bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic circular dichroism of Nd3+ and Yb3+ ions in LiNbO3 crystals
Authors: Bonardi C. , Carvalho R. A. , Basso H. C. , Terrile M. C. , Cruz G. K. , Bausá L. E. , Solé J. Garcı́a
Published: July 2002

Magnetic circular dichroism (MCD) of trivalent rare earth doped lithium niobate crystals is reported for the first time. Magneto-optical signals of Nd3+ and Yb3+ ions have been studied at 2 K as a function of the magnetic field strength up to 5 T. This study allows the identification of the Zeeman sublevels of these ions, which can be labeled by irreducible representations (Γ4, Γ5, or Γ6), so that the sign and allowance of transitions can be predicted. From the dependence of suitable MCD spectral lines on the magnetic field strength, the effective gyromagnetic factor of the ground state has been determined for both ions: (gNd)∥=1.4±0.1 and; (gYb)∥=4.7±0.1.
Ion association and condensation in primitive models of electrolyte solutions
Authors: Camp Philip J. , Patey G. N.
Published: July 2002

Monte Carlo simulations have been used to investigate condensation and ion association in primitive models of electrolyte solutions. We have examined models in which the ions have equal diameter, the cation charge is +Zq, and the anion charge is −q, with Z=1, Z=2, and Z=4. Z=1 corresponds to the familiar restricted primitive model. Rough estimates of the critical temperatures have been obtained. In all cases the vapor phase is highly associated, with the ions forming electroneutral pairs, triples, and quintuples for Z=1, Z=2, and Z=4, respectively. Ion association has been investigated using a nearest-neighbor distribution function. For each value of Z the onset of ion dissociation in the low-density vapor is shown to occur at a temperature very close to the critical temperature. This raises the possibility of ion association affecting the critical behavior of ionic fluids. We have compared the simulation results for the restricted primitive model with the predictions of Debye–Hückel-based theories and the pairing mean spherical approximation. All of the theories perform well in the vapor phase, but are less successful in the liquid phase.
Effect of pressure on the segmental and chain dynamics of polyisoprene. Molecular weight dependence
Authors: Floudas G. , Gravalides C. , Reisinger T. , Wegner G.
Published: July 2002

We report on the effect of pressure on the relaxation modes of the type-A polymer polyisoprene as a function of the chain length for M/Me ratio’s (Me is the entanglement molecular weight): 15, 12, 23, 2, and 5. We find that the spectral shape of the segmental and normal modes is invariant under temperature (T) and pressure (P) variations, but the time–pressure superposition fails as a consequence of the stronger P dependence of the segmental mode. The segmental mode activation volume for the different molecular weights was found to scale with the temperature difference from the respective glass transition temperature. Pressure is found to slow down both modes in a quantitatively similar way, irrespective of the chain length, implying that Me is largely independent of pressure.
The diffraction signatures of individual vibrational modes in polyatomic molecules
Authors: Ryu Seol , Weber Peter M. , Stratt Richard M.
Published: July 2002

Though one normally thinks of single-molecule diffraction studies as tools for eliciting molecular geometry, molecular diffraction patterns are really the Fourier transforms of complete molecular wave functions. There is thus at least the possibility of imaging the vibrational wave functions of polyatomic molecules by means of a pump–probe diffraction experiment: the pump laser could prepare a specific vibrational state and an electron or x-ray could then be diffracted off the molecule some short time later. The present paper develops the general theory of diffraction signatures for individual vibrational wave functions in polyatomic molecules and investigates the feasibility of seeing such signatures experimentally using the example of a linear triatomic molecule modeled after CS2. Although aligned molecules in specific vibrational quantum states turn out to exhibit very characteristic diffraction signatures, the signatures of the vibrational wave functions are partially washed out for the complete isotropy expected from gas phase molecules. Nonetheless, it is possible to design a diffraction experiment using a pump–dump sequence with a polarized laser beam which will select a nonisotropic sample of vibrationally excited molecules. We show that the resulting level of anisotropy should enhance the diffraction signature, helping to distinguish different vibrational components. These model calculations therefore suggest the possibility of observing the dynamics of vibrational wave packets using experimentally realizable diffraction techniques.
MCSCF calculations of NMR spin–spin coupling constant of the HF molecule
Authors: San Fabián J. , Casanueva J. , San Fabián E. , Guilleme J.
Published: July 2002

The dependence of spin–spin NMR coupling constants on the basis set and electron correlation has been investigated for the hydrogen fluoride using Hartree–Fock (HF-SCF) and multiconfigurational self-consistent field (MCSCF) wave functions. The effect of the size, contraction, and tight s-type, augmented and polarization functions in the basis sets is analyzed. MCSCF wave functions with different number of active orbitals and excited electrons were used within the frozen-core approximation and with all-electron calculations. The correlation effect associated with the core electrons is not negligible. An approximation to determine spin–spin coupling constants at high level of electron correlation and reduced computational cost is applied satisfactorily. The best calculated and estimated 1JFH couplings are 544.20 and 536.63 Hz, respectively, with all electron correlation. Both values agree with the experimental one within the error bars (525±20 Hz).
A combined discrete/continuum solvation model: Application to glycine
Authors: Bandyopadhyay Pradipta , Gordon Mark S.
Published: July 2002

