A new model for the cross spectrum and wavenumber-frequency spectrum of turbulent pressure fluctuations in a boundary layer

A model is developed for the statistical characteristics of the turbulent pressure field in a boundary layer. It is shown that the coherence scales are limited at low frequencies by the finiteness of the boundary layer thickness and at high frequencies by the effect of viscous forces acting in the flow. A relationship between the behavior of the coherence scales and that of the power spectra is demonstrated. The model is characterized by convertibility, which allows changes from cross spectra to wavenumber-frequency spectra and back via Fourier transformations. Calculations performed using the proposed model agree well with experimental results.     

Illumination of the shadow zone in a two-channel oceanic waveguide with a fine structure of sound speed inhomogeneities

The experimental data on the sound field structure, which were obtained by emitting a continuous pseudonoise signal (a midfrequency of 3.2 kHz) in a two-channel oceanic waveguide, are compared with the calculations performed by the wave program with allowance for the fine structure of the sound speed inhomogeneities. A considerable increase in the intensity of the sound field with a definite angular spectrum is observed in the upper channel in the first shadow zone, and, in the experiment, the increase begins nearer to the sound source than predicted by the wave and ray calculations for a smooth sound speed profile. These features of the field structure are explained by the illumination of the shadow zone by the regular scattering of signals from highly anisotropic fine-structure inhomogeneities of the sound speed profile, which are clearly pronounced in the region of the given oceanic experiment.     

Detection of hepatocarcinoma in rats by integration of the fluorescence spectrum: Experimental model

The incorporation of spectroscopic techniques into diagnostic procedures may greatly improve the chances for precise diagnostics. One promising technique is fluorescence spectroscopy, which has recently been used to detect many different types of diseases. In this work, we use laser-induced tissue fluorescence to detect hepatocarcinoma in rats using excitation light at wavelengths of 443 and 532 nm. Hepatocarcinoma was induced chemically in Wistar rats. The collected fluorescence spectrum ranges from the excitation wavelength up to 850 nm. A mathematical procedure carried out on the spectrum determines a figure of merit value, which allows the detection of hepatocarcinoma. The figure of merit involves a procedure which evaluates the ratio between the backscattered excitation wavelength and the broad emission fluorescence band. We demonstrate that a normalization allowed by integration of the fluorescence spectra is a simple operation that may allow the detection of hepatocarcinoma.     

Resonances in Muon Transfer from Muonic Hydrogen to Oxygen and Neon

The influence of resonances on the muon transfer processes from muonic hydrogen to oxygen and neon (pμ)_1s + O⁸⁺ → p + (μO)_nl⁷⁺ and (pμ)_1s + Ne¹⁰⁺ → p + (μNe)_nl⁹⁺ is considered using the Smith lifetime matrix formalism. It is shown that the existence of a long lived resonance in the case of Ne induces a stronger dependence on collision energy of the muon transfer cross-section for this system. Present address: Chemistry Department, Vancouver University, Vancouver, BC V6T 1Z3, Canada     

Study of the structural and optical properties of GaN/AlN quantum dot superlattices

We study GaN/AlN Quantum Dot (QD) superlattices utilizing the STREL environment which allows the building of atomistic models, relaxation of the structures, the calculation of the electronic states and optical transitions and the visualization of the results. The forces are calculated using an appropriate Keating or Stillinger–Weber interatomic potential model and the electronic states and optical transitions using a tight-binding formulation which is economical and produces realistic electronic properties. The relaxed structure has strains mainly in the GaN region which are compressive and small tensile strains in the AlN region, mainly below the QD. In the calculation of the electronic states and of the optical transitions the strains are included realistically at the atomistic level. The study of the wavefunctions close to the fundamental gap show how these strains influence the form and spatial extent of the wavefunction. Very close to the fundamental gap the valence and some conduction states are confined in the QD and have considerable oscillator strength.     

Excess noise peaks in porous silicon-based diode structures

Results of an experimental observation of the voltage oscillations associated with a discrete tunneling of holes in porous silicon at room temperature are presented. The noise characteristics of diode structures with a porous silicon interlayer formed on heavily boron-doped silicon single crystals are studied. Peaks of excessive noise are observed at frequencies of ～1 MHz, at which single-electron oscillations should be expected. The peak noise power is found to increase with current according to the ～2.5 power law and, at a current density of 0.15 A/cm², to exceed the noise power of the receiver by three to four orders of magnitude. The complex shape of the noise spectrum and its extension to the higher frequency region with increasing current are explained by the three-dimensionality of the system of nanometer-sized silicon grains embedded in insulating silicon dioxide of porous silicon.     

Phase modulation of the signal beam in three-wave forward mixing in photorefractive bismuth silicon oxide

The interaction of three forward beams in a BSO crystal is investigated under conditions when the two pump beams are anti-symmetrically detuned and the signal beam is phase modulated. For sinusoidal phase modulation the signal gain is shown to be dependent on the instantaneous frequency detuning. Single and double maxima in gain are obtained depending on the voltage amplitude applied to the piezoelectric mirror. For triangular phase modulation a slight asymmetry is found in the gain versus detuning curve.