Showing papers by "Russian Academy of Sciences published in 1998"

Effect of Mechanical Boundary Conditions on Phase Diagrams of Epitaxial Ferroelectric Thin Films

Abstract: A phenomenological thermodynamic theory of ferroelectric thin films epitaxially grow on cubic substrates is developed using a new form of the thermodynamic potential. which corresponds to the ac tual mechanical boundary conditions of the problem, For single-domain BaTiO3 and PbTiO3 films, the "misfit-temperature" phase diagrams are constructed. It is found that the 2D clamping of the films, apart from a shift of the temperature of the ferroelectric transition, results in a change of its order. A change of the sequence of the phases and the appearance of phases forbidden in the bulk crystals are predicted. [S0031-9007(98)05421-0].

Synaptogenesis via dendritic filopodia in developing hippocampal area CA1.

Abstract: To determine the role of dendritic filopodia in the genesis of excitatory synaptic contacts and dendritic spines in hippocampal area CA1, serial section electron microscopy and three-dimensional analysis of 16 volumes of neuropil from nine male rat pups, aged postnatal day 1 (P1) through P12, were performed. The analysis revealed that numerous dendritic filopodia formed asymmetric synaptic contacts with axons and with filopodia extending from axons, especially during the first postnatal week. At P1, 22 +/- 5.5% of synapses occurred on dendritic filopodia, with 19 +/- 5.9% on filopodia at P4, 20 +/- 8.0% at P6, decreasing to 7.2 +/- 4.7% at P12 (p < 0.02). Synapses were found at the base and along the entire length of filopodia, with many filopodia exhibiting multiple synaptic contacts. In all, 162 completely traceable dendritic filopodia received 255 asymmetric synaptic contacts. These synapses were found at all parts of filopodia with equal frequency, usually occurring on fusiform swellings of the diameter. Most synaptic contacts (53 +/- 11%) occurred directly on dendritic shafts during the first postnatal week. A smaller but still substantial portion (32 +/- 12%) of synapses were on shafts at P12 (p < 0.036). There was a highly significant (p < 0.0002) increase in the proportion of dendritic spine synapses with age, rising from just 4.9 +/- 4.3% at P1 to 37 +/- 14% at P12. The concurrence of primarily shaft and filopodial synapses in the first postnatal week suggests that filopodia recruit shaft synapses that later give rise to spines through a process of outgrowth.

Two-dimensional ferroelectric films

Abstract: Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films1,2, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature3. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature4. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 A in diameter5, in perovskite films 100 A thick6 and in crystalline ferroelectric polymers as thin as 25 A (7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size11,12,13. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 A (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

Reduced sensitivity of recent tree-growth to temperature at high northern latitudes

Abstract: Tree-ring chronologies that represent annual changes in the density of wood formed during the late summer can provide a proxy for local summertime air temperature1. Here we undertake an examination of large-regional-scale wood-density/air-temperature relationships using measurements from hundreds of sites at high latitudes in the Northern Hemisphere. When averaged over large areas of northern America and Eurasia, tree-ring density series display a strong coherence with summer temperature measurements averaged over the same areas, demonstrating the ability of this proxy to portray mean temperature changes over sub-continents and even the whole Northern Hemisphere. During the second half of the twentieth century, the decadal-scale trends in wood density and summer temperatures have increasingly diverged as wood density has progressively fallen. The cause of this increasing insensitivity of wood density to temperature changes is not known, but if it is not taken into account in dendroclimatic reconstructions, past temperatures could be overestimated. Moreover, the recent reduction in the response of trees to air-temperature changes would mean that estimates of future atmospheric CO2 concentrations, based on carbon-cycle models that are uniformly sensitive to high-latitude warming, could be too low.

Tetrachromacy, oil droplets and bird plumage colours

Abstract: There is a growing body of data on avian eyes, including measurements of visual pigment and oil droplet spectral absorption, and of receptor densities and their distributions across the retina. These data are sufficient to predict psychophysical colour discrimination thresholds for light-adapted eyes, and hence provide a basis for relating eye design to visual needs. We examine the advantages of coloured oil droplets, UV vision and tetrachromacy for discriminating a diverse set of avian plumage spectra under natural illumination. Discriminability is enhanced both by tetrachromacy and coloured oil droplets. Oil droplets may also improve colour constancy. Comparison of the performance of a pigeon's eye, where the shortest wavelength receptor peak is at 410 nm, with that of the passerine Leiothrix, where the ultraviolet-sensitive peak is at 365 nm, generally shows a small advantage to the latter, but this advantage depends critically on the noise level in the sensitivity mechanism and on the set of spectra being viewed.

