355 research outputs found
Ballistic propagation of thermal excitations near a vortex in superfluid He3-B
Andreev scattering of thermal excitations is a powerful tool for studying
quantized vortices and turbulence in superfluid He3-B at very low temperatures.
We write Hamilton's equations for a quasiparticle in the presence of a vortex
line, determine its trajectory, and find under wich conditions it is Andreev
reflected. To make contact with experiments, we generalize our results to the
Onsager vortex gas, and find values of the intervortex spacing in agreement
with less rigorous estimates
Cross-sections of Andreev scattering by quantized vortex rings in 3He-B
We studied numerically the Andreev scattering cross-sections of
three-dimensional isolated quantized vortex rings in superfluid 3He-B at
ultra-low temperatures. We calculated the dependence of the cross-section on
the ring's size and on the angle between the beam of incident thermal
quasiparticle excitations and the direction of the ring's motion. We also
introduced, and investigated numerically, the cross-section averaged over all
possible orientations of the vortex ring; such a cross-section may be
particularly relevant for the analysis of experimental data. We also analyzed
the role of screening effects for Andreev reflection of quasiparticles by
systems of vortex rings. Using the results obtained for isolated rings we found
that the screening factor for a system of unlinked rings depends strongly on
the average radius of the vortex ring, and that the screening effects increase
with decreasing the rings' size.Comment: 11 pages, 8 figures ; submitted to Physical Review
Quasiclassical and ultraquantum decay of superfluid turbulence
This letter addresses the question which, after a decade-long discussion,
still remains open: what is the nature of the ultraquantum regime of decay of
quantum turbulence? The model developed in this work reproduces both the
ultraquantum and the quasiclassical decay regimes and explains their
hydrodynamical natures. In the case where turbulence is generated by forcing at
some intermediate lengthscale, e.g. by the beam of vortex rings in the
experiment of Walmsley and Golov [Phys. Rev. Lett. {\bf 100}, 245301 (2008)],
we explained the mechanisms of generation of both ultraquantum and
quasiclassical regimes. We also found that the anisotropy of the beam is
important for generating the large scale motion associated with the
quasiclassical regime
The saturation of decaying counterflow turbulence in helium II
We are concerned with the problem of the decay of a tangle of quantized
vortices in He II generated by a heat current. Direct application of Vinen's
equation yields the temporal scaling of vortex line density .
Schwarz and Rozen [Phys. Rev. Lett. {\bf 66}, 1898 (1991); Phys. Rev. B {\bf
44}, 7563 (1991)] observed a faster decay followed by a slower decay. More
recently, Skrbek and collaborators [Phys. Rev. E {\bf 67}, 047302 (2003)] found
an initial transient followed by the same classical scaling observed
in the decay of grid-generated turbulence. We present a simple theoretical
model which, we argue, contains the essential physical ingredients, and
accounts for these apparently contradictory results.Comment: 19 pages, 5 figure
Vortex density fluctuations in quantum turbulence
We compute the frequency spectrum of turbulent superfluid vortex density
fluctuations and obtain the same Kolmogorov scaling which has been observed in
a recent experiment in Helium-4. We show that the scaling can be interpreted in
terms of the spectrum of reconnecting material lines. The calculation is
performed using a vortex tree algorithm which considerably speeds up the
evaluation of Biot-Savart integrals.Comment: 7 Pages, 7 figure
A Dynamical Self-Consistent Finite Temperature Kinetic Theory: The ZNG Scheme
We review a self-consistent scheme for modelling trapped weakly-interacting
quantum gases at temperatures where the condensate coexists with a significant
thermal cloud. This method has been applied to atomic gases by Zaremba, Nikuni,
and Griffin, and is often referred to as ZNG. It describes both
mean-field-dominated and hydrodynamic regimes, except at very low temperatures
or in the regime of large fluctuations. Condensate dynamics are described by a
dissipative Gross-Pitaevskii equation (or the corresponding quantum
hydrodynamic equation with a source term), while the non-condensate evolution
is represented by a quantum Boltzmann equation, which additionally includes
collisional processes which transfer atoms between these two subsystems. In the
mean-field-dominated regime collisions are treated perturbatively and the full
distribution function is needed to describe the thermal cloud, while in the
hydrodynamic regime the system is parametrised in terms of a set of local
variables. Applications to finite temperature induced damping of collective
modes and vortices in the mean-field-dominated regime are presented.Comment: Unedited version of chapter to appear in Quantum Gases: Finite
Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms Series). N.P.
Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds. Imperial
College Press, London (in press). See
http://www.icpress.co.uk/physics/p817.htm
- …