bluuebunny/crossref_metadata_2025 huggingface dataset:
The frustrated translational mode of CO on Cu(110): Azimuthal anisotropy studied by helium atom scattering—A comparison with time-of-flight electron stimulated desorption of ion angular distribution measurements
Authors: Braun J. , Weckesser J. , Ahner J. , Mocuta D. , Yates J. T. , Wöll Ch.
Published: July 2002

The frustrated translation of isolated CO molecules parallel to the surface, or T mode, has been studied on a Cu(110) substrate using helium atom scattering. A small but significant anisotropy in T-mode vibrational frequencies was found for the on-top site species with oscillator frequencies of 3.6 and 3.2 meV along the [11̄0] and the [001] directions, respectively. These results are compared with recent results from time-of-flight electron stimulated desorption of ion angular distribution measurements [Ahner et al., Phys. Rev. Lett. 79, 1889 (1997)], where also a significant anisotropy has been observed.
Liquid-crystalline behavior of rod-coil diblock copolymers
Authors: Matsen M. W. , Barrett C.
Published: July 2002

The exact mean-field phase behavior of the Semenov–Vasilenko model for rod-coil diblock copolymers is studied by applying self-consistent field techniques. The behavior depends on three quantities: the rod/coil immiscibility χN, the coil volume fraction f, and the ratio ν of the characteristic coil to rod dimensions. When χN≲5, the rods and coils mix producing a nematic phase, and at larger χN, they microphase separate forming a lamellar phase. The nonlamellar phases expected at f≳0.7 are not treated here. The lamellar phase is typically a smectic-C structure with monolayers of tilted rods. A thorough understanding of the model is achieved by closely examining segment distributions and various contributions to the free energy. The tilt angle θ is generally controlled by a competition between rod/coil interfacial tension and stretching of the coils. Lowering f reduces the latter contribution, causing θ→0 and producing a continuous transition to a smectic-A structure. Beyond that, there is a tendency to form structures with the rods arranged in bilayers, but this is strongly suppressed by a large steric penalty. We suggest that small amounts of solvent can greatly alleviate this penalty, and therefore could significantly affect some aspects of the phase behavior.
Calculation of macroscopic first-, second-, and third-order optical susceptibilities for the urea crystal
Authors: Reis H. , Papadopoulos M. G. , Munn R. W.
Published: July 2002

Macroscopic first-, second-, and third-order susceptibilities of the urea crystal are calculated using static and frequency-dependent ab initio molecular (hyper)polarizabilities at the self-consistent field (SCF) and second-order-Mo/ller–Plesset (MP2) levels. Environmental effects are taken into account using a rigorous local-field theory and are compared with the anisotropic Lorentz field factor approximation. The electric field arising from the permanent dipoles of the surrounding molecules is explicitly taken into account using a self-consistent approach. The dipole moment and the first hyperpolarizability are particularly strongly dependent on this field, but the crystal structure leads to a fortuitous cancellation of the field effect on the second-order susceptibility. The experimental linear susceptibility is accurately reproduced, while differences remain for the quadratic susceptibility. Dispersion curves for the first-order susceptibility, and results for quadratic electrooptic effect (QEO), electric-field-induced second-harmonic generation (EFISH), and third-harmonic generation (THG) experiments are predicted. The (hyper)polarizabilities of a linear dimer of urea molecules are calculated and used to estimate the effect of hydrogen bonding on the susceptibilities, which proves to be small. Semiempirically calculated (hyper)polarizabilities methods yield unreliable results for the susceptibilities compared with those from the ab initio method. This deficiency can be overcome by recourse to additional experimental data.
Formation of argon dimers in ternary monomer collisions—A classical trajectory study
Authors: Kalus René
Published: July 2002

Classical trajectories are used to study formation of both stable and metastable dimers in dilute argon over a 15–500 K temperature range. The results are compared with previously published trajectory data [R. E. Howard, T. L. Planck, S. R. Trussell, B. Saadevandi, Chem. Phys. Lett. 142, 33 (1987)] and with molecular dynamics data [W. C. Schieve, and H. W. Harrison, J. Chem. Phys. 61, 700 (1974)]. There are serious differences between the results reported by Howard et al. and our trajectory data, whereas good agreement is achieved with the molecular-dynamics results reported by Schieve and Harrison. The computed recombination rate coefficients for argon are used to assess atom–atom recombination rates in other noble gases.
Infrared cavity ringdown spectroscopy of water clusters: O–D stretching bands
Authors: Paul J. B. , Provencal R. A. , Chapo C. , Petterson A. , Saykally R. J.
Published: July 2002

The infrared O–D stretching spectrum of fully deuterated jet-cooled water clusters is reported. Sequential red-shifts in the single donor O–D stretches, which characterize the cooperative effects in the hydrogen bond network, were accurately measured for clusters up to (D2O)8. Detailed comparisons with corresponding data obtained for (H2O)n clusters are presented. Additionally, rotational analyses of two D2O dimer bands are presented. These measurements were made possible by the advent of infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) using Raman-shifted pulsed dye lasers, which creates many new opportunities for gas phase IR spectroscopy.
A density matrix description of 14N overtone nuclear magnetic resonance in static and spinning solids
Authors: Marinelli Laura , Wi Sungsool , Frydman Lucio
Published: July 2002

