Showing papers by "Russian Academy of Sciences published in 1996"

Detection, mixing, and frequency multiplication of terahertz radiation by two-dimensional electronic fluid

Abstract: We show that a short channel High Electron Mobility Transistor (HEMT) has a resonance response to electromagnetic radiation at the plasma oscillation frequencies of the two dimensional electrons in the device. This response can be used for new types of detectors, mixers, and multipliers. These devices should operate at much higher frequencies than conventional, transit-time limited devices, since the plasma waves propagate much faster than electrons. The responsivities of such devices may greatly exceed the responsivities of Schottky diodes currently used as detectors and mixers in the terahertz range. A long channel HEMT has a nonresonant response to electromagnetic radiation and can be used as a broadband detector for frequencies up to several tens of terahertz.

Intermediate-spin state and properties of LaCoO3.

Abstract: The electronic structure of the perovskite ${\mathrm{LaCoO}}_{3}$ for different spin states of Co ions was calculated in the local-density approximation LDA+U approach The ground state is found to be a nonmagnetic insulator with Co ions in a low-spin state Somewhat higher in energy, we find two intermediate-spin states followed by a high-spin state at significantly higher energy The calculations show that Co 3d states of ${\mathit{t}}_{2\mathit{g}}$ symmetry form narrow bands which could easily localize, while ${\mathit{e}}_{\mathit{g}}$ orbitals, due to their strong hybridization with the oxygen 2p states, form a broad \ensuremath{\sigma}* band With temperature variation which is simulated by a corresponding change of the lattice parameters, a transition from the low- to intermediate-spin state occurs This intermediate-spin (occupation ${\mathit{t}}_{2\mathit{g}}^{5}$${\mathit{e}}_{\mathit{g}}^{1}$) can develop an orbital ordering which can account for the nonmetallic nature of ${\mathrm{LaCoO}}_{3}$ at 90 KT500 K Possible explanations of the magnetic behavior and gradual insulator-metal transition are suggested \textcopyright{} 1996 The American Physical Society

Synthesis of Novel Thin-Film Materials by Pulsed Laser Deposition

Abstract: Pulsed laser deposition (PLD) is a conceptually and experimentally simple yet highly versatile tool for thin-film and multilayer research. Its advantages for the film growth of oxides and other chemically complex materials include stoichiometric transfer, growth from an energetic beam, reactive deposition, and inherent simplicity for the growth of multilayered structures. With the use of PLD, artificially layered materials and metastable phases have been created and their properties varied by control of the layer thicknesses. In situ monitoring techniques have provided information about the role of energetic species in the formation of ultrahard phases and in the doping of semiconductors. Cluster-assembled nanocrystalline and composite films offer opportunities to control and produce new combinations of properties with PLD.

Structural and Dynamic Parameters Obtained from 17O NMR, EPR, and NMRD Studies of Monomeric and Dimeric Gd3+ Complexes of Interest in Magnetic Resonance Imaging: An Integrated and Theoretically Self-Consistent Approach1

Abstract: We present the results of new and previously published 17O NMR, EPR, and NMRD studies of aqueous solutions of the Gd3+ octaaqua ion and the commercial MRI contrast agents [Gd(DTPA)(H2O)]2- (MAGNEVIST, Schering AG, DTPA = 1,1,4,7,7-pentakis(carboxymethyl)-1,4,7-triazaheptane), [Gd(DTPA-BMA)(H2O)] (OMNISCAN, Sanofi Nycomed, DTPA-BMA = 1,7-bis[(N-methylcarbamoyl)methyl]-1,4,7-tris(carboxymethyl)-1,4,7-triazaheptane), and [Gd(DOTA)(H2O)]- (DOTAREM, Guerbet, DOTA = 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). High-field EPR measurements at different concentrations give evidence of an intermolecular dipole−dipole electronic relaxation mechanism that has not previously been described for Gd3+ complexes. For the first time, the experimental data from the three techniques for each complex have been treated using a self-consistent theoretical model in a simultaneous multiple parameter least-squares fitting procedure. The lower quality of the fits compared to separate fits of the data for each o...

