bluuebunny/crossref_metadata_2025 huggingface dataset:
Representation-theoretic aspects of two-dimensional quantum systems in singular vector potentials: Canonical commutation relations, quantum algebras, and reduction to lattice quantum systems
Authors: Arai Asao
Published: July 2002

Some representation-theoretic aspects of a two-dimensional quantum system of a charged particle in a vector potential A, which may be singular on an infinite discrete subset D of R2 are investigated. For each vector v in a set V(D)⊂R2\{0}, the projection Pv of the physical momentum operator P≔p−αA to the direction of v is defined by Pv≔v⋅P as an operator acting in L2(R2), where p=(−iDx,−iDy)[(x,y)∈R2] with Dx (resp., Dy) being the generalized partial differential operator in the variable x (resp., y) and α∈R is a parameter denoting the charge of the particle. It is proven that Pv is essentially self-adjoint and an explicit formula is derived for the strongly continuous one-parameter unitary group {eitP̄v}t∈R generated by the self-adjoint operator P̄v (the closure of Pv), i.e., the magnetic translation to the direction of the vector v. The magnetic translations along curves in R2\D are also considered. Conjugately to Pv and Pw [w∈V(D)], a self-adjoint multiplication operator Qv,w is introduced, which is a linear combination of the position operators x and y, such that, if A is flat on R2\D, then πv,wA≔{Qv,w,Qw,v,Pv,Pw} gives a representation of the canonical commutation relations (CCR) with two degrees of freedom. Properties of the representation πv,wA are analyzed. In particular, a necessary and sufficient condition for πv,wA to be unitarily equivalent (or inequivalent) to the Schrödinger representation of CCR is established. The case where πv,wA is inequivalent to the Schrödinger representation corresponds to the Aharonov–Bohm effect. Quantum algebraic structures [quantum plane and the quantum group Uq(sl2)] associated with the pair {P̄v,P̄w} are also discussed. Moreover, for every A in a class of vector potentials having singularities on the infinite lattice L(ω1,ω2)≔{mω1+nω2|m,n∈Z} [the case D=L(ω1,ω2)], where ω1∈R2 and ω2∈R2 are linearly independent, it is shown that the magnetic translations eiP̄ωj, j=1,2, with A replaced by a modified vector potential are reduced by the Hilbert space l2(L(ω1,ω2)) identified with a closed subspace of L2(R2). This result, which may be regarded as one of the most important novel results of the present paper, establishes a connection of continuous quantum systems in vector potentials to lattice ones.
Connections and metrics respecting purification of quantum states
Authors: Dittmann J. , Uhlmann A.
Published: July 2002

Standard purification interlaces Hermitian and Riemannian metrics on the space of density operators with metrics and connections on the purifying Hilbert–Schmidt space. We discuss connections and metrics which are well adopted to purification, and present a selected set of relations between them. A connection, as well as a metric on state space, can be obtained from a metric on the purification space. We include a condition, with which this correspondence becomes one to one. Our methods are borrowed from elementary *-representation and fiber space theory. We lift, as an example, solutions of a von Neumann equation, write down holonomy invariants for cyclic ones, and “add noise” to a curve of pure states.
On form-preserving transformations for the time-dependent Schrödinger equation
Authors: Finkel Federico , González-López Artemio , Kamran Niky , Rodrı́guez Miguel A.
Published: July 2002

In this paper we point out a close connection between the Darboux transformation and the group of point transformations which preserve the form of the time-dependent Schrödinger equation (TDSE). In our main result, we prove that any pair of time-dependent real potentials related by a Darboux transformation for the TDSE may be transformed by a suitable point transformation into a pair of time-independent potentials related by a usual Darboux transformation for the stationary Schrödinger equation. Thus, any (real) potential solvable via a time-dependent Darboux transformation can alternatively be solved by applying an appropriate form-preserving point transformation of the TDSE to a time-independent potential. The pre-eminent role of the latter type of transformations in the solution of the TDSE is illustrated with a family of quasi-exactly solvable time-dependent anharmonic potentials.
Solutions with intersecting p-branes related to Toda chains
Authors: Ivashchuk V. D. , Kim Sung-Won
Published: July 2002

Solutions in multidimensional gravity with m p-branes related to Toda-type systems (of general type) are obtained. These solutions are defined on a product of n+1 Ricci-flat spaces M0×M1×⋯×Mn and are governed by one harmonic function on M0. The solutions are defined up to the solutions of Laplace and Toda-type equations and correspond to null-geodesics of the (sigma-model) target-space metric. Special solutions relating to Am Toda chains (e.g., with m=1,2) are considered.
Bondi-type systems near spacelike infinity and the calculation of the Newman–Penrose constants
Authors: Friedrich Helmut , Kánnár János
Published: July 2002