Overtone NMR is an experiment introduced by LeGros, Bloom, Tycko, and Opella, capable of providing 14N powder spectra devoid of first-order quadrupole broadenings by irradiation and observation of the nuclear spins at twice their Larmor frequency. This technique constitutes one of the most promising alternatives for the acquisition of high resolution solid 14N NMR spectra from random powders, particularly if it can be combined with strategies capable of removing the substantial second-order quadrupole broadenings remaining in the overtone line shapes. In order to facilitate the search for these averaging manipulations, we present here a theoretical description of the overtone experiment based on the time-domain propagation of density matrices. It is shown that by combining perturbation methods with appropriate rotating-frame transformations and diagonalizations, overtone spin-1 phenomena can be described using a single set of fictitious spin-1/2 operators. By contrast to conventional spin-1/2 irradiation and detection processes, however, overtone manipulations involve an unusual angular dependence on the azimuthal angle defining rotations about the main Zeeman magnetic field. This behavior introduces unexpected complications toward the narrowing of overtone resonances by conventional sample spinning techniques. Nevertheless, it can still be shown that the removal of all spin-1 anisotropies by certain forms of dynamic-angle spinning overtone NMR remains feasible.
Monotonically convergent algorithm for quantum optimal control with dissipation
Authors: Ohtsuki Yukiyoshi , Zhu Wusheng , Rabitz Herschel
Published: July 2002

This paper extends a monotonically convergent algorithm for quantum optimal control to treat systems with dissipation. The algorithm working with the density matrix is proved to exhibit quadratic and monotonic convergence. Several numerical tests are implemented in three-level model systems. The algorithm is exploited to control various targets, including the expectation value of a Hermitian operator, the modulus square of the expectation value of a non-Hermitian operator, and off-diagonal elements of the density matrix.
Flexing analysis of ethane internal rotation energetics
Authors: Goodman Lionel , Gu Hongbing , Pophristic Vojislava
Published: July 2002

A flexing analysis of the ethane barrier energy in terms of structural (ΔEstruct), steric exchange (ΔEsteric), and hyperconjugative charge-transfer (ΔEdeloc) energy contributions has been carried out using natural bond orbitals. No evidence is found for the view that the ethane staggered equilibrium geometry or the C–C bond expansion that accompanies rotation results from steric exchange repulsion interactions. The analysis shows that ΔEstruct and ΔEdeloc have very different stereoelectronic dependencies, but that the ΔEsteric and ΔEdeloc dependencies are antagonistic. All of their contributions are strongly affected by the C–C bond expansion, with the result that the barrier mechanism cannot be understood without taking into account their different relaxation dependencies. Neglect of C–C expansion leaves the charge-transfer interactions paramount by subduing the steric and structural contributions. These interactions are found to be an important determinant for the expansion. The strong expansion dependence found for ΔEstruct is largely controlled by weakening of the C–C bond, and to a lesser extent by concomitant strengthening of the C–H bonds. Most of this dependence can be mimicked by C–C expansion in the absence of methyl torsion indicating that C–C bond weakening does not arise from the symmetry change accompanying ethane torsion.
Density-functional study of hydration of ammonium in water clusters
Authors: Brugé Filippo , Bernasconi Marco , Parrinello Michele
Published: July 2002

The structure and hydration energy of small water clusters containing an ammonium ion are investigated within density functional theory by using the Becke–Lee–Yang–Parr (BLYP) generalized gradient corrections to the local density approximation to the exchange and correlation energy, norm-conserving pseudopotentials, and a plane-wave expansion of Kohn–Sham orbitals. The calculated hydration energies are in good agreement with experimental data and with available quantum chemical calculations. The result of this study furnishes us with further evidence of the ability of the BLYP functional to provide a good description of hydrated systems.
Residual dipolar couplings by 1H dipolar-encoded longitudinal magnetization, double- and triple-quantum nuclear magnetic resonance in cross-linked elastomers
Authors: Schneider M. , Gasper L. , Demco D. E. , Blümich B.
Published: July 2002

The measurements of residual dipolar couplings in elastomer system is desirable, because they reflect the hindrance to molecular motions by the cross-linking, topological constraints and the external factors like mechanical stress. Dipolar-encoded longitudinal magnetization nuclear magnetic resonance (NMR) decay curves, double-quantum and triple-quantum NMR buildup intensities for measuring the residual dipolar couplings, and the associated dynamic order parameters are introduced. It is shown that in the short excitation time regime the effective dipolar network is simplified. In the limit of this model based on localized dipolar couplings, the spin response to two-dimensional pulse sequences used to record multiple-quantum (MQ) NMR coherences was evaluated for longitudinal magnetization, double-, and triple-quantum coherences of methylene, and methyl protons in synthetic 1,4-cis-polyisoprene. The dynamic order parameters can be evaluated from this NMR response using a classical scale-invariant polymer model. These dynamic order parameters were measured for a cross-link series of synthetic polyisoprene and correlated with the cross-link density. The decay rates of the Hahn-echo amplitudes reflecting residual dipolar couplings as well as effects of molecular motion are also measured for the same cross-link series. The contribution of molecular motions to the transverse relaxation can be separated from the residual dipolar couplings using a train of magic echoes. The sensitivity of these transverse relaxation rates to the cross-link density is compared to that of residual dipolar couplings. The NMR time scale is shorter for the dipolar-encoded longitudinal magnetization and MQ experiments as compared to transverse relaxation experiments leading to an increased sensitivity to cross-link density of the former approaches.
Changes in the dynamics of supercooled systems revealed by dielectric spectroscopy
Authors: Corezzi S. , Campani E. , Rolla P. A. , Capaccioli S. , Fioretto D.
Published: July 2002

