6,745 research outputs found
Ultracold, radiative charge transfer in hybrid Yb ion - Rb atom traps
Ultracold hybrid ion-atom traps offer the possibility of microscopic
manipulation of quantum coherences in the gas using the ion as a probe.
However, inelastic processes, particularly charge transfer can be a significant
process of ion loss and has been measured experimentally for the Yb ion
immersed in a Rb vapour. We use first-principles quantum chemistry codes to
obtain the potential energy curves and dipole moments for the lowest-lying
energy states of this complex. Calculations for the radiative decay processes
cross sections and rate coefficients are presented for the total decay
processes. Comparing the semi-classical Langevin approximation with the quantum
approach, we find it provides a very good estimate of the background at higher
energies. The results demonstrate that radiative decay mechanisms are important
over the energy and temperature region considered. In fact, the Langevin
process of ion-atom collisions dominates cold ion-atom collisions. For spin
dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole
interaction and the second-order spin-orbit coupling can play important roles,
inducing couplingbetween the spin and the orbital motion. They measured the
spin-relaxing collision rate to be approximately 5 orders of magnitude higher
than the charge-exchange collision rate \cite{kohl13}. Regarding the measured
radiative charge transfer collision rate, we find that our calculation is in
very good agreement with experiment and with previous calculations.
Nonetheless, we find no broad resonances features that might underly a strong
isotope effect. In conclusion, we find, in agreement with previous theory that
the isotope anomaly observed in experiment remains an open question.Comment: 7 figures, 1 table accepted for publication in J. Phys. B: At. Mol.
Opt. Phys. arXiv admin note: text overlap with arXiv:1107.114
Dynamics of bubbles in a two-component Bose-Einstein condensate
The dynamics of a phase-separated two-component Bose-Einstein condensate are
investigated, in which a bubble of one component moves through the other
component. Numerical simulations of the Gross--Pitaevskii equation reveal a
variety of dynamics associated with the creation of quantized vortices. In two
dimensions, a circular bubble deforms into an ellipse and splits into fragments
with vortices, which undergo the Magnus effect. The B\'enard--von K\'arm\'an
vortex street is also generated. In three dimensions, a spherical bubble
deforms into toruses with vortex rings. When two rings are formed, they exhibit
leapfrogging dynamics.Comment: 6 pages, 7 figure
Capillary-Gravity Waves on Depth-Dependent Currents: Consequences for the Wave Resistance
We study theoretically the capillary-gravity waves created at the water-air
interface by a small two-dimensional perturbation when a depth-dependent
current is initially present in the fluid. Assuming linear wave theory, we
derive a general expression of the wave resistance experienced by the
perturbation as a function of the current profile in the case of an inviscid
fluid. We then analyze and discuss in details the behavior of the wave
resistance in the particular case of a linear current, a valid approximation
for some wind generated currents.Comment: Submitted to EP
Two-Pulse Propagation in a Partially Phase-Coherent Medium
We analyze the effects of partial coherence of ground state preparation on
two-pulse propagation in a three-level medium, in contrast to
previous treastments that have considered the cases of media whose ground
states are characterized by probabilities (level populations) or by probability
amplitudes (coherent pure states). We present analytic solutions of the
Maxwell-Bloch equations, and we extend our analysis with numerical solutions to
the same equations. We interpret these solutions in the bright/dark dressed
state basis, and show that they describe a population transfer between the
bright and dark state. For mixed-state media with partial ground
state phase coherence the dark state can never be fully populated. This has
implications for phase-coherent effects such as pulse matching, coherent
population trapping, and electromagnetically induced transparency (EIT). We
show that for partially phase-coherent three-level media, self induced
transparency (SIT) dominates EIT and our results suggest a corresponding
three-level area theorem.Comment: 29 pages, 12 figures. Submitted to Phys. Rev.
