8,061 research outputs found
Motion of a condensate in a shaken and vibrating harmonic trap
The dynamics of a Bose-Einstein condensate (BEC) in a time-dependent harmonic
trapping potential is determined for arbitrary variations of the position of
the center of the trap and its frequencies. The dynamics of the BEC wavepacket
is soliton-like. The motion of the center of the wavepacket, and the spatially
and temporally dependent phase (which affects the coherence properties of the
BEC) multiplying the soliton-like part of the wavepacket, are analytically
determined.Comment: Accepted for publication in J. Phys. B: At Mol Opt Phy
Calculation of the microcanonical temperature for the classical Bose field
The ergodic hypothesis asserts that a classical mechanical system will in
time visit every available configuration in phase space. Thus, for an ergodic
system, an ensemble average of a thermodynamic quantity can equally well be
calculated by a time average over a sufficiently long period of dynamical
evolution. In this paper we describe in detail how to calculate the temperature
and chemical potential from the dynamics of a microcanonical classical field,
using the particular example of the classical modes of a Bose-condensed gas.
The accurate determination of these thermodynamics quantities is essential in
measuring the shift of the critical temperature of a Bose gas due to
non-perturbative many-body effects.Comment: revtex4, 10 pages, 1 figure. v2: updated to published version. Fuller
discussion of numerical results, correction of some minor error
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
Manifestation of superfluidity in an evolving Bose-condensed gas
We study the generation of excitations due to an ''impurity''(static
perturbation) placed into an oscillating Bose-condensed gas in the
time-dependent trapping field. It is shown that there are two regions for the
position of the local perturbation. In the first region the condensate flows
around the ''impurity'' without generation of excitations demonstrating
superfluid properties. In the second region the creation of excitations occurs,
at least within a limited time interval, revealing destruction of
superfluidity. The phenomenon can be studied by measuring the damping of
condensate oscillations at different positions of the ''impurity''
Zero-Temperature Structures of Atomic Metallic Hydrogen
Ab initio random structure searching with density functional theory was used
to determine the zero-temperature structures of atomic metallic hydrogen from
500 GPa to 5 TPa. Including zero point motion in the harmonic approximation, we
estimate that molecular hydrogen dissociates into a monatomic body-centered
tetragonal structure near 500 GPa (r_s = 1.225), which then remains stable to
2.5 TPa (r_s = 0.969). At higher pressures, hydrogen stabilizes in an
...ABCABC... planar structure that is remarkably similar to the ground state of
lithium, which compresses to the face-centered cubic lattice beyond 5 TPa (r_s
< 0.86). At this level of theory, our results provide a complete ab initio
description of the atomic metallic structures of hydrogen, resolving one of the
most fundamental and long outstanding issues concerning the structures of the
elements.Comment: 9 pages; 4 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
Long-Term Clustering, Scaling, and Universality in the Temporal Occurrence of Earthquakes
Scaling analysis reveals striking regularities in earthquake occurrence. The
time between any one earthquake and that following it is random, but it is
described by the same universal-probability distribution for any spatial region
and magnitude range considered. When time is expressed in rescaled units, set
by the averaged seismic activity, the self-similar nature of the process
becomes apparent. The form of the probability distribution reveals that
earthquakes tend to cluster in time, beyond the duration of aftershock
sequences. Furthermore, if aftershock sequences are analysed in an analogous
way, yet taking into account the fact that seismic activity is not constant but
decays in time, the same universal distribution is found for the rescaled time
between events.Comment: short paper, only 2 figure
Exact Results for Three-Body Correlations in a Degenerate One-Dimensional Bose Gas
Motivated by recent experiments we derive an exact expression for the
correlation function entering the three-body recombination rate for a
one-dimensional gas of interacting bosons. The answer, given in terms of two
thermodynamic parameters of the Lieb-Liniger model, is valid for all values of
the dimensionless coupling and contains the previously known results
for the Bogoliubov and Tonks-Girardeau regimes as limiting cases. We also
investigate finite-size effects by calculating the correlation function for
small systems of 3, 4, 5 and 6 particles.Comment: 4 pages, 2 figure
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