65 research outputs found
Shape oscillations in non-degenerate Bose gases - transition from the collisionless to the hydrodynamic regime
We investigate collective oscillations of non-degenerate clouds of Rb-87
atoms as a function of density in an elongated magnetic trap. For the low-lying
M=0 monopole-quadrupole shape oscillation we measure the oscillation
frequencies and damping rates. At the highest densities the mean-free-path is
smaller than the axial dimension of the sample, which corresponds to
collisionally hydrodynamic conditions. This allows us to cover the cross-over
from the collisionless to the hydrodynamic regime. The experimental results
show good agreement with theory. We also analyze the influence of trap
anharmonicities on the oscillations in relation to observed temperature
dependencies of the dipole and quadrupole oscillation frequencies. We present
convenient expressions to quantify these effects.Comment: 10 pages, 5 figure
A laser based accelerator for ultracold atoms
We present first results on our implementation of a laser based accelerator
for ultracold atoms. Atoms cooled to a temperature of 420 nK are confined and
accelerated by means of laser tweezer beams and the atomic scattering is
directly observed in laser absorption imaging. The optical collider has been
characterized using Rb87 atoms in the |F=2,mF=2> state, but the scheme is not
restricted to atoms in any particular magnetic substates and can readily be
extended to other atomic species as well.Comment: (c) 2012 The Optical Society, 3 pages, 4 figures, 1 movie lin
Biomarkers as Proxies to Analyse Land-Use History in Northern Jordan
In the semi-arid 'Decapolis region' in northern Jordan, due severe land degradation in the past, 'barren' and 'impoverished' landscapes can be found today. It is widely believed that land degradation in these regions was caused by ancient land use, e.g. overgrazing due to ‘Arab mismanagement'. However, the connection of degradation with land use is far from certain. The 'Decapolis region' is located in an approximately 100 km wide transition zone from Mediterranean to steppe and desert climate. Therefore, the landscape in this region is highly sensitive to climate variations. A major sedimentation phase in the late 6th century AD appears to represent a significant climate change towards more aridity, and might be connected with a cluster of heavy rainfall events in northern Jordan. In fact, more recent studies have found that periods of predominantly pastoral land use in northern Jordan were connected with natural reforestation. Since a dating of sedimentation alone does not deliver clues about the precise reason of deposition, a multidisciplinary team is analyzing the land-use history in the ‘Decapolis’ region. This presentation focusses on ongoing biomarker analyses. Samples were selected considering geoarchaeological data, including phosphorus concentrations, archaeological data, including distribution of potsherds and other fragments on ancient fields and data of further disciplines. Vegetation changes are investigated by analyses of n-alkanes and terpenoids. Manuring with faeces is analysed by specific steroids that are indicative for faeces deposition. Preliminary results showed a high input of omnivorous (pigs, humans) faeces in some areas. Manuring with faeces of herbivores seemed to be less important
Observation of Quantum Effects in sub Kelvin Cold Reactions
There has been a long-standing quest to observe chemical reactions at low
temperatures where reaction rates and pathways are governed by quantum
mechanical effects. So far this field of Quantum Chemistry has been dominated
by theory. The difficulty has been to realize in the laboratory low enough
collisional velocities between neutral reactants, so that the quantum wave
nature could be observed. We report here the first realization of merged
neutral supersonic beams, and the observation of clear quantum effects in the
resulting reactions. We observe orbiting resonances in the Penning ionization
reaction of argon and molecular hydrogen with metastable helium leading to a
sharp increase in the absolute reaction rate in the energy range corresponding
to a few degrees kelvin down to 10 mK. Our method is widely applicable to many
canonical chemical reactions, and will enable a breakthrough in the
experimental study of Quantum Chemistry
Hydrodynamic behavior in expanding thermal clouds of Rb-87
We study hydrodynamic behavior in expanding thermal clouds of Rb-87 released
from an elongated trap. At our highest densities the mean free path is smaller
than the radial size of the cloud. After release the clouds expand
anisotropically. The cloud temperature drops by as much as 30%. This is
attributed to isentropic cooling during the early stages of the expansion. We
present an analytical model to describe the expansion and to estimate the
cooling. Important consequences for time-of-flight thermometry are discussed.Comment: 7 pages with 2 figure
A Quantum Scattering Interferometer
The collision of two ultra-cold atoms results in a quantum-mechanical
superposition of two outcomes: each atom continues without scattering and each
atom scatters as a spherically outgoing wave with an s-wave phase shift. The
magnitude of the s-wave phase shift depends very sensitively on the interaction
between the atoms. Quantum scattering and the underlying phase shifts are
vitally important in many areas of contemporary atomic physics, including
Bose-Einstein condensates, degenerate Fermi gases, frequency shifts in atomic
clocks, and magnetically-tuned Feshbach resonances. Precise measurements of
quantum scattering phase shifts have not been possible until now because, in
scattering experiments, the number of scattered atoms depends on the s-wave
phase shifts as well as the atomic density, which cannot be measured precisely.
Here we demonstrate a fundamentally new type of scattering experiment that
interferometrically detects the quantum scattering phase shifts of individual
atoms. By performing an atomic clock measurement using only the scattered part
of each atom, we directly and precisely measure the difference of the s-wave
phase shifts for the two clock states in a density independent manner. Our
method will give the most direct and precise measurements of ultracold
atom-atom interactions and will place stringent limits on the time variations
of fundamental constants.Comment: Corrected formatting and typo
The Long Shadow of History. Roman Legacy and Economic Development -- Evidence from the German Limes
Differentiation of plant derived organic matter in soil, loess and rhizoliths based on n-alkane molecular proxies
- …