bluuebunny/crossref_metadata_2025 huggingface dataset:
The temperature dependence of the conduction current in Ba0.5Sr0.5TiO3 thin-film capacitors for memory device applications
Authors: Lin Yung-Bin , Lee Joseph Ya-min
Published: July 2002

The temperature dependence of the current–voltage characteristics of Ba0.5Sr0.5TiO3 (BST) thin-film capacitors was studied from 300 to 433 K. An Au/BST/Pt/Ti/SiO2/Si structure was used. The BST films were deposited by rf magnetron sputtering. The conduction current was dominated by Schottky emission at room temperature and by Poole–Frenkel emission above 403 K. The dominant conduction mechanism changed from Schottky emission to Poole–Frenkel emission in the temperature range from 373 to 403 K. The BST/Pt and the BST/Au Schottky barrier heights were found to be 0.88 and 0.72 eV, respectively. The trap energy level in BST was 0.83 eV.
Reactions of ground state chlorine atoms with fluorinated methanes and ethanes
Authors: Tschuikow-Roux E. , Yano T. , Niedzielski J.
Published: July 2002

The reactions of photochemically generated Cl(2PJ) atoms with a number of fluorohydrocarbons have been investigated in the temperature range 8–95 °C by the competitive photochlorination technique using CH4 as a primary standard. Relative and absolute rate parameters are reported for CH3F, CH2F2, CH3CH2F, CH2FCH2F, CH3CHF2, CH3CF3, CH2FCHF2, CHF2CHF2, and several auxiliary reactions including CH3Cl, C2H6, and C3H8. The internal competition for hydrogen abstraction in asymmetric fluorethanes is examined in detail. The reactivity trends are discussed and it is found that the activation energies in the fluoromethane series correlate with the known C–H bond dissociation energies. The hydrogen reactivity in the fluoroethane series for which a sufficient data base of DH°(C–H) values is not available is best rationalized in terms of inductive effects and resonance interactions.
The effects of restructuring on the geometry of clusters formed by diffusion-limited, ballistic, and reaction-limited cluster–cluster aggregation
Authors: Meakin Paul , Jullien Remi
Published: July 2002

The effects of simple restructuring mechanisms on the geometry of clusters formed by diffusion-limited, ballistic, and reaction-limited cluster–cluster aggregation have been investigated using three-dimensional off-lattice models. In these models restructuring is allowed to take place immediately after two clusters have contacted each other, but no subsequent restructuring is allowed. Restructuring takes place in three distinct stages and the process can be stopped after each stage. In the case of diffusion-limited cluster–cluster aggregation, the fractal dimensionality is increased from about 1.8 to about 2.1 after the first stage. Complete restructuring increases the fractal dimensionality to about 2.2. Similar but somewhat smaller increases in the fractal dimensionality are found for ballistic and reaction-limited aggregation.
The electron density and chemical bonding: A reinvestigation of Berlin’s theorem
Authors: Silberbach H.
Published: July 2002

Berlin’s theorem concerning the force that acts upon the nuclei in a diatomic molecule is generalized in the form of an invariance property of the electron density of the molecule. It is shown that consequently the notion of binding and antibinding regions in a molecule cannot be maintained, and the use of a promolecular charge density in discussing dynamical aspects of the chemical bond turns out not to be properly founded and to yield doubtful results.
The rheology of n alkanes: Decane and eicosane
Authors: Morriss Gary P. , Daivis Peter J. , Evans Denis J.
Published: July 2002

We examine the rheological behavior of n alkanes, in particular n decane (C10H22) and n eicosane (C20H42). We observe, for the first time in a molecular dynamics simulation, the phenomenon of shear thickening. Over the accessible range of strain rates we see a regime of shear thinning, and then shear thickening in both fluids. The transition from shear thinning to shear thickening is correlated with a qualitative change in the single molecule properties; in particular the distribution of alignment angles and end-to-end distances.
A theoretical analysis of a diamond (100)-(2×1) dimer bond
Authors: Weiner Brian , Skokov Sergei , Frenklach Michael
Published: July 2002

The nature of the bonding between the carbon atoms of reconstructed dimers on a dehydrogenated (100)-(2×1) diamond surface is examined. Utilizing a complete active space self-consistent field (CASSCF) description and a 6-31G** basis set it is demonstrated that such dimer units form a biradical and are not purely π bonded as has often been conjectured. The significance of these findings for growth mechanisms on diamond surfaces involving radical sites is discussed.
The adsorption and photodesorption of oxygen on the TiO2(110) surface
Authors: Lu Guangquan , Linsebigler Amy , Yates John T.
Published: July 2002

We have investigated the adsorption and thermal conversion of molecular oxygen (O2) states on the TiO2(110) surface by making use of the distinct photodesorption behavior of each adsorption state. Oxygen chemisorbs at the oxygen vacancy defect sites on the annealed TiO2(110) surface at 105 K to a saturation coverage of less than 0.12 monolayers (ML), producing mostly the α-O2 species which is observed to undergo slow photodesorption. Upon heating this surface to above 250 K, the α-O2 is converted to the β-O2 state which can photodesorb at a significantly higher rate. The β-O2 species dissociates above 400 K to produce atomic oxygen, eliminating the oxygen anionic vacancies. Both the α- and β-photodesorption processes have a threshold energy at the TiO2 band gap (3.1 eV), indicating a substrate excitation mediated process. The photodesorption time-profile is fitted with an exponential decay function with a cross section of ∼8×10−17 cm2 for the α-O2 and ∼1.5×10−15 cm2 for the β-O2 species at a photon energy of 3.94 eV.
Partially coherent anti-Stokes Raman scattering (PCARS): A comparative band-shape analysis of the spontaneous Raman and PCARS spectra of toluene
Authors: Ishibashi Taka-aki , Hamaguchi Hiro-o
Published: July 2002

