36 research outputs found
Condensation Energy of a Spin-1/2 Strongly Interacting Fermi Gas
We report a measurement of the condensation energy of a two-component Fermi
gas with tunable interactions. From the equation of state of the gas, we infer
the properties of the normal phase in the zero-temperature limit. By comparing
the pressure of the normal phase at T=0 to that of the low-temperature
superfluid phase, we deduce the condensation energy, i.e. the energy gain of
the system in being in the superfluid rather than normal state. We compare our
measurements to a ladder approximation description of the normal phase, and to
a fixed node Monte-Carlo approach, finding excellent agreement. We discuss the
relationship between condensation energy and pairing gap in the BEC-BCS
crossover.Comment: 4 figure
Balancing photosynthetic electron flow is critical for cyanobacterial acclimation to nitrogen limitation
AbstractNitrogen limitation forces photosynthetic organisms to reallocate available nitrogen to essential functions. At the same time, it increases the probability of photo-damage by limiting the rate of energy-demanding metabolic processes, downstream of the photosynthetic apparatus. Non-diazotrophic cyanobacteria cope with this situation by decreasing the size of their phycobilisome antenna and by modifying their photosynthetic apparatus. These changes can serve two purposes: to provide extra amino-acids and to decrease excitation pressure. We examined the effects of nitrogen limitation on the form and function of the photosynthetic apparatus. Our aim was to study which of the two demands serve as the driving force for the remodeling of the photosynthetic apparatus, under different growth conditions. We found that a drastic reduction in light intensity allowed cells to maintain a more functional photosynthetic apparatus: the phycobilisome antenna was bigger, the activity of both photosystems was higher and the levels of photosystem (PS) proteins were higher. Pre-acclimating cells to Mn limitation, under which the activity of both PSI and PSII is diminished, results in a very similar response. The rate of PSII photoinhibition, in nitrogen limited cells, was found to be directly related to the activity of the photosynthetic apparatus. These data indicate that, under our experimental conditions, photo-damage avoidance was the more prominent determinant during the acclimation process. The combinations of limiting factors tested here is by no means artificial. Similar scenarios can take place under environmental conditions and should be taken into account when estimating nutrient limitations in nature
Dynamics and Thermodynamics of the Low-Temperature Strongly Interacting Bose Gas
We measure the zero-temperature equation of state of a homogeneous Bose gas
of Li atoms by analyzing the \emph{in-situ} density distributions of
trapped samples. For increasing repulsive interactions our data shows a clear
departure from mean-field theory and provides a quantitative test of the
many-body corrections first predicted in 1957 by Lee, Huang and Yang. We
further probe the dynamic response of the Bose gas to a varying interaction
strength and compare it to simple theoretical models. We deduce a lower bound
for the value of the universal constant that would characterize
the universal Bose gas at the unitary limit
Novel Adaptive Photosynthetic Characteristics of Mesophotic Symbiotic Microalgae within the Reef-Building Coral, Stylophora pistillata
Photosynthetic coral reef structures extend from the shallow sundrenched waters to the dimly lit, “twilight” mesophotic depths. For their resident endosymbiotic dinoflagellates, primarily from the genus Symbiodinium spp., this represents a photic environment that varies ~15-fold in intensity and also differs in spectral composition. We examined photosynthesis in the scleractinian coral Stylophora pistillata in shallow (3 m) and mesophotic settings (65 m) in the northern Red Sea. Symbiodinium spp. in corals originating from the mesophotic environment consistently performed below their photosynthetic compensation point and also exhibited distinct light harvesting antenna organization. In addition, the non-photochemical quenching activity of Symbiodinium spp. from mesophotic corals was shown to be considerably lower than those found in shallow corals, showing they have fewer defenses to high-light settings. Over a period of almost 4 years, we extensively utilized closed circuit Trimix rebreather diving to perform the study. Phylogenetic analysis showed that shallow corals (3 m) transplanted to a deep reef environment (65 m) maintained their initial Symbiodinium spp. community (clade A), rather than taking on deep low-light clades (clade C), demonstrating that shallow S. pistillata acclimate to low-light mesophotic environments while maintaining their shallow photosynthetic traits. Mesophotic corals exhibited static depth-related chlorophyll content per cell, a decrease in PSI activity and enhanced sigmoidal fluorescence rise kinetics. The sigmoidal fluorescence rise kinetics we observed in mesophotic corals is an indication of energy transfer between photosynthetic units. We postulate that at mesophotic depths, a community of adapted Symbiodinium spp. utilize a unique adaptation to lower light conditions by shifting their light harvesting to a PSII based system, where PSII is structured near PSI, with additional PCP soluble antenna also trapping light that is funneled to the PSI reaction center. In this study, we provide evidence that mesophotic Symbiodinium spp. have developed novel adaptive low-light characteristics consisting of a cooperative system for excitation energy transfer between photosynthetic units that maximizes light utilization
