11,399 research outputs found
Non-exponential one-body loss in a Bose-Einstein condensate
We have studied the decay of a Bose-Einstein condensate of metastable helium
atoms in an optical dipole trap. In the regime where two- and three-body losses
can be neglected we show that the Bose-Einstein condensate and the thermal
cloud show fundamentally different decay characteristics. The total number of
atoms decays exponentially with time constant tau; however, the thermal cloud
decays exponentially with time constant (4/3)tau and the condensate decays much
faster, and non-exponentially. We show that this behaviour, which should be
present for all BECs in thermal equilibrium with a considerable thermal
fraction, is due to a transfer of atoms from the condensate to the thermal
cloud during its decay.Comment: The intuitive explanation of the atomic transfer effect has been
correcte
Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal Fixed Point
Critical dynamics of an ultracold Bose gas far from equilibrium is studied in
two spatial dimensions. Superfluid turbulence is created by quenching the
equilibrium state close to zero temperature. Instead of immediately
re-thermalizing, the system approaches a meta-stable transient state,
characterized as a non-thermal fixed point. A focus is set on the vortex
density and vortex-antivortex correlations which characterize the evolution
towards the non-thermal fixed point and the departure to final
(quasi-)condensation. Two distinct power-law regimes in the vortex-density
decay are found and discussed in terms of a vortex binding-unbinding transition
and a kinetic description of vortex scattering. A possible relation to decaying
turbulence in classical fluids is pointed out. By comparing the results to
equilibrium studies of a two-dimensional Bose gas, an intuitive understanding
of the location of the non-thermal fixed point in a reduced phase space is
developed.Comment: 11 pages, 13 figures; PRA versio
Cold Collision Frequency Shift in Two-Dimensional Atomic Hydrogen
We report a measurement of the cold collision frequency shift in atomic
hydrogen gas adsorbed on the surface of superfluid 4He at T<=90 mK. Using
two-photon electron and nuclear magnetic resonance in 4.6 T field we separate
the resonance line shifts due to the dipolar and exchange interactions, both
proportional to surface density sigma. We find the clock shift Delta v_c =
-1.0(1)x10^-7 Hz cm^-2 x sigma, which is about 100 times smaller than the value
predicted by the mean field theory and known scattering lengths in the 3D case.Comment: 4 pages, 3 figure
Adaptively Smoothed Seismicity Earthquake Forecasts for Italy
We present a model for estimating the probabilities of future earthquakes of
magnitudes m > 4.95 in Italy. The model, a slightly modified version of the one
proposed for California by Helmstetter et al. (2007) and Werner et al. (2010),
approximates seismicity by a spatially heterogeneous, temporally homogeneous
Poisson point process. The temporal, spatial and magnitude dimensions are
entirely decoupled. Magnitudes are independently and identically distributed
according to a tapered Gutenberg-Richter magnitude distribution. We estimated
the spatial distribution of future seismicity by smoothing the locations of
past earthquakes listed in two Italian catalogs: a short instrumental catalog
and a longer instrumental and historical catalog. The bandwidth of the adaptive
spatial kernel is estimated by optimizing the predictive power of the kernel
estimate of the spatial earthquake density in retrospective forecasts. When
available and trustworthy, we used small earthquakes m>2.95 to illuminate
active fault structures and likely future epicenters. By calibrating the model
on two catalogs of different duration to create two forecasts, we intend to
quantify the loss (or gain) of predictability incurred when only a short but
recent data record is available. Both forecasts, scaled to five and ten years,
were submitted to the Italian prospective forecasting experiment of the global
Collaboratory for the Study of Earthquake Predictability (CSEP). An earlier
forecast from the model was submitted by Helmstetter et al. (2007) to the
Regional Earthquake Likelihood Model (RELM) experiment in California, and, with
over half of the five-year experiment over, the forecast performs better than
its competitors.Comment: revised manuscript. 22 pages, 3 figures, 2 table