Partially coherent anti-Stokes Raman scattering (PCARS) spectrum of liquid toluene has been measured. The observed shape of the 1004 cm−1 band is asymmetric unlike that of the spontaneous Raman spectrum. A least-squares fitting shows that this asymmetric PCARS band-shape is fitted well to ‖χ(3)‖2 but not to the imaginary part of χ(3). The origin of the observed band asymmetry is the nonresonant term of χ(3), which is real and does not affect the spontaneous Raman band shapes. These results support our previous interpretation that the PCARS intensity originates from the relaxation of the CARS phase matching condition due to the optical inhomogeneity of the sample.
A Fock-space coupled-cluster method fully utilizing valence universal strategy
Authors: Meissner Leszek
Published: July 2002

An alternative formulation of the Fock-space coupled-cluster method using the hole-particle model spaces that fully exploits valence universal hierarchy of solving the coupled-cluster equations is presented. Formalism employs similarity transformations to separate eigenvalue problems for valence sectors with different numbers of quasiparticles but the same number of particles and that leads to certain simplifications in comparison with the previous formulations. These simplifications appear in the higher sectors that recently have been a subject of growing interest. The linked-cluster theorem is proven for the method.
Microscopic formulation of Marcus’ theory of electron transfer
Authors: Zhou Huan-Xiang , Szabo Attila
Published: July 2002

A microscopic theory for the rate of nonadiabatic electron transfer is developed and its relation to classical Marcus theory is analyzed. The focus is on how the nonlinear response of a molecular solvent to a change in the charge distribution of the donor–acceptor pair influences the rate; quantum mechanical and solvent dynamical effects are ignored. Under these restrictions, the rate is determined by the probability density of the energy gap, which is defined as the instantaneous change in solvation energy upon moving an electron from the donor to the acceptor. It is shown how this probability density can be obtained from the free energies of transferring varying amounts of charge between the donor and acceptor (as specified by a charging parameter). A simple algorithm is proposed for calculating these free-energy changes (and hence the energy gap probability density) from computer simulations on just three states: the reactant, the product, and an ‘‘anti’’-product formed by transferring a positive unit charge from the donor to the acceptor. Microscopic generalizations of the Marcus nonequilibrium free-energy surfaces for the reactant and the product, constructed as functions of the charging parameter, are presented. Their relation to surfaces constructed as functions of the energy gap is also established. The Marcus relation (i.e., the activation energy as a parabolic function of the free-energy change of reaction) is derived in a way that clearly shows that it is a good approximation in the normal region even when the solvent response is significantly nonlinear. A simple generalization of this relation, in which the activation energy is given by parabolic functions with different curvatures in the normal and inverted regions, is proposed. These curvatures are inversely proportional to the reorganization energies of the product and the antiproduct, respectively. Computer simulations of a simple model system are performed to illustrate and test these results and procedures.
Stability analysis for solutions of the closed shell Kohn–Sham equation
Authors: Bauernschmitt Rüdiger , Ahlrichs Reinhart
Published: July 2002

The stability conditions for restricted closed shell Kohn–Sham density functional treatments are derived. In close analogy to the Hartree–Fock method we obtain the equivalent of singlet and triplet instabilities. The nonreal instability now becomes trivial and just expresses the aufbau principle. Demonstrative applications confirm the expected increased stability of density functional treatments as compared to Hartree–Fock. This is an improvement but the unphysical bifurcations do persist.
Grafted macromolecules with adsorbing ends
Authors: Skvortsov A. M. , Pavlushkov I. V. , Gorbunov A. A. , Zhulina E. B.
Published: July 2002

Using a self-consistent field approach we analyze the equilibrium characteristics of macromolecules grafted at one end onto a planar surface and having an adsorbing group at the second end. Three different systems are considered: (1) isolated end-adsorbing Gaussian chain, (2) end-adsorbing chains admixed into a polymer brush with nonadsorbing free ends, and (3) polymer brush formed by the end-adsorbing macromolecules. The dependences of the average height and dispersion of the adsorbing ends are calculated as functions of the adsorption energy and the chain length. The experimental results on relevant systems are discussed.
Absolute partial cross sections for electron-impact ionization of CO2 from threshold to 1000 eV
Authors: Straub H. C. , Lindsay B. G. , Smith K. A. , Stebbings R. F.
Published: July 2002

Absolute partial cross sections for the production of CO+2, CO+, CO2+2, O+, C+, O2+, and C2+ from electron-impact ionization of CO2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides clear evidence that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±3.5% for singly charged parent ions and is slightly greater for fragment and doubly charged ions. For the fragment ion cross sections, all but one of the previous measurements are observed to be significantly lower than the present results.
Collisional mixing among the z3D
J and z3F
J states of Fe atoms in He and Ar
Authors: Goo J. S. , Lee K. , Bae S. C. , Ku J. K.
Published: July 2002

