5,675 research outputs found
Comment on "Collective dynamics in liquid lithium, sodium, and aluminum"
In a recent paper, S. Singh and K. Tankeshwar (ST), [Phys. Rev. E
\textbf{67}, 012201 (2003)], proposed a new interpretation of the collective
dynamics in liquid metals, and, in particular, of the relaxation mechanisms
ruling the density fluctuations propagation. At variance with both the
predictions of the current literature and the results of recent Inelastic X-ray
Scattering (IXS) experiments, ST associate the quasielastic component of the
to the thermal relaxation, as it holds in an ordinary adiabatic
hydrodynamics valid for non-conductive liquids and in the limit. We
show here that this interpretation leads to a non-physical behaviour of
different thermodynamic and transport parameters.Comment: 4 pages, 1 figure, to appear in PRE (scheduled in 1 June issue
Is the energy density of the ground state of the sine-Gordon model unbounded from below for beta^2 > 8 pi ?
We discuss Coleman's theorem concerning the energy density of the ground
state of the sine-Gordon model proved in Phys. Rev. D 11, 2088 (1975).
According to this theorem the energy density of the ground state of the
sine-Gordon model should be unbounded from below for coupling constants beta^2
> 8 pi. The consequence of this theorem would be the non-existence of the
quantum ground state of the sine-Gordon model for beta^2 > 8 pi. We show that
the energy density of the ground state in the sine-Gordon model is bounded from
below even for beta^2 > 8 pi. This result is discussed in relation to Coleman's
theorem (Comm. Math. Phys. 31, 259 (1973)), particle mass spectra and
soliton-soliton scattering in the sine-Gordon model.Comment: 22 pages, Latex, no figures, revised according to the version
accepted for publication in Journal of Physics
Long Distance Coupling of a Quantum Mechanical Oscillator to the Internal States of an Atomic Ensemble
We propose and investigate a hybrid optomechanical system consisting of a
micro-mechanical oscillator coupled to the internal states of a distant
ensemble of atoms. The interaction between the systems is mediated by a light
field which allows to couple the two systems in a modular way over long
distances. Coupling to internal degrees of freedom of atoms opens up the
possibility to employ high-frequency mechanical resonators in the MHz to GHz
regime, such as optomechanical crystal structures, and to benefit from the rich
toolbox of quantum control over internal atomic states. Previous schemes
involving atomic motional states are rather limited in both of these aspects.
We derive a full quantum model for the effective coupling including the main
sources of decoherence. As an application we show that sympathetic ground-state
cooling and strong coupling between the two systems is possible.Comment: 14 pages, 5 figure
Filling the holes: Evolving excised binary black hole initial data with puncture techniques
We follow the inspiral and merger of equal-mass black holes (BHs) by the
moving puncture technique and demonstrate that both the exterior solution and
the asymptotic gravitational waveforms are unchanged when the initial interior
solution is replaced by constraint-violating ``junk'' initial data. We apply
this result to evolve conformal thin-sandwich (CTS) binary BH initial data by
filling their excised interiors with arbitrary, but smooth, initial data and
evolving with standard puncture gauge choices. The waveforms generated for both
puncture and filled-CTS initial data are remarkably similar, and there are only
minor differences between irrotational and corotational CTS BH binaries. Even
the interior solutions appear to evolve to the same constraint-satisfying
solution at late times, independent of the initial data.Comment: 5 pages, 5 figures, accepted by PRD Rapid Communications, RevTe
Quasiequilibrium sequences of black-hole--neutron-star binaries in general relativity
We construct quasiequilibrium sequences of black hole-neutron star binaries
for arbitrary mass ratios by solving the constraint equations of general
relativity in the conformal thin-sandwich decomposition. We model the neutron
star as a stationary polytrope satisfying the relativistic equations of
hydrodynamics, and account for the black hole by imposing equilibrium boundary
conditions on the surface of an excised sphere (the apparent horizon). In this
paper we focus on irrotational configurations, meaning that both the neutron
star and the black hole are approximately nonspinning in an inertial frame. We
present results for a binary with polytropic index n=1, mass ratio
M_{irr}^{BH}/M_{B}^{NS}=5 and neutron star compaction
M_{ADM,0}^{NS}/R_0=0.0879, where M_{irr}^{BH} is the irreducible mass of the
black hole, M_{B}^{NS} the neutron star baryon rest-mass, and M_{ADM,0}^{NS}
and R_0 the neutron star Arnowitt-Deser-Misner mass and areal radius in
isolation, respectively. Our models represent valid solutions to Einstein's
constraint equations and may therefore be employed as initial data for
dynamical simulations of black hole-neutron star binaries.Comment: 5 pages, 1 figure, revtex4, published in Phys.Rev.
Quasiequilibrium black hole-neutron star binaries in general relativity
We construct quasiequilibrium sequences of black hole-neutron star binaries
in general relativity. We solve Einstein's constraint equations in the
conformal thin-sandwich formalism, subject to black hole boundary conditions
imposed on the surface of an excised sphere, together with the relativistic
equations of hydrostatic equilibrium. In contrast to our previous calculations
we adopt a flat spatial background geometry and do not assume extreme mass
ratios. We adopt a Gamma=2 polytropic equation of state and focus on
irrotational neutron star configurations as well as approximately nonspinning
black holes. We present numerical results for ratios of the black hole's
irreducible mass to the neutron star's ADM mass in isolation of
M_{irr}^{BH}/M_{ADM,0}^{NS} = 1, 2, 3, 5, and 10. We consider neutron stars of
baryon rest mass M_B^{NS}/M_B^{max} = 83% and 56%, where M_B^{max} is the
maximum allowed rest mass of a spherical star in isolation for our equation of
state. For these sequences, we locate the onset of tidal disruption and, in
cases with sufficiently large mass ratios and neutron star compactions, the
innermost stable circular orbit. We compare with previous results for black
hole-neutron star binaries and find excellent agreement with third-order
post-Newtonian results, especially for large binary separations. We also use
our results to estimate the energy spectrum of the outgoing gravitational
radiation emitted during the inspiral phase for these binaries.Comment: 17 pages, 15 figures, published in Phys. Rev.
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