Codes, Bent Functions and Permutations Suitable For DES-likeCryptosystems

Abstract: Almost bent functions oppose an optimum resistance to linear and differential cryptanalysis. We present basic properties of almost bent functions; particularly we give an upper bound on the degree. We develop the "coding theory" point of view for studying the existence of almost bent functions, showing explicitly the links with cyclic codes. We also give new characterizations of almost bent functions by means of associated Boolean functions.

Stepwise Polyelectrolyte Assembly on Particle Surfaces: a Novel Approach to Colloid Design

Abstract: Polyelectrolyte multilayers were deposited onto polystyrene and melamine formaldehyde latex particles by means of consecutive adsorption. Two different methods of multilayer growth were employed. First, adsorption of polyelectrolytes at a concentration exceeding saturation amounts was combined with the removal of the nonbound polyelectrolyte by means of centrifugation. Second, adsorption of polyelectrolyte was performed at a concentration just sufficient for saturation coverage. Both methods yielded continuous layer growth. The process of film formation was followed by electrophoresis, dynamic light scattering, single particle light scattering and fluorescence intensity measurements. Layer deposition onto partially crosslinked melamine resin latex particles, which were soluble at pH values of less than 1.6, resulted in the production of three-dimensional thin polyelectrolyte shells upon dissolving the core. The ultrathin shells were observed by means of scanning and transmission electron microscopy. © 1998 John Wiley & Sons, Ltd.

The LX-T Relation and Temperature Function for Nearby Clusters Revisited

Abstract: The X-ray luminosity-temperature relation for nearby T 3.5-10 keV clusters is rederived using new ASCA temperatures and ROSAT luminosities. Both quantities are derived by directly excluding the cooling flow regions. This correction results in a greatly reduced scatter in the LX-T relation; cooling flow clusters are similar to others outside the small cooling flow regions. For a fit of the form Lbol ∝ Tα, we obtain α = 2.64 ± 0.27 (90%) and a residual rms scatter in log Lbol of 0.10. The derived relation can be directly compared to theoretical predictions that do not include radiative cooling. It also provides an accurate reference point for future evolution searches and comparison to cooler clusters. The new temperatures and LX-T relation together with a newly selected cluster sample are used to update the temperature function at z ~ 0.05. The resulting function is generally higher and flatter than, although within the errors of, the previous estimates by Edge and coworkers and Henry and Arnaud (as rederived by Eke and coworkers). For a qualitative estimate of constraints that the new data place on the density fluctuation spectrum, we apply the Press-Schechter formalism for Ω0 = 1 and 0.3. For Ω0 = 1, assuming cluster isothermality, the temperature function implies σ8 = 0.55 ± 0.03, while taking into account the observed cluster temperature profiles, σ8 = 0.51 ± 0.03, consistent with the previously derived range. The dependence of σ8 on Ω0 is different from the earlier results because of our treatment of the slope of the fluctuation spectrum, n, as a free parameter. For the considered values of Ω0, n = -(2.0-2.3) ± 0.3, somewhat steeper than that derived from the earlier temperature function data, in agreement with the local slope of the galaxy fluctuation spectrum from the Automatic Plate Measuring Facility (APM) survey, and significantly steeper than the standard cold dark matter prediction.

Viability and activity in readily culturable bacteria: a review and discussion of the practical issues

Abstract: In microbiology the terms 'viability' and 'culturability' are often equated. However, in recent years the apparently self-contradictory expression 'viable-but-nonculturable' ('VBNC') has been applied to cells with various and often poorly defined physiological attributes but which, nonetheless, could not be cultured by methods normally appropriate to the organism concerned. These attributes include apparent cell integrity, the possession of some form of measurable cellular activity and the apparent capacity to regain culturability. We review the evidence relating to putative VBNC cells and stress our view that most of the reports claiming a return to culturability have failed to exclude the regrowth of a limited number of cells which had never lost culturability. We argue that failure to differentiate clearly between use of the terms 'viability' and 'culturability' in an operational versus a conceptual sense is fuelling the current debate, and conclude with a number of proposals that are designed to help clarify the major issues involved. In particular, we suggest an alternative operational terminology that replaces 'VBNC' with expressions that are internally consistent.

