bluuebunny/crossref_metadata_2025 huggingface dataset:
Modulatory effect of cAMP on fungal ergosterol level and inhibitory activity of azole drugs
Authors: Sardari S , Mori Y , Kurosawa T , Daneshtalab M
Published: July 2003

The functions and biosynthesis of sterols have been effective targets for fungal control in different areas, including pharmaceutical and agricultural applications. Fungi are among the organisms that synthesize sterols, principally ergosterol. In this paper, the effect of dibutyryl-cAMP (db-cAMP) on ergosterol level and the interaction of drugs that would change the concentration of cAMP with antifungal drugs have been investigated. Sterols were extracted from Candida albicans, and ergosterol was measured using the gas chromatography method. The interaction of different agents was measured by the broth dilution method. It was found that phosphodiesterase inhibitors reverse the inhibitory activity of azole antifungal drugs. Evaluating the ergosterol level of C. albicans incubated with db-cAMP revealed that it increased ergosterol level. Further experiments provided evidence attributing the observed interaction between azoles and phosphodiesterase inhibitors to the relationship between ergosterol and cAMP. The possible significance of this interaction includes potentiation of antifungal activity of drugs by manipulating the cAMP level.Key words: ergosterol, cAMP, phosphodiesterase inhibitor, antifungal.
Prevalence ofEscherichia coliO157:H7 andSalmonellaspp. in surface waters of southern Alberta and its relation to manure sources
Authors: Johnson J YM , Thomas J E , Graham T A , Townshend I , Byrne J , Selinger L B , Gannon V PJ
Published: July 2003

The Oldman River watershed in southern Alberta, Canada, is an extensively irrigated region in which intensive agricultural practices have flourished. Concern over water quality in the basin has been expressed because of high levels of enteric disease indigenous to the region. To address these concerns, we conducted a 2-year study to estimate the prevalence of Escherichia coli O157:H7 and Salmonella spp. in surface water within the basin. This study is the first of its kind to identify E. coli O157:H7 repeatedly in surface water collected from a Canadian watershed. Prevalence of E. coli O157:H7 and Salmonella spp. in water samples was 0.9% (n = 1483) and 6.2% (n = 1429), respectively. While data examined at a regional level show a relationship between high livestock density and high pathogen levels in southern Alberta, statistical analysis of point source data indicates that predicted manure output from bovine, swine, and poultry feeding operations was not directly associated with either Salmonella spp. or E. coli O157:H7 prevalence. However, geography and weather variables, which are likely to influence bacterial runoff, were not considered in this model. We also postulate that variations in time, amount, and frequency of manure application onto agricultural lands may have influenced levels of surface-water contamination with these bacterial pathogens.Key words: water, Salmonella, E. coli O157:H7, manure output, agriculture.
Wild-type Escherichia coli producing microcins B17, D93, J25, and L; cloning of genes for microcin L production and immunity
Authors: Sablé S , Duarte M , Bravo D , Lanneluc I , Pons A M , Cottenceau G , Moreno F
Published: July 2003

For the first time, an Escherichia coli strain producing four microcins (Mcc), B17, D93, J25, and L, and showing immunity to Mcc V was isolated and characterized. Each of the gene clusters encoding the production of Mcc B17, D93, and L was cloned separately. The gene cluster for Mcc L was cloned within a 13.5-kb HindIIISalI fragment, which includes the Mcc V immunity gene, cvi.Key words: microcin B17, microcin D93, microcin L, multiproduction, cloning.
High TNT-transforming activity by a mixed culture acclimated and maintained on crude-oil-containing media
Authors: Popesku Jason T , Singh Ajay , Zhao Jian-Shen , Hawari Jalal , Ward Owen P
Published: July 2003

A mixed microbial culture originating from a petroleum-contaminated site and maintained on crude oil exhibited high 2,4,6-trinitrotoluene (TNT) transformation activity. Cultivation of the mixed culture in glucose-containing medium for 29 h resulted in almost complete transformation of 100 ppm TNT. TNT transformation was observed with both growing and resting cells. With subculturing, it was found that TNT could support growth of the mixed culture when supplied as sole carbon source, sole nitrogen source, or sole carbon and nitrogen source. The finding that a mixed microbial culture maintained on crude oil exhibited high TNT transformation activity without prior subculture on TNT-containing media is novel and may have potential practical applications in the bioremediation of munitions-contaminated soil and wastewater.Key words: 2,4,6-trinitrotoluene, mixed culture, transformation, co-metabolism, surfactant.
Singular Value Decomposition Learning on Double Stiefel Manifold
Authors: Fiori Simone
Published: July 2003

The aim of this paper is to present a unifying view of four SVD-neural-computation techniques found in the scientific literature and to present some theoretical results on their behavior. The considered SVD neural algorithms are shown to arise as Riemannian-gradient flows on double Stiefel manifold and their geometric and dynamical properties are investigated with the help of differential geometry.
Associative Memory Design Using Overlapping Decomposition and Generalized Brain-State-in-a-Box Neural Networks
Authors: Oh Cheolhwan , Żak Stanislaw H.
Published: July 2003

