4,169 research outputs found
A simple spectral condition implying separability for states of bipartite quantum systems
For two qubits and for general bipartite quantum systems, we give a simple
spectral condition in terms of the ordered eigenvalues of the density matrix
which guarantees that the corresponding state is separable.Comment: 5 pages Revised 31 May 200
Small deformations of extreme Kerr black hole initial data
We prove the existence of a family of initial data for Einstein equations
which represent small deformations of the extreme Kerr black hole initial data.
The data in this family have the same asymptotic geometry as extreme Kerr. In
particular, the deformations preserve the angular momentum and the area of the
cylindrical end.Comment: 26 pages, 4 figure
Extremal black hole initial data deformations
We study deformations of axially symmetric initial data for Einstein-Maxwell
equations satisfying the time-rotation (-) symmetry and containing one
asymptotically cylindrical end and one asymptotically flat end. We find that
the - symmetry implies the existence of a family of deformed data
having the same horizon structure. This result allows us to measure how close
solutions to Lichnerowicz equation are when arising from nearby free data.Comment: 21 pages, no figures, final versio
Rheology of a sonofluidized granular packing
We report experimental measurements on the rheology of a dry granular
material under a weak level of vibration generated by sound injection. First,
we measure the drag force exerted on a wire moving in the bulk. We show that
when the driving vibration energy is increased, the effective rheology changes
drastically: going from a non-linear dynamical friction behavior - weakly
increasing with the velocity- up to a linear force-velocity regime. We present
a simple heuristic model to account for the vanishing of the stress dynamical
threshold at a finite vibration intensity and the onset of a linear
force-velocity behavior. Second, we measure the drag force on spherical
intruders when the dragging velocity, the vibration energy, and the diameters
are varied. We evidence a so-called ''geometrical hardening'' effect for
smaller size intruders and a logarithmic hardening effect for the velocity
dependence. We show that this last effect is only weakly dependent on the
vibration intensity.Comment: Accepted to be published in EPJE. v3: Includes changes suggested by
referee
Topologically massive gravito-electrodynamics: exact solutions
We construct two classes of exact solutions to the field equations of
topologically massive electrodynamics coupled to topologically massive gravity
in 2 + 1 dimensions. The self-dual stationary solutions of the first class are
horizonless, asymptotic to the extreme BTZ black-hole metric, and regular for a
suitable parameter domain. The diagonal solutions of the second class, which
exist if the two Chern-Simons coupling constants exactly balance, include
anisotropic cosmologies and static solutions with a pointlike horizon.Comment: 15 pages, LaTeX, no figure
Dynamic and instability of submarine avalanches
We perform a laboratory-scale experiment of submarine avalanches on a rough
inclined plane. A sediment layer is prepared and thereafter tilted up to an
angle lower than the spontaneous avalanche angle. The sediment is scrapped
until an avalanche is triggered. Based on the stability diagram of the sediment
layer, we investigate different structures for the avalanche front dynamics.
First we see a straight front descending the slope, and then a transverse
instability occurs. Eventually, a fingering instability shows up similar to
rivulets appearing for a viscous fluid flowing down an incline. The mechanisms
leading to this new instability and the wavelength selection are discussed.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and
Grains 200
Horizon area--angular momentum inequality for a class of axially symmetric black holes
We prove an inequality between horizon area and angular momentum for a class
of axially symmetric black holes. This class includes initial conditions with
an isometry which leaves fixed a two-surface. These initial conditions have
been extensively used in the numerical evolution of rotating black holes. They
can describe highly distorted black holes, not necessarily near equilibrium. We
also prove the inequality on extreme throat initial data, extending previous
results.Comment: 23 pages, 5 figures. We improved the hypothesis of the main theore
Black branes on the linear dilaton background
We show that the complete static black p-brane supergravity solution with a
single charge contains two and only two branches with respect to behavior at
infinity in the transverse space. One branch is the standard family of
asymptotically flat black branes, and another is the family of black branes
which asymptotically approach the linear dilaton background with antisymmetric
form flux (LDB). Such configurations were previously obtained in the
near-horizon near-extreme limit of the dilatonic asymptotically flat p-branes,
and used to describe the thermal phase of field theories involved in the DW/QFT
dualities and the thermodynamics of little string theory in the case of the
NS5-brane. Here we show by direct integration of the Einstein equations that
the asymptotically LDB p-branes are indeed exact supergravity solutions, and we
prove a new uniqueness theorem for static p-brane solutions satisfying cosmic
censorship. In the non-dilatonic case, our general non-asymptotically flat
p-branes are uncharged black branes on the background supported by the form flux. We develop the general formalism of
quasilocal quantities for non-asymptotically flat supergravity solutions with
antisymmetric form fields, and show that our solutions satisfy the first law of
theormodynamics. We also suggest a constructive procedure to derive rotating
asymptotically LDB brane solutions.Comment: 16 pages, revtex4, v2 - references added, "authors" metatag correcte
Evidence of Raleigh-Hertz surface waves and shear stiffness anomaly in granular media
Due to the non-linearity of Hertzian contacts, the speed of sound in granular
matter increases with pressure. Under gravity, the non-linear elastic
description predicts that acoustic propagation is only possible through surface
modes, called Rayleigh-Hertz modes and guided by the index gradient. Here we
directly evidence these modes in a controlled laboratory experiment and use
them to probe the elastic properties of a granular packing under vanishing
confining pressure. The shape and the dispersion relation of both transverse
and sagittal modes are compared to the prediction of non-linear elasticity that
includes finite size effects. This allows to test the existence of a shear
stiffness anomaly close to the jamming transition.Comment: 4 pages 4 figure
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