1,027 research outputs found
Numerical Study of Velocity Statistics in Steady Counterflow Quantum Turbulence
We investigate the velocity statistics by calculating the Biot--Savart
velocity induced by vortex filaments in steady counterflow turbulence
investigated in a previous study [Phys. Rev. B {\bf 81}, 104511 (2010)]. The
probability density function (PDF) obeys a Gaussian distribution in the
low-velocity region and a power-law distribution in the high-velocity
region. This transition between the two distributions occur at the velocity
characterized by the mean inter-vortex distance. Counterflow turbulence causes
anisotropy of the vortex tangle, which leads to a difference in the PDF for the
velocities perpendicular to and parallel to the counterflow.Comment: 4 pages, 7 figure
Self-Organization of Vortex Length Distribution in Quantum Turbulence: An Approach from the Barabasi-Albert Model
The energy spectrum of quantum turbulence obeys Kolmogorov's law. The vortex
length distribution (VLD), meaning the size distribution of the vortices, in
Kolmogorov quantum turbulence also obeys a power law. We propose here an
innovative idea to study the origin of the power law of the VLD. The nature of
quantized vortices allows one to describe the decay of quantum turbulence with
a simple model that is similar to the Barabasi-Albert model of large networks.
We show here that such a model can reproduce the power law of the VLD well.Comment: 4 pages including 5 figure
Phase Separation of a Fast Rotating Boson-Fermion Mixture in the Lowest-Landau-Level Regime
By minimizing the coupled mean-field energy functionals, we investigate the
ground-state properties of a rotating atomic boson-fermion mixture in a
two-dimensional parabolic trap. At high angular frequencies in the
mean-field-lowest-Landau-level regime, quantized vortices enter the bosonic
condensate, and a finite number of degenerate fermions form the
maximum-density-droplet state. As the boson-fermion coupling constant
increases, the maximum density droplet develops into a lower-density state
associated with the phase separation, revealing characteristics of a
Landau-level structure
Ferromagnetic Resonance in Spinor Dipolar Bose--Einstein Condensates
We used the Gross--Pitaevskii equations to investigate ferromagnetic
resonance in spin-1 Bose--Einstein condensates with a magnetic dipole-dipole
interaction. By introducing the dipole interaction, we obtained equations
similar to the Kittel equations used to represent ferromagnetic resonance in
condensed matter physics. These equations indicated that the ferromagnetic
resonance originated from dipolar interaction, and that the resonance frequency
depended upon the shape of the condensate. Furthermore, spin currents driven by
spin diffusions are characteristic of this system.Comment: 8 pages, 10 figure
Dynamics of vortex tangle without mutual friction in superfluid He
A recent experiment has shown that a tangle of quantized vortices in
superfluid He decayed even at mK temperatures where the normal fluid was
negligible and no mutual friction worked. Motivated by this experiment, this
work studies numerically the dynamics of the vortex tangle without the mutual
friction, thus showing that a self-similar cascade process, whereby large
vortex loops break up to smaller ones, proceeds in the vortex tangle and is
closely related with its free decay. This cascade process which may be covered
with the mutual friction at higher temperatures is just the one at zero
temperature Feynman proposed long ago. The full Biot-Savart calculation is made
for dilute vortices, while the localized induction approximation is used for a
dense tangle. The former finds the elementary scenario: the reconnection of the
vortices excites vortex waves along them and makes them kinked, which could be
suppressed if the mutual friction worked. The kinked parts reconnect with the
vortex they belong to, dividing into small loops. The latter simulation under
the localized induction approximation shows that such cascade process actually
proceeds self-similarly in a dense tangle and continues to make small vortices.
Considering that the vortices of the interatomic size no longer keep the
picture of vortex, the cascade process leads to the decay of the vortex line
density. The presence of the cascade process is supported also by investigating
the classification of the reconnection type and the size distribution of
vortices. The decay of the vortex line density is consistent with the solution
of the Vinen's equation which was originally derived on the basis of the idea
of homogeneous turbulence with the cascade process. The obtained result is
compared with the recent Vinen's theory.Comment: 16 pages, 16 figures, submitted to PR
Spin textures in rotating two-component Bose-Einstein condensates
We investigate two kinds of coreless vortices with axisymmetric and
nonaxisymmetric configurations in rotating two-component Bose-Einstein
condensates. Starting from the Gross-Pitaevskii energy functional in a rotating
frame, we derive a nonlinear sigma model generalized to the two-component
condensates. In terms of a pseudospin representation, an axisymmetric vortex
and a nonaxisymmetric one correspond to spin textures referred to as a
"skyrmion" and a "meron-pair", respectively. A variational method is used to
investigate the dependence of the sizes of the stable spin textures on system
parameters, and the optimized variational function is found to reproduce well
the numerical solution. In the SU(2) symmetric case, the optimal skyrmion and
meron-pair are degenerate and transform to each other by a rotation of the
pseudospin. An external rf-field that couples coherently the hyperfine states
of two components breaks the degeneracy in favor of the meron-pair texture due
to an effective transverse pseudomagnetic field. The difference between the
intracomponent and intercomponent interactions yields a longitudinal
pseudomagnetic field and a ferromagnetic or an antiferromagnetic pseudospin
interaction, leading to a meron-pair texture with an anisotropic distribution
of vorticity.Comment: 14 pages, 15 figure
Instability of vortex array and transitions to turbulent states in rotating helium II
We consider superfluid helium inside a container which rotates at constant
angular velocity and investigate numerically the stability of the array of
quantized vortices in the presence of an imposed axial counterflow. This
problem was studied experimentally by Swanson {\it et al.}, who reported
evidence of instabilities at increasing axial flow but were not able to explain
their nature. We find that Kelvin waves on individual vortices become unstable
and grow in amplitude, until the amplitude of the waves becomes large enough
that vortex reconnections take place and the vortex array is destabilized. The
eventual nonlinear saturation of the instability consists of a turbulent tangle
of quantized vortices which is strongly polarized. The computed results compare
well with the experiments. Finally we suggest a theoretical explanation for the
second instability which was observed at higher values of the axial flow
Drag force on an oscillating object in quantum turbulence
This paper reports results of the computation of the drag force exerted on an
oscillating object in quantum turbulence in superfluid He. The drag force
is calculated on the basis of numerical simulations of quantum turbulent flow
about the object. The drag force is proportional to the square of the magnitude
of the oscillation velocity, which is similar to that in classical turbulence
at high Reynolds number. The drag coefficient is also calculated, and its value
is found to be of the same order as that observed in previous experiments. The
correspondence between quantum and classical turbulences is further clarified
by examining the turbulence created by oscillating objects.Comment: 7 pages, 5 figures, 1 tabl
- …