16,789 research outputs found
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
Multiple Reflections and Diffuse Scattering in Bragg Scattering at Optical Lattices
We study Bragg scattering at 1D atomic lattices. Cold atoms are confined by
optical dipole forces at the antinodes of a standing wave generated inside a
laser-driven cavity. The atoms arrange themselves into an array of lens-shaped
layers located at the antinodes of the standing wave. Light incident on this
array at a well-defined angle is partially Bragg-reflected. We measure
reflectivities as high as 30%. In contrast to a previous experiment devoted to
the thin grating limit [S. Slama, et al., Phys. Rev. Lett. 94, 193901 (2005)]
we now investigate the thick grating limit characterized by multiple
reflections of the light beam between the atomic layers. In principle multiple
reflections give rise to a photonic stop band, which manifests itself in the
Bragg diffraction spectra as asymmetries and minima due to destructive
interference between different reflection paths. We show that close to
resonance however disorder favors diffuse scattering, hinders coherent multiple
scattering and impedes the characteristic suppression of spontaneous emission
inside a photonic band gap
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
Dimensional Crossover in Bragg Scattering from an Optical Lattice
We study Bragg scattering at 1D optical lattices. Cold atoms are confined by
the optical dipole force at the antinodes of a standing wave generated inside a
laser-driven high-finesse cavity. The atoms arrange themselves into a chain of
pancake-shaped layers located at the antinodes of the standing wave. Laser
light incident on this chain is partially Bragg-reflected. We observe an
angular dependence of this Bragg reflection which is different to what is known
from crystalline solids. In solids the scattering layers can be taken to be
infinitely spread (3D limit). This is not generally true for an optical lattice
consistent of a 1D linear chain of point-like scattering sites. By an explicit
structure factor calculation we derive a generalized Bragg condition, which is
valid in the intermediate regime. This enables us to determine the aspect ratio
of the atomic lattice from the angular dependance of the Bragg scattered light.Comment: 4 pages, 5 figure
Diffraction of a Bose-Einstein condensate from a Magnetic Lattice on a Micro Chip
We experimentally study the diffraction of a Bose-Einstein condensate from a
magnetic lattice, realized by a set of 372 parallel gold conductors which are
micro fabricated on a silicon substrate. The conductors generate a periodic
potential for the atoms with a lattice constant of 4 microns. After exposing
the condensate to the lattice for several milliseconds we observe diffraction
up to 5th order by standard time of flight imaging techniques. The experimental
data can be quantitatively interpreted with a simple phase imprinting model.
The demonstrated diffraction grating offers promising perspectives for the
construction of an integrated atom interferometer.Comment: 4 pages, 4 figure
Oxygen superstructures throughout the phase diagram of
Short-range lattice superstructures have been studied with high-energy x-ray
diffuse scattering in underdoped, optimally doped, and overdoped . A new four-unit-cell superstructure was observed in
compounds with . Its temperature, doping, and material dependence
was used to attribute its origin to short-range oxygen vacancy ordering, rather
than electronic instabilities in the layers. No significant diffuse
scattering is observed in YBaCuO. The oxygen superstructures must
be taken into account when interpreting spectral anomalies in
Displacement Operator Formalism for Renormalization and Gauge Dependence to All Orders
We present a new method for determining the renormalization of Green
functions to all orders in perturbation theory, which we call the displacement
operator formalism, or the D-formalism, in short. This formalism exploits the
fact that the renormalized Green functions may be calculated by displacing by
an infinite amount the renormalized fields and parameters of the theory with
respect to the unrenormalized ones. With the help of this formalism, we are
able to obtain the precise form of the deformations induced to the Nielsen
identities after renormalization, and thus derive the exact dependence of the
renormalized Green functions on the renormalized gauge-fixing parameter to all
orders. As a particular non-trivial example, we calculate the gauge-dependence
of at two loops in the framework of an Abelian Higgs model, using a
gauge-fixing scheme that preserves the Higgs-boson low-energy theorem for
off-shell Green functions. Various possible applications and future directions
are briefly discussed.Comment: 41 pages, 8 figure
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
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MP3 - A meteorology and physical properties package to explore air-sea interaction on Titan
The exchange of mass, heat and momentum at the air:sea interface are profound influences on the terrestrial environment, affecting the intensity of hurricanes, the size of waves and lake-effect precipitation. Titan presents us with an opportunity to study these processes in a novel physical context, with a different sea, atmosphere and gravity. The MP3 instrument, under development for the proposed Discovery mission TiME (Titan Mare Explorer [1,2]) is an integrated suite of small, simple sensors that combines the function of traditional meteorology packages with liquid physical properties and depth-sounding : these latter functions follow the concept of - and indeed use spare elements from - the Huygens Surface Science Package (SSP,[3]). However, unlike Huygens’ brief and dynamic 3 hours of measurement, in TiME’s 6-Titan-day (96 Earth day) nominal mission enabled by radioisotope power, MP3 will have an unprecedented long-term measurement opportunity in one of the most evocative environments in the solar system, Titan’s sea Ligeia Mare
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