Baryogenesis via neutrino oscillations

Abstract: We propose a new mechanism of leptogenesis in which the asymmetries in lepton numbers are produced through the $\mathrm{CP}$-violating oscillations of ``sterile'' (electroweak singlet) neutrinos. The asymmetry is communicated from singlet neutrinos to ordinary leptons through their Yukawa couplings. The lepton asymmetry is then reprocessed into baryon asymmetry by electroweak sphalerons. We show that the observed value of baryon asymmetry can be generated in this way, and the masses of ordinary neutrinos induced by the seesaw mechanism are in the astrophysically and cosmologically interesting range. Except for singlet neutrinos, no physics beyond the standard model is required.

Estimation of blood-brain barrier crossing of drugs using molecular size and shape, and H-bonding descriptors.

Abstract: The influence of physicochemical properties, including lipophilicity, H-bonding capacity and molecular size and shape descriptors on brain uptake has been investigated using a selection of marketed CNS and CNS-inactive drugs. It is demonstrated that the polar surface area of a drug can be used as a suitable descriptor for the drugs' H-bonding potential. A combination of a H-bonding and a molecular size descriptor, i.e., the major components of lipophilicity and permeability, avoiding knowledge of distribution coefficients, is proposed to estimate brain penetration potential of new drug candidates. Previously reported experimental surface activity data appear to be strongly correlated to molecular size of the drug compounds. Present analysis offers a modern basis for property-based design and targeting of CNS drugs.

The Temperature Structure of 30 Nearby Clusters Observed with ASCA: Similarity of Temperature Profiles

Abstract: We present an analysis of ASCA spatially resolved spectroscopic data for a nearly complete sample of bright clusters with redshifts between 0.04 and 0.09. Together with several clusters analyzed elsewhere using the same method, this sample consists of 30 objects with Te ∼> 3.5 keV for which we obtained projected temperature profiles and, when possible, crude two-dimensional temperature maps. The clusters are A85, A119, A399, A401, A478, A644, A754, A780, A1650, A1651, A1795, A2029, A2065, A2142, A2256, A2319, A2597, A2657, A3112, A3266, A3376, A3391, A3395, A3558, A3571, A3667, A4059, Cygnus A, MKW3S, and Triangulum Australis. All clusters, with the possible exception of a few with insufficiently accurate data, are found to be nonisothermal with spatial temperature variations (apart from cooling flows) by a factor of 1.3–2. ASCA temperature maps for many clusters reveal merger shocks. The most notable of these are A754, A2065, A3558, A3667, and Cygnus A; merging can also be inferred with lower confidence from the A85, A119, and A2657 temperature maps and from the A3395 and Triangulum Australis entropy maps. About half of the sample shows signs of merging; in about 60% of the sample, we detect cooling flows. Nearly all clusters show a significant radial temperature decline at large radii. For a typical 7 keV cluster, the observed temperature decline between 1 and 6 X-ray core radii (0.15 and 0.9 h−1 Mpc) can be approximately quantified by a polytropic index of 1.2–1.3. Assuming such a polytropic temperature profile and hydrostatic equilibrium, the gravitating mass within 1 and within 6 core radii is approximately 1.35 and 0.7 times the isothermal β-model estimates, respectively. Most interestingly, we find that temperature profiles, excluding those for the most asymmetric clusters, appear remarkably similar when the temperature is plotted against radius in units of the estimated virial radius. We compare the composite temperature profile to a host of published hydrodynamic simulations. The observed profiles appear steeper than predictions of most Lagrangian simulations (Evrard, Metzler, & Navarro 1996; Eke, Navarro, & Frenk 1997). The predictions for Ω = 1 cosmological models are most discrepant, while models with low Ω are closer to our data. We note, however, that at least one Ω = 1 Lagrangian simulation (Katz & White 1993) and the recent high-resolution Eulerian simulation (Bryan & Norman 1997) produced clusters with temperature profiles similar to or steeper than those observed. Our results thus provide a new constraint for adjusting numerical simulations and, potentially, discriminating among models of cluster formation. Subject headings: Cosmology — galaxies: clusters: individual — intergalactic medium — X-rays: galaxies

Transient States of Matter during Short Pulse Laser Ablation

Abstract: Short pulse laser ablation of semiconductors and metals is studied by means of ultrafast timeresolved microscopy. The characteristic stages of the conversion of solid material into hot fluid matter undergoing ablation are identified. Initially metallic material transforms during the expansion into a transparent state with a high index of refraction. [S0031-9007(98)06502-8]

