THE STUDY OF THE INFLUENCE OF X-RAY IRRADIATION ON DISLOCATION CHARACTERISTICS IN LiF CRYSTALS

The dependences of the absorption α and the ultrasound velocity in LiF single crystals with residual deformation ε = 0.65% at 300 K in the range of radiation doses 0...1057 R were studied using the acoustic pulse echo method at a frequency of 7.5 MHz. Based on the results of measurements of the acoustic characteristics, the absolute values of the parameters of the dislocation structure – the average effective length of the dislocation loop L and the dislocation density Λ and their dependences on the irradiation time are determined. The calculated characteristics are compared with the previously obtained results for the high-frequency branch of the damped dislocation resonance and using the selective etching method. The revealed noticeable discrepancy in the values of these parameters is explained by the impossibility of describing a single attenuation mechanism for acoustic measurements carried out in a wide frequency range.

Collisional and resonance absorption of electromagnetic waves in a weakly collisional, inhomogeneous magnetoplasma slab

Absorbance of normally incident electromagnetic wave on a cold, weakly collisional, and inhomogeneous magnetoplasma slab is investigated. The plasma density is Budden-like sinusoidal profile, where the inhomogeniety is treated as a multilayered system of homogeneous sub-cells within the transfer matrix technique. For incident wave frequencies much above the ion cyclotron frequency, only right hand circularly polarized waves are relevant for wave propagation parallel to a static magnetic field. Calculations are performed in normalized parameters, that make the results suitable for many applications including atmospheric and laboratory plasmas. The presence of the dc-magnetic field leads to the formation of two absorption bands explained by plasma collisional dissipation and electron cyclotron resonance in the low frequency branch of the $R$-wave below the electron cyclotron frequency. The transmittance shows the emergence of the low frequency electron cyclotron wave, which becomes a Whistler mode at very low frequency. More detailed discussion on the effect of plasma collisionality, inhomogeneity, and dc-magnetic field on the propagation characteristics is given at the relevant place within the body of the manuscript.

Specifications of Resonant Acoustic Rotating Waves in Circular and Annular Domains

Circular and annular domains of hydroacoustic vibration are very common in modern technology due to their simplicity. On the other hand it turns out that such a shape possesses remarkable vibration properties. It is determined that there are two classes of resonant rotating waves, predominantly tangential and predominantly radial, in terms of prevalence of tangential or radial components of the vectors of vibrational velocities and displacements. The complete map of resonant angular velocities shows that all predominantly tangential angular velocities for all values of ring thickness are assembled into the self-isolating unique single low-frequency branch, whereas predominantly radial ones fill the entire high-frequency region very densely.

Raman Spectroscopic Characterizations of Self-Catalyzed InP/InAs/InP One-Dimensional Nanostructures on InP(111)B Substrate using a Simple Substrate-Tilting Method

We report optical phonon vibration modes in ensembles of self-catalyzed InP/InAs/InP multi core-shell one-dimensional nanostructures (nanopillars and nanocones) grown on InP(111)B substrates using liquid indium droplets as a catalyst via metal-organic chemical vapor deposition. We characterized the Raman vibration modes of InAs E1(TO), InAs A1(TO), InAs E1(LO), InP E1(TO), InP A1(LO), and InP E1(LO) from the ensemble of as-grown nanostructures. We also identified second-order Raman vibration modes, associated with InP E1(2TO), E1(LO+TO), and E1(2LO), in the InP/InAs/InP core-shell nanopillars and nanocones. Raman spectra of InP/InAs/InP nanopillars showed redshift and broadening of LO modes at low-frequency branches of InAs and InP. Due to the polar nature in groups III–V nanowires, we observed strong frequency splitting between InAs E1(TO) and InAs A1(LO) in InP/InAs/InP nanocones. The Raman resonance intensities of InP and InAs LO modes are found to be changed linearly with an excitation power. By tilting the substrate relative to the incoming laser beam, we observed strong suppression of low-frequency branch of InP and InAs LO phonon vibrations from InP/InAs/InP nanocones. The integrated intensity ratio of InP E1(TO)/E1(LO) for both nanostructures is almost constant at 0-degree tilt, but the ratio of the nanocones is dramatically increased at 30-degree tilt. Our results suggest that Raman spectroscopy characterization with a simple substrate tilting method can provide new insights into non-destructive characterization of the shape, structure, and composition of the as-grown nanostructures for the wafer-scale growth and integration processing of groups III–V semiconducting hetero-nanostructures into nanoelectronics and photonics applications.

