Showing papers by "Moscow State University published in 1993"

Superconductivity at 94 K in HgBa 2 Cu0 4+δ

Abstract: FOLLOWING the discovery1 of high-transition-temperature (high-Tc) superconductivity in doped La2CuO4, several families of related compounds have been discovered which have layers of CuO2 as the essential requirement for superconductivity: the highest transition temperatures so far have been found for thallium-bearing compounds2. Recently the mercury-bearing compound HgBa2Rcu2O6+δ (Hg-1212) was synthesized3 (where R is a rare-earth element), with a structure similar to the thallium-bearing superconductor TlBa2CaCu2O7 (Tl-1212), which has one T1O layer and two CuO2 layers per unit cell, and a Tc of 85 K (ref. 2). But in spite of its resemblance to Tl-1212, Hg-1212 was found not to be superconducting. Here we report the synthesis of the related compound HgBa2CuO4+δ (Hg-1201), with only one CuO2 layer per unit cell, and show that it is superconducting below 94 K. Its structure is similar to that of Tl-1201 (which has a Tc of < 10 K)4, but its transition temperature is considerably higher. The availability of a material with high Tc but only a single metal oxide (HgO) layer may be important for technological applications, as it seems that a smaller spacing between CuO2 planes leads to better superconducting properties in a magnetic field5.

Localization at large disorder and at extreme energies: an elementary derivation

Abstract: The work presents a short proof of localization under the conditions of either strong disorder (λ > λ0) or extreme energies for a wide class of self adjoint operators with random matrix elements, acting inl 2 spaces. A prototypical example is the discrete Schrodinger operatorH=−Δ+U 0(x)+λV x onZ d ,d≧1, withU 0(x) a specified background potential and {V x } generated as random variables. The general results apply to operators with −Δ replaced by a non-local self adjoint operatorT whose matrix elements satisfy: ∑ y |T x,y | S ≦Const., uniformly inx, for somes<1. Localization means here that within a specified energy range the spectrum ofH is of the pure-point type, or equivalently — the wave functions do not spread indefinitely under the unitary time evolution generated byH. The effect is produced by strong disorder in either the potential or in the off-diagonal matrix elementsT x, y . Under rapid decay ofT x, y , the corresponding eigenfunctions are also proven to decay exponentially. The method is based on resolvent techniques. The central technical ideas include the use of low moments of the resolvent kernel, i.e. <|G E (x, y)| s > withs small enough (<1) to avoid the divergence caused by the distribution's Cauchy tails, and an effective use of the simple form of the dependence ofG E (x, y) on the individual matrix elements ofH in elucidating the implications of the fundamental equation (H−E)G E (x,x 0)=δ x,x0 . This approach simplifies previous derivations of localization results, avoiding the small denominator difficulties which have been hitherto encountered in the subject. It also yields some new results which include localization under the following sets of conditions: i) potentials with an inhomogeneous non-random partU 0 (x), ii) the Bethe lattice, iii) operators with very slow decay in the off-diagonal terms (T x,y≈1/|x−y|(d+e)), and iv) localization produced by disordered boundary conditions.

Superconductivity above 120 K in HgBa2CaCu2O6+δ

Abstract: The second member (Hg-1212) of the recently discovered homologous series HgBa2Can−1CunO2n+2+δ has been synthesized by solid state reaction, carried out at 800°C under 60 kbar. The X-ray diffraction powder pattern of one of the samples was refined by the Rietveld analysis. The final results clearly showed that it corresponded to the structural arrangement of the Hg-1212 compound with lattice parameters a = 3.8556 (8) A , c = 12.652 (4) A . No major impurities were observed in the X-ray pattern. AC magnetic susceptibility measurements showed that these samples undergo a paramagnetic-to-diamagnetic transition well above 120 K. The multi-transition aspect of the susceptibility curves showed that the samples contained some intergrowth. In a few samples a transition as high as 132 K was observed which might correspond to the third member of the Hg-series.

