773 research outputs found
Linking Ultracold Polar Molecules
We predict that pairs of polar molecules can be weakly bound together in an
ultracold environment, provided that a dc electric field is present. The field
that links the molecules together also strongly influences the basic properties
of the resulting dimer, such as its binding energy and predissociation
lifetime. Because of their long-range character these dimers will be useful in
disentangling cold collision dynamics of polar molecules. As an example, we
estimate the microwave photoassociation yield for OH-OH cold collisions.Comment: 4 pages 2 figure
Feshbach spectroscopy and analysis of the interaction potentials of ultracold sodium
We have studied magnetic Feshbach resonances in an ultracold sample of Na
prepared in the absolute hyperfine ground state. We report on the observation
of three s-, eight d-, and three g-wave Feshbach resonances, including a more
precise determination of two known s-wave resonances, and one s-wave resonance
at a magnetic field exceeding 200mT. Using a coupled-channels calculation we
have improved the sodium ground-state potentials by taking into account these
new experimental data, and derived values for the scattering lengths. In
addition, a description of the molecular states leading to the Feshbach
resonances in terms of the asymptotic-bound-state model is presented.Comment: 11 pages, 4 figure
Novel technique for the CO<sub>2</sub> laser fabrication of optical devices with sub-micrometer ablation depth precision
We present novel techniques for the processing of fibre end face and cladding surfaces using a 9.6 µm CO2 laser. We investigate the effects of pulse duration on process parameters
Creation and manipulation of Feshbach resonances with radio-frequency radiation
We present a simple technique for studying collisions of ultracold atoms in
the presence of a magnetic field and radio-frequency radiation (rf). Resonant
control of scattering properties can be achieved by using rf to couple a
colliding pair of atoms to a bound state. We show, using the example of 6Li,
that in some ranges of rf frequency and magnetic field this can be done without
giving rise to losses. We also show that halo molecules of large spatial extent
require much less rf power than deeply bound states. Another way to exert
resonant control is with a set of rf-coupled bound states, linked to the
colliding pair through the molecular interactions that give rise to
magnetically tunable Feshbach resonances. This was recently demonstrated for
87Rb [Kaufman et al., Phys. Rev. A 80:050701(R), 2009]. We examine the
underlying atomic and molecular physics which made this possible. Lastly, we
consider the control that may be exerted over atomic collisions by placing
atoms in superpositions of Zeeman states, and suggest that it could be useful
where small changes in scattering length are required. We suggest other species
for which rf and magnetic field control could together provide a useful tuning
mechanism.Comment: 21 pages, 8 figures, submitted to New Journal of Physic
Photoassociation spectroscopy of cold alkaline earth atoms near the intercombination line
The properties of photoassociation (PA) spectra near the intercombination
line (the weak transition between and states) of group
II atoms are theoretically investigated. As an example we have carried out a
calculation for Calcium atoms colliding at ultra low temperatures of 1 mK, 1
K, and 1 nK. Unlike in most current photoassociation spectroscopy the
Doppler effect can significantly affect the shape of the investigated lines.