Collisional mixing among the z3DJ and z3FJ states of Fe[3d64s(a4D)4p] atoms was investigated in He and Ar by laser-induced fluorescence method. The z3DJ and z3FJ states of Fe atoms were generated directly by photodissociation of Fe(CO)5 followed by single photon absorption within a laser pulse using an unfocussed laser beam with atomic transition frequencies of Fe. When the z3D3 level was excited, the emissions from this level showed a double exponential decay. The fast and slow components of the decay constants from the z3D3 level were 10.7×10−10 and 0.3×10−10 cm3 molecule−1 s−1 in He, and 8.8×10−10 and 1.6×10−10 cm3 molecule−1 s−1 in Ar, respectively. When the z3F4 level was pumped, the emissions from this level showed a single exponential decay and the decay constants were the same as those of the slow components of z3D3. The emissions from higher-lying levels were single exponential at low pressures and the decay constants were in the range of 0.7–3.6×10−10 cm3 molecule−1 s−1. It is found that the collisional mixing between the z3D3 and z3F4 levels is very fast in both buffer gases while the mixing among the higher-lying four levels is relatively slow. The radiative lifetimes of the z3DJ and z3FJ levels were 280–370 and 770–1100 ns, respectively, depending on J. Kinetic simulations of time profiles from the laser excited and collisional product levels revealed that intermultiplet mixing appeared to be more efficient than intramultiplet mixing.
Slippage of initial conditions for the Redfield master equation
Authors: Gaspard P. , Nagaoka M.
Published: July 2002

For a slow open quantum subsystem weakly coupled to a fast thermal bath, we derive the general form of the slippage to be applied to the initial conditions of the Redfield master equation. This slippage is given by a superoperator which describes the non-Markovian dynamics of the subsystem during the short-time relaxation of the thermal bath. We verify in an example that the Redfield equation preserves positivity after the slippage superoperator has been applied to the initial density matrix of the subsystem. For δ-correlated baths, the Redfield master equation reduces to the Lindblad master equation and the slippage of initial conditions vanishes consistently.
Configuration interaction singles, time-dependent Hartree–Fock, and time-dependent density functional theory for the electronic excited states of extended systems
Authors: Hirata So , Head-Gordon Martin , Bartlett Rodney J.
Published: July 2002

A general formalism for time-dependent linear response theory is presented within the framework of linear-combination-of-atomic-orbital crystalline orbital theory for the electronic excited states of infinite one-dimensional lattices (polymers). The formalism encompasses those of time-dependent Hartree–Fock theory (TDHF), time-dependent density functional theory (TDDFT), and configuration interaction singles theory (CIS) (as the Tamm–Dancoff approximation to TDHF) as particular cases. These single-excitation theories are implemented by using a trial-vector algorithm, such that the atomic-orbital-based two-electron integrals are recomputed as needed and the transformation of these integrals from the atomic-orbital basis to the crystalline-orbital basis is avoided. Convergence of the calculated excitation energies with respect to the number of unit cells taken into account in the lattice summations (N) and the number of wave vector sampling points (K) is studied taking the lowest singlet and triplet exciton states of all-trans polyethylene as an example. The CIS and TDHF excitation energies of polyethylene show rapid convergence with respect to K and they are substantially smaller than the corresponding Hartree–Fock fundamental band gaps. In contrast, the excitation energies obtained from TDDFT and its modification, the Tamm–Dancoff approximation to TDDFT, show slower convergence with respect to K and the excitation energies to the lowest singlet exciton states tend to collapse to the corresponding Kohn–Sham fundamental band gaps in the limit of K→∞. We consider this to be a consequence of the incomplete cancellation of the self-interaction energy in the matrix elements of the TDDFT matrix eigenvalue equation, and to be a problem inherent to the current approximate exchange–correlation potentials that decay too rapidly in the asymptotic region.
Nuclear magnetic resonance quantum logic gates using quadrupolar nuclei
Authors: Khitrin A. K. , Fung B. M.
Published: July 2002

The use of nuclei with spin >1/2 and quadrupolar splitting of the spin energy levels has certain advantages over systems of interacting spins 1/2 for building quantum logic gates. Experimental demonstration has been made using 23Na (spin 3/2) NMR in a lyotropic liquid crystal system. The pulse sequences for establishing effective pure quantum states are given, and the resulting NMR spectra are presented.
Area-preserving diffeomorphisms of Riemann surfaces
Authors: Wolski A. , Dowker J. S.
Published: July 2002

A number of Riemann surfaces are constructed by factoring the plane under the action of discontinuous groups. Expressions are found for the algebras and curvatures of the groups of area-preserving diffeomorphisms of the Riemann surfaces. The difficulties involved in finding finite algebras, which yield the diffeomorphism algebras in the infinite-dimension limit, are discussed.
Knot polynomial states of quantum gravity in terms of loops and extended loops: Some remarks
Authors: Di Bartolo Cayetano , Gambini Rodolfo , Griego Jorge , Pullin Jorge
Published: July 2002

In this paper we review the status of several solutions to all the constraints of quantum gravity that have been proposed in terms of loops and extended loops, based on knot polynomials. We discuss pitfalls of several of the results, and in particular the issues of covariance and regularization of the constraints in terms of extended loops. We also propose a formalism for ‘‘thickened out loops,’’ which does not face the covariance problems of extended loops and may allow to regularize expressions in a consistent manner.
Determinant structure of the rational solutions for the Painlevé II equation
Authors: Kajiwara Kenji , Ohta Yasuhiro
Published: July 2002

