bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarizabilities and first hyperpolarizabilities of HF, Ne, and BH from full configuration interaction and coupled cluster calculations
Authors: Larsen Helena , Olsen Jeppe , Hättig Christof , Jo/rgensen Poul , Christiansen Ove , Gauss Jürgen
Published: July 2002

Static and frequency-dependent polarizabilities and first hyperpolarizabilities have been calculated for HF and Ne using full configuration interaction (FCI) and a hierarchy of coupled cluster models: coupled cluster singles (CCS), an approximate coupled cluster singles and doubles model (CC2), coupled cluster singles and doubles (CCSD), an approximate coupled cluster singles, doubles, and triples model (CC3), and coupled cluster singles, doubles, and triples (CCSDT). A previous study of BH concerning FCI benchmarking has been extended to include CC3 and static CCSDT values. Systematic improvements of the polarizabilities and the hyperpolarizabilities are found going from CCS to CCSD and from CCSD to CC3 or CCSDT. Little or no improvement of the polarizabilities and no improvement of the hyperpolarizabilities are seen when going from CCS to CC2. The CCSD results represent a significant improvement over CCS and CC2 but are again surpassed by the CC3 results which agree very well with the FCI values. The relative error for the static polarizability at the CC3 level is 0.11% for Ne and, respectively, 0.16% and 0.20% for αxx and αzz of HF. For βzzz and βzxx the errors are 0.50% and 1.7%, respectively. Only in the challenging case of BH does CCSDT improve the CC3 values. The dispersion for the polarizabilities and hyperpolarizabilities is predicted with increasing accuracy in the CCS–CC2–CCSD–CC3 sequence as expected from the increasing accuracy of the electronic excitation energies. For all molecules the effect of orbital relaxation has been investigated for the static properties. The inclusion of orbital relaxation gives results that are somewhat different from the unrelaxed results but are in general no improvement.
Two-order-parameter description of liquids. II. Criteria for vitrification and predictions of our model
Authors: Tanaka Hajime
Published: July 2002

In our previous paper (paper I), we proposed a simple physical model that may universally describe glass-transition phenomena from the strong to fragile limit. It is based on the idea that in any liquid there always exist two competing orderings, which lead to two types of local structures frustrated with each other: (i) normal-liquid structures and (ii) locally favored structures. Here we demonstrate that this frustration, which causes an extra energy barrier for crystal nucleation, can be an additional physical factor to make vitrification easier. It can be regarded as impurity effects on crystallization. This idea provides us with a simple physical criterion for vitrification, which is consistent with the well-known empirical laws. We also check several main predictions of our model. According to our model, the melting temperature of the corresponding pure system free from disorder effects, Tm*, is a key temperature: Below it, a system starts to have special dynamic features peculiar to the Griffiths phase known in the field of random-spin systems, which is characterized by a complex free-energy landscape. We stress that this prediction is specific in the sense that Tm* is directly related to the real melting point Tm, which is an intrinsic physical property of the material. In our view, a stronger liquid suffers from stronger disorder effects due to a higher concentration of locally favored structures. This leads to a larger distance between Tm* and the Vogel–Fulcher temperature T0 for a stronger liquid, which is consistent with experimental results. Finally, the effect of pressure on the fragility is discussed in the light of our two-order-parameter model of liquids.
The role of quantum and thermal fluctuations upon properties of lithium clusters
Authors: Rousseau Roger , Marx Dominik
Published: July 2002

The impact of quantum effects and thermal fluctuations on properties of small lithium clusters, Li8 and Li20, is investigated at low temperatures with ab initio path integrals. The static optimized structures, i.e., the minima of the potential energy surface, are characterized by packings of multicenter bonded tetrahedral building blocks featuring short and long Li–Li bonds. Quantum effects at 10 K are found to be induced by zero-point motion, whereas tunneling turns out to be irrelevant. The simulations uncover that these quantum effects destroy the distinction between short and long chemical bonds, while preserving the overall structural shape of the clusters.
Monte Carlo investigations of dense copolymer systems II, Properties of ABA triblocks
Authors: Zifferer Gerhard , Neubauer Brigitte , Olaj Oskar Friedrich
Published: July 2002

In the present article we give a detailed analysis of ABA triblock copolymers in a selective solvent that is a good one for the outer blocks and a theta solvent for the inner one. A lattice model is used for the investigations and the concentration ranges from a volume fraction φ=0 up to φ=0.8. The results are compared to the behavior of the constituent homopolymers, i.e., homopolymers in athermal (AAA) and theta solvent (BBB). In the limit φ→0 the concentration dependence of properties of the whole chain as well as properties of individual blocks, may be deduced from pair-data with excellent precision for all systems under consideration. For large concentrations mean square dimensions of ABA markedly exceed that of AAA or BBB, respectively, due to microphase separation which becomes evident for concentrations larger than φ≈0.4 which corresponds to the point where intermolecular and intramolecular interactions are equally important.
Multiple lines of conical intersections and nondegenerate ground state in T⊗t2 Jahn–Teller systems
Authors: Koizumi Hiroyasu , Bersuker Isaac B. , Boggs James E. , Polinger Victor Z.
Published: July 2002

