bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimized production of the Diels-Alderase antibody 1E9 as a chimeric Fab
Authors: Piatesi Andrea , Hilvert Donald
Published: July 2002

Monoclonal antibody 1E9, which catalyzes the [4+2] cycloaddition between tetrachlorothiophene dioxide and N-ethylmaleimide, has been re-engineered for production as a chimeric humanmurine Fab fragment in Escherichia coli. Stabilizing point mutations in the variable regions of the antibody were identified by replacing residues that rarely occur at individual positions in aligned immunoglobulin sequences with their consensus counterparts. By combining favorable substitutions, double (MetH87ThrGlyL63Ser) and triple (MetH87ThrGlyL63SerPheL95Pro) mutants were created, which can be produced in good yield (4 and 17 mg L1cell culture, respectively). The triple mutant exhibits a modest fourfold drop in the apparent kcatvalue for the cycloaddition reaction, but the kinetic properties of the double mutant are indistinguishable from those of the parent murine IgG. The availability of recombinant versions of this catalytic antibody will facilitate efforts to determine the origins of its selectivity and catalytic efficiency through mutagenesis.Key words: catalytic antibody, Fab fragment, bacterial production.
Efficient synthesis of the optically active dihydropyrimidinone of a potent α1A- selective adrenoceptor antagonist
Authors: Sidler Daniel R , Barta Nancy , Li Wenjie , Hu Essa , Matty Louis , Ikemoto Nori , Campbell Jeffrey S , Chartrain Michel , Gbewonyo Kozo , Boyd Russell , Corley Edward G , Ball Richard G , Larsen Robert D , Reider Paul J
Published: July 2002

The convergent synthesis of a potent α1A-selective adrenoceptor antagonist is described. Salient features of the synthesis include the enzymatic resolution of a racemic dihydropyrimidinone and the use of a palladium coupling reaction in the synthesis of 2,4'-dipyridyl.Key words: dihydropyrimidinone, enzymatic resolution, palladium coupling.
Structure-based rationalization of aldolase-catalyzed asymmetric synthesis
Authors: Liu Junjie , DeSantis Grace , Wong Chi-Huey
Published: July 2002

This paper describes a structure-based approach to elucidate the stereospecificity, including inversion of enantioselectivity, of the 2-deoxyribose-5-phosphate aldolase-catalyzed asymmetric aldol addition reaction using unnatural substrates designed for the total synthesis of epothilones. In addition, an aldolase variant with Ser-238 being altered for Asp was found to be 2.5 times more effective than the wild type in accepting the unphosphorylated substrate D-glyceraldehyde. A new H-bonding interaction between the Asp-238 carboxylate and the 3-hydroxyl of the substrate was identified and was used to rationalize the rate enhancements.Key words: aldol reaction, 2-deoxyribose-5-phosphate aldolase, mutagenesis, inversion of enantioselectivity.
Syntheses and X-ray structures of boraamidinate complexes of lithium, phosphorus, and tellurium
Authors: Chivers Tristram , Fedorchuk Chantall , Schatte Gabriele , Brask Justin K
Published: July 2002

The dilithiated boraamidinate complexes {Li2[RB(N-t-Bu)2]}2 (1a, R = Ph; 1b, R = t-Bu), prepared by the reaction of B[N(H)-t-Bu]3 with three equivalents of LiR, are shown by X-ray crystallography to have dimeric structures consisting of a distorted Li4N4 cube capped on two opposite faces by RB units. Reactions of 1a with TeCl4 or PX3 (X = Cl, Br) yielded complexes PhB(μ-N-t-Bu)2TeCl2 (2) and PhB(μ-N-t-Bu)2PX (4a, X = Cl; 4b, X = Br), respectively. The structures of 2 and 4b were determined by X-ray crystallography. In the solid state, complex 2 forms a dimer with weak Te···Cl contacts (3.2411(6) Å). Complex 4b is monomeric with a PBr bond length of 2.3047(11) Å. The reactions of 2 and 4a with the appropriate amounts of Li[N(H)-t-Bu] produce the monomeric tellurium imide PhB(μ-N-t-Bu)2Te=N-t-Bu and the amido derivative PhB(μ-N-t-Bu)2PN(H)-t-Bu (4c), respectively. The X-ray structure of 4c was determined. Deprotonation of 4c with n-BuLi produces the dimeric monolithium derivative {Li[PhB(μ-N-t-Bu)2PN-t-Bu]}2 (5), which was shown by X-ray crystallography to have a centrosymmetric structure with a central transoid Li2N2 ring.Key words: boraamidinates, lithium, tellurium, phosphorus, X-ray structures.
Climate, canopy disturbance, and radial growth averaging in a second-growth mixed-oak forest in West Virginia, U.S.A.
Authors: Rentch James S , Desta Fekedulegn , Miller Gary W
Published: July 2002

This study evaluated the use of radial growth averaging as a technique of identifying canopy disturbances in a thinned 55-year-old mixed-oak stand in West Virginia. We used analysis of variance to determine the time interval (averaging period) and lag period (time between thinning and growth increase) that best captured the growth increase associated with different levels of crown release of Quercus prinus L. and Quercus rubra L. A lag of 3 years and an interval of 7 years yielded the best fit of percent growth change and percent crown release, respectively, for Q. prinus; for Q. rubra, the radial growth response did not differ significantly when lag and interval were varied from 1 to 3 and 6 to 15 years, respectively. The relationship between percent crown release and percent growth change was linear for both species. This method provides a suitable means of detecting canopy disturbances affecting overstory trees and is potentially applicable to other tree species. When combined with fire histories, these data can provide the basis for reconstructing long-term disturbance regimes. This estimate may also provide a framework for scheduling the rate of stand entry for silvicultural treatments (e.g., thinning) that is consistent with its historic stand development.
Tree seedling growth and mortality responses to manipulations of calcium and aluminum in a northern hardwood forest
Authors: Kobe Richard K , Likens Gene E , Eagar Christopher
Published: July 2002