Observation of Phase Fluctuations in Bose-Einstein Condensates
The occurrence of phase fluctuations due to thermal excitations in
Bose-Einstein condensates (BECs) is studied for a variety of temperatures and
trap geometries. We observe the statistical nature of the appearence of phase
fluctuations and characterize the dependence of their average value on
temperature, number of particles and the trapping potential. We find pronounced
phase fluctuations for condensates in very elongated traps in a broad
temperature range. The results are of great importance for the realization of
BEC in quasi 1D geometries, for matter wave interferometry with BECs, as well
as for coherence properties of guided atom laser beams.Comment: 4 pages, 4 figure
Possible role of 3He impurities in solid 4He
We use a quantum lattice gas model to describe essential aspects of the
motion of 4He atoms and of 3He impurities in solid 4He. This study suggests
that 3He impurities bind to defects and promote 4He atoms to interstitial sites
which can turn the bosonic quantum disordered crystal into a metastable
supersolid. It is suggested that defects and interstitial atoms are produced
during the solid 4He nucleation process where the role of 3He impurities (in
addition to the cooling rate) is known to be important even at very small (1
ppm) impurity concentration. It is also proposed that such defects can form a
glass phase during the 4He solid growth by rapid cooling.Comment: 4 two-column Revtex pages, 4 figures. Europhysics Letters (in Press
Adsorption and two-body recombination of atomic hydrogen on He-He mixture films
We present the first systematic measurement of the binding energy of
hydrogen atoms to the surface of saturated He-He mixture films.
is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the
population of the ground surface state of He grows from zero to
cm, yielding the value K cm
for the mean-field parameter of H-He interaction in 2D. The experiments
were carried out with overall He concentrations ranging from 0.1 ppm to 5 %
as well as with commercial and isotopically purified He at temperatures
70...400 mK. Measuring by ESR the rate constants and for
second-order recombination of hydrogen atoms in hyperfine states and we
find the ratio to be independent of the He content and to
grow with temperature.Comment: 4 pages, 4 figures, all zipped in a sigle file. Submitted to Phys.
Rev. Let
Nonlinear Dynamics of a Bose-Einstein Condensate in a Magnetic Waveguide
We have studied the internal and external dynamics of a Bose-Einstein
condensate in an anharmonic magnetic waveguide. An oscillating condensate
experiences a strong coupling between the center of mass motion and the
internal collective modes. Due to the anharmonicity of the magnetic potential,
not only the center of mass motion shows harmonic frequency generation, but
also the internal dynamics exhibit nonlinear frequency mixing. We describe the
data with a theoretical model to high accuracy. For strong excitations we test
the experimental data for indications of a chaotic behavior.Comment: 4 pages, 4 figure
Simulations of thermal Bose fields in the classical limit
We demonstrate that the time-dependent projected Gross-Pitaevskii equation
derived earlier [Davis, et al., J. Phys. B 34, 4487 (2001)] can represent the
highly occupied modes of a homogeneous, partially-condensed Bose gas. We find
that this equation will evolve randomised initial wave functions to
equilibrium, and compare our numerical data to the predictions of a gapless,
second-order theory of Bose-Einstein condensation [S. A. Morgan, J. Phys. B 33,
3847 (2000)]. We find that we can determine the temperature of the equilibrium
state when this theory is valid.
Outside the range of perturbation theory we describe how to measure the
temperature of our simulations. We also determine the dependence of the
condensate fraction and specific heat on temperature for several interaction
strengths, and observe the appearance of vortex networks. As the
Gross-Pitaevskii equation is non-perturbative, we expect that it can describe
the correct thermal behaviour of a Bose gas as long as all relevant modes are
highly occupied.Comment: 15 pages, 12 figures, revtex4, follow up to Phys. Rev. Lett. 87
160402 (2001). v2: Modified after referee comments. Extra data added to two
figures, section on temperature determination expande
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