bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-consistent field theory for the nucleation of micelles
Authors: Besseling N. A. M. , Cohen Stuart M. A.
Published: July 2002

We report the first molecular model for the transition states (critical nuclei) that determine the rate of micelle formation and breakdown. These processes, and homogeneous nucleation of macroscopic phase separation are handled in a unified way. It turns out that with micelle-forming amphiphiles several new features appear that do not occur with ordinary phase separation. We predict that the activation free-energy barrier for micelle formation is maximal but finite at the critical micelle concentration (CMC) and decreases with concentration. The barrier for micelle breakdown vanishes at the CMC and increases with concentration. Furthermore, our results indicate formation of transient large aggregates upon quenches far beyond the CMC.
Intramolecular energy transfer in highly vibrationally excited methanol. II. Multiple time scales of energy redistribution
Authors: Boyarkin O. V. , Rizzo T. R. , Perry David S.
Published: July 2002

State-selected spectra of the OH stretch overtones of methanol in the range of v1=3–8 reveal spectral splittings and broadenings that result from vibrational couplings within the molecule. We employ a two-color excitation technique in which an infrared pulse promotes jet-cooled methanol molecules to a single rotational state in v1=1 or 2 and a second visible or near-infrared laser pulse is scanned to record a vibrational overtone spectrum. The final vibrationally excited species are detected by infrared laser assisted photofragment spectroscopy. The implications of the spectra for vibrational dynamics in the time domain can be understood in terms of a hypothetical coherent excitation of relevant portions of the spectrum. The observed splittings and widths correspond to three time scales. The largest splittings imply subpicosecond oscillation of energy between the OH stretch and a combination with the C–H stretch (5ν1⇔4ν1+ν2 and 6ν1⇔5ν1+ν2) or a combination with the COH bend (7ν1⇔6ν1+2ν6). Secondary time scales correspond to finer splittings and are thought to arise from low-order resonances with other vibrational states. We argue that the nonmonotonic energy dependence of the presence and extent of such secondary structure throughout the recorded spectra reflects the requirement of resonance with important zeroth-order states. The third time scale, represented by the widths of the narrowest features at each overtone level, reflects the onset of vibrational energy randomization. These widths increase exponentially with vibrational energy in the range 2ν1 up to 8ν1. At the highest energy (25 000 cm−1) the three time scales begin to converge, implying an irreversible decay of the OH stretch overtone in 300 fs.
Attractive interactions and phase transitions in solutions of similarly charged rod-like polyelectrolytes
Authors: Carri Gustavo A. , Muthukumar M.
Published: July 2002

We have calculated the effective interaction and thermodynamic behavior of semidilute solutions of rod-like polyelectrolytes. The model is based on a bare potential energy constructed from the Debye–Hückel potential but extended to include the anisotropic contributions arising from the rod-like conformation. We have calculated the bare and effective potential energy between two Kuhn segments in different polyelectrolyte rods, the isotropic-to-nematic transition, the orientational order parameter of the nematic phase and the phase diagram of the system. Moreover, the effects of molecular weight, monomer, and salt concentration on the different quantities are discussed.
Thermodynamics of finite magnetic two-isomer systems
Authors: Borrmann Peter , Stamerjohanns Heinrich , Hilf Eberhard R. , Jund Philippe , Kim Seong Gon , Tománek David
Published: July 2002

We use Monte Carlo simulations to investigate the thermodynamical behavior of aggregates consisting of few superparamagnetic particles in a colloidal suspension. The potential energy surface of this classical two-isomer system with a stable and a metastable “ring” and “chain” configuration is tunable by an external magnetic field and temperature. We determine the complex “phase diagram” of this system and analyze thermodynamically the nature of the transition between the ring and the chain “phase.”
Laser-induced fluorescence and Optical/Stark spectroscopy of PtC
Authors: Beaton S. A. , Steimle T. C.
Published: July 2002

Optical/Stark measurements have been performed on the (0,0) bands of both the A″ 1Σ+–X 1Σ+ system (ν0≈12 643 cm−1) and the A′ 1Π–X Σ+ system (ν0≈13 196 cm−1) of platinum monocarbide. The PtC molecules were produced in a pulsed supersonic molecular beam source following the reaction of laser ablated platinum vapor with a mixture of a few percent of methane in argon. The newly determined permanent electric dipole moments obtained are 1.94(2)D (A″ 1Σ+) and 1.919(9)D (A′ 1Π). These results are discussed in terms of a proposed molecular orbital correlation diagram for platinum containing diatomics. The laser-induced fluorescence spectrum of the (0,0)A″ 1Σ+–X 1Σ+ transition of PtC has been re-recorded at high resolution (full width of half-maximum ∼40 MHz) and analyzed to yield rotational constants for the four most abundant isotopomers of PtC, extending the previous analysis [Appelblad, Nilsson, and Scullman, Phys. Scr. 7, 65 (1973)]. The anomalously large value (∼15 MHz) for the newly derived nuclear-spin rotation parameter, CI(195Pt), for the A″ 1Σ+ state is discussed.
Colloidal aggregation under steric interactions: Simulation and experiments
Authors: Romero-Cano M. S. , Puertas A. M. , de las Nieves F. J.
Published: July 2002

