871 research outputs found
Photoassociation spectroscopy of a Spin-1 Bose-Einstein condensate
We report on the high resolution photoassociation spectroscopy of a Rb
spin-1 Bose-Einstein condensate to the excited
molecular states. We demonstrate the use of spin dependent photoassociation to
experimentally identify the molecular states and their corresponding initial
scattering channel. These identifications are in excellent agreement with the
eigenvalues of a hyperfine-rotational Hamiltonian. Using the observed spectra
we estimate the change in scattering length and identify photoassociation laser
light frequency ranges that maximize the change in the spin-dependent
mean-field interaction energy.Comment: 5 pages, 4 figure
A numerical renormalization group study of laser induced freezing
We study the phenomenon of laser induced freezing, within a numerical
renormalization scheme which allows explicit comparison with a recent defect
mediated melting theory. Precise values for the `bare' dislocation fugacities
and elastic moduli of the 2-d hard disk system are obtained from a constrained
Monte Carlo simulation sampling only configurations {\em without} dislocations.
These are used as inputs to appropriate renormalization flow equations to
obtain the equilibrium phase diagram which shows excellent agreement with
earlier simulation results. We show that the flow equations need to be correct
at least up to third order in defect fugacity to reproduce meaningful results.Comment: Minor Corrections; Combined version of Europhys. Lett. 67 (2004) p.
814 and Europhys. Lett. 68 (2004) p. 16
Direct test of defect mediated laser induced melting theory for two dimensional solids
We investigate by direct numerical solution of appropriate renormalization
flow equations, the validity of a recent dislocation unbinding theory for laser
induced freezing/melting in two dimensions. The bare elastic moduli and
dislocation fugacities which are inputs to the flow equations are obtained for
three different 2-d systems (hard disk, inverse power and the
Derjaguin-Landau-Verwey-Overbeek potentials) from a restricted Monte Carlo
simulation sampling only configurations {\em without} dislocations. We conclude
that (a) the flow equations need to be correct at least up to third order in
defect fugacity to reproduce meaningful results, (b) there is excellent
quantitative agreement between our results and earlier conventional Monte Carlo
simulations for the hard disk system and (c) while the qualitative form of the
phase diagram is reproduced for systems with soft potentials there is some
quantitative discrepancy which we explain.Comment: 11 pages, 14 figures, submitted to Phys. Rev.
Observation of spinor dynamics in optically trapped 87Rb Bose-Einstein Condensates
We measure spin mixing of F=1 and F=2 spinor condensates of 87Rb atoms
confined in an optical trap. We determine the spin mixing time to be typically
less than 600 ms and observe spin population oscillations. The equilibrium spin
configuration in the F=1 manifold is measured for different magnetic fields and
found to show ferromagnetic behavior for low field gradients. An F=2 condensate
is created by microwave excitation from F=1 manifold, and this spin-2
condensate is observed to decay exponentially with time constant 250 ms.
Despite the short lifetime in the F=2 manifold, spin mixing of the condensate
is observed within 50 ms.Comment: 4 pages, 6 figure
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Self-assembly, nematic phase formation and organocatalytic behaviour of a proline-functionalized lipopeptide
The self-assembly of the amphiphilic lipopeptide PAEPKI-C16 (P = proline, A = alanine, E = glutamic acid, K = lysine, I = isoleucine, C16 = hexadecyl) was investigated using a combination of spectroscopic, microscopic and scattering methods and compared to C16-IKPEAP with the same (reversed) peptide sequence and the alkyl chain positioned N-terminally and which lacks a free N-terminal proline residue. The catalytic activity of these peptides were then compared using a model aldol reaction system. For PAEPKI-C16, Cryo-TEM images showed the formation of micrometer length fibers, which by Small-angle X-ray scattering (SAXS) were found to have a radius of 2.5 - 2.6 nm. Spectroscopic analysis shows these fibers are built from -sheets. This behaviour is in complete contrast to that of C16-IKPEAP which forms spherical micelles with peptides in a disordered conformation [Hutchinson, J. A. et al. J. Phys. Chem. B 2019, 123, 613]. For PAEPKI-C16, the spontaneous alignment of fibers was observed upon increasing pH, which was accompanied by observed birefringence and anisotropy of SAXS patterns. This shows the formation of a nematic liquids and unprecedented nematic hydrogel formation was also observed these lipopeptides at sufficiently high concentrations. SAXS shows retention of an ultrafine (1.7 nm core radius) fibrillar network within the hydrogel. PAEPKI-C16 with free N-terminal proline shows enhanced anti:syn diastereoselectivity and better conversion compared to C16-IKPEAP. The cytotoxicity of PAEPKI-C16 was also lower than C16-IKPEAP for both fibroblast and cancer cell lines. These results highlight the sensitivity of lipopeptide properties to the presence of a free proline residue. The spontaneous nematic phase formation by PAEPKI-C16 points to the highly anisotropy of its ultrafine fibrillar structure and the formation of such a phase at low concentration in aqueous solution may be valuable for future applications
