity measurements in an earlier paper, and expanded to
encompass linear polarization in a companion paper to this one. We use PSR
B0943+10 for the initial tests of our model because it has a simple geometry,
it has been exhaustively studied in the literature, and its behavior is
well-documented. As prelude to quantitative fitting, we have reanalyzed
previously published archival data of PSR B0943+10 and uncovered subtle but
significant behavior that is difficult to explain in the framework of the
drifting spark model. Our fits of a non-radial oscillation model are able to
successfully reproduce the observed behavior in this pulsar.Comment: 45 pages, 16 figures, accepted Ap
Systems of interacting diffusions with partial annihilation through membranes
We introduce an interacting particle system in which two families of
reflected diffusions interact in a singular manner near a deterministic
interface . This system can be used to model the transport of positive and
negative charges in a solar cell or the population dynamics of two segregated
species under competition. A related interacting random walk model with
discrete state spaces has recently been introduced and studied in Chen and Fan
(2014). In this paper, we establish the functional law of large numbers for
this new system, thereby extending the hydrodynamic limit in Chen and Fan
(2014) to reflected diffusions in domains with mixed-type boundary conditions,
which include absorption (harvest of electric charges). We employ a new and
direct approach that avoids going through the delicate BBGKY hierarchy.Comment: Published at http://dx.doi.org/10.1214/15-AOP1047 in the Annals of
Probability (http://www.imstat.org/aop/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Fingering convection induced by atomic diffusion in stars: 3D numerical computations and applications to stellar models
Iron-rich layers are known to form in the stellar subsurface through a
combination of gravitational settling and radiative levitation. Their presence,
nature and detailed structure can affect the excitation process of various
stellar pulsation modes, and must therefore be modeled carefully in order to
better interpret Kepler asteroseismic data. In this paper, we study the
interplay between atomic diffusion and fingering convection in A-type stars,
and its role in the establishment and evolution of iron accumulation layers. To
do so, we use a combination of three-dimensional idealized numerical
simulations of fingering convection, and one-dimensional realistic stellar
models. Using the three-dimensional simulations, we first validate the mixing
prescription for fingering convection recently proposed by Brown et al. (2013),
and identify what system parameters (total mass of iron, iron diffusivity,
thermal diffusivity, etc.) play a role in the overall evolution of the layer.
We then implement the Brown et al. (2013) prescription in the Toulouse-Geneva
Evolution code to study the evolution of the iron abundance profile beneath the
stellar surface. We find, as first discussed by Th\'eado et al. (2009), that
when the concurrent settling of helium is ignored, this accumulation rapidly
causes an inversion in the mean molecular weight profile, which then drives
fingering convection. The latter mixes iron with the surrounding material very
efficiently, and the resulting iron layer is very weak. However, taking helium
settling into account partially stabilizes the iron profile against fingering
convection, and a large iron overabundance can accumulate. The opacity also
increases significantly as a result, and in some cases ultimately triggers
dynamical convection.Comment: 38 pages, 16 figures, submitted to Ap
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