To assess potential forest compositional responses to exchangeable soil calcium (Caexch) and aluminum (Alexch), we characterized light-dependent growth and mortality of tree seedlings under amendments of CaCl2 and AlCl3 at Hubbard Brook Experimental Forest (HBEF), New Hampshire, U.S.A. Seedlings of Acer saccharum Marsh., Fagus grandifolia Ehrh., Betula alleghaniensis Britton, Abies balsamea (L.) Mill., and Picea rubens Sarg. were transplanted into field plots, which were randomly assigned to control, CaCl2, or AlCl3 treatments and stratified across <1 to 35% full sun. Acer saccharum and P. rubens exhibited significantly higher mortality in Al-amended than Ca-amended or control plots. Acer saccharum showed significant increases in relative diameter growth in Ca-amended plots versus controls; all other species showed nonsignificantly higher relative diameter growth under Ca amendments. We incorporated significant seedling responses into a model of forest dynamics (SORTIE) to assess potential changes in species composition under Alexch increases and Caexch losses. SORTIE predicts that further increases in Alexch would have negligible effects on canopy composition within 200 years but that the estimated Caexch depleted from HBEF between 1968 and 1995 and its influence on seedling dynamics could lead to substantial decreases in A. saccharum canopy dominance within a single forest generation (<125 years).
Canopy disturbances over the five-century lifetime of an old-growth Douglas-fir stand in the Pacific Northwest
Authors: Winter Linda E , Brubaker Linda B , Franklin Jerry F , Miller Eric A , DeWitt Donald Q
Published: July 2002

The history of canopy disturbances over the lifetime of an old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand in the western Cascade Range of southern Washington was reconstructed using tree-ring records of cross-dated samples from a 3.3-ha mapped plot. The reconstruction detected pulses in which many western hemlock (Tsuga heterophylla (Raf.) Sarg.) synchronously experienced abrupt and sustained increases in ringwidth, i.e., "growth-increases", and focused on medium-sized or larger ([Formula: see text]0.8 ha) events. The results show that the stand experienced at least three canopy disturbances that each thinned, but did not clear, the canopy over areas [Formula: see text]0.8 ha, occurring approximately in the late 1500s, the 1760s, and the 1930s. None of these promoted regeneration of the shade-intolerant Douglas-fir, all of which established 15001521. The disturbances may have promoted regeneration of western hemlock, but their strongest effect on tree dynamics was to elicit western hemlock growth-increases. Canopy disturbances are known to create patchiness, or horizontal heterogeneity, an important characteristic of old-growth forests. This reconstructed history provides one model for restoration strategies to create horizontal heterogeneity in young Douglas-fir stands, for example, by suggesting sizes of areas to thin in variable-density thinnings.
Assessing Clark's nutcracker seed-caching flights using maternally inherited mitochondrial DNA of whitebark pine
Authors: Richardson Bryce A , Klopfenstein Ned B , Brunsfeld Steven J
Published: July 2002

Maternally inherited mitochondrial DNA haplotypes in whitebark pine (Pinus albicaulis Engelm.) were used to examine the maternal genetic structure at three hierarchical spatial scales: fine scale, coarse scale, and inter population. These data were used to draw inferences into Clark's nutcracker (Nucifraga columbiana Wilson) seed-caching flight distances. Statistical analyses of fine-scale and coarse-scale distribution of haplotypes showed no apparent signs of deviation from a random pattern. This suggests nutcrackers are effective in dispersal of seed within populations, which is consistent with data gathered on nutcracker seed-caching behavior. However, the lack of homogeneity in haplotype frequencies among populations indicates nutcrackers rarely disperse seeds across large gaps (>20 km) in subalpine habitat.
Phosphate absorption byAplysia californicagut: thyroid hormone stimulation
Authors: Gerencser George A , Levin Randy , Zhang Jianliang
Published: July 2002

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium/phosphate symporter. Triiodothyronine stimulated the absorptive activity of the sodium/phosphate symporter, whereas reverse triiodothyronine had no effect on the sodium/phosphate symporter. It appears that thyroid hormone or its molluscan equivalent plays a role in the overall regulation of phosphate homeostasis by the Aplysia californica gut.Key words: phosphate absorption, sodium/phosphate symporter, thyroid hormone, regulation.
Effect of purinergic agonists and antagonists on insulin secretion from INS-1 cells (insulinoma cell line) and rat pancreatic islets
Authors: Verspohl E J , Johannwille B , Waheed A , Neye H
Published: July 2002