A new solvation model that combines discrete and continuum descriptions of the solvent has been developed. The discrete solvent molecules are represented by effective fragment potentials (EFP), while the continuum is represented by the Onsager model. This (EFP+Onsager) model has been applied to the relative stabilities of the neutral and zwitterionic forms of glycine. Other supermolecule-continuum calculations were also performed, using quantum mechanical discrete waters and the isodensity polarizable continuum model (IPCM) or solvation model 5.42R (SM5.42R) for the continuum. It is shown that the Onsager model provides a poor description of the solvent in the supermolecule-continuum calculations. On the other hand, more sophisticated models can predict the correct energy order of the glycine isomers. Thus, the development of mixed methods that combine sophisticated continuum models with the discrete EFP model appear to be promising.
Bessel function expansions of Coulomb wave functions
Authors: Humblet J.
Published: July 2002

From the convergence properties of the expansion of the function Φl∝r−l−1Fl in powers of the energy, we successively obtain the expansions of Fl and Gl as single series of modified Bessel functions I2l+1+n and K2l+1+n, respectively, as well as corresponding asymptotic approximations of Gl for ‖η‖→∞. Both repulsive and attractive fields are considered for real and complex energies as well. The expansion of Fl is not new, but its convergence is given a simpler and corrected proof. The simplest form of the asymptotic approximations obtained for Gl, in the case of a repulsive field and for low positive energies, is compared to an expansion obtained by Abramowitz.
Unitary highest weight representations of noncompact supergroups
Authors: Günaydin Murat
Published: July 2002

The oscillator method for the construction of unitary highest (or lowest) weight representations of noncompact groups and supergroups is generalized. Within this generalization, the method yields unitary highest weight representations of all simple supergroups whose even subgroups are in the form of a direct product of a compact group with a simple noncompact group. The method is illustrated by studying in detail the unitary highest weight representations of the supergroup OSp (2n+1/2m,R). The generalized supercoherent states associated with these unitary representations are also defined.
Normal forms of an abstract Dirac operator and applications to scattering theory
Authors: Thaller Bernd
Published: July 2002

The unitary transformations which convert an abstract Dirac operator into an ‘‘even’’ (resp. ‘‘odd’’) operator are determined. The problem is formulated and solved completely within the general setup of supersymmetric quantum mechanics. This leads to some apparently new applications in relativistic quantum mechanics, where the transformations are known as the Foldy–Wouthuysen (resp. Cini–Touschek) transformations. The scattering theory for abstract Dirac operators is discussed and the utility of the general theory is illustrated by proving existence of relativistic Mo/ller operators for scattering from long-range magnetic fields.
Reflection principles for biased correlated walks. Simple applications
Authors: Godoy S. , Fujita S.
Published: July 2002

The new type of reflection principle applicable to the Laplace transform of the arrival probability for biased walks developed by Khantha and Balakrishnan is extended to the cases of biased correlated walks with absorbing and/or reflecting walls. An exact expression for the mean escape time <t≳ for the linear discrete walk bounded by absorbing walls at 0 and N is obtained as follows:   <t≳=N(1−δ)[2(N−1)(a−d)]−1[(aN+dN)/ (aN−dN)−(1+δ)/(a−d)]     +δ[(N−1)(a−d)]−1{1−[N(a−d)dN−1]/ (aN−dN)},   where a=1/2(a+δ+ε) and d=1/2(1+δ−ε), and δ and ε are the degrees of correlation and anisotropy, respectively.
New relations for two-dimensional Hermite polynomials
Authors: Dodonov V. V. , Man’ko V. I.
Published: July 2002

The effective formulas reducing the two-dimensional Hermite polynomials to the classical one-dimensional orthogonal polynomials by Jacobi, Gegenbauer, Legendre, Laguerre, and Hermite are given. New one-parameter generating functions for the ‘‘diagonal’’ multidimensional Hermite polynomials are derived. The factorial moments and cumulants of the distribution functions related to the Hermite polynomials of two variables with equal indices are expressed in terms of the Legendre and Chebyshev polynomials. Asymptotical formulas for the two-dimensional polynomials with large values of indices and zero arguments are found. The applications to the squeezed one-mode states and to the time-dependent quantum harmonic oscillator are considered.
A new perspective on functional integration
Authors: Cartier Pierre , DeWitt-Morette Cécile
Published: July 2002

The core of this article is a general theorem with a large number of specializations. Given a manifold N and a finite number of one-parameter groups of point transformations on N with generators Y,X(1),...,X(d), we obtain, via functional integration over spaces of pointed paths on N (paths with one fixed point), a one-parameter group of functional operators acting on tensor or spinor fields on N. The generator of this group is a quadratic form in the Lie derivatives ℒX(α) in the X(α)-direction plus a term linear in ℒY. The basic functional integral is over L2,1 paths x:T→N (continuous paths with square integrable first derivative). Although the integrator is invariant under time translation, the integral is powerful enough to be used for systems which are not time translation invariant. Seven nontrivial applications of the basic formula are given, and the semiclassical expansion is computed. The methods of proof are rigorous and combine Albeverio–Ho/egh-Krohn oscillatory integrals with Elworthy’s parametrization of paths in a curved space. Unlike other approaches, Schrödinger type equations are solved directly, rather than solving first diffusion equations and then using analytic continuation.
Cohomology of Lie superalgebras and their generalizations
Authors: Scheunert M. , Zhang R. B.
Published: July 2002