This paper is concerned with large scale associative memory design. A serious problem with neural associative memories is the quadratic growth of the number of interconnections with the problem size. An overlapping decomposition algorithm is proposed to attack this problem. Specifically, a pattern to be processed is decomposed into overlapping sub-patterns. Then, neural sub-networks are constructed that process the sub-patterns. An error correction algorithm operates on the outputs of each sub-network in order to correct the mismatches between sub-patterns that are obtained from the independent recall processes of individual sub-networks. The performance of the proposed large scale associative memory is illustrated using two-dimensional images. It is shown that the proposed method reduces the computing cost of the design of the associative memories compared with non-interconnected associative memories.
Global and Robust Stability of Interval Hopfield Neural Networks with Time-Varying Delays
Authors: Liao Xiaofeng , Wang Jun , Cao Jinde
Published: July 2003

In this paper, we investigate the problem of global and robust stability of a class of interval Hopfield neural networks that have time-varying delays. Some criteria for the global and robust stability of such networks are derived, by means of constructing suitable Lyapunov functionals for the networks. As a by-product, for the conventional Hopfield neural networks with time-varying delays, we also obtain some new criteria for their global and asymptotic stability.
Global Convergence of Delayed Neural Network Systems
Authors: Lu Wenlian , Rong Libin , Chen Tianping
Published: July 2003

In this paper, without assuming the boundedness, strict monotonicity and differentiability of the activation functions, we utilize a new Lyapunov function to analyze the global convergence of a class of neural networks models with time delays. A new sufficient condition guaranteeing the existence, uniqueness and global exponential stability of the equilibrium point is derived. This stability criterion imposes constraints on the feedback matrices independently of the delay parameters. The result is compared with some previous works. Furthermore, the condition may be less restrictive in the case that the activation functions are hyperbolic tangent.
A Hybrid Neural Network of Addressable and Content-Addressable Memory
Authors: Ma Jinwen
Published: July 2003

We investigate the memory structure and retrieval of the brain and propose a hybrid neural network of addressable and content-addressable memory which is a special database model and can memorize and retrieve any piece of information (a binary pattern) both addressably and content-addressably. The architecture of this hybrid neural network is hierarchical and takes the form of a tree of slabs which consist of binary neurons with the same array. Simplex memory neural networks are considered as the slabs of basic memory units, being distributed on the terminal vertexes of the tree. It is shown by theoretical analysis that the hybrid neural network is able to be constructed with Hebbian and competitive learning rules, and some other important characteristics of its learning and memory behavior are also consistent with those of the brain. Moreover, we demonstrate the hybrid neural network on a set of ten binary numeral patters.
Cost Component Analysis
Authors: Lörincz András , Póczos Barnabás
Published: July 2003

In optimizations the dimension of the problem may severely, sometimes exponentially increase optimization time. Parametric function approximatiors (FAPPs) have been suggested to overcome this problem. Here, a novel FAPP, cost component analysis (CCA) is described. In CCA, the search space is resampled according to the Boltzmann distribution generated by the energy landscape. That is, CCA converts the optimization problem to density estimation. Structure of the induced density is searched by independent component analysis (ICA). The advantage of CCA is that each independent ICA component can be optimized separately. In turn, (i) CCA intends to partition the original problem into subproblems and (ii) separating (partitioning) the original optimization problem into subproblems may serve interpretation. Most importantly, (iii) CCA may give rise to high gains in optimization time. Numerical simulations illustrate the working of the algorithm.
Quintessence Problem and Brans–Dicke Theory
Authors: Chakraborty Subenoy , Chakraborty N. C. , Debnath Ujjal
Published: July 2003

It has been shown that Brans–Dicke (BD) theory in anisotropic cosmological model can solve the quintessence problem and we have accelerated expanding universe without any quintessence matter. Also the flatness problem has been discussed in this context.
Hidden Degeneracy in the Brick Wall Model of Black Holes
Authors: Gupta Kumar S. , Sen Siddhartha
Published: July 2003

Quantum field theory in the near-horizon region of a black hole predicts the existence of an infinite number of degenerate modes. Such a degeneracy is regulated in the brick wall model by the introduction of a short distance cutoff. In this paper we show that states of the brick wall model with nonzero energy admit a further degeneracy for any given finite value of the cutoff. The black hole entropy is calculated within the brick wall model taking this degeneracy into account. Modes with complex frequencies however do not exhibit such a degeneracy.
Three-Neutrino Mass Matrices with Two Texture Zeros
Authors: Desai Bipin R. , Vaucher Alexander R. , Roy D. P.
Published: July 2003

Out of the 15 3 × 3 neutrino mass matrices with two texture zeros, seven are compatible with the neutrino oscillation data. While two of them correspond to hierarchical neutrino masses and 1 to an inverted hierarchy, the remaining four correspond to degenerate masses. Moreover only the first three of the seven mass matrices are compatible with the maximal mixing angle of atmospheric neutrino and hence favored by data. We give compact expressions for mass matrices in terms of mass eigenvalues and study phenomenological implications for the seven cases. Similarity of the textures of the neutrino, charged-lepton mass matrices with those of quark mass matrices is also discussed.
Diluting Solutions of the Cosmological Constant Problem
Authors: Corradini Olindo , Iglesias Alberto , Kakushadze Zurab
Published: July 2003

We discuss the cosmological constant problem in the context of higher codimension brane world scenarios with infinite-volume extra dimensions.
Gauge Model of Quark–Lepton Nonuniversality
Authors: Li Xiao-Yuan , Ma Ernest
Published: July 2003