We have investigated the T⊗t2 Jahn–Teller problem with linear and quadratic vibronic coupling including a fourth-order term. First, numerical calculations of the lowest vibronic states were carried out by direct diagonalization of the Hamiltonian. The results show that the energy level of the ground vibronic state, which is triply degenerate T for small quadratic coupling g values, intersects the next A level with increasing g, thus realizing the nondegenerate ground state at sufficiently large g values. This result reverses the long-standing belief that the ground vibronic state for the T⊗t2 system has the same degeneracy and symmetry T as the initial electronic state. To explain these results in terms of Berry phase requirements and conical intersections, the adiabatic potential energy surface of the system is analyzed, and the relationships among the type and number of minima, conical intersections, and relevant tunneling paths are revealed. Depending on the vibronic coupling parameter values, there are four trigonal minima and six orthorhombic saddle points, which become minima at large g values, plus ten lines of conical intersections on the lowest potential energy surface. The barriers between the minima are significantly increased near the lines of conical intersections where the Born–Huang terms diverge. For small enough quadratic coupling, only four lines of conical intersections that originate from the highest symmetry point and proceed along the four trigonal directions are significant in determining the Berry phase, and the triply degenerate ground T state is obtained. By increasing the quadratic coupling parameter, the remaining six lines of conical intersections approach the point of highest symmetry, thus allowing for alternative tunneling paths and Berry phases which lead to the nondegenerate A ground state. This explanation of the origin of the nondegenerate ground state for some range of values of the vibronic coupling parameters is strengthened by model calculations of tunneling splitting.
Theoretical study on the structure of Na+-doped helium clusters: Path integral Monte Carlo calculations
Authors: Nakayama Akira , Yamashita Koichi
Published: July 2002

The microscopic structure of Na+-doped helium clusters (Na+⋅HeN) is investigated by employing the path integral Monte Carlo (PIMC) method. Our primary interest is in determining the nature of superfluidity in these clusters by examining the temperature and size dependences of several physical quantities such as energy and superfluid fraction. Comparison with Boltzmann statistics has also been made to clarify the effects of superfluidity. It is found that clusters of N⩾100 have a triple-layer structure where the first shell is rigid-body and solidlike and the third shell is almost like liquid helium. Between T=1.0 K and T=1.25 K, a superfluid transition is observed in the second shell. This transition temperature shifts to a lower temperature than that observed in bulk liquid helium because the finite system size and strong binding to the cation cause superfluidity to be suppressed. By computing the effective moment of inertia as a function of cluster size, it is found that there exists a specific number of helium atoms needed for the formation of the complex, the components of which are in a state of normal liquid.
Auto-Bäcklund transformation, Lax pairs, and Painlevé property of a variable coefficient Korteweg–de Vries equation. I
Authors: Nirmala N. , Vedan M. J. , Baby B. V.
Published: July 2002

Using the Painlevé property of partial differential equations, the auto-Bäcklund transformation and Lax pairs for a Korteweg–de Vries (KdV) equation with time-dependent coefficients are obtained. The Lax pair criterion also makes it possible for some new models of the variable coefficient KdV equation to be found that can represent nonsoliton dynamical systems. This can explain the wave breaking phenomenon in variable depth shallow water.
Modular representations as a possible basis of finite physics
Authors: Lev Felix
Published: July 2002

The approach in which physical systems are described by the elements of a linear space over a finite field, and operators of physical quantities by linear operators in this space, is discussed. The mathematical formulation of the correspondence between such a description and the conventional one is given for a case when the characteristic of the finite field is sufficiently large. The above correspondence is considered in the examples of finite analogs of representations of so(2,3), so(1,4), and osp(1,4) algebras. Finite analogs of representations of infinite-dimensional algebras are briefly discussed.
Bäcklund transformations for surfaces in Minkowski space
Authors: Palmer Bennett
Published: July 2002

A Bäcklund transformation is constructed between spacelike surfaces of constant negative curvature and timelike surfaces of constant negative curvature in three-dimensional Minkowksi space. The transformation gives a differential geometric interpretation to a Bäcklund transformation between the elliptic sine-Gordon equation and the elliptic sinh-Gordon equation studied by Leibbrandt [J. Math. Phys. 19, (1978)].
Exact response functions of a 2-D Fermion fluid
Authors: Zhang M. Q.
Published: July 2002

Exact k-space density response of a perfect Fermion (spinless) fluid in the ground state to external potentials is derived in a closed form to all orders in two dimensions. The results are expressed in terms of geometrical invariants. The most distinct features in two dimensions are that no short wave-vector response depends on kF (the Fermi wave vector), and the usual first-order linear response function R̃(k) is a constant for k≤2kF.
O(3,3)-like symmetries of coupled harmonic oscillators
Authors: Han D. , Kim Y. S. , Noz Marilyn E.
Published: July 2002

In classical mechanics, the system of two coupled harmonic oscillators is shown to possess the symmetry of the Lorentz group O(3,3) or SL(4,r) in the four-dimensional phase space. In quantum mechanics, the symmetry is reduced to that of O(3,2) or Sp(4), which is a subgroup of O(3,3) or SL(4,r), respectively. It is shown that among the six Sp(4)-like subgroups, only one possesses the symmetry which can be translated into the group of unitary transformations in quantum mechanics.
The symplectic geometry of a parametrized scalar field on a curved background
Authors: Hájíček P. , Isham C. J.
Published: July 2002