The effects of purinergic agonists on insulin release are controversial in the literature. In our studies (mainly using INS-1 cells, but also using rat pancreatic islets), ATP had a dual effect on insulin release depending on the ATP concentration: increasing insulin release (EC50[Formula: see text] 0.0032 μM) and inhibiting insulin release (EC50[Formula: see text] 0.32 μM) at both 5.6 and 8.3 mM glucose. This is compatible with the view that either two different receptors are involved, or the cells desensitize and (or) the effect of an inhibitory degradation product such as adenosine (ectonucleotidase effect) emerges. The same dual effects of ATP on insulin release were obtained using rat pancreatic islets instead of INS-1 cells. ADPβS, which is less degradable than ATP and rather specific for P2Y1receptors, had a dual effect on insulin release at 8.3 mM glucose: stimulatory (EC50[Formula: see text] 0.02 μM) and inhibitory (EC50[Formula: see text] 0.32 μM). The effectiveness of this compound indicates the possible involvement of a P2Y1receptor. 2-Methylthio-ATP exhibited an insulinotropic effect at very high concentrations (EC50[Formula: see text] 15 μM at 8.3 mM glucose). This indicated that distinct P2Xor the P2Y1receptor may be involved in these insulin-secreting cells. UTP increased insulin release (EC50[Formula: see text] 2 μM) very weakly, indicating that a P2Ureceptor (P2X3or possibly a P2Y2or P2Y4) are not likely to be involved. Suramin (50 μM) antagonized the insulinotropic effect of ATP (0.01 μM) and UTP (0.32 μM). Since suramin is not selective, the data indicated that various P2Xand P2Yreceptors may be involved. PPADS (100 μM), a P2Xand P2Y1,4,6receptor antagonist, was ineffective using either low or high concentrations of ATP and ADPβS, which combined with the suramin data hints at a P2Yreceptor effect of the compounds. Adenosine inhibited insulin release in a concentration-dependent manner. DPCPX (100 μM), an adenosine (A1) receptor antagonist, inhibited the inhibitory effects of both adenosine and of high concentrations of ATP. Adenosine deaminase (1 U/mL) abolished the inhibitory effect of high ATP concentrations, indicating the involvement of the degradation product adenosine. Repetitive addition of ATP did not desensitize the stimulatory effect of ATP. U-73122 (2 μM), a PLC inhibitor, abolished the ATP effect at low concentrations. The data indicate that ATP at low concentrations is effective via P2Yreceptors and the PLC-system and not via P2Xreceptors; it inhibits insulin release at high concentrations by being metabolized to adenosine.Key words: purinergic receptors, insulin release, ATP.
Magnesium supplementation and deoxycorticosterone acetate  salt hypertension: effect on arterial mechanical properties and on activity of endothelin-1
Authors: Berthon Nathalie , Laurant Pascal , Hayoz Daniel , Fellmann Dominique , Brunner Hans R , Berthelot Alain
Published: July 2002

The aim of this study was to show whether the decrease in blood pressure induced by Mg supplementation in deoxycorticosterone acetate  salt (DOCAsalt) hypertensive rats is associated with mechanical modifications of blood vessels and (or) changes in tissular production and (or) vasoconstrictor activity to endothelin-1. DOCAsalt treatment increased blood pressure, media thickness, cross-sectional area, and lumen diameter of carotid arteries. Distensibility and incremental elastic modulus versus stress were not altered in carotid arteries, suggesting that the DOCAsalt vessel wall adapts structurally to preserve its blood pressure buffering capacity. Magnesium supplementation attenuated DOCAsalt hypertension. In comparison with normotensive rats, systolic, mean, and pulse pressures were higher whereas diastolic pressure was not different in Mg-supplemented DOCA-salt rats. Magnesium supplementation did not significantly modify the elastic parameters of carotid arteries. In resistance mesenteric arteries, DOCAsalt hypertension induces an inward hypertrophic remodeling. Magnesium supplementation attenuates wall hypertrophy and increases lumen diameter to the normotensive diameter, suggesting a decrease in peripheral resistance. Magnesium supplementation normalizes the altered vasoconstrictor activity of endothelin-1 in mesenteric arteries and attenuates endothelin-1 overproduction in kidney, left ventricle, and aorta of DOCA-salt rats. These findings suggest that Mg supplementation prevents blood pressure elevation by attenuating peripheral resistance and by decreasing hypertrophic effect of endothelin-1 via inhibition of endothelin-1 production.Key words: hypertension, resistance, distensibility, blood vessels, magnesium.
Characterization of Ca2+release from heterogeneous Ca2+stores in sarcoplasmic reticulum isolated from arterial and gastric smooth muscle
Authors: Stout Marguerite A , Raeymaekers Luc , Smedt Humbert De , Casteels Rik
Published: July 2002

Ca2+transport was investigated in vesicles of sarcoplasmic reticulum subfractionated from bovine main pulmonary artery and porcine gastric antrum using digitonin binding and zonal density gradient centrifugation. Gradient fractions recovered at 1533% sucrose were studied as the sarcoplasmic reticulum component using Fluo-3 fluorescence or45Ca2+Millipore filtration. Thapsigargin blocked active Ca2+uptake and induced a slow Ca2+release from actively loaded vesicles. Unidirectional45Ca2+efflux from passively loaded vesicles showed multicompartmental kinetics. The time course of an initial fast component could not be quantitatively measured with the sampling method. The slow release had a half-time of several minutes. Both components were inhibited by 20 μM ruthenium red and 10 mM Mg2+. Caffeine, inositol 1,4,5-trisphosphate, ATP, and diltiazem accelerated the slow component. A Ca2+release component activated by ryanodine or cyclic adenosine diphosphate ribose was resolved with Fluo-3. Comparison of tissue responses showed that the fast Ca2+release was significantly smaller and more sensitive to inhibition by Mg2+and ruthenium red in arterial vesicles. They released more Ca2+in response to inositol 1,4,5-trisphosphate and were more sensitive to activation by cyclic adenosine diphosphate ribose. Ryanodine and caffeine, in contrast, were more effective in gastric antrum. In each tissue, the fraction of the Ca+2store released by sequential application of caffeine and inositol 1,4,5-trisphosphate depended on the order applied and was additive. The results indicate that sarcoplasmic reticulum purified from arterial and gastric smooth muscle represents vesicle subpopulations that retain functional Ca2+channels that reflect tissue-specific pharmacological modulation. The relationship of these differences to physiological responses has not been determined.Key words: calcium channels, smooth muscle, sarcoplasmic reticulum.
Characteristics of resident and wandering prairie voles, Microtus ochrogaster
Authors: Solomon Nancy G , Jacquot Joseph J
Published: July 2002