We consider a gauge model where quark–lepton universality is an accidental symmetry which is only approximate, in analogy to the well-accepted notion that strong isospin is accidental and approximate. This is a natural framework for explaining possible small deviations of quark–lepton universality which is applicable to the recently reported apparent nonunitarity of the quark mixing matrix. As a result, small departures from quark–lepton universality are expected in Z decays as well as in the recent neutrino data of the NuTeV collaboration and in future low-energy experiments. New physics is predicted at the TeV scale.
MSW Effect in Loop Quantum Gravity and Constraints on Parameters from Neutrino–Antineutrino Transitions
Authors: Lambiase G.
Published: July 2003

The MSW effect is studied in the context of loop quantum gravity. In this theory the dispersion relation of fermion depends on the helicity of particles. Bounds on parameters related to helicity-terms, which violate the Lorentz invariance, are derived in the framework of spin-flip conversion of neutrino flavors.
Exploring CP-Violation Through B Decays
Authors: Fleischer Robert
Published: July 2003

The B-meson system provides many strategies to perform stringent tests of the Standard-Model description of CP-violation. In this brief review, we discuss implications of the currently available B-factory data on the angles α, β and γ of the unitarity triangle, emphasize the importance of Bsstudies at hadronic B experiments, and discuss new, theoretically clean strategies to determine γ.
Charged Coherent States Related to suq(2) Covariance
Authors: Li Yun , Jing Sicong
Published: July 2003

A new kind of q-deformed charged coherent states is constructed in Fock space of two-mode q-boson system with su q(2) covariance and a resolution of unity for these states is derived. We also present a simple way to obtain these coherent states using state projection method.
On Marginal Deformation of WZNW Model and PP-Wave Limit of Deformed AdS3 × S3 String Geometry
Authors: Manvelyan Ruben
Published: July 2003

We discuss the Penrose limit of the classical string geometry obtained from a truly marginal deformation of SL (2) ⊗ SU (2) WZNW model.
Renormalizable Abelian-Projected Effective Gauge Theory Derived from Quantum Chromodynamics II
Authors: Shinohara Toru
Published: July 2003

In the previous paper,1 we derived the Abelian projected effective gauge theory as a low energy effective theory of the SU (N) Yang–Mills theory by adopting the maximal Abelian gauge. At that time, we have demonstrated the multiplicative renormalizability of the propagators for the diagonal gluon and the dual Abelian antisymmetric tensor field. In this paper, we show the multiplicative renormalizability of the Green's functions also for the off-diagonal gluon. Moreover, we complement the previous results by calculating the anomalous dimension and the renormalization group functions which are undetermined in the previous paper.
Matter Determinants in Background Fields Using Random Walk World Line Loops on the Lattice
Authors: Schmidt Michael G. , Stamatescu Ion-Olimpiu
Published: July 2003

The continuum World Line Formalism permits a transparent discussion of bosonic and fermionic determinants in some background field. For general, nontrivial backgrounds numerical evaluations must be envisaged. In this work we implement this formalism on the lattice by using statistically generated random walk world line loops. We illustrate the method by applying it to special cases and discuss the results in comparison with known analytic solutions in continuum.
Neutral Higgs $H^0\to
h^0(A^0)l^-_il^+_j$ Decay
Authors: Iltan E. O.
Published: July 2003

We study the lepton flavor violating [Formula: see text] and the lepton flavor conserving [Formula: see text] decays in the framework of the general two-Higgs doublet model, the so-called model III. We estimate the decay width of LFV (LFC) process at the order of magnitude of 10-5 GeV (10-4 GeV), for mH0 = 150 GeV , and the intermediate values of the coupling [Formula: see text]. The experimental result of the process under consideration can ensure comprehensive information about the standard model Higgs boson H0 and the model III free parameters.
A Large-D Weyl Invariant String in Anti-de Sitter Space
Authors: Davies Ian , Mansfield Paul
Published: July 2003

The bosonic string propagating in AdSD+1 is made Weyl invariant to leading and sub-leading order in large D by a ghost–matter coupling that despite modifying the target-space still preserves the Poincaré symmetry of the boundary.
Remarks on N = 4 Superconformal Extension of the Calogero Model
Authors: Galajinsky Anton V.
Published: July 2003

Recently it was conjectured by Gibbons and Townsend that the large n limit of an N = 4 superconformal extension of the n-particle Calogero model might provide a microscopic description of the extreme Reissner–Nordström black hole near the horizon. In this paper a possibility to construct an SU(1,1|2) invariant extension of the Calogero model is considered. We treat in detail the two-particle case and comment on some peculiarities intrinsic to n > 2 generalizations.
Effective Photon Mass in Nuclear Matter and Finite Nuclei
Authors: Sun Bao-Xi , Lu Xiao-Fu , Shen Peng-Nian , Zhao En-Guang
Published: July 2003

Electromagnetic field in nuclear matter and nuclei are studied. In the nuclear matter, because the expectation value of the electric charge density operator is not zero, different in vacuum, the U(1) local gauge symmetry of electric charge is spontaneously broken, and consequently, the photon gains an effective mass through the Higgs mechanism. An alternative way to study the effective mass of photon is to calculate the self-energy of photon perturbatively. It shows that the effective mass of photon is about 5.42 MeV in the symmetric nuclear matter at the saturation density ρ0 = 0.16 fm -3 and about 2.0 MeV at the surface of 238U. It seems that the two-body decay of a massive photon causes the sharp lines of electron–positron pairs in the low energy heavy ion collision experiments of 238 U + 232 Th .
Strong-Electroweak Unification at About 4 TeV
Authors: Frampton Paul H.
Published: July 2003

