49 research outputs found
Reconnections of quantized vortex rings in superfluid He at very low temperatures
Collisions in a beam of unidirectional quantized vortex rings of nearly
identical radii in superfluid He in the limit of zero temperature (0.05
K) were studied using time-of-flight spectroscopy. Reconnections between two
primary rings result in secondary vortex loops of both smaller and larger
radii. Discrete steps in the distribution of flight times, due to the limits on
the earliest possible arrival times of secondary loops created after either one
or two consecutive reconnections, are observed. The density of primary rings
was found to be capped at the value independent of
the injected density. This is due to collisions between rings causing piling-up
of many other vortex rings. Both observations are in quantitative agreement
with our theory.Comment: 7 pages, 4 figures, includes supplementary materia
No Effect of Steady Rotation on Solid He in a Torsional Oscillator
We have measured the response of a torsional oscillator containing
polycrystalline hcp solid He to applied steady rotation in an attempt to
verify the observations of several other groups that were initially interpreted
as evidence for macroscopic quantum effects. The geometry of the cell was that
of a simple annulus, with a fill line of relatively narrow diameter in the
centre of the torsion rod. Varying the angular velocity of rotation up to
2\,rad\,s showed that there were no step-like features in the resonant
frequency or dissipation of the oscillator and no history dependence, even
though we achieved the sensitivity required to detect the various effects seen
in earlier experiments on other rotating cryostats. All small changes during
rotation were consistent with those occurring with an empty cell. We thus
observed no effects on the samples of solid He attributable to steady
rotation.Comment: 8 pages, 3 figures, accepted in J. Low Temp. Phy
Dissipation of Quasiclassical Turbulence in Superfluid He
We compare the decay of turbulence in superfluid He produced by a moving
grid to the decay of turbulence created by either impulsive spin-down to rest
or by intense ion injection. In all cases the vortex line density decays at
late time as . At temperatures above 0.8 K, all methods
result in the same rate of decay. Below 0.8 K, the spin-down turbulence
maintains initial rotation and decays slower than grid turbulence and ion-jet
turbulence. This may be due to a decoupling of the large-scale superfluid flow
from the normal component at low temperatures, which changes its effective
boundary condition from no-slip to slip.Comment: Main article: 5 pages, 3 figures. Supplemental material: 4 pages, 3
figures. Accepted for publication in Physical Review Letter
Longitudinal NMR and Spin States in the A-like Phase of 3He in Aerogel
It was found that two different spin states of the A-like phase can be
obtained in aerogel sample. In one of these states we have observed the signal
of the longitudinal NMR, while in another state no trace of such a signal was
found. The states also have different properties in transverse NMR experiments.
Longitudinal NMR signal was also observed in the B-like phase of 3He in
aerogel.Comment: 8 pages, 7 figure
Simultaneous Measurements of the Torsional Oscillator Anomaly and Thermal Conductivity in Solid 4He
In these torsional oscillator experiments the samples of solid He were
characterized by measuring their thermal conducitvity. Polycrystalline samples
of helium of either high isotopic purity or natural concentration of He
were grown in an annular container by the blocked-capillary method and
investigated before and after annealing. No correlation has been found between
the magnitude of the low-temperature shift of the torsional oscillator
frequency and the amount of crystalline defects as measured by the thermal
conductivity. In samples with the natural He concentration a substantial
excess thermal conductivity over the usual dependence was observed below
120 mK.Comment: 4 pages, 3 figure
Soliton-like Spin State in the A-like Phase of 3He in Anisotropic Aerogel
We have found a new stable spin state in the A-like phase of superfluid 3He
confined to intrinsically anisotropic aerogel. The state can be formed by
radiofrequency excitation applied while cooling through the superfluid
transition temperature and its NMR properties are different from the standard
A-like phase obtained in the limit of very small excitation. It is possible
that this new state is formed by textural domain walls pinned by aerogel.Comment: 9 pages, 3 figures. Submitted to J. of Low Tem. Phys. (QFS2007
Proceedings
A low-frequency, high-amplitude, torsional oscillator for turbulence studies in quantum fluids
We describe a new type of torsional oscillator, suitable for studies of quantum fluids at frequencies of ∼ 100 Hz, but capable of reaching high velocities of up to several cm\,s−1. This requires the oscillator amplitude to exceed 100 μm, which is much too large for a conventional capacitor-driven device. We describe the new geometry for the oscillator, discuss its design, and report our initial tests of its performance
Observation of Crossover from Ballistic to Diffusion Regime for Excimer Molecules in Superfluid He
We have measured the temperature dependence of the time of flight of helium
excimer molecules He2* in superfluid 4He and find that the molecules behave
ballistically below 100mK and exhibit Brownian motion above 200 mK. In the
intermediate temperature range the transport cannot be described by either of
the models.Comment: 8 pages, 6 figures, submitted to the Proceedings of the International
Conference on Quantum Fluids and Solids 201