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

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

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

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.

Supersolid behavior in confined geometry

We have carried out torsional oscillator (TO) and heat capacity (HC) measurements on solid 4He samples grown within a geometry which restricts the helium to thin (150 um) cylindrical discs. In contrast to previously reported values from Rittner and Reppy of 20% non-classical rotational inertia (NCRI) for similar confining dimensions, 0.9% NCRI (consistent with that found in bulk samples and samples imbedded in porous media) was observed in our TO cell. In this confined geometry the heat capacity peak is consistent with that found in bulk solid samples of high crystalline quality

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