8,565 research outputs found
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
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
Small-scale phase separation in doped anisotropic antiferromagnets
We analyze the possibility of the nanoscale phase separation manifesting
itself in the formation of ferromagnetic (FM) polarons (FM droplets) in the
general situation of doped anisotropic three- and two-dimensional
antiferromagnets. In these cases, we calculate the shape of the most
energetically favorable droplets. We show that the binding energy and the
volume of a FM droplet in the three-dimensional (3D) case depend only upon two
universal parameters and , where and are effective
antiferromagnetic (AFM) exchange and hopping integrals, respectively. In the
two-dimensional (2D) case, these parameters have the form and . The most favorable shape of a
ferromagnetic droplet corresponds to an ellipse in the 2D case and to an
ellipsoid in the 3D case.Comment: 6 pages, 1 figure, RevTe
Evolution of a Bose-condensed gas under variations of the confining potential
We discuss the dynamic properties of a trapped Bose-condensed gas under
variations of the confining field and find analytical scaling solutions for the
evolving coherent state (condensate). We further discuss the characteristic
features and the depletion of this coherent state.Comment: 4 pages, no postscript figure
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