Observation of the Talbot effect for ultrasonic waves

The diffraction of ultrasonic radiation on an amplitude diffraction grating in the near-field area (Fresnel diffraction) has been studied. The effect of self-imaging of the grating (Talbot effect) has been detected for ultrasonic radiation at distances from the grating in the range from z = 0 to z = 2L _T, where L _T is the Talbot length. The fractional Talbot effect, i.e., the ultrasonic image of the grating with the period d/2, has been observed.     

Purcell effect in one-dimensional photonic quasicrystals

The change in probability of spontaneous emission for emitter placed in one-dimensional photonic quasicrystal (optical Fibonacci lattice) was examined. When the dipole is placed in Fibonacci lattice two different scenarios can be expected: enhancing (if frequency and direction of the dipole emission correspond to optical eigenmode of structure, and position corresponds to maximum value of modes electric field profile) or suppression (in case of photonic band gap) of spontaneous emission rate. Fact that both effects are expressed in quasicrystals less than in the Bragg reflectors and in the microcavities was demonstrated.     

Laser-induced breakdown threshold and fracture versus the nanostructure of two-phase glass

The radiation hardness and mechanical strength of single-and two-phase glasses are studied for the case when nanosecond laser pulses (λ= 1.06 μm, τ_0.5 ≈ 12.5 ns) are focused inside the material. Laser interferometry is applied to measure the displacement of the free surface, find optical breakage thresholds, and carry out the fractographic analysis of damaged regions. It is shown that breakdown channels and damage regions develop in a nonlinear manner according to optical breakdown mechanisms, changing each other with an increase in the laser energy. The strength of the two-phase glass is found to be more than four times that of the single-phase glass, although their elastic properties differ insignificantly. Such a considerable difference in the hardness of these materials with chemically similar constitutents is attributed to the presence of the double-lattice nanometer-scale structure of the two-phase glass.     

Which stresses affect the glide of screw dislocations in bcc metals?

By direct application of stress in molecular statics calculations we identify the stress components that affect the glide of 1/2?111? screw dislocations in bcc tungsten. These results prove that the hydrostatic stress and the normal stress parallel to the dislocation line do not play any role in the dislocation glide. Therefore, the Peierls stress of the dislocation cannot depend directly on the remaining two normal stresses that are perpendicular to the dislocation but, instead, on their combination that causes an equibiaxial tension-compression (and thus shear) in the plane perpendicular to the dislocation line. The Peierls stress of 1/2?111? screw dislocations then depends only on the orientation of the plane in which the shear stress parallel to the Burgers vector is applied and on the magnitude and orientation of the shear stress perpendicular to the slip direction.     

On the initiation of detonation in high explosives by a high-current electron beam

A critical analysis of experimental work on the initiation of high explosives (HE), high-current electron beam of nanosecond duration. It is demonstrated that there is no unambiguous evidence for the initiation of detonation in HE by the electron beams described in the reviewed works. The results of experiments on the effects of a pulsed electron beam on model paraffin samples are presented, which show that the geometric parameters of the sample and the presence of copper oxide nanopowder in it influence its resistance to the action of the beam. It is shown that the main effect on the sample is produced by cathode plasma beam and that the selection of the sample geometry and introduction of nanopowders can enhance the action of a high-current electron beam.     

Phonon spectra of La-containing Bi2Sr2CaCu2O8-based solid solutions measured by inelastic neutron scattering

Phonon spectra of Bi₂Sr_2?y Ca_1?x La _{x + y} Cu₂O_{8 + δ} solid solutions (x=y=0; x=0, y=0.25; x=0.25, y=0; x=0, y=0.5) were measured by inelastic neutron scattering on a DIN-2PI direct-geometry spectrometer. A full-profile analysis of the x-ray diffraction data yielded an estimate of the distribution of alkaline-earth (AE) and La atoms over the Ca and Sr positions in the Bi₂Sr₂CaCu₂O₈ structure. An assignment of the main features in the phonon spectrum to dominant vibrations of certain atoms was made. In particular, the correlation of the x-ray structural data with spectroscopic data implies that the feature due to cation vibrations in the Sr crystallographic positions of the basic 2212 structure (near 11 meV) is single-mode in behavior. Substitution of La for an AE atom modifies the high-frequency part (>40 meV) of the phonon spectrum (which corresponds to vibrations of oxygen in the SrO and CuO₂ layers of the Bi₂Sr₂CaCu₂O₈ structure) and affects the cut-off frequencies in the spectra of solid solutions of various compositions. A comparison of the results obtained in this study with earlier spectra of the 2212 compound, in which Ca was substituted for by Nd, indicates that the shape and intensity of the high-frequency vibration spectra of the samples studied are related to the average cation charge in the SrO layer of the Bi₂Sr₂CaCu₂O₈ structure.     

On mechanical characteristics of nanocrystals

The dependence of the elastic moduli of a nanocrystal on its size is investigated theoretically with reference to a two-dimensional single-crystal strip. It is shown that the uncertainty (of a fundamental nature) in the size of a nanocrystal causes the determination of many of its mechanical characteristics to be ambiguous. It is found that the Cauchy-Green relations are modified and the elastic-constant tensor ceases to be symmetric; the size and shape of a nanocrystal render its mechanical properties more anisotropic. For a single-crystal strip, the Poisson ratio decreases and the Young modulus increases with decreasing thickness of the strip; in the case of a very thin crystal film (two atomic layers thick), these elastic moduli can differ from their macroscopic values by a factor of two. The size effects which make the continuum elasticity theory inapplicable to nanocrystals are estimated. The size effects that occur when the molecular dynamics method is applied for modeling macroscopic objects are also discussed.     

Effects of anisotropic diffraction on quadratic multisoliton excitation in noncritically p hase-matched crystals

The generation of multiple quadratic spatial solitons aligned approximately along a crystal axis in a biaxial noncritically phase-matched crystal is shown theoretically and experimentally to be the consequence of anisotropic diffraction.     

Phase Diagrams of Semicrystalline Polymer–Crystalline Substances: Polyolefins–1,2,4,5-Tetrachlorobenzene

The full phase diagrams of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and isotactic polypropylene (i-PP) mixtures with 1,2,4,5-tetrachlorobenzene (TeCB), including the solubility curve of TeCB in a solid polymer, were constructed by an optical method. The diagrams contain a eutectic point that corresponds to the situation when the crystallization of TeCB out of its solution in a polyolefin is accompanied by the crystallization of monomer units of the macromolecules. As a result, the polymer acquires a gel structure with crystallites as crosslinks and amorphous regions saturated with TeCB. It is demonstrated that the eutectic point position on the phase diagram can be used for ranking polymers with respect to their thermodynamic affinity to a solvent. For the studied systems, the affinity to TeCB was decreased in the order i-PP, HDPE, and LDPE. Direct experimental evidence was obtained that TeCB crystals can be dissolved in a solid polymer via a vapor phase mechanism, which leads to the polymer amorphization.