We relate Bondi systems near spacelike infinity to another type of gauge conditions. While the former are based on null infinity, the latter are defined in terms of Einstein propagation, the conformal structure, and data on some Cauchy hypersurface. For a certain class of time symmetric space–times we study an expansion which allows us to determine the behavior of various fields arising in Bondi systems in the region of space–time where null infinity touches spacelike infinity. The coefficients of these expansions can be read off from the initial data. We obtain, in particular, expressions for the constants discovered by Newman and Penrose in terms of the initial data. For this purpose we calculate a certain expansion introduced by Friedrich [J. Geom. Phys. 24, 83–163 (1998)] up to third order.
A simple expression for the terms in the Baker–Campbell–Hausdorff series
Authors: Reinsch Matthias W.
Published: July 2002

A simple expression is derived for the terms in the Baker–Campbell–Hausdorff series. One formulation of the result involves a finite number of operations with matrices of rational numbers. Generalizations are discussed.
Turbulence in a randomly stirred fluid
Authors: Panda Raj , Sonnad Vijay , Clementi Enrico , Orszag Steven A. , Yakhot Victor
Published: July 2002

The results of direct numerical simulations of a fluid stirred by the Gaussian random force 〈 fifj 〉 ∝k−3 are reported. It is shown that the mean properties of the random force generated turbulence are reasonably close to those observed in physical experiments. It is shown that higher-order moments of temporal and spatial velocity derivatives are dominated by different factors. It is shown that there is an intermediate range of wavenumbers with spatial and temporal spectra that fit E(k)∝k−5/3 and E(ω)∝ω−2. This suggests the breakdown of the random Taylor hypothesis in the inertial range of homogenous, isotropic turbulence at moderate Reynolds numbers.
The delay of turbulent boundary layer separation by oscillatory active control
Authors: Katz Y. , Nishri B. , Wygnanski I.
Published: July 2002

The flow outside a solid wedge that abruptly diverges at an angle of 18° was investigated experimentally. A turbulent boundary layer, which separated at the discontinuity and turned into a mixing layer downstream of it, reattached as a result of harmonic excitation at the apex of the wedge. Preliminary results indicate that this might be an effective way to delay separation of turbulent and laminar boundary layers.
Monte Carlo study of Knudsen layers in evaporation from elemental and binary media
Authors: Sibold Dieter , Urbassek Herbert M.
Published: July 2002

By Monte Carlo simulation the Knudsen layer in front of a surface from which atoms evaporate is studied. Evaporation into a vacuum is simulated by means of an evaporation–condensation geometry. Hard sphere interaction cross sections are employed. With the help of the present simulation data, the Knudsen layer is defined as that region adjacent to the evaporating surface, where the temperature of the flow parallel and perpendicular to the flow direction deviate by at least a given resolution δ. Taking δ=1%, it is found that the Knudsen layer is established after 800 mean-free flight times; it has an extension of 20 mean-free paths. It takes 60 monolayers to desorb before a Knudsen layer is formed. The data are generally in good agreement with predictions of analytical theory, where available. The differences observed in the case of evaporation from a binary target are discussed.
The structure and dynamics of electrostatic and magnetostatic drift holes
Authors: Terry P. W. , Diamond P. H. , Hahm T. S.
Published: July 2002

Localized, nonwavelike phase-space density fluctuations with self-trapping potentials, called drift holes, are studied for systems with density gradients. The three-dimensional structure of maximally probable, isolated electrostatic drift holes is determined and shown to satisfy a condition for self-trapping. The dynamics of such structures is examined. It is shown that under a broad set of conditions the potential amplitude, hole depth, and velocity-space width grow nonlinearly while maintaining the self-trapping structure. The nonlinear instability responsible for growth requires dissipative coupling between the trapped particles and the passing particles of the opposite species. Two saturation mechanisms for this instability are described and saturated amplitude estimates are given. It is shown that when the hole velocity is small compared to the speed of light, self-consistent coupling of stationary magnetic perturbations to the electrostatic potential is possible and produces structures called magnetostatic drift holes. The role of electrostatic and magnetostatic drift holes as a constituent of intermittent plasma turbulence is discussed.
Numerical calculation of axisymmetric non-neutral plasma equilibria
Authors: Spencer Ross L. , Rasband S. N. , Vanfleet Richard R.
Published: July 2002

