Raman scattering of Ge dot superlattices

Self-organised Ge dot superlattices grown by molecular beam epitaxy of Ge and Si layers utilizing Stranski-Krastanov growth mode were investigated by Raman spectroscopy. An average size of Ge quantum dots was obtained from transmission electron microscopy measurements. The strain and interdiffusion of Ge and Si atoms in Ge quantum dots were estimated from the analysis of frequency positions of optical phonons observed in the Raman spectra. Raman scattering by folded longitudinal acoustic phonons in the Ge dot superlattices was observed and explained using of elastic continuum theory. Received 25 January 2000     

On the central limit theorem for lacunary trigonometric series

Хорошо известно, что в ероятностное поведе ние лакунарного тригоно метрического ряда {cos 2πn _kx} тесно связ ано с «критическим» у словием лакунарности (*) $$\frac{{n_{k + 1} }}{{n_k }} \geqq 1 + \frac{{c_k }}{{\sqrt k }},c_k \to \infty $$ . Например, если выполн ено условие (*), то последовательность {cos2πn _kx} удовлетворяет центральной предель ной теореме, и при этом условие (*) не может быть ослабле но. Для последовательносте й, удовлетворяющих (*), и звестны и другие результаты по добного рода, в то время как для более медленно расту щих последовательносте й {n_k} не известно, по-видимому, ничего. В с татье развит метод, ко торый при помощи мартингально й техники позволяет проводить исследование систем {cos 2πn_kx} для последовательно стей, не удовлетворяю щих условию (*). Получено про стое объяснение условия (*), изучено, как «пропа-дает» центральная предель ная теорема при посте пенном ослаблении условия (*) и дока-заны некоторые центральн ые предельные теорем ы в отсутствие этого усл овия. Получены другие предельные те оремы для {cos 2πn_kx}, напри мер, закон повторного лог арифма и принципы инвариантн ости.     

Enhanced robustness of dispersion-managed solitons with respect to polarization mode dispersion

Polarization mode dispersion causes solitons to emit background radiation, which degrades transmission quality. It is shown theoretically and confirmed by numerical simulations that dispersion-managed solitons can trap part of that radiation into localized eigenmodes, thus yielding improvement in transmission quality compared with that of conventional solitons.     

The accuracy of whole brain N-acetylaspartate quantification

A non-localizing pulse sequence to quantify the total amount of N-acetylaspartate (NAA) in the whole brain (WBNAA) was introduced recently [Magn. Reson. Med. 40, 684–689 (1998)]. However, it is known that regional magnetic field inhomogeneities, ΔB₀s, arising from susceptibility differences at tissue interfaces, shift and broaden local resonances to outside the integration window, leading to an underestimation of the true amount of NAA in the entire brain. To quantify the upper limit of this loss, the whole-head proton MR spectrum (¹H-MRS) of the water was integrated over the same frequency width as the NAA. The ratio of this area/total-water-line was 75 ± 5% in 5 volunteers. The procedure was repeated with the brain-only water peak, obtained by summing signals only from voxels within that organ from a three-dimensional chemical-shift-imaging (3D CSI) set. It indicated that <10% of the water signal loss occurred in the brain. Therefore, by analogy, WBNAA accounts for >90% of that metabolite.     

Multi-gigabit WDM optical networking for next generation avionics system communications

It is envisaged that photonic networking will play a significant role in improving performance and reliability in both civil and military avionics systems. Of all the available photonic multiplexing technologies, wavelength-division multiplexing (WDM) has been the primary focus of attention within mainstream telecommunications offering increased throughput at a reasonable cost, with scope for enhanced routing flexibility, connectivity and network survivability. A direct mapping of techniques and devices from the maturing telecommunications sector is, however, not possible because of the stringent requirements of systems operating in the hostile aerospace environment. This paper gives an outline of these requirements and discusses, in detail, the design and development of a multi-gigabit, broadband optical WDM network architecture, specifically for use on aerospace platforms. The paper will also discuss a key element in the system, the arrayed-waveguide grating (AWG) wavelength multiplexing component, which has been designed to allow operation over the full military temperature specification without environmental conditioning.     

Structural analysis of composite metal-insulator materials by small-angle x-ray scattering

A small-angle x-ray scattering study of the structure of Cu: SiO₂ composite films obtained by magnetron cosputtering is reported. The experimental spectra are analyzed by direct numerical simulation of scattering from a polydisperse system of spherical particles with a high volume concentration. The calculated scattering spectra were found to fit well to the experiment if a log-normal particle distribution in size is assumed, and the parameters of this distribution were determined.     

Finitary Spacetime Sheaves of Quantum Causal Sets: Curving Quantum Causality

A locally finite, causal, and quantal substitute for a locally Minkowskian principal fiber bundle of modules of Cartan differential forms over a bounded region X of a curved C -smooth spacetime manifold M with structure group G that of orthochronous Lorentz transformations L ⁺ := SO(1,3), is presented. is usually regarded as the kinematical structure of classical Lorentzian gravity when the latter is viewed as a Yang-Mills type of gauge theory of a sl(2, {})-valued connection 1-form . The mathematical structure employed to model this replacement of is a principal finitary spacetime sheaf of quantum causal sets with structure group G _n, which is a finitary version of the continuous group G of local symmetries of General Relativity, and a finitary Lie algebra g _n-valued connection 1-form on it, which is a section of its subsheaf . is physically interpreted as the dynamical field of a locally finite quantum causality, whereas its associated curvature as some sort of finitary and causal Lorentzian quantum gravity.     

Yakutsk EAS array data on the variations in intensities of the highest energy cosmic rays

The results of analysis of arrival frequency of cosmic rays with energies E ₀≥4×10¹⁷ eV are presented based on the data collected on the Yakutsk array during its 24 years of continuous operation (1977–2000). It is shown that the intensity of cosmic rays is variable. At E ₀≤(3?5)×10¹⁸ eV, the (2–3)-month data show many deviations by (3–4)σ from the mean level. At E ₀≥10¹⁹ eV, the intensities steadily decrease, on the average, by 1.5 times during the time period considered.