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 $L \sim t^{-1}$. 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 $t^{-3/2}$ 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

Quantum vortex reconnections

We study reconnections of quantum vortices by numerically solving the governing Gross-Pitaevskii equation. We find that the minimum distance between vortices scales differently with time before and after the vortex reconnection. We also compute vortex reconnections using the Biot-Savart law for vortex filaments of infinitesimal thickness, and find that, in this model, reconnection are time-symmetric. We argue that the likely cause of the difference between the Gross-Pitaevskii model and the Biot-Savart model is the intense rarefaction wave which is radiated away from a Gross-Pitaeveskii reconnection. Finally we compare our results to experimental observations in superfluid helium, and discuss the different length scales probed by the two models and by experiments.Comment: 23 Pages, 12 Figure