Many studies have documented intraspecific differences in the behavior of males or females. In some species, many adults are territorial while others have larger home ranges encompassing multiple territories. Although these two types of behavior have been documented, they are not well understood in mammals. Therefore, in the mono gamous prairie vole (Microtus ochrogaster) we characterized individuals that engage in these two behavior patterns as residents and wanderers. We monitored populations enclosed in 0.1-ha. enclosures at Miami University's Ecology Research Center. As many as 26% of animals were wanderers: animals captured frequently, but less than 75% of the time, at one nest. As expected, wanderers had larger home ranges than residents. Wanderers were primarily adult males (70%) but included some adult females. This behavior pattern was not fixed, since some wanderers previously had been residents and at least 31% of males and 57% of adult female wanderers became residents during the same field season. Wanderers were not in worse physical condition, as estimated by body mass, and survived for slightly longer than residents. Thus, it does not appear that wanderers are making the best of a bad situation, but analysis of parentage is critical to validate this conclusion.
Metabolic costs of sound production in the oyster toadfish,Opsanus tau
Authors: Amorim Maria Clara P. , McCracken Marti L , Fine Michael L
Published: July 2002

The energetics of mate calling has been studied in insects, frogs, birds, and mammals, but not in fishes. The oyster toadfish, Opsanus tau, produces a boatwhistle advertisement call using one of the fastest muscles known in vertebrates. Because toadfish will not boatwhistle in a respirometer, we measured oxygen consumption after eliciting sound production by electrically stimulating the sonic swim bladder muscle nerve. Induced sounds were similar to a male calling at a rapid rate. Stimulation of the sonic nerve increased the respiration rate by 4060% in males, but they became agitated. Repeating the experiment decreased agitation, and in most fish respiration rates approximated control levels by the second or third replication. Elicited sounds and therefore sonic-muscle performance were similar in all repetitions, hence it appears that the increased oxygen consumption in the first trial was caused by the fish's agitation. Controls indicated that electrode implantation and electrical stimulation of the body cavity did not affect the respiration rate. We suggest that allocation of a small amount of the total energy budget to sound production is reasonable in toadfish, and probably most other fish species, because of the small amount of time that the sonic muscles actually contract and their small size (about 1% of body mass).
Thermoregulation in the box turtles Terrapene carolina and Terrapene ornata
Authors: do Amaral José Pedro Sousa , Marvin Glenn , Hutchison Victor H
Published: July 2002

Terrapene ornata and Terrapene carolina are box turtles that live in different habitats, the former in grasslands and desert edges and the latter in forested areas. Considering these species' habitat selection, we predicted that T. ornata would select a higher body temperature (Tb) and would be a more precise thermoregulator than T. carolina. We recorded time series of cloacal Tb's in thigmothermal linear gradients from acclimatized (12 h light : 12 h dark; 10 or 20°C) box turtles. We used three analytical methods to evaluate and characterize turtles' activity: a ratio-dependent index that measured activity as an indirect function of changes in Tb, a comparison of hourly mean variances of Tb (ratio-independent), and autocorrelation. We tested the thermoregulatory differences between active T. carolina and T. ornata with a factorial ANOVA and characterized the turtles' thermoregulatory cycles with correlograms. Overall, T. ornata had significantly higher mean Tb's than T. carolina. The two species had similar diel thermoregulatory cycles with a period of approximately 24 h. No clear differences in absolute thermoregulatory precision of Tb's were detected. These species' thermal behaviours were consistent with those reported from field studies, suggesting that there are intrinsically determined differences in thermal preference that may help explain the different habitat choices.
BIPARTITE GRAPHS TOTALLY DECOMPOSABLE BY CANONICAL DECOMPOSITION
Authors: FOUQUET JEAN-LUC , GIAKOUMAKIS VASSILIS , VANHERPE JEAN-MARIE
Published: July 2002

We describe here a technique of decomposition of bipartite graphs which seems to be as interesting within this context as the well known modular and split techniques for the decomposition of general graphs. In particular, we characterize by forbidden subgraphs the family of bipartite graphs which are totally decomposable (i.e. reducible to single vertices) with respect to our decomposition. This family contains previously known families of graphs such as bicographs and P6-free bipartite graphs. As an application we provide polynomial solutions of optimization problems, some of them being NP-complete for general bipartite graphs.
GROWTH, STRUCTURES, AND OPTICAL PROPERTIES OF III-NITRIDE QUANTUM DOTS
Authors: HUANG DAMING , RESHCHIKOV MICHAEL A. , MORKOÇ HADIS
Published: July 2002

This article reviews the advances in the growth of III-nitride quantum dots achieved in the last few years and their unique properties. The growth techniques and the strcutural and optical properties associated with quantum confinement, strain, and polarization in GaN/Al x Ga 1-x N and In x Ga 1-x N/GaN quantum dots are discussed in detail.
GENERAL CUNTZ–KRIEGER UNIQUENESS THEOREM
Authors: SZYMAŃSKI WOJCIECH
Published: July 2002