We show how an SU (N)M quiver gauge theory can accommodate the standard model with three chiral families and unify all of SU (3)C, SU (2)L and U(1)Y couplings with high accuracy at one unique scale estimated as M ≃ 4 TeV .
Transverse Mass Distribution Characteristics of π0 Production in 208Pb-Induced Reactions and the Combinational Approach
Authors: De Bhaskar , Bhattacharyya S.
Published: July 2003

The nature of invariant cross-sections and multiplicities in some 208Pb-induced reactions and some important ratio behaviors of the invariant multiplicities for various centralities of the collision will be dealt with here in the light of a combinational approach which has been built up recently by us. Next, the results would be compared with the outcome of some of the simulation-based standard models for multiple production in nuclear collisions at high energies. Finally, the implications of all this would be discussed.
|Vub| Determination by B → Dsπ
Authors: Hayakawa Hideaki , Hosokawa Kaoru , Kurimoto Takeshi
Published: July 2003

We investigate [Formula: see text] decay in perturbative QCD approach which has recently been applied to B-meson decays. [Formula: see text] decay (and its charge conjugated mode) can be one of the hopeful modes to determine |Vub| since it occurs through b → u transition only. We estimate both factorizable and non-factorizable contribution, and show that the non-factorizable contribution is much less than the factorizable one. Our calculation gives [Formula: see text].
Graviton-Scalar Interaction in the pp-Wave Background
Authors: Bobkov K.
Published: July 2003

We compute the graviton two scalar off-shell interaction vertex at tree level in Type IIB superstring theory on the pp-wave background using the light-cone string field theory formalism. We then show that the tree level vertex vanishes when all particles are on-shell and conservation of p+ and p- are imposed. We reinforce our claim by calculating the same vertex starting from the corresponding SUGRA action expanded around the pp-wave background in the light-cone gauge.
Noncommutativity Parameter and Composite Fermions
Authors: Jellal Ahmed
Published: July 2003

We determine some particular values of the noncommutativity parameter θ and show that the Murthy–Shankar approach is in fact a particular case of a more general one. Indeed, using the fractional quantum Hall effect (FQHE) experimental data, we give a measurement of θ. This measurement can be obtained by considering some values of the filling factor ν and other ingredients, magnetic field B and electron density ρ. Moreover, it is found that θ can be quantized either fractionally or integrally in terms of the magnetic length l0 and the quantization is exactly what Murthy and Shankar formulated recently for the FQHE. On the other hand, we show that the mapping of the FQHE in terms of the composite fermion basis has a noncommutative geometry nature and therefore there is a more general way than the Murthy–Shankar method to do this mapping.
Area Quantization in Quasi-Extreme Black Holes
Authors: Abdalla E. , Castello-Branco K. H. C. , Lima-Santos A.
Published: July 2003

We consider quasi-extreme Kerr and quasi-extreme Schwarzschild–de Sitter black holes. From the known analytical expressions obtained for their quasi-normal modes frequencies, we suggest an area quantization prescription for those objects.
Phase Diagram and Electronic Properties of High-TcSuperconducting Oxides
Authors: Pavuna Davor
Published: July 2003

We firstly briefly summarize some of the most relevant recent results and open questions across rather complex electronic phase diagram of cuprates. We continue with a discussion of results on thin superconducting oxide films grown by laser ablation. Systematic studies show that BSCCO-phases and LSCO-214 exhibit conductor-like Fermi edge, whereas materials containing "chains" (like YBCO-123) are prone to very rapid surface degradation, most likely related to critical oxygen loss at the outermost layers. Recently, direct ARPES dispersion measurements on in-situ grown, strained 10UC thin LSCO-214 films (Tc= 44 K ) have shown the band crossing of Fermi level well before the Brillouin zone boundary. This is in contrast to the flat band observed in unstrained single crystals — and to the band flattening predicted by band calculations for in-plane compressive strain. In spite of density of states reduction near the Fermi level, the critical temperature increases in strained films with respect to unstrained crystals; this poses further challenge to HTSC theory.
Energy Gap Evolution over Wide Temperature and Doping Ranges in Bi2Sr2CaCu2O8+δ
Authors: Momono Naoki , Oda Migaku , Ido Masayuki , Sakai A.
Published: July 2003

In this article we report the energy gap evolution in Bi 2 Sr 2 CaCu 2 O 8+δ( Bi2212 ) on the basis of recent tunneling and Raman scattering experiments over wide temperature and hole-doping (p) ranges. It has been demonstrated that in the normal-state electronic excitations, there exist two kinds of pseudogaps (LPG and SPG) with different characteristic energies. The LPG, which is 3 to 4 times larger than the superconducting (SC) gap at T ≪ T c , develops below ~ T max , where the magnetic susceptibility starts to decrease because of the gradual development of antiferromagnetic spin fluctuations. On the other hand, the SPG, whose magnitude is comparable to the SC gap, develops progressively, in addition to the LPG, below the mean-field characteristic temperature T co for d-wave superconductors, and then evolves into the SC gap below T c , suggesting that it will be some kind of precursor of superconductivity. It has also been demonstrated that the maximal gap Δ0 or the gap at the positions near (±π, 0) and (0,±π), determined at T ≪ T c by tunneling technique, does not scale with T c , while the gap around (±π/2,±π/2), determined at T ≪ T c from the coherence peak energy in B 2g Raman spectra, scales with T c , suggesting that the latter gap will function as an effective SC gap Δeff in determining T c . Furthermore, we will report that Δeff is given by the product of p and Δ0; Δ eff ~ 5.3pΔ0 in Bi2212, and discuss some scenarios for the SC transition which is consistent with the present result on the effective SC gap.
Three-Spin-Polarons and Their Elastic Interaction in Cuprates
Authors: Kochelaev B. I. , Safina A. M. , Shengelaya A. , Keller H. , Müller K. A. , Conder K.
Published: July 2003