Efficient techniques for computing axisymmetric non-neutral plasma equilibria are described. These equilibria may be obtained either by requiring global thermal equilibrium, by specifying the midplane radial density profile, or by specifying the radial profile of ∫n dz. Both splines and finite-differences are used, and the accuracy of the two is compared by using a new characterization of the thermal equilibrium density profile which gives a simple formula for estimating the radial and axial gradient scale lengths of thermal equilibria. It is found that for global thermal equilibrium 1% accuracy is achieved with splines if the distance between neighboring splines is about two Debye lengths while finite differences require a grid spacing of about one-half Debye length to achieve the same accuracy.
Effect of finite ion gyroradius on the fire-hose instability in a high beta plasma
Authors: Yoon Peter H. , Wu C. S. , de Assis A. S.
Published: July 2002

In this paper, a generalized kinetic dispersion equation that supports various hydromagnetic waves and instabilities is derived. The general dispersion equation is derived under the usual assumption of hydromagnetic perturbations [i.e., ‖ω‖2≪Ωi2, and (kzνA/Ωi)2≪β∥i, where Ωi and νA are the ion gyrofrequency and Alfvén speed, respectively, and β∥i is the parallel ion beta], but for arbitrary values of the quantity λi=(k⊥ρ⊥i)2/2=(k⊥νA/Ωi)2 β⊥i/2 that appears in the dielectric tensor. Here, ρ⊥i refers to the mean ion gyroradius, and β⊥i is the perpendicular ion beta. Otherwise, the dispersion equation is fairly general with no additional approximation, such as ignoring certain off-diagonal dielectric tensor elements (which is usually done in the literature) employed. In the subsequent numerical analysis, special attention is paid to the fire-hose instability in a high beta plasma. The numerical results reveal that the conventional treatment of the fire-hose instability (i.e., taking zero ion gyroradius limit at the outset) is not adequate, and that the effect of finite ion gyroradius results in a significant enhancement of the growth rate over a large range of wave numbers.
Viscous flow induced by counter-rotating vortices
Authors: Ersoy S. , Walker J. D. A.
Published: July 2002

The nature of the viscous boundary-layer flow induced near a wall, caused by a pair of counter-rotating vortices above the wall, is investigated. Solutions for the unsteady flow that develops near the wall are obtained numerically for a variety of cases. The vortices are taken to be of equal strength with cores located at equal distances from the wall; depending upon the assumed sense of rotation, the vortices either move toward the wall or recede from it, creating a region of either inflow or outflow near the wall, respectively. The calculated results show that the adverse pressure gradient near the wall induced by the vortex motion gives rise to explosively growing regions of recirculating flow near the wall. The possible relevance of these results to Görtler vortex instability and turbulent flow near walls is discussed.
Chaotic behavior of interacting elliptical instability modes
Authors: Hellberg Carl S. , Orszag Steven A.
Published: July 2002

A three-mode projection of the Navier–Stokes equations for nonlinear perturbations to an elliptical vortex is studied numerically. It is found that, as the Reynolds number increases, the perturbations undergo a sequence of period doublings leading to chaos according to the Feigenbaum scenario [J. Statis. Phys. 19, 25 (1978); Phys. Lett. 74 A, 375 (1979)].
A three-dimensional description of solitary waves and their interaction in Marangoni–Bénard layers
Authors: Nepomnyashchy Alexander A. , Velarde Manuel G.
Published: July 2002

A dissipation-modified Boussinesq-like system of equations governing three-dimensional long wavelength Marangoni–Bénard oscillatory convection in a shallow layer heated from the air side is presented. Solitary waves and their oblique and head-on interactions are considered, thus leading to results that compare well with available experimental data.
The role of stationary cross-flow vortices in boundary-layer transition on swept wings
Authors: Joslin R. D. , Streett C. L.
Published: July 2002

