159 research outputs found
Dipolar Relaxation in an ultra-cold Gas of magnetically trapped chromium atoms
We have investigated both theoretically and experimentally dipolar relaxation
in a gas of magnetically trapped chromium atoms. We have found that the large
magnetic moment of 6 results in an event rate coefficient for dipolar
relaxation processes of up to cms at a magnetic
field of 44 G. We present a theoretical model based on pure dipolar coupling,
which predicts dipolar relaxation rates in agreement with our experimental
observations. This very general approach can be applied to a large variety of
dipolar gases.Comment: 9 pages, 9 figure
Spontaneous formation of persistent square pattern in a driven superfluid
The emergence of patterns from simple physical laws belongs to the most
striking topics in natural science. In particular, the spontaneous formation of
structures from an initially homogeneous state can eventually lead to stable,
non-homogeneous states of matter. Here we report on the spontaneous formation
of square lattice patterns in a rotationally symmetric and driven Bose-Einstein
condensate, confined in a two-dimensional box potential with absorptive
boundaries. The drive is realized by globally modulating the two-particle
interaction periodically in time. After a primary phase of randomly oriented
stripes that emerge as a consequence of the Faraday instability, we observe the
subsequent formation of persistent square lattice patterns in the highly
occupied regime, where phonon-phonon interactions become relevant. We show
theoretically that this state can be understood as an attractive fixed point of
coupled nonlinear amplitude equations. Establishing the existence of this fixed
point opens the perspective for engineering new, highly correlated states of
matter in driven superfluids.Comment: 9 pages, 5 figure
Condensate Heating by Atomic Losses
Atomic Bose-Einstein condensate is heated by atomic losses. Predicted
depletion ranges from 1% for a uniform 3D condensate to around 10% for a
quasi-1D condensate in a harmonic trap.Comment: 4 pages in RevTex, 1 eps figur
Square Pattern Formation as Stable Fixed Point in Driven Two-Dimensional Bose-Einstein Condensates
We investigate pattern formation in two-dimensional Bose-Einstein condensates
(BECs) caused by temporal periodic modulation of the interatomic interaction.
Temporal modulation of the interaction causes the so-called Faraday instability
in the condensate, which we show generically leads to a stable square grid
density pattern. We take the amplitudes in each of the two directions spanning
the two-dimensional density pattern as order parameters in pattern formation
and derive a set of simultaneous time evolution equations for those order
parameters from the Gross--Pitaevskii (GP) equation with a time-periodic
interaction. We identify the fixed points of the time evolution and show by
stability analysis that the inhomogeneous density exhibits a square grid
pattern as a stable fixed point.Comment: 7 pages, 3 figures. Supplemental material: 9 page
Enhancement and suppression of spontaneous emission and light scattering by quantum degeneracy
Quantum degeneracy modifies light scattering and spontaneous emission. For
fermions, Pauli blocking leads to a suppression of both processes. In contrast,
in a weakly interacting Bose-Einstein condensate, we find spontaneous emission
to be enhanced, while light scattering is suppressed. This difference is
attributed to many-body effects and quantum interference in a Bose-Einstein
condensate.Comment: 4 pages 1 figur
Stability of rotating states in a weakly-interacting Bose-Einstein condensate
We investigate the lowest state of a rotating, weakly-interacting
Bose-Einstein condensate trapped in a harmonic confining potential that is
driven by an infinitesimally asymmetric perturbation. Although in an
axially-symmetric confining potential the gas has an axially-symmetric
single-particle density distribution, we show that in the presence of the small
asymmetric perturbation its lowest state is the one given by the mean-field
approximation, which is a broken-symmetric state. We also estimate the rate of
relaxation of angular momentum when the gas is no longer driven by the
asymmetric perturbation and identify two regimes of "slow" and "fast"
relaxation. States of certain symmetry are found to be more robust.Comment: 6 pages, RevTe
Domain wall propagation in Permalloy nanowires with a thickness gradient
The domain wall nucleation and motion processes in Permalloy nanowires with a
thickness gradient along the nanowire axis have been studied. Nanowires with
widths, w = 250 nm to 3 um and a base thickness of t = 10 nm were fabricated by
electron-beam lithography. The magnetization hysteresis loops measured on
individual nanowires are compared to corresponding nanowires without a
thickness gradient. The Hc vs. t/w curves of wires with and without a thickness
gradient are discussed and compared to micromagnetic simulations. We find a
metastability regime at values of w, where a transformation from transverse to
vortex domain wall type is expected
Two-species mixture of quantum degenerate Bose and Fermi gases
We have produced a macroscopic quantum system in which a Li-6 Fermi sea
coexists with a large and stable Na-23 Bose-Einstein condensate. This was
accomplished using inter-species sympathetic cooling of fermionic Li-6 in a
thermal bath of bosonic Na-23
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