A bacterial cytokine

Abstract: Viable cells of Micrococcus luteus secrete a factor, which promotes the resuscitation and growth of dormant, nongrowing cells of the same organism. The resuscitation-promoting factor (Rpf) is a protein, which has been purified to homogeneity. In picomolar concentrations, it increases the viable cell count of dormant M. luteus cultures at least 100-fold and can also stimulate the growth of viable cells. Rpf also stimulates the growth of several other high G+C Gram-positive organisms, including Mycobacterium avium, Mycobacterium bovis (BCG), Mycobacterium kansasii, Mycobacterium smegmatis, and Mycobacterium tuberculosis. Similar genes are widely distributed among high G+C Gram-positive bacteria; genome sequencing has uncovered examples in Mycobacterium leprae and Mb. tuberculosis and others have been detected by hybridization in Mb. smegmatis, Corynebacterium glutamicum, and Streptomyces spp. The mycobacterial gene products may provide different targets for the detection and control of these important pathogens. This report is thus a description of a proteinaceous autocrine or paracrine bacterial growth factor or cytokine.

In vivo OCT imaging of hard and soft tissue of the oral cavity

Abstract: We use optical coherence tomography (OCT) to perform a comprehensive program of in vivo and in vitro structural imaging of hard and soft tissues within the oral cavity. We have imaged the different types of healthy oral mucosa as well as normal and abnormal tooth structure. OCT is able to differentiate between the various types of keratinized and non-keratinized mucosa with high resolution. OCT is also able to provide detailed structural information on clinical abnormalities (caries and non-caries lesions) in teeth and provide guidance in dental restorative procedures. Our investigations demonstrate the utility of OCT as a diagnostic imaging modality in clinical and research dentistry.

CrO2: a self-doped double exchange ferromagnet

Abstract: With the help of band structure calculations $(\mathrm{LSDA}+U)$ a clear picture of the physics behind the metallic ferromagnetic properties of ${\mathrm{CrO}}_{2}$ is revealed. It is concluded that ${\mathrm{CrO}}_{2}$ is a negative charge transfer gap material which leads to self-doping and explains why it is a metal in spite of the large Coulomb interactions. We find that there exist in ${\mathrm{CrO}}_{2}$ both localized and itinerant $d$ electrons, resulting in ferromagnetic ordering due to double exchange similar to colossal magnetoresistance manganates.

Liquid-crystal adaptive lenses with modal control.

Abstract: We report on a novel approach to the realization of nematic liquid-crystal (LC) phase correctors to form spherical and cylindrical wave fronts. A LC cell with a distributed reactive electrical impedance was driven by an ac voltage applied to the cell boundary to yield the desired spatial distribution of the refractive index. The two-dimensional function of the phase delay introduced into the light beam depends on the frequency of the ac control voltage, the geometry of the boundary electrode surrounding the LC cell, and the electrical parameters of the cell. We realized a cylindrical adaptive lens with a clear aperture of 15 mm x 4mm and a spherical adaptive lens with circular aperture of 6.5 mm. Both devices are capable of focusing collimated light in the range infinity...0.5 m.

Optical Modes in Photonic Molecules

Abstract: Photonic molecules have been fabricated by coupling pairs of micrometer-sized semiconductor cavities via a narrow channel. The optical modes in these structures have been studied spectroscopically as a function of the coupling and the mode energies are compared to detailed calculations. These results provide a rich picture of photonic modes in these molecules.

Quantum dot heterostructures: Fabrication, properties, lasers (Review)

Abstract: In the present review we summarize original results where 1) we have experimentally discovered a novel class of spontaneously ordered nanostructures, namely equilibrium arrays of threedimensional, coherently strained islands on crystal surfaces; 2) we have developed a theory of spontaneous formation of semiconductor nanostructures in heteroepitaxial systems; 3) we have experimentally demonstrated the existence of a novel class of semiconductor heterostructures, namely perfect quantum dots having an atom-like energy spectrum; we have performed a detailed investigation of the optical properties of quantum dots; 4) we have fabricated quantum dot-based injection lasers demonstrating unique charactristics, namely high-temperature stability of the threshold current and ultra-high material gain.

