17,792 research outputs found
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
Microelectromagnets for Trapping and Manipulating Ultracold Atomic Quantum Gases
We describe the production and characterization of microelectromagnets made
for trapping and manipulating atomic ensembles. The devices consist of 7
fabricated parallel copper conductors 3 micrometer thick, 25mm long, with
widths ranging from 3 to 30 micrometer, and are produced by electroplating a
sapphire substrate. Maximum current densities in the wires up to 6.5 * 10^6 A /
cm^2 are achieved in continuous mode operation. The device operates
successfully at a base pressure of 10^-11 mbar. The microstructures permit the
realization of a variety of magnetic field configurations, and hence provide
enormous flexibility for controlling the motion and the shape of Bose-Einstein
condensates.Comment: 4 pages, 3 figure
A 492 GHz cooled Schottky receiver for radio-astronomy
We developed a 492 GHz cooled GaAs Schottky receiver driven by a solid state local oscillator with a DSB noise temperature of 550 K measured at the telescope. The receiver-bandwidth is approx. equal to 1.0 GHz. Quasi-optical mirrors focus the sky and local oscillator radiation into the mixer. Stability analysis via the Allan variance method shows that the total system including a 1 GHz bandwidth acousto-optical spectrometer built in Cologne allows integration times up to 100 sec per half switching cycle. We successfully used the receiver at the KOSMA 3 m telescope on Gornergrat (3150m) located in the central Swiss Alps near Zermatt during January-February 1992 for observations of the 492 GHz, (CI) (3)P1 to (3)P0 fine structure line in several galactic sources. These observations confirm that Gornergrat is an excellent winter submillimeter site in accordance with previous predictions based on the atmospheric opacity from KOSMA 345 GHz measurements
Enhanced charge stripe order of superconducting La(2-x)Ba(x)CuO(4) in a magnetic field
The effect of a magnetic field on the charge stripe order in
La(2-x)Ba(x)CuO(4) has been studied by means of high energy (100 keV) x-ray
diffraction for charge carrier concentrations ranging from strongly underdoped
to optimally doped. We find that charge stripe order can be significantly
enhanced by a magnetic field applied along the c-axis, but only at temperatures
and dopings where it coexists with bulk superconductivity at zero field. The
field also increases stripe correlations between the planes, which can result
in an enhanced frustration of the interlayer Josephson coupling. Close to the
famous x=1/8 compound, where zero field stripe order is pronounced and bulk
superconductivity is suppressed, charge stripe order is independent of a
magnetic field. The results imply that static stripe order and
three-dimensionally coherent superconductivity are competing ground states.Comment: 6 pages, 4 figure
Particles held by springs in a linear shear flow exhibit oscillatory motion
The dynamics of small spheres, which are held by linear springs in a low
Reynolds number shear flow at neighboring locations is investigated. The flow
elongates the beads and the interplay of the shear gradient with the nonlinear
behavior of the hydrodynamic interaction among the spheres causes in a large
range of parameters a bifurcation to a surprising oscillatory bead motion. The
parameter ranges, wherein this bifurcation is either super- or subcritical, are
determined.Comment: 4 pages, 5 figure
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
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