We study the real, massive Klein–Gordon field on a C∞ globally-hyperbolic background space–time with compact Cauchy hypersurfaces. In particular, the parametrization of this system as initiated by Dirac and Kuchař is put on a rigorous basis. The discussion is focussed on the structure of the set of spacelike embeddings of the Cauchy manifold into the space–time, and on the associated e-tensor density bundles and their tangent and cotangent bundles. The dynamics of the field is expressed as a set of automorphisms of the space of initial data in which each pair of embeddings defines one such automorphism. Using these results, the extended phase space of the system is shown to be a weak-symplectic manifold, and the Kuchař constraint is shown to define a smooth constraint submanifold which is foliated smoothly by the constraint orbits. The pull-back of the symplectic form to the constraint surface is a presymplectic form which is singular on the tangent spaces to the constraint orbits. Thus, the geometric structure of this infinite-dimensional system is analogous to that of a finite-dimensional, first-class parametrized system, and hence many of the results for the latter can be transferred to the infinite-dimensional case without difficulty.
Large-order perturbation theory for a non-Hermitian 𝓟𝓣-symmetric Hamiltonian
Authors: Bender Carl M. , Dunne Gerald V.
Published: July 2002

A precise calculation of the ground-state energy of the complex 𝒫𝒯-symmetric Hamiltonian H=p2+14x2+iλx3, is performed using high-order Rayleigh–Schrödinger perturbation theory. The energy spectrum of this Hamiltonian has recently been shown to be real using numerical methods. Here we present convincing numerical evidence that the Rayleigh–Schrödinger perturbation series is Borel summable, and show that Padé summation provides excellent agreement with the real energy spectrum. Padé analysis provides strong numerical evidence that the once-subtracted ground-state energy considered as a function of λ2 is a Stieltjes function. The analyticity properties of this Stieltjes function lead to a dispersion relation that can be used to compute the imaginary part of the energy for the related real but unstable Hamiltonian H=p2+14 x2−εx3.
1/N
expansions in nonrelativistic quantum mechanics
Authors: Bjerrum-Bohr Niels Emil Jannik
Published: July 2002

An extensive number of numerical computations of energy 1/N series using a recursive Taylor series method are presented in this paper. The series are computed to a high order of approximation and their behavior on increasing the order of approximation is examined.
Density variations in weakly compressible flows
Authors: Bayly B. J. , Levermore C. D. , Passot T.
Published: July 2002

Density variations in real fluids are related to both pressure and entropy variations, even in the incompressible limit. The behavior of density variations depends crucially on the relative sizes of the pressure, temperature, and entropy fluctuations. It is shown how this arises from a formal asymptotic expansion of the compressible Navier–Stokes equations about a uniform state. Direct numerical simulations of the full compressible equations verify the consistency of different asymptotic regimes. In the case of turbulent flow, the Kolmogorov–Obukhov theory allows one to predict inertial range scalings for the density fluctuation power spectra in various situations. The results may be relevant to observations of the interstellar medium.
Dynamics of passively advected impurities in simple two-dimensional flow models
Authors: Crisanti Andrea , Falcioni Massimo , Provenzale Antonello , Tanga Paolo , Vulpiani Angelo
Published: July 2002

The motion of passively advected impurities with density ρp different from the fluid density ρf in simple models of two-dimensional velocity fields is studied. The impurity dynamics strongly depends on the parameter δ=ρf/ρp. For a stationary streamfunction, impurities that are lighter than the fluid undergo regular motions and converge to the centers of the advection cells. Particles denser than the fluid exhibit chaotic trajectories and standard diffusion at large times. For isotropic velocity fields, the diffusion coefficients display a scaling dependence upon the parameter ε=1−δ. For heavy impurities in weakly anisotropic velocity fields, the diffusion coefficients in the x and y directions may be different by several orders of magnitude. These features bear several resemblances to the motion of fluid particles in the presence of additive noise. For a time-periodic streamfunction, the heavy particles undergo chaotic trajectories and standard diffusion; light particles may either display chaotic behavior and diffusion or regular motions ending with periodic trajectories, depending upon the values of the control parameters of the Eulerian flow. At intermediate times, the particle distribution displays complex caustic structures. Except that at very short times, the distribution of advected impurities trace neither the density nor the velocity of the advecting flow, even for δ only slightly different from one.
The rigid ballooning mode in finite-beta axisymmetric plasmas with diffuse pressure profiles
Authors: LoDestro L. L.
Published: July 2002

The ballooning equation for the m=1 mode in an axisymmetric plasma with a diffuse pressure profile is developed within the framework of the arbitrary-β, dominant-finite-Larmor-radius analysis of Kaiser and Pearlstein [Phys. Fluids 28, 1003 (1985)]. Diffuse profiles preserve the sharp-boundary result that only the vacuum curvature appears in the drive. A diffuse profile is shown to reduce (reproduce) the volume-averaged pressure required for stability for an isotropic (ideal) pressure in the limit as a conducting wall approaches the plasma/vacuum boundary and the mirror ratio approaches one.
The effective thermal conductivity and elongational viscosity of a nondilute suspension of aligned slender rods
Authors: Acrivos Andreas , Shaqfeh E. S. G.
Published: July 2002