Properties of quasiparticles in doped cuprates formed by an oxygen hole and two adjacent copper holes are investigated on the basis of the extended Hubbard model. The ground state energy, wave functions and the polaron-phonon coupling are calculated. We also analyzed the polaron-polaron interaction via the phonon field. It was found that this interaction is highly anisotropic and can explain the experimentally observed phase separation in the strongly underdoped LaSrCuO:Mn system.
Theoretical Studies on Many-Band Effects in Superconductivity by Using Renormalization Group Approach
Authors: Nagao Hidemi , Kawabe Hiroyuki , Kruchinin Sergei P. , Manske Dirk , Yamaguchi Kizashi
Published: July 2003

We present a renormalization equations for two-band superconductivity by using a two-band model and present phase diagrams for the two-band superconductivity. In the framework of two-band model, the present results predict that superconductivity appears, even if electron-electron interaction is positive. We discuss the possibility of a cooperative mechanism in the two-band superconductivity in relation to high-Tc superconductivity.
Magnetic Properties of the Doped Two-Dimensional Antiferromagnet
Authors: Sherman A. , Schreiber M.
Published: July 2003

The variety of the normal-state magnetic properties of cuprate high-Tc superconductors is interpreted based on the self-consistent solution of the self-energy equations for the two-dimensional t-J model. The observed variations of the spin correlation length with the hole concentration x, of the spin susceptibility with x and temperature T and the scaling of the static uniform susceptibility are well reproduced by the calculated results. The nonmonotonic temperature dependence of the Cu spin-lattice relaxation rate is connected with two competing tendencies in the low-frequency susceptibility: its temperature decrease due to the increasing spin gap and the growth of the susceptibility in this frequency region with the temperature broadening of the maximum in the susceptibility.
Three-Center Interactions and Magnetic Mechanism of Superconductivity with dx2-y2-Symmetry in the t - J*-Model
Authors: Val'kov V. V. , Val'kova T. A. , Dzebisashvili D. M. , Ovchinnikov S. G.
Published: July 2003

The effect of three-center interactions on the formation of a superconducting phase with dx2-y2-symmetry is considered using the diagram technique for Hubbard operators and irreducible Green's functions. It is shown that these interactions lead to a decrease in Tc by a factor of several tens.
Two Pseudogaps in Underdoped Cuprates
Authors: Kristoffel Nikolai
Published: July 2003

Cuprate superconductivity gaps are investigated applying the interband pairing scheme to the itinerant band and a defect "band" which is created by hole doping. A pseudogap is naturally present in this model at underdoping. Vibronic hybridization of bands renormalizes the pseudogap and two superconductivity gaps. Now two pseudogaps of electronic and vibronic origin are present in the underdoped normal state as recently observed. Entering the optimally doped region (overlapping bands) there appears a joint normal state pseudogap of vibronic origin.
Phase Transitions and Spin Relaxation in La0.95Sr0.05MnO3
Authors: Kochelaev B. I. , Shilova E. , Deisenhofer J. , Krug von Nidda H.-A. , Loidl A. , Mukhin A. A. , Balbashov A. M.
Published: July 2003

We present a model calculation of the paramagnetic-resonance linewidth in La 0.95 Sr 0.05 MnO 3. Both the temperature and the angular dependencies can be described by contributions from the crystal field due to the cooperative Jahn–Teller transition.
Orbital Order and Spin Relaxation in La0.95Sr0.05MnO3
Authors: Kochelaev B. I. , Shilova E. , Deisenhofer J. , Krug von Nidda H.-A. , Loidl A. , Mukhin A. A. , Balbashov A. M.
Published: July 2003

We analyze the temperature and angular dependence of the paramagnetic-resonance linewidth of La 0.95 Sr 0.05 MnO 3. The observed anisotropy can be well described by the crystal field contributions originated by the Jahn–Teller distortion. The critical behavior of the linewidth at the Jahn–Teller transition is found to be directly related with the orbital order.
Doping Dependence of the Band Structure and Chemical Potential in Cuprates by the Generalized Tight-Binding Method
Authors: Borisov A. A. , Gavrichkov V. A. , Ovchinnikov S. G.
Published: July 2003

Quasiparticle band structure in hole doped CuO2 layer is calculated with account for strong electron correlations in the framework of multiband p–d model. For undoped layer we obtain the charge-transfer antiferromagnetic insulator. With doping unusual impurity-like quasiparticle appears at the top of the valence band with spectral weight proportional to doping concentration. In the overdoped regime the band structure in the paramagnetic phase results in the doping dependent Fermi surface in agreement to ARPES data.
Magnetic Properties and Spin Kinetics of Two-Dimensional Heisenberg Antiferromagnets
Authors: Belov S. I. , Kochelaev B. I.
Published: July 2003

In this report we present our study of two-dimensional Heisenberg magnets based on the picture of thermally excited skyrmions. The local order parameter, energy spectrum of elementary spin excitations above the skyrmion background, the skyrmion averaged radius and static magnetic susceptibility were obtained by the Green functions method. There was calculated the rate of nuclear relaxation caused by the interaction of nuclear spins with above mentioned spin excitations. The contribution of Moriya–Dsyloshinskii and anisotropic symmetric interactions into EPR line was considered. A comparison with NMR and neutron scattering experiments in cuprates and with the results obtained by other methods is given. We discuss the consequences of an interaction between skyrmions as the result of the spin fluctuations.
Spin Injection Into YBa2Cu3Ox Films
Authors: Mikheenko P. , Allsworth M. D. , Chakalov R. , Colclough M. S. , Muirhead C. M.
Published: July 2003