The influence of steric interactions in the initial stages of aggregation kinetics in colloidal systems has been studied by simulation and experiments. A simulation model has been proposed to study the initial stages of aggregation under short range repulsive potentials. A polystyrene latex was used as model colloid and the steric interaction was provided by adsorption of a nonionic surfactant. Depending on the range or strength of the interactions, sensitive or insensitive systems to electrolyte concentration can be observed. At low κa, the long-range electrostatic repulsion dominates the system behavior, stabilizing the colloidal system. In conditions of screened electrostatic potential, particle collision is the result of a competition between van der Waals attraction and steric repulsion, leading to a decrease in the dimer formation constant as the range or strength of the steric interaction increases. The steric interaction energy has been included in the theoretical calculation of the aggregation rate constant by means of Fuchs’ stability factor. Good agreement between simulation, experimental, and theoretical calculations was obtained.
The effect of inhomogeneous broadening on optical strong field spectroscopy
Authors: Morillo M. , Cukier R. I.
Published: July 2002

We show that a recent theory of strong field spectroscopy (SFS) [R. I. Cukier and M. Morillo, Phys. Rev. B 57, 6972 (1998), M. Morillo and R. I. Cukier, J. Chem. Phys. 110, 7966 (1999)] can be used to circumvent the effects of inhomogeneous broadening on this spectroscopy. In SFS, a strong external field is used to connect, with the transition dipole, two electronic states of a solute immersed in a medium. The electronic dephasing due to the medium is characterized via the power absorbed by the solute. The average absorbed power P̄(t) for resonant, strong fields exhibits an oscillatory decay in time, reflecting the finite change in the population difference of the electronic states and the dephasing arising from the coupling to the medium. The decay rate is characterized by d≡Δ2τc, where Δ and τc are, respectively, the strength and time constant of the correlation function characterizing the solute–medium coupling. The decay can be very rapid, on a 10–100 fs time scale, and this necessitates an indirect procedure to experimentally probe P̄(t) that we develop. For strong, off-resonance fields, P̄(t) returns to an exponential decay regime. The contrasting behavior of resonant and nonresonant strong fields can be used to avoid the loss of information about the homogeneous properties due to inhomogeneous broadening of the optical transition, when this broadening arises from inhomogeneity in the optical transition frequency.
The semi-classical expansion for a charged particle on a curved space background
Authors: Elworthy David , Truman Aubrey , Watling Keith
Published: July 2002

We give the semi-classical expansion, with remainder to any order in ℏ, for the wave function of a nonrelativistic quantum particle in a classical external magnetic field on a curved space background. The basic assumption is of a ‘‘no caustics condition’’ on the underlying classical mechanics, at least up to the time in question. The gauge invariance of the result is emphasized together with a discussion of the geometric meaning of the classical mechanical quantities involved.
Generalized quasispin for supergroups
Authors: Jarvis P. D. , Yang Mei , Wybourne B. G.
Published: July 2002

The embedding of the dynamical algebra U(M/N) of nuclear supersymmetries in larger algebraic structures is studied. A noncompact Z2⊕Z2 graded color superalgebra SpO(2M/1/2N/0) is identified as a receptacle for various chains containing boson and fermion (super) algebras. The existence of a generalized quasispin algebra is demonstrated and discussed.
A renormalization group analysis of infrared QED
Authors: Dimock J. , Hurd T. R.
Published: July 2002

Euclidean quantum electrodynamics in the Feynman gauge is considered in dimensions d≥2 with an ultraviolet cutoff and large fermion mass. The fermions are integrated out giving an effective photon interaction. For this interaction a complete renormalization group analysis is given and it is shown that the flow of the effective interactions is toward a free photon field (infrared asymptotic freedom). This result is a special case of a general method for gauge invariant perturbations of the free photon field.
A simple recurrence for the higher derivatives of the Hurwitz zeta function
Authors: Elizalde E.
Published: July 2002

A recurrent formula which allows the calculation of the asymptotic series expansion of any derivative, ζ(m)(z,a)=∂mζ(z,a)/∂zm, of the Hurwitz zeta function ζ(z,a) is obtained. In particular, the first terms of the series corresponding to ζ″(−n,a) in inverse powers of a are explicitly given, for n=0,1,2,3. Knowledge of these expressions is fundamental, e.g., in the zeta-function regularization procedure.
Uniform density theorem for the Hubbard model
Authors: Lieb Elliott H. , Loss Michael , McCann Robert J.
Published: July 2002

A general class of hopping models on a finite bipartite lattice is considered, including the Hubbard model and the Falicov–Kimball model. For the half-filled band, the single-particle density matrix ρ(x,y) in the ground state and in the canonical and grand canonical ensembles is shown to be constant on the diagonal x=y, and to vanish if x≠y and if x and y are on the same sublattice. For free electron hopping models, it is shown in addition that there are no correlations between sites of the same sublattice in any higher order density matrix. Physical implications are discussed.
Local geometric invariants of integrable evolution equations
Authors: Langer Joel , Perline Ron
Published: July 2002

The integrable hierarchy of commuting vector fields for the localized induction equation of 3D hydrodynamics, and its associated recursion operator, are used to generate families of integrable evolution equations which preserve local geometric invariants of the evolving curve or swept-out surface.
Quantum canonical transformations and exact solution of the Schrödinger equation
Authors: Mostafazadeh Ali
Published: July 2002