The spatial evolution of stationary cross-flow-vortex packets in a laminar boundary layer on a swept wing is computed by direct numerical simulation with the incompressible Navier–Stokes equations. Steady suction and blowing at the wing surface is used to generate disturbances that are periodic and equally spaced in the spanwise direction. The initiated disturbances become unstable and lead to distinct stages of instability evolution. Initially, each vortex packet undergoes a region of chordwise and spanwise linear independent growth; then the individual packets coalesce downstream and lead to spanwise wave adjustments and linear superposition of adjacent packets; finally, the vortex packets reach sufficiently large amplitudes in later stages of disturbance development to nonlinearly interact, which results in the rapid growth of the disturbances. In this later stage, the low-velocity fluid near the wing surface is lifted out into the boundary layer and rolled over the high-speed fluid in the direction of the positive spanwise velocity component. Highly inflectional chordwise and spanwise velocity profiles are recorded in this region. Previous studies have demonstrated that these inflectional profiles spawn the rapid growth of secondary instabilities, which lead to the catastrophic breakdown to turbulence.
On the velocity field and tracer patterns in a twisted duct flow
Authors: Duchêne Ch. , Peerhossaini H. , Michard P. J.
Published: July 2002

The laminar flow in a twisted square duct is studied numerically to analyze the formation and reorganization of Dean roll cells under abrupt curvature change. The flow geometry consists of four 90° bends, the curvature plane of each making a 90° angle with that of its neighbor. Straight ducts are inserted between successive bends. The flow geometry studied here is a basic element of a twisted duct flow in which chaotic advection has been observed [Jones, Thomas, and Aref, J. Fluid Mech. 209, 335 (1989)]. Steady incompressible full Navier–Stokes equations have been solved by an approximate factorization technique. Vector plots of the secondary flow (Dean roll cells) show upstream diffusion of curvature effects. The upstream propagation is weak, however, and the Dean roll cells adopt the rotation protocol of the bend immediately at its entrance. Farther downstream, roll cells continue to develop and reach an axially invariant state before leaving the bend. A passive tracer was also introduced at the center of the entrance to the twisted duct. Tracer distribution was followed in the downstream direction and was visualized every 30°. Comparison of the secondary flow velocity vector plots and the corresponding tracer patterns shows that the tracer distribution lags behind the velocity field in adjusting to changes in curvature. Tracer patterns show strong stretching and folding, which lead to mixing enhancement.
Acoustic waves in unbounded shear flows
Authors: Chagelishvili G. D. , Khujadze G. R. , Lominadze J. G. , Rogava A. D.
Published: July 2002

The linear evolution of acoustic waves in a fluid flow with uniform mean density and uniform shear of velocity is investigated. The process of the mean flow energy extraction by the three-dimensional acoustic waves, stimulated by the non-normal character of the linear dynamics in the shear flow, is analyzed. The thorough examination of the dynamics of different physical variables characterizing the wave evolution is presented. The physics of gaining of the shear energy by acoustic waves is described.
Passive scalar spectrum in isotropic turbulence: Prediction by the Lagrangian direct-interaction approximation
Authors: Goto Susumu , Kida Shigeo
Published: July 2002

The Lagrangian direct-interaction approximation developed previously by the present authors [S. Kida and S. Goto, J. Fluid Mech. 345, 307 (1997)] is applied to a passive scalar field in isotropic turbulence. We examine the behavior of solutions to the resultant closure equations for the correlation function of the scalar field for arbitrary values of the Schmidt number, and show systematically that the solutions are completely consistent with the phenomenological theories on the scalar spectral function by Obukhov (1949), Corrsin (1951), Batchelor et al. (1959), and Batchelor (1959). The universal forms of the function in the statistically stationary state are obtained by solving the closure equations numerically in the whole wave number range for each case of moderate, extremely large, and small values of the Schmidt number.
Experiments on the motion of a solid sphere toward a wall: From viscous dissipation to elastohydrodynamic bouncing
Authors: Gondret P. , Hallouin E. , Lance M. , Petit L.
Published: July 2002

In the present study, we investigate experimentally the motion of a macroscopic (non-Brownian and noncolloidal) solid sphere falling under gravity into a viscous liquid toward a solid wall. We observe the transition from a nonbouncing to a bouncing regime when increasing the Stokes number St which characterizes the particle inertia. In the bouncing regime, the recording of the particle trajectories allows us to determine the coefficient of “elastic” restitution e above the transition. We observe that e first increases with St as predicted by Davis, Serayssol, and Hinch (1986) and seems to reach a plateau at high St.
Breakup of drops and bubbles translating through cylindrical capillaries
Authors: Borhan A. , Pallinti J.
Published: July 2002