For an arbitrary directed graph E and a *-homomorphism ϕ of the graph C*-algebra C*(E) we give a necessary and sufficient condition of injectivity of ϕ.
DEGENERATE TORSION-FREE G3-CONNECTIONS
Authors: ChI QUO-SHIN
Published: July 2002

Using exterior differential systems (EDS), Bryant proved that the moduli space of nondegenerate analytic torsion-free G3-connections depends on four functions in three variables. Here nondegeneracy is a technical condition which imposes the nonvanishing everywhere of a certain determinant pertinent to a torsion-free G3-connection for EDS to carry through. The finite-dimensional moduli of homogeneous torsion-free G3-connections are degenerate examples, where the determinant vanishes identically. We establish in fact that the moduli space of analytic inhomogeneous degenerate torsion-free G3-connections is infinite-dimensional.
EFFECTS OF SINUSOIDAL ELECTROPHORETIC FIELD ON CHAIN MACROMOLECULES IN A HETEROGENEOUS MEDIUM: STRUCTURE AND DYNAMICS BY A HYBRID COMPUTER SIMULATION
Authors: FOO GRACE M. , PANDEY R. B.
Published: July 2002

Using a hybrid simulation method, structural and dynamic response properties of chain macromolecules are studied in a heterogeneous gel matrix in presence of an electrophoretic field E(t) = Eb + Es sin (2π ft), where Eb is the static field and Es is the amplitude of sinusoidal field of frequency f. We find that the oscillating field enhances the macromolecular mobility at Es > Eb while it becomes ineffective at Es < Eb, consistent with the experimental work of Masubuchi et al. Enhanced segmental and global movement of molecules at Es > Eb leads to interesting effects such as (i) a significant decrease in molecular clustering, (ii) reduced radius of gyration, and (iii) its linear response to Eb. Power law behavior of molecular motion, i.e., the variation rms displacement (R) with the time steps (t), R ∝ tν, is sensitive to temperature, molecular weight, and field.
PURELY AFFINE ELEMENTARY su(N) FUSIONS
Authors: RASMUSSEN JØRGEN , WALTON MARK A.
Published: July 2002

We consider three-point couplings in simple Lie algebras — singlets in triple tensor products of their integrable highest weight representations. A coupling can be expressed as a linear combination of products of finitely many elementary couplings. This carries over to affine fusion, the fusion of Wess–Zumino–Witten conformal field theories, where the expressions are in terms of elementary fusions. In the case of su(4) it has been observed that there is a purely affine elementary fusion, i.e. an elementary fusion that is not an elementary coupling. In this paper we show by construction that there is at least one purely affine elementary fusion associated to every su (N > 3).
SIMULATION OF ATMOSPHERIC SECONDARY HADRON AND LEPTON FLUX FROM SATELLITE TO UNDERGROUND EXPERIMENTS
Authors: BUÉNERD M.
Published: July 2002

The successful simulation results obtained in the interpretation of the recent AMS measurements of proton, leptons and light nuclei particle flux, are reviewed. A similar success in being met in the analysis of secondary particle flux measured in teh atmosphere (antiprotons, muons) and in underground experiments (neutrinos).
REVIEW ON MODELLING OF THE RADIATION BELTS
Authors: HEYNDERICKX D.
Published: July 2002

The Earth's trapped radiation belts were discovered at the beginning of the space age and were immediately recognised as a considerable hazard to space missions. Consequently, considerable effort was invested in building models of the trapped proton and electron populatinos, culminating in the NASA AP-8 and AE-8 models which have been the de facto standards since the seventies. The CRRES mission has demonstrated that the trapped radiation environment is much more complex than the static environment described by the old models. Spatial and especially temporal variations were shown to be much more important that previously thought, and to require more complex models than those in use at that time. Such models are now becoming available, but they are limited in spatial or temporal coverage, and no global, dynamic, trapped radiation belt model is forthcoming. It is therefore vital to co-ordinate future modelling efforts in order to develop new standard models.
Bethe Ansatz and Classical Hirota Equation
Authors: Wiegmann P.
Published: July 2002

We discuss an interrelation between quantum integrable models and classical soliton equations with discretized time. It appeared that spectral characteristics of quantum integrable systems may be obtained from entirely classical set up. namely, the eigenvalues of the quantum transfer matrix and the scattering S-matrix itself are identified with a certain τ-functions of the discrete Liouville equation. The Bethe ansatz equations are obtained as dynamics of zeros. For comparison we also present the Bethe ansatz equations for elliptic solutions of the classical discrete Sine-Gordon equation. The paper is based on the recent study of classical integrable structures in quantum integrable systems.1
Avalanche in Adhesion at Metal Interfaces
Authors: Banerjea Amitava , Good Brian S.
Published: July 2002

Simulations have shown that as two metal surfaces approach each other, the surface layers can avalanche together when the rigid interfacial spacing falls below a critical distance. This is accompanied by a discontinuous decrease in the adhesive energy. Here we present an examination of this phenomenon for the bcc metals Fe and W using the Equivalent Crystal Theory. In order to identify the circumstances under which avalanche might be inhibited, the effect of loss of registry between the two surfaces is investigated in detail. The avalanche is inhibited when the two surfaces are sufficiently far out of registry and when only a few layers near the surface are allowed to relax. As the relaxing slabs get thicker a sharp avalanche reappears. However, as the loss of registry increases the energy released in the avalanche decreases.
Electronic States of Nano-Systems
Authors: Rieth M. , Schommers W.
Published: July 2002