The cohomology groups of Lie superalgebras and, more generally, of ε Lie algebras, are introduced and investigated. The main emphasis is on the case where the module of coefficients is nontrivial. Two general propositions are proved, which help to calculate the cohomology groups. Several examples are included to show the peculiarities of the super case. For L=sl(1|2), the cohomology groups H1(L,V) and H2(L,V), with V a finite-dimensional simple graded L-module, are determined, and the result is used to show that H2(L,U(L)) [with U(L) the enveloping algebra of L] is trivial. This implies that the superalgebra U(L) does not admit any nontrivial formal deformations (in the sense of Gerstenhaber). Garland’s theory of universal central extensions of Lie algebras is generalized to the case of ε Lie algebras.
Variational and conformal structure of nonlinear metric-connection gravitational Lagrangians
Authors: Cotsakis Spiros , Miritzis John , Querella Laurent
Published: July 2002

We examine the variational and conformal structures of higher-order theories of gravity that are derived from a metric-connection Lagrangian that is an arbitrary function of the curvature invariants. We show that the constrained first-order formalism when applied to these theories may lead consistently to a new method of reduction of order of the associated field equations. We show that the similarity of the field equations that are derived from appropriate actions via this formalism to those produced by Hilbert varying purely metric Lagrangians is not merely formal but is implied by the diffeomorphism covariant property of the associated Lagrangians. We prove that the conformal equivalence theorem of these theories with general relativity plus a scalar field, holds in the extended framework of Weyl geometry with the same forms of field and self-interacting potential but, in addition, there is a new “source term” that plays the role of a stress. We point out how these results may be further exploited and address a number of new issues that arise from this analysis.
A unified treatment of cubic invariants at fixed and arbitrary energy
Authors: Karlovini Max , Rosquist Kjell
Published: July 2002

Cubic invariants for two-dimensional Hamiltonian systems are investigated using the Jacobi geometrization procedure. This approach allows for a unified treatment of invariants at both fixed and arbitrary energy. In the geometric picture the invariant generally corresponds to a third rank Killing tensor, whose existence at a fixed energy value forces the metric to satisfy a nonlinear integrability condition expressed in terms of a Kähler potential. Further conditions, leading to a system of equations which is overdetermined except for singular cases, are added when the energy is arbitrary. As solutions to these equations we obtain several new superintegrable cases in addition to the previously known cases. We also discover a superintegrable case where the cubic invariant is of a new type which can be represented by an energy-dependent linear invariant. A complete list of all known systems which admit a cubic invariant at arbitrary energy is given.
Axisymmetric shapes and stability of charged drops in an external electric field
Authors: Basaran O. A. , Scriven L. E.
Published: July 2002

A highly conducting charged drop that is surrounded by a fluid insulator of another density can be levitated by suitably applying a uniform electric field. Axisymmetric equilibrium shapes and stability of the levitated drop are found by solving simultaneously the augmented Young–Laplace equation for surface shape and the Laplace equation for the electric field, together with constraints of fixed drop volume, charge, and center of mass. The means are a method of subdomains, finite element basis functions, and Galerkin’s method of weighted residuals, all facilitated by a large-scale computer. Shape families of fixed charge are treated systematically by first-order continuation. Previous analyses by Abbas et al. in 1967 and Abbas and Latham in 1969, in which the shapes of levitated drops are approximated as spheroids, are corrected. The new analysis shows that drops charged to less than the Rayleigh limit lose shape stability at turning points, with respect to external field strength, and that the instability seen in experiments of Doyle et al. in 1964 and others is not a bifurcation to a family of two-lobed shapes, but rather is a related imperfect bifurcation.
Transport by capillary waves. Part I. Particle trajectories
Authors: Ramshankar R. , Berlin D. , Gollub J. P.
Published: July 2002

The transport of particles on capillary waves generated by the Faraday instability is studied experimentally. The motion of a single particle on the fluid surface is determined over 4×104 wave periods by particle tracking. Erratic particle motion over large distances occurs even for small wave amplitudes, where the pattern is ordered, though spatially modulated. This implies that the wave field is accompanied by significant flows with velocities approximately 30 times smaller than the instantaneous local fluid velocities. The particle motion is characterized statistically. The displacements of the particle in the two orthogonal directions follow independent Gaussian distributions. It is found that the variance of the particle’s displacement grows with the elapsed time interval as V(τ)∼τ2H over times long compared with the correlation time of the motion. The exponent H depends on the wave amplitude, decreasing gradually from about 0.7 to an asymptotic value of 0.5 (corresponding to classical Brownian motion) at large wave amplitudes.
Monte Carlo implementation and analytic solution of an inert-scalar turbulent-mixing test problem using a mapping closure
Authors: Valiño Luis , Ros Javier , Dopazo César
Published: July 2002