The Bering Sea Green Belt: shelf-edge processes and ecosystem production

Abstract: The concept of a highly productive habitat, or Green Belt, along the edge of the continental shelf in the Bering Sea is based upon compelling but fragmentary and often anecdotal observations of a variety of physical and biological features acquired from many sources over many years. Enhanced production at continental margins is not a novel concept, but in the case of the Bering Sea its importance has been overlooked during studies of the unusually broad continental shelf. The limited data reported from the vicinity of the shelf edge in the Bering Sea indicate that annual primary production can be as high as 175 to 275 g C m˜ year-, or approximately 60% greater than production in the adjacent outer shelf domain and 270% greater than in the oceanic domain. Estimates of annual secondary production at the eastern shelf edge also average approximately 60% higher than estimates for the outer domain and 260% higher than those for the oceanic domain. Physical processes at the shelf edge, such as intensive tidal mixing and transverse circulation and eddies in the Bering Slope Current, bring nutrients into the euphoric zone and contribute to enhanced primary and secondary production and elevated biomass of phytoplankton and zooplankton. Fishes and squids concentrate in this narrow corridor because of favourable feeding conditions and because of a thermal refuge from cold shelf-bottom temperatures that can be found at the shelf edge from fall to spring. The abundance of zooplankton, fishes and squids, in turn, attracts large numbers of marine birds and mammals. In aggregate, the observations suggest that sustained primary productivity, intense food web exchange and high transfer efficiency at the shelf edge are important to biomass yield at numerous trophic levels and to ecosystem production of the Bering Sea.

Relatively recent construction of the Tien Shan inferred from GPS measurements of present-day crustal deformation rates

Abstract: THE Tien Shan—a high, seismically active intracontinental mountain belt, 1,000–2,000 km north of the Himalaya—has grown as a result of India's collision with Asia1. The crustal shortening (~ 200 ± 50 km; refs 2, 3) and thickening that gave rise to the Tien Shan accommodates only a small fraction of India's total penetration into Asia (2,000—3,000km), and the temporal relationship of deformation in this belt to the India–Asia collision remains unclear. Here we report geodetic measurements of the Tien Shan, using the Global Positioning System (GPS), that indicate that the current crustal shortening rate is nearly half of India's convergence rate with Eurasia in this area4. We infer a total shortening rate for the Tien Shan of ~20 mm yr−1, which is approximately twice that inferred previously from the extrapolation of slip rates in the Holocene3 and earthquake-induced displacements during this century5, suggesting that the rate of mountain building in this region has accelerated several-fold since the onset of collision ~50–55 Myr ago6,7. If, as we argue, the current shortening rate can be extrapolated to geological timescales, then our results suggest that most of the Tien Shan has been constructed during the past 10 Myr, perhaps in response to an increased horizontal force following an abrupt rise of the Tibetan plateau8,9.

Electroweak baryon number non-conservation in the early Universe and in high-energy collisions

Abstract: We review recent progress in the study of the anomalous baryon number non-conservation at high temperatures and in high energy collisions. Recent results on high temperature phase transitions are described, and applications to electroweak baryogenesis are considered. The current status of the problem of electroweak instanton-like processes at high energies is outlined. This paper is written on the occasion of Sakharov’s 75th anniversary and will appear in the memorial volume of Uspekhi (Usp. Fiz. Nauk, volume 166, No 5, May 1996).

Human minisatellite mutation rate after the Chernobyl accident

Abstract: Germline mutation at human minisatellite loci has been studied among children born in heavily polluted areas of the Mogilev district of Belarus after the Chernobyl accident and in a control population. The frequency of mutation was found to be twice as high in the exposed families as in the control group. Mutation rate in the Mogilev families was correlated with the level of caesium-137 surface contamination, consistent with radiation induction of germline mutation.