Time-dependent unitary transformations are used to study the Schrödinger equation for explicitly time-dependent Hamiltonians of the form H(t)=R(t)⋅J, where R is an arbitrary real vector-valued function of time and J is the angular momentum operator. The solution of the Schrödinger equation for the most general Hamiltonian of this form is shown to be equivalent to the special case R=(1,0,ν(t)). This corresponds to the problem of a driven two-level atom for the spin half representation of J. It is also shown that by requiring the magnitude of R to depend on its direction in a particular way, one can solve the Schrödinger equation exactly. In particular, it is shown that for every Hamiltonian of the form H(t)=R(t)⋅J there is another Hamiltonian with the same eigenstates for which the Schrödinger equation is exactly solved. The application of the results to the exact solution of the parallel transport equation and exact holomony calculation for SU(2) principal bundles (Yang–Mills gauge theory) is also pointed out.
Three-dimensional Lorentzian manifolds with constant principal Ricci curvatures ρ1=ρ2≠ρ3
Authors: Bueken Peter
Published: July 2002

The aim of this paper is the study of (nonhomogeneous) three-dimensional Lorentzian manifolds whose Ricci curvature tensor is diagonalizable with two distinct constant eigenvalues. Two mistakes in a recent paper by D. McManus [J. Math. Phys. 36, 362–369 (1995)] are pointed out and corrected, and a complete (local) classification of the nonhomogeneous manifolds of this type is given, thereby generalizing some results of O. Kowalski [Nagoya Math. J. 132, 1–36 (1993)] to the framework of Lorentzian geometry.
Non-negative Feynman–Kac kernels in Schrödinger’s interpolation problem
Authors: Blanchard Philippe , Garbaczewski Piotr , Olkiewicz Robert
Published: July 2002

The local formulations of the Markovian interpolating dynamics, which is constrained by the prescribed input-output statistics data, usually utilize strictly positive Feynman–Kac kernels. This implies that the related Markov diffusion processes admit vanishing probability densities only at the boundaries of the spatial volume confining the process. We discuss an extension of the framework to encompass singular potentials and associated non-negative Feynman–Kac-type kernels. It allows us to deal with a class of continuous interpolations admitted by general non-negative solutions of the Schrödinger boundary data problem. The resulting nonstationary stochastic processes are capable of both developing and destroying nodes (zeros) of probability densities in the course of their evolution, also away from the spatial boundaries. This observation conforms with the general mathematical theory (due to M. Nagasawa and R. Aebi) that is based on the notion of multiplicative functionals, extending in turn the well known Doob’s h-transformation technique. In view of emphasizing the role of the theory of non-negative solutions of parabolic partial differential equations and the link with “Wiener exclusion” techniques used to evaluate certain Wiener functionals, we give an alternative insight into the issue, that opens a transparent route towards applications.
Abelian Chern–Simons theory. II. A functional integral approach
Authors: Manoliu Mihaela
Published: July 2002

Following Witten, [Commun. Math. Phys. 21, 351–399 (1989)] we approach the Abelian quantum Chern–Simons (CS) gauge theory from a Feynman functional integral point of view. We show that for 3-manifolds with and without a boundary the formal functional integral definitions lead to mathematically proper expressions that agree with the results from the rigorous construction [J. Math. Phys. 39, 170–206 (1998)] of the Abelian CS topological quantum field theory via geometric quantization.
Convective instability in superposed fluid and anisotropic porous layers
Authors: Chen Falin , Chen C. F. , Pearlstein Arne J.
Published: July 2002

The onset of thermal convection due to heating from below in a system consisting of a fluid layer overlying a porous layer with anisotropic permeability and thermal diffusivity is studied. Flow in the porous medium is assumed to be governed by Darcy’s law; the Beavers–Joseph condition is applied at the interface between the two layers. The linear perturbation equations are solved numerically. It is found that the effects of anisotropy on the onset of thermal convection are most profound for small values of the depth ratio ζ (ratio of fluid layer thickness to porous layer thickness), since in that case, the onset of convection corresponds to significant motion in both layers. For fixed values of the vertical permeability in the porous medium, decreasing the value of ξ (ratio of horizontal to vertical permeability) leads to stabilization of the superposed layer configuration because of increased resistance to motion in the porous medium. For larger values of ζ, the onset of motion is increasingly confined to the fluid layer, with the transport of heat through the porous layer occurring primarily by conduction. Accordingly, the influence of ξ on the stability characteristics for larger ζ is less significant than the effects of an anisotropic thermal conductivity.
The mean flow of a laminar wall-jet subjected to blowing or suction
Authors: Amitay M. , Cohen J.
Published: July 2002

The mean flow of an incompressible laminar two-dimensional air wall jet subjected to boundary suction or blowing is investigated experimentally. Hot-wire measurements of the streamwise mean velocity profiles, the downstream development of the local boundary-layer thickness, and the local maximum streamwise velocity, confirm previous theoretical self-similar solutions in which the vertical velocity at the wall varies as a power of the downstream distance.
The dynamics of small, heavy, rigid spherical particles in a periodic Stuart vortex flow
Authors: Tio Kek-Kiong , Gañán-Calvo Alfonso M. , Lasheras Juan C.
Published: July 2002

