8,433 research outputs found
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
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
Phase diagram of the Kohn-Luttinger superconducting state for bilayer graphene
The effect of the intersite and interplane Coulomb interactions between the
Dirac fermions on the formation of the Kohn-Luttinger superconductivity in
bilayer doped graphene is studied disregarding the effects of the van der Waals
potential of the substrate and both magnetic and non-magnetic impurities. The
phase diagram determining the boundaries of superconductive domains with
different types of symmetry of the order parameter is built using the extended
Hubbard model in the Born weak-coupling approximation with allowance for the
intratomic, interatomic, and interlayer Coulomb interactions between electrons.
It is shown that the Kohn-Luttinger polarization contributions up to the second
order of perturbation theory in the Coulomb interaction inclusively and an
account for the long-range intraplane Coulomb interactions significantly affect
the competition between the superconducting , , and wave
pairings. It is demonstrated that the account for the interplane Coulomb
interaction enhances the critical temperature of the transition to the
superconducting phase.Comment: 10 pages, 7 figure
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