We report a range of experiments designed to test the efficacy of spin polarized injection from colossal magnetoresistive material (CMR) into high temperature superconductors (HTS). These include traditional transport measurements, studies of CMR/YBCO contacts, measurements of penetration depth and studies of the effect of spin-injection on the critical state of square thin films and rings. We have used pulsed currents to minimize heating and have made comparisons with the non-magnetic homologue of CMR LaNiO3. Our experiments provide evidence that there is a measurable effect, but that it is small and mainly confined to 10–20 nm of the YBCO surface. Current gain of about 0.2 has been observed.
Zero Bias Conductance Peak in HTS above Critical Temperature
Authors: Mikheenko P. , Chakalov R. , Chakalova R. I. , Colclough M. S. , Muirhead C. M.
Published: July 2003

It has recently been proposed that if phase incoherent preformed pairs exist above critical temperature (Tc), Andreev reflection should be observable above Tc too. A distinctive feature of Andreev reflection is a zero bias conductance peak (ZBCP) in the conductance/voltage (G/V) characteristics. We have performed an extensive search for the ZBCP in HTS/normal-metal planar junctions including samples which have been annealed in Argon in an attempt to induce underdoping and enhancement of the pseudogap temperature. We have observed a maximum in the G/V characteristics above Tc in a number of samples and contact configurations. In some samples this behavior persists up to room temperature, but has a very broad voltage scale, much greater than the gap voltage. In YBa 2 Cu 3 O x/ Au and YBa 2 Cu 3 O x/ Ag contacts the form and amplitude of the ZBCP is close to the theoretical prediction, but the origin of this effect could also be in sample heating. One particularly curious result comes from an Au/LSMO/YBCO sample that shows a clear sub-gap ZBCP over the temperature range 15–266 K.
Flux Pinning by Magnetic Bubble Domains
Authors: Lange M. , Moshchalkov V. V. , van Bael M. J.
Published: July 2003

Flux pinning is investigated in a thin type-II superconducting film covering a ferromagnetic Co/Pt film with perpendicular anisotropy. Stable bubble domains with varying size and density can be created in the ferromagnet by means of different magnetization procedures. The flux pinning is strongly depending on this magnetic domain structure, resulting in asymmetric magnetization curves with respect to the field polarity.
Multiply Connected Mesoscopic Superconductors
Authors: Baelus B. J. , Peeters F. M.
Published: July 2003

Multiply connected mesoscopic superconductors are considered within the framework of the nonlinear Ginzburg–Landau theory. The two coupled nonlinear equations are solved numerically and we investigated the properties of a superconducting ring, two concentric rings, and an asymmetric ring. We find that (i) for a mesoscopic superconducting ring the flux through the hole is not quantized, (ii) two concentric mesoscopic superconducting rings are magnetically coupled and the interaction energy increases with increasing sample thickness, and (iii) in asymmetric rings, a stationary phase slip state is predicted.
Vortex Configurations in Mesoscopic Superconducting Nanowires
Authors: Stenuit G. , Michotte S. , Govaerts J. , Piraux L. , Bertrand D.
Published: July 2003

Beyond the well-known Abrikosov and giant vortex configurations, new solutions to the Ginzburg–Landau model corresponding to vortices of integer and half-integer winding number are described. Phase diagrams (Bext, Energy) and magnetization curves have been determined, aiming towards an understanding of the magnetic properties of lead nanowires and the possible consequences of such solutions with respect to the switching mechanism between vortex states in mesoscopic superconductors.
Hot-Electron Instability in Superconductors
Authors: Kunchur Milind N. , Knight James M.
Published: July 2003

High flux velocities in a superconductor can distort the quasiparticle distribution function and elevate the electronic temperature. Close to T c , a non-thermal distribution function shrinks the vortex core producing the well-known Larkin-Ovchinnikov flux instability. In the present work we consider the opposite limit of low temperatures, where electron-electron scattering is more rapid than electron-phonon, resulting in an electronic temperature rise with a thermal-like distribution function. This produces a different kind of flux instability, due to a reduction in condensate and expansion of the vortex core. Measurements in YBCO films confirm the distinct predictions ofthis mechanism.
The Effect of Magnetic Insulator in Tunneling Junction with the Structure of Super/Insulator/Super
Authors: Hori Hidenobu , Imai Syozo , Seko Kazuyuki , Itaya Shin-Ichi , Terajima Ryo , Yamamoto Yoshiyuki
Published: July 2003

Magnetic properties of the composite film structure of NiO x/ Nb are investigated by using ESR data, because NiO x/ Nb is a key structure in Super/Normal/Super of Nb / NiO x/ Nb and Nb / AlO x/ NiO x/ Nb . The ESR signals were not observed below liquid He temperature, while they were observed above about 7 K. The result is explained by temperature dependence of H c2 -curve of Nb. The suppression of Josephson current is related to the local moment observed by the X-band ESR. The existence of the local moment may explained by the model of "AF-domains" of the micro-crystals on polycrystalline film. The disappearance of ESR signals below liquid He temperature shows the change of the magnetic effect by the quasi-particle tunneling.
Superconducting Current-Phase Relation and Andreev Bound States in Nb/Au/YBa2Cu3Ox Josephson Heterojunctions
Authors: Ovsyannikov G. A. , Komissinski P. V. , Kislinski Yu. V. , Ivanov Z. G.
Published: July 2003