The dynamics of monoepoxy, diepoxy, and triepoxy glass-formers from below to above the glass transition temperature, Tg, has been investigated through the temperature behavior of relaxation times, strengths, and conductivity, determined in a wide frequency range (102–2×1010 Hz). In all systems the main and secondary relaxations define a splitting temperature TS∼1.3×Tg; moreover, a crossover temperature TB∼TS is recognized, marking the separation between two different Vogel–Fulcher regimes for the structural dynamics. The strengths behavior reflects the distribution of the overall energy between the relaxation processes and no peculiar behavior is revealed at TS. A strong increase characterizes the strength of the secondary relaxation on crossing the glass transition from the lower temperatures. Conductivity data have been analyzed to test the dynamics in terms of the Debye–Stokes–Einstein (DSE) diffusion law. The prediction of the DSE model is well verified for mono- and diepoxide up to the high viscosity regime, while a fractional DSE law with exponent ∼0.81, accounting for a decoupling between translational and rotational motions, replaces the DSE relation in triepoxide for temperatures below TS. The change of the structural dynamics, the splitting between main and secondary relaxation and the breakdown of the DSE behavior, all occur within a narrow temperature range around TS; this finding argues in favor of the existence of a change of the dynamics in the supercooled liquid state well above the glass transition temperature.
Designing new thermoreversible gels by molecular tailoring of hydrophilic-hydrophobic interactions
Authors: Varghese S. , Lele A. K. , Mashelkar R. A.
Published: July 2002

We have shown that the lattice fluid hydrogen bond (LFHB) model can successfully quantify the first-order volume transition in hydrogels. The model predicts that a critical balance of hydrophilic and hydrophobic interactions is required for a gel to exhibit a discontinuous volume transition. In this work we will report the swelling behavior of a new thermoreversible copolymer hydrogel, which has been synthesized from two monomers, whose homopolymers do not show any volume transition in water in the observable range of temperatures. The discontinuous volume transition phenomenon in the copolymer gel was observed only at a critical balance of hydrophilic-hydrophobic interactions. The discontinuous nature of the volume transition is lost with a subtle change in the hydrophilic-hydrophobic balance. The copolymer gel was synthesized from 2-acrylamido 2-methyl propane sulfonic acid (AMPS), which is a hydrophilic monomer, and N-tertiary butylacrylamide (N-t-BAm), which is a hydrophobic monomer. The hydrophilic-hydrophobic balance in the gel was altered by either changing the composition of the co-monomers or by substituting the N-t-BAm with another hydrophobic monomer, N-isopropylacrylamide (NIPAm).
Fock space description of simple spinors
Authors: Budinich Paolo , Trautman Andrzej
Published: July 2002

Cartan’s simple—often called pure—spinors corresponding to even-dimensional complex vector spaces are defined in terms of the associated maximal totally null planes. Their geometrical properties are derived and described using notions familiar to physicists: Dirac and Weyl spinors, gamma matrices, tensors formed bilinearly from pairs of spinors, and creation and annihilation operators of Fermi states. A new theorem characterizes a simple spinor φ by the properties of the vector tψBγμφ, where ψ is an arbitrary spinor and B is the matrix connecting the gamma matrices with their transposes. The Cartan constraint equations on the components of simple spinors are given a new, geometrically transparent derivation based on the action on simple spinors of a maximal Abelian subgroup of the group Spin.
Huygens’ principle and characteristic propagation property for waves in curved space-times
Authors: Sonego S. , Faraoni V.
Published: July 2002

Fields satisfying a wave equation on a curved space-time can be accompanied by tails propagating inside the light cone. A formal characterization of tails is given, and the relationship of the tail-free property with the characteristic propagation property (CPP) discussed in the literature is clarified. The scalar wave equation satisfies the CPP independently of the tail-free property only in the case of minimal coupling in two space-time dimensions. The peculiar feature of electromagnetic radiation, for which pure gauge tails in the potential can occur, is pointed out.
A Sato formula for reflectionless finite difference operators
Authors: van Diejen J. F.
Published: July 2002

An explicit parametrization of the Jost functions of the reflectionless Jacobi operators in terms of their spectral data is presented. Interpolation produces a Sato formula for the eigenfunctions of a class of reflectionless finite difference operators on the line.
E
6
unification model building. I. Clebsch–Gordan coefficients of 27⊗27¯
Authors: Anderson Gregory W. , Blažek Tomáš
Published: July 2002

