Mesoscopic wave turbulence

We report results of sumulation of wave turbulence. Both inverse and direct cascades are observed. The definition of "mesoscopic turbulence" is given. This is a regime when the number of modes in a system involved in turbulence is high enough to qualitatively simulate most of the processes but significantly smaller then the threshold which gives us quantitative agreement with the statistical description, such as kinetic equation. Such a regime takes place in numerical simulation, in essentially finite systems, etc.Comment: 5 pages, 11 figure

Self-similarity of wind-driven seas

International audienceThe results of theoretical and numerical study of the Hasselmann kinetic equation for deep water waves in presence of wind input and dissipation are presented. The guideline of the study: nonlinear transfer is the dominating mechanism of wind-wave evolution. In other words, the most important features of wind-driven sea could be understood in a framework of conservative Hasselmann equation while forcing and dissipation determine parameters of a solution of the conservative equation. The conservative Hasselmann equation has a rich family of self-similar solutions for duration-limited and fetch-limited wind-wave growth. These solutions are closely related to classic stationary and homogeneous weak-turbulent Kolmogorov spectra and can be considered as non-stationary and non-homogeneous generalizations of these spectra. It is shown that experimental parameterizations of wind-wave spectra (e.g. JONSWAP spectrum) that imply self-similarity give a solid basis for comparison with theoretical predictions. In particular, the self-similarity analysis predicts correctly the dependence of mean wave energy and mean frequency on wave age Cp / U10. This comparison is detailed in the extensive numerical study of duration-limited growth of wind waves. The study is based on algorithm suggested by Webb (1978) that was first realized as an operating code by Resio and Perrie (1989, 1991). This code is now updated: the new version is up to one order faster than the previous one. The new stable and reliable code makes possible to perform massive numerical simulation of the Hasselmann equation with different models of wind input and dissipation. As a result, a strong tendency of numerical solutions to self-similar behavior is shown for rather wide range of wave generation and dissipation conditions. We found very good quantitative coincidence of these solutions with available results on duration-limited growth, as well as with experimental parametrization of fetch-limited spectra JONSWAP in terms of wind-wave age Cp / U10

Spine-sheath polarization structures in four active galactic nuclei jets

We present the results of multifrequency (15 + 8 + 5 GHz) polarization Very Long Baseline Array (VLBA) observations of the three BL Lacertae objects 0745 + 241, 1418 + 546 and 1652 + 398 together with 5-GHz VLBI Space Observatory Programme (VSOP) observations of 1418 + 546 and 1.6- and 5-GHz VSOP observations of the blazar 1055 + 018. The jets of all these sources have polarization structure transverse to the jet axis, with the polarization E vectors aligned with the jet along the jet spine and 'sheaths' of orthogonal E vectors at one or both edges of the jet. The presence of polarization aligned with the jet near the 'spine' may indicate that the jets are associated with helical B fields that propogate outward with the jet flow; the presence of orthogonal polarization near the edges of the jet may likewise be a consequence of a helical jet B field, or may be owing to an interaction with the ambient medium on parsec scales. We have tentatively detected interknot polarization in 1055 + 018 with E aligned with the local jet direction, consistent with the possibility that the jet of this source is associated with a helical B field

Cluster superconductivity in the magnetoelectric Pb(Fe1/2Sb1/2)O3 ceramics

We report the observation of cluster (local) superconductivity in the magnetoelectric Pb(Fe1/2Sb1/2)O3 ceramics prepared at a hydrostatic pressure of 6 GPa and temperatures 1200-1800 K to stabilize the perovskite phase. The superconductivity is manifested by an abrupt drop of the magnetic susceptibility at the critical temperature TC 7 K. Both the magnitude of this drop and TC decrease with magnetic field increase. Similarly, the low-field paramagnetic absorption measured by EPR spectrometer drops significantly below TC as well. The observed effects and their critical magnetic field dependence are interpreted as manifestation of the superconductivity and Meissner effect in metallic Pb nanoclusters existing in the ceramics. Their volume fraction and average size were estimated as 0.1-0.2% and 140-150 nm, respectively. The superconductivity related effects disappear after oxidizing annealing of the ceramics.Comment: 9 pages, 5 figure

Diffusion model of interacting gravity waves on the surface of deep fluid

International audienceA simple phenomenological model for nonlinear interactions of gravity waves on the surface of deep water is developed. The Snl nonlinear interaction term in the kinetic equation for wave action is replaced by the nonlinear second-order diffusion-type operator. Analytical and numerical studies show that the new model gives a reasonably good description of a real situation, consuming three order of magnitude less computer time

An effective spin model on the honeycomb lattice for the description of magnetic properties in two-dimensional Fe3_3GeTe2_2

Fe$_3$GeTe$_2$ attracts significant attention due to technological perspectives of realizing room temperature ferromagnetism in two-dimensional materials. Here we show that due to structural peculiarities of the Fe$_3$GeTe$_2$ monolayer, short distance between the neighboring iron atoms induces a strong exchange coupling. This strong coupling allows us to consider them as an effective cluster with a magnetic moment $\sim$5 $\mu_B$, giving rise to a simplified spin model on a bipartite honeycomb lattice with the reduced number of long-range interactions. The simplified model perfectly reproduces the results of the conventional spin model, but allows for a more tractable description of the magnetic properties of Fe$_3$GeTe$_2$, which is important, e.g., for large-scale simulations. Also, we discuss the role of biaxial strain in the stabilization of ferromagnetic ordering in Fe$_3$GeTe$_2$.Comment: 7 pages, 7 figure

Spontaneous and induced ferroelectricity in the BiFe1−xScxO3 perovskite ceramics

High-pressure synthesis method allows obtaining single-phase perovskite BiFe1-xScxO3 ceramics in the entire concentration range. As-prepared compositions with x from 0.30 to 0.55 have the antipolar orthorhombic Pnma structure but can be irreversible converted into the polar rhombohedral R3c or the polar orthorhombic Ima2 phase via annealing at ambient pressure. Microstructure defects and large conductivity of the high-pressure-synthesized ceramics make it difficult to study and even verify their ferroelectric properties. These obstacles can be overcome using piezoresponse force microscopy (PFM) addressing ferroelectric behavior inside single grains. Herein, the PFM study of the BiFe1-xScxO3 ceramics (0.30 ≤ x ≤ 0.50) is reported. The annealed samples show a strong PFM contrast. Switching of domain polarity by an electric field confirms the ferroelectric nature of these samples. The as-prepared BiFe0.5Sc0.5O3 ceramics demonstrate no piezoresponse in accordance with the antipolar character of the Pnma phase. However, application of a strong enough electric field induces irreversible transition to the ferroelectric state. The as-prepared BiFe0.7Sc0.3O3 ceramics show coexistence of ferroelectric and antiferroelectric grains without poling. It is assumed that mechanical stress caused by the sample polishing can be also a driving force of phase transformation in these materials alongside temperature and external electric field.publishe