Nb / Au / YBa 2 Cu 3 O x heterojunctions in (001) and (1 1 20)-oriented YBa 2 Cu 3 O x films have been fabricated. I-V curves of both kinds of heterojunctions show pronounced superconducting current. As affected by microwave radiation Shapiro steps at voltages hf/ 4e arise in the I-V curves of Nb / Au / YBa 2 Cu 3 O x (001) heterojunctions implying the double periodic (~ sin 2φ) superconducting current-phase relation. However, in the case of Nb / Au / YBa 2 Cu 3 O x (1 1 20) heterojunctions Shapiro steps have been observed in the I-V curves at voltages hf/ 2e only supporting a sinusoidal (~ sin φ) superconducting current-phase relation within measurement accuracy. A zero bias conductance peak associated with zero-energy Andreev bound states occurs in the I-V curve of the Nb / Au / YBa 2 Cu 3 O x (1 1 20) heterojunctions. We discuss the experimental results in terms of the d-wave symmetrical superconducting order parameter in YBCO.
Vortex Lattice Structure in Superconducting Films with Periodic Pinning Arrays
Authors: Pogosov W. V. , Rakhmanov A. L. , Moshchalkov V. V.
Published: July 2003

The structure of the flux-line lattice in the superconducting film with the periodic system of artificial pinning sites is studied theoretically using the London model. The existence of different vortex phases (pinned square phase, depinned deformed triangular phase, and "intermediate" phase) is revealed. The phase diagrams of the vortex lattice are obtained.
Instability of the Vortex-Antivortex System in Anisotropic Hard Superconductors with Nonlinear Current-Voltage Characteristics
Authors: Fisher L. M. , Rakhmanov A. L. , Levchenko A. A. , Yampol'Skii V. A. , Bazilevich M. , Johansen T. H.
Published: July 2003

The stability of the anisotropic flux flow in the vortex-antivortex system of a superconductor is theoretically studied. The vortex motion is assumed to be governed by the power current-voltage (I-V) characteristic, E ∝ Jm. It is shown that the instability of the interface between vortices and antivortices occurs even at relatively small anisotropy of the current capability of a sample if the exponent m is large. The dispersion equation defining the dependence of the increment of the instability on the wave number is obtained and analyzed. The instability leads to the macroturbulence of the flux flow which is observed in superconductors of the system Y-123.
Collapse of the Static Magnetization in Anisotropic Hard Superconductors
Authors: Fisher L. M. , Kalinov A. V. , Voloshin I. F. , Levchenko A. A. , Yampol'Skii V. A.
Published: July 2003

The suppression (collapse) of the static magnetization of an anisotropic hard superconductor under effect of ac magnetic fields was studied experimentally and theoretically. The measurements were carried out in mutually orthogonal dc and ac magnetic fields, [Formula: see text] and [Formula: see text]. Both fields were parallel to the sample surface. Superconducting Y-123 plate-like single crystals were cut from a textured ingot in such a way that the c axis was in the sample plane. The amplitude of the ac field that suppresses completely the static magnetization was shown to be dependent on the orientation of the magnetic fields with respect to the crystallographic axes of the sample. The dynamics of the collapse of the magnetization was investigated. In some cases, the magnetic moment was found to change non-monotonically with the increase of the amplitude of the ac magnetic field. An unusual phenomenon of the transition of the superconductor from the paramagnetic to diamagnetic state induced by the transverse ac field is discovered. The observed results are interpreted within the framework of the critical state model generalized to the anisotropic case.
Flux Compression in HTS Films
Authors: Mikheenko P. , Colclough M. S. , Chakalov R. , Kawano K. , Muirhead C. M.
Published: July 2003

We report on experimental investigation of the effect of flux compression in superconducting YBa 2 Cu 3 O x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers. The flux compression produces positive magnetic moment (m) upon the cooling in a field from above to below the critical temperature. We found effect of compression in all measured films and multilayers. In accordance with theoretical calculations, m is proportional to applied magnetic field. The amplitude of the effect depends on the cooling rate, which suggests the inhomogeneous cooling as its origin. The positive moment is always very small, a fraction of a percent of the ideal diamagnetic response. A CMR layer in contact with HTS decreases the amplitude of the effect. The flux compression weakly depends on sample size, but sensitive to its form and topology. The positive magnetic moment does not appear in bulk samples at low rates of the cooling. Our results show that the main features of the flux compression are very different from those in Paramagnetic Meissner effect observed in bulk high temperature superconductors and Nb disks.
Singular and Nonsingular Vortex Structures in High-Temperature Superconductors
Authors: Bystrov A. S. , Mel'Nikov A. S. , Ryzhov D. A. , Nefedov I. M. , Shereshevskii I. A. , Vysheslavtsev P. P.
Published: July 2003