Electronic properties of open but well-defined nano-systems have been studied quantum-mechanically. The energy states, electronic densities and the quantum-mechanical currents of such systems are dependent on the wave functions at the boundaries of the systems, the number of electrons inside the system, and the size of the system.
Mode Fluctuations as Fingerprints of Chaotic and Non-Chaotic Systems
Authors: Aurich R. , Bäcker A. , Steiner F.
Published: July 2002

The mode-fluctuation distribution P(W) is studied for chaotic as well as for non-chaotic quantum billiards. This statistic is discussed in the broader framework of the E(k,L) functions being the probability of finding k energy levels in a randomly chosen interval of length L, and the distribution of n(L), where n(L) is the number of levels in such an interval, and their cumulants ck(L). It is demonstrated that the cumulants provide a possible measure for the distinction between chaotic and non-chaotic systems. The vanishing of the normalized cumulants Ck, k ≥ 3, implies a Gaussian behaviour of P(W), which is realized in the case of chaotic systems, whereas non-chaotic systems display non-vanishing values for these cumulants leading to a non-Gaussian behaviour of P(W). For some integrable systems there exist rigorous proofs of the non-Gaussian behaviour which are also discussed. Our numerical results and the rigorous results for integrable systems suggest that a clear fingerprint of chaotic systems is provided by a Gaussian distribution of the mode-fluctuation distribution P(W).
Off-Diagonal Long-Range Order in Generalized Hubbard Models
Authors: Michielsen Kristel , De Raedt Hans
Published: July 2002

We present stochastic diagonalization results for the ground-state energy and the largest eigenvalue of the two-fermion density matrix of the BCS reduced Hamiltonian, the Hubbard model, and the Hubbard model with correlated hopping. The system-size dependence of this eigenvalue is used to study the existence of Off-Diagonal Long-Range Order in these models. We show that the model with correlated hopping and repulsive on-site interaction can exhibit Off-Diagonal Long-Range Order. Analytical results for some special limiting cases indicate that Off-Diagonal Long-Range Order not always implies superconductivity.
Exact Solution to the Moment Problem for the XY Chain
Authors: Witte N. S.
Published: July 2002

We present the exact solution to the moment problem for the spin-1/2 isotropic antiferromagnetic XY chain with explicit forms for the moments with respect to the Néel state, the cumulant generating function, and the Resolvent Operator. We verify the correctness of the Horn–Weinstein Theorems, but the analytic structure of the generating function  in the complex t-plane is quite different from that assumed by the t-Expansion and the Connected Moments Expansion due to the vanishing gap. This function has a finite radius of convergence about t=0, and for large t has a leading descending algebraic series E(t)-E0~ At-2. The Resolvent has a branch cut and essential singularity near the ground state energy of the form G(s)/s~ B|s+1|-3/4 exp (C|s+1|1/2). Consequently extrapolation strategies based on these assumptions are flawed and in practise we find that the CMX methods are pathological, and cannot be applied, while numerical evidence for two of the t-expansion methods indicates a clear asymptotic convergence behaviour with truncation order.
Thermodynamics of Two-Dimensional Quantum Magnets with Collinear Order Parameter
Authors: Elstner Norbert
Published: July 2002

We review recent theoretical investigations of the thermodynamics of two-dimensional quantum magnets on bipartite lattices. Despite the simple, semiclassical nature of the ground state, different types of low temperature scaling can occur in these systems. None of these scaling proposals allows for a conclusive description of experimental and numerical data. However, these are well reproduced assuming a wide nonuniversal crossover region smoothly connected to the high temperature limit. This provides evidence that for a quantitative description of experimental results and numerical data at finite temperatures nonuniversal effects are much more important then previously thought.
Umklapp Scattering in Fermi Liquids: A Renormalisation Group Approach
Authors: Kumar Deepak , Rajaraman R.
Published: July 2002

The role of Umklapp scattering for electrons on a lattice is studied. Using renormalisation group methods it is shown that the effective low energy interaction involves not only forward scattering, but also non-forward Umklapp scattering. We show that the latter can happen to almost all of the electrons even when they have a filling of around one-half. The bahavior of these Umklapp couplings is studied in the loop expansion. It is shown that they remain marginal to all loop orders. The discussion is given in the context of two-dimensional lattices, but generalisation to three-dimensions is mentioned. We also briefly discuss the possible consequences of this on Fermi liquid theory and on physical properties of such electronic systems.
Persistent Currents in Continuous One-Dimensional Disordered Rings within the Hartree–Fock Approximation
Authors: Cohen A. , Berkovits R. , Heinrich A.
Published: July 2002

We present numerical results for the zero temperature persistent currents carried by interacting spinless electrons in disordered one-dimensional continuous rings. The disorder potential is described by a collection of δ-functions at random locations and strengths. The calculations are performed by a self-consistent Hartree–Fock (HF) approximation. Because the HF approximation retains the concept of single-electron levels, we compare the statistics of energy levels of noninteracting electrons with those of interacting electrons as well as of the level persistent currents. We find that the e–e interactions alter the levels and samples persistent currents and introduces a preffered diamagnetic current direction. In contrast to the analogous calculations that recently appeared in the literature for interacting spinless electrons in the presence of moderate disorder in tight-binding models we find no suppression of the persistent currents due to the e–e interactions.
E⊗e Jahn–Teller Effect in ${\rm C}_{70}^{3-}$ Systems
Authors: Tian Lin , Yi Ya-Sha , Wang Chui-Lin , Su Zhao-Bin
Published: July 2002