How to determine an effective potential for a variable cosmological term

Abstract: It is shown that if a variable cosmological term in the present Universe is described by a scalar field with minimal coupling to gravity and with some phenomenological self-interaction potential V(ϕ), then this potential can be unambiguously determined from the following observational data: either from the behavior of density perturbations in dustlike matter component as a function of redshift (given the Hubble constant additionally), or from the luminosity distance as a function of redshift (given the present density of dustlike matter in terms of the critical value).

The history and future of semiconductor heterostructures

Abstract: The history of the development of semiconductor heterostructures and their applications in various electron devices is presented, along with a brief historical survey of the physics, production technology, and applications of quantum wells and superlattices. Advances in recent years in the fabrication of structures utilizing quantum wires and especially quantum dots are discussed. An outline of future trends and prospects for the development and application of these latest types of heterostructures is presented.

Calculus of variations and optimal control

Abstract: First order conditions: First order conditions Theory of a weak minimum for the problem on a fixed time interval Theory of the maximum principle Extremals and the Hamiltonian of a control system Hamilton-Jacobi equation and field theory Transformations of problems and invariance of extremals Quadratic conditions: Quadratic conditions and conjugate points for broken extremals Quadratic conditions for a Pontryagin minimum and sufficient conditions for a strong minimum: Proofs Quadratic conditions in the general problem of the calculus of variations and related optimal control problems Investigation of extremals by quadratic conditions: Examples Bibliography.

Trees tell of past climates: But are they speaking less clearly today?

Abstract: The annual growth of trees, as represented by a variety of ringwidth, densitometric, or chemical parameters, represents a combined record of different environmental forcings, one of which is climat...

A Seven-Gene Locus for Synthesis of Phenazine-1-Carboxylic Acid by Pseudomonas fluorescens 2-79

Abstract: Pseudomonas fluorescens 2-79 produces the broad-spectrum antibiotic phenazine-1-carboxylic acid (PCA), which is active against a variety of fungal root pathogens. In this study, seven genes designated phzABCDEFG that are sufficient for synthesis of PCA were localized within a 6.8-kb BglII-XbaI fragment from the phenazine biosynthesis locus of strain 2-79. Polypeptides corresponding to all phz genes were identified by analysis of recombinant plasmids in a T7 promoter/polymerase expression system. Products of the phzC, phzD, and phzE genes have similarities to enzymes of shikimic acid and chorismic acid metabolism and, together with PhzF, are absolutely necessary for PCA production. PhzG is similar to pyridoxamine-5*-phosphate oxidases and probably is a source of cofactor for the PCA-synthesizing enzyme(s). Products of the phzA and phzB genes are highly homologous to each other and may be involved in stabilization of a putative PCA-synthesizing multienzyme complex. Two new genes, phzX and phzY, that are homologous to phzA and phzB, respectively, were cloned and sequenced from P. aureofaciens 30-84, which produces PCA, 2-hydroxyphenazine-1-carboxylic acid, and 2-hydroxyphenazine. Based on functional analysis of the phz genes from strains 2-79 and 30-84, we postulate that different species of fluorescent pseudomonads have similar genetic systems that confer the ability to synthesize PCA. Certain members of the genus Pseudomonas produce diverse low-molecular-weight (“secondary”) metabolites including nitrogen-containing heterocyclic pigments known as phenazine compounds (5, 19). Phenazines are synthesized by a limited number of bacterial genera including Pseudomonas, Burkholderia, Brevibacterium, and Streptomyces (38). Almost all phenazines exhibit broad-spectrum activity against various species of bacteria and fungi (32). This activity is connected with the ability of phenazine compounds to undergo oxidation-reduction transformations and thus cause the accumulation of toxic superoxide radicals in the target cells (15). Some phenazine compounds can act as bacterial virulence factors. For example, pyocyanin, produced by the opportunistic pathogen Pseudomonas aeruginosa during cystic fibrosis, has been shown to inhibit the ciliary function of respiratory epithelial cells (40). Phenazine antibiotics produced by the biocontrol strains P. fluorescens 2-79 and P. aureofaciens 30-84 are major factors in the ability of these strains to inhibit the growth of fungal root pathogens. Moreover, studies involving phenazine-deficient mutants have clearly demonstrated that antibiotic production in natural habitats plays an important role in the ecological competence and long-term survival of these strains in the environment (21). Over 50 naturally occurring phenazine compounds have been described, and certain bacterial producers are able to synthesize mixtures of as many as 10 different phenazine derivatives at one time (32, 38). Growth conditions