Showing papers by "Russian Academy of Sciences published in 1997"

Modern magnetooptics and magnetooptical materials

Abstract: Modern Magnetooptics and Magnetooptical Materials provides a comprehensive account of the principles and applications of magnetooptics, bridging the gap between textbooks and specialist accounts in the research and review literature. The book is aimed at the graduate physicist and electrical engineer, but assumes no specialist knowledge of magnetooptics. Chapters have been designed to be reasonably independent, so that readers in search of information on a particular topic can go straight to the appropriate place in the book, with only occasional reference to material elsewhere.Divided into three main parts, the book begins with the principles of magnetooptics to provide the necessary theoretical background. This section's emphasis is on introducing practical considerations through examples taken from real-life situations. The next part surveys a wide range of magnetooptic materials, including metals, alloys, and granular structures. The final part explores applications of magnetooptics in practical devices, such as modulators, switches, memory devices, and waveguides. This book provides a thorough introduction for graduate students of physics and electrical engineering, and a useful reference for researchers.

InGaAs-GaAs quantum-dot lasers

Abstract: Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A/spl middot/cm/sup -2/ at 77 K and of 62 A/spl middot/cm/sup -2/ at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of /spl sim/80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to /spl sim/50 meV, ultrahigh material gain of /spl sim/10/sup 5/ cm/sup -1/, differential gain of /spl sim/10/sup -13/ cm/sup 2/ and strong nonlinear gain effects with a gain compression coefficient of /spl sim/10/sup -16/ cm/sup 3/. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is /spl sim/0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 /spl mu/m and 1.37 /spl mu/m at 300 K.

Polypyrrole : a conducting polymer; its synthesis, properties and applications

Abstract: The synthesis, properties and applications of polypyrrole (PPy), a representative of conducting polymers, are reviewed. Chemical and electrochemical methods for the preparation of PPy and PPy films and the effect of the reaction conditions on the properties of the polymer produced are discussed. The mechanisms of electrochemical polymerisation of pyrrole are compared. The physicomechanical properties of the polymer, its morphology and structure are discussed. The mechanism of charge transfer in PPy is considered within the framework of the model of quasi-equilibrium concentrations of polarons and bipolarons. The methods for modification of the electrochemical performance of PPy are reviewed, in particular, synthesis of PPy from pyrrole derivatives with the appropriate electroactive groups and introduction in the PPy of electroactive dopant anions and the effect of those on the electrochemical performance of PPy films are considered. Examples of the use of PPy are given and the prospects for its application in various areas are discussed. The bibliography includes 254 references.

Sensitivity Measures, ANOVA-like Techniques and the Use of Bootstrap

Abstract: Sobol' sensitivity indices,used in variance based global sensitivity analysis of model output,are compared with the Analysis of Variance in classical factorial design. Monte Carlo computation of Sobol' indices is described briefly, and a bootstrap approach is presented,which can be used to produce a confidence interval for the true,unknown indices.

Equilibrium programming using proximal-like algorithms

Abstract: We compute constrained equilibria satisfying an optimality condition. Important examples include convex programming, saddle problems, noncooperative games, and variational inequalities. Under a monotonicity hypothesis we show that equilibrium solutions can be found via iterative convex minimization. In the main algorithm each stage of computation requires two proximal steps, possibly using Bregman functions. One step serves to predict the next point; the other helps to correct the new prediction. To enhance practical applicability we tolerate numerical errors.