In this paper we focus on the comparison of the equilibrium and transport properties of singular and nonsingular vortex structures in high-temperature compounds. Using the time-dependent Ginzburg–Landau theory we study the dynamics of vortex structures in superconductors with (d+s)-wave pairing. We calculate the angular dependent correction to the viscosity tensor of a singular flux line, which appears due to the admixture of subdominant s-wave order parameter component in the vortex core. The second order phase transition between singular and nonsingular vortices in high-temperature superconductors was simulated within the time-dependent Ginzburg-Landau equations. The semiclassical analysis of the positions of gap nodes in the vortex core is performed both for singular and nonsingular flux lines.
Peculiarities of the Resistive State in Mo/Si Superlattices in a Magnetic Field
Authors: Aladyshkin A. Yu. , Klimov A. Yu. , Kucherenko M. A. , Kurin V. V. , Mironov V. L. , Nozdrin Yu. N. , Pestov E. E.
Published: July 2003

The experimental investigations of the transport properties of the superconducting Mo/Si multilayered structures are presented. The observed anisotropy of the electrical resistivity of Mo/Si bridges in a weak magnetic fields H ≲ 5 × 102 Oe is in agreement with the data reported earlier. However, for the Mo/Si microbridges with the width 2 μm in a transverse magnetic field H ≳ 5 × 102 Oe we have found out that the temperature dependence of the in-plane resistivity R(T) demonstrates a sharp change of the resistivity at a certain critical temperature T* < Tc, where Tc is the superconducting transition temperature in a zero magnetic field. For a parallel magnetic field this step-like behavior has not been revealed; we neither observed this effect in wider bridges. The comparison of the transport properties and the nonlinear microwave characteristics is presented. A possible origin of this unusual temperature dependence of R(T) is proposed.
Sweep-Rate Dependent Reversal of Magnetic Moment in Superconducting Films
Authors: Mikheenko P. , Colclough M. S. , Chakalov R. , Kawano K. , Muirhead C. M.
Published: July 2003

We report on a dynamical effect in YBa 2 Cu 3 O x (YBCO) films and YBCO/CMR (Colossal Magnetoresistive) multilayers, in which variations in the magnetic field sweep rate can change, or reverse the sign of, the magnetic moment. The effect occurs in thin films having a low critical current density, either intrinsically or as a result of operation near the critical temperature. We attribute this effect to the large demagnetizing field present in the films, leading to a relaxation in the non-equilibrium vortex distribution, in which current near the edge of the film changes direction. We have investigated this effect in a variety of systems by magnetometery, and provide some magneto-optical evidence of its mechanism.
Point Normal Metal-Superconductor (NS) Contact in Nonballistic Regime
Authors: Askerzade I. N. , Kulik I. O.
Published: July 2003

We analyze the point NS contact conductivity taking into account the depression of superconductivity at high-injection current density and Andreev reflection at the adaptive NS boundary. The dependence of the excess current on the voltage, as well as conductivity of contact at arbitrary voltage is obtained.
Study of the Electron-Phonon Interaction in Metal DiboridesMeB2(Me=Zr, Nb, Ta, Mg) by Point-Contact Spectroscopy
Authors: Yanson I. K. , Naidyuk Yu. G. , Kvitnitskaya O. E. , Fisun V. V. , Bobrov N. L. , Chubov P. N. , Ryabovol V. V. , Behr G. , Kang W. N. , Choi E.-M. , Kim H.-J. , Lee S.-I. , Aizawa T. , Otani S. , Drechsler S.-L.
Published: July 2003

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB2(Me = Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter λ ≲ 0.1 were estimated. The data are consistent with the measured surface phonon dispersion curves. The low λ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB2films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB2a two-band model is exploited and the proximity effect in the k-space is suggested.
MgB2 Superconductivity Properties in a Two-Gap Model
Authors: Örd T. , Kristoffel N. , Rägo K.
Published: July 2003

For MgB2 where coexist two coupled superconductivity gaps a two-band scheme has been developed. Three interaction channels have been taken into account: a pair-transfer type σ-π-interband repulsion, a σ-intraband effective attraction of electron-phonon nature, and a σ-intraband Coulomb interaction. By means of simultaneous fitting of Tc, specific heat jump and zero-temperature gaps ratio the optimal values of interaction constants have been found. The Tc isotope effect exponent, the temperature dependencies of gaps and the thermodynamic characteristics (specific heat, Hc2) calculated agree with the experimental findings. The theoretical curve of Tc vs chemical potential reproduces qualitatively the influence of doping on superconductivity phase transition in MgB2.
Magnetic Flux Penetration into MgB2
Authors: Mikheenko P. , Chakalov R. , Chakalova R. I. , Colclough M. S. , Muirhead C. M.
Published: July 2003

We report magnetization and transport measurements on MgB2 in the form of powder, bulk ceramic, wire made by diffusion of Mg into B, and pulsed laser deposited thin films. Ceramic and wire forms show strong intergranular links, and we compare their properties with those of single crystals. The powder shows a magnetic moment versus temperature curve that scales with the moment at the lowest temperature, consistent with a distribution of grain sizes, on the scale of the London penetration depth. The ceramics shows anisotropic magnetization behavior, which is probably a consequence of the anisotropic compressional forces used in its manufacture. In both powder and ceramic, we have observed intriguing negative magnetic moments and steps therein upon changing temperature, well above the obvious superconducting transition. These could indicate small amounts of some higher Tc superconducting phases. However, magnetization loops measured in this regime show ferromagnetism, which we suggest is the origin of the magnetic properties above Tc. The wire shows a linear diamagnetic response up to an Hc1 of 236 Oe, has a normal state resistivity of 2.6 × 10-6 Ω·cm just above the transition and a resistivity ratio of 21, which is also similar to those of single crystals. The thin films are composed of large crystalline platelets, have a Tc of 35 K, and are diamagnetic.