56 research outputs found
Gyroscopic motion of superfluid trapped atomic condensates
The gyroscopic motion of a trapped Bose gas containing a vortex is studied.
We model the system as a classical top, as a superposition of coherent
hydrodynamic states, by solution of the Bogoliubov equations, and by
integration of the time-dependent Gross-Pitaevskii equation. The frequency
spectrum of Bogoliubov excitations, including quantum frequency shifts, is
calculated and the quantal precession frequency is found to be consistent with
experimental results, though a small discrepancy exists. The superfluid
precession is found to be well described by the classical and hydrodynamic
models. However the frequency shifts and helical oscillations associated with
vortex bending and twisting require a quantal treatment. In gyroscopic
precession, the vortex excitation modes are the dominant features
giving a vortex kink or bend, while the is found to be the dominant
Kelvin wave associated with vortex twisting.Comment: 18 pages, 7 figures, 1 tabl
Consequence of superfluidity on the expansion of a rotating Bose-Einstein condensate
We study the time evolution of a rotating condensate, that expands after
being suddenly released from the confining trap, by solving the hydrodynamic
equations of irrotational superfluids. For slow initial rotation speeds,
, we find that the condensate's angular velocity increases rapidly
to a maximum value and this is accompanied by a minimum in the deformation of
the condensate in the rotating plane. During the expansion the sample makes a
global rotation of approximately , where the exact value depends on
. This minimum deformation can serve as an easily detectable
signature of superfluidity in a Bose--Einstein condensate.Comment: 4 pages, 3 figures, submitted to PR
Matter wave solitons at finite temperatures
We consider the dynamics of a dark soliton in an elongated harmonically
trapped Bose-Einstein condensate. A central question concerns the behavior at
finite temperatures, where dissipation arises due to the presence of a thermal
cloud. We study this problem using coupled Gross-Pitaevskii and -body
simulations, which include the mean field coupling between the condensate and
thermal cloud. We find that the soliton decays relatively quickly even at very
low temperatures, with the decay rate increasing with rising temperature.Comment: 6 pages, 2 figures, submitted to the Proceedings of QFS '0
Moment of Inertia and Quadrupole Response Function of a Trapped Superfluid
We derive an explicit relationship between the moment of inertia and the
quadrupole response function of an interacting gas confined in a harmonic trap.
The relationship holds for both Bose and Fermi systems and is well suited to
reveal the effects of irrotationality of the superfluid motion. Recent
experimental results on the scissors mode are used to extract the value of the
moment of inertia of a trapped Bose gas and to point out the deviations from
the rigid value due to superfluidity.Comment: 6 page
Pinning of quantized vortices in helium drop by dopant atoms and molecules
Using a density functional method, we investigate the properties of liquid
4He droplets doped with atoms (Ne and Xe) and molecules (SF_6 and HCN). We
consider the case of droplets having a quantized vortex pinned to the dopant. A
liquid drop formula is proposed that accurately describes the total energy of
the complex and allows one to extrapolate the density functional results to
large N. For a given impurity, we find that the formation of a
dopant+vortex+4He_N complex is energetically favored below a critical size
N_cr. Our result support the possibility to observe quantized vortices in
helium droplets by means of spectroscopic techniques.Comment: Typeset using Revtex, 3 pages and 5 figures (4 Postscript, 1 jpeg
Superfluid and Dissipative Dynamics of a Bose-Einstein Condensate in a Periodic Optical Potential
We create Bose-Einstein condensates of 87-rubidium in a static magnetic trap
with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic
trap center we are able to control the condensate evolution. We observe a
change in the frequency of the center-of-mass oscillation in the harmonic
trapping potential, in analogy with an increase in effective mass. For fluid
velocities greater than a local speed of sound, we observe the onset of
dissipative processes up to full removal of the superfluid component. A
parallel simulation study visualizes the dynamics of the BEC and accounts for
the main features of the observed behavior.Comment: 4 pages, including figure
Vortex stabilization in a small rotating asymmetric Bose-Einstein condensate
We use a variational method to investigate the ground-state phase diagram of
a small, asymmetric Bose-Einstein condensate with respect to the dimensionless
interparticle interaction strength and the applied external rotation
speed . For a given , the transition lines between no-vortex
and vortex states are shifted toward higher relative to those for the
symmetric case. We also find a re-entrant behavior, where the number of vortex
cores can decrease for large . In addition, stabilizing a vortex in a
rotating asymmetric trap requires a minimum interaction strength. For a given
asymmetry, the evolution of the variational parameters with increasing
shows two different types of transitions (sharp or continuous), depending on
the strength of the interaction. We also investigate transitions to states with
higher vorticity; the corresponding angular momentum increases continuously as
a function of
Graphite flake self-retraction response based on potential seeking
The high elastic modulus and interlayer strengths of graphite flakes make them a durable solid superlubricant. Apart from this, they have configurable electrical properties, exhibit quantum Hall effects, and possess a myriad of useful photonic properties. The self-retraction behavior of graphite flakes can have significant impact on the creation of ordered stacks for various applications because any accidental or intentional displacement of the top flake over the stacks below may result in a misalignment of the carbon-carbon atomic arrangement which, in turn, can have influence over the electrical and photonic properties. It has also been revealed that there was a tendency of the displaced microflake to fail at times to return to its original starting position and orientation. Here, we elucidate this behavior by considering the influence of the interlayer potential forces based on minimal potential energy seeking. The maps of the parameters interrogated here provide the ability for precautions to be undertaken. They also potentially permit the creation of an array of microflake stacks in which the metastable states permit different information to be encoded by virtue of the differentiated photonic or electrical characteristics readable from each array site
Spectroscopy of Dark Soliton States in Bose-Einstein Condensates
Experimental and numerical studies of the velocity field of dark solitons in
Bose-Einstein condensates are presented. The formation process after phase
imprinting as well as the propagation of the emerging soliton are investigated
using spatially resolved Bragg-spectroscopy of soliton states in Bose-Einstein
condensates of Rubidium87. A comparison of experimental data to results from
numerical simulations of the Gross-Pitaevskii equation clearly identifies the
flux underlying a dark soliton propagating in a Bose-Einstein condensate. The
results allow further optimization of the phase imprinting method for creating
collective exitations of Bose-Einstein condensates.Comment: 14 pages, 9 figure
Liquid-Phase Exfoliated Indium–Selenide Flakes and Their Application in Hydrogen Evolution Reaction
Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of β-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g L−1. Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. It is demonstrated that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few micrometers, and thicknesses ranging from 1 to 20 nm, with a maximum population centered at ≈5 nm, corresponding to 4 Se–In–In–Se quaternary layers. It is also shown that no formation of further InSe-based compounds (such as In2Se3) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for the hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. The dependence of the InSe flakes' morphologies, i.e., surface area and thickness, on the HER performances is highlighted, achieving the best efficiencies with small flakes offering predominant edge effects. The theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
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