The primordial helium abundance: systematic effects and a new determination

Abstract: We present new high-quality spectrophotometric observations of 27 supergiant H II regions in 23 low-metallicity blue compact galaxies (BCGs) with oxygen abundance 12 + log (O/H) between 7.22 and 8.17 (Z☉/50 ≤ Z ≤ Z☉/6), most of which were selected from the first and second Byurakan objective-prism surveys. Combining this new data with our previous sample of 10 low-metallicity BCGs, we extract a subsample of 27 H II regions in 24 BCGs most appropriate for the determination of the primordial helium abundance Yp. We find that the most metal-deficient BCG known, I Zw 18 (Z ~ Z☉/50), cannot be used for this purpose because of its abnormally low He I line intensities. We have examined critically the systematic effects that may influence the determination of Yp. We find that the effects of the corrections for neutral helium and for underlying stellar absorption in the He I lines, possible deviations from case B recombination theory, fluorescent enhancement of the He I line intensities, temperature fluctuations in H II regions, Wolf-Rayet stellar winds, and supernova shock waves to be small. The main effect comes from the particular set of atomic data used. The best set of atomic data for use in the determination of Yp is composed of Smits's He I emissivities and Kingdon & Ferland's collisional enhancement correction factors. This set gives the smallest dispersion of the data in the Y-O/H and Y-N/H planes and corrects best the data for collisional enhancement effects. By extrapolating the Y versus O/H and Y versus N/H linear regressions to O/H = N/H = 0, we obtain Yp = 0.243 ± 0.003 for both regressions, considerably larger than the values derived before. In the framework of the standard hot big bang nucleosynthesis model with a number of neutrino families Nν = 3 and a neutron half-lifetime τn = 887 s, our new Yp gives a baryon-to-photon number ratio η=3.51.0−0.7 × 10-10, or a baryonic mass fraction Ωh250=0.05±0.01. Our Yp determination is fully consistent with measurements of the primordial 7Li in Galactic halo stars and of (D +3He) in the local interstellar medium and the solar system, and consistent at the 2 σ level with the primordial D abundance derived in a quasar absorption system by Tytler, Fan, & Burles. We derive a slope dY/dZ = 1.7 ± 0.9, considerably smaller than the slopes obtained before and consistent with chemical evolution models for star-forming dwarf galaxies with an outflow of well-mixed material. Extrapolation to solar metallicity with such a slope gives the correct solar helium mass fraction within the errors.

Dust plasma crystals, drops, and clouds

Abstract: The experimental and theoretical aspects of the dust particle phenomenon are discussed. The subjects include dust particle attraction in open systems ( in spite of charges of up to 105e on individual particles); dust molecule formation; large (100 eV and higher) values of the dust-plasma crystal binding energy; self-contraction instabilities (similar to and operating together with gravitational instability in cosmic structures); free boundary dust-plasma crystals; new dust attraction mechanisms; the growth and agglomeration of dust particles; and the development of long order in dust plasmas. New estimates for understanding the fireball phenomenon and star production are given.

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; I, Results from X-ray diffraction and selected-area electron diffraction

Abstract: Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal birnessite (HBi), were studied by X-ray and selected-area electron diffraction (XRD, SAED). SAED patterns were also obtained for synthetic Sr-exchanged birnessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic structure of NaBi and the one-layer hexagonal structure of HBi with subcell parameters a = 5.172 Aa, b = 2.849 Aa, c = 7.34 Aa, beta = 103.3 degrees and a = 2.848 Aa, c = 7.19 Aa, gamma = 120 degrees , respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental observations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mn octahedral layers. The departure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of Mn (super 3+) for Mn (super 4+) . The supercell A = 3a parameter arises from the ordered distribution of Mn (super 3+) -rich rows parallel to [010] and separated from each other along [100] by two Mn (super 4+) rows. The superstructure in the b direction of NaBi type II (B = 3b) comes from the ordered distribution of Na cations in the interlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when Mn (super 3+) -rich rows are free of Mn (super 4+) . The idealized structural formula proposed for NaBi type II is Na (sub 0.333) (Mn (super 4+) (sub 0.722) Mn (super 3+) (sub 0.222) Mn (super 2+) (sub 0.055) )O 2 . NaBi type I has a lower amount of Mn (super 3+) and its ideal composition would vary from Na (sub 0.167) (Mn (super 4+) (sub 0.833) Mn (super 3+) (sub 0.167) )O 2 to Na (sub 0.25) (Mn (super 4+) (sub 0.75) Mn (super 3+) (sub 0.25) )O 2 . Satellites in SAED patterns of NaBi crystals result from the ordered distribution of Mn (super 4+) and Mn (super 2+) pairs in Mn (super 3+) -rich rows with a periodicity of 6b. The structure of HBi consists of hexagonal octahedral layers containing predominantly Mn (super 4+) with variable amounts of Mn (super 3+) and layer vacancies. The distribution of layer vacancies is inherited from the former Mn (super 3+) distribution in NaB. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is probably the destabilization of Mn (super 3+) -rich rows at low pH.