The effective thermal conductivity and elongational viscosity of a nondilute suspension of infinitely conducting perfectly aligned slender rods of slenderness ratio ε≪1 are determined using an effective medium theory together with a self-consistent scheme. The expressions thereby obtained cover the range in particle concentration c from infinite dilution c/ε2≪1, to the semidilute regime, c/ε2≫1, but c≪1. In the latter case, the present results agree to leading order with a formula derived by Batchelor [J. Fluid Mech. 46, 813 (1971)] even though the analyses employ physical arguments that appear to be quite distinct.
Parallel flow convection in a tilted two-dimensional porous layer heated from all sides
Authors: Sen Mihir , Vasseur P. , Robillard L.
Published: July 2002

In this work natural convection within a two-dimensional fluid saturated Darcy porous layer is considered. The porous material is in a large aspect ratio rectangle with its sides inclined with respect to the gravity vector. All four faces are exposed to uniform heat fluxes, opposite faces being heated and cooled, respectively. Analytical solutions for the streamfunction and temperature fields in the central region are deduced using a parallel flow assumption. Numerical confirmation of the analytical results is also obtained. For high enough Rayleigh numbers multiple steady states around the rest state are found. These states represent unsymmetrical flows in opposite directions. The critical Rayleigh number for the onset of motion determined from a stability analysis corresponds to that for the existence of unicellular convection using the parallel flow approximation. Linear stability limits of each of the multiple states are also calculated. One of the convective flows is found to be stable to higher Rayleigh numbers than the other.
Freeze-out and the effect of compressibility in the Richtmyer–Meshkov instability
Authors: Mikaelian Karnig O.
Published: July 2002

Perturbations of small-amplitude η at the interface between two fluids grow linearly in time after the passage of a shock. According to Richtmyer’s prescription, the growth rate is proportional to the Atwood number after the interface has been shocked, η̇∼Aafter. The focus is on highly compressible fluids starting with Abefore≥0. By carrying out two-dimensional numerical simulations, several exceptions to Richtmyer’s prescription are found, in particular, when Aafter≤0. Neither the expected freeze-out when Aafter=0 nor the phase reversal when Aafter<0 is observed. The results, however, are in agreement with Fraley’s analysis, which is compared and contrasted with Richtmyer’s prescription. Previous calculations and experiments on the Richtmyer–Meshkov instability are analyzed, it is explained why they have not detected the failure of Richtmyer’s prescription, and several new numerical and physical experiments are proposed.
Rapid relaxation of an axisymmetric vortex
Authors: Bernoff Andrew J. , Lingevitch Joseph F.
Published: July 2002

In this paper it is argued that a two-dimensional axisymmetric large Reynolds number (Re) monopole when perturbed will return to an axisymmetric state on a time scale (Re1/3) that is much faster than the viscous evolution time scale (Re). It is shown that an arbitrary perturbation can be broken into three pieces; first, an axisymmetric piece corresponding to a slight radial redistribution of vorticity; second, a translational piece which corresponds to a small displacement of the center of the original vortex; and finally, a nonaxisymmetric perturbation which decays on the Re1/3 time scale due to a shear/diffusion averaging mechanism studied by Rhines and Young [J. Fluid Mech. 133, 133 (1983)] for a passive scalar and Lundgren [Phys. Fluids 25, 2193 (1982)] for vorticity. This mechanism is verified numerically for the canonical example of a Lamb monopole. This result suggests a physical explanation for the persistence of monopole structures in large Reynolds flows, such as decaying turbulence.
Translational instability of a bubble undergoing shape oscillations
Authors: Feng Z. C. , Leal L. G.
Published: July 2002

This paper studies the translational instability of an oscillating bubble. It is shown that when a spherical bubble undergoing volume oscillation becomes unstable, giving rise to shape oscillations of two neighboring modes, the translational mode is intimately coupled with the two shape modes and this results in translational instability of the bubble. The main contribution is twofold. First, the integral relations for motions of bubbles in an infinite perfect liquid are not relied on, hence result is applicable to liquids with weak viscous effect. Second, the method of deriving the amplitude equations, which is similar to that of normal form calculations for ordinary differential equations, has not been applied to partial differential equations before.
Collision of dipolar vortices on a β plane
Authors: Velasco Fuentes O. U. , van Heijst G. J. F.
Published: July 2002

The interaction of two dipoles moving perpendicularly to the gradient of background vorticity is studied both numerically and experimentally. In the numerical computations the vorticity distribution is represented either by four point vortices (point-vortex model) or by thousands of them (vortex-in-cell method). The simplest model is used to study the dynamics and the advection of fluid particles in two kinds of interaction: coaxial couples of equal strength but different size, and equal parallel couples with a nonzero impact parameter (the distance between the dipoles’ axis). As a result of the interaction fluid masses are exchanged between the two dipoles and between each dipole and the ambient fluid. In the case of equal coaxial couples the amount of fluid exchanged depends on the gradient of ambient vorticity, with the largest mass exchange occurring always between the eastward traveling dipole and the ambient fluid. The collision of parallel couples with nonzero impact parameter leads to a large mass exchange, either because several interactions may occur or because when two independent couples arise, they have a nonuniform motion. Laboratory experiments in a rotating fluid (with a flat sloping bottom providing the β effect), confirm that an elastic interaction is a rare event. The unstable trajectory of the westward traveling dipole, as well as small perturbations unavoidable in the laboratory, invariably lead to collisions of nonaligned dipoles. The gross features of the vortex motion, as well as of the mass exchange, are well modeled using the point-vortex model, whereas the vortex-in-cell method reproduces many details of the vortex motion, the evolution of the vorticity field, and the exchange of mass.
Kinetic theoretical study of a simply sheared two-dimensional granular gas to Burnett order
Authors: Sela N. , Goldhirsch I. , Noskowicz S. H.
Published: July 2002

