21,334 research outputs found
Spatially resolved photo ionization of ultracold atoms on an atom chip
We report on photo ionization of ultracold magnetically trapped Rb atoms on
an atom chip. The atoms are trapped at 5 K in a strongly anisotropic
trap. Through a hole in the chip with a diameter of 150 m two laser beams
are focussed onto a fraction of the atomic cloud. A first laser beam with a
wavelength of 778 nm excites the atoms via a two photon transition to the 5D
level. With a fiber laser at 1080 nm the excited atoms are photo ionized.
Ionization leads to depletion of the atomic density distribution observed by
absorption imaging. The resonant ionization spectrum is reported. The setup
used in this experiment is not only suitable to investigate BEC ion mixtures
but also single atom detection on an atom chip
The Frequency Dependence of Critical-velocity Behavior in Oscillatory Flow of Superfluid Helium-4 Through a 2-micrometer by 2-micrometer Aperture in a Thin Foil
The critical-velocity behavior of oscillatory superfluid Helium-4 flow
through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil
has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up
to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during
which the frequency remained below 400 Hz, the critical velocity was a
nearly-linearly decreasing function of increasing temperature throughout the
region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi
phase slips could be observed at the onset of dissipation. In runs with
frequencies higher than 400 Hz, downward curvature was observed in the decrease
of critical velocity with increasing temperature. In addition, above 500 Hz an
alteration in supercritical behavior was seen at the lower temperatures,
involving the appearance of large energy-loss events. These irregular events
typically lasted a few tens of half-cycles of oscillation and could involve
hundreds of times more energy loss than would have occurred in a single
complete 2 Pi phase slip at maximum flow. The temperatures at which this
altered behavior was observed rose with frequency, from ~ 0.6 K and below, at
500 Hz, to ~ 1.0 K and below, at 1880 Hz.Comment: 35 pages, 13 figures, prequel to cond-mat/050203
Combined chips for atom-optics
We present experiments with Bose-Einstein condensates on a combined atom
chip. The combined structure consists of a large-scale "carrier chip" and
smaller "atom-optics chips", containing micron-sized elements. This allows us
to work with condensates very close to chip surfaces without suffering from
fragmentation or losses due to thermally driven spin flips. Precise
three-dimensional positioning and transport with constant trap frequencies are
described. Bose-Einstein condensates were manipulated with submicron accuracy
above atom-optics chips. As an application of atom chips, a direction sensitive
magnetic field microscope is demonstrated.Comment: 9 pages, 9 figure
Coevolution of dynamical states and interactions in dynamic networks
We explore the coupled dynamics of the internal states of a set of
interacting elements and the network of interactions among them. Interactions
are modeled by a spatial game and the network of interaction links evolves
adapting to the outcome of the game. As an example we consider a model of
cooperation, where the adaptation is shown to facilitate the formation of a
hierarchical interaction network that sustains a highly cooperative stationary
state. The resulting network has the characteristics of a small world network
when a mechanism of local neighbor selection is introduced in the adaptive
network dynamics. The highly connected nodes in the hierarchical structure of
the network play a leading role in the stability of the network. Perturbations
acting on the state of these special nodes trigger global avalanches leading to
complete network reorganization.Comment: 4 pages, 5 figures, for related material visit
http:www.imedea.uib.es/physdept
Stripe-hexagon competition in forced pattern forming systems with broken up-down symmetry
We investigate the response of two-dimensional pattern forming systems with a
broken up-down symmetry, such as chemical reactions, to spatially resonant
forcing and propose related experiments. The nonlinear behavior immediately
above threshold is analyzed in terms of amplitude equations suggested for a
and ratio between the wavelength of the spatial periodic forcing
and the wavelength of the pattern of the respective system. Both sets of
coupled amplitude equations are derived by a perturbative method from the
Lengyel-Epstein model describing a chemical reaction showing Turing patterns,
which gives us the opportunity to relate the generic response scenarios to a
specific pattern forming system. The nonlinear competition between stripe
patterns and distorted hexagons is explored and their range of existence,
stability and coexistence is determined. Whereas without modulations hexagonal
patterns are always preferred near onset of pattern formation, single mode
solutions (stripes) are favored close to threshold for modulation amplitudes
beyond some critical value. Hence distorted hexagons only occur in a finite
range of the control parameter and their interval of existence shrinks to zero
with increasing values of the modulation amplitude. Furthermore depending on
the modulation amplitude the transition between stripes and distorted hexagons
is either sub- or supercritical.Comment: 10 pages, 12 figures, submitted to Physical Review
Direct measurement of shear-induced cross-correlations of Brownian motion
Shear-induced cross-correlations of particle fluctuations perpendicular and
along stream-lines are investigated experimentally and theoretically. Direct
measurements of the Brownian motion of micron-sized beads, held by optical
tweezers in a shear-flow cell, show a strong time-asymmetry in the
cross-correlation, which is caused by the non-normal amplification of
fluctuations. Complementary measurements on the single particle probability
distribution substantiate this behavior and both results are consistent with a
Langevin model. In addition, a shear-induced anti-correlation between
orthogonal random-displacements of two trapped and hydrodynamically interacting
particles is detected, having one or two extrema in time, depending on the
positions of the particles.Comment: 4 pages, 4 figure
Vibrational States of Glassy and Crystalline Orthotherphenyl
Low-frequency vibrations of glassy and crystalline orthoterphenyl are studied
by means of neutron scattering. Phonon dispersions are measured along the main
axes of a single crystal, and the corresponding longitudinal and transversal
sound velocities are obtained. For glassy and polycrystalline samples, a
density of vibrational states is determined and cross-checked against other
dynamic observables. In the crystal, low-lying zone-boundary modes lead to an
excess over the Debye density of states. In the glass, the boson peak is
located at even lower frequencies. With increasing temperature, both glass and
crystal show anharmonicity.Comment: 7 pages of LaTeX (svjour), 2 tables, 10 figures accepted in Eur.
Phys. J.
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