A simple generalization, using a time-dependent Gaussian scalar field, of a mapping closure recently proposed by Chen et al. [Phys. Rev. Lett. 63, 2657 (1989)] to model molecular mixing, enhanced by turbulent straining, for a fluctuating scalar field is presented and Monte Carlo implemented. An arbitrary parameter β allows the adjustment of the Gaussian-scalar dissipation rate; the actual scalar field evolution is, however, independent of β. An analytic solution is obtained for the turbulent mixing of cells with two different values of the scalar content and simple symmetric initial conditions. Numerical results for this problem are produced, combining the Monte Carlo simulation and the analytic solution, in order to show the feasibility of this technique and its accuracy. Future extensions are outlined.
Statistics of an advected passive scalar
Authors: Kimura Yoshifumi , Kraichnan Robert H.
Published: July 2002

An elementary argument shows that non-Gaussian fluctuations in the temperature at a point in space are induced by random advection of a passive temperature field that has a nonlinear mean gradient, whether or not there is molecular diffusion. This is corroborated by exact analysis for the nondiffusive case and by direct numerical simulation for diffusive cases. Eulerian mapping closure gives results close to the simulation data. Non-Gaussian fluctuations of temperature at a point also are induced by a more subtle mechanism that requires both advection and molecular diffusion and is effective even when the statistics are strictly homogeneous. It operates through selectively strong dissipation of regions where intense temperature gradients have been induced by advective straining. This phenomenon is demonstrated by simulations and explored by means of an idealized analytical model.
Framework of the parametric instabilities in the presence of space-time fluctuations in homogeneous and inhomogeneous plasma. II. Applications
Authors: Lu Liang
Published: July 2002

Based on the formalism developed in another paper [Phys. Fluids 31, 3362 (1988)], three analytical results are obtained relating to (1) a WKB type of equation in inhomogeneous plasma, (2) the Raman process at (1/4) nc, and (3) two plasma decay processes at (1/4) nc. Interesting scaling results are found.
Determination of diffusive and nondiffusive transport in modulation experiments in plasmas
Authors: Jacchia A. , Mantica P. , De Luca F. , Gorini G.
Published: July 2002

The problem of the propagation of a harmonic temperature perturbation in a plasma with both diffusive and nondiffusive energy transport is addressed. The energy flux is modeled by two (radially varying) effective coefficients for the diffusive and nondiffusive transport, and the effects of perturbed energy sinks and of cylindrical geometry are taken into account. A simple, local relationship is found between the two transport coefficients and the gradients of the phase and amplitude of the temperature perturbation. This relationship can be used for the interpretation of heating modulation experiments, provided data at different modulation frequencies are available. Since a harmonic density perturbation in a plasma follows a similar linearized transport equation, a similar model can be applied also to density modulation experiments.
A possible excitation mechanism for observed superthermal ion cyclotron emission from tokamak plasmas
Authors: Dendy R. O. , Lashmore-Davies C. N. , Kam K. F.
Published: July 2002

Intense superthermal ion cyclotron emission (ICE) has been observed from tokamak plasmas. The power spectrum displays narrow peaks at multiple harmonics of the background ion cyclotron frequency [Cottrell and Dendy, Phys. Rev. Lett. 60, 33 (1988)] in Ohmic deuterium plasmas, and the radiation appears to be driven by the fusion ion population in the edge plasma. Theoretical investigation of this phenomenon may be rewarding, in terms of the information about the behavior of energetic ions in tokamaks that can be extracted from ICE observations. The interpretation presented here is based on the resonant excitation of fast Alfvén waves with ion Bernstein waves supported by an energetic ion species (α), in the presence of a more numerous thermal ion species (i). Because the ion cyclotron frequencies may be commensurate (lΩα=sΩi for some low integers l,s), and observations indicate that ω is comparable in magnitude to Ωi, the standard theory [Belikov and Kolesnichenko, Sov. Phys. Tech. Phys. 20, 1146 (1976)] which assumes ω≫Ωi is not immediately applicable, and is accordingly extended here to the low-frequency regime. The results show that excitation of the fast Alfvén wave at proton cyclotron harmonics can occur for fusion proton concentrations nα/ni as low as 10−7, and that multiple cyclotron harmonics can be simultaneously unstable. Furthermore, while fusion protons born at 3.0 MeV are above the energy threshold required to drive the instability, the other primary fusion products in deuterium—1.0 MeV tritons and 0.82 MeV helium-3 nuclei—fall below it, consistent with the observation that radiation at cyclotron harmonics of the latter is not detected. However, the mechanism considered here cannot easily explain the generation of radiation which is observed at deuterium cyclotron harmonics that are degenerate with proton half-harmonics.
Internal kink stabilization by high-energy ions with nonstandard orbits
Authors: Porcelli F. , Stankiewicz R. , Berk H. L. , Zhang Y. Z.
Published: July 2002