Two types of determinant representations of the rational solutions for the Painlevé II equation are discussed by using the bilinear formalism. One of them is a representation by the Devisme polynomials, and another one is Hankel determinant representation. They are derived from the determinant solutions of the KP hierarchy and Toda lattice, respectively.
The structure of spherically symmetric su(n) Yang–Mills fields
Authors: Bartnik Robert
Published: July 2002

We summarize the algebraic structure of spherically symmetric Yang–Mills potentials for a general compact gauge group, and investigate the particular case of gauge groups with Lie algebra su(n) in detail. We develop techniques that lead to a complete classification of the possible spherical symmetry ansätze, including descriptions of the reduced gauge group 𝒵, the space of magnetic potentials ℋ, and for those ansätze that admit extensions across the symmetry axis, a description of the space of vacuum potentials H0 and its little group Z0. These results are illustrated by listing all irreducible models for su(n), n⩽6.
Extensions of Neveu–Schwarz conformal modules
Authors: Cheng Shun-Jen , Kac Victor G. , Wakimoto Minoru
Published: July 2002

In this paper we study extensions between two finite-rank irreducible conformal modules over the Neveu–Schwarz algebra.
On the effect of dislocation arrays on the superconducting transition temperature in HTSC
Authors: Gurevich A. V. , Pashitskiı̆ É. A.
Published: July 2002

It is shown that arrays of edge dislocations with parallel Burgers vectors in the slip plane can lead to significant local changes in the transition temperature Tc of the deformed crystals of high-temperature superconductors. These changes are due to the redistribution of free charge carriers in the elastic strain fields of the ionic crystal lattice and a strong nonmonotonic dependence of Tc on the concentration of charge carriers if the characteristic length of dislocation arrays is much larger than the coherence length and screening radius.
Temperature dependence of high-field magnetization of dilute spinels with cluster-type magnetic structures
Authors: Efimova N. N.
Published: July 2002

The isotherms σT(H) and the high-field magnetization polytherms σH(T) of polycrystals of the dilute spin-glass system Li0.5Fe2.5−xGaxO4 are studied in the temperature range 4.2–380 K in fields up to 25 kOe. The nonmagnetic Ga3+ ions in the crystals have concentrations x=1.4 and 1.6 in the vicinity of the multicritical point x0=1.5 on the x-T phase diagram, near which all types of magnetic states have cluster-type spatially inhomogeneous structures. It is found that the dependences σH(T) obey Bloch’s T3/2 law in the paramagnet temperature range T>Tf=18 K, x=1.6 and T>TC=160 K, x=1.4 for H⩾10 kOe. This is attributed to spin wave excitations in individual noninteracting clusters. The T3/2 law is violated at T<Tf for x=1.6, but is obeyed at T<TC for x=1.4. The clearly manifested phase transition at the Curie point in the form of a kink on the linear dependence σH(T3/2) suggests that the spin wave excitations are cooperative: their spectrum is formed at T⩽TC by the entire crystal, i.e., by the clusters and the matrix which is responsible for the long-range exchange coupling between clusters. At low temperatures, the T3/2 law is violated at T∼50 K, where the sample with x=1.4 previously displayed a first-order phase-transition.
Nonlinear electromagnetic waves in metals under strong magnetism of conduction electrons
Authors: Peschanskiı̆ V. G. , Stepanenko D. I.
Published: July 2002

Wave processes in noncompensated metals in a quantizing magnetic fields are investigated theoretically. It is shown that small-amplitude nonlinear electromagnetic waves can propagate when the magnetic susceptibility is close to 1/4π. Nonlinear solutions of the system of Maxwell equations are obtained under the conditions of strong magnetism of conduction electrons.
Electromagnetic waves in organic conductors under strong magnetism of conduction electrons
Authors: Peschansky V. G. , Stepanenko D. I.
Published: July 2002

Wave processes in layered organic conductors in strong magnetic fields are investigated theoretically at low temperatures when the de Haas–van Alphen effect is manifested strongly. The spectrum and amplitude of weakly attenuating waves are determined in the vicinity of the electron phase transition where the formation of diamagnetic domains takes place.
A statistical theory for the distribution of energy dissipation in intermittent turbulence
Authors: Andrews L. C. , Phillips R. L. , Shivamoggi B. K. , Beck J. K. , Joshi M. L.
Published: July 2002

A new statistical theory involving the gamma distribution is presented for the local average rate of dissipation εr. The gamma distribution for εr leads to results for the high-order moments of the velocity structure function that lie between those predicted by the lognormal model and β model and closely follow those of the multifractal model. Comparisons of results predicted by the gamma distribution are made with previously published experimental data, showing excellent agreement. Based on the gamma distribution for εr, the one-dimensional K distribution is developed as a plausible model for the distribution of energy dissipation ε. Some laboratory measurements of velocity fluctuations and certain derived quantities from a modified airjet are also discussed. A good agreement is found by making comparisons of the measured cumulative probability distribution of the squared time derivative of the streamwise velocity component with that predicted by this new model.
Three-dimensional numerical simulation of turbulent mixing by Rayleigh–Taylor instability
Authors: Youngs David L.
Published: July 2002