Fermi-Liquid Behavior of the Normal Phase of a Strongly Interacting Gas of Cold Atoms
International audienceWe measure the magnetic susceptibility of a Fermi gas with tunable interactions in the low-temperature limit and compare it to quantum Monte Carlo calculations. Experiment and theory are in excellent agreement and fully compatible with the Landau theory of Fermi liquids. We show that these measure- ments shed new light on the nature of the excitations of the normal phase of a strongly interacting Fermi gas
The equation of state of ultracold Bose and Fermi gases: a few examples
We describe a powerful method for determining the equation of state of an
ultracold gas from in situ images. The method provides a measurement of the
local pressure of an harmonically trapped gas and we give several applications
to Bose and Fermi gases. We obtain the grand-canonical equation of state of a
spin-balanced Fermi gas with resonant interactions as a function of
temperature. We compare our equation of state with an equation of state
measured by the Tokyo group, that reveals a significant difference in the
high-temperature regime. The normal phase, at low temperature, is well
described by a Landau Fermi liquid model, and we observe a clear thermodynamic
signature of the superfluid transition. In a second part we apply the same
procedure to Bose gases. From a single image of a quasi ideal Bose gas we
determine the equation of state from the classical to the condensed regime.
Finally the method is applied to a Bose gas in a 3D optical lattice in the Mott
insulator regime. Our equation of state directly reveals the Mott insulator
behavior and is suited to investigate finite-temperature effects.Comment: 14 pages, 6 figure
The scientific payload of the Ultraviolet Transient Astronomy Satellite (ULTRASAT)
The Ultraviolet Transient Astronomy Satellite (ULTRASAT) is a space-borne
near UV telescope with an unprecedented large field of view (200 sq. deg.). The
mission, led by the Weizmann Institute of Science and the Israel Space Agency
in collaboration with DESY (Helmholtz association, Germany) and NASA (USA), is
fully funded and expected to be launched to a geostationary transfer orbit in
Q2/3 of 2025. With a grasp 300 times larger than GALEX, the most sensitive UV
satellite to date, ULTRASAT will revolutionize our understanding of the hot
transient universe, as well as of flaring galactic sources. We describe the
mission payload, the optical design and the choice of materials allowing us to
achieve a point spread function of ~10arcsec across the FoV, and the detector
assembly. We detail the mitigation techniques implemented to suppress
out-of-band flux and reduce stray light, detector properties including measured
quantum efficiency of scout (prototype) detectors, and expected performance
(limiting magnitude) for various objects.Comment: Presented in the SPIE Astronomical Telescopes + Instrumentation 202
Safety of procuring research tissue during a clinically indicated kidney biopsy from patients with lupus: data from the Accelerating Medicines Partnership RA/SLE Network
Objectives In lupus nephritis the pathological diagnosis from tissue retrieved during kidney biopsy drives treatment and management. Despite recent approval of new drugs, complete remission rates remain well under aspirational levels, necessitating identification of new therapeutic targets by greater dissection of the pathways to tissue inflammation and injury. This study assessed the safety of kidney biopsies in patients with SLE enrolled in the Accelerating Medicines Partnership, a consortium formed to molecularly deconstruct nephritis.Methods 475 patients with SLE across 15 clinical sites in the USA consented to obtain tissue for research purposes during a clinically indicated kidney biopsy. Adverse events (AEs) were documented for 30 days following the procedure and were determined to be related or unrelated by all site investigators. Serious AEs were defined according to the National Institutes of Health reporting guidelines.Results 34 patients (7.2%) experienced a procedure-related AE: 30 with haematoma, 2 with jets, 1 with pain and 1 with an arteriovenous fistula. Eighteen (3.8%) experienced a serious AE requiring hospitalisation; four patients (0.8%) required a blood transfusion related to the kidney biopsy. At one site where the number of cores retrieved during the biopsy was recorded, the mean was 3.4 for those who experienced a related AE (n=9) and 3.07 for those who did not experience any AE (n=140). All related AEs resolved.Conclusions Procurement of research tissue should be considered feasible, accompanied by a complication risk likely no greater than that incurred for standard clinical purposes. In the quest for targeted treatments personalised based on molecular findings, enhanced diagnostics beyond histology will likely be required