Cellular precision of orientation and spatial frequency maps in macaque V1

Functional organization of neuronal response properties along the surface of the neocortex is a fundamental guiding principle of neural computation in the brain. Despite this importance, the cellular precision of functional maps is still largely unknown. We address the challenge by using two-photon calcium imaging to measure cell-specific orientation and spatial frequency (SF) responses across fields of macaque V1 superficial layers. The cellular orientation maps confirm iso-orientation domains, but rarely show pinwheels. Pinwheels obtained through conventional Gaussian smoothing and vector summation of orientation responses mostly overlap with blood vessel regions, suggesting false singularities. Cellular SF maps clarify existing controversies by showing weak iso-frequency clusters, which also suggests a weak geometric relationship between orientation and SF maps. Most neurons are tuned to medium frequencies, but the tuning functions are often asymmetric with a wider low- or high-frequency branch, which may help encode low or high SF information for later decoding.

Parametric instability and wave turbulence driven by tidal excitation of internal waves

We investigate the stability of stratified fluid layers undergoing homogeneous and periodic tidal deformation. We first introduce a local model which allows us to study velocity and buoyancy fluctuations in a Lagrangian domain periodically stretched and sheared by the tidal base flow. While keeping the key physical ingredients only, such a model is efficient in simulating planetary regimes where tidal amplitudes and dissipation are small. With this model, we prove that tidal flows are able to drive parametric subharmonic resonances of internal waves, in a way reminiscent of the elliptical instability in rotating fluids. The growth rates computed via direct numerical simulations (DNSs) are in very good agreement with Wentzel–Kramers–Brillouin analysis and Floquet theory. We also investigate the turbulence driven by this instability mechanism. With spatio-temporal analysis, we show that it is weak internal wave turbulence occurring at small Froude and buoyancy Reynolds numbers. When the gap between the excitation and the Brunt–Väisälä frequencies is increased, the frequency spectrum of this wave turbulence displays a $-2$ power law reminiscent of the high-frequency branch of the Garett and Munk spectrum (Geophys. Fluid Dyn., vol. 3 (1), 1972, pp. 225–264) which has been measured in the oceans. In addition, we find that the mixing efficiency is altered compared to what is computed in the context of DNS of stratified turbulence excited at small Froude and large buoyancy Reynolds numbers and is consistent with a superposition of waves.

Electromagnetic Waves Dispersion and Interaction of an Annular Beam-Ion Channel System in Plasma Waveguide

A linear theory for the electromagnetic properties and interactions of an annular beam-ion channel system in plasma waveguide is presented. The dispersion relations for two families of propagating modes, including the electrostatic and transverse magnetic modes, are derived. The dependencies of the dispersion behavior and interaction for different wave modes on the thickness of the annular beam and betatron oscillation frequency are studied in detail by numerical calculations. The results show that the inner and outer radii of the beam have different influences on propagation properties of the electrostatic and electromagnetic modes with different betatron oscillation parameters. In the weak ion channel situation, the two types of electrostatic waves, that is, space charge and betatron modes, have no interaction with the transverse magnetic modes. However, in the strong ion channel situation, the transverse magnetic modes will have two branches and a low frequency mode emerged as the new branch. In this case, compared with the solid beam case, the betatron modes not only can interact with the high frequency branch at small wavenumber but also can interact with the low frequency branch at large wavenumber.