Three-dimensional simulation of the mixing of miscible fluids by Rayleigh–Taylor instability is described for density ratios, ρ1/ρ2, in the range 1.5 to 20. Significant dissipation of density fluctuations and kinetic energy occurs via the cascade to high wave numbers. Comparison is made with experimental measurements of the overall growth of the mixing zone and of the magnitude of density fluctuations. The differences between 2-D and 3-D simulation are discussed.
Microbubble skin friction reduction on an axisymmetric body
Authors: Deutsch S. , Castano J.
Published: July 2002

A reduction in skin friction drag is shown when gas is introduced into the liquid turbulent boundary layer of a submerged axisymmetric body. The 89 mm diameter, 632 mm long body has a cylindrical balance 273 mm long. Free stream speeds in the 305 mm diameter tunnel are as high as 17 m/sec, giving length Reynolds number of up to 10 million. In general, skin friction reduction is shown to increase with increasing free stream speed. At high speeds, helium injection is shown to be more effective at reducing skin friction than is air injection. Maximum skin friction reduction is near 80%—a value in good agreement with the maximum value observed in the flat plate work of Madavan et al. [Phys. Fluids 27, 356 (1984)]. While maximum skin friction reduction was found at a free stream speed of 5 m/sec for the flat plate geometry, maximum skin friction reduction was at a free stream speed of 17 m/sec for the axisymmetric geometry.
Model consistency in large eddy simulation of turbulent channel flows
Authors: Piomelli Ugo , Moin Parviz , Ferziger Joel H.
Published: July 2002

Combinations of filters and subgrid scale stress models for large eddy simulation of the Navier–Stokes equations are examined by a priori tests and numerical simulations. The structure of the subgrid scales is found to depend strongly on the type of filter used, and consistency between model and filter is essential to ensure accurate results. The implementation of consistent combinations of filter and model gives more accurate turbulence statistics than those obtained in previous investigations in which the models were chosen independently from the filter. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
Comparison of flow characteristics in the near field of two parallel plane jets and an offset plane jet
Authors: Nasr A. , Lai J. C. S.
Published: July 2002

Two parallel plane air jets and offset air jets have several common features such as the existence of a subatmospheric pressure region and the formation of a flow recirculation zone adjacent to the nozzle plate. In particular, the symmetry plane that exists between two parallel plane jets may appear to affect the flow field in much the same way as a solid wall does in a reattaching offset jet. It is obvious, however, that there are significant differences far downstream from the nozzles because the two parallel plane jets will combine to form a single free jet while the offset jet will develop into a wall jet. Differences in the near field between these two jet configurations with a separation ratio of 2.125 are examined here under identical initial flow exit conditions through laser Doppler anemometer measurements of the mean velocity components, turbulence intensities, and Reynolds shear stress. The results indicate that the wall exerts significant retarding and turbulence suppression effects on the offset jet in the flow development region. Through the use of comparisons, the interaction of two inner shear layers on both sides of the symmetry plane in two parallel plane jets results in a much more turbulent near field than that of the offset jet.
Propagation of acoustic waves in disordered flows composed of many vortices. II. Examples
Authors: Boyer Denis , Lund Fernando
Published: July 2002

The theory of acoustic wave propagation through systems of many vortices randomly distributed, developed in Part I, is applied to specific examples in two and three dimensions. Two classes of vortex blobs are considered; vortices with an axisymmetric distribution of vorticity, such as disks or tubes, and vortices with a nonvanishing dipolar moment such as dipoles or rings. The index of refraction and attenuation length are numerically computed as a function of wavelength for various values of vortex parameters. The asymptotic behavior of the dispersion relation for very short and very long wavelengths is also derived analytically. At short wavelengths λ the attenuation length scales as λ−2 in all examples studied. At long wavelengths the scaling depends on the lowest nonvanishing multipole moment of the vorticity distribution; say, for vortex rings, it is λ−4 as in Thomson scattering. For an ideal gas, the phase velocity of the coherent acoustic wave is greater than in the undisturbed flow for long wavelengths and smaller than in the undisturbed flow for short wavelengths. This appears to be a robust feature. When properly normalized, the attenuation length does not depend very strongly on the ratio l/ε, where l is a vortex length scale and ε the thickness of the vorticity bearing region, both in two and three dimensions. The effective index of refraction, however, does depend on this ratio. The conditions of applicability of the results, which rely on a Born approximation scheme, are also determined. The expressions obtained in this paper for the scattering cross sections are used to discuss the properties of sound localization in two dimensional disordered flows.
Plasma–particulate interactions in nonuniform plasmas with finite flows
Authors: Hamaguchi S. , Farouki R. T.
Published: July 2002

The polarization force on a charged particulate or ‘‘dust’’ grain in a nonuniform plasma with finite ion flows and an external electric field is derived, based on a fluid approximation for the background plasma. This polarization force is proportional to the magnitude of the spatial gradient of the Debye length, and acts in the direction of decreasing Debye length. When the ion flow velocity is sufficiently large compared to ion thermal velocities, ions do not participate in the formation of sheaths around negatively charged particulates and the electron Debye length must be employed, since sheaths comprise only a deficiency of electrons. If the ion flow velocity is small, the contribution of the ion flow to the polarization force is proportional to the spatial gradient of the flow kinetic energy and is thus usually negligible. The expressions describing the plasma–particulate interaction may be applied to the modeling of contaminant behavior in materials-processing plasmas.
Influence of electromagnetic effects on the two-stream instability in a relativistic electron beam
Authors: Uhm Han S.
Published: July 2002