Plasma wave electronics: novel terahertz devices using two dimensional electron fluid

Abstract: We discuss how the propagation of plasma waves in a High Electron Mobility Transistor (HEMT) can be used to implement a new generation of terahertz devices, including sources, resonant detectors, broad band detectors, and frequency multipliers. Our estimates show that these devices should outperform conventional terahertz devices, which use deep submicron Schottky diodes.

Quantum-size effects in the thermodynamic properties of metallic nanoparticles

Abstract: THE properties of nanometre-scale metallic particles differ from those of the same material1,2 in bulk. Conduction electrons, because of their wave-like nature, can have only certain discrete values of kinetic energy or wavelength. Such 'quantum-size' effects have been observed in two-dimensional electron gases in semiconductors3,4, and in atomic-scale metallic point contacts5. Also present are 'Coulomb-charging' effects: these are purely classical in origin, and occur when the energy required to add one electron to a conducting sphere exceeds the mean thermal energy kBT. Thermal fluctuations in the total charge on the particle are then suppressed6. In theory, the combination of quantum-size and Coulomb-charging effects should cause the properties of small metallic particles to depend sensitively on whether they have an odd or even number of electrons7. Odd–even effects have been observed in experiments on tunnelling between discrete electronic levels of single metal particles8, but their influence on thermodynamic properties remains to be demonstrated. Here we report measurements of the heat capacity and electronic magnetic susceptibility of small metallic clusters. Our results show definitive evidence for odd–even effects, thus confirming that quantum and classical size effects strongly influence the thermodynamic properties of small particles.

Detection of coloured stimuli by honeybees: minimum visual angles and receptor specific contrasts

Abstract: Honeybees Apis mellifera were trained to distinguish between the presence and the absence of a rewarded coloured spot, presented on a vertical, achromatic plane in a Y-maze. They were subsequently tested with different subtended visual angles of that spot, generated by different disk diameters and different distances from the decision point in the device. Bees were trained easily to detect bee-chromatic colours, but not an achromatic one. Chromatic contrast was not the only parameter allowing learning and, therefore, detection: α min, the subtended visual angle at which the bees detect a given stimulus with a probability P 0 = 0.6, was 5° for stimuli presenting both chromatic contrast and contrast for the green photoreceptors [i.e. excitation difference in the green photoreceptors, between target and background (green contrast)], and 15° for stimuli presenting chromatic but no green contrast. Our results suggest that green contrast can be utilized for target detection if target recognition has been established by means of the colour vision system. The green-contrast signal would be used as a far-distance signal for flower detection. This signal would always be detected before chromatic contrast during an approach flight and would be learned in compound with chromatic contrast, in a facilitation-like process.

A large deep freshwater lake beneath the ice of central East Antarctica

Abstract: IN 1974–75, an airborne radio-echo survey of ice depths over central East Antarctica led to the discovery of a sub-ice lake of unknown depth and composition, with an area of about 10,000km2 and lying beneath ∼4km of ice1. In 1993, altimetric data from satellite measurements2 provided independent evidence of the lake's areal extent, thus confirming it to be the largest known sub-ice lake by an order of magnitude. Here we analyse new altimetric and radio-echo data, along with existing seismic data3, to show that the lake is deep (mean depth of 125 m or more) and fresh, and that it has an area that exceeds previous estimates by about 50%—dimensions comparable with those of Lake Ontario. We estimate that the residence time of the water in the lake is of the order of tens of thousands of years, and that the mean age of water in the lake, since deposition as surface ice, is about one million years. Regional ice-dynamics can be explained in terms of steady-state ice flow along and over the lake.