The kinetics of a collection of inelastically colliding smooth disks in a plane, in a state of constant shear rate, is studied by performing an analysis of the pertinent Boltzmann equation. The fact that the granular temperature T satisfies T∝γ2ℓ2/ε, where ℓ is the mean free path, γ is the shear rate and ε≡1−e2, where e is the coefficient of normal restitution, leads to the observation that when γ∝√ε the limit ε→0 of the above problem corresponds to a system of elastically colliding particles in equilibrium (at temperature T). This observation enables the construction of a systematic perturbative expansion (for the single particle distribution function) in powers of √ε , in which the equilibrium (Maxwellian) distribution function serves as zeroth order. The limitations of this expansion are discussed alongside possible generalizations. Explicit expressions for the single particle distribution function to O(ε) and expressions for the corresponding stress tensor are obtained. The phenomenon of normal stress difference is shown to be of O(ε), i.e. of second (Burnett) order in the shear-rate and its calculated magnitude compares well with results of numerical simulations. A comparison of the present theory with that of Jenkins and Richman is presented as well.
Three-dimensional instability of anticyclonic swirling flow in rotating fluid: Laboratory experiments and related theoretical predictions
Authors: Afanasyev Ya. D. , Peltier W. R.
Published: July 2002

We present results from a new series of experiments on the geophysically important issue of the instability of anticyclonic columnar vortices in a rotating fluid in circumstances such that the Rossby number exeeds unity. The core of the vortex is modeled as a solid cylinder rotating in a fluid that is itself initially in a state of solid-body rotation. When the cylinder rotates cyclonically the flow induced by the differential rotation is stable except for a brief initial period. When the cylinder rotates anticyclonically, however, intense perturbations spontaneously appear and amplify in the flow. The experimental results demonstrate that secondary motions appear in an annular region of finite width surrounding the cylinder (in accord with the prediction of the generalized Rayleigh criterion) and are governed by the process of three-dimensional centrifugal instability. These motions are characterized by a definite wave number in the coordinate direction parallel to the axis of the cylinder. Both the width of the unstable annular region and the vertical wavelength of the motions induced by centrifugal instability are determined by the main nondimensional parameter of the flow—the Rossby number. The evolution of the secondary motions gives rise to the appearance of tertiary motions—which are Kelvin–Helmholtz-like (barotropic) vortices that develop at the periphery of the unstable annulus, thus leading to the formation of exceedingly complex dynamical structures. If the rotating cylinder is withdrawn vertically from the fluid, the instability rapidly destroys the core of the vortex. During its initial phase of development the flow evolves in a way that is strongly analogous to the cylindrical Couette case. An appropriate theory is employed to explain the results of the laboratory experiments.
Orientation distribution in a dilute suspension of fibers subject to simple shear flow
Authors: Chen Shing Bor , Jiang Li
Published: July 2002

The orientation distribution in a dilute suspension of Brownian fibers subject to simple shear flow is investigated theoretically. The fiber has a large aspect ratio r and may carry charge. The flow strength is characterized by the rotary Peclet number Pe=γ/D, where γ is the shear rate and D is the rotary diffusivity of the fiber. Emphasis is placed on the microstructure of the suspension in strong flows, in which the advection and diffusion are of equal importance in a small region of angular space near the flow direction. A new computational method based on a finite difference scheme is developed to calculate the orientation distribution function and then evaluate the orientation moments. For the case of uncharged rods and 1≪Pe1/3≪r, the obtained orientation moments compare favorably with those deduced from the spherical harmonic method. The effects of the fiber aspect ratio and charge are also investigated when they become crucial. It is found that at a given large Pe, the orientation distribution of uncharged fibers with r∼O(Pe1/3) becomes broad and its peak shifts down towards the flow axis, compared with the case of thin rods with r≫Pe1/3. The microstructure of a suspension of charged fibers depends on the charge distribution and the double layer thickness relative to the fiber length.
Modeling ionization by helicon waves
Authors: Degeling A. W. , Boswell R. W.
Published: July 2002

The response of the electron distribution function in one dimension to a traveling wave electric field is modeled for parameters relevant to a low-pressure helicon wave plasma source, and the resulting change in the ionization rate calculated. This is done by calculating the trajectories of individual electrons in a given wave field and assuming no collisions to build up the distribution function as the distance from the antenna is increased. The ionization rate is calculated for argon by considering the ionization cross section and electron flux at a specified position and time relative to the left-hand boundary, where the distribution function is assumed to be Maxwellian and the wave travels to the right. The simulation shows pulses in the ionization rate that move away from the antenna at the phase velocity of the wave, demonstrating the effect of resonant electrons trapped in the wave’s frame of reference. It is found that the ionization rate is highest when the phase velocity of the wave is between 2 and 3×106 m/s, where the electrons interacting strongly with the wave (i.e., electrons with velocities inside the wave’s “trapping width”) have initial energies just below the ionization threshold. Results from the model are compared with experimental data and show reasonable qualitative agreement.
Shock formation in a negative ion plasma
Authors: Luo Q-Z. , D’Angelo N. , Merlino R. L.
Published: July 2002