Cabauw Experimental Results from the Project for Intercomparison of Land-Surface Parameterization Schemes

Abstract: In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m22 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (6 10 Wm 2 2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of

Reaction-induced phase separation in modified thermosetting polymers

Abstract: Thermosetting polymers are frequently used in formulations, including rubbers, thermoplastic polymers or oils, etc, in an amount of the order of 2–50 wt% with respect to the thermoset. This extra component, called the modifier, may initially be immiscible or may phase-separate during cure. This last process, i.e the reaction-induced phase separation, is the subject of this review. A thermodynamic description of the process is made, using the Flory-Huggins equation at two approximation levels, i.e. a quasi-binary approach and a multicomponent treatment taking polydispersity of constituents into account. Thermodynamic factors affecting the phase separation process are thus established. Nucleation and growth (NG) and spinodal demixing (SD) are considered as possible phase separation mechanisms. Factors promoting one or the other process are discussed. The control of morphologies generated is analyzed on the basis of thermodynamic and kinetic arguments. Ideas for obtaining particular morphologies enhancing particular properties are put forward.

Bjerknes forces between small cavitation bubbles in a strong acoustic field

Abstract: The mutual interaction between small oscillating cavitation bubbles ${(R}_{0}l10\ensuremath{\mu}\mathrm{m})$ in a strong acoustic field (${P}_{a}g1\mathrm{bar},$ $f=20\mathrm{kHz}$) is investigated numerically. We assume spherical symmetry and a coupling of the bubble oscillations. Our results show that the strength and even the directions of the resulting secondary Bjerknes forces differ considerably from predictions of the well-known linear theory. This is of immediate consequence for understanding and modeling structure formation processes in acoustic cavitation and multibubble sonoluminescence.

The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism

Abstract: The fossil Kimberella quadrata was originally described from late Precambrian rocks of southern Australia1. Reconstructed as a jellyfish2, it was later assigned to the cubozoans (‘box jellies’), and has been cited as a clear instance of an extant animal lineage present before the Cambrian3,4,5,6,7. Until recently, Kimberella was known only from Australia, with the exception of some questionable north Indian specimens8. We now have over thirty-five specimens of this fossil from the Winter Coast of the White Sea in northern Russia. Our study of the new material does not support a cnidarian affinity. We reconstruct Kimberella as a bilaterally symmetrical, benthic animal with a non-mineralized, univalved shell, resembling a mollusc in many respects. This is important evidence for the existence of large triploblastic metazoans in the Precambrian and indicates that the origin of the higher groups of protostomes lies well back in the Precambrian.

Relic gravitational waves produced after preheating

Abstract: We show that gravitational radiation is produced quite efficiently in interactions of classical waves created by resonant decay of a coherently oscillating field. As an important example we consider simple models of chaotic inflation, where we find that today's ratio of energy density in gravitational waves per octave to the critical density of the Universe can be as large as ${10}^{\ensuremath{-}12}$ at the maximal wavelength of order ${10}^{5}$ cm. In the pure $\ensuremath{\lambda}{\ensuremath{\varphi}}^{4}/4$ model with inflaton self-coupling $\ensuremath{\lambda}{=10}^{\ensuremath{-}13}$, the maximal today's wavelength of gravitational waves produced by this mechanism is of order ${10}^{6}$ cm, close to the upper bound of operational LIGO and TIGA frequencies. The energy density of waves in this model, though, is likely to be well below the sensitivity of LIGO or TIGA at such frequencies. We discuss the possibility that in other models the interaction of classical waves can lead to an even stronger gravitational radiation background.

