8,950 research outputs found
Ehrlich-Schwoebel barrier controlled slope selection in epitaxial growth
We examine the step dynamics in a 1+1 dimensional model of epitaxial growth
based on the BCF-theory. The model takes analytically into account the
diffusion of adatoms, an incorporation mechanism and an Ehrlich-Schwoebel
barrier at step edges. We find that the formation of mounds with a stable slope
is closely related to the presence of an incorporation mechanism. We confirm
this finding using a Solid-On-Solid model in 2+1 dimensions. In the case of an
infinite step edge barrier we are able to calculate the saturation profile
analytically. Without incorporation but with inclusion of desorption and
detachment we find a critical flux for instable growth but no slope selection.
In particular, we show that the temperature dependence of the selected slope is
solely determined by the Ehrlich-Schwoebel barrier which opens a new
possibility in order to measure this fundamental barrier in experiments.Comment: 17 pages, 4 figure
Formation of correlations and energy-conservation at short time scales
The formation of correlations due to collisions in an interacting nucleonic
system is investigated. Results from one-time kinetic equations are compared
with the Kadanoff and Baym two-time equation with collisions included in Born
approximation. A reasonable agreement is found for a proposed approximation of
the memory effects by a finite duration of collisions. This form of collision
integral is in agreement with intuitive estimates from Fermi's golden rule. The
formation of correlations and the build up time is calculated analytically for
the high temperature and the low temperature limit. Different approximate
expressions are compared with the numerical results. We present analytically
the time dependent interaction energy and the formation time for Gau\ss{}- and
Yukawa type of potentials.Comment: Europ. Lournal Physics A accepte
Correlations in Many-Body Systems with Two-time Green's Functions
The Kadanoff-Baym (KB) equations are solved numerically for infinite nuclear
matter. In particular we calculate correlation energies and correlation times.
Approximating the Green's functions in the KB collision kernel by the free
Green's functions the Levinson equation is obtained. This approximation is
valid for weak interactions and/or low densities. It relates to the extended
quasi-classical approximation for the spectral function. Comparing the
Levinson, Born and KB calculations allows for an estimate of higher order
spectral corrections to the correlations. A decrease in binding energy is
reported due to spectral correlations and off-shell parts in the reduced
density matrix
Making Cold Molecules by Time-dependent Feshbach Resonances
Pairs of trapped atoms can be associated to make a diatomic molecule using a
time dependent magnetic field to ramp the energy of a scattering resonance
state from above to below the scattering threshold. A relatively simple model,
parameterized in terms of the background scattering length and resonance width
and magnetic moment, can be used to predict conversion probabilities from atoms
to molecules. The model and its Landau-Zener interpretation are described and
illustrated by specific calculations for Na, Rb, and Cs
resonances. The model can be readily adapted to Bose-Einstein condensates.
Comparison with full many-body calculations for the condensate case show that
the model is very useful for making simple estimates of molecule conversion
efficiencies.Comment: 11 pages, 11 figures; talk for Quantum Challenges Symposium, Warsaw,
Poland, September 4-7, 2003. Published in Journal of Modern Optics 51,
1787-1806 (2004). Typographical errors in Journal article correcte
Spontaneous Dissociation of 85Rb Feshbach Molecules
The spontaneous dissociation of 85Rb dimers in the highest lying vibrational
level has been observed in the vicinity of the Feshbach resonance which was
used to produce them. The molecular lifetime shows a strong dependence on
magnetic field, varying by three orders of magnitude between 155.5 G and 162.2
G. Our measurements are in good agreement with theoretical predictions in which
molecular dissociation is driven by inelastic spin relaxation. Molecule
lifetimes of tens of milliseconds can be achieved close to resonance.Comment: 4 pages, 3 figure
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