An experimental investigation of the effect of negative ions on shock formation in a (collisional) Q machine plasma is described. Shock formation was observed only when the ratio of the negative ion to positive ion density, ε, exceeded about 0.9.
Low defect density CdTe(111)-GaAs(001) heterostructures by molecular beam epitaxy
Authors: Ballingall J. M. , Takei W. J. , Feldman Bernard J.
Published: July 2002

(111) oriented CdTe has been grown on (001) oriented GaAs substrates by molecular beam epitaxy. Double crystal x-ray diffraction rocking curve and photoluminescence measurements indicate that the CdTe is of exceptional quality despite the large lattice mismatch of 14.6%.
Raman spectroscopy of intrinsic defects in electron and neutron irradiated GaAs
Authors: Berg R. S. , Yu P. Y. , Weber E. R.
Published: July 2002

We report Raman scattering measurements in GaAs that has been damaged by irradiation with either high-energy electrons or neutrons. We observe new and relatively sharp peaks which are attributed to vibrational modes of intrinsic point defects, most likely an As vacancy.
Void formation in thin Al films
Authors: Colgan E. G. , Li C.-Y. , Mayer J. W.
Published: July 2002

Al films deposited on Pt layers developed voids after annealing between 200 and 300 °C. Void formation in the Al was also observed when the Pt layer was deposited above the Al film. The amount of Al surrounding the voids increased as the voids grew. The void growth seems to saturate when all the Pt is consumed to form Pt2Al3, and the rate of void growth decreases as the thickness of the initial Al film increases. We believe the controlling mechanism is diffusion along the Al/Pt interface made possible by compound formation there.
Enhanced beam coupling modulation using the polarization properties of photorefractive GaAs
Authors: Partovi Afshin , Garmire Elsa M. , Cheng Li-Jen
Published: July 2002

We report observation of a rotation in the polarization of the two photorefractive recording beams in GaAs for a configuration with the internally generated space-charge field along the 〈110〉 crystallographic orientation. This rotation is a result of simultaneous constructive and destructive beam coupling in each beam for the optical electric field components along the two electro-optically induced principal dielectric axes of the crystal. By turning one of the beams on and off, the intensity of the other beam after the crystal and a polarization analyzer can be modulated by as much as 500%. This result is of particular importance for optical information processing applications.
Pulsed photothermal radiometry for depth profiling of layered media
Authors: Long Frederick H. , Anderson R. Rox , Deutsch Thomas F.
Published: July 2002

Pulsed photothermal radiometry (PPTR) has been used to analyze depth profile of layered materials, including biological tissue. An analytic theory for the time dependence of the PPTR signal has been developed and compared with experimental results on inert materials and living human skin.
Penetration depths of high T
c films measured by two-coil mutual inductances
Authors: Fiory A. T. , Hebard A. F. , Mankiewich P. M. , Howard R. E.
Published: July 2002

A fundamental parameter of sample quality in epitaxial films of high Tc oxides is the effective penetration depth λ∥ of the superconducting sheet. A contactless audio-frequency method is described, in which an epitaxial YBa2Cu3O7 film is sandwiched between sets of stacked coils, and the procedure is given for computing the complex sheet impedance and hence λ∥ from the change in mutual inductance produced by screening currents in the film. Temperature dependence of the complex impedance is presented.
The use of dimetal building blocks in convergent syntheses of large arrays
Authors: Cotton F. Albert , Lin Chun , Murillo Carlos A.
Published: July 2002

Within the broad field called “supramolecular chemistry,” there is a sector that is based on the use of metal atoms or ions as key elements in promoting the assembly and dictating the main structural features of the supramolecular products. Considerable success has been achieved by using M—M bonded dimetal entities in this role. Metal–metal bonded cationic complexes of the [M
2
(DAniF)

n

(MeCN)

8–2
n

]

(4−
n
)+

type, where M = Mo or Rh and DAniF is an
N,N′
-di-
p
-anisylformamidinate anion, have been used as subunit precursors and then linked by various equatorial and axial bridging groups such as polycarboxylate anions, polypyridyls, and polynitriles. Characterization of the products by single-crystal x-ray diffraction, cyclic voltammetry, differential pulse voltammetry, NMR, and other spectroscopic techniques has revealed the presence of discrete tetranuclear (pairs or loops), hexanuclear (triangles), octanuclear (squares), and dodecanuclear (cages) species and one-, two-, or three-dimensional molecular nanotubes. These compounds display a rich electrochemical behavior that is affected by the nature of the linkers.
An electric lobe suppressor for a yeast choline transport mutation belongs to a new family of transporter-like proteins
Authors: O'Regan Seana , Traiffort Elisabeth , Ruat Martial , Cha Nathalie , Compaoré Désiré , Meunier François-Marie
Published: July 2002