We examine the shape deformation and breakup of air bubbles and viscous drops moving through vertical cylindrical capillaries under the action of pressure and/or buoyancy forces. Experimental observations of fluid particle shape are reported over a wide range of particle sizes and capillary numbers in a variety of two-phase systems. Four different modes of breakup are identified, and the critical conditions for the onset of various modes are examined. It is found that buoyancy forces can have a stabilizing effect on the breakup mechanism observed by Olbricht and Kung [Phys. Fluids 4, 134, (1992)] for low viscosity-ratio drops, wherein a growing indentation at the trailing end of the drop develops into a penetrating jet of outer phase fluid.
Spatial autoresonance cyclotron accelerator
Authors: Friedland Lazar
Published: July 2002

A mechanism of electron acceleration scheme utilizing the spatial autoresonance phenomenon in combined axial guide magnetic field and a traveling electromagnetic wave with adiabatically varying parameters is analyzed. The acceleration is achieved due to the self-tendency of the particles to stay in the relativistic gyroresonance with the wave, despite the variation of systems parameters. As compared to other traveling wave cyclotron acceleration schemes, the spatial autoresonance accelerator does not require a precise initial frequency matching and a rigid tuning of the guide magnetic field for sustaining the resonance. The method is illustrated in the case of a microwave driver with a tapered axisymmetric waveguide geometry.
Pulsed sheath dynamics in a small cylindrical bore
Authors: Sheridan T. E.
Published: July 2002

The temporal evolution of the plasma sheath in small cylindrical bores and planar gaps is calculated for zero-rise-time voltage pulses. The ions are modeled as a cold, collisionless fluid, and the problem is reduced to the solution of two coupled, first-order, ordinary differential equations. These equations are solved analytically for the planar case, and numerically for the cylindrical case. The maximum ion impact energies are 50% and 36.8% of the maximum potential drop in the planar and cylindrical cases, respectively. Ion impact energy decreases with the square of the radius of the bore.
Low-order longitudinal modes of single-component plasmas
Authors: Tinkle M. D. , Greaves R. G. , Surko C. M.
Published: July 2002

The low-order modes of spheroidal, pure electron plasmas have been studied experimentally, both in a cylindrical electrode structure and in a quadrupole trap. Comparison is made between measurements of mode frequencies, recent analytical theories, and numerical simulations. Effects considered include trap anharmonicity, image charges, and temperature. Quantitative agreement is obtained between the predictions and these measurements for spheroidal plasmas in the quadrupole trap. In many experiments on single-component plasmas, including antimatter plasmas, the standard diagnostic techniques used to measure the density and temperature are not appropriate. A new method is presented for determining the size, shape, average density, and temperature of a plasma confined in a Penning trap from measurements of the mode frequencies.
Modulational instability in dusty plasmas
Authors: Benkadda S. , Tsytovich V. N.
Published: July 2002

Self-consistent equations for ion density, envelope electric fields, and charge variations of dust particles are obtained for a dusty plasma. A new type of modulational instability of Langmuir waves related to dust charging processes is found.
An experimental counter-example to the local transport paradigm
Authors: Gentle K. W. , Bravenec R. V. , Cima G. , Gasquet H. , Hallock G. A. , Phillips P. E. , Ross D. W. , Rowan W. L. , Wootton A. J. , Crowley T. P. , Heard J. , Ouroua A. , Schoch P. M. , Watts C.
Published: July 2002

The response of a tokamak discharge to a sharp drop in edge temperature differs significantly from that expected from typical local transport models in several important respects. Laser ablation of carbon induces large (ΔT/T≤70%), rapid (<200 μs) electron temperature drops in the outermost region of the plasma, r/a≥0.9. This cold pulse proceeds through the outer plasma (r/a≥0.75), rapidly compared with power balance or sawtooth predictions. However, the pulse shrinks markedly thereafter, disappearing near r/a∼0.5. Within r/a∼0.3, the temperature rises promptly. The results are inconsistent with conventional local transport models; a nonlocal phenomenology, in which transport coefficients increase in the edge and decrease in the core, is suggested. The turbulence levels measured with a heavy ion beam probe increase near the edge but are unchanged in the core.
Spontaneous formation of zero magnetic field region near the axis of a high-β mirror device
Authors: Lotov K. V.
Published: July 2002

It is shown that in open field line geometry with high expansion ratios of the flux conserver there appears a region of zero magnetic field near the axis. Once the zero-field region is formed, all magnetic field is concentrated in the thin near-wall layer, where high gradients of plasma pressure are present as well. These gradients cause enhanced plasma and heat diffusion.
Linear perturbation growth at a shocked interface
Authors: Wouchuk Juan Gustavo , Nishihara Katsunobu
Published: July 2002