Spectra are obtained using Ca--Ca and Ca--Ca short-range {\it ab initio}
potentials and long-range van der Waals and resonance dipole potentials. The
similar van der Waals coefficients of ground and
excited states cause the PA to differ greatly from
those of strong, allowed transitions with resonant dipole interactions. The
density of spectral lines is lower, the Condon points are at relatively short
range, and the reflection approximation for the Franck-Condon factors is not
applicable, and the spontaneous decay to bound ground-state molecules is
efficient. Finally, the possibility of efficient production of cold molecules
is discussed
Acrylamides with hydrolytically labile carbonate ester side chains as versatile building blocks for well-defined block copolymer micelles via RAFT polymerization
En route towards improved delivery systems for targeted chemotherapy, we propose a straightforward approach for the hydrophobic modification of the acrylamide N-(2-Hydroxyethyl) acrylamide (HEAm). An ethyl or benzyl group was introduced via a hydrolytically sensitive carbonate ester yielding HEAm-EC and HEAm-BC, respectively. Block copolymers of HEAm, respectively PEG and HEAm-EC or HEAm-BC were successfully synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, obtaining a library of well-defined block copolymers with different degrees of polymerization (DP). To further explore the versatility of our approach in terms of polymer synthesis, self-assembly, drug solubilization and in vitro cell interaction, polyethylene glycol (PEG) and polyHEAm as hydrophilic polymer blocks were compared. The block copolymers formed micellar nanoparticles (10-100 nm) in PBS and could efficiently solubilize hydrophobic dyes and anti-cancer drugs. Benzyl carbonate ester side chains increased micellar stability and drug loading capacity. Moreover, PEG as hydrophilic block showed in comparison to HEAm more promising results concerning both colloidal stability and drug loading capacity. Confocal microscopy showed that the micelles could efficiently deliver a hydrophobic dye inside the cells. Finally, we also demonstrated efficient formulation of the anti-cancer drug paclitaxel with an in vitro cancer cell killing performance comparable or even better than the two commercial PTX nano-formulations Abraxane and Genexol-PM at equal drug dose. In conclusion, modification of HEAm through carbonate linkages offers a versatile platform for the design of degradable polymers with potential for biomedical applications
Atom-molecule equilibration in a degenerate Fermi gas with resonant interactions
We present a nonequilibrium kinetic theory describing atom-molecule
population dynamics in a two-component Fermi gas with a Feshbach resonance. Key
collision integrals emerge that govern the relaxation of the atom-molecule
mixture to chemical and thermal equilibrium. Our focus is on the pseudogap
regime where molecules form above the superfluid transition temperature. In
this regime, we formulate a simple model for the atom-molecule population
dynamics. The model predicts the saturation of molecule formation that has been
observed in recent experiments, and indicates that a dramatic enhancement of
the atom-molecule conversion efficiency occurs at low temperatures.Comment: Updated manuscript on July 5, 2004. Four pages with three embedded
figure
Prediction of Feshbach resonances from three input parameters
We have developed a model of Feshbach resonances in gases of ultracold alkali
metal atoms using the ideas of multichannel quantum defect theory. Our model
requires just three parameters describing the interactions - the singlet and
triplet scattering lengths, and the long range van der Waals coefficient - in
addition to known atomic properties. Without using any further details of the
interactions, our approach can accurately predict the locations of resonances.
It can also be used to find the singlet and triplet scattering lengths from
measured resonance data. We apply our technique to Li--K and
K--Rb scattering, obtaining good agreement with experimental
results, and with the more computationally intensive coupled channels
technique.Comment: 5 pages, 2 figures, revised versio
Tree-body loss of of trapped ultracold Rb atoms due to a Feshbach resonance
The loss of ultracold trapped atoms in the vicinity of a Feshbach resonance
is treated as a two-stage reaction, using the Breit-Wigner theory. The first
stage is the formation of a resonant diatomic molecule, and the second one is
its deactivation by inelastic collisions with other atoms. This model is
applied to the analysis of recent experiments on Rb, leading to an
estimated value of cms for the deactivation rate
coefficient.Comment: LaTeX, 4 pages with 1 figures, uses REVTeX4, uses improved
experimental dat
Microscopic theory of atom-molecule oscillations in a Bose-Einstein condensate
In a recent experiment at JILA [E.A. Donley et al., Nature (London) 417, 529
(2002)] an initially pure condensate of Rb-85 atoms was exposed to a specially
designed time dependent magnetic field pulse in the vicinity of a Feshbach
resonance. The production of new components of the gas as well as their
oscillatory behavior have been reported. We apply a microscopic theory of the
gas to identify these components and determine their physical properties. Our
time dependent studies allow us to explain the observed dynamic evolution of
all fractions, and to identify the physical relevance of the pulse shape. Based
on ab initio predictions, our theory strongly supports the view that the
experiments have produced a molecular condensate.Comment: 18 pages, 20 figure
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