The electron–phonon interaction in C 70 anions is studied by making use of a lattice relaxation approach. We find there exists a Jahn–Teller effect in [Formula: see text] system, due to an extra electron being doped to the double degenerate [Formula: see text] state. As a result of this effect, the original D5h symmetry of the ground state becomes unstable, which causes distortion of the lattice configuration. The only symmetry maintained in the final state of the relaxation is the x–y plane reflection symmetry. We further find that besides the Jahn–Teller active [Formula: see text] modes, the non-Jahn–Teller active [Formula: see text] vibrations also contribute to the relaxation process. The [Formula: see text] components come from the nonlinear effect and are two or three orders smaller than those of the Jahn–Teller active modes. We suggest that the [Formula: see text] molecule is a promising Berry Phase candidate in this effective E⊗e Jahn–Teller system.
Raman Scattering in Cuprate Superconductors
Authors: Devereaux Thomas Peter , Kampf Arno Paul
Published: July 2002

A theory for electronic Raman scattering in the cuprate superconductors is presented with a specific emphasis on the polarization dependence of the spectra which can infer the symmetry of the energy gap. Signatures of the effects of disorder on the low frequency and low temperature behavior of the Raman spectra for different symmetry channels provide detailed information about the magnitude and the phase of the energy gap. Properties of the theory for finite T will be discussed and compared to recent data concerning the doping dependence of the Raman spectra in cuprate superconductors, and remaining questions will be addressed.
Formation, Characterization and Magnetic Properties of Some Ternary Al-Cu-M (M = Transition Metal) Quasicrystals Prepared by Conventional Solidification
Authors: Athanasiou N. S.
Published: July 2002

We present a new semi-empirical criterion responsible for the formation and stability of the quasicrystalline state of conventionally solidified ternary Al 80-x Cu 20 M x (M = Ru, Rh, Re, Ir) alloys with x=7.5, 15, 20 at.%. Generally we found that decagonal (D-phase) and icosahedral (I-phase) quasicrystals exist in neighboring composition ranges, when one of the reported criteria examined in terms of average valence electron concentration (e/a) and effective atomic radius ratio (a eff ) is fulfilled. By conventional cooling (dT/dt≈ 103 to 102 Ksec -1) decagonal and icosahedral quasicrystals were formed in all the systems, with the exception of the Al-Cu-Re system, where only crystalline phases were found. In the ternary Al-Cu-Ir system we found large (1–1.3 mm) and stable D- and I-phases. Further on, magnetic susceptibility and magnetization curves of the alloys were investigated in the temperature range from 5 to 300 K and for magnetic fields between -50 kOe ≤ H≤50 kOe . From the resulting magnetization curves the I-Al60Cu20Ru20 alloy shows only paramagnetic behavior in the field range -50 kOe ≤ H≤50 kOe . The other samples show a paramagnetic or a diamagnetic behavior depending on the field intensity. For the samples I-Al72.5Cu20Ru7.5 , I-Al65Cu20Ru15 , D-Al65Cu20Rh15 and D-Al60Cu20Rh20 a paramagnetic behavior was observed in the field range of -1 kOe ≤ H≤1 kOe . Outside this field region a transition to diamagnetism takes place. On the basis of the resulting magnetic susceptibility curves we found that in the range of 20–300 K the magnetic susceptibility is temperature independent for all samples. On the contrary, in the range of 5–20 K the magnetic susceptibility is temperature dependent (the Curie–Weiss law is valid), with the exception of the samples I-Al65Cu20Ru15 , D-Al65Cu20Rh15 , D-Al65Cu20Ir15 and D-Al60Cu20Rh20 . The measured magnetic susceptibility values of conventionally solidified ternary Al-Cu-M (M = Ru, Rh, Re, Ir) alloys lie between the pointed values of icosahedral Al55Cu9.2Li35.8 and Al65Cu20Cr15 alloys.
POSITRON STATES IN Si1-xGex ALLOYS: DEVIATION FROM VEGARD'S LAW
Authors: BOUARISSA N.
Published: July 2002

Electron-positron momentum densities along different crystallographic directions and positron bulk lifetime in Si 1-x Ge x alloys have been investigated within the pseudopotential formalism employing the independent particle model. Special attention has been given to the effect of the deviation of the alloy lattice parameters from Vegard's rule on the studied quantities. It is found that using Vegard's law leads to an underestimation of the total positron annihilation rate indicating therefore an overestimation of the positron bulk lifetime. This result could not be checked using the Siethoff relation (H. Siethoff, Phys. Stat. Sol.B205, R3 (1998)). Moreover, this relation predicts a monotonic dependence of the positron bulk lifetime on the alloy composition which disagrees with the positron lifetime measurement.
MAGNETIC PROPERTIES OF Pr6O11-B2O3-CdO GLASSES
Authors: ARDELEAN I.
Published: July 2002

Magnetic susceptibility measurements have been performed on x Pr 6 O 11 · (100 - x)- [2 B 2 O 3 · CdO ] glasses with 0 < x ≤ 30 mol%. The data give evidence that for x > 1 mol% the praseodymium ions in these glasses are in Pr3+ and Pr4+ valence states, which explains their magnetic behavior.
LIMITATIONS OF DELAYED STATE FEEDBACK: A NUMERICAL STUDY
Authors: MICHIELS W. , ROOSE D.
Published: July 2002