The influence of the electromagnetic effects on the two-stream instability in a relativistic electron beam propagating through a collisionless plasma channel is investigated, and a closed algebraic dispersion relation of the two-stream instability for the complex eigenfrequency ω is obtained. It is shown that electromagnetic effects have a strong stabilizing influence on the two-stream instability in a relativistic electron beam, drastically enhancing the critical beam current for instability.
Tumor cell surface heparan sulfate as cryptic promoters or inhibitors of tumor growth and metastasis
Authors: Liu Dongfang , Shriver Zachary , Venkataraman Ganesh , El Shabrawi Yosuf , Sasisekharan Ram
Published: July 2002

Heparan sulfate glycosaminoglycans, present at the cell surface and in the extracellular matrix that surrounds cells, are important mediators of complex biological processes. Furthermore, it is now apparent that cells dynamically regulate the structure of their heparan sulfate “coat” to differentially regulate extracellular signals. In the present study, the importance of sequence information contained within tumor cell-surface heparan sulfate was investigated. Herein, we demonstrate that the heparan sulfate glycosaminoglycan coat present on tumor cells contains bioactive sequences that impinge on tumor-cell growth and metastasis. Importantly, we find that growth promoting as well as growth inhibiting sequences are contained within the polysaccharide coat. Furthermore, we find that the dynamic balance between these distinct polysaccharide populations regulates specific intracellular signal-transduction pathways. This study not only provides a framework for the development of polysaccharide-based anti-cancer molecules but also underscores the importance of understanding a cell's polysaccharide array in addition to its protein complement, to understand how genotype translates to phenotype in this postgenomic age.
Rationally designed mutations convertde novoamyloid-like fibrils into monomeric β-sheet proteins
Authors: Wang Weixun , Hecht Michael H.
Published: July 2002

Amyloid fibrils are associated with a variety of neurodegenerative maladies including Alzheimer's disease and the prion diseases. The structures of amyloid fibrils are composed of β-strands oriented orthogonal to the fibril axis (“cross β” structure). We previously reported the design and characterization of a combinatorial library ofde novoβ-sheet proteins that self-assemble into fibrillar structures resembling amyloid. The libraries were designed by using a “binary code” strategy, in which the locations of polar and nonpolar residues are specified explicitly, but the identities of these residues are not specified and are varied combinatorially. The initial libraries were designed to encode proteins containing amphiphilic β-strands separated by reverse turns. Each β-strand was designed to be seven residues long, with polar (○) and nonpolar (●) amino acids arranged with an alternating periodicity (○●○●○●○). The initial design specified the identical polar/nonpolar pattern for all of the β-strands; no strand was explicitly designated to form the edges of the resulting β-sheets. With all β-strands preferring to occupy interior (as opposed to edge) locations, intermolecular oligomerization was favored, and the proteins assembled into amyloid-like fibrils. To assess whether explicit design of edge-favoring strands might tip the balance in favor of monomeric β-sheet proteins, we have now redesigned the first and/or last β-strands of several sequences from the original library. In the redesigned β-strands, the binary pattern is changed from ○●○●○●○ to ○●○K○●○ (K denotes lysine). The presence of a lysine on the nonpolar face of a β-strand should disfavor fibrillar structures because such structures would bury an uncompensated charge. The nonpolar → lysine mutations, therefore, would be expected to favor monomeric structures in which the ○●○K○●○ sequences form edge strands with the charged lysine side chain accessible to solvent. To test this hypothesis, we constructed several second generation sequences in which the central nonpolar residue of either the N-terminal β-strand or the C-terminal β-strand (or both) is changed to lysine. Characterization of the redesigned proteins shows that they form monomeric β-sheet proteins.
Discovery of a spermatogenesis stage-specific ornithine decarboxylase antizyme: Antizyme 3
Authors: Ivanov Ivaylo P. , Rohrwasser Andreas , Terreros Daniel A. , Gesteland Raymond F. , Atkins John F.
Published: July 2002

Previous studies with mice overproducing ornithine decarboxylase have demonstrated the importance of polyamine homeostasis for normal mammalian spermatogenesis. The present study introduces a likely key player in the maintenance of proper polyamine homeostasis during spermatogenesis. Antizyme 3 is a paralog of mammalian ornithine decarboxylase antizymes. Like its previously described counterparts, antizymes 1 and 2, it inhibits ornithine decarboxylase, which catalyzes the synthesis of putrescine. Earlier work has shown that the coding sequences for antizymes 1 and 2 are in two different, partially overlapping reading frames. Ribosomes translate the first reading frame, and just before the stop codon for that frame, they shift to the second reading frame to synthesize a trans-frame product. The efficiency of this frameshifting depends on polyamine concentration, creating an autoregulatory circuit. Antizyme 3 cDNA has the same arrangement of reading frames and a potential shift site with definite, although limited, homology to its evolutionarily distant antizyme 1 and 2 counterparts. In contrast to antizymes 1 and 2, which are widely expressed throughout the body, antizyme 3 transcription is restricted to testis germ cells. Expression starts early in spermiogenesis and finishes in the late spermatid phase. The potential significance of antizyme 3 expression during spermatogenesis is discussed in this paper.
PTEN controls tumor-induced angiogenesis
Authors: Wen Shenghua , Stolarov Javor , Myers Michael P. , Su Jing Dong , Wigler Michael H. , Tonks Nicholas K. , Durden Donald L.
Published: July 2002