A generalized energy principle that takes into account the nonstandard, potato-shaped particle orbits of high-energy ions in the central region of a tokamak is derived. It is shown that, in the limit of zero orbit width, this energy principle reduces to the one formulated by Van Dam et al. [Phys. Fluids 25, 1349 (1982)]. The modification of hot particle stabilization theory when such orbit effects are important is investigated. In particular, a model distribution function is chosen to describe high-energy trapped ions produced by ion cyclotron resonant frequency (ICRF) heating applied near the axis of a tokamak. Standard banana orbit theory predicts that, for fixed total stored energy of energetic particles peaked about the magnetic axis, the stabilizing influence on internal kink modes is inversely proportional to the spatial spread of the hot particles. However, this scaling saturates when the spatial spread of the distribution function approaches the width of a typical nonstandard orbit. Hence, ICRF heating is most efficient in producing stabilization when the heating zone is comparable to the orbit width, while the tendency to stabilize does not improve if the heating zone is narrower than the orbit width. Further, it is shown that, if particle orbits can extend close to the q=1 surface, the tendency for stabilization is inhibited.
Spontaneous poloidal spin-up of tokamak plasmas: Reduced equations, physical mechanism, and sonic regimes
Authors: Hassam A. B. , Drake J. F.
Published: July 2002

A theoretical description of the spontaneous (Stringer) poloidal spin-up of tokamak plasmas is given. A set of reduced equations is derived to investigate various aspects of the phenomenon. A simple physical description of the instability mechanism is given. The theory of the instability is extended to show that the spin-up persists even under conditions where the growth rate is sonic. A nonlinear evolution equation for unstable poloidal rotation is obtained which shows that the instability mechanism loses its efficacy when the poloidal speed exceeds the poloidal sound speed, thus resulting in nonlinear saturation of the rotation at this level.
Analysis of the current-diffusive ballooning mode
Authors: Yagi M. , Itoh K. , Itoh S.-I. , Fukuyama A. , Azumi M.
Published: July 2002

The current-diffusive ballooning mode is analyzed in the tokamak plasma. This mode is destabilized by the current diffusivity (i.e., the electron viscosity) and stabilized by the thermal conductivity and ion viscosity. By use of the ballooning transformation, the eigenmode equation is solved. An analytic solution is obtained by the strong ballooning limit. Numerical calculation is also performed to confirm the analytic theory. The growth rate of the mode and the mode structure are analyzed. The stability boundary is derived in terms of the current diffusivity, thermal conductivity, ion viscosity, and the pressure gradient for the given shear parameter. This result is applied to express the thermal conductivity in terms of the pressure gradient, magnetic configurational parameters (such as the safety factor, shear, and aspect ratio), and the Prandtl numbers.
Design and testing of axisymmetric nozzles for ion-molecule reaction studies between 20 °K and 160 °K
Authors: Dupeyrat G. , Marquette J. B. , Rowe B. R.
Published: July 2002

Axisymmetric contoured Laval nozzles have been calculated in order to obtain uniform supersonic flow. For nitrogen and oxygen gases the calculation uses only similarity techniques, but for helium, complete calculations of the isentropic core and the boundary layer were performed for two sets of boundary conditions. Experimental testing has shown that the nitrogen or oxygen flows exhibit a good uniformity but that the helium flow was uniform only for one set of boundary conditions. The purpose of this work is to provide flow reactors for studies of ion-molecule reactions in the real temperature range of interstellar clouds (T<80 °K).
The interaction of quasiperpendicular shock waves in a collisionless plasma
Authors: Cargill P. J. , Goodrich C. C.
Published: July 2002

Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the interaction of collisionless shocks. The interaction between pairs of sub- and supercritical shocks are studied for a wide range of parameters relevant to both the solar and interplanetary plasma. These parameters include the shock Mach number, the electron and ion plasma betas, and the angle between the shock normal and the magnetic field. In the collision of two subcritical quasiperpendicular shocks, the shock electromagnetic field structures pass through each other, but do not carry the plasma associated with them. When unequal shocks collide, there is no evidence of the contact discontinuity predicted by fluid theory. In the collision of supercritical quasiperpendicular shocks, significant acceleration of ions occurs. This acceleration arises when ions reflected at one shock encounter the electric fields of the other shock and takes place in a number of distinct phases. For a wide range of parameters, the maximum ion energy is roughly an order of magnitude larger than the ram kinetic energy of the colliding shocks. This ion energization will subsequently manifest itself as a high ion to electron temperature ratio in the doubly shocked plasma. Finally, it was found that the values of the Mach number and angle between the shock normal and magnetic field play a significant role in the collisions. For shock geometries significantly different from perpendicular, ions from each oncoming shock can escape upstream and undergo considerable acceleration.
The sedimentation rate of disordered suspensions
Authors: Brady John F. , Durlofsky Louis J.
Published: July 2002

An explicit expression for the sedimentation velocity at low particle Reynolds number in a concentrated suspension is derived and evaluated for two different approximations to the hydrodynamic interactions: a strict pairwise additive approximation and a far-field, or Rotne–Prager, approximation. It is shown that the simple Rotne–Prager approximation gives a very accurate prediction for the sedimentation velocity of random suspensions from the dilute limit all the way up to close packing. The pairwise additive approximation, however, fails completely, predicting an aphysical negative sedimentation velocity above a volume fraction φ≊0.23. The explanation for these different behaviors is shown to be linked to the ‘‘effective medium’’ behavior of the suspensions. It is shown analytically and by Stokesian dynamics simulation that a suspension of neutrally buoyant particles may be modeled as a homogeneous fluid with an effective viscosity, but a sedimenting suspension cannot. As a result, the Rotne–Prager approximation actually captures the correct features of the many-body interactions in sedimentation. An analytical expression for the sedimentation rate, which is in good agreement with experiment, is obtained using the Percus–Yevick hard-sphere distribution function.
Secondary instability in the wake of a circular cylinder
Authors: Henderson Ronald D. , Barkley Dwight
Published: July 2002