Choline is an important metabolite in all cells due to the major contribution of phosphatidylcholine to the production of membranes, but it takes on an added role in cholinergic neurons where it participates in the synthesis of the neurotransmitter acetylcholine. We have cloned a suppressor for a yeast choline transport mutation from a Torpedo electric lobe yeast expression library by functional complementation. The full-length clone encodes a protein with 10 putative transmembrane domains, two of which contain transporter-like motifs, and whose expression increased high-affinity choline uptake in mutant yeast. The gene was calledCTL1for its choline transporter-like properties. The homologous rat gene, rCTL1, was isolated and found to be highly expressed as a 3.5-kb transcript in the spinal cord and brain and as a 5-kb transcript in the colon.In situhybridization showed strong expression of rCTL1in motor neurons and oligodendrocytes and to a lesser extent in various neuronal populations throughout the rat brain. High levels of rCTL1were also identified in the mucosal cell layer of the colon. Although the sequence of theCTL1gene shows clear homology with a single gene inCaenorhabditis elegans, several homologous genes are found in mammals (CTL2–4). These results establish a new family of genes for transporter-like proteins in eukaryotes and suggest that one of its members,CTL1, is involved in supplying choline to certain cell types, including a specific subset of cholinergic neurons.
The crystal structure and mutational analysis of a novel RNA-binding domain found in the human Tap nuclear mRNA export factor
Authors: Ho Dona N. , Coburn Glen A. , Kang Yibin , Cullen Bryan R. , Georgiadis Millie M.
Published: July 2002

The Tap protein mediates the sequence nonspecific nuclear export of cellular mRNAs as well as the sequence-specific export of retroviral mRNAs bearing the constitutive transport element (CTE). Previously, the structures of individual Tap subdomains, including ribonucleoprotein and leucine-rich repeat domains, have been described. Here, we report the crystal structure of a functional CTE RNA-binding domain of human Tap, including the N-terminal arm of the ribonucleoprotein domain and interdomain linking polypeptide. To identify residues that interact with the CTE, we have introduced 38 alanine substitutions for surface residues in the Tap CTE-binding domain and tested these mutants for their ability to support CTE-dependent nuclear RNA export and CTE binding. Four residues that cluster on a concave surface in the leucine-rich repeat domain were found to be critical for CTE binding and define a CTE-interacting surface on this domain. The second critical CTE-interacting surface on Tap is defined by three previously identified residues on the surface of the ribonucleoprotein domain. The structural and mutational data define a novel RNA-binding site on the Tap protein.
Enhancer action in trans is permitted throughout the
Drosophila
genome
Authors: Chen Ji-Long , Huisinga Kathryn L. , Viering Michaela M. , Ou Sharon A. , Wu C.-ting , Geyer Pamela K.
Published: July 2002

Interactions between paired homologous genes can lead to changes in gene expression. Such trans-regulatory effects exemplify transvection and are displayed by many genes in
Drosophila
, in which homologous chromosomes are paired somatically. Transvection involving the
yellow
cuticle pigmentation gene can occur by at least two mechanisms, one involving the trans-action of enhancers on a paired promoter and a second involving pairing-mediated bypass of a chromatin insulator. A system was developed to evaluate whether the action of the
yellow
enhancers in trans could be reconstituted outside of the natural near telomeric location of the
yellow
gene. To this end, transgenic flies were generated that carried a
yellow
gene modified by the inclusion of strategically placed recognition sites for the Cre and FLP recombinases. Independent action of the recombinases produced a pair of derivative alleles, one enhancerless and the other promoterless, at each transgene location. Transvection between the derivatives was assessed by the degree of interallelic complementation. Complementation was observed at all eight sites tested. These studies demonstrate that
yellow
transvection can occur at multiple genomic locations and indicate that the
Drosophila
genome generally is permissive to enhancer action in trans.
Chronic morphine induces the concomitant phosphorylation and altered association of multiple signaling proteins: A novel mechanism for modulating cell signaling
Authors: Chakrabarti Sumita , Oppermann Martin , Gintzler Alan R.
Published: July 2002

Traditional mechanisms thought to underlie opioid tolerance
include receptor phosphorylation/down-regulation, G-protein
uncoupling, and adenylyl cyclase superactivation. A parallel line of
investigation also indicates that opioid tolerance development results
from a switch from predominantly opioid receptor G
iα
inhibitory to G
β
γ
stimulatory signaling. As
described previously, this results, in part, from the increased
relative abundance of G
β
γ
-stimulated
adenylyl cyclase isoforms as well as from a profound increase in their
phosphorylation [Chakrabarti, S., Rivera, M., Yan, S.-Z., Tang, W.-J.
& Gintzler, A. R. (1998)
Mol. Pharmacol.
54,
655–662; Chakrabarti, S., Wang, L., Tang, W.-J. & Gintzler, A. R.
(1998)
Mol. Pharmacol.
54, 949–953]. The present study
demonstrates that chronic morphine administration results in the
concomitant phosphorylation of three key signaling proteins, G protein
receptor kinase (GRK) 2/3, β-arrestin, and G
β
, in the
guinea pig longitudinal muscle myenteric plexus tissue. Augmented
phosphorylation of all three proteins is evident in immunoprecipitate
obtained by using either anti-GRK2/3 or G
β
antibodies,
but the phosphorylation increment is greater in immunoprecipitate
obtained with G
β
antibodies. Analyses of
coimmunoprecipitated proteins indicate that phosphorylation of
GRK2/3, β-arrestin, and G
β
has varying consequences
on their ability to associate. As a result, increased availability of
and signaling via G
β
γ
could occur without
compromising the membrane content (and presumably activity) of
GRK2/3. Induction of the concomitant phosphorylation of multiple
proteins in a multimolecular complex with attendant modulation of their
association represents a novel mechanism for increasing
G
β
γ
signaling and opioid tolerance
formation.
The structures of anthranilate synthase of
Serratia marcescens
crystallized in the presence of (
i
) its substrates, chorismate and glutamine, and a product, glutamate, and (
ii
) its end-product inhibitor,
l
-tryptophan
Authors: Spraggon Glen , Kim Choel , Nguyen-Huu Xuong , Yee Muh-Ching , Yanofsky Charles , Mills Stanley E.
Published: July 2002

