Reconnections of quantized vortex rings in superfluid 4^4He at very low temperatures

Collisions in a beam of unidirectional quantized vortex rings of nearly identical radii $R$ in superfluid $^4$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 $500{\rm \,cm}^{-2} R^{-1}$ 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 4^4He in a Torsional Oscillator

We have measured the response of a torsional oscillator containing polycrystalline hcp solid $^{4}$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$^{-1}$ 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 $^4$He attributable to steady rotation.Comment: 8 pages, 3 figures, accepted in J. Low Temp. Phy

Dissipation of Quasiclassical Turbulence in Superfluid 4^4He

We compare the decay of turbulence in superfluid $^4$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 $L$ decays at late time $t$ as $L \propto t^{-3/2}$. 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

Simultaneous Measurements of the Torsional Oscillator Anomaly and Thermal Conductivity in Solid 4He

In these torsional oscillator experiments the samples of solid $^4$He were characterized by measuring their thermal conducitvity. Polycrystalline samples of helium of either high isotopic purity or natural concentration of $^3$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 $^3$He concentration a substantial excess thermal conductivity over the usual $T^3$ dependence was observed below 120 mK.Comment: 4 pages, 3 figure

UML representation of object-oriented design antipatterns

Nowadays the ability to apply, implement and modify patterns of design and architecture has become a one of primary skills for software engineers. Competence of pattern design and implementation involves detecting and correcting inefficient solutions known as antipatterns. However, unlike patterns, very few antipatterns have a graphical representation so that an inefficient solution to a specific problem can be detected visually and refactored. Detecting antipatterns is not simple even with full set of technical documentation. This paper proposes a graphical UML representation of antipatterns to detect them at various stages of the software lifecycle. It proposes a method to refactor described antipatterns to improve software design quality and avoid software development process risks. UML diagrams modeling of 18 antipatterns is presented and refactoring method for all of them was described. Most of antipatterns were diagrammed using information from text descriptions and additional notes about arguable properties of antipatterns were included

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

Thermal conductivity and torsional oscillations of solid ⁴He

Polycrystalline samples of hcp ⁴He of molar volume Vm = 19.5 cm³ with small amount of ³He impurities were grown in an annular container by the blocked-capillary method. Three concentrations of ³He, x₃, were studied: isotopically purified ⁴He with the estimated x₃ < 10⁻¹⁰, commercial ‘well-grade’ helium with x₃ ~ 3⋅10⁻⁷ and a mixture with x₃ = 2.5⋅10⁻⁶. Torsional oscillations at two frequencies, 132.5 and 853.6 Hz, and thermal conductivity were investigated before and after annealing. The solid helium under investigation was located not only in the annular container but also in the axial fill line inside two torsion rods and dummy bob of the doublefrequency torsional oscillator. The analysis of the frequency shifts upon loading with helium and changing temperatures of different parts of the oscillator suggests that the three techniques probe the properties of solid helium in three different locations: the two different torsion modes respond to the changes of the shear modulus of solid helium in either of the two torsion rods while the thermal conductivity probes the phonon mean free path in solid helium inside the annular container. The temperature and width of the torsional anomaly increase with increasing frequency and x₃. The phonon mean free path increases with increasing x3. Annealing typically resulted in an increased phonon mean free path but often in little change in the torsional oscillator response. While the magnitude of the torsional anomaly and phonon mean free path can be very different in different samples, no correlation was found between them

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