Secondary instability of flow past a circular cylinder is examined using highly accurate numerical methods. The critical Reynolds number for this instability is found to be Rec=188.5. The secondary instability leads to three-dimensionality with a spanwise wavelength at onset of 4 cylinder diameters. Three-dimensional simulations show that this bifurcation is weakly subcritical.
Axisymmetrization of an isolated vortex region by splitting and partial merging of satellite depletion perturbations
Authors: Yao H. B. , Zabusky N. J.
Published: July 2002

In numerical studies, we observe the essentially inviscid approach to axisymmetry of an isolated-and-perturbed monopolar and uniformly decreasing distributed vortex region in a two-dimensional incompressible fluid. In particular, an initial small-but-finite amplitude, 3-fold, ‘‘edge’’-located hole or depletion perturbation of a monopole evolves into a 2-fold state. This occurs through the nonlinear processes of hole stretching, splitting and partial merger. The initial growth rate for this downward cascade is proportional to the initial perturbation magnitude. Near-inviscid simulations are made with surgery-regularized contour dynamics codes (CDS), using from 5 to 11 contours. Pseudospectral (PS) simulations with varying Newtonian and hyperviscosities, that is considering continuum and dissipation effects, yield consistent results. Our numerical results are in agreement with small-but-finite amplitude perturbations that were used in recent laboratory experiments on magnetized electron columns. This process may also occur in late time evolutions associated with the ‘‘bump-on-tail’’ initial condition in Vlasov plasmas.
Sawteeth and energy confinement in the Madison Symmetric Torus reversed-field pinch
Authors: Chapman B. E. , Almagri A. F. , Cekic M. , Den Hartog D. J. , Prager S. C. , Sarff J. S.
Published: July 2002

Most Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] reversed-field pinch discharges exhibit sawtooth oscillations with a period of 2–5 ms, corresponding to magnetohydrodynamic (MHD) instability and increased transport. However, in discharges where the plasma-facing wall has been boronized, the plasma resistivity is reduced, and sawteeth are often suppressed for periods up to 20 ms. The energy confinement time during these sawtooth-free periods is triple the normal value, corresponding to a higher plasma temperature and lower Ohmic input power. In addition, the steady growth of the dominant magnetic fluctuations normally observed between sawtooth crashes is absent.
Interpretation of ion cyclotron emission from sub-Alfvénic fusion products in the Tokamak Fusion Test Reactor
Authors: McClements K. G. , Dendy R. O. , Lashmore-Davies C. N. , Cottrell G. A. , Cauffman S. , Majeski R.
Published: July 2002

Ion cyclotron emission (ICE) has been observed during neutral beam-heated supershots in the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 72, 3526 (1994)] deuterium–tritium campaign at fusion product cyclotron harmonics. The emission originates from the outer midplane edge plasma, where fusion products initially have an anisotropic velocity distribution, sharply peaked at a sub-Alfvénic speed. It is shown that the magnetoacoustic cyclotron instability, resulting in the generation of obliquely propagating fast Alfvén waves at fusion product cyclotron harmonics, can occur under such conditions. The time evolution of the growth rate closely follows that of the observed ICE amplitude. Instability is suppressed if the fusion products undergo a moderate degree of thermalization, or are isotropic. In contrast, the super-Alfvénic fusion products present in the outer midplane of the Joint European Torus (JET) [Nucl. Fusion 33, 1365 (1993)] can drive the instability if they are isotropic or have a broad speed distribution. This may help to account for the observation that fusion product-driven ICE in JET persists for longer than fusion product-driven ICE in TFTR supershots.
Laser-plasma filamentation and the spatially periodic nonlinear Schrödinger equation approximation
Authors: Johnston T. W. , Vidal F. , Fréchette D.
Published: July 2002

For understanding self-focusing and filamentation of electromagnetic beams in plasmas (and other media) when the beam power is well over critical, considerable success has recently been achieved using the well-known nonlinear Schrödinger equation with saturating nonlinearities. Sufficiently large isolated high-power beams with noticeable structure can break up into numerous filaments, which emerge from the phase of filament creation rather close to the known filament equilibria having lost excess power while forming and pulsating. However, the periodic boundary cases more characteristic of laser beam coverage of inertial confinement fusion targets show asymptotic states more complicated than a noninteracting ensemble of equilibrium filaments. While the filament density can be estimated in terms of the average intensity, considerable filament interaction and activity is the usual result. At extremely high intensities very complicated self-focusing structures are formed.
Asymptotical and mapping methods in study of ergodic divertor magnetic field in a toroidal system
Authors: Abdullaev S. S. , Finken K. H. , Spatschek K. H.
Published: July 2002