An analytic model for the Richtmyer-Meshkov instability in the linear regime is presented. Approximate formulas for the interface asymptotic velocity are obtained for weak incident shocks. When a rarefaction is reflected, a comparison with recent experiments is shown. The phase reversal is also discussed. The areal mass density perturbations are studied and they are found to grow asymptotically linearly with time. For strong shocks there is a shift due to the entropy disturbances behind the shock(s). The impulsive model is modified to consider the different accelerations exerted by the reflected and transmitted fronts. It agrees with the compressible solution in the weak shock limit.
Supernova hydrodynamics experiments on the Nova laser
Authors: Remington B. A. , Kane J. , Drake R. P. , Glendinning S. G. , Estabrook K. , London R. , Castor J. , Wallace R. J. , Arnett D. , Liang E. , McCray R. , Rubenchik A. , Fryxell B.
Published: July 2002

In studying complex astrophysical phenomena such as supernovae, one does not have the luxury of setting up clean, well-controlled experiments in the universe to test the physics of current models and theories. Consequently, creating a surrogate environment to serve as an experimental astrophysics testbed would be highly beneficial. The existence of highly sophisticated, modern research lasers, developed largely as a result of the world-wide effort in inertial confinement fusion, opens a new potential for creating just such an experimental testbed utilizing well-controlled, well-diagnosed laser-produced plasmas. Two areas of physics critical to an understanding of supernovae are discussed that are amenable to supporting research on large lasers: (1) compressible nonlinear hydrodynamic mixing and (2) radiative shock hydrodynamics.
Reduction of crossed-field diode transmitted current due to anode secondary emission
Authors: Gopinath V. P. , Vanderberg B. H.
Published: July 2002

The limiting current theory for planar crossed-field diodes has long been studied extensively for various emission energies and temperatures. However, experimental measurements of transmitted current have shown significant departure from theory. This paper attempts to explain the reduction in transmitted current from that expected in theory in terms of secondary electron emission created by electrons hitting the anode. It is proposed that the presence of the secondary electrons increases the charge density in the gap, thereby reducing the amount of current transmitted. A detailed secondary emission model is implemented in a particle-in-cell code to study current reduction. The effect of secondary electrons on charge density, and on the resultant electric field and potential is also presented.
A numerical study of rotating magnetic fields as a current drive for field reversed configurations
Authors: Milroy Richard D.
Published: July 2002

A fixed ion model has been developed to study the use of a Rotating Magnetic Field (RMF) as a current drive mechanism in a Field Reversed Configuration (FRC). This model is used to investigate the physics of RMF current drive in a parameter range of interest to two experiments at the University of Washington. Empirical expressions are found to characterize the critical RMF magnitude required for full penetration and the rate of RMF penetration. It is shown that in the presence of a strong anisotropic plasma resistivity, the direction and magnitude of the axial bias field can have a strong influence on the penetration of an RMF. Calculations that include the effects of realistic RMF antennae at finite radius are used to find the effects of coil spacing and positioning.
Mass-action control of AlGaAs and GaAs growth in molecular beam epitaxy
Authors: Van Hove J. M. , Cohen P. I.
Published: July 2002

GaAs evaporation during molecular beam epitaxy (MBE) is measured using reflection high-energy electron diffraction (RHEED). On the (001) surface there is a first-order phase transition from a 2 × 4 to 1 × 1 reconstruction. Upon crossing the phase boundary into the 1 × 1 structure, layer-by-layer evaporation of GaAs is observed. This evaporation affects the rate of growth of GaAs and AlxGa1−xAs by MBE. The dependence of the growth rate on substrate temperature and on As, Al, and Ga fluxes is followed by measuring RHEED intensity oscillations. The results agree quantitatively with the mass-action analysis of Heckingbottom [R. Heckingbottom, J. Vac. Sci. Technol. B 3, 572 (1985)]. At substrate temperatures above 850 K no differences between As2 and As4 incident fluxes are observed.
Near threshold sputtering of Si and SiO2 in a Cl2 environment
Authors: Oostra D. J. , van Ingen R. P. , Haring A. , de Vries A. E. , van Veen G. N. A.
Published: July 2002

Si sputtering yields and Si to SiO2 etch rate ratios have been determined by measuring the depth of the etched craters after Ar+ ion bombardment. The experiments have been performed with energies down to 50 eV both with and without Cl2. Surprisingly high Si sputtering yields are obtained in a Cl2 environment by low-energy Ar+ ions. Hence, the influence of Cl