Faraday waves and vortices on the surface of superfluid He II

It has been shown that the interaction between Faraday waves excited on the surface of He II by intense vertical vibrations of the sample cell can lead to the emergence of macroscopic vortices on the surface of the superfluid liquid, as in the case of an ordinary classical liquid. Visualization of phenomena on the surface of He II by light glass tracers with a diameter of ~0.2 mm localized beneath the surface of He II has allowed observing the emergence of a vortex lattice on the surface of the vibrating superfluid liquid in square and cylindrical cells and the evolution of the vortex structure after switching off the pumping.     

Magnetic crystallization waves in He

The melting and growth of ³He crystals spin-polarized by an external magnetic field at temperatures below the Neel temperature are accompanied by spin supercurrents in addition to mass flows. In fields weak in comparison with exchange fields, the crystallization waves change in nature, because the spin currents begin to play a dominent role. The wave spectrum becomes linear with a velocity inversely proportional to the magnetic field. The attenuation of the waves at low enough temperatures is mainly due to the interaction of the moving crystal-liquid interface with thermal spin waves in the crystal. The waves could be weakly damped at temperatures below a few hundred microkelvins.     

Kelvin waves cascade in superfluid turbulence

We study numerically the interaction of four initial superfluid vortex rings in the absence of any dissipation or friction. We find evidence for a cascade of Kelvin waves generated by individual vortex reconnection events which transfers energy to higher and higher wave numbers k. After the vortex reconnections occur, the energy spectrum scales as k(-1) and the curvature spectrum becomes flat. These effects highlight the importance of Kelvin waves and reconnections in the transfer of energy within a turbulent vortex tangle.     

The observation of viscous waves in a He I film

The transverse acoustic impedance of a ⁴He film has been measured at 3.11 K and 20.5 MHz and shows the occurence of standing shear waves in the film. This is in good agreement with calculations using transmission line theory and gives a new method of measuring the thickness of liquid films.     

Kelvin-wave cascade on a vortex in superfluid 4He at a very low temperature

A study by computer simulation is reported of the behavior of a quantized vortex line at a very low temperature when there is continuous excitation of low-frequency Kelvin waves. There is no dissipation except by phonon radiation at a very high frequency. It is shown that nonlinear coupling leads to a net flow of energy to higher wave numbers and to the development of a simple spectrum of Kelvin waves that is insensitive to the strength and frequency of the exciting drive. The results are likely to be relevant to the decay of turbulence in superfluid 4He at very low temperatures.     

Temperature dependent specular reflection of 3He quasiparticles from a wall in He II

Measurements of the transverse acoustic impedance of dilute solutions of ³He in superfluid ⁴He show that the fraction of ³He quasiparticles which are specularly reflected at a plane wall is temperature dependent between 0.1 and 1 K. Values of the ³He relaxation time are also obtained.     

Relaxation-time analysis of second-sound data in NaF

In an earlier paper, starting from the phonon Boltzmann equation, expressions were derived which give velocityc ₂ and damping of second sound in a Debye solid in terms of mean relaxation times for various scattering processes. Using those results, heat-pulse data showing second sound in NaF were analyzed. The experimental values ofc ₂ (T) can only be reproduced if \(\bar \tau _N^{ - 1}\) , the mean relaxation rate for normal phonon collisions, varies with temperature asT ³. The mean Umklapp collision time also shows some unexpected behavior.     

Isotropization of two-dimensional hydrodynamic turbulence in the direct cascade

We present results of numerical simulation of the direct cascade in two-dimensional hydrodynamic turbulence (with spatial resolution up to ). If at the earlier stage (at the time of order of the inverse pumping growth rate τ-Γ_max ^?1), the turbulence develops according to the same scenario as in the case of a freely decaying turbulence [1, 2]: quasi-singular distribution of di-vorticity are formed, which in k-space correspond to jets, leading to a strong turbulence anisotropy, then for times of the order of 10τ turbulence becomes almost isotropic. In particular, at these times any significant anisotropy in the angular fluctuations for the energy spectrum (for a fixed k) is not visible, while the probability distribution function of vorticity for large arguments has the exponential tail with the exponent linearly dependent on vorticity, in the agreement with the theoretical prediction [3].     

Dineutron Formation and Breaking in 8He

We investigate the structures of the 0⁺ states in ⁸He from the viewpoint of dineutron correlation, i.e., spatial correlation between two neutrons coupled to a spin singlet. In this paper, we mainly discuss the \({0^+_2}\) state which is a candidate for the two-dineutron condensate state. This state has one α and two compact dineutrons expanded widely with little correlation with each other to form a gaslike structure.     

Fluctuations in turbulent He II counterflow

We have measured power spectra of vortex line density fluctuations in steady turbulent counterflow in a high aspect ratio channel using a second sound burst technique as a local probe. Our results are compared with previous measurements and theoretical predictions.     