In an effort to develop tools for grand unified model building for the Lie group E6, in this paper we present the computation of the Clebsch–Gordan coefficients for the product (100000)⊗(000010), where (100000) is the fundamental 27-dimensional representation of E6 and (000010) is its charged conjugate. The results are presented in terms of the dominant weight states of the irreducible representations in this product. These results are necessary for the group analysis of E6 operators also involving higher representations, which is the next step in this project. In this paper we apply the results to the construction of the operator 273.
The effects of intermittency on statistical characteristics of turbulence and scale similarity of breakdown coefficients
Authors: Novikov E. A.
Published: July 2002

The influence of intermittency on turbulent diffusion is expressed in terms of the statistics of the dissipation field. The high-order moments of relative diffusion are obtained by using the concept of scale similarity of the breakdown coefficients (bdc). The method of bdc is useful for obtaining new models and general results, which then can be expressed in terms of multifractals. In particular, the concavity and other properties of spectral codimension are proved. Special attention is paid to the logarithmically periodic modulations. The parametrization of small-scale intermittent turbulence, which can be used for large-eddy simulation, is presented. The effect of molecular viscosity is taken into account in the spirit of the renorm group, but without spectral series, ε expansion, and fictitious random forces.
Destabilization of the Karman vortex street by frequency-modulated excitation
Authors: Nakano M. , Rockwell D.
Published: July 2002

A two-dimensional cylinder is subjected to frequency-modulated excitation in the cross-stream direction. Transition from a locked-in response to a completely destabilized response can be attained by lowering the value of modulation frequency while all other parameters are maintained constant. This transition involves progression through a series of identifiable states including lock-in at the modulation frequency, period doubling at the modulation frequency, and spectral-broadened response.
The elliptical model of two-dimensional vortex dynamics. I: The basic state
Authors: Legras Bernard , Dritschel David G.
Published: July 2002

An approximate model of nonaxisymmetric vortices in a two-dimensional fluid is developed in which each vortex is assumed to consist of a nested stack of elliptical regions of uniform vorticity. The model is governed by a set of coupled nonlinear ordinary differential equations for the aspect ratio, orientation, and centroid of the bounding contour of each region. It also admits a Hamiltonian formulation and possesses all the invariants of the original system. In Part II [Phys. Fluids A 3, XXX (1991)], the model is extended to include disturbances to the elliptical shape of each contour.
The periodic instability of thermocapillary convection in cylindrical liquid bridges
Authors: Velten R. , Schwabe D. , Scharmann A.
Published: July 2002

Thermocapillary convection (TC) in cylindrical liquid bridges (floating zones) of liquids with Prandtl numbers Pr=1, 7, and 49 is investigated experimentally. The zones have been heated from above or from below to study the influence of buoyant forces. Fourier analyses of temperature signals from zones covering systematically wide ranges of aspect ratios A and Marangoni numbers Ma have shown the existence of various forms of periodic and nonperiodic TC. This paper reports on periodic TC existing under certain conditions between the onset of time-dependent TC at the critical Marangoni number Mac and 7×Mac. From the measurements of the onset of periodic TC the dependence is reported for the threshold Mac and the period near the threshold τc on the aspect ratio. The development of periodic TC when further increasing Ma is shown by typical examples from measurements of the frequency and the amplitude of the oscillations. By correlation analyses from three temperature signals, different structures of periodic TC in the investigated A–Ma–Pr range were identified. Both the running waves with an azimuthal component (m≥1) and the axially running waves (m=0) were found and the findings of these and various other spatiotemporal structures of periodic TC were displayed in A–Ma/Mac state maps. These maps indicate as well the influence of the Prandtl number and of the buoyant forces on the preferred spatial structure of periodic TC, as discussed in the light of an already existing theory.
Motion of an elliptical vortex under applied periodic strain
Authors: Dhanak Manhar R. , Marshall Marilyn P.
Published: July 2002

The effect of subjecting a uniform elliptical vortex to a periodically varying plane straining field is considered. For plane steady straining fields, it is known that a Rankine-type vortex core of uniform vorticity and elliptical shape can exist in a state in which it (a) is steady and stationary, (b) rotates about its axis or nutates about a fixed axis, or (c) elongates indefinitely, smearing the core into a thin layer. In state (a), for sufficiently weak straining fields, a vortex of small enough aspect ratio of the ellipse persists under such a plane strain, being robust to small two-dimensional disturbances present in the flow field. It is shown that if, however, a small periodically varying component is added to the basic straining field, then at frequencies corresponding to the natural frequency of vibration of the vortex and their harmonics, a resonance phenomenon takes place which destabilizes the apparently stable stationary vortex in finite time, causing it to flip from state (a) into states (b) or (c), in which action of instabilities associated with a higher, nonelliptical, mode of deformation of the vortex boundary by disturbances in the flow field would lead to disintegration of the vortex structure. In fact it is found that the vortex also flips to states (b) and (c) for a range of non-natural frequencies of oscillation of the straining field. The effect of the periodic plane straining on the three-dimensional Crow instability is also discussed.
Zero crossings of velocity fluctuations in turbulent boundary layers
Authors: Kailasnath P. , Sreenivasan K. R.
Published: July 2002