Mutations of the tumor suppressor PTEN, a phosphatase with
specificity for 3-phosphorylated inositol phospholipids, accompany
progression of brain tumors from benign to the most malignant forms.
Tumor progression, particularly in aggressive and malignant tumors, is
associated with the induction of angiogenesis, a process termed the
angiogenic switch. Therefore, we tested whether PTEN regulates tumor
progression by modulating angiogenesis. U87MG glioma cells stably
reconstituted with PTEN cDNA were tested for growth in a nude mouse
orthotopic brain tumor model. We observed that the reconstitution of
wild-type PTEN had no effect on
in vitro
proliferation
but dramatically decreased tumor growth
in vivo
and
prolonged survival in mice implanted intracranially with these tumor
cells. PTEN reconstitution diminished phosphorylation of AKT within the
PTEN-reconstituted tumor, induced thrombospondin 1 expression, and
suppressed angiogenic activity. These effects were not observed in
tumors reconstituted with a lipid phosphatase inactive G129E mutant of
PTEN, a result that provides evidence that the lipid phosphatase
activity of PTEN regulates the angiogenic response
in
vivo
. These data provide evidence that PTEN regulates
tumor-induced angiogenesis and the progression of gliomas to a
malignant phenotype via the regulation of phosphoinositide-dependent
signals.
A cardiac myocyte vascular endothelial growth factor paracrine pathway is required to maintain cardiac function
Authors: Giordano Frank J. , Gerber Hans-Peter , Williams Simon-Peter , VanBruggen Nicholas , Bunting Stuart , Ruiz-Lozano Pilar , Gu Yusu , Nath Anjali K. , Huang Yan , Hickey Reed , Dalton Nancy , Peterson Kirk L. , Ross John , Chien Kenneth R. , Ferrara Napoleone
Published: July 2002

The role of the cardiac myocyte as a mediator of paracrine
signaling in the heart has remained unclear. To address this issue, we
generated mice with cardiac myocyte-specific deletion of the vascular
endothelial growth factor gene, thereby producing a
cardiomyocyte-specific knockout of a secreted factor. The hearts of
these mice had fewer coronary microvessels, thinned ventricular walls,
depressed basal contractile function, induction of hypoxia-responsive
genes involved in energy metabolism, and an abnormal response to
β-adrenergic stimulation. These findings establish the critical
importance of cardiac myocyte-derived vascular endothelial growth
factor in cardiac morphogenesis and determination of heart function.
Further, they establish an adult murine model of hypovascular
nonnecrotic cardiac contractile dysfunction.
Plant biology in the future
Authors: Bazzaz F. A.
Published: July 2002

In the beginning of modern plant biology, plant biologists
followed a simple model for their science. This model included
important branches of plant biology known then. Of course, plants had
to be identified and classified first. Thus, there was much work on
taxonomy, genetics, and physiology. Ecology and evolution were
approached implicitly, rather than explicitly, through paleobotany,
taxonomy, morphology, and historical geography. However, the burgeoning
explosion of knowledge and great advances in molecular biology, e.g.,
to the extent that genes for specific traits can be added (or deleted)
at will, have created a revolution in the study of plants. Genomics in
agriculture has made it possible to address many important issues in
crop production by the identification and manipulation of genes in crop
plants. The current model of plant study differs from the previous one
in that it places greater emphasis on developmental controls and on
evolution by differential fitness. In a rapidly changing environment,
the current model also explicitly considers the phenotypic variation
among individuals on which selection operates. These are calls for the
unity of science. In fact, the proponents of “Complexity Theory”
think there are common algorithms describing all levels of
organization, from atoms all the way to the structure of the universe,
and that when these are discovered, the issue of scaling will be
greatly simplified! Plant biology must seriously contribute to, among
other things, meeting the nutritional needs of the human population.
This challenge constitutes a key part of the backdrop against which
future evolution will occur. Genetic engineering technologies are and
will continue to be an important component of agriculture; however, we
must consider the evolutionary implications of these new technologies.
Meeting these demands requires drastic changes in the undergraduate
curriculum. Students of biology should be trained in molecular,
cellular, organismal, and ecosystem biology, including all living
organisms.
Drosophila
phosphoinositide-dependent kinase-1 regulates apoptosis and growth via the phosphoinositide 3-kinase-dependent signaling pathway
Authors: Cho Kyoung Sang , Lee Jun Hee , Kim Sunhong , Kim Dohoon , Koh Hyongjong , Lee Jihyun , Kim Changsoo , Kim Jaeseob , Chung Jongkyeong
Published: July 2002