The synthesis and characterization of the HgBa2Ca2Cu3O8+δ and HgBa2Ca3Cu4O10+δ phases

Abstract: The third (Hg-1223) and the fourth (Hg-1234) members of the recently-discovered homologous series HgBa2Can−1CunO2n+2+δ have been synthesized by solid state reaction, carried out at 950°C under 50 kbar at different annealing times. These phases have a tetragonal cell with lattice parameters: a = 3.8532 (6) A , c = 15.818(2) A , and a = 3.8540(3) A , c = 19.006 (3) A , respectively. The c parameters are in agreement with the formula c ≌ 9.5 + 3.2 (n − 1). Electron microscopy study showed similar lattice parameters as well as the occurence of different intergrowths and stacking faults. A periodicity of 22 A has also been detected, which may be attributed to the existence of the Hg-1245 phase. EDS analysis data of several grains of Hg-1223 and Hg-1234 are in agreement with the proposed chemical formulae. AC susceptibility measurements show that an increase of the superconducting transition temperature with n in the HgBa2Can−1CunO2n+2+δ series occurs till the third member, after which a saturation seems to be achieved.

Role of the Acylated Amino Terminus of Recoverin in Ca 2+ -Dependent Membrane Interaction

Abstract: Recoverin, a calcium ion (Ca2+)-binding protein of vertebrate photoreceptors, binds to photoreceptor membranes when the Ca2+ concentration is greater than 1 micromolar. This interaction requires a fatty acyl residue covalently linked to the recoverin amino (NH2)-terminus. Removal of the acyl residue, either by proteolytic cleavage of the NH2-terminus or by production of nonacylated recoverin, prevented recoverin from binding to membranes. The acylated recoverin NH2-terminus could be cleaved by trypsin only when Ca2+ was bound to recoverin. These results suggest that the hydrophobic NH2-terminus is constrained in Ca(2+)-free recoverin and liberated by Ca2+ binding. The hydrophobic acyl moiety of recoverin may interact with the membrane only when recoverin binds Ca2+.

Lambek grammars are context free

Abstract: Basic categorial grammars are the context-free ones. Another kind of categorial grammars was introduced by J. Lambek (1958). These grammars are based on a syntactic calculus, known as the Lambek calculus. Chomsky (1963) conjectured that these grammars are also equivalent to context-free ones. Every basic categorial grammar (and thus every context-free grammar) is equivalent to a Lambek grammar. Conversely, some special kinds of Lambek grammars are context-free. These grammars use weakly unidirectional types, or types of order at most two. The main result of this paper says that Lambek grammars generate only context-free languages. Thus they are equivalent to context-free grammars and also to basic categorial grammars. The Chomsky conjecture, that all languages recognized by the Lambek calculus are context-free, is thus proved. >

Mitochondrial benzodiazepine receptor linked to inner membrane ion channels by nanomolar actions of ligands

Abstract: The mitochrondrial benzodiazepine receptor (mBzR) binds a subset of benzodiazepines and isoquinoline carboxamides with nanomolar affinity and consists of the voltage-dependent anion channel, the adenine nucleotide translocator, and an 18-kDa protein. The effect of ligands of the mBzR on two inner mitochondrial membrane channel activities was determined with patch-clamp techniques. The relative inhibitory potencies of the drugs resemble their binding affinities for the mBzR. Ro5-4864 and protoporphyrin IX inhibit activity of the multiple conductance channel (MCC) and the mitochondrial centum-picosiemen (mCtS) channel activities at nanomolar concentrations. PK11195 inhibits mCtS activity at similar levels. Higher concentrations of protoporphyrin IX induce MCC but possibly not mCtS activity. Clonazepam, which has low affinity for mBzR, is at least 500 times less potent at both channel activities. Ro15-1788, which also has a low mBzR affinity, inhibits MCC at very high concentrations (16 microM). The findings indicate an association of these two channel activities with the proteins forming the mBzR complex and are consistent with an interaction of inner and outer membrane channels.

Conformational transitions in polymer gels: Theory and experiment

Abstract: We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account. Experimental results demonstrate a good correlation with theoretical analysis.

Pressure-induced enhancement of tc above 150 k in hg-1223.

Abstract: The recently discovered homologous series HgBa 2 Ca n -1 Cu n O 2 n +2+δ possesses remarkable properties. A superconducting transition temperature, T c , as high as 133 kelvin has been measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable to the Hg-1223 compound. Temperature-dependent electrical resistivity measurements under pressure on a (> 95%) pure Hg-1223 phase are reported. These data show that T c increases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15 gigapascals, then more slowly and reaches a T c = 150 kelvin, with the onset of the transition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as compared to the small effects observed in similar compounds with the optimal T c ) strongly suggests that higher critical temperatures could be obtained at atmospheric pressure.