Asymptotical and mapping methods to study the structure of magnetic field perturbations and magnetic field line dynamics in a tokamak ergodic divertor in toroidal geometry are developed. The investigation is applied to the Dynamic Ergodic Divertor under construction for the Torus Experiment for the Technology Oriented Research (TEXTOR-94) Tokamak at Jülich [Fusion Eng. Design 37, 337 (1997)]. An ideal coil configuration designed to create resonant magnetic perturbations at the plasma edge is considered. In cylindrical geometry, the analytical expressions for the vacuum magnetic field perturbations of such a coil system are derived, and its properties are studied. Corrections to the magnetic field due to the toroidicity are presented. The asymptotical analysis of transformation of magnetic perturbation into the Hamiltonian perturbation in toroidal geometry is carried out, and the asymptotic formulas for the spectrum of the Hamiltonian perturbations are found. A new method of integration of Hamiltonian equations is developed. It is based on a canonical transformation of variables that replaces the dynamics of a continuous Hamiltonian system by a symplectic mapping. The form of the mapping is established in the first order of perturbation theory. It is shown that the mapping well reproduces Poincaré sections of field lines, as well as their statistical properties in an ergodic zone obtained by the numerical integration of field line equations. The mapping is applied to study, in particular, the formation of a stochastic layer and the statistical properties of field lines at the plasma edge.
The plasma condensation: Liquid and crystalline plasmas
Authors: Morfill G. E. , Thomas H. M. , Konopka U. , Zuzic M.
Published: July 2002

Colloidal plasmas may “condense” under certain conditions into liquid and crystalline states, while retaining their essential plasma properties. This “plasma condensation” therefore leads to new states of matter: “liquid plasmas” and “plasma crystals.” The experimental discovery was first reported in 1994, and since then many researchers have begun to investigate the properties of condensed plasma states. In this paper we describe some of the basic physics required to understand colloidal plasmas and discuss experiments conducted to investigate the details of the interaction between the plasma particles (in particular, the interaction potential), the melting phase transition, and the thermodynamics of this new state of matter.
Nonlinear modeling of kinetic plasma instabilities
Authors: Candy J. , Berk H. L. , Breizman B. N. , Porcelli F.
Published: July 2002

Many kinetic plasma instabilities, in quite different physical systems, share a genuinely similar mathematical structure near isolated phase-space islands. For this reason, dynamical features such as faster-than-exponential growth of the instability, as well as nonlinear frequency sweeping, are found to be universal. Numerical δf methods, which follow the evolution of the (nonlinear) perturbed distribution function along single-particle orbits, have been applied to analytic models, which include a continuous particle source, resonant particle collisions, and wave damping. The result is a series of codes that can reliably model the nonlinear evolution of kinetic instabilities, including some specific to tokamak plasmas, over experimentally relevant time scales. New results include (i) nonlinear simulations of two-species, one-degree-of-freedom plasmas; (ii) simulations of fishbone bursts in tokamak plasmas; (iii) nonlinear modeling of beam-driven toroidal Alfvén eigenmode activity in tokamaks.
Influence of light-induced defects in hydrogenated amorphous silicon on charge carrier dynamics
Authors: Werner A. , Kunst M.
Published: July 2002

Charge carrier decay processes in hydrogenated amorphous silicon after extended illumination are studied by the time-resolved microwave conductivity method. It is shown that creation of metastable defects depends on illumination time. Two different defects can be distinguished: one which decreases the charge carrier lifetime in the bulk, whereas the other one increases the subband gap absorption at the surface.
Reflection high-energy electron diffraction oscillations from vicinal surfaces—a new approach to surface diffusion measurements
Authors: Neave J. H. , Dobson P. J. , Joyce B. A. , Zhang Jing
Published: July 2002

A simple extension of the reflection high-energy electron diffraction oscillation technique to vicinal surfaces provides a means of studying surface diffusion during molecular beam epitaxial growth. The basis of the method is described and some preliminary results for Ga diffusion during the growth of GaAs films with (001) 2×4 and 3×1 reconstructed surfaces are presented.
High quality Si-on-SiO2 films by large dose oxygen implantation and lamp annealing
Authors: Celler G. K. , Hemment P. L. F. , West K. W. , Gibson J. M.
Published: July 2002

Ion beam synthesis of a buried SiO2 layer is an attractive silicon-on-insulator technology for high-speed complementary metal-oxide-semiconductor circuits and radiation hardened devices. We demonstrate here a new annealing procedure at 1405 °C that produces silicon films of excellent quality, essentially free of oxygen precipitates and with sharp interfaces between the Si and the SiO2. Buried oxide layers have been formed in Si (100) wafers by implanting 400 keV molecular oxygen at 500 °C to a dose of 1.8×1018 cm−2. Annealing was performed by radiative heating of the back side of each sample to the melt temperature of silicon, TM=1412 °C, so that the buried oxide structure was at 1405 °C. The temperature control relies entirely on the change in optical properties of silicon upon melting. This ensures, without any external feedback, that the surface exposed to the photon flux will remain at TM.
Phonon shifts and strains in strain-layered (Ga1−xInx)As
Authors: Burns Gerald , Wie C. R. , Dacol F. H. , Pettit G. D. , Woodall J. M.
Published: July 2002