Electromagnetic properties of composites containing elongated conducting inclusions

Abstract: We present a detailed theoretical study of the dielectric and magnetic response of composites containing elongated conducting inclusions---sticks. These composites are widely used as engineering materials. They can be also considered as a model to describe many processes occurring in nature, e.g., dielectric enhancement in grain-saturated porous rocks. An approach is proposed that is based on the idea of a scale-dependent local dielectric constant. We develop an effective-medium approximation and derive an equation to calculate an effective dielectric constant of the composites in the quasistatic case and for the high frequency when there is a strong skin effect in the conducting sticks. Our theory predicts very large values of the effective dielectric constant in a wide range of the stick concentration. We find that the dielectric constant can exhibit various dispersive behaviors. It can have relaxation behavior, power-law scaling behavior, or resonance dependence on the frequency. The resonance dependence occurs when the skin effect is strong and wavelength is comparable to the stick length. Then the real part of the dielectric constant has negative values in some frequency ranges. The possibility of a wave localization is discussed in that case. We consider effective magnetic properties of the conducting stick composites. We propose that the composites with nonmagnetic components will have a giant paramagnetic response as a result of a collective interaction of the sticks with an external magnetic field. \textcopyright{} 1996 The American Physical Society.

The X-ray structure of human serum ceruloplasmin at 3.1.Å: nature of the copper centres

Abstract: The X-ray structure of human serum ceruloplasmin has been solved at a resolution of 3.1 A. The structure reveals that the molecule is comprised of six plastocyanin-type domains arranged in a triangular array. There are six copper atoms; three form a trinuclear cluster sited at the interface of domains 1 and 6, and there are three mononuclear sites in domains 2, 4 and 6. Each of the mononuclear coppers is coordinated to a cysteine and two histidine residues, and those in domains 4 and 6 also coordinate to a methionine residue; in domain 2, the methionine is replaced by a leucine residue which may form van der Waals type contacts with the copper. The trinuclear centre and the mononuclear copper in domain 6 form a cluster essentially the same as that found in ascorbate oxidase, strongly suggesting an oxidase role for ceruloplasmin in the plasma.

Chemistry and electronic properties of metal-organic semiconductor interfaces: Al, Ti, In, Sn, Ag, and Au on PTCDA.

Abstract: The chemistry and electronic properties of interfaces formed between thin films of the archetype molecular organic semiconductor 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA) and reactive and nonreactive metals are investigated via synchrotron radiation photoemission spectroscopy. In, Al, Ti, and Sn react at room temperature with the anhydride group of the PTCDA molecule, producing heavily oxidized interface metal species and thick interfacial layers with a high density of states in the PTCDA band gap. The penetration of the reactive metal species in the PTCDA film is found to be inversely related to their first ionization energy. The noble metals Ag and Au form abrupt, unreacted interfaces. The chemical and structural results correlate well with the electrical properties of the interfaces that show Ohmic behavior with the reactive metal contacts and blocking characteristics with the noble metals. The Ohmic behavior of the reactive metal contacts is ascribed to carrier hopping and/or tunneling through the reaction-induced interface states. \textcopyright{} 1996 The American Physical Society.

Cloud feedback in atmospheric general circulation models: An update

Abstract: Six years ago, we compared the climate sensitivity of 19 atmospheric general circulation models and found a roughly threefold variation among the models; most of this variation was attributed to differences in the models' depictions of cloud feedback. In an update of this comparison, current models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the models still have physical disagreements.

Gene recognition via spliced sequence alignment

Abstract: Gene recognition is one of the most important problems in computational molecular biology. Previous attempts to solve this problem were based on statistics, and applications of combinatorial methods for gene recognition were almost unexplored. Recent advances in large-scale cDNA sequencing open a way toward a new approach to gene recognition that uses previously sequenced genes as a clue for recognition of newly sequenced genes. This paper describes a spliced alignment algorithm and software tool that explores all possible exon assemblies in polynomial time and finds the multiexon structure with the best fit to a related protein. Unlike other existing methods, the algorithm successfully recognizes genes even in the case of short exons or exons with unusual codon usage; we also report correct assemblies for genes with more than 10 exons. On a test sample of human genes with known mammalian relatives, the average correlation between the predicted and actual proteins was 99%. The algorithm correctly reconstructed 87% of genes and the rare discrepancies between the predicted and real exon-intron structures were caused either by short (less than 5 amino acids) initial/terminal exons or by alternative splicing. Moreover, the algorithm predicts human genes reasonably well when the homologous protein is nonvertebrate or even prokaryotic. The surprisingly good performance of the method was confirmed by extensive simulations: in particular, with target proteins at 160 accepted point mutations (PAM) (25% similarity), the correlation between the predicted and actual genes was still as high as 95%.