In this paper some results are presented on the statistical properties of zero crossings of turbulent velocity fluctuations in boundary layers over a wide range of Reynolds numbers. The earlier finding that the probability density function (pdf) of the intervals between successive zero crossings of the streamwise velocity fluctuation u can be approximated by two exponentials, each with its own characteristic scale, is confirmed. The cross-stream variation of these characteristic scales is investigated. One of these scales, corresponding to the large zero-crossing intervals, is independent of the Reynolds number, while the other for the viscous-dominated small-scale crossings varies with as Rλ−1/2, where Rλ is the Reynolds number based on the Taylor microscale, λ. The pdf’s for the normal velocity component v and the fluctuating part of the Reynolds stress uv are essentially exponential over the whole range of zero-crossing scales, and each possesses just one characteristic scale. The mean and the standard deviation of the zero-crossing scales of u and v, when normalized by their respective Taylor microscales, are roughly unity and essentially independent of the cross-stream position. Similar data are also presented for the Reynolds stress fluctuations. A brief discussion of the results as well as an example of the application of the zero-crossing pdf are given.
Linear and nonlinear theory of cyclotron autoresonance masers with multiple waveguide modes
Authors: Chen Chiping , Wurtele Jonathan S.
Published: July 2002

The interaction of multiple waveguide modes with a relativistic electron beam in an overmoded, single-frequency, cyclotron autoresonance maser amplifier is analyzed using a nonlinear self-consistent model and kinetic theory. It is shown analytically, and confirmed by simulation, that all of the coupled waveguide modes grow at the spatial growth rate of the dominant unstable mode, but suffer different launching losses which depend upon detuning. The phases of coupled modes are locked in the exponential gain regime, and remain approximately locked for some finite interaction length beyond saturation. The saturated power in each mode is found to be insensitive to the input modal radio-frequency (rf) power distribution, but sensitive to detuning. Simulations indicate that the saturated fractional rf power in a given mode reaches a maximum at its resonant magnetic field, and then decreases rapidly off resonance. Good agreement is found between the simulations and the kinetic theory in the linear regime.
Map model for nonlinear alpha particle interaction with toroidal Alfvén waves
Authors: Berk H. L. , Breizman B. N. , Ye Huanchun
Published: July 2002

A map model has been developed for studying the nonlinear interaction of alpha particles with the toroidal Alfvén eigenmodes. The map is constructed by assuming a linear interaction during a single poloidal transit, which allows the study of the nonlinear interaction over many transits. By using this map, analytic expressions are obtained for the particle nonlinear bounce frequency, and the wave amplitude threshold for the onset of particle orbit stochasticity. The map model can also facilitate self-consistent simulations which incorporate the time variation of the waves.
The effect of surface tension on the shape of a Hele–Shaw cell bubble
Authors: Tanveer Saleh
Published: July 2002

Numerical and asymptotic solutions are found for the steady motion of a symmetrical bubble through a parallel-sided channel in a Hele–Shaw cell containing a viscous liquid. The degeneracy of the Taylor–Saffman zero surface-tension solution is shown to be removed by the effect of surface tension. An apparent contradiction between numerics and perturbation arises here as it does for the finger. This contradiction is resolved analytically for small bubbles and is shown to be the result of exponentially small terms. Numerical results suggest that this is true for bubbles of arbitrary size. The limit of infinite surface tension is also analyzed.
Electron acceleration in localized plasma waves
Authors: Colunga M. , Luciani J. F. , Mora P.
Published: July 2002

Particle acceleration in a localized electrostatic wave packet is studied. A weak field regime and a strong field regime are displayed. In the weak field limit a quasilinear relation for the velocity perturbation is derived. When the particles cross the accelerating field several times, a quasilinear equation for the distribution function is established. The theory is illustrated by numerical results from a model of resonance absorption of laser light by a plasma. Finally a model of electron reheating in resonance absorption including collisions is presented, leading to a nearly Maxwellian behavior for the hot component of the electron distribution function, f∼exp−(v/v0)8/3.
New solutions for steady bubbles in a Hele–Shaw cell
Authors: Tanveer S.
Published: July 2002

Exact solutions are presented for steadily moving bubbles in a Hele–Shaw cell when the effect of surface tension is neglected. These solutions form a three-parameter family. For specified area, both the speed of the bubble and the distance of its centroid from the channel centerline remain arbitrary when surface tension is ignored. However, numerical evidence suggests that this twofold arbitrariness is removed by the effect of surface tension, i.e., for given bubble area and surface tension, solutions exist only when the bubble velocity and the centroid distance from the channel centerline attain one or more isolated values. From a limited numerical search, no nonsymmetric solutions could be found; however, a branch of symmetric bubble solutions that was not found in earlier work was found. This branch corresponds to one of the Romero–Vanden-Broeck branch of finger solutions when the bubble size is large. A new procedure for numerical calculations of bubble solutions in the presence of surface tension is presented and is found to work very well for reasonably large bubbles, unlike the previous method of Tanveer [Phys. Fluids 29, 3537 (1986)]. The precise power law dependence of bubble velocity on surface tension for small surface tension is explored for bubbles of different area. Agreement is noted with recent analytical results for a finger.
Nonuniversality of sublayer streaks in turbulent flow
Authors: Bradshaw Peter , Langer Carlos A.
Published: July 2002

