Vacancy-Impurity Nanoclusters in Solid Solutions 3He−4He^3He - ^4He

The existence of vacancy--impurity clusters due to quantum properties of vacancies in phase separated solid solutions of 4He in 3He is analyzed and discussed. Additional mechanism called vacancy assisted nucleation is proposed. According to this assumption the vacancy-impurity clusters should have b.c.c. structure.Comment: 5 pages, 1 figure, Late

Observation of acoustic turbulence in a system of nonlinear second sound waves in superfluid 4He.

We discuss the results of recent studies of acoustic turbulence in a system of nonlinear second sound waves in a high-quality resonator filled with superfluid 4He. It was found that, when the driving amplitude was sufficiently increased, a steady-state direct wave cascade is formed involving a flux of energy towards high frequencies. The wave amplitude distribution follows a power law over a wide range of frequencies. Development of a decay instability at high driving amplitudes results in the formation of subharmonics of the driving frequency, and to a backflow of energy towards the low-frequency spectral domain, in addition to the direct cascade

Statistical properties of strongly nonlinear waves within a resonator.

An experimental investigation of nonlinear waves is reported for a system of one-dimensional second sound waves in superfluid helium within a cylindrical resonator of high Q quality factor. The strong nonlinear dependence of the wave velocity on amplitude distorts the wave shape and leads to the formation of multiple harmonics. The restricted geometry of the resonator results in a discrete energy spectrum, where the energy is transmitted from the driving frequency to the high-frequency edge of the spectrum, where dissipation occurs—a Kolmogorov-like energy distribution. It is found that the main resonance occurs at the driving frequency, and that the next few harmonics are approximately sinusoidal, coherent with the driving force, but that higher harmonics appear to be chaotic and are no longer phase coherent with the drive. For developed turbulence, the probability density function of the high-frequency harmonics is well approximated by a Gaussian distribution. Thus, the nonlinear acoustic waves exhibit the statistical properties distinctive of weak turbulence, confirming that they can properly be treated in terms of a statistical description

Critical Casimir force in 4^4He films: confirmation of finite-size scaling

We present new capacitance measurements of critical Casimir force-induced thinning of $^4$He films near the superfluid/normal transition, focused on the region below $T_{\lambda}$ where the effect is the greatest. $^4$He films of 238, 285, and 340 \AA thickness are adsorbed on N-doped silicon substrates with roughness $\approx 8 {\AA}$. The Casimir force scaling function $\vartheta$, deduced from the thinning of these three films, collapses onto a single universal curve, attaining a minimum $\vartheta = -1.30 \pm 0.03$ at $x=td^{1/\nu}=-9.7\pm 0.8 {\AA}^{1/\nu}$. The collapse confirms the finite-size scaling origin of the dip in the film thickness. Separately, we also confirm the presence down to $2.13 K$ of the Goldstone/surface fluctuation force, which makes the superfluid film $\sim 2 {\AA}$ thinner than the normal film.Comment: 4 pages, 3 figures, submitted to PR

Neutron reflection from the liquid helium surface.

The reflection of neutrons from a helium surface has been observed for the first time. The 4He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer ~200 Å thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface. The application of neutron reflectometry described in this paper creates new and exciting opportunities for the surface and interfacial study of quantum fluids

Global Lagrangian atmospheric dispersion model

The Global Lagrangian Atmospheric Dispersion Model (GLADIM) is described. GLADIM is based on the global trajectory model, which had been developed earlier and uses fields of weather parameters from different atmospheric reanalysis centers for calculations of trajectories of air mass that include trace gases. GLADIM includes the parameterization of turbulent diffusion and allows the forward calculation of concentrations of atmospheric tracers at nodes of a global regular grid when a source is specified. Thus, GLADIM can be used for the forward simulation of pollutant propagation (volcanic ash, radionuclides, and so on). Working in the reverse direction, GLADIM allows the detection of remote sources that mainly contribute to the tracer concentration at an observation point. This property of Lagrangian models is widely used for data analysis and the reverse modeling of emission sources of a pollutant specified. In this work we describe the model and some results of its validation through a comparison with results of a similar model and observation data

Experiments on wave turbulence : the evolution and growth of second sound acoustic turbulence in superfluid 4He confirm self-similarity.

We report our experiments on the formation of second sound acoustic turbulence in superfluid 4He. The initial growth in spectral amplitude follows power laws that steepen rapidly with increasing harmonic number n, corresponding to a propagating front in frequency space. The lower growth exponents agree well with analytic predictions and numerical modeling. The observed increase in the formation delay with n validates the concept of selfsimilarity in the growth of wave turbulence