The equation of motion of small, heavy, rigid spherical particles in a periodic Stuart vortex flow is studied as a four-dimensional nonlinear dynamical system with a parametric space of five dimensions. The five parameters are a scaled Reynolds number, the Stokes number, the fluid-to-particle density ratio, the vorticity distribution in the flow, and a gravitational parameter. Depending on the values of these parameters, heavy particles may either settle or remain indefinitely suspended against gravity. When suspension occurs, suspended particles asymptotically collect along periodic, quasiperiodic, or chaotic open trajectories located above or below the vortices. The nature of these asymptotic paths is investigated using the standard tools of power spectrum and bifurcation diagram (Poincaré sections). Furthermore, the basins of attraction in the physical and parametric spaces are also computed for both types of suspensions (above and below the vortices). In addition to the two types of upper and lower asymptotic orbits, the dynamical system of this study also exhibits the phenomenon of intermittency, whereby a particle remains suspended alternately above and below the vortices. Apart from open trajectories, closed orbits encircling one or several vortices are also observed in this work.
Diffusion of magnetic field lines in a toroidal geometry
Authors: Mendonça J. T.
Published: July 2002

Diffusion of stochastic magnetic field lines is studied in a toroidal geometry. General expressions for the radial diffusion coefficient are obtained in various physically important situations. The Poincaré representation of magnetic field lines is reduced to discrete mappings and the corresponding diffusion coefficients are derived. The diffusion processes taking place near the threshold to global stochasticity are also discussed.
Wave heating at the third harmonic of the ion-cyclotron frequency
Authors: Swanson D. G.
Published: July 2002

For heating of fusion reactors, heating at the third harmonic of deuterium is shown to be an attractive possibility since absorption is still strong and the plasma core is accessible. The higher frequency also makes the application of waveguide launchers more attractive because of the smaller size. The calculation model is a deuterium–tritium plasma with a small α component. The dispersion relation leads to a sixth-order mode conversion equation solved by the Green function formalism which converts it to an integral equation. Numerical studies for the effects of absorption find the analytic transmission coefficient to be exact, and empirical formulas are given for the reflection and conversion coefficients and the power balance between the deuterium and helium. Conservation of energy is also discussed in some detail.
Analytic theory of resistive ballooning modes
Authors: Drake J. F. , Antonsen T. M.
Published: July 2002

The linearized resistive magnetohydrodynamic (MHD) equations have been solved analytically in a large-aspect-ratio toroidal plasma to obtain a dispersion relation for resistive ballooning modes. The calculation is limited to values of β=8πnT/B2 below the threshold for ideal ballooning modes. The dispersion relation yields a universal curve which can be used to determine mode stability as a function of the plasma parameters. Because of the strong stabilizing influence of plasma compressibility, resistive-ballooning modes are only unstable in tokamaks which are either quite cold or are very close to the ideal threshold.
Rapid growth and collapse of single bubbles in polymer solutions undergoing shear
Authors: Kezios Peter S. , Schowalter W. R.
Published: July 2002

The dynamics of single bubbles have been studied when the bubbles are generated by focusing the energy discharged from a Q-switched laser into a small volume of dilute polymer solution undergoing shear between coaxial cylinders, one of which is rotating at a constant rate. Because of the shearing the bubbles are nonspherical. In accord with theoretical predictions the nonsphericity is reduced by approximately a factor of two when polymer is present at concentrations below 0.1% and the rate of shear in the fluid at the time of bubble inception is below 300 sec−1. A bubble is formed from the laser discharge on a time scale of nanoseconds. Bubble growth and collapse occur in approximately 1 msec. Because the event occurs over such a short time, it is believed that the results are relevant to the large alteration in cavitation behavior that is observed when polymer is added to fluid flowing past a bluff body in a water tunnel.
Linear stability analysis of compact toroidal plasmas using particle simulation
Authors: Mikić Zoran , Morse Edward C.
Published: July 2002

A technique is presented for the linear three-dimensional stability analysis of plasmas in which ion kinetic effects are important. This technique is appropriate for the analysis of compact toroidal plasmas such as spheromaks, field-reversed mirrors and theta pinches, field-reversed configurations, and ion rings and layers. The plasma is modeled by the hybrid quasineutral model, in which the ions are represented by particles. An initial value approach is used to find the most rapid instabilities, in conjuction with the numerical integration of the equations. The model is verified against known analytic and numerical results for the linear stability of ion layers and theta pinches. The tilt instability in the spheromak is investigated, and comparison is made to magnetohydrodynamic (MHD) stability results for an equilibrium with a low ion beta.
One-and-a-quarter-dimensional transport modeling of the field-reversed configuration
Authors: Werley K. A.
Published: July 2002

A radial time-dependent plasma transport code to be used for numerically modeling field-reversed configurations is described. The plasma model assumes two-fluid magnetohydrodynamics and includes a description of plasma on both the closed and open magnetic field lines, as well as such physics effects as axial contraction of the separatrix, flux surface averaging, and particle and energy losses along the open magnetic field lines. This code was applied to two typical discharges (5 and 20 mTorr initial fill pressures) of the FRX-C field-reversed configuration[Fusion Tech. 9, 13 (1986); Phys. Fluids 27, 1514 (1984)] experiment at Los Alamos. Simulation results and values of the transport coefficients are reported.
Turbulent and inertial roll waves in inclined film flow
Authors: Hwang Shyh-Hong , Chang Hsueh-Chia
Published: July 2002

Conditions for the onset of high Reynolds number roll waves on inclined interfaces are sought. The model equations of Dressler [Commun. Pure Appl. Math. 2, 149 (1949)] and Needham and Merkin [Proc. R. Soc. London Ser. A 394, 259 (1984)] are analyzed using dynamic singularity theory (normal form techniques) and numerical methods. A new family of roll-wave solutions is discovered. They provide bounds and averages of the velocities of all roll waves at a given Froude number. These are favorably compared to the data of Brock [Proc. Am. Soc. Civ. Eng. 12, 2565 (1970)] and Brauner and Maron [Int. J. Heat Mass Transfer 25, 99 (1982)]. The average wave velocity is demonstrated to be approximately 1.5 times the average fluid velocity.
Low-frequency stability for field reversed configuration parameters
Authors: Krall N. A.
Published: July 2002