Cavity QED with optically transported atoms
Ultracold Rb atoms are delivered into a high-finesse optical
micro-cavity using a translating optical lattice trap and detected via the
cavity field. The atoms are loaded into an optical lattice from a magneto-optic
trap (MOT) and transported 1.5 cm into the cavity. Our cavity satisfies the
strong-coupling requirements for a single intracavity atom, thus permitting
real-time observation of single atoms transported into the cavity. This
transport scheme enables us to vary the number of intracavity atoms from 1 to
100 corresponding to a maximum atomic cooperativity parameter of 5400, the
highest value ever achieved in an atom--cavity system. When many atoms are
loaded into the cavity, optical bistability is directly measured in real-time
cavity transmission.Comment: 4 figures, 4 page
Structural Uncertainty in Onchocerciasis Transmission Models Influences the Estimation of Elimination Thresholds and Selection of Age Groups for Seromonitoring
Background. The World Health Organization recommends monitoring Ov16 serologyin children aged <10 years for stopping mass ivermectin administration. Transmission models can help to identify the most informative age groups for serological monitoring and investigate the discriminatory power of serology-based elimination thresholds.Model predictions will depend on assumed age-exposure patterns and transmission efficiency at low infection levels. Methods. The individual-based transmission model, EPIONCHO-IBM, was used toassess: i) the most informative age groups for serological monitoring using receiveroperator characteristic curves for different elimination thresholds under various age-dependent exposure assumptions, including those of ONCHOSIM (another widely-used model), and ii) the influence of within-human density-dependent parasite establishment (included in EPIONCHO-IBM but not in ONCHOSIM) on positive predictive values for different serological thresholds.Results. When assuming EPIONCHO-IBM exposure patterns, under-10s are themost informative age group for seromonitoring; when assuming ONCHOSIM’s exposure patterns, 5–15-year olds are the most informative (as published elsewhere).Omitting density-dependent parasite establishment results in more lenientseroprevalence thresholds, even for higher baseline infection prevalence and shorter treatment durations.Conclusions. Selecting appropriate seromonitoring age groups depends critically onage-dependent exposure patterns. The role of density dependence on elimination thresholds largely explains differing EPIONCHO-IBM and ONCHOSIM elimination predictions
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Polymorphism of asymmetric catalysts based on amphiphilic lipopeptides in solution
The self-assembly of model [P]RWG lipopeptides (P: L-proline, R: L-arginine, W: L-tryptophan, G: L-glycine), containing one or two aliphatic octadecyl (C18) chains in water and cyclohexanone/water solutions was examined. The self-assembly of mixtures of these RWG and PRWG lipopeptides was also investigated. These materials presented a similar critical aggregation concentration of ∼4.0 × 10−4 wt% and were characterized by unordered secondary structures with some β-sheet content. TEM and cryo-TEM revealed the presence of mainly nanotape structures with micelles observed for systems rich in PRWG(C18H37). Analysis of detailed SAXS form factor measurements revealed the presence of bilayers 3–4 nm thick while the PRWG(C18H37) micelles have a core radius of approximately 3 nm, and a shell thickness of 2 nm. For the cyclohexanone/water systems polymorphs containing cluster aggregates (with radius of 0.25 nm to 0.50 nm) and some elongated structures (with radius of 5.7 nm to 26.1 nm) were seen. Longer structures were formed with the increase of the proline-containing lipopeptide content. The catalytic activity of these peptides was assessed using a model nitro-aldol reaction. The concentration of water in the reaction system influenced the conversion, higher content promoted better efficiency for the water systems, but the opposite was observed for the cyclohexanone/water samples
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