The impedance of the Ag-CeF<Subscript>3</Subscript>/CeF<Subscript>3</Subscript> bicrystal-Ag system

The impedance spectra of CeF₃/CeF₃ bicrystal (two single crystals separated by a single intercrystalline boundary) between Ag-electrodes are studied over a 135 to 410 K temperature interval (including temperatures below room temperature). The bicrystal was prepared by thermal-diffusion welding under a pressure of 1.5 × 10⁷ Pa at 1473 K in vacuum (∼10⁻² Pa). It is shown that the intercrystalline boundary affects but insignificantly the bicrystal bulk impedance. The CeF₃/CeF₃ ionic conductivity is 3 × 10⁻⁶ S/cm at 293 K; it is mainly determined by transfer processes in the single crystal bulk.     

X-ray imaging of structural defects in Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> single crystals using a white synchrotron beam

Nonmonochromatic (white) synchrotron radiation with a high spatial coherence makes it possible to use different types of interaction of X-rays with matter simultaneously: diffraction, refraction, absorption, and fluorescence. In this case, the structure of materials is studied by the real-time recording of high-resolution images of different types under the same conditions. The use of X-ray images for studying the structural quality is demonstrated by the example of Czochralski-grown Si_{1 −x} Ge _x single crystals. The effect that the germanium content has on the formation and evolution of the defect structure is analyzed and the relationship between the structure and properties is investigated. The experiments were performed on the Pohang Light Source (Pohang, Republic of Korea).     

Tunable continuous-wave operation of a Cr3+,Li+:Mg2SiO4 laser

Room-temperature tunable cw laser oscillation of Cr3+ ions in forsterite crystals (Mg2SiO4) codoped with chromium and lithium has been demonstrated, for the first time to the authors' knowledge, in pumping a crystal with a Ti:sapphire laser into the first excited level of Cr3+ ions (4A2-4T2 transition). The laser is smoothly tunable in the region of 1095-1170 nm. A slope efficiency as great as 4.4% has been achieved.     

On the effect of damping on dispersion curves in plates

This paper presents a study on quantitative prediction and understanding of time-harmonic wave characteristics in damped plates. Material dissipation is modelled by using complex-valued velocities of free dilatation and shear waves in an unbounded volume. As a numerical example, solution of the classical Rayleigh–Lamb problem for a viscoelastic plate is presented to illustrate and discuss the role of dissipation in the cut-off phenomenon and in the phenomenon of veering for dispersion curves. These phenomena are explained in more detail considering a simple model, which allows accurate asymptotic analysis of the perturbation of dispersion curves in the regions of cut-off and veering.     

Flow instability in the zone of laminar axisymmetric boundary layer separation

The stability of flow with laminar boundary layer separation from a body of revolution aligned with an incompressible gas stream is investigated in a wind tunnel. In several experimental regimes with respect to the Reynolds number hot-wire anemometry is used to determine the main parameters of disturbances which grow behind the separation line, thus initiating transition to the turbulent flow state. The relations between the frequencies, the spatial growth rates of the most “hazardous” disturbances, and the integral characteristics of velocity profiles obtained in the study are in good agreement with the analogous data for plane separation flows.     

Measurement of the Dynamic Displacement Current as a New Method of Study of the Dynamics of Formation of a Streamer at a Breakdown of Gases at a High Pressure

The dynamics of formation of streamers in a “needle–plane” gap filled with air at atmospheric pressure has been studied. It has been shown that the time dependence of the dynamic displacement current measured by either a current shunt or a collector placed behind a grid electrode is determined by the rate of variation of the shape and dimensions of a streamer. The presence of a single peak on oscillograms of the dynamic displacement current means that a ball streamer is formed in the gap and does not cross the gap during the time of voltage application. The presence of two peaks on oscillograms of the dynamic displacement current means that the ball streamer crosses the gap and reaches the opposite electrode. In this case, the ball streamer is usually transformed to a cylindrical one. It has been shown that the measurement of the dynamic displacement current makes it possible to determine the time dependence of the electric field strength near the planar electrode.     

Critical and multicritical behavior in the Ising–Heisenberg universality class

Critical behavior of three-dimensional classical frustrated antiferromagnets with a collinear spin ordering and with an additional twofold degeneracy of the ground state is studied. We consider two lattice models, whose continuous limit describes a single phase transition with a symmetry class differing from the class of non-frustrated magnets as well as from the classes of magnets with non-collinear spin ordering. A symmetry breaking is described by a pair of independent order parameters, which are similar to order parameters of the Ising and O(N) models correspondingly. Using the renormalization group method, it is shown that a transition is of first order for non-Ising spins. For Ising spins, a second order phase transition from the universality class of the O(2) model may be observed. The lattice models are considered by Monte Carlo simulations based on the Wang–Landau algorithm. The models are a ferromagnet on a body-centered cubic lattice with the additional antiferromagnetic exchange interaction between next-nearest-neighbor spins and an antiferromagnet on a simple cubic lattice with the additional interaction in layers. We consider the cases N = 1, 2, 3 and in all of them find a first-order transition. For the N = 1 case we exclude possibilities of the second order or pseudo-first order of a transition. An almost second order transition for large N is also discussed.     

Two-velocity flow containing surfactant

A hydrodynamic theory of two-velocity fluid with surfactant is constructed in the present paper. The model takes into account both surface effects and impact of the concentration gradient on relative velocity of the components. The dependence of the nonstationary two-phase flow regimes on the surface tension gradient and the dependence of the relative drop velocity on the gradient of the surfactant concentration are investigated numerically on the base of volume control method.     

Thermal conductivity and expansion of PbF2 single crystals

In order to check a phenomenon of the negative correlation between ionic and thermal conductivities of solid substances, we studied the thermal conductivity and expansion of cubic PbF₂ single crystals at 50–300 and 5.6–317 K, respectively. We found that lead difluoride had a thermal expansion coefficient α that was equal to (28.5 ± 0.3)10⁻⁶ K⁻¹ at 300 K, and a thermal conductivity coefficient k(T) was equal to 1.40 ± 0.07 W/(m K) at the same temperature. Thus, the thermal conductivity for PbF₂ is the lowest among fluorite-type MF₂ (M = Ca, Sr, Ba, Cd, Pb) thermal conductivities, whereas its fluoride-ion conductivity is the highest one among MF₂ (M = Ca, Sr, Ba, Cd, Pb) ionic conductors.

     

Magnetoresonance EMF in thin manganite films

The effect of magnetoresonance emf (MREMF) in thin epitaxial films of rare-earth manganites La_0.67Sr_0.33MnO₃ is detected and investigated. The effect is manifested in the occurrence of a constant voltage under the action of microwave pumping in magnetic fields corresponding to ferromagnetic resonance conditions. The MREMF signal includes symmetric and antisymmetric components and changes its polarity upon switching of the external magnetic field. The temperature dependence of the effect (including the range in the neighborhood of the phase transition) is analyzed. The experimental data including the shape of the signal and its dependence of the field orientation are in good agreement with the results obtained in the theoretical model based on the mechanism of anisotropic magnetoresistance. It is shown that the magnetoresistance anisotropy in the manganite under investigation is negative and sharply attenuates as the temperature approaches the Curie point, almost vanishing in the paramagnetic phase.