Stabilization of a class of linear time-delay systems can be achieved by a numerical procedure, called the continuous pole placement method [Michiels et al., 2000]. This method can be seen as an extension of the classical pole placement algorithm for ordinary differential equations to a class of delay differential equations. In [Michiels et al., 2000] it was applied to the stabilization of a linear time-invariant system with an input delay using static state feedback. In this paper we study the limitations of such delayed state feedback laws. More precisely we completely characterize the class of stabilizable plants in the 2D-case. For that purpose we make use of numerical continuation techniques. The use of delayed state feedback in various control applications and the effect of its limitations are briefly discussed.
BASIN CONFIGURATION OF A SIX-DIMENSIONAL MODEL OF AN ELECTRIC POWER SYSTEM
Authors: UEDA YOSHISUKE , AMANO HIROYUKI , ABRAHAM RALPH H. , STEWART H. BRUCE
Published: July 2002

As part of an ongoing project on the stability of massively complex electrical power systems, we discuss the global geometric structure of contacts among the basins of attraction of a six-dimensional dynamical system. This system represents a simple model of an electrical power system involving three machines and an infinite bus. Apart from the possible occurrence of attractors representing pathological states, the contacts between the basins have a practical importance, from the point of view of the operation of a real electrical power system. With the aid of a global map of basins, one could hope to design an intervention strategy to boot the power system back into its normal state. Our method involves taking two-dimensional sections of the six-dimensional state space, and then determining the basins directly by numerical simulation from a dense grid of initial conditions. The relations among all the basins are given for a specific numerical example, that is, choosing particular values for the parameters in our model.
SPATIAL INTERMITTENCY IN AN INHOMOGENEOUS COUPLED MAP LATTICE
Authors: SHARMA ASHUTOSH , GUPTE NEELIMA
Published: July 2002

We observe purely spatial intermittency accompanied by temporally periodic behavior in an inhomogeneous lattice of coupled logistic maps where the inhomogenity appears in the form of different values of the map parameters at distinct sites. Linear analysis shows that the spatial intermittency appears in the neighborhood of tangent period doubling bifurcation points. The intermittency near the bifurcation points is associated with a power-law distribution for the laminar lengths. The scaling exponent ζ for the laminar length distribution is obtained.
ASYMPTOTIC BEHAVIOR OF GLOBAL SMOOTH SOLUTIONS TO THE FULL 1D HYDRODYNAMIC MODEL FOR SEMICONDUCTORS
Authors: HSIAO LING , WANG SHU
Published: July 2002

In this paper, we study the asymptotic behavior of smooth solutions to the initial boundary value problem for the full one-dimensional hydrodynamic model for semiconductors. We prove that the solution to the problem converges to the unique stationary solution time asymptotically exponentially fast.
A STOCHASTIC MODEL OF THE EVOLUTION DERIVED FROM ELASTIC VELOCITY PROCESS WITH MIXED DIFFUSION-JUMP CHARACTERISTICS
Authors: PAPIEZ LECH , TULOVSKY VLADIMIR
Published: July 2002

The phenomenon of phase space evolution is modeled by the diffusional change in particles directions that is interrupted by independent collisions. Transition densities of stochastic process defined by this dynamical evolution are equivalent to solutions of a particular Boltzmann integro-differential equation. The general construction of solutions of this equation is developed. Specific methods of calculations of transition densities are detailed for the small angle approximation of the Boltzmann equation.
KINETICS MODELS OF INELASTIC GASES
Authors: BALDASSARRI ANDREA , PUGLISI ANDREA , MARCONI UMBERTO MARINI BETTOLO
Published: July 2002

In the present paper we review some recent progresses in the study of the dynamics of cooling granular gases, obtained using idealized models to address different issues of their kinetics. The inelastic Maxwell gas is studied as an introductory mean field model that has the major advantage of being exactly resoluble in the case of scalar velocities, showing an asymptotic velocity distribution with power law tails |v|-4. More realistic models can be obtained placing the same process on a spatial lattice. Two regimes are observed: an uncorrelated transient followed by a dynamical stage characterized by correlations in the velocity field in the form of shocks and vortices. The lattice models, in one and two dimensions, account for different numerical measurements: some of them agree with the already known results, while others have never been efficiently measured and shed light on the deviation from homogeneity. In particular in the velocity-correlated regime the computation of structure factors gives indication of a dynamics similar to that of a diffusion process on large scales with a more complex behavior at shorter scales.
INTERLEAVED BACKTRACKING IN DISTRIBUTED CONSTRAINT NETWORKS
Authors: HAMADI YOUSSEF
Published: July 2002

The adaptation of software technology to distributed environments is an important challenge today. In this work we combine parallel and distributed search. By this way we add the potential speed-up of a parallel exploration in the processing of distributed problems. This paper extends DIBT, a distributed search procedure operating in distributed constraint networks.11 The extension is threefold. First, the ordered hierarchies used during backtracking are extended to remove partial orders. Second the procedure is updated to face delayed information problems upcoming in heterogeneous (We would like to thank here M. Yokoo for rising this potential problem.)Third, the search is extended to simultaneously explore independent parts of a distributed search tree. The first and third points were first presented in 1999.7 The third improvement introduces parallelism into distributed search, which brings to Interleaved Distributed Intelligent BackTracking (IDIBT). Our results show that 1) insoluble problems do not greatly degrade performance over DIBT and 2) super-linear speed-up can be achieved when the distribution of solutions is non-uniform.