Spectroscopy of barium ions in He II

The excitation and emission spectra of Ba⁺ immersed in superfluid helium have been measured. The two absorption bands - several nm wide — are slightly blue shifted with respect to the D₁ and D₂ lines of the free ion. Three emission bands at λ₁ = 491 nm, λ₂ = 523 nm and λ₃ = 648 nm have been observed.     

Formation of transverse waves in oblique detonations

The structure of oblique detonation waves stabilized on a hypersonic wedge in mixtures characterized by a large activation energy is investigated via steady method of characteristics (MoC) calculations and unsteady computational flowfield simulations. The steady MoC solutions show that, after the transition from shock-induced combustion to an overdriven oblique detonation, the shock and reaction complex exhibit a spatial oscillation. The degree of overdrive required to suppress this oscillation was found to be nearly equal to the overdrive required to force a one-dimensional piston-driven detonation to be stable, demonstrating the equivalence of two-dimensional steady oblique detonations and one-dimensional unsteady detonations. Full unsteady computational simulations of the flowfield using an adaptive refinement scheme showed that these spatial oscillations are transient in nature, evolving in time into transverse waves on the leading shock front. The formation of left-running transverse waves (facing upstream) precedes the formation of right-running transverse waves (facing downstream). Both sets of waves are convected downstream away from the wedge in the supersonic flow behind the leading oblique front, such that the mechanism of instability must continuously generate new transverse waves from an initially uniform flow. Together, these waves define a cellular structure that is qualitatively similar to a normal propagating detonation.     

Formation of nitrogen oxides in detonation waves

Based on detailed kinetic calculations and experimental data, it is demonstrated that the emission of nitrogen oxides from detonation burner units (DBUs) is significantly lower than that from powerful conventional burners with similar characteristics. Under certain conditions, realized largely in DBUs with rotating detonation, the main component of the nitrogen oxides may turn out to be N₂O.     

Decay of a turbulent cascade of capillary waves at the surface of liquid hydrogen

Free decay of a turbulent cascade of capillary waves was experimentally studied at the surface of liquid hydrogen after switching off harmonic pumping. It was found that the cascade starts to decay at the high-frequency side of the frequency spectrum and proceeds in the quasi-stationary regime. The characteristic relaxation time of the whole cascade proves to be close to the viscous-damping time of the wave, whose frequency coincides with the surface-excitation frequency.     

Spinor waves in a space-time lattice (II)

In a previous note, an exceptional space-time lattice was found by a roundabout heuristic process. This process was far from convincing; here a more translucent characterization of the lattice is presented. A cornerstone is the consideration of pairs of reciprocal lattices, together with the basic symmetry (S ₄) of the metric tensor. The basic requirement is that one member of a pair of reciprocal lattices contains the other as a sublattice. One preferred lattice is discussed in some detail; it contains three copies of its reciprocal lattice, and it is the simplest example satisfying the requirements. In the expression of the metric tensor in terms of the lattice generators a possible topology on the lattice is suggested. By means of this topology, propagation of spinor waves can be formulated. This proposed—the simplest—propagation mechanism is inhibited, though, by the fact that the three sublattices are required to carry the two types of spinors alternatively. This inhibition can be lifted by introducing a second type of elementary propagation, to next nearest neighbors. If this inhibition is only feebly lifted, this would result in particles with mass small as compared to the inverse of the lattice constant, presumably the Planck mass. Including the propagation to next nearest neighbors leads to spinor waves with six components, two components for each sublattice. In the long-wavelength limit four of them obey a massive Dirac equation, while the remaining two obey a Weyl equation. These considerations conceivably provide a root for the lack of parity invariance in nature, and for the joint occurrence of pairs of massive and massless spinor waves. The construction, furthermore, allows one to accommodate just three different families of spinor waves of this type. Extension of the above arguments outside the realm of the long-wavelength limit forcibly makes the lattice concept independent of the original continuous Minkowski spacetime: the latter is no longer a unique embedding space for the lattice, but appears as anapproximate interpolation, valid near the long-wavelength limit. This may be the minimal requirement to be imposed on a lattice theory in the light of the empirical evidence, if the scale of the lattice structure is, compared to the empirical scales, as small as the Planck scale.     

Disintegration of the primordial 4He by electromagnetic cascade in the early Universe

The results of calculations of ³He+³H and D production through the disintegration of the primordial ⁴He by electromagnetic cascade in the early Universe are presented. An electromagnetic cascade initiated by a high-energy particle and developing in cosmic plasma starts to disintegrate ⁴He intensively after t～4×10⁴ s. For certain parameters of the source (sorts of particles, energy, and time of appearance in the Universe), this mechanism dominates in the extra production of ³He and ³H. For example, an electromagnetic cascade originating from the thermal proton-antiproton annihilation is mainly responsible for the ⁴He disintegration in a period of ～4×10⁴?5×10⁸ s.