Coefficients of series expansions of turbulent velocity fluctuations in the viscous wall region are used to generate an arbitrary but quantitative measure of the time-average strength of the near-wall quasi-streamwise vortices, which appear as ‘‘streaks’’ in flow visualization. Existing databases from direct numerical simulations of wall bounded turbulence are used to compute some estimates. The results show that the strength of the streaks is Reynolds-number-dependent, even in simple flows, as well as flow-dependent, contrary to traditional law-of-the-wall arguments.
Chaotic advection in creeping flow of viscoelastic fluids between slowly modulated eccentric cylinders
Authors: Kumar Satish , Homsy G. M.
Published: July 2002

Recent experiments show that very low levels of elasticity can either enhance or diminish the area over which chaotic advection occurs in creeping flows [T. C. Niederkorn and J. M. Ottino, J. Fluid Mech. 256, 243 (1993)]. No mechanistic explanation of this phenomenon is currently available. This has motivated us to consider the problem of two-dimensional flow between counter-rotating eccentric cylinders where the angular velocities are subject to slow, continuous modulation. Regular perturbation theory for low levels of elasticity is used to semi-analytically determine the viscoelastic correction to the Newtonian flow field based on the Oldroyd-B constitutive model. The geometric theory of Kaper and Wiggins [J. Fluid Mech. 253, 211 (1993)] is then applied to make predictions about how elasticity affects chaotic advection in quasi-steady flows. It is found that elasticity can act to either increase or decrease the area over which chaotic advection occurs, depending on the boundary motion. This is accomplished through three distinct mechanisms: (1) area changes of the maximum area over which chaotic advection can occur, the potential mixing zone (PMZ); (2) area changes of the region in the PMZ where fluid particles execute non-chaotic trajectories below a critical modulation frequency; (3) area changes of the region between the extrema of the Newtonian stagnation streamlines which does not belong to the PMZ. The mechanism responsible for these area changes is a modified pressure gradient in the angular direction, which in turn appears to be due to first normal stress differences caused by shearing. Numerical calculations of fluid particle trajectories confirm the predictions of the geometric theory. For the boundary motions considered here, the calculations yield two additional results about the effect of low levels of elasticity on chaotic advection. First, the critical modulation frequency is decreased. Second, the rate of chaotic mixing, as measured by the largest Liapunov exponent, is increased for modulation fre- quencies greater than the critical Newtonian value.
Laser–plasma interactions in ignition-scale hohlraum plasmas
Authors: MacGowan B. J. , Afeyan B. B. , Back C. A. , Berger R. L. , Bonnaud G. , Casanova M. , Cohen B. I. , Desenne D. E. , DuBois D. F. , Dulieu A. G. , Estabrook K. G. , Fernandez J. C. , Glenzer S. H. , Hinkel D. E. , Kaiser T. B. , Kalantar D. H. , Kauffman R. L. , Kirkwood R. K. , Kruer W. L. , Langdon A. B. , Lasinski B. F. , Montgomery D. S. , Moody J. D. , Munro D. H. , Powers L. V. , Rose H. A. , Rousseaux C. , Turner R. E. , Wilde B. H. , Wilks S. C. , Williams E. A.
Published: July 2002

Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low-Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high-Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).
Time-dependent filamentation and stimulated Brillouin forward scattering in inertial confinement fusion plasmas
Authors: Schmitt Andrew J. , Afeyan Bedros B.
Published: July 2002

Numerical simulations of the temporal evolution of laser light filamentation and stimulated Brillouin forward scattering (SBFS) in plasmas, under conditions that are relevant to laser fusion, are presented and analyzed. Long term unsteady behavior of filaments is observed to be the norm. Temporal and spatial incoherence due to filamentation and SBFS are impressed upon time-independent incident laser beams. The bandwidth and angular divergence imposed upon the beam increase with the strength of the interaction. In addition, the spectrum of the transmitted light is redshifted by an amount that increases with the interaction strength. Spectral analysis of the transmitted light reveals that SBFS plays a role in the generation of the observed temporal incoherence. Incident beams with some spatial incoherence but no temporal smoothing are compared to those with ab initio temporal beam smoothing (TBS). Under typical conditions, TBS beams will undergo far less angular and spectral spreading and far less SBFS than unsmoothed beams.
Experimental studies of magnetic reconnection
Authors: Brown M. R.
Published: July 2002

