80 research outputs found
Ultracold gases far from equilibrium
Ultracold atomic quantum gases belong to the most exciting challenges of
modern physics. Their theoretical description has drawn much from classical
field equations. These mean-field approximations are in general reliable for
dilute gases in which the atoms collide only rarely with each other, and for
situations where the gas is not too far from thermal equilibrium. With
present-day technology it is, however, possible to drive and observe a system
far away from equilibrium. Functional quantum field theory provides powerful
tools to achieve both, analytical understanding and numerical computability,
also in higher dimensions, of far-from-equilibrium quantum many-body dynamics.
In the article, an outline of these approaches is given, including methods
based on the two-particle irreducible effective action as well as on
renormalisation-group theory. Their relation to near-equilibrium kinetic theory
is discussed, and the distinction between quantum and classical statistical
fluctuations is shown to naturally emerge from the functional-integral
description. Example applications to the evolution of an ultracold atomic Bose
gas in one spatial dimension underline the power of the methods. The article is
compiled from the notes for lectures held at 46. Internationale
Universitaetswochen fuer Theoretische Physik 2008 in Schladming, Austria.Comment: 59 pages, 26 figures; Compiled from notes for lectures held at 46.
Internationale Universitaetswochen fuer Theoretische Physik 2008 in
Schladming, Austria. To be published in Eur. Phys. J. Special Topic
High light intensity photoassociation in a Bose-Einstein condensate
We investigate theoretically the molecular yield in photoassociation of
Bose-Einstein condensed sodium atoms for light intensities of the order of and
above those applied in a recent experiment. Our results show that the rate at
which ground state molecules may be formed saturates at high light intensities
whereas the loss rate of condensate atoms does not. This is caused by the
opposing roles of the short and long range pair correlations present near
resonance under the influence of the laser and is crucial for the development
of efficient photoassociation procedures in a condensate.Comment: 4 pages RevTeX, 4 Figures, numerical errors corrected in revised
versio
Universal scaling at non-thermal fixed points of a two-component Bose gas
Quasi-stationary far-from-equilibrium critical states of a two-component Bose
gas are studied in two spatial dimensions. After the system has undergone an
initial dynamical instability it approaches a non-thermal fixed point. At this
critical point the structure of the gas is characterised by ensembles of
(quasi-)topological defects such as vortices, skyrmions and solitons which give
rise to universal power-law behaviour of momentum correlation functions. The
resulting power-law spectra can be interpreted in terms of
strong-wave-turbulence cascades driven by particle transport into
long-wave-length excitations. Scaling exponents are determined on both sides of
the miscible-immiscible transition controlled by the ratio of the intra-species
to inter-species couplings. Making use of quantum turbulence methods, we
explain the specific values of the exponents from the presence of transient
(quasi-)topological defects.Comment: 13 pages, 12 figure
Superfluid Turbulence: Nonthermal Fixed Point in an Ultracold Bose Gas
Nonthermal fixed points of far-from-equilibrium dynamics of a dilute
degenerate Bose gas are analysed in two and three spatial dimensions. For such
systems, universal power-law distributions, previously found within a
nonperturbative quantum-field theoretic approach, are shown to be related to
vortical dynamics and superfluid turbulence. The results imply an
interpretation of the momentum scaling at the nonthermal fixed points in terms
of independent vortex excitations of the superfluid. Long-wavelength acoustic
excitations on the top of these are found to follow a non-thermal power law.
The results shed light on fundamental aspects of superfluid turbulence and have
strong potential implications for related phenomena studied, e.g., in
early-universe inflation or quark-gluon plasma dynamics.Comment: 5 pages, 5 figure
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