The stability of a slab plasma with β, connection length, and trapped particle fraction comparable to that found in a field reversed configuration (FRC) [in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1978), Vol. II, pp. 187 and 447] is examined with emphasis on density and temperature gradient driven modes. Three distinct persistent low-frequency instabilities are found for the weak gradient situation common to the FRC interior. Two of these modes specifically require the high β>1 and short connection distance found in the FRC. It is suggested that these low-frequency modes contribute to the observed anomalous flux loss and electron heat loss in FRC.
Beam optics of exploding foil plasma x-ray lasers
Authors: London Richard A.
Published: July 2002

In soft x-ray lasers, amplification is achieved as the x rays propagate down a long narrow plasma column. Refraction, due to electron density gradients, tends to direct the x-rays out of high density regions. This can have the undesirable effect of shortening the distance that the x ray stay within the plasma, thereby limiting the amount of amplification. The exploding foil design lessens refraction, but does not eliminate it. In this paper, a quantitative analysis of propagation and amplification in an exploding foil x-ray laser is presented. The density and gain profiles within the plasma are modeled in an approximate manner, which enables considerable analytic progress. It is found that refraction introduces a loss term to the laser amplification. The beam pattern from a parabolic gain profile laser has a dominant peak on the x-ray laser axis. The pattern from a quartic gain profile having a dip on-axis can produce a profile with off-axis peaks, in better agreement with recent experimental data.
Scaling properties of differential molecular diffusion effects in turbulence
Authors: Kerstein A. R. , Cremer M. A. , McMurtry P. A.
Published: July 2002

Scaling properties of a normalized concentration difference in a turbulent flow containing two scalars of unequal diffusivity are determined by similarity analysis and numerical simulation. Similarity hypotheses applied to the power spectrum of the normalized concentration difference, termed the differential diffusion, yield predicted dependences of the variance of the differential diffusion on the turbulence Reynolds number (Re) and on the Schmidt numbers (Sc) of the scalars. In particular, the variance is found to be proportional to Re−1/2. This and other predictions are supported by numerical simulations of multiple scalar mixing using a one-dimensional stochastic mixing model. The analysis and numerical results indicate fundamental distinctions between the physical mechanisms governing the scalar spectral cascade and those governing spectral transfer of the differential diffusion. The relationships of predicted scalings to passive mixing measurements that have been reported and to behaviors expected in reacting flow are noted.
The influence of hydrodynamic instability on the structure of cellular flames
Authors: Kadowaki Satoshi
Published: July 2002

The influence of hydrodynamic instability on the structure of two-dimensional (2D) and three-dimensional (3D) cellular flames is numerically investigated. The equation used is the compressible Navier–Stokes equation including a one-step irreversible chemical reaction. We superimpose an infinitesimal disturbance on the stationary plane flame and calculate the evolution of the disturbed flame front to obtain the relation between the growth rate and the wave number, i.e., the dispersion relation. With an increase in flame temperature, the growth rate increases since hydrodynamic instability becomes stronger. The unstable range normalized by the preheat zone thickness hardly changes, even though the flame temperature increases. The critical wave number, which corresponds to the maximum growth rate, is almost constant. Therefore, the normalized spacing between cells of the cellular flame is independent of the flame temperature. Moreover, we superimpose the disturbance with the critical wave number to investigate the structure of cellular flames. The stationary cellular flame is obtained when the inlet-flow velocity is set to the flame velocity of the cellular flame. The higher the flame temperature, the deeper the cell and the broader the flame surface. In addition, the cell depth and the surface area of the 3D flame are larger than those of the 2D flame. This is caused by the difference in the disposition of cells.
Two-dimensional particle simulation of Langmuir probe sheaths with oblique magnetic field
Authors: Bergmann A.
Published: July 2002

Two-dimensional particle simulations of the sheath in front of a flat Langmuir probe mounted into a particle absorbing plate are performed to study the influence of a strong magnetic field (relation between Larmor radii and Debye length: ρe≤λD, ρi≫λD) which is oriented obliquely to the probe surface. Ion-attracting probes are considered and the sheath is assumed to be collisionless. The full particle orbits in the homogeneous magnetic field and the self-consistent electric potential are calculated with a particle-in-cell (PIC) code with two spatial coordinates and three velocity components (2d,3v). The main results are: In the sheath the ion trajectories are bent towards the normal to the probe surface so that the ion flow is focused to the edges of the probe. This leads to an enhancement of the ion current as compared to the current flowing in the flux tube subtended by the probe. As a consequence the current does not saturate at large (negative) probe voltage, because the thickness of the Debye sheath, and thus the effective probe size, grows with increasing probe voltage. This effect is particularly strong if the sheath thickness is about as large, or larger than, the projection of the probe size along the magnetic field lines. These results can help to explain the ion-current nonsaturation found in recent measurements with Langmuir probes in the boundary layer of magnetically confined plasmas.
Tomography of (2, 1) and (3, 2) magnetic island structures on Tokamak Fusion Test Reactor
Authors: Nagayama Y. , Taylor G. , Fredrickson E. D. , Budny R. V. , Janos A. C. , Mansfield D. K. , McGuire K. M. , Yamada M.
Published: July 2002

