204 research outputs found
Coordination polymers and isomerism; a study using silver(I) and a ∏-stacked ligand
The ligand 2,5-bis(2-pyridylmethylsulfanylmethyl)pyrazine (L) was prepared by the base coupling of 2-(sulfanylmethyl)pyridine and 2,5-bis(chloromethyl)pyrazine. This new ligand was treated with AgClO₄ in a 1 1 metal-to-ligand ratio and with AgNO₃in a 2 1 metal-to-ligand ratio to give coordination polymers. The crystal structures of {[Ag(L)]ClO₄}∞ ( 1) and {[Ag₂(L)](NO₃)₂}∞ ( 2) were determined. The Ag(I) ions in the one-dimensional polymeric chains of 1 adopted square-pyramidal geometries with the pyridine and pyrazine N donors coordinated in an extremely bent fashion. The structure of 2 revealed two isomeric polymer chains in the one crystal forming a single supramolecular array. The isomeric polymers differed in the donor atoms about the Ag(I) ions and in the arrangement of adjacent ligands along the chain. A feature of both structures was that L adopted a three-layer ∏-stacked arrangement
Three-dimensional vortex dynamics in Bose-Einstein condensates
We simulate in the mean-field limit the effects of rotationally stirring a
three-dimensional trapped Bose-Einstein condensate with a Gaussian laser beam.
A single vortex cycling regime is found for a range of trap geometries, and is
well described as coherent cycling between the ground and the first excited
vortex states. The critical angular speed of stirring for vortex formation is
quantitatively predicted by a simple model. We report preliminary results for
the collisions of vortex lines, in which sections may be exchanged.Comment: 4 pages, 4 figures, REVTeX 3.1; Submitted to Physical Review A (6
March 2000
Fourier transforming a trapped Bose-Einstein condensate by waiting a quarter of the trap period: simulation and applications
We investigate the property of isotropic harmonic traps to Fourier transform a weakly interacting Bose–Einstein condensate (BEC) every quarter of a trap period. We solve the Gross–Pitaevskii equation numerically to investigate the time evolution of interacting BECs in the context of the Fourier transform, and we suggest potential applications
Bis(μ-di-2-pyridyl disulfide-κ3 N,S:N′)di-μ3-iodido-di-μ2-iodido-tetracopper(I)
In the centrosymmetric tetranuclear title compound, [Cu4I4(C10H8N2S2)2], there are two different CuI atoms with tetrahedral coordination geometries. One is chelated by a pyridine N atom and an S-donor from one di-2-pyridyl disulfide ligand and coordinated by two I atoms, while the second CuI atom is coordinated by a pyridine-N and three I atoms. Iodine bridges between the CuI atoms form a tetranuclear structure
Pade approximations of solitary wave solutions of the Gross-Pitaevskii equation
Pade approximants are used to find approximate vortex solutions of any
winding number in the context of Gross-Pitaevskii equation for a uniform
condensate and condensates with axisymmetric trapping potentials. Rational
function and generalised rational function approximations of axisymmetric
solitary waves of the Gross-Pitaevskii equation are obtained in two and three
dimensions. These approximations are used to establish a new mechanism of
vortex nucleation as a result of solitary wave interactions.Comment: In press by Journal of Physics: Mathematics and Genera
Coherent Dynamics of Vortex Formation in Trapped Bose-Einstein Condensates
Simulations of a rotationally stirred condensate show that a regime of simple
behaviour occurs in which a single vortex cycles in and out of the condensate.
We present a simple quantitative model of this behaviour, which accurately
describes the full vortex dynamics, including a critical angular speed of
stirring for vortex formation. A method for experimentally preparing a
condensate in a central vortex state is suggested.Comment: 4 pages, 4 figures, REVTeX 3.1; Submitted to Physical Review Letters
(5 February 1999); See http://www.physics.otago.ac.nz/research/bec/vortex for
MPEG movies and further information; Accepted for Physical Review Letters (24
June 1999); Changes: updated Figs 1 and 2 (new style), minor typos fixed,
more discussion at en
Structural basis for hemoglobin capture by Staphylococcus aureus cell-surface protein, IsdH
Pathogens must steal iron from their hosts to establish infection. In mammals, hemoglobin (Hb) represents the largest reservoir of iron, and pathogens express Hb-binding proteins to access this source. Here, we show how one of the commonest and most significant human pathogens, Staphylococcus aureus, captures Hb as the first step of an iron-scavenging pathway. The x-ray crystal structure of Hb bound to a domain from the Isd (iron-regulated surface determinant) protein, IsdH, is the first structure of a Hb capture complex to be determined. Surface mutations in Hb that reduce binding to the Hb-receptor limit the capacity of S. aureus to utilize Hb as an iron source, suggesting that Hb sequence is a factor in host susceptibility to infection. The demonstration that pathogens make highly specific recognition complexes with Hb raises the possibility of developing inhibitors of Hb binding as antibacterial agents. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc
Dynamic instability of a rotating Bose-Einstein condensate
We consider a Bose-Einstein condensate subject to a rotating harmonic
potential, in connection with recent experiments leading to the formation of
vortices. We use the classical hydrodynamic approximation to the non-linear
Schr\"odinger equation to determine almost analytically the evolution of the
condensate. We predict that this evolution can exhibit dynamical instabilities,
for the stirring procedure previously demonstrated at ENS and for a new
stirring procedure that we put forward. These instabilities take place within
the range of stirring frequency and amplitude for which vortices are produced
experimentally. They provide therefore an initiating mechanism for vortex
nucleation.Comment: 4 pages, 3 figures, last version including comparison with
experiment
Observation of Superfluid Flow in a Bose-Einstein Condensed Gas
We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred
by a macroscopic object, a blue detuned laser beam, using nondestructive {\em
in situ} phase contrast imaging. A critical velocity for the onset of a
pressure gradient has been observed, and shown to be density dependent. The
technique has been compared to a calorimetric method used previously to measure
the heating induced by the motion of the laser beam.Comment: 4 pages, 5 figure
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