58,778 research outputs found
Optical excitation of nonlinear spin waves
We demonstrate a technique for exciting spin waves in an ultracold gas of
Rb-87 atoms based on tunable AC Stark potentials. This technique allows us to
excite normal modes of spin waves with arbitrary amplitudes in the trapped gas,
including dipole, quadrupole, octupole, and hexadecapole modes. These modes
exhibit strong nonlinearities, which manifest as amplitude dependence of the
excitation frequencies and departure from sinusoidal behavior. Our results are
in good agreement with a full treatment of a quantum Boltzmann transport
equation.Comment: 11 pages, 5 figure
Kinematic Density Waves in Accretion Disks
When thin accretion disks around black holes are perturbed, the main
restoring force is gravity. If gas pressure, magnetic stresses, and radiation
pressure are neglected, the disk remains thin as long as orbits do not
intersect. Intersections would result in pressure forces which limit the growth
of perturbations. We find that a discrete set of perturbations is possible for
which orbits remain non-intersecting for arbitrarily long times. These modes
define a discrete set of frequencies. We classify all long-lived perturbations
for arbitrary potentials and show how their mode frequencies are related to
pattern speeds computed from the azimuthal and epicyclic frequencies. We show
that modes are concentrated near radii where the pattern speed has vanishing
radial derivative. We explore these modes around Kerr black holes as a possible
explanation for the high-frequency quasi-periodic oscillations of black hole
binaries such as GRO J1655-40. The long-lived modes are shown to coincide with
diskoseismic waves in the limit of small sound speed. While the waves have long
lifetime, they have the wrong frequencies to explain the pairs of
high-frequency quasi-periodic oscillations observed in black hole binaries.Comment: 28 pages, 6 figures; extended comparison with diskoseismology; added
reference to astro-ph/060368
7Be(p,gamma)8B S-factor from ab initio no-core shell model wave functions
Nuclear structure of 7Be, 8B and 7,8Li is studied within the ab initio
no-core shell model (NCSM). Starting from high-precision nucleon-nucleon (NN)
interactions, wave functions of 7Be and 8B bound states are obtained in basis
spaces up to 10 hbar Omega and used to calculate channel cluster form factors
(overlap integrals) of the 8B ground state with 7Be+p. Due to the use of the
harmonic oscillator (HO) basis, the overlap integrals have incorrect asymptotic
properties. We fix this problem in two alternative ways. First, by a
Woods-Saxon (WS) potential solution fit to the interior of the NCSM overlap
integrals. Second, by a direct matching with the Whittaker function. The
corrected overlap integrals are then used for the 7Be(p,gamma)8B S-factor
calculation. We study the convergence of the S-factor with respect to the NCSM
HO frequency and the model space size. Our S-factor results are in agreement
with recent direct measurement data. We also test the spectroscopic factors and
the corrected overlap integrals from the NCSM in describing the momentum
distributions in knockout reactions with 8B projectiles. A good agreement with
the available experimental data is also found, attesting the overall
consistency of the calculations.Comment: 18 pages, 26 figure
Tensor Spectra Templates for Axion-Gauge Fields Dynamics during Inflation
gauge fields can generate large gravitational waves during inflation,
if they are coupled to an axion which can be either the inflaton or a spectator
field. The shape of the produced tensor power spectrum depends
on the form of the axion potential. We derive analytic expressions and provide
general templates for for various types of the spectator axion
potential. Furthermore, we explore the detectability of the oscillatory
feature, which is present in in the case of an axion monodromy
model, by possible future CMB B-mode polarization observations.Comment: 31 pages, 11 figure
Directivity patterns of laser-generated sound in solids: Effects of optical and thermal parameters
In the present paper, directivity patterns of laser-generated sound in solids
are investigated theoretically. Two main approaches to the calculation of
directivity patterns of laser-generated sound are discussed for the most
important case of thermo-optical regime of generation. The first approach,
which is widely used in practice, is based on the simple modelling of the
equivalent thermo-optical source as a mechanical dipole comprising two
horizontal forces applied to the surface in opposite directions. The second
approach is based on the rigorous theory that takes into account all
acoustical, optical and thermal parameters of a solid material and all
geometrical and physical parameters of a laser beam. Directivity patterns of
laser-generated bulk longitudinal and shear elastic waves, as well as the
amplitudes of generated Rayleigh surface waves, are calculated for different
values of physical and geometrical parameters and compared with the directivity
patterns calculated in case of dipole-source representation. It is demonstrated
that the simple approach using a dipole-source representation of
laser-generated sound is rather limited, especially for description of
generated longitudinal acoustic waves. A practical criterion is established to
define the conditions under which the dipole-source representation gives
predictions with acceptable errors. It is shown that, for radiation in the
normal direction to the surface, the amplitudes of longitudinal waves are
especially sensitive to the values of thermal parameters and of the acoustic
reflection coefficient from a free solid surface. A discussion is given on the
possibility of using such a high sensitivity to the values of the reflection
coefficient for investigation of surface properties of real solids.Comment: 14 pages, 7 figure
A microscopic equation of state for neutron-rich matter and its effect on neutron star properties
Chapter prepared for the book "Astrophysics", ISBN 979-953-307-389-6, INTECH
Publishers (in press).Comment: 34 pages, 20 figure
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