373 research outputs found
Simulations of Sisyphus cooling including multiple excited states
We extend the theory for laser cooling in a near-resonant optical lattice to
include multiple excited hyperfine states. Simulations are performed treating
the external degrees of freedom of the atom, i.e., position and momentum,
classically, while the internal atomic states are treated quantum mechanically,
allowing for arbitrary superpositions. Whereas theoretical treatments including
only a single excited hyperfine state predict that the temperature should be a
function of lattice depth only, except close to resonance, experiments have
shown that the minimum temperature achieved depends also on the detuning from
resonance of the lattice light. Our results resolve this discrepancy.Comment: 7 pages, 6 figure
Internal Structure of Ariebreen, Spitsbergen, from radio-echo sounding data
Ariebreen (77º 01' N, 15º 29' E) is a small valley glacier (ca. 0.36 km2 in August 2007) located at Hornsund, Spitsbergen, Svalbard, ca. 2.5 km to the west of Hornsund Polish Polar Station. Ariebreen, like many other Svalbard glaciers, has experienced a significant recession at least since the 1930s, and most likely since the end of Little Ice Age (LIA) in the early part of the 20th century. Moreover, the thinning rate of western Svalbard glaciers has shown an acceleration during the most recent decades. Ariebreen follows this general retreat pattern, as is shown in another contribution to this workshop (Petlicki et al., 2008). Most investigated glaciers in Hornsund area, in the neighbourhood of Ariebreen, are known to be polythermal (e.g. Hansbreen and Werenskioldbreen, Pälli et al., 2003). It has been suggested (Macheret et al., 1992) that the thinning of polythermal glaciers may result in a switch to cold thermal structure under appropriate conditions. The strong thinning experienced by Ariebreen during the recent decades makes it an ideal candidate to undergo such change. The main aims of this contribution are to understand the internal structure of Ariebreen, in particular, its hydrothermal regime, and to determine whether the glacier is undergoing or has already experienced a transition from polythermal to cold structure. The main tool to accomplish this will be the analysis of radio-echo sounding data
A nonadiabatic semi-classical method for dynamics of atoms in optical lattices
We develop a semi-classical method to simulate the motion of atoms in a
dissipative optical lattice. Our method treats the internal states of the atom
quantum mechanically, including all nonadiabatic couplings, while position and
momentum are treated as classical variables. We test our method in the
one-dimensional case. Excellent agreement with fully quantum mechanical
simulations is found. Our results are much more accurate than those of earlier
semi-classical methods based on the adiabatic approximation.Comment: 7 pages, 5 figures, submitted to European Physical Journal
Experimental measurement of efficiency and transport coherence of a cold atom Brownian motor in optical lattices
The rectification of noise into directed movement or useful energy is
utilized by many different systems. The peculiar nature of the energy source
and conceptual differences between such Brownian motor systems makes a
characterization of the performance far from straightforward. In this work,
where the Brownian motor consists of atoms interacting with dissipative optical
lattices, we adopt existing theory and present experimental measurements for
both the efficiency and the transport coherence. We achieve up to 0.3% for the
efficiency and 0.01 for the P\'eclet number
Four-boson scale near a Feshbach resonance
We show that an independent four-body momentum scale drives the
tetramer binding energy for fixed trimer energy (or three-body scale
) and large scattering length (). The three- and four-body forces
from the one-channel reduction of the atomic interaction near a Feshbach
resonance disentangle and . The four-body independent
scale is also manifested through a family of Tjon-lines, with slope given by
for . There is the possibility of a new
renormalization group limit cycle due to the new scale
The role of antihydrogen formation in the radial transport of antiprotons in positron plasmas
Simulations of the behaviour of antiprotons in positron clouds during antihydrogen formation.The work added to our understanding of the underlying processes, including the dominant role played by antihydrogen formation itself
Modelling the evolution and nucleosynthesis of carbon-enhanced metal-poor stars
We present the results of binary population simulations of carbon-enhanced
metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers
of the origin of CEMP stars, and conclude that the observed paucity of very
nitrogen-rich stars puts strong constraints on possible modifications of the
initial mass function at low metallicity. The large number fraction of CEMP
stars may instead require much more efficient dredge-up from low-metallicity
asymptotic giant branch stars.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU Symposium 252
"The Art of Modelling Stars in the 21st Century", April 6-11, 2008, Sanya,
Chin
On the formation of trappable antihydrogen
Study, using extensive simulations, of the fraction of trappable antihydrogen under typical experimental conditions. Discovery that positron collisions can influence the magnetic moment of the antihydrogen after formation to promote the trappable fraction. Thus attempting experiments at higher positron densities should be beneficial
Resonant Phenomena in Antihydrogen-Hydrogen Scattering
We present a treatment of cold hydrogen-antihydrogen collisions based on the
asymptotic properties of atom-antiatom interactions. We derive general formulas
for the elastic and inelastic cross sections and for the scattering lengths and
analyze their sensitivity to the parameters characterizing the inelasticity of
the collision process. Given the inelasticity, we obtain bounds for the complex
scattering length. We investigate the influence of strong nuclear forces and
the isotope effects in and collisions
and demonstrate enhancement of these effects due to the presence of the
near-threshold narrow () states. The
values of the elastic and inelastic cross-sections with simultaneous account of
rearrangement and strong forces are presented. General expressions for the
(complex) energies of the near-threshold states are
obtained.Comment: 26 pages 7 figure
Demonstration of a controllable three-dimensional Brownian motor in symmetric potentials
We demonstrate a Brownian motor, based on cold atoms in optical lattices,
where isotropic random fluctuations are rectified in order to induce controlled
atomic motion in arbitrary directions. In contrast to earlier demonstrations of
ratchet effects, our Brownian motor operates in potentials that are spatially
and temporally symmetric, but where spatiotemporal symmetry is broken by a
phase shift between the potentials and asymmetric transfer rates between them.
The Brownian motor is demonstrated in three dimensions and the noise-induced
drift is controllable in our system.Comment: 5 pages, 4 figure
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