690 research outputs found
Mechanical effect of van der Waals interactions observed in real time in an ultracold Rydberg gas
We present time-resolved spectroscopic measurements of Rydberg-Rydberg
interactions in an ultracold gas, revealing the pair dynamics induced by
long-range van der Waals interactions between the atoms. By detuning the
excitation laser, a specific pair distribution is prepared. Penning ionization
on a microsecond timescale serves as a probe for the pair dynamics under the
influence of the attractive long-range forces. Comparison with a Monte Carlo
model not only explains all spectroscopic features but also gives quantitative
information about the interaction potentials. The results imply that the
interaction-induced ionization rate can be influenced by the excitation laser.
Surprisingly, interaction-induced ionization is also observed for Rydberg
states with purely repulsive interactions
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Isoprenoid Biosynthesis in Plasmodium falciparum
Malaria kills nearly 1 million people each year, and the protozoan parasite Plasmodium falciparum has become increasingly resistant to current therapies. Isoprenoid synthesis via the methylerythritol phosphate (MEP) pathway represents an attractive target for the development of new antimalarials. The phosphonic acid antibiotic fosmidomycin is a specific inhibitor of isoprenoid synthesis and has been a helpful tool to outline the essential functions of isoprenoid biosynthesis in P. falciparum. Isoprenoids are a large, diverse class of hydrocarbons that function in a variety of essential cellular processes in eukaryotes. In P. falciparum, isoprenoids are used for tRNA isopentenylation and protein prenylation, as well as the synthesis of vitamin E, carotenoids, ubiquinone, and dolichols. Recently, isoprenoid synthesis in P. falciparum has been shown to be regulated by a sugar phosphatase. We outline what is known about isoprenoid function and the regulation of isoprenoid synthesis in P. falciparum, in order to identify valuable directions for future research
Observation of the transition from lasing driven by a bosonic to a fermionic reservoir in a GaAs quantum well microcavity
We show that by monitoring the free carrier reservoir in a GaAs-based quantum well microcavity under non-resonant pulsed optical pumping, lasing supported by a fermionic reservoir (photon lasing) can be distinguished from lasing supported by a reservoir of bosons (polariton lasing). Carrier densities are probed by measuring the photocurrent between lateral contacts deposited directly on the quantum wells of a microcavity that are partially exposed by wet chemical etching. We identify two clear thresholds in the input-output characteristic of the photoluminescence signal which can be attributed to polariton and photon lasing, respectively. The power dependence of the probed photocurrent shows a distinct kink at the threshold power for photon lasing due to increased radiative recombination of free carriers as stimulated emission into the cavity mode sets in. At the polariton lasing threshold on the other hand, the nonlinear increase of the luminescence is caused by stimulated scattering of exciton-polaritons to the ground state which do not contribute directly to the photocurrent.PostprintPeer reviewe
Rabi oscillations between ground and Rydberg states and van der Waals blockade in a mesoscopic frozen Rydberg gas
We present a detailed analysis of our recent observation of synchronous Rabi
oscillations between the electronic ground state and Rydberg states in a
mesoscopic ensemble containing roughly 100 ultracold atoms [M. Reetz-Lamour
\textit{et al.}, submitted, arXiv:0711.4321]. The mesoscopic cloud is selected
out of a sample of laser-cooled Rb atoms by optical pumping. The atoms are
coupled to a Rydberg state with principal quantum number around 30 by a
two-photon scheme employing flat-top laser beams. The influence of residual
spatial intensity fluctuations as well as sources of decoherence such as
redistribution to other states, radiative lifetime, and laser bandwidth are
analysed. The results open up new possibilities for the investigation of
coherent many-body phenomena in dipolar Rydberg gases. As an example we
demonstrate the van der Waals blockade, a variant of the dipole blockade, for a
mesoscopic atom sample
Evidence for a change in the nuclear mass surface with the discovery of the most neutron-rich nuclei with 17<Z <25
The results of measurements of the production of neutron-rich nuclei by the
fragmentation of a 76-Ge beam are presented. The cross sections were measured
for a large range of nuclei including fifteen new isotopes that are the most
neutron-rich nuclides of the elements chlorine to manganese (50-Cl, 53-Ar,
55,56-K, 57,58-Ca, 59,60,61-Sc, 62,63-Ti, 65,66-V, 68-Cr, 70-Mn). The enhanced
cross sections of several new nuclei relative to a simple thermal evaporation
framework, previously shown to describe similar production cross sections,
indicates that nuclei in the region around 62-Ti might be more stable than
predicted by current mass models and could be an indication of a new island of
inversion similar to that centered on 31-Na.Comment: 4 pages, 3 figures, to be published in Physical Review Letters, 200
Coherent Population Trapping with Controlled Interparticle Interactions
We investigate Coherent Population Trapping in a strongly interacting
ultracold Rydberg gas. Despite the strong van der Waals interactions and
interparticle correlations, we observe the persistence of a resonance with
subnatural linewidth at the single-particle resonance frequency as we tune the
interaction strength. This narrow resonance cannot be understood within a
meanfield description of the strong Rydberg--Rydberg interactions. Instead, a
many-body density matrix approach, accounting for the dynamics of interparticle
correlations, is shown to reproduce the observed spectral features
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Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: Intermodel comparisons and relationships to field measurements
Nine ecosystem process models were used to predict CO2 and water vapor exchanges by a 150-year-old black spruce forest in central Canada during 1994â1996 to evaluate and improve the models. Three models had hourly time steps, five had daily time steps, and one had monthly time steps. Model input included site ecosystem characteristics and meteorology. Model predictions were compared to eddy covariance (EC) measurements of whole-ecosystem CO2exchange and evapotranspiration, to chamber measurements of nighttime moss-surface CO2release, and to ground-based estimates of annual gross primary production, net primary production, net ecosystem production (NEP), plant respiration, and decomposition. Model-model differences were apparent for all variables. Model-measurement agreement was good in some cases but poor in others. Modeled annual NEP ranged from â11 g C mâ2 (weak CO2source) to 85 g C mâ2 (moderate CO2 sink). The models generally predicted greater annual CO2sink activity than measured by EC, a discrepancy consistent with the fact that model parameterizations represented the more productive fraction of the EC tower âfootprint.â At hourly to monthly timescales, predictions bracketed EC measurements so median predictions were similar to measurements, but there were quantitatively important model-measurement discrepancies found for all models at subannual timescales. For these models and input data, hourly time steps (and greater complexity) compared to daily time steps tended to improve model-measurement agreement for daily scale CO2 exchange and evapotranspiration (as judged by root-mean-squared error). Model time step and complexity played only small roles in monthly to annual predictions
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