High-resolution electron cyclotron emission (ECE) image reconstruction has been used to observe (m,n)=(2,1) and (3, 2) island structures on Tokamak Fusion Test Reactor [Plasma Phys. Controlled. Fusion 33, 1509 (1991)], where m and n are the poloidal and the toroidal mode number, respectively. The observed island structure is compared with other diagnostics, such as soft x-ray tomography and magnetic measurements. A cold elliptic island is observed after lithium pellet injection. Evidence for the enhancement of the heat transfer due to the island is observed. A relaxation phenomenon due to the m=2 mode is newly observed in Ohmic plasmas.
Dependence of stimulated Brillouin scattering on focusing optic F number in long scale-length plasmas
Authors: Watt R. G. , Cobble J. , DuBois D. F. , Fernandez J. C. , Rose Harvey A. , Drake R. P. , Bauer Bruno S.
Published: July 2002

The onset of stimulated Brillouin scattering (SBS) has been examined by varying the effective F number (lens focal length/incident beam diameter) of the optical system used to irradiate a performed 1 mm long, longitudinally uniform hydrocarbon (CH) plasma. SBS onset occurs at higher average laser intensity for smaller F, in qualitative agreement with theory. Quantitative comparison between theory and experiment, and implications for the National Ignition Facility [Phys. Plasmas 2, 2480 (1995)] are discussed.
Tomography of full sawtooth crashes on the Tokamak Fusion Test Reactor
Authors: Nagayama Y. , Yamada M. , Park W. , Fredrickson E. D. , Janos A. C. , McGuire K. M. , Taylor G.
Published: July 2002

Full sawtooth crashes in high temperature plasmas have been investigated on the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Fusion 33, 1509 (1991)]. A strong asymmetry in the direction of major radius, a feature of the ballooning mode, and a remaining m=1 region after the crash have been observed with electron cyclotron emission image reconstructions. The TFTR data is not consistent with two-dimensional (2-D) models; it rather suggests a three-dimensional (3-D) localized reconnection arising on the bad curvature side. This process explains the phenomenon of fast heat transfer which keeps the condition q0<1.
Angular momentum injection into a Penning–Malmberg trap
Authors: Fitzpatrick Richard , Yu Edmund P.
Published: July 2002

It is demonstrated using conventional fluid theory that angular momentum can be injected into a single component plasma confined in a Penning–Malmberg trap via an externally generated, oscillating, nonaxisymmetric, electric field. The torque exerted on the plasma by the electric field is a highly nonmonotonic function of the plasma angular rotation velocity. The torque vs angular velocity curve is dominated by sharp resonances at which the angular phase velocity of a particular poloidal harmonic of the external field matches the plasma angular rotation velocity. The torque exerted on the plasma by a given poloidal harmonic is negative when the field rotates faster than the plasma, and vice versa. This rather surprising behavior is shown to be entirely consistent with a standard result in hydrodynamic theory, but is generally not observed in present-day experiments.
Observation of resonant energy transfer between identical-frequency laser beams
Authors: Wharton K. B. , Kirkwood R. K. , Glenzer S. H. , Estabrook K. G. , Afeyan B. B. , Cohen B. I. , Moody J. D. , MacGowan B. J. , Joshi C.
Published: July 2002

Enhanced transmission of a low intensity laser beam is observed when crossed with an identical-frequency beam in a plasma with a flow velocity near the ion sound speed. The time history of the enhancement and the dependence on the flow velocity strongly suggest that this is due to energy transfer between the beams via a resonant ion wave with zero frequency in the laboratory frame. The maximum energy transfer has been observed when the beams cross in a region with Mach 1 flow. The addition of frequency modulation on the crossing beams is seen to reduce the energy transfer by a factor of 2. Implications for indirect-drive fusion schemes are discussed.
Impact ionization coefficients in (111) InP
Authors: Tabatabaie N. , Robbins V. M. , Pan N. , Stillman G. E.
Published: July 2002

High quality avalanche photodetectors with optical access to both sides of the junction have been fabricated in (111) InP. Impact ionization measurements were performed on these devices yielding the ionization coefficients for electrons and holes along 〈111〉. The results are similar to the measured values in the 〈100〉 direction indicating that the ionization coefficients are isotropic.
Molecular beam epitaxial growth and transmission electron microscopy studies of thin GaAs/InAs(100) multiple quantum well structures
Authors: Grunthaner F. J. , Yen M. Y. , Fernandez R. , Lee T. C. , Madhukar A. , Lewis B. F.
Published: July 2002

We report successful molecular beam epitaxial growth of thin multiple quantum well structures of GaAs/InAs(100) involving 7.4% lattice mismatch. Cross-sectional transmission electron microscopy studies reveal well formed interfaces and low defect density.
Use of a rapid anneal to improve CaF2:Si (100) epitaxy
Authors: Pfeiffer Loren , Phillips Julia M. , Smith T. P. , Augustyniak W. M. , West K. W.
Published: July 2002