New RSFQ circuits (Josephson junction digital devices)

Abstract: Several novel circuits of the rapid single-flux-quantum (RSFQ) family of Josephson-junction digital devices have been designed, fabricated using a 2.5- mu m 1000-A/cm/sup 2/ Nb trilayer technology, and tested at low frequencies. Numerical simulation and measurements have shown that these circuits have considerably wider parameter margins, due to application of several novel design methods. The authors have also carried out an experiment to measure the rate of errors in a simple RSFQ circuit including an inverter, confluence buffer, and Josephson transmission line. Near the middle of the parameter window at 4.2 K, the error probability was definitely lower than 3*10/sup -15/ per logic operation, despite experimentation with rudimentary shielding and filtering. >

On the Theory of Trajectory Tubes — A Mathematical Formalism for Uncertain Dynamics, Viability and Control

Abstract: This paper is a survey on the theory of trajectory tubes for differential inclusions, which appears to be a relevant tool for modeling uncertain dynamics. It is motivated by results in nonlinear analysis, particularly, in viability theory for differential inclusions, as well as by recent achievements in the theory of estimation and control for systems with unknown but bounded uncertainties. The motivations for these studies come from applied areas and the rapidly increasing number of applications range from computer sciences and engineering to economics as well as ecological and biomedical modeling. The theory is constructive and has led to the development of effective tools of computational and graphic animation.

On the computer simulation of the deformation and breakup of colloidal aggregates in shear flow

Abstract: Computer simulations are used to investigate the deformation and breakup of colloidal aggregates in shear flow. It is argued that the aggregate radius depends on the shear rate via a power law S -m . The behavior of the aggregates strongly depends on the parameters of the particle interactions. In general the interaction potential of spherical particles is a superposition of central and noncentral components. The former depends only upon the distance between the centers of neighboring particles, while the latter can be described as a function of the angles between adjacent bonds. If the noncentral component is present, the aggregate is rigid ( m = m rigid ); i.e., its internal structure responds elastically to small deformations and can be characterized by the elastic moduli (e.g., Young modulus E ) and yield strength, σ a , depending upon the radius of gyration, R , and the internal volume fraction of particles, φ int , via power laws, E ∼ σ a ∝ R -γ ∝ φ γ1 int . The exponent m in this case can be identified with m rigid ≡ 1/γ. The exponent γ 1 is identical to the exponent characterizing the power-law dependence of the moduli of a colloid gel network consisting of interconnected fractal clusters upon volume fraction. The exponents m rigid , γ and γ 1 can be related to the exponents characterizing geometrical properties of the internal structure of the aggregate and its skeleton (such as fractal dimension d f , chemical dimension d 1 , etc.). For one special type of aggregate (fractal trees without loops with d f = 1.85-2.5, d 1 = 1.1-1.8) we found m rigid = 0.23-0.29, γ 1 = 3.4-7.0 in 3D. The values of γ 1 are in good agreement with the experimental data recently obtained for colloid gels. If the forces of the particles' interactions are purely central, the aggregate is soft ( m = m soft ); i.e., its internal structure does not respond elastically to small deformations. In this case the simulations of the disaggregation process in shear flow were carried out in the free draining approximation. The initial aggregate is broken into two or more secondary aggregates. It is established that the mean radius of these aggregates also depends on S via a power law with m soft = 0.4-0.5. Comparing the results of our analyses with the experimental data we found that in general m rigid and m soft can be considered the lower and upper bounds of m .

Studies of acoustical and shock waves in the pulsed laser ablation of biotissue

Abstract: Quantitative studies are conducted into the absolute pressure values of the acoustical and shock waves generated and propagating in a biotissue under pulsed (tau p = 50 ns) UV (lambda = 308 nm) laser irradiation (below and above the ablation threshold). Powerful (several hundreds of bars in pressure) high-frequency (f approximately 10(7) Hz) acoustic compression and rarefaction pulses are found to be generated in the biotissue. The amplitudes and profiles of the acoustic pulses developing in atherosclerotic human aorta tissues and an aqueous CuCl2 solution under laser irradiation are investigated as a function of the laser pulse energy fluence. The results obtained point to the absence of the cold spallation of the objects of study by rarefaction waves. Based on experimental data, the rise rates, pressure gradients, and propagation velocities of shock waves in the biotissue are calculated. The experimental data are found to agree well with the theoretical estimates.

