92 research outputs found
Measuring the Superfluid Fraction of an Ultracold Atomic Gas
We propose a method to measure the superfluid fraction of an atomic gas. The
method involves the use of a vector potential generated by optical beams with
non-zero angular momentum to simulate uniform rotation. The induced change in
angular momentum of the atomic gas can be measured spectroscopically. This
allows a direct determination of the superfluid fraction.Comment: 4 pages, 2 figure
Structural change of vortex patterns in anisotropic Bose-Einstein condensates
We study the changes in the spatial distribution of vortices in a rotating
Bose-Einstein condensate due to an increasing anisotropy of the trapping
potential. Once the rotational symmetry is broken, we find that the vortex
system undergoes a rich variety of structural changes, including the formation
of zig-zag and linear configurations. These spatial re-arrangements are well
signaled by the change in the behavior of the vortex-pattern eigenmodes against
the anisotropy parameter. The existence of such structural changes opens up
possibilities for the coherent exploitation of effective many-body systems
based on vortex patterns.Comment: 5 pages, 4 figure
Static spectroscopy of a dense superfluid
Dense Bose superfluids, as HeII, differ from dilute ones by the existence of
a roton minimum in their excitation spectrum. It is known that this roton
minimum is qualitatively responsible for density oscillations close to any
singularity, such as vortex cores, or close to solid boundaries. We show that
the period of these oscillations, and their exponential decrease with the
distance to the singularity, are fully determined by the position and the width
of the roton minimum. Only an overall amplitude factor and a phase shift are
shown to depend on the details of the interaction potential. Reciprocally, it
allows for determining the characteristics of this roton minimum from static
"observations" of a disturbed ground state, in cases where the dynamics is not
easily accessible. We focus on the vortex example. Our analysis further shows
why the energy of these oscillations is negligible compared to the kinetic
energy, which limits their influence on the vortex dynamics, except for high
curvatures.Comment: 14 pages, 4 figures, extended version, published in J. Low Temp. Phy
Vibrations of a Columnar Vortex in a Trapped Bose-Einstein Condensate
We derive a governing equation for a Kelvin wave supported on a vortex line
in a Bose-Einstein condensate, in a rotating cylindrically symmetric parabolic
trap. From this solution the Kelvin wave dispersion relation is determined. In
the limit of an oblate trap and in the absence of longitudinal trapping our
results are consistent with previous work. We show that the derived Kelvin wave
dispersion in the general case is in quantitative agreement with numerical
calculations of the Bogoliubov spectrum and offer a significant improvement
upon previous analytical work.Comment: 5 pages with 1 figur
On the origin of the decrease in the torsional oscillator period of solid He4
A decrease in the rotational period observed in torsional oscillator
measurements was recently taken as a possible indication of a supersolid state
of helium. We reexamine this interpretation and note that the decrease in the
rotation period is also consistent with a solidification of a small liquid-like
component into a low-temperature glass. Such a solidification may occur by a
low-temperature quench of topological defects (e.g., grain boundaries or
dislocations) which we examined in an earlier work. The low-temperature glass
can account for not only a monotonic decrease in the rotation period as the
temperature is lowered but also explains the peak in the dissipation occurring
near the transition point. Unlike the non-classical rotational inertia
scenario, which depends on the supersolid fraction, the dependence of the
rotational period on external parameters, e.g., the oscillator velocity,
provides an alternate interpretation of the oscillator experiments. Future
experiments might explore this effect.Comment: 10 pages, 3 figures; to appear in Phys. Rev.
Changes of the topological charge of vortices
We consider changes of the topological charge of vortices in quantum
mechanics by investigating analytical examples where the creation or
annihilation of vortices occurs. In classical hydrodynamics of non-viscous
fluids the Helmholtz-Kelvin theorem ensures that the velocity field circulation
is conserved. We discuss applicability of the theorem in the hydrodynamical
formulation of quantum mechanics showing that the assumptions of the theorem
may be broken in quantum evolution of the wavefunction leading to a change of
the topological charge.Comment: 5 pages, 2 figures, version accepted for publication in J. Phys.
Experiments on the twisted vortex state in superfluid 3He-B
We have performed measurements and numerical simulations on a bundle of
vortex lines which is expanding along a rotating column of initially
vortex-free 3He-B. Expanding vortices form a propagating front: Within the
front the superfluid is involved in rotation and behind the front the twisted
vortex state forms, which eventually relaxes to the equilibrium vortex state.
We have measured the magnitude of the twist and its relaxation rate as function
of temperature above 0.3Tc. We also demonstrate that the integrity of the
propagating vortex front results from axial superfluid flow, induced by the
twist.Comment: prepared for proceedings of the QFS2007 symposium in Kaza
The glassy response of solid He-4 to torsional oscillations
We calculated the glassy response of solid He-4 to torsional oscillations
assuming a phenomenological glass model. Making only a few assumptions about
the distribution of glassy relaxation times in a small subsystem of otherwise
rigid solid He-4, we can account for the magnitude of the observed period shift
and concomitant dissipation peak in several torsion oscillator experiments. The
implications of the glass model for solid He-4 are threefold: (1) The dynamics
of solid He-4 is governed by glassy relaxation processes. (2) The distribution
of relaxation times varies significantly between different torsion oscillator
experiments. (3) The mechanical response of a torsion oscillator does not
require a supersolid component to account for the observed anomaly at low
temperatures, though we cannot rule out its existence.Comment: 9 pages, 4 figures, presented at QFS200
Path integral Monte Carlo simulation of charged particles in traps
This chapter is devoted to the computation of equilibrium (thermodynamic)
properties of quantum systems. In particular, we will be interested in the
situation where the interaction between particles is so strong that it cannot
be treated as a small perturbation. For weakly coupled systems many efficient
theoretical and computational techniques do exist. However, for strongly
interacting systems such as nonideal gases or plasmas, strongly correlated
electrons and so on, perturbation methods fail and alternative approaches are
needed. Among them, an extremely successful one is the Monte Carlo (MC) method
which we are going to consider in this chapter.Comment: 18 pages, based on talks on Hareaus school on computational methods,
Greifswald, September 200
NMR Experiments on Rotating Superfluid 3He-A : Evidence for Vorticity
Experiments on rotating superfluid 3He-A in an open cylindrical geometry show a change in the NMR line shape as a result of rotation: The amplitude of the peak decreases in proportion to f(T)g(Ω), where Ω is the angular velocity of rotation; at the same time the line broadens. Near Tc, f(T) is a linear function of 1−T/Tc. At small velocities g(Ω)∝Ω. These observations are consistent with the existence of vortices in rotating 3He-A.Peer reviewe
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