28 research outputs found
Scattering length of Andreev reflection from quantized vortices in He-
Andreev reflection of thermal quasiparticles from quantized vortices is an
important technique to visualize quantum turbulence in low temperature
He-. We revisit a problem of Andreev reflection from the isolated,
rectilinear vortex line. For quasiparticle excitations whose impact parameters,
defined as distances of the closest approach to the vortex core, do not exceed
some arbitrary value, , we calculate exactly the reflected fraction of the
total flux of excitations incident upon the vortex in the direction orthogonal
to the vortex line. We then define and calculate exactly, as a function of ,
the scattering length, that is the scattering cross-section per unit length of
the vortex line. We also define and calculate the scattering lengths for the
flux of energy carried by thermal excitations, and for the net energy flux
resulting from a (small) temperature gradient, and analyze the dependence of
these scattering lengths on temperature.Comment: 8 pages, 4 figure
Visualizing Pure Quantum Turbulence in Superfluid He: Andreev Reflection and its Spectral Properties
Superfluid He-B in the zero-temperature limit offers a unique means of
studying quantum turbulence by the Andreev reflection of quasiparticle
excitations by the vortex flow fields. We validate the experimental
visualization of turbulence in He-B by showing the relation between the
vortex-line density and the Andreev reflectance of the vortex tangle in the
first simulations of the Andreev reflectance by a realistic 3D vortex tangle,
and comparing the results with the first experimental measurements able to
probe quantum turbulence on length scales smaller than the inter-vortex
separation.Comment: 5 pages, 4 figures, and Supplemental Material (2 pages, 2 figures
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
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
Interaction of ballistic quasiparticles and vortex configurations in superfluid He3-B
The vortex line density of turbulent superfluid He3-B at very low temperature
is deduced by detecting the shadow of ballistic quasiparticles which are
Andreev reflected by quantized vortices. Until now the measured total shadow
has been interpreted as the sum of shadows arising from interactions of a
single quasiparticle with a single vortex. By integrating numerically the
quasi-classical Hamiltonian equations of motion of ballistic quasiparticles in
the presence of nontrivial but relatively simple vortex systems (such as
vortex-vortex and vortex-antivortex pairs and small clusters of vortices) we
show that partial screening can take place, and the total shadow is not
necessarily the sum of the shadows. We have also found that it is possible
that, upon impinging on complex vortex configurations, quasiparticles
experience multiple reflections, which can be classical, Andreev, or both.Comment: To appear in Phys Rev
Orbital viscosity in superfluid He-3-B in a magnetic field.
Orbital viscosity is usually associated with the A phase of superfluid He-3 which has a finite orbital angular momentum even in zero magnetic field. The B phase has no orbital angular momentum in zero magnetic field, but both spin and orbital angular momenta are induced by a field. The Leggett equations for spin dynamics assume that the orbital angular momentum can only change on timescales much longer than those involved in spin dynamics. We calculate the orbital viscosity of the B phase in both the hydrodynamic and ballistic limits. At low temperatures the orbital viscosity becomes vanishingly small which gives rise to the possibility of coupled spin-orbit dynamics
Coherently precessing spin and orbital states in superfluid He-3-B.
The Leggett equations for the spin dynamics of superfluid He-3 give a good description of the whole range of NMR phenomena observed at relatively high temperatures. However these equations assume that the orbital angular momentum of the condensate may only change on timescales much longer than the spin precession period. At the lowest achievable temperatures, the orbital viscosity of the B-phase of superfluid He-3 becomes vanishingly small, giving rise to the possibility of rapid orbital motion. We have reformulated Leggett's equations for the B-phase to allow for fast orbital dynamics in the absence of dissipation. The resulting non-linear equations of motion couple spin and orbital degrees of freedom resulting in qualitatively new phenomena. In particular, they allow for phase-locked precession of the spin and orbital angular momentum around an applied magnetic field. The coupled spin-orbit dynamics may eventually explain the exotic ultra long-lived NMR signals found at the lowest temperatures in He-3-B