Dynamic Remanent Vortices in Superfluid 3He-B

We investigate the decay of vortices in a rotating cylindrical sample of ³He-B, after rotation has been stopped. With decreasing temperature vortex annihilation slows down as the damping in vortex motion, the mutual friction dissipation α(T), decreases almost exponentially. Remanent vortices then survive for increasingly long periods, while they move towards annihilation in zero applied flow. After a waiting period Δt at zero flow, rotation is reapplied and the remnants evolve to rectilinear vortices. By counting these lines, we measure at temperatures above the transition to turbulence ∼0.6 T _c the number of remnants as a function of α(T) and Δt. At temperatures below the transition to turbulence T≲0.55 T _c, remnants expanding in applied flow become unstable and generate in a turbulent burst the equilibrium number of vortices. Here we measure the onset temperature T _on of turbulence as a function of Δt, applied flow velocity v=v _n−v _s, and length of sample L.     

Superfluid Spin-down with Random Unpinning of the Vortices

The so-called creeping motion of the pinned vortices in a rotating superfluid involves random unpinning and vortex motion as two physically separate processes. We argue that such a creeping motion of the vortices need not be (biased) in the direction of an existing radial Magnus force, nor should a constant microscopic radial velocity be assigned to the vortex motion, in contradiction with the basic assumptions of the vortex creep model. We point out internal inconsistencies in the predictions of this model which arise due to this unjustified foundation that ignores the role of the actual torque on the superfluid. The proper spin-down rate of a pinned superfluid is then calculated and turns out to be much less than that suggested in the vortex creep model, hence being of even less observational significance for its possible application in explaining the post-glitch relaxations of the radio pulsars.     

Numerical Calculation of the Interaction of Superfluid Vortices and a Rigid Sphere

We implement a method for the calculation of the effects of a solid sphere on the dynamics of superfluid vortices. We present in detail the derivation of the mathematical formulae used in a computational algorithm which avoids numerical singularities, as well as an adaptive algorithm for convergence tests and diagnostics of the method’s accuracy.     

Dynamics of Quantized Vortices in Superfluid Helium and Rotating Bose-Einstein Condensates

No Heading We study theoretically and numerically the dynamics of quantized vortices in superfluid helium and rotating Bose-Einstein condensates. After reviewing briefly the recent motivation, we discuss these topics with the emphasis on the research done by our group. One of the modern important problems is how superfluid turbulence relates to classical turbulence. First, we show that superfluid turbulence consisting of a vortex tangle has an energy spectrum consistent with the Kolmogorov law. Second, we discuss the vortex states that appear in a rotating channel with counterflow. In the field of atomic-gas Bose-Einstein condensation, the dramatic observations of quantized vortices were made for rotating condensates. By solving numerically the Gross-Pitaevskii equation, we found a whole story of the vortex lattice formation consistent with the observations.PACS numbers: 67.40.Vs, 47.32.Cc, 47.37.+q.     

Charged Tangles of Quantized Vortices in Superfluid 4He

We show that turbulence in superfluid ⁴He at low temperatures can be generated and probed by injected ions trapped on vortex cores. The results of our recent experiments, in which negative ions were injected during short and long periods, in different quantities, and into different applied electric fields, are outlined. Three very different mechanisms of vortex-assisted transport of trapped ions were observed: one is on isolated vortex rings while two others are associated with tangles of vortex lines. It seems there are two different types of vortex tangles that can be characterized by the velocity of ion motion through them: a drifting polarized tangle in a low-dissipation state that mainly advects trapped ions, and a more isotropic tangle in a highly-dissipative state, sustained by a continuous forcing by the ion current in an applied electric field, through which trapped ions move rapidly.     

Time-of-Flight Measurements of Vortices Emitted from Quantum Turbulence in Superfluid 4He

An oscillating obstacle generates quantum turbulence in superfluids, when vortices remained attached to obstacle surfaces or vortex rings collided with it during oscillation. Turbulence provides a source of vortices; however, the characteristics of these vortices are not clear. In the present work, we report the flight of vortices emitted from quantum turbulence in superfluid ⁴He at low temperatures, using vibrating wires as a generator and a detector of vortices. A vortex-free vibrating wire can detect only the first colliding vortex ring, though it will be refreshed after low vibration and be able to detect a vortex ring again. By measuring a period from the start of turbulence generation to the vortex detection repeatedly, we find an exponential distribution of time-of-flights with a non-detection period t ₀ and a mean detection period t ₁, suggesting a Poisson process. Both periods t ₀ and t ₁ increase with increasing distance between a generator and a detector. A vortex flight velocity estimated from period t ₀ suggests that the sizes of the emitted vortex rings distribute to a range smaller than a generator thickness or a generator vibration amplitude. Vortices are emitted radially from a turbulence region, at least in the direction of oscillator vibration.     

The Vortex Generation in Superfluid Helium and the Role of Neighbouring Vortices

The thermal activation or quantum tunneling is believed to be necessary for the vortex nucleated near a wall to escape from its vicinity, and for the vortex nucleated in the bulk to reach the critical size above which it can increase according to its own dynamics. This paper suggests the idea that there exists the possibility to understand these processes (without leaving the framework of hydrodynamics) as the result of the simultaneous presence of several vortices or the interaction between various sections of vortex lines.     