Classical Decay of the Inflaton

Abstract: We present the first fully nonlinear calculation of inflaton decay. We map inflaton decay onto an equivalent classical problem and solve the latter numerically. In the $\ensuremath{\lambda}{\ensuremath{\varphi}}^{4}$ model, we find that parametric resonance develops slower and ends at smaller values of fluctuating fields, as compared to estimates existing in literature. We also observe a number of qualitatively new phenomena, including a stage of semiclassical thermalization, during which the decay of inflaton is essentially as effective as during the resonance stage.

Wavelet analysis: basic theory and some applications

Abstract: The basic theory of the wavelet transform, an effective investigation tool for inhomogeneous processes involving widely different scales of interacting perturbations, is presented. In contrast to the Fourier transform, with the analysing function extending over the entire axis of time, the two-parametric analysing function of the one-dimensional wavelet transform is well localised in both time and frequency. The potential of the method is illustrated by analysing familiar model series (such as harmonic, fractal, and those with various types of singularities) and the long-term variation of some meteorological characteristics (Southern Oscillation index and global and hemispheric temperatures). The analysis of a number of El Nino events and of the temporal behaviour of the Southern Oscillation index reveals periodic components, local periodicity features and time scales on which self-similarity structures are seen. On the whole, both stochastic and regular components seem to be present. The global and hemispheric temperatures are qualitatively similar in structure, the main difference — presumably due to the greater amount of land and stronger anthropogenic factor — being that the warming trend in the Northern Hemisphere is slightly stronger and goes first in time.

Determination of Protein Secondary Structure

Abstract: Circular dichroism (CD) is one of the most sensitive physical techniques for determining structures and monitoring structural changes of biomolecules. It can directly interpret the changes of protein secondary structure, even though the method is empirical. The far-ultraviolet (far-UV) CD specra (below 250 nm) of proteins are extremely sensitive toward protein structure, and the near-UV spectra reflect the contributions of aromatic side chains, disulfide bonds, and induced CD bands of prosthetic groups. Together these measurements provide information about the overall structure of a protein molecule as well as its local conformation around the aromatic and prosthetic groups and disulfide linkages. The ease of CD measurements is attractive, but CD, unlike two other powerful techniques—x-ray diffraction of protein crystals and NMR for protein solutions—cannot determine the three-dimensional structure of a protein. In this chapter we will discuss several methods of CD analysis of proteins, which can provide estimates of α helix, β sheet, β turn, and unordered form. These empirical methods utilize a set of reference proteins of known structure from x-ray diffraction studies. Thus, proteins are presumed to have the same structure in the crystalline state and in aqueous solution.

An investigation of grain boundaries in submicrometer-grained al-mg solid solution alloys using high-resolution electron microscopy

Abstract: High-resolution electron microscopy was used to examine the structural features of grain boundaries in Al–1.5% Mg and Al–3% Mg solid solution alloys produced with submicrometer grain sizes using an intense plastic straining technique. The grain boundaries were mostly curved or wavy along their length, and some portions were corrugated with regular or irregular arrangements of facets and steps. During exposure to high-energy electrons, grain boundary migration occurred to reduce the number of facets and thus to reduce the total boundary energy. The observed features demonstrate conclusively that the grain boundaries in these submicrometer-grained materials are in a high-energy nonequilibrium configuration.