34,675 research outputs found
Long-Wavelength Anomalies in the Asymptotic Behavior of Mode-Coupling Theory
We discuss the dynamic behavior of a tagged particle close to a classical
localization transition in the framework of the mode-coupling theory of the
glass transition. Asymptotic results are derived for the order parameter as
well as the dynamic correlation functions and the mean-squared displacement
close to the transition. The influence of an infrared cutoff is discussed.Comment: 15 pages, 8 figures, to appear in J Phys Condens Matte
The Dynamical Cluster Approximation (DCA) versus the Cellular Dynamical Mean Field Theory (CDMFT) in strongly correlated electrons systems
We are commenting on the article Phys. Rev. {\bf B 65}, 155112 (2002) by G.
Biroli and G. Kotliar in which they make a comparison between two cluster
techniques, the {\it Cellular Dynamical Mean Field Theory} (CDMFT) and the {\it
Dynamical Cluster Approximation} (DCA). Based upon an incorrect implementation
of the DCA technique in their work, they conclude that the CDMFT is a faster
converging technique than the DCA. We present the correct DCA prescription for
the particular model Hamiltonian studied in their article and conclude that the
DCA, once implemented correctly, is a faster converging technique for the
quantities averaged over the cluster. We also refer to their latest response to
our comment where they argue that instead of averaging over the cluster, local
observables should be calculated in the bulk of the cluster which indeed makes
them converge much faster in the CDMFT than in the DCA. We however show that in
their original work, the authors themselves use the cluster averaged quantities
to draw their conclusions in favor of using the CDMFT over the DCA.Comment: Comment on Phys. Rev. B 65, 155112 (2002). 3 pages, 2 figure
Tracing the development of dust around evolved stars: The case of 47 Tuc
We observed mid-infrared (7.5-22 mum) spectra of AGB stars in the globular
cluster 47 Tuc with the Spitzer telescope and find significant dust features of
various types. Comparison of the characteristics of the dust spectra with the
location of the stars in a logP-K-diagram shows that dust mineralogy and
position on the AGB are related. A 13 mum feature is seen in spectra of low
luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 mum.
The spectra of the most luminous stars are dominated by the amorphous silicate
bending vibration centered at 9.7 mum. For 47 Tuc AGB stars, we conclude that
early on the AGB dust consisting primarily of Mg-, Al- and Fe oxides is formed.
With further AGB evolution amorphous silicates become the dominant species.Comment: 2 figures, accepted for publication in ApJ Letter
Core Collapse and Then? The Route to Massive Star Explosions
The rapidly growing base of observational data for supernova explosions of
massive stars demands theoretical explanations. Central of these is a
self-consistent model for the physical mechanism that provides the energy to
start and drive the disruption of the star. We give arguments why the delayed
neutrino-heating mechanism should still be regarded as the standard paradigm to
explain most explosions of massive stars and show how large-scale and even
global asymmetries can result as a natural consequence of convective overturn
in the neutrino-heating region behind the supernova shock. Since the explosion
is a threshold phenomenon and depends sensitively on the efficiency of the
energy transfer by neutrinos, even relatively minor differences in numerical
simulations can matter on the secular timescale of the delayed mechanism. To
enhance this point, we present some results of recent one- and two-dimensional
computations, which we have performed with a Boltzmann solver for the neutrino
transport and a state-of-the-art description of neutrino-matter interactions.
Although our most complete models fail to explode, the simulations demonstrate
that one is encouragingly close to the critical threshold because a modest
variation of the neutrino transport in combination with postshock convection
leads to a weak neutrino-driven explosion with properties that fulfill
important requirements from observations.Comment: 14 pages; 3 figures. Invited Review, in: ``From Twilight to
Highlight: The Physics of Supernovae'', Eds. W. Hillebrandt and B.
Leibundgut, Springer Series ``ESO Astrophysics Symposia'', Berli
Global Anisotropies in Supernova Explosions and Pulsar Recoil
We show by two-dimensional and first three-dimensional simulations of
neutrino-driven supernova explosions that low (l=1,2) modes can dominate the
flow pattern in the convective postshock region on timescales of hundreds of
milliseconds after core bounce. This can lead to large global anisotropy of the
supernova explosion and pulsar kicks in excess of 500 km/s.Comment: 3 pages, 2 figures, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Killing spinors in supergravity with 4-fluxes
We study the spinorial Killing equation of supergravity involving a torsion
3-form \T as well as a flux 4-form \F. In dimension seven, we construct
explicit families of compact solutions out of 3-Sasakian geometries, nearly
parallel \G_2-geometries and on the homogeneous Aloff-Wallach space. The
constraint \F \cdot \Psi = 0 defines a non empty subfamily of solutions. We
investigate the constraint \T \cdot \Psi = 0, too, and show that it singles
out a very special choice of numerical parameters in the Killing equation,
which can also be justified geometrically
Nucleosynthesis and Clump Formation in a Core Collapse Supernova
High-resolution two-dimensional simulations were performed for the first five
minutes of the evolution of a core collapse supernova explosion in a 15 solar
mass blue supergiant progenitor. The computations start shortly after bounce
and include neutrino-matter interactions by using a light-bulb approximation
for the neutrinos, and a treatment of the nucleosynthesis due to explosive
silicon and oxygen burning. We find that newly formed iron-group elements are
distributed throughout the inner half of the helium core by Rayleigh-Taylor
instabilities at the Ni+Si/O and C+O/He interfaces, seeded by convective
overturn during the early stages of the explosion. Fast moving nickel mushrooms
with velocities up to about 4000 km/s are observed. This offers a natural
explanation for the mixing required in light curve and spectral synthesis
studies of Type Ib explosions. A continuation of the calculations to later
times, however, indicates that the iron velocities observed in SN 1987 A cannot
be reproduced because of a strong deceleration of the clumps in the dense shell
left behind by the shock at the He/H interface.Comment: 8 pages, LaTeX, 2 postscript figures, 2 gif figures, shortened and
slightly revised text and references, accepted by ApJ Letter
Observing the Profile of an Atom Laser Beam
We report on an investigation of the beam profile of an atom laser extracted
from a magnetically trapped Rb Bose-Einstein condensate. The transverse
momentum distribution is magnified by a curved mirror for matter waves and a
momentum resolution of 1/60 of a photon recoil is obtained. We find the
transverse momentum distribution to be determined by the mean-field potential
of the residing condensate, which leads to a non-smooth transverse density
distribution. Our experimental data are compared with a full 3D simulation of
the output coupling process and we find good agreement.Comment: 4 pages, 4 figure
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