Phosphoinositide-dependent kinase-1 (PDK-1) is a central mediator
of the cell signaling between phosphoinositide 3-kinase (PI3K) and
various intracellular serine/threonine kinases including
Akt/protein kinase B (PKB), p70 S6 kinases, and protein kinase C.
Recent studies with cell transfection experiments have implied that
PDK-1 may be involved in various cell functions including cell growth
and apoptosis. However, despite its pivotal role in cellular
signalings, the
in vivo
functions of PDK-1 in a
multicellular system have rarely been investigated. Here, we have
isolated
Drosophila
PDK-1 (
dPDK-1
)
mutants and characterized the
in vivo
roles of the
kinase.
Drosophila
deficient in the
dPDK-1
gene exhibited lethality and an apoptotic
phenotype in the embryonic stage. Conversely, overexpression of
dPDK-1
increased cell and organ size in a
Drosophila
PI3K-dependent manner.
dPDK-1
not only could activate
Drosophila
Akt/PKB
(
Dakt1
), but also substitute the
in vivo
functions of its mammalian ortholog to activate Akt/PKB. This
functional interaction between
dPDK-1
and
Dakt1
was further confirmed through genetic analyses in
Drosophila
. On the other hand, cAMP-dependent protein
kinase, which has been proposed as a possible target of
dPDK-1
, did not interact with
dPDK-1
. In
conclusion, our findings provide direct evidence that
dPDK-1
regulates cell growth and apoptosis
during
Drosophila
development via the PI3K-dependent
signaling pathway and demonstrate our
Drosophila
system
to be a powerful tool for elucidating the
in vivo
functions and targets of PDK-1.
Amplification of the human dihydrofolate reductase gene via double minutes is initiated by chromosome breaks
Authors: Singer Michael J. , Mesner Larry D. , Friedman Cynthia L. , Trask Barbara J. , Hamlin Joyce L.
Published: July 2002

DNA sequence amplification is one of the most frequent manifestations of genomic instability in human tumors. We have shown previously that amplification of the dihydrofolate reductase (DHFR) gene in Chinese hamster cells is initiated by chromosome breaks, followed by bridge-breakage-fusion cycles that generate large intrachromosomal repeats; these are ultimately trimmed by an unknown process to smaller, more homogenous units manifested as homogenously staining chromosome regions (HSRs). However, in most human tumor cells, amplified DNA sequences are borne on unstable, extrachromosomal double minutes (DMs), which suggests the operation of a different amplification mechanism. In this study, we have isolated a large number of independent methotrexate-resistant human cell lines, all of which contained DHFR-bearing DMs. Surprisingly, all but one of these also had suffered partial or complete loss of one of the parental DHFR-bearing chromosomes. Cells in a few populations displayed what could be transient intermediates in the amplification process, including an initial HSR, its subsequent breakage, the appearance of DHFR-containing fragments, and, finally, DMs. Our studies suggest that HSRs and DMs both are initiated by chromosome breaks, but that cell types differ in how the extra sequences ultimately are processed and/or maintained.
Hormone binding induces rapid proteasome-mediated degradation of thyroid hormone receptors
Authors: Dace Alexandra , Zhao Li , Park Kyung Soo , Furuno T. , Takamura N. , Nakanishi M. , West Brian L. , Hanover John A. , Cheng Sheue-yann
Published: July 2002

The thyroid hormone 3,3′,5-triiodo-
l
-thyronine (T3) is essential for growth, differentiation, and development. Its biological activities are mediated by T3 nuclear receptors (TRs). At present, how T3 regulates TR proteins and the resulting functional consequences are still unknown. Immunofluorescence analyses of endogenous TR in the growth hormone-producing GC cells showed that the T3-induced rapid degradation of TR was specifically blocked by lactacystin, a selective inhibitor of the ubiquitin–proteasome degradation pathway. Immunoblots demonstrated that the transfected TRβ1 was ubiquitinated and that the ubiquitination was T3 independent. Studies with a series of truncated TRβ1 showed that the hormone-binding domain was sufficient for the T3-induced rapid degradation of TRβ1 by the proteasome degradation pathway. T3 also induced rapid degradation of TRβ2 and TRα1. In contrast, the stability of the non-T3-binding TRα2 and naturally occurring TRβ1 mutants that do not bind T3 was not affected by T3 treatment, indicating that hormone binding to receptor was essential for the degradation of the wild-type receptors. In the presence of proteasome protease inhibitors, the levels of both total and ubiquitinated TRβ1 protein increased, yet T3-dependent transcriptional activation and the expression of the growth hormone gene were diminished, suggesting that proteasome-mediated degradation played a novel role in modulating transcriptional activation by TR. The present study reveals a role of T3 in modulating the functions of TR by regulating its receptor level via the ubiquitin–proteasome degradation pathway.
Critical role for the docking-protein FRS2α in FGF receptor-mediated signal transduction pathways
Authors: Hadari Y. R. , Gotoh N. , Kouhara H. , Lax I. , Schlessinger J.
Published: July 2002

The docking protein FRS2α has been implicated as a mediator of signaling via fibroblast growth factor receptors (FGFRs). We have demonstrated that targeted disruption of FRS2α gene causes severe impairment in mouse development resulting in embryonal lethality at E7.0–E7.5. Experiments with FRS2α-deficient fibroblasts demonstrate that FRS2α plays a critical role in FGF-induced mitogen-activated protein (MAP) kinase stimulation, phosphatidylinositol-3 (PI-3) kinase activation, chemotactic response, and cell proliferation. Following FGF stimulation, tyrosine phosphorylated FRS2α functions as a site for coordinated assembly of a multiprotein complex that includes Gab1 and the effector proteins that are recruited by this docking protein. Furthermore, we demonstrate that different tyrosine phosphorylation sites on FRS2α are responsible for mediating different FGF-induced biological responses. These experiments establish the central role of FRS2α in signaling via FGFRs and demonstrate that FRS2α mediates multiple FGFR-dependent signaling pathways critical for embryonic development.