442 research outputs found
Dark soliton collisions in a toroidal Bose-Einstein condensate
We study the dynamics of two gray solitons in a Bose-Einstein condensate
confined by a toroidal trap with a tight confinement in the radial direction.
Gross-Pitaevskii simulations show that solitons can be long living objects
passing through many collisional processes. We have observed quite different
behaviors depending on the soliton velocity. Very slow solitons, obtained by
perturbing the stationary solitonic profile, move with a constant angular
velocity until they collide elastically and move in the opposite direction
without showing any sign of lowering their energy. In this case the density
notches are always well separated and the fronts are sharp and straight. Faster
solitons present vortices around the notches, which play a central role during
the collisions. We have found that in these processes the solitons lose energy,
as the outgoing velocity turns out to be larger than the incoming one. To study
the dynamics, we model the gray soliton state with a free parameter that is
related to the soliton velocity. We further analyze the energy, soliton
velocity and turning points in terms of such a free parameter, finding that the
main features are in accordance with the infinite one-dimensional system.Comment: 15 pages, 11 figures. Accepted in PR
Two-mode effective interaction in a double-well condensate
We investigate the origin of a disagreement between the two-mode model and
the exact Gross-Pitaevskii dynamics applied to double-well systems. In general
this model, even in its improved version, predicts a faster dynamics and
underestimates the critical population imbalance separating Josephson and
self-trapping regimes. We show that the source of this mismatch in the dynamics
lies in the value of the on-site interaction energy parameter. Using simplified
Thomas-Fermi densities, we find that the on-site energy parameter exhibits a
linear dependence on the population imbalance, which is also confirmed by
Gross-Pitaevskii simulations. When introducing this dependence in the two-mode
equations of motion, we obtain a reduced interaction energy parameter which
depends on the dimensionality of the system. The use of this new parameter
significantly heals the disagreement in the dynamics and also produces better
estimates of the critical imbalance.Comment: 5 pages, 4 figures, accepted in PR
The variety generated by order algebras
Every ordered set can be considered as an algebra in a natural way. We investigate the variety generated by order algebras. We prove, among other things, that this variety is not finitely based and, although locally finite, it is not contained in any finitely generated variety; we describe the bottom of the lattice of its subvarieties
Important amino acid residues of potato plant uncoupling protein (StUCP)
Chemical modifications were used to identify some of the functionally important amino acid residues of the potato plant uncoupling protein (StUCP). The proton-dependent swelling of potato mitochondria in K+-acetate in the presence of linoleic acid and valinomycin was inhibited by mersalyl (Ki = 5 µM) and other hydrophilic SH reagents such as Thiolyte MB, iodoacetate and 5,5'-dithio-bis-(2-nitrobenzoate), but not by hydrophobic N-ethylmaleimide. This pattern of inhibition by SH reagents was similar to that of brown adipose tissue uncoupling protein (UCP1). As with UCP1, the arginine reagent 2,3-butadione, but not N-ethylmaleimide or other hydrophobic SH reagents, prevented the inhibition of StUCP-mediated transport by ATP in isolated potato mitochondria or with reconstituted StUCP. The results indicate that the most reactive amino acid residues in UCP1 and StUCP are similar, with the exception of N-ethylmaleimide-reactive cysteines in the purine nucleotide-binding site.1413142
Quantum cavitation in liquid He: dissipation effects
We have investigated the effect that dissipation may have on the cavitation
process in normal liquid He. Our results indicate that a rather small
dissipation decreases sizeably the quantum-to-thermal crossover temperature
for cavitation in normal liquid He. This is a possible explanation
why recent experiments have not yet found clear evidence of quantum cavitation
at temperatures below the predicted by calculations which neglect
dissipation.Comment: To be published in Physical Review B6
Longitudinal and transverse forces on a vortex in superfluid He
We examine the transverse and longitudinal components of the drag force upon
a straight vortex line due to the scattering of liquid He excitations. For
this purpose, we consider a recently proposed Hamiltonian that describes the
dissipative motion of a vortex, giving an explicit expression for the
vortex-quasiparticle interaction. The involved dissipative coefficients are
obtained in terms of the reservoir correlation function. Most of our explicit
calculations are concerned to the range of temperatures below 0.4 K, at which
the reservoir is composed by phonon quasiparticle excitations. We also discuss
some important implications in the determination of possible scattering
processes leading to dissipation, according to the values of vortex mass found
in the literature.Comment: 10 pages, 2 figures, latex2e (iopart.cls), enlarged version submitted
to J. Phys.: Condensed Matte
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