Massive particles in acoustic space-times emergent inertia and passive gravity
I show that massive-particle dynamics can be simulated by a weak, spherical,
external perturbation on a potential flow in an ideal fluid. The effective
Lagrangian is of the form mc^2L(U^2/c^2), where U is the velocity of the
particle relative to the fluid and c the speed of sound. This can serve as a
model for emergent relativistic inertia a la Mach's principle with m playing
the role of inertial mass, and also of analog gravity where it is also the
passive gravitational mass. m depends on the particle type and intrinsic
structure, while L is universal: For D dimensional particles L is proportional
to the hypergeometric function F(1,1/2;D/2;U^2/c^2). Particles fall in the same
way in the analog gravitational field independent of their internal structure,
thus satisfying the weak equivalence principle. For D less or equal 5 they all
have a relativistic limit with the acquired energy and momentum diverging as U
approaches c. For D less or equal 7 the null geodesics of the standard acoustic
metric solve our equation of motion. Interestingly, for D=4 the dynamics is
very nearly Lorentzian. The particles can be said to follow the geodesics of a
generalized acoustic metric of a Finslerian type that shares the null geodesics
with the standard acoustic metric. In vortex geometries, the ergosphere is
automatically the static limit. As in the real world, in ``black hole''
geometries circular orbits do not exist below a certain radius that occurs
outside the horizon. There is a natural definition of antiparticles; and I
describe a mock particle vacuum in whose context one can discuss, e.g.,
particle Hawking radiation near event horizons.Comment: 15 page: version published in Physical Review
Capillary-gravity waves: The effect of viscosity on the wave resistance
The effect of viscosity on the wave resistance experienced by a 2d
perturbation moving at uniform velocity over the free surface of a fluid is
investigated. The analysis is based on Rayleigh's linearized theory of
capillary-gravity waves. It is shown in particular that the wave resistance
remains bounded as the velocity of the perturbation approches the minimun phase
speed, unlike what is predicted by the inviscid theory.Comment: Europhysics Letters, in pres
Solar Magnetic Tracking. I. Software Comparison and Recommended Practices
Feature tracking and recognition are increasingly common tools for data
analysis, but are typically implemented on an ad-hoc basis by individual
research groups, limiting the usefulness of derived results when selection
effects and algorithmic differences are not controlled. Specific results that
are affected include the solar magnetic turnover time, the distributions of
sizes, strengths, and lifetimes of magnetic features, and the physics of both
small scale flux emergence and the small-scale dynamo. In this paper, we
present the results of a detailed comparison between four tracking codes
applied to a single set of data from SOHO/MDI, describe the interplay between
desired tracking behavior and parameterization of tracking algorithms, and make
recommendations for feature selection and tracking practice in future work.Comment: In press for Astrophys. J. 200
On some geometric features of the Kramer interior solution for a rotating perfect fluid
Geometric features (including convexity properties) of an exact interior
gravitational field due to a self-gravitating axisymmetric body of perfect
fluid in stationary, rigid rotation are studied. In spite of the seemingly
non-Newtonian features of the bounding surface for some rotation rates, we
show, by means of a detailed analysis of the three-dimensional spatial
geodesics, that the standard Newtonian convexity properties do hold. A central
role is played by a family of geodesics that are introduced here, and provide a
generalization of the Newtonian straight lines parallel to the axis of
rotation.Comment: LaTeX, 15 pages with 4 Poscript figures. To be published in Classical
and Quantum Gravit
Suppressing the Rayleigh-Taylor instability with a rotating magnetic field
The Rayleigh-Taylor instability of a magnetic fluid superimposed on a
non-magnetic liquid of lower density may be suppressed with the help of a
spatially homogeneous magnetic field rotating in the plane of the undisturbed
interface. Starting from the complete set of Navier-Stokes equations for both
liquids a Floquet analysis is performed which consistently takes into account
the viscosities of the fluids. Using experimentally relevant values of the
parameters we suggest to use this stabilization mechanism to provide controlled
initial conditions for an experimental investigation of the Rayleigh-Taylor
instability
Self-gravitating astrophysical mass with singular central density vibrating in fundamental mode
The fluid-dynamical model of a self-gravitating mass of viscous liquid with
singular density at the center vibrating in fundamental mode is considered in
juxtaposition with that for Kelvin fundamental mode in a homogeneous heavy mass
of incompressible inviscid liquid. Particular attention is given to the
difference between spectral formulae for the frequency and lifetime of -mode
in the singular and homogeneous models. The newly obtained results are
discussed in the context of theoretical asteroseismology of pre-white dwarf
stage of red giants and stellar cocoons -- spherical gas-dust clouds with dense
star-forming core at the center.Comment: Mod. Phys. Lett. A, Vol. 24, No. 40 (2009) pp. 3257-327
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