We have measured the phonon frequencies (Raman technique) and the strains (x-ray rocking curve technique) of (Ga1−xInx)As films on GaAs (100) substrates. Films with various x values and various thicknesses were studied. The films range from perfect epitaxial to those that have relaxed by different amounts. The strain-induced frequency shift was calculated for each sample using the phonon deformation constants and measured strains. From the measurements and calculation, it is shown that the frequency shifts due to strain and the equivalent bulk frequencies for the phonon give internal agreement. This indicates that the Raman technique can be used for in situ monitoring of the growth process.
Preparation of superconducting thin films of Ba2YCu3O7 by a novel spin-on pyrolysis technique
Authors: Rice C. E. , van Dover R. B. , Fisanick G. J.
Published: July 2002

A technique is described for preparing superconducting cuprate thin films using wet chemical precursors. This new method is simple and inexpensive, and allows facile manipulation of film stoichiometry. A solution of metal acetates in aqueous acetic acid is spun on a substrate, pyrolyzed, and heat treated. A 1600-Å film of Ba2YCu3O7 prepared by this method had a room-temperature resistivity of 500 μΩ cm, Tc(onset) of 90 K, and R=0 at 58 K. It showed preferential orientation with the orthorhombic c axis perpendicular to the film, and random azimuthal orientation. The preparation and electrical and microstructural characterization of these films are discussed.
Identification of an interstitial carbon-interstitial oxygen complex in silicon
Authors: Trombetta J. M. , Watkins G. D.
Published: July 2002

An electron paramagnetic resonance spectrum, observed in electron irradiated silicon and labeled Si-G15, is shown to originate from the same carbon-oxygen complex as does the well studied C-line photoluminescence spectrum with zero-phonon line at 0.79 eV. Both the g tensor and the 13C hyperfine tensor for this center are remarkably similar to those for the isolated interstitial carbon atom. Stress-induced alignment experiments reveal the role of oxygen and indicate a unique structure for an interstitial carbon-interstitial oxygen pair.
DNA sequence recognition in the minor groove by crosslinked polyamides: The effect of N-terminal head group and linker length on binding affinity and specificity
Authors: O'Hare C. Caroline , Mack Darcy , Tandon Manju , Sharma Sanjay K. , Lown J. William , Kopka Mary L. , Dickerson Richard E. , Hartley John A.
Published: July 2002

Development of sequence-reading polyamides or “lexitropsins”
with comparable DNA-binding affinities to cellular proteins raises the
possibility of artificially regulated gene expression. Covalent
linkage of polyamide ligands, with either a hairpin motif or
crosslinking methylene bridge, has greatly improved binding affinity by
ensuring their side-by-side register. Whereas hairpin polyamides have
been investigated extensively, the optimized structure of crosslinked
polyamides remains to be determined. This study examines a series of
thiazole-imidazole-pyrrole (TIP) monomers and crosslinked dimers to
evaluate the effects on selectivity and binding affinity of different
N-terminal head groups attached to the leading thiazole ring and
differing methylene linker lengths. Quantitative footprinting of a DNA
sequence, containing potential match and mismatch sites for both
maximum overlap and one-residue stagger binding modes, allowed
measurement of binding constants at each putative site. Within an
N-terminal amino TIP series, C7 and C8-linked compounds bound most
strongly to these sites, whereas maximum binding affinity was observed
for a C6 linker with a formyl head group. A C5 linker gave weak binding
with either head group. A hydrogen or acetyl head group abrogated
binding. Binding was confirmed by gel shift analyses. The highest
specificity for the maximum overlap site over the one-residue stagger
was observed with TIP-C7-amino. Selectivity of the leading thiazole was
modulated by the head group, with N-terminal formyl TIP exhibiting up
to 3-fold specificity for AGT over TGT, suggesting that
N
-formyl-thiazole may provide sequence discrimination of
adenine over thymine. Moreover, the leading head group and methylene
linker length significantly influences the binding characteristics of
crosslinked polyamides.
Dominant-interfering forms of MEF2 generated by caspase cleavage contribute to NMDA-induced neuronal apoptosis
Authors: Okamoto Shu-ichi , Li Zhen , Ju Chung , Schölzke Marion N. , Mathews Emily , Cui Jiankun , Salvesen Guy S. , Bossy-Wetzel Ella , Lipton Stuart A.
Published: July 2002

Myocyte enhancer factor-2 (MEF2) transcription factors are activated by p38 mitogen-activated protein kinase during neuronal and myogenic differentiation. Recent work has shown that stimulation of this pathway is antiapoptotic during development but proapoptotic in mature neurons exposed to excitotoxic or other stress. We now report that excitotoxic (
N
-methyl-D-aspartate) insults to mature cerebrocortical neurons activate caspase-3, -7, in turn cleaving MEF2A, C, and D isoforms. MEF2 cleavage fragments containing a truncated transactivation domain but preserved DNA-binding domain block MEF2 transcriptional activity via dominant interference. Transfection of constitutively active MEF2 (MEF2C-CA) rescues MEF2 transcriptional activity after
N
-methyl-D-aspartate insult and prevents neuronal apoptosis. Conversely, dominant-interfering MEF2 abrogates neuroprotection by MEF2C-CA. These results define a pathway to excitotoxic neuronal stress/apoptosis via caspase-catalyzed cleavage of MEF2. Additionally, we show that similar MEF2 cleavage fragments are generated
in vivo
during focal stroke damage. Hence, this pathway appears to have pathophysiological relevance
in vivo
.