Laboratory magnetic reconnection experiments have been performed for nearly 20 years. Elegant experiments by Stenzel and Gekelman [R. L. Stenzel and W. Gekelman, Phys. Rev. Lett. 42, 1055 (1979); W. Gekelman and R. L. Stenzel, Phys. Rev. Lett. 54, 2414 (1985)] focused on the measurement of field quantities with a single movable probe in a highly reproducible plasma. Observations included a very thin current sheet (on the order of c/ωpe), accelerated electrons, and whistler waves. The argon ions were unmagnetized in these experiments. Recent magnetohydrodynamic (MHD) experiments by Yamada and Ono have used merging plasmoids [M. Yamada, Y. Ono, A. Hayakawa, M. Katsurai, and F. W. Perkins, Phys. Rev. Lett. 65, 721 (1990); Y. Ono, M. Yamada, T. Akao, T. Tajima, and R. Matsumoto, Phys. Rev. Lett. 76, 3328 (1996)] and have measured three dimensional effects and ion acceleration. We have observed correlations between magnetic reconnection and energetic ion flow events with merging force free spheromaks at the Swarthmore Spheromak Experiment (SSX) [T. W. Kornack, P. K. Sollins, and M. R. Brown, Phys. Rev. E 58, R36 (1998)]. The reconnection layer is measured with linear and two dimensional probe arrays and ion flow is directly measured with a retarding grid energy analyzer. Flow has been measured both in the plane of the reconnection layer and out of the plane. The outflow velocity is nearly Alfvénic in the reconnection plane and the scale of the magnetic structures is consistent with collisionless reconnection theories (on the order of c/ωpi). Results from the two dimensional array show the formation of magnetic islands correlated with super-Alfvénic ions accelerated normal to the layer.
Dominant moving species in the formation of amorphous NiZr by solid-state reaction
Authors: Cheng Y.-T. , Johnson W. L. , Nicolet M.-A.
Published: July 2002

The displacements of W and Hf markers have been monitored by backscattering of MeV He to study the growth of the amorphous NiZr phase by solid-state reaction. We find that the Ni is the dominant moving species in this reaction.
Selective Ge deposition on Si using thermal decomposition of GeH4
Authors: Ishii Hiromu , Takahashi Yasuo , Murota Junichi
Published: July 2002

Deposition characteristics of Ge using thermal decomposition of GeH4 are studied. The deposition rate of Ge in a surface reaction region at temperatures below 410 °C is formulated as a function of the deposition temperature and GeH4 partial pressure. Selective deposition of Ge occurs reproducibly in the lower temperature range below 410 °C. The cause of the suppression of selectivity and epitaxial growth at deposition temperatures above 450 °C is confirmed as oxide contamination on the substrate.
Superconductivity in Nb-Ge-Al-O films above 44 K
Authors: Ōgushi Tetsuya , Ōsono Yoshihisa
Published: July 2002

Niobium-germanium-aluminum-oxygen films have been found to remain superconducting up to 44.5 K. The diamagnetic susceptibility of this material was confirmed close to χ∼−0.95 experimentally and the transition from a zero resistance state to a finite resistance state was induced by current.
Observation of magnetic dead layers at the surface of iron oxide films
Authors: Parkin S. S. P. , Sigsbee R. , Felici R. , Felcher G. P.
Published: July 2002

Depth profiles of the magnetization of sputtered films of Fe3O4 and its oxidized form, γ-Fe2O3, were obtained by the new polarized neutron reflection technique. We find that the magnetization of Fe3O4 is constant throughout the thickness (2600 Å) of the film except for a surface region about 25 Å deep corresponding to the surface roughness. In contrast the oxidized sample shows a magnetically dead layer ∼150 Å thick at the surface. X-ray and transmission electron microscopy studies give no evidence that the surface layers differ structurally from the bulk of the film, although there is the possibility of a highly defective surface region formed during the oxidation process.
Proposed structure for large quantum interference effects
Authors: Datta S. , Melloch M. R. , Bandyopadhyay S. , Lundstrom M. S.
Published: July 2002

In this letter we propose and analyze a new semiconductor structure that can be fabricated by present day technology and can lead to large quantum interference effects with potential device applications.
Influence of Ga-As-Te interfacial phases on the orientation of epitaxial CdTe on GaAs
Authors: Feldman R. D. , Austin R. F. , Kisker D. W. , Jeffers K. S. , Bridenbaugh P. M.
Published: July 2002

When CdTe is grown by molecular beam or organometallic vapor phase epitaxy on (100) GaAs, the layer can grow with either a (100) or (111) orientation. Reflection high-energy electron diffraction and Auger studies are presented here which show that adsorption of different submonolayer amounts of Te on a GaAs surface can change the surface symmetry and the resulting CdTe orientation. A precursor surface to (111) growth results from the formation of a relatively Te-poor Ga-As-Te surface phase. A relatively Te-rich structure yields a surface with (100) symmetry and lead to (100) growth.
Sequential resonant tunneling through a multiquantum well superlattice
Authors: Capasso Federico , Mohammed Khalid , Cho Alfred Y.
Published: July 2002

We report the observation of two negative conductance regions in the low-temperature photocurrent-voltage characteristic of tight-binding multiquantum well (35 periods) 1-μm-thick Al0.48 In0.52As/ Ga0.47 In0.53As superlattices grown by molecular beam epitaxy. The two peaks occur at voltages corresponding to a potential energy drop across the superlattice period equal to the energy differences between the first two excited states and the ground state of the quantum wells. This provides direct evidence of sequential resonant tunneling between the ground and excited states of adjacent wells alternated with intrawell energy relaxation.
Study of ballistic transport in Si-CoSi2-Si metal base transistors
Authors: Rosencher E. , Badoz P. A. , Pfister J. C. , d’Avitaya F. Arnaud , Vincent G. , Delage S.
Published: July 2002

