3,538 research outputs found
Rolling and sliding of a nanorod between two planes: Tribological regimes and control of friction
The motion of a cylindrical crystalline nanoparticle sandwiched between two
crystalline planes, one stationary and the other pulled at a constant velocity
and pressed down by a normal load, is considered theoretically using a planar
model. The results of our model calculations show that, depending on load and
velocity, the nanoparticle can be either rolling or sliding. At sufficiently
high normal loads, several sliding states characterized by different friction
forces can coexist, corresponding to different orientations of the
nanoparticle, and allowing one to have low or high friction at the same pulling
velocity and normal load.Comment: 5 figure
Rotating binary Bose-Einstein condensates and vortex clusters in quantum droplets
Quantum droplets may form out of a gaseous Bose-Einstein condensate,
stabilized by quantum fluctuations beyond mean field. We show that multiple
singly-quantized vortices may form in these droplets at moderate angular
momenta in two dimensions. Droplets carrying these precursors of an Abrikosov
lattice remain self-bound for certain timescales after switching off an initial
harmonic confinement. Furthermore, we examine how these vortex-carrying
droplets can be formed in a more pertubation-resistant setting, by starting
from a rotating binary Bose-Einstein condensate and inducing a metastable
persistent current via a non-monotonic trapping potential.Comment: 5 page, 4 figure
Phase diagram of a rapidly-rotating two-component Bose gas
We derive analytically the phase diagram of a two-component Bose gas confined
in an anharmonic potential, which becomes exact and universal in the limit of
weak interactions and small anharmonicity of the trapping potential. The
transitions between the different phases, which consist of vortex states of
single and multiple quantization, are all continuous because of the addition of
the second component.Comment: 5 pages, 3 figure
Rotational properties of non-dipolar and dipolar Bose-Einstein condensates confined in annular potentials
We investigate the rotational response of both non-dipolar and dipolar
Bose-Einstein condensates confined in an annular potential. For the non-dipolar
case we identify certain critical rotational frequencies associated with the
formation of vortices. For the dipolar case, assuming that the dipoles are
aligned along some arbitrary and tunable direction, we study the same problem
as a function of the orientation angle of the dipole moment of the atoms.Comment: 5 pages, 4 figure
Finite-size effects in the dynamics of few bosons in a ring potential
We study the temporal evolution of a small number of ultra-cold bosonic
atoms confined in a ring potential. Assuming that initially the system is in a
solitary-wave solution of the corresponding mean-field problem, we identify
significant differences in the time evolution of the density distribution of
the atoms when it instead is evaluated with the many-body Schr\"odinger
equation. Three characteristic timescales are derived: the first is the period
of rotation of the wave around the ring, the second is associated with a
"decay" of the density variation, and the third is associated with periodic
"collapses" and "revivals" of the density variations, with a factor of separating each of them. The last two timescales tend to infinity in the
appropriate limit of large , in agreement with the mean-field approximation.
These findings are based on the assumption of the initial state being a
mean-field state. We confirm this behavior by comparison to the exact solutions
for a few-body system stirred by an external potential. We find that the exact
solutions of the driven system exhibit similar dynamical features.Comment: To appear in Journal of Physics
Hexagons become second if symmetry is broken
Pattern formation on the free surface of a magnetic fluid subjected to a
magnetic field is investigated experimentally. By tilting the magnetic field
the symmetry can be broken in a controllable manner. When increasing the
amplitude of the tilted field, the flat surface gives way to liquid ridges. A
further increase results in a hysteretic transition to a pattern of stretched
hexagons. The instabilities are detected by means of a linear array of magnetic
hall sensors and compared with theoretical predictions.Comment: accepted for publication by Physical Review E/Rapid Communicatio
Resonant Activation Phenomenon for Non-Markovian Potential-Fluctuation Processes
We consider a generalization of the model by Doering and Gadoua to
non-Markovian potential-switching generated by arbitrary renewal processes. For
the Markovian switching process, we extend the original results by Doering and
Gadoua by giving a complete description of the absorption process. For all
non-Markovian processes having the first moment of the waiting time
distributions, we get qualitatively the same results as in the Markovian case.
However, for distributions without the first moment, the mean first passage
time curves do not exhibit the resonant activation minimum. We thus come to the
conjecture that the generic mechanism of the resonant activation fails for
fluctuating processes widely deviating from Markovian.Comment: RevTeX 4, 5 pages, 4 figures; considerably shortened version accepted
as a brief report to Phys. Rev.
Slice Stretching at the Event Horizon when Geodesically Slicing the Schwarzschild Spacetime with Excision
Slice-stretching effects are discussed as they arise at the event horizon
when geodesically slicing the extended Schwarzschild black-hole spacetime while
using singularity excision. In particular, for Novikov and isotropic spatial
coordinates the outward movement of the event horizon (``slice sucking'') and
the unbounded growth there of the radial metric component (``slice wrapping'')
are analyzed. For the overall slice stretching, very similar late time behavior
is found when comparing with maximal slicing. Thus, the intuitive argument that
attributes slice stretching to singularity avoidance is incorrect.Comment: 5 pages, 2 figures, published version including minor amendments
suggested by the refere
Spin-orbit-coupled Bose-Einstein-condensed atoms confined in annular potentials
A spin-orbit-coupled Bose-Einstein-condensed cloud of atoms confined in an
annular trapping potential shows a variety of phases that we investigate in the
present study. Starting with the non-interacting problem, the homogeneous phase
that is present in an untrapped system is replaced by a sinusoidal density
variation in the limit of a very narrow annulus. In the case of an untrapped
system there is another phase with a striped-like density distribution, and its
counterpart is also found in the limit of a very narrow annulus. As the width
of the annulus increases, this picture persists qualitatively. Depending on the
relative strength between the inter- and the intra-components, interactions
either favor the striped phase, or suppress it, in which case either a
homogeneous, or a sinusoidal-like phase appears. Interactions also give rise to
novel solutions with a nonzero circulation.Comment: Final, slightly revised versio
Fluid pumped by magnetic stress
A magnetic field rotating on the free surface of a ferrofluid layer is shown
to induce considerable fluid motion toward the direction the field is rolling.
The measured flow velocity i) increases with the square of the magnetic field
amplitude, ii) is proportional to the thickness of the fluid layer, and iii)
has a maximum at a driving frequency of about 3 kHz. The pumping speed can be
estimated with a two-dimensional flow model.Comment: 3 pages, 4 figure
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