Spin state tomography

Abstract: A scheme for measuring the quantum state for an arbitrary spin is proposed that is analogous to the symplectic tomography scheme used to measure quantum states associated with continuous observables such as position and momentum An invariant form for the spin state density operator is derived in terms of an integral, over the angles which specify the quantization axis, of a product of the measured probability of the values of the spin along a chosen direction and spherical harmonics summed with Clebsch-Gordan functions

XRD measurement of mean crystallite thickness of illite and illite/smectite; reappraisal of the Kubler index and the Scherrer equation

Abstract: The standard form of the Scherrer equation, which has been used to calculate the mean thickness of the coherent scattering domain (CSD) of illite crystals from X-ray diffraction (XRD) full width data at half maximum (FWHM) intensity, employs a constant, Ksh, of 0.89. Use of this constant is unjustified, even if swelling has no effect on peak broadening, because this constant is valid only if all CSDs have a single thickness. For different thickness distributions, the Scherrer “constant” has very different values. Analysis of fundamental particle thickness data (transmission electron microscopy, TEM) for samples of authigenic illite and illite/smectite from diagenetically altered pyroclastics and filamentous illites from sandstones reveals a unique family of lognormal thickness distributions for these clays. Experimental relations between the distributions’ lognormal parameters and mean thicknesses are established. These relations then are used to calculate the mean thickness of CSDs for illitic samples from XRD FWHM, or from integral XRD peak widths (integrated intensity/maximum intensity). For mixed-layer illite/smectite, the measured thickness of the CSD corresponds to the mean thickness of the mixed-layer crystal. Using this measurement, the mean thickness of the fundamental particles that compose the mixed-layer crystals can be calculated after XRD determination of percent smectitic interlayers. The effect of mixed layering (swelling) on XRD peak width for these samples is eliminated by using the 003 reflection for glycolated samples, and the 001, 002 or 003 reflection for dehydrated, K-sa-turated samples. If this technique is applied to the 001 reflection of air-dried samples (Kubler index measurement), mean CSD thicknesses are underestimated due to the mixed-layering effect. The technique was calibrated using NEWMOD©-simulated XRD profiles of illite, and then tested on well-characterized illite and illite/smectite samples. The XRD measurements are in good agreement with estimates of the mean thickness of fundamental particles obtained both from TEM measurements and from fixed cations content, up to a mean value of 20 layers. Correction for instrumental broadening under the conditions employed here is unnecessary for this range of thicknesses.

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy

Abstract: Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal (H 1 -exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural Na 1 with solution H 1 and the partial loss of Mn 21 to the aqueous phase. The desorption of Na 1 in two temporally distinct steps indicates the existence of two types of binding sites for this cation. Mn 21 EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing MnO6 octahedra, with no evidence for triple- corner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-shar- ing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly Mn 31 . The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer Mn 31 to interlayer TC-sharing positions and re-adsorption of Mn 21 from solution onto layer vacancy sites. The migration of Mn 31 cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of Zn 21 -exchanged birnessite shows that Zn 21 also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral.

On self-similar traffic in ATM queues: definitions, overflow probability bound, and cell delay distribution

Abstract: Recent traffic measurements in corporate local-area networks (LANs), variable-bit-rate video sources, ISDN control-channels, and other communication systems, have indicated traffic behaviour of self-similar nature. This paper first discusses some definitions and properties of (second-order) self-similarity and gives simpler criteria for it. It then gives a model of self-similar traffic suitable for queuing system analysis of an asynchronous transfer mode (ATM) queue. A lower bound to the overflow probability of a finite ATM buffer is obtained, as also a lower bound to the cell loss probability. Finally, the stationary distribution of the cell delay in an infinite ATM buffer is obtained.