The crystal structure of anthranilate synthase (AS) from
Serratia marcescens,
a mesophilic bacterium, has been
solved in the presence of its substrates, chorismate and glutamine, and
one product, glutamate, at 1.95 Å, and with its bound feedback
inhibitor, tryptophan, at 2.4 Å. In comparison with the AS structure
from the hyperthermophile
Sulfolobus solfataricus
, the
S. marcescens
structure shows similar subunit structures
but a markedly different oligomeric organization. One crystal form of
the
S. marcescens
enzyme displays a bound pyruvate as
well as a putative anthranilate (the nitrogen group is ambiguous) in
the TrpE subunit. It also confirms the presence of a covalently bound
glutamyl thioester intermediate in the TrpG subunit. The
tryptophan-bound form reveals that the inhibitor binds at a site
distinct from that of the substrate, chorismate. Bound tryptophan
appears to prevent chorismate binding by a demonstrable conformational
effect, and the structure reveals how occupancy of only one of the two
feedback inhibition sites can immobilize the catalytic activity of both
TrpE subunits. The presence of effectors in the structure provides a
view of the locations of some of the amino acid residues in the active
sites. Our findings are discussed in terms of the previously described
AS structure of
S. solfataricus
, mutational data
obtained from enteric bacteria, and the enzyme's mechanism of action.
Sustained activation of Ras/Raf/mitogen-activated protein kinase cascade by the tumor suppressor p53
Authors: Lee Sam W. , Fang Li , Igarashi Makoto , Ouchi Toru , Lu Kun Ping , Aaronson Stuart A.
Published: July 2002

The
p53
tumor suppressor gene can inhibit proliferation transiently, induce permanent cell-cycle arrest/senescence, or cause apoptosis depending on the cellular context. The mitogen-activated protein kinase (MAPK) cascade is known to play a crucial role in cell proliferation and differentiation. Moreover, the duration and intensity of MAPK activation can profoundly influence the biological response observed. We demonstrated that a sustained activation of MAPK cascade could be induced by wild-type
p53
expression but not by p21
Waf1/Cip1
. Furthermore, exposure of normal cells to DNA-damaging agents induced MAPK activation in a
p53
-dependent manner. Tumor-derived p53 mutants defective in DNA binding failed to activate MAPK, implying that p53 transcriptional activity is essential for this function. Finally, activation of MAPK by p53 was inhibited by expression of dominant-negative Ras (N17Ras) and Raf1 mutants, indicating that MAPK activation by p53 is mediated at a level upstream of Ras. All of these findings establish a biochemical link between p53 signaling and the Ras/Raf/MAPK cascade.
Cyclosporine A-induced hypertension involves synapsin in renal sensory nerve endings
Authors: Zhang Weiguo , Li Jia-Ling , Hosaka Masahiro , Janz Roger , Shelton John M. , Albright George M. , Richardson James A. , Südhof Thomas C. , Victor Ronald G.
Published: July 2002

The calcineurin inhibitor cyclosporine A (CsA) has emerged as a major cause of secondary hypertension in humans, but the underlying pathogenetic mechanisms have remained enigmatic. Synapsins are a family of synaptic vesicle phosphoproteins that are essential for normal regulation of neurotransmitter release at synapses. In addition to synaptic vesicles, synapsins and other vesicle proteins are found on microvesicles in sensory nerve endings in peripheral tissues. However, the functions of the sensory microvesicles in general, and of synapsins in particular, are unknown. We now demonstrate in a mouse model that CsA raises blood pressure by stimulating renal sensory nerve endings that contain synapsin-positive microvesicles. In knockout mice lacking synapsin I and II, sensory nerve endings are normally developed but not stimulated by CsA whereas a control stimulus, capsaicin, is fully active. The reflex activation of efferent sympathetic nerve activity and the increase in blood pressure by CsA seen in control are greatly attenuated in synapsin-deficient mice. These results provide a mechanistic explanation for CsA-induced acute hypertension and suggest that synapsins could serve as a drug target in this refractory condition. Furthermore, these data establish evidence that synapsin-containing sensory microvesicles perform an essential role in sensory transduction and suggest a role for synapsin phosphorylation in this process.