11,683 research outputs found
Universal dynamics on the way to thermalisation
It is demonstrated how a many-body system far from thermal equilibrium can
exhibit universal dynamics in passing a non-thermal fixed point. As an example,
the process of Bose-Einstein (BE) condensation of a dilute cold gas is
considered. If the particle flux into the low-energy modes, induced, e.g., by a
cooling quench, is sufficiently strong, the Bose gas develops a characteristic
power-law single-particle spectrum , and critical slowing down
in time occurs. The fixed point is shown to be marked by the creation and
dilution of tangled vortex lines. Alternatively, for a weak cooling quench and
particle flux, the condensation process runs quasi adiabatically, passing by
the fixed point in far distance, and signatures of critical scaling remain
absent.Comment: 5+2 pages, 8 figure
Formation of a condensed state with macroscopic number of phonons in ultracold Bose gases
A mechanism for the formation of a new type of stationary state with
macroscopical number of phonons in condensed atomic gases is proposed. This
mechanism is based on generating longitudinal phonons as a result of parametric
resonance caused by a permanent modulation of the transverse trap frequency in
an elongated trap. The phonon-phonon interaction predetermines the
self-consistent evolution which is completed with macroscopic population of one
from all levels within the energy interval of parametric amplification. This
level proves to be shifted to the edge of this interval. All other levels end
the evolution with zero population.Comment: 9 pages, 8 figure
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''
Collapse and Bose-Einstein condensation in a trapped Bose-gas with negative scattering length
We find that the key features of the evolution and collapse of a trapped Bose
condensate with negative scattering length are predetermined by the particle
flux from the above-condensate cloud to the condensate and by 3-body
recombination of Bose-condensed atoms. The collapse, starting once the number
of Bose-condensed atoms reaches the critical value, ceases and turns to
expansion when the density of the collapsing cloud becomes so high that the
recombination losses dominate over attractive interparticle interaction. As a
result, we obtain a sequence of collapses, each of them followed by dynamic
oscillations of the condensate. In every collapse the 3-body recombination
burns only a part of the condensate, and the number of Bose-condensed atoms
always remains finite. However, it can comparatively slowly decrease after the
collapse, due to the transfer of the condensate particles to the
above-condensate cloud in the course of damping of the condensate oscillations.Comment: 11 pages, 3 figure
Accidental suppression of Landau damping of the transverse breathing mode in elongated Bose-Einstein condensates
We study transverse radial oscillations of an elongated Bose-Einstein
condensate using finite temperature simulations, in the context of a recent
experiment at ENS. We demonstrate the existence of a mode corresponding to an
in-phase collective oscillation of both the condensate and thermal cloud.
Excitation of this mode accounts for the very small damping rate observed
experimentally, and we find excellent quantitative agreement between experiment
and theory. In contrast to other condensate modes, interatomic collisions are
found to be the dominant damping mechanism in this case.Comment: 4 pages, 3 figure
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
Measurement of positive and negative scattering lengths in a Fermi gas of atoms
An exotic superfluid phase has been predicted for an ultracold gas of
fermionic atoms. This phase requires strong attractive interactions in the gas,
or correspondingly atoms with a large, negative s-wave scattering length. Here
we report on progress toward realizing this predicted superfluid phase. We
present measurements of both large positive and large negative scattering
lengths in a quantum degenerate Fermi gas of atoms. Starting with a
two-component gas that has been evaporatively cooled to quantum degeneracy, we
create controllable, strong interactions between the atoms using a
magnetic-field Feshbach resonance. We then employ a novel rf spectroscopy
technique to directly measure the mean-field interaction energy, which is
proportional to the s-wave scattering length. Near the peak of the resonance we
observe a saturation of the interaction energy; it is in this strongly
interacting regime that superfluidity is predicted to occur. We have also
observed anisotropic expansion of the gas, which has recently been suggested as
a signature of superfluidity. However, we find that this can be attributed to a
purely collisional effect
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
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