Some theoretical aspects of thin-film optics and their applications.

Abstract: The principal focus of my report is on the theoretical study of the properties of spectral coefficients in a complex wave-number plane. The basic results of the study are described, and their application to the synthesis of a rugate filter and to inhomogeneous layer recognition problems are considered. General results concerning the existence of solutions to synthesis problems are also presented. The close analogy between synthesis problems in thin-film optics and optimal control problems is outlined, and some applications of Pontryagin's maximum principle are considered.

Neutron powder diffraction study at room temperature and at 10 K of the crystal structure of the 133 K superconductor HgBa2Ca2Cu3O8+δ

Abstract: The crystal structure of the superconductor HgBa2Ca2Cu3O8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8501(1), c=15.7837(9) A . The structure is described by the sequence of layers: …[(CuO2)(Ca)(CuO2)(BaO)(HgOδ(BaO)(CuO2)(Ca)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2Ca2Cu3Ox at a pressure of 18 kbar and 880°C, has an oxygen composition corresponding to δ=0.41 (2). This extra oxygen is located on the layer of the mercury atoms at the sites 1 2 , 1 2 , 1 2 . The superconducting transition temperature for the as-prepared sample is Tc=133 K. No evidence has been found of orthorhombic distortions, nor for substitution of some of the Hg atoms by Cu. The copper and oxygen atoms of the layers (CuO2) are coplanar for the Z=0 (CuO2 plane and almost perfectly coplanar for the others. No phase transition occurs down to 10 K.

Neutron powder diffraction study of the crystal structures of HgBa2CuO4+δ and HgBaO2

Abstract: The crystal structures of HgBa2CuO4+δ and HgBaO2 have been refined at room temperature using neutron powder diffraction data obtained from a sample containing 44% of the first phase, 42% of the second and 14% of Ba2Cu3O5+δ. the compound HgBa2CuO4+δ crystallizes with the symmetry of space group P4/mmm and lattice parameters a = 3.8829 (6), c = 9.5129 (14) A . The unit cell contains only one CuO2 layer and the material is a superconductor with a value of Tc of 94 K. The oxygen in excess of the O4 stoichiometry (δ = 0.063 in our sample) is located in interstitial positions on the HgOδ layer, and it is the only extra oxygen present in the structure. No mixing of the cations has been detected in this study. The compound HgBaO2 crystallizes with the symmetry of space group R 3 m and lattice parameters (hexagonal axes) a = 4.0991 (6), c = 19.355 (3) A . In this new polymorph of HgBaO2 the Ba atoms are octahedrally coordinated while Hg has two-fold coordination.

The RNA polymerase I-specific transcription initiation factor UBF is associated with transcriptionally active and inactive ribosomal genes

Abstract: We have characterized an anti-NOR (nucleolar organizer region) serum (P419) from a patient with rheumatoid arthritis and show that it contains antibodies directed against the RNA polymerase I-specific transcription initiation factor UBF. This serum reacts with UBF from a variety of vertebrate cells as revealed both by immunoblotting and by indirect immunofluorescence. We have used the P419 serum to study the intracellular localization of this transcription factor at the light and electron microscopic level. In interphase cells, UBF exhibits a pronounced punctate pattern and is found to be associated with necklace-like structures, which appear to reflect the transcriptionally active state of the nucleolus. Inhibition of rRNA synthetic activity caused either by nutritional starvation or by actinomycin D treatment resulted in a marked decrease in the number and in a significant increase in the size of UBF-positive granules. Under all experimental conditions applied, UBF was exclusively found within the nucleolus and was not released into the nucleoplasm or cytoplasm. During mitosis, UBF was found to be concentrated at the chromosomal NOR indicating that a significant quantity, if not all, of this factor remains bound to the ribosomal transcription units. From this we conclude that UBF is associated both with transcriptionally active and inactive rRNA genes and, therefore, changes in the intracellular localization of UBF are very likely not involved in rDNA transcription regulation.