20,142 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
Dephasing in Quantum Dots: Quadratic Coupling to Acoustic Phonons
A microscopic theory of optical transitions in quantum dots with
carrier-phonon interaction is developed. Virtual transitions into higher
confined states with acoustic phonon assistance add a quadratic phonon coupling
to the standard linear one, thus extending the independent Boson model. Summing
infinitely many diagrams in the cumulant, a numerically exact solution for the
interband polarization is found. Its full time dependence and the absorption
lineshape of the quantum dot are calculated. It is the quadratic interaction
which gives rise to a temperature-dependent broadening of the zero-phonon line,
being here calculated for the first time in a consistent scheme.Comment: 4 pages, 2 figure
High-temperature ferroelectric order and magnetic field-cooled effect driven magnetoelectric coupling in R2BaCuO5 (R= Er, Dy, Sm)
The high-temperature ferroelectric order and a remarkable magnetoelectric
effect driven by the magnetic field cooling are reported in R2BaCuO5 (R = Er,
Dy, Sm) series. The ferroelectric (FE) orders are observed at much higher
temperatures than their magnetic orders for all three members. The value of FE
Curie temperature (TFE) is considerably high as ~ 235 K with the polarization
value (P) of ~ 1410 {\mu}C/m2 for a 4 kV/cm poling field in case of Er2BaCuO5,
whereas the values of TFE and P are also promising as ~ 232 K and ~ 992
{\mu}C/m2 for Dy2BaCuO5, and ~ 184 K and ~ 980 {\mu}C/m2 for Sm2BaCuO5. The
synchrotron diffraction studies of Dy2BaCuO5 confirm a structural transition at
TFE to a polar Pna21 structure, which correlates the FE order. An unusual
magnetoelectric coupling is observed below the R order for Er and Dy compounds
and below the Cu order for Sm compound, when the pyroelectric current is
recorded only with the magnetic field both in heating and cooling cycles i.e.
typical magnetic field cooled effect. The magnetic field cooled effect driven
emergence of polarization is ferroelectric in nature, as it reverses due to the
opposite poling field. The unexplored R2BaCuO5 series attracts the community
for large TFE, high P value, and strange magnetoelectric consequences.Comment: 9 figures and 2 supporting figure
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
Freezing of parallel hard cubes with rounded edges
The freezing transition in a classical three-dimensional system of parallel
hard cubes with rounded edges is studied by computer simulation and
fundamental-measure density functional theory. By switching the rounding
parameter s from zero to one, one can smoothly interpolate between cubes with
sharp edges and hard spheres. The equilibrium phase diagram of rounded parallel
hard cubes is computed as a function of their volume fraction and the rounding
parameter s. The second order freezing transition known for oriented cubes at s
= 0 is found to be persistent up to s = 0.65. The fluid freezes into a
simple-cubic crystal which exhibits a large vacancy concentration. Upon a
further increase of s, the continuous freezing is replaced by a first-order
transition into either a sheared simple cubic lattice or a deformed
face-centered cubic lattice with two possible unit cells: body-centered
orthorhombic or base-centered monoclinic. In principle, a system of parallel
cubes could be realized in experiments on colloids using advanced synthesis
techniques and a combination of external fields.Comment: Submitted to JC
Phase-sensitive detection of Bragg scattering at 1D optical lattices
We report on the observation of Bragg scattering at 1D atomic lattices. Cold
atoms are confined by optical dipole forces at the antinodes of a standing wave
generated by the two counter-propagating modes of a laser-driven high-finesse
ring cavity. By heterodyning the Bragg-scattered light with a reference beam,
we obtain detailed information on phase shifts imparted by the Bragg scattering
process. Being deep in the Lamb-Dicke regime, the scattered light is not
broadened by the motion of individual atoms. In contrast, we have detected
signatures of global translatory motion of the atomic grating.Comment: 4 pages, 4 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
Carrier-wave Rabi flopping signatures in high-order harmonic generation for alkali atoms
We present the first theoretical investigation of carrier-wave Rabi flopping
in real atoms by employing numerical simulations of high-order harmonic
generation (HHG) in alkali species. Given the short HHG cutoff, related to the
low saturation intensity, we concentrate on the features of the third harmonic
of sodium (Na) and potassium (K) atoms. For pulse areas of 2 and Na atoms,
a characteristic unique peak appears, which, after analyzing the ground state
population, we correlate with the conventional Rabi flopping. On the other
hand, for larger pulse areas, carrier-wave Rabi flopping occurs, and is
associated with a more complex structure in the third harmonic. These new
characteristics observed in K atoms indicate the breakdown of the area theorem,
as was already demonstrated under similar circumstances in narrow band gap
semiconductors
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