108,480 research outputs found
Thermalization and temperature distribution in a driven ion chain
We study thermalization and non-equilibrium dynamics in a dissipative quantum
many-body system -- a chain of ions with two points of the chain driven by
thermal bath under different temperature. Instead of a simple linear
temperature gradient as one expects from the classical heat diffusion process,
the temperature distribution in the ion chain shows surprisingly rich patterns,
which depend on the ion coupling rate to the bath, the location of the driven
ions, and the dissipation rates of the other ions in the chain. Through
simulation of the temperature evolution, we show that these unusual temperature
distribution patterns in the ion chain can be quantitatively tested in
experiments within a realistic time scale.Comment: 5 pages, 5 figure
Superradiance in spin- particles: Effects of multiple levels
We study the superradiance dynamics in a dense system of atoms each of which
can be generally a spin- particle with an arbitrary half-integer. We
generalize Dicke's superradiance point of view to multiple-level systems, and
compare the results based on a novel approach we have developed in {[}Yelin
\textit{et al.}, arXiv:quant-ph/0509184{]}. Using this formalism we derive an
effective two-body description that shows cooperative and collective effects
for spin- particles, taking into account the coherence of transitions
between different atomic levels. We find that the superradiance, which is
well-known as a many-body phenomenon, can also be modified by multiple level
effects. We also discuss the feasibility and propose that our approach can be
applied to polar molecules, for their vibrational states have multi-level
structure which is partially harmonic.Comment: 11 pages, 7 figure
Three-dimensional waves generated at Lindblad resonances in thermally stratified disks
We analyze the linear, 3D response to tidal forcing of a disk that is thin
and thermally stratified in the direction normal to the disk plane. We model
the vertical disk structure locally as a polytrope which represents a disk of
high optical depth. We solve the 3D gas-dynamic equations semi-analytically in
the neighborhood of a Lindblad resonance. These solutions match asymptotically
on to those valid away from resonances and provide solutions valid at all
radii. We obtain the following results. 1) A variety of waves are launched at
resonance. However, the f mode carries more than 95% of the torque exerted at
the resonance. 2) These 3D waves collectively transport exactly the amount of
angular momentum predicted by the 2D torque formula. 3) Near resonance, the f
mode occupies the full vertical extent of the disk. Away from resonance, the f
mode becomes confined near the surface of the disk, and, in the absence of
other dissipation mechanisms, damps via shocks. The radial length scale for
this process is roughly r_L/m (for resonant radius r_L and azimuthal wavenumber
m), independent of the disk thickness H. This wave channeling process is due to
the variations of physical quantities in r and is not due to wave refraction.
4) However, the inwardly propagating f mode launched from an m=2 inner Lindblad
resonance experiences relatively minor channeling.
We conclude that for binary stars, tidally generated waves in highly
optically thick circumbinary disks are subject to strong nonlinear damping by
the channeling mechanism, while those in circumstellar accretion disks are
subject to weaker nonlinear effects. We also apply our results to waves excited
by young planets for which m is approximately r/H and conclude that the waves
are damped on the scale of a few H.Comment: 15 pages, 3 figures, 2 colour plates, to be published in the
Astrophysical Journa
First Lattice Study of the - Transition Form Factors
Experiments at Jefferson Laboratory, MIT-Bates, LEGS, Mainz, Bonn, GRAAL, and
Spring-8 offer new opportunities to understand in detail how nucleon resonance
() properties emerge from the nonperturbative aspects of QCD. Preliminary
data from CLAS collaboration, which cover a large range of photon virtuality
show interesting behavior with respect to dependence: in the region
, both the transverse amplitude, , and the
longitudinal amplitude, , decrease rapidly. In this work, we
attempt to use first-principles lattice QCD (for the first time) to provide a
model-independent study of the Roper-nucleon transition form factor.Comment: 4 pages, 2 figures, double colum
Eccentricity Evolution of Extrasolar Multiple Planetary Systems due to the Depletion of Nascent Protostellar Disks
Most extrasolar planets are observed to have eccentricities much larger than
those in the solar system. Some of these planets have sibling planets, with
comparable masses, orbiting around the same host stars. In these multiple
planetary systems, eccentricity is modulated by the planets' mutual secular
interaction as a consequence of angular momentum exchange between them. For
mature planets, the eigenfrequencies of this modulation are determined by their
mass and semi-major axis ratios. But, prior to the disk depletion, self gravity
of the planets' nascent disks dominates the precession eigenfrequencies. We
examine here the initial evolution of young planets' eccentricity due to the
apsidal libration or circulation induced by both the secular interaction
between them and the self gravity of their nascent disks. We show that as the
latter effect declines adiabatically with disk depletion, the modulation
amplitude of the planets' relative phase of periapse is approximately invariant
despite the time-asymmetrical exchange of angular momentum between planets.
However, as the young planets' orbits pass through a state of secular
resonance, their mean eccentricities undergo systematic quantitative changes.
For applications, we analyze the eccentricity evolution of planets around
Upsilon Andromedae and HD168443 during the epoch of protostellar disk
depletion. We find that the disk depletion can change the planets' eccentricity
ratio. However, the relatively large amplitude of the planets' eccentricity
cannot be excited if all the planets had small initial eccentricities.Comment: 50 pages including 11 figures, submitted to Ap
Geometric phases for neutral and charged particles in a time-dependent magnetic field
It is well known that any cyclic solution of a spin 1/2 neutral particle
moving in an arbitrary magnetic field has a nonadiabatic geometric phase
proportional to the solid angle subtended by the trace of the spin. For neutral
particles with higher spin, this is true for cyclic solutions with special
initial conditions. For more general cyclic solutions, however, this does not
hold. As an example, we consider the most general solutions of such particles
moving in a rotating magnetic field. If the parameters of the system are
appropriately chosen, all solutions are cyclic. The nonadiabatic geometric
phase and the solid angle are both calculated explicitly. It turns out that the
nonadiabatic geometric phase contains an extra term in addition to the one
proportional to the solid angle. The extra term vanishes automatically for spin
1/2. For higher spin, however, it depends on the initial condition. We also
consider the valence electron of an alkaline atom. For cyclic solutions with
special initial conditions in an arbitrary strong magnetic field, we prove that
the nonadiabatic geometric phase is a linear combination of the two solid
angles subtended by the traces of the orbit and spin angular momenta. For more
general cyclic solutions in a strong rotating magnetic field, the nonadiabatic
geometric phase also contains extra terms in addition to the linear
combination.Comment: revtex, 18 pages, no figur
Characteristics of Bose-Einstein condensation in an optical lattice
We discuss several possible experimental signatures of the Bose-Einstein
condensation (BEC) transition for an ultracold Bose gas in an inhomogeneous
optical lattice. Based on the commonly used time-of-flight imaging technique,
we show that the momentum-space density profile in the first Brillouin zone,
supplemented by the visibility of interference patterns, provides valuable
information about the system. In particular, by crossing the BEC transition
temperature, the appearance of a clear bimodal structure sets a qualitative and
universal signature of this phase transition. Furthermore, the momentum
distribution can also be applied to extract the condensate fraction, which may
serve as a promising thermometer in such a system.Comment: 12 pages, 13 figures; Revised version with new figures; Phys. Rev. A
77, 043626 (2008
- …