The hot-electron transfer ratio from the Schottky emitter diode to the Schottky collector diode through the thin base layer of Si-CoSi2-Si metal base transistors has been measured in devices for which the pinhole contribution to the current gain is shown by transconductance measurements to be negligible. The common-base current gain α of the transistors, measured as a function of temperature and base thickness, exhibits an exponential dependence on CoSi2 film thickness, from which a ballistic mean free path is deduced. This value is in good agreement with the mean free path deduced from conductivity data, both at 77 and 300 K. The behavior of the pre-exponential coefficient, i.e., the gain extrapolated to zero base thickness, is not completely understood and shows evidence for hot-electron and space-charge effects in the epitaxial overgrown Si material of the emitter.
High-speed dual-wavelength demultiplexing and detection in a monolithic superlattice p-i-n waveguide detector array
Authors: Larsson A. , Andrekson P. A. , Andersson P. , Eng S. T. , Salzman J. , Yariv A.
Published: July 2002

We demonstrate high-speed (1 Gbit/s) dual-wavelength demultiplexing and detection in a monolithic linear array of superlattice p-i-n photodetectors in a waveguide configuration. A crosstalk attenuation of 28 dB was achieved between two digital transmission channels with an interchannel wavelength spacing of 30 nm. The device performance is a result of an enhanced electroabsorption due to the quantum-confined Stark effect in the superlattice p-i-n diodes.
Electro-optical effects of externally applied 〈100〉 uniaxial stress on InGaAsP 1.3 and 1.5 μm injection lasers
Authors: Swaminathan V. , Parayanthal P. , Hartman R. L.
Published: July 2002

The changes in the polarization of the emission, spectrum, and light-current (L-I) behavior of a 1.3 and 1.5 μm InGaAsP channeled substrate buried heterostructure laser and double-channel planar buried heterostructure laser under an external uniaxial compressive stress perpendicular to the junction were studied at room temperature. It was found that the TM emission (electric vector perpendicular to the junction) reached threshold even at a stress level of ∼1×108 dyn cm−2. For stress ≤4×108 dyn cm−2, the TM emission, although reaching threshold first, was found to exist only over a limited range of currents. At high currents only TE emission (electric vector parallel to the junction) was observed. The appearance of the TE emission gave rise to nonlinear L-I characteristics. When both TM and TE emissions were seen, the former occurred at a higher energy. The limited stability of TM emission with current at low stresses is explained by invoking spectral hole burning effects which reduce the gain saturation power for TM mode than the TE mode. At high stresses ≥4×108 dyn cm−2, the increased gain for TM-polarized light ensures its stability at all currents. From our results it can be concluded that to avoid TM emission, and the associated spectral changes and nonlinear L-I characteristics, the tensile stress in the plane of the active layer should be less than 108 dyn cm−2.
Observation of grating-induced intersubband emission from GaAs/AlGaAs superlattices
Authors: Helm M. , Colas E. , England P. , DeRosa F. , Allen S. J.
Published: July 2002

We have observed far-infrared emission due to electronic transitions between subbands in GaAs/AlGaAs superlattices. Population of higher subbands is achieved by applying an electric field in the plane of the layers. The radiation is coupled out of the sample by a metallic grating on the surface.
CD3-mediated activation of tumor-reactive lymphocytes from patients with advanced cancer
Authors: Hellström Ingegerd , Ledbetter Jeffrey A. , Scholler Nathalie , Yang Yi , Ye Zhengmao , Goodman Gary , Pullman Janice , Hayden-Ledbetter Martha , Hellström Karl Erik
Published: July 2002

Lymphocytes from blood or tumors of patients with advanced cancer did not proliferate and produced very low levels of tumor necrosis factor and IFN-γ when cultured with autologous tumor cells. Proliferation and lymphokine production dramatically increased in the presence of beads conjugated with mAbs to CD3 plus mAbs to CD28 and/or CD40, and the lymphocytes destroyed the tumor cells. Expression density of CD3 concomitantly increased from low to normal levels. Furthermore, beads providing a CD3 signal (in combination with CD28 or CD28 plus CD40) gave partial protection against the inhibitory effect of transforming growth factor type β1 on lymphocyte proliferation and production of tumor necrosis factor and IFN-γ. MHC class I-restricted cytolytic T cells lysing autologous tumor cells in a 4-h Cr51release assay were generated when peripheral blood leukocytes were activated in the presence of autologous tumor cells and anti-CD3/CD28 or anti-CD3/CD28/CD40 beads. Experiments performed in a model system using anti-V-β1 or anti-V-β2 mAbs to activate subsets of T cells expressing restricted T cell receptor showed that lymphocytes previously activated by anti-V-β can respond to CD3 stimulation with vigorous proliferation and lymphokine production while retaining their specificity, also in the presence of transforming growth factor type β1. Our results suggest that T lymphocytes from cancer patients can proliferate and form Th1 type lymphokines in the presence of autologous tumor cell when properly activated, and that antigen released from killed tumor cells and presented by antigen-presenting cells in the cultures facilitates the selective expansion of tumor-directed, CD8+cytolytic T cells.