Observation of Remanent Vortices Attached to Rough Boundaries in Superfluid 4He

We report the study on remanent vortices attached to rough boundaries in superfluid ⁴He after the turbulent transition. We used 2.6 µm vibrating wires with smooth surfaces and rough surfaces, a cover box and slow cooling method, in order to investigate the effect of surface roughness on the condition and the number of vortices attached to a wire. The responses of the wire with smooth surfaces show large hysteresis at the turbulent transition. This result indicates that remanent vortices attached between the wire and surrounding boundaries cause turbulence. At first sweep of driving force of the wire with rough surfaces, we also observed hysteresis as large as the case of the smooth wire: at the other sweeps, however, small hysteresis was observed. These results indicate that once turbulence is generated at a wire velocity during first sweep, vortex lines newly attach between rough surfaces of the wire, which easily cause turbulence at a low wire velocity. Therefore, we conclude that a smooth wire can reduce the number of vortices attached to a wire.     

Half-Quantum Vortices in Polar Phase of Superfluid ^{3}He

The magnetic dipole-dipole interaction does not prevent existence of half-quantum vortices in the polar phase of superfluid \(^{3}\) He which can be stable in uniaxial anisotropic aerogel. Here we discuss this exotic possibility. After developing a phenomenological theory of phase transition from the normal to the polar superfluid state and then to axipolar superfluid state we calculate the NMR properties of such type superfluids at the rest or under rotation when liquid is filled by an array either single quantum or half quantum vortices.     

On Vortices Induced in Superfluid Films by Crossed Electric and Magnetic Fields

The behaviour of a superfluid Bose-gas in crossed electric E and magnetic H fields is studied. It is shown that in the case when the two-dimensional divergence div ₂[E×H] differs from zero, the crossed fields induce the appearance of quantized vortex threads in the electroneutral superfluid system.     

Propagation of Quantized Vortices Driven by an Oscillating Sphere in Superfluid 4He

We performed numerical simulation of quantum turbulence at 0 K generated from remnant vortices attached to an oscillating sphere. The remnant vortices are extended by the sphere motion and form a tangle with emitting vortex loops. As time passes, the length of vortices in a computational volume becomes statistically steady. We investigate in the statistical steady state the distribution of the length of vortex loops and anisotropy of their propagation direction caused by the sphere oscillation. The propagation direction of the emitted vortex loops is anisotropic along the oscillation direction of the sphere. The obtained results are consistent with results obtained in the experimental study using vibrating wires in superfluid ⁴He.     

Superfluid 3He in Presence of Spin-Polarized Scattering Centers

The influence of spin-exchange scattering centers on triplet Cooper pairing is considered by exploring the behavior of superfluid ³He in high porosity aerogel, containing ³He atoms localized on the surface of silica strands. The homogeneously distributed isotropic scattering system of spin-polarized impurity centers is adopted as a simple model to investigate the contribution of the spin-exchange scattering channel for quasiparticles to the formation of non-unitary superfluid A₁-phase in the aerogel environment. It is demonstrated that an interference between the potential and exchange parts of quasiparticle scattering from spin-polarized impurity centers can change considerably the temperature width and the spin structure of the A₁-phase in aerogel. PACS numbers: 05.70 Ln, 05.70 Jk, 64.     

Experiments on the Dynamics of Vortices in Superfluid 4He with No Normal Component

Negative ions, probe particles of radius ~ 10–20 ?, can be injected into helium, manipulated and detected. They can be trapped by quantized vortices and hence used as vortex detectors. We show that by observing the change of the ion current caused by rotation of helium one can learn about the presence and dynamics of vortices even at very low-temperatures.     

Coherent Bragg Scattering of Light by a Vortex Lattice in Rotating Superfluid He

Proposed observation of coherent Bragg scattering by a lattice of vortices in uniformly rotating superfulid Helium is examined. We present the calculations of the cross section for different parameter values, as well as simulations of the detected intensity of the Bragg peaks for triangular arrays with varied degrees of distortion. Within this frame, a potentially feasible experiment is outlined to provide direct evidence of vortices with an atomic-scale core in superfluid Helium.     

Energy Spectrum of the Superfluid Velocity Made by Quantized Vortices in Two-Dimensional Quantum Turbulence

We discuss the configurations of vortices in two-dimensional quantum turbulence, studying energy spectrum of superfluid velocity and correlation functions with the distance between two vortices. We apply the above method to quantum turbulence described by Gross-Pitaevskii equation in Bose-Einstein condensates. We make two-dimensional quantum turbulence from many dark solitons through the dynamical instability. A dark soliton is unstable and decays into vortices in two- and three-dimensional systems. In our work, we propose a method of discriminating between the uncorrelated turbulence and the correlated turbulence. We decompose the energy spectrum into two terms, namely the self-energy spectrum E _self (k) made by individual vortices and the interactive energy spectrum E _int (k) made by interference of two vortices. The uncorrelated turbulence is defined as turbulence with E _int (k)?E _self (k), while the correlated turbulence is turbulence where E _int (k) is not much smaller than E _self (k). Our simulations show that in the decay of dark solitons, the vortices created consist of correlated pairs of opposite circulation vortices, leading to the correlated turbulence.     

A Study of the Motion of Particles in Superfluid Helium-4 and Interactions with Vortices

We describe experiments in which a sound pulse is used to inject small particles into liquid helium at temperatures between 1 and 3 K. The motion of these particles is then observed. We are able to observe the effect of applying an impulse to a particle that is attached to a vortex. We also report on a number of surprising motions of particles and discuss possible explanations.     

Superfluid in the Space Between Rotating Spheres II: The Shape of Vortices and Nonequilibrium States

Vortex dynamics is used to consider the following problems concerning vortices in the space between concentric spheres: the equilibrium shape of vortices (they are almost rectilinear if their number is large), the metastable clusters with nonequilibrium number of vortices, the processes of vortex breaking and connection in the vicinity of the inner sphere equator, the annihilation in the vicinity of the outer sphere equator, and the behavior of an almost freely rotating double-sphere device during the variation of the superfluid angular momentum caused by the changes of vortex configuration (similar phenomena are among the possible reasons of pulsars self-acceleration).