Post-anneals of short duration at high temperature are shown to improve significantly the quality of CaF2 films on Si (100). An anneal at 1100 °C for 20 s in an Ar ambient reduced χmin, the ratio of backscattered 1.8-MeV 4He+ ions in the aligned to random direction, from 0.26 for an as-grown CaF2 film to 0.03 following the post-anneal. This is the best χmin yet reported for the CaF2:Si (100) system. The post-anneal films also show improved chemical, mechanical, and electrical properties.
High resolution electron microscope study of epitaxial CdTe-GaAs interfaces
Authors: Otsuka N. , Kolodziejski L. A. , Gunshor R. L. , Datta S. , Bicknell R. N. , Schetzina J. F.
Published: July 2002

CdTe films have been grown on (100) GaAs substrates with two different epitaxial relations: (111)CdTe∥(100)GaAs and (100)CdTe∥(100)GaAs. High resolution electron microscope observation of these two types of interfaces was carried out in order to investigate the role of the substrate surface microstructure in determining which type of epitaxy occurs. The interface of the former type shows a direct contact between the CdTe and GaAs crystals, while the interface of the latter type has a very thin layer (∼10 Å in thickness), which is most likely an oxide, between the two crystals. These observations suggest that the GaAs substrate preheating cycle prior to CdTe film growth is crucial in determining which type of epitaxy occurs in this system.
Antiphase boundaries in GaAs
Authors: Cho N.-H. , De Cooman B. C. , Carter C. B. , Fletcher R. , Wagner D. K.
Published: July 2002

Antiphase boundaries in GaAs have been produced by growing the GaAs on {001} Ge substrates. The GaAs was grown by the technique of organometallic vapor phase epitaxy to a thickness in excess of 1 μm. The antiphase boundaries are shown to be faceted with facets parallel to the {110} planes being particularly common. The rigid-body translation at the different facet planes is shown to be small for the {110} planes but it can be large for other facet planes.
Use of a plasma temporal attenuator to investigate the pulse length dependence of CO2 laser induced damage
Authors: Gibson D. R. , Wilson A. D.
Published: July 2002

A surface initiated plasma temporal attenuator enabling production of CO2 laser pulses with tunable durations (20–90 ns), similar temporal profiles, and near Gaussian spatial profiles, is described. Use of the technique to investigate the pulse length dependence and thereby elucidate the mechanism of CO2 laser damage is demonstrated for uncoated and diamondlike carbon antireflection coated n-type Ge.
GaInAs/InP quantum wells grown by organometallic vapor phase epitaxy
Authors: Kuo C. P. , Fry K. L. , Stringfellow G. B.
Published: July 2002

Single quantum well InP/Ga0.47In0.53As/InP structures have been fabricated using atmospheric pressure organometallic vapor phase epitaxy. The reactants were trimethylgallium, trimethylindium, arsine, and phosphine. The structures were characterized mainly using low-temperature (4 K) photoluminescence (PL). Three major changes in the PL emission were observed as the well thickness was decreased from 500 to 62 Å. The PL peak energy was observed to increase by 41 meV, slightly less than predicted from simple calculations. The PL intensity was found to increase, being proportional to the reciprocal well thickness. The PL intensity from the 62 Å well was found to be approximately 100× higher than for 1–2-μm-thick Ga0.47In0.53 As epitaxial layers. Finally, the half-width of the single PL peak was found to increase with decreasing well width. This was tentatively interpreted in terms of well size fluctuations due to an approximately 20 Å compositional transition at each interface.
Hole-mediated chemisorption of atomic hydrogen in silicon
Authors: Pankove J. I. , Magee C. W. , Wance R. O.
Published: July 2002

It has been shown that at 120 °C atomic hydrogen diffuses into p-type silicon where it ties to a Si dangling bond near an acceptor. However, a thin n-type surface layer blocks the entry of atomic hydrogen. This demonstrates that free holes in the surface layer are needed to permit the entry of atomic hydrogen.
New high-speed long-wavelength Al0.48In0.52As/Ga0.47In0.53As multiquantum well avalanche photodiodes
Authors: Mohammed K. , Capasso F. , Allam J. , Cho A. Y. , Hutchinson A. L.
Published: July 2002

We report the operation of a new long-wavelength (λ=1.3 μm) superlattice avalanche photodiode. The p+in+ structure, grown by molecular beam epitaxy, consists of a 35 period Al0.48In0.52As (139 Å)/Ga0.47In0.53As (139 Å) multiquantum well i region sandwiched between p+ and n+-Al0.48In0.52As transparent layers. dc and high-frequency multiplications of 32 and 12, respectively, have been measured; the dark current at unity gain is 70 nA. High speed of response with full width at half-maximum of 220 ps at a gain of 12 and the absence of tails are demonstrated, indicating that carrier trapping in the wells is negligible. In addition, detailed studies of the spectral response for samples with thin layers (103 Å) show extremely clearly the effect of the quantum states of the wells on the photocurrent at room temperature.
Lateral p-n junction formation in GaAs molecular beam epitaxy by crystal plane dependent doping
Authors: Miller D. L.
Published: July 2002

Lateral p-n junctions have been formed during the growth of GaAs by molecular beam epitaxy. Using a GaAs substrate which consisted of side-by-side {100} and {111} A planes formed by plane-selective etching, Si-doped GaAs was found to grow with p-type conductivity on the {111} A planes, while n-type material was grown simultaneously on the {100} planes. n-p-n structures with p-type regions as narrow as 2 μm were grown. Conductivity measurements on narrow stripes of p-type material formed by GaAs growth on {111} A planes indicate that Si was efficiently incorporated as an acceptor under the growth conditions used for this experiment.
Thin SiO2 insulators grown by rapid thermal oxidation of silicon
Authors: Moslehi Mehrdad M. , Shatas Steven C. , Saraswat Krishna C.
Published: July 2002

Rapid thermal oxidation of 〈100〉 silicon in dry oxygen ambient has been performed in a lamp-heated rapid thermal processing system. For the first time we report fairly extensive experimental results on the initial regime of thermal oxidation of silicon by the rapid thermal oxidation technique. The results clearly indicate the nonlinear behavior of the rapid thermal oxidation growth kinetics in the short time regime, in contrast to some recent rapid thermal oxidation data in the literature which suggested linear growth kinetics. The kinetics data show an increasing growth rate as the rapid thermal oxidation time is reduced, the highest oxidation rate being for the shortest oxidation time. It is also revealed that simple extrapolation of the existing long time thin oxide growth models to the short time rapid thermal oxidation regime does not give a sufficiently precise prediction of the kinetics in the short time regime where other transient physical processes besides simple one-species oxidant diffusion and interface reaction may play an important role.
Remote monitoring of ammonia using a CO2 laser and infrared fibers
Authors: Simhony S. , Katzir A.
Published: July 2002

Silver halide (AgCl:AgBr) fibers were used for transmitting infrared radiation from a CO2 laser to an absorption cell, and from the cell to an infrared detector. The absorption of CO2 laser lines in ammonia (NH3) gas was measured in the remote cell. Use of infrared transmitting fibers for spectroscopy in the mid and far infrared and for remote sensing of gases, was demonstrated for the first time.
Temperature dependence and hysteresis of Young’s modulus in a graphite/aluminum metal matrix composite
Authors: Liu John M.
Published: July 2002

Some unusual behavior in the temperature dependence of the Young’s modulus along the direction of the reinforcing graphite fibers in a graphite/aluminum composite has been observed. The decrease of the Young’s modulus with decreasing temperature as measured by ultrasonic techniques, utilizing noncontacting electromagnetic acoustic transducers, is shown to be due to the strong dependence of the fiber modulus on stress, which existed as a result of the mismatch between the coefficients of thermal expansion for the fiber and the matrix. Results appear significant in the selection of suitable combination of fibers and matrices for achieving thermally stable composites.
Interaction of TiSi2 layers with polycrystalline Si
Authors: Zheng L. R. , Hung L. S. , Feng S. Q. , Revesz P. , Mayer J. W. , Miles G.
Published: July 2002

Interactions of silicide films with undoped polycrystalline layers of Si grown by chemical vapor deposition at 630 °C were investigated by MeV He ion backscattering spectrometry, scanning electron microscopy, and transmission electron microscopy. For TiSi2, heat treatment in vacuum at temperatures above 850 °C results in erosion of the polycrystalline Si layer and growth of Si crystallites in the silicide film. The same phenomenon is observed for NiSi, Pd2Si, and CrSi2 at temperatures above one-half of melting point of the corresponding silicide.
Molecular beam epitaxial growth of a novel strained layer type III superlattice system: HgTe-ZnTe
Authors: Faurie J. P. , Sivananthan S. , Chu X. , Wijewarnasuriya P. S.
Published: July 2002

HgTe-ZnTe strained type III superlattices have been grown for the first time using the molecular beam epitaxy technique. Three superlattices grown at 185 °C have been characterized by electron and x-ray diffraction, infrared absorption, and Hall measurements. The presence of satellite peaks in the x-ray spectra shows that the superlattices are of good quality despite the large lattice mismatch between HgTe and ZnTe (Δa/a=6.5%). These superlattices are p type and the hole mobilities are very high compared to those of the corresponding alloy. Such a phenomenon has already been reported for HgTe-CdTe superlattices. Infrared transmission spectra show that HgTe-ZnTe superlattices have narrower band gaps than equivalent HgZnTe alloys.
New laterally selective growth technique by metalorganic chemical vapor deposition
Authors: Bedair S. M. , Tischler M. A. , Katsuyama T.
Published: July 2002

Laterally selective growth of III-V compounds has been successfully demonstrated by metalorganic chemical vapor deposition. This was achieved by using a specially designed growth chamber and susceptor that allows the substrate to move with respect to a stationary GaAs or Si mask. We have used this technique to selectively deposit GaAs1−xPx with different values of x and a GaAs-GaAsP superlattice on a single GaAs substrate. We have also selectively grown multiple color light-emitting diodes on a GaAs substrate.
AlGaAs/GaAs(111) heterostructures grown by molecular beam epitaxy
Authors: Vina L. , Wang W. I.
Published: July 2002

We have grown AlGaAs/GaAs heterostructures on (111) oriented GaAs substrates by molecular beam epitaxy. Materials with good optical and electrical properties, including mobility enhancement in two-dimensional electron and hole gases, have been obtained for the first time.
Ridge waveguide AlGaAs/GaAs distributed feedback lasers with multiple quantum well structure
Authors: Noda S. , Kojima K. , Mitsunaga K. , Kyuma K. , Hamanaka K. , Nakayama T.
Published: July 2002

Ridge waveguide AlGaAs/GaAs distributed feedback lasers with multiple quantum well structure were fabricated for the first time. The threshold current of 28 mA, which is the lowest ever reported among AlGaAs/GaAs distributed feedback lasers, was obtained at room temperature. Stable single longitudinal and transverse mode oscillation was observed over the wide temperature range. The dynamic linewidth was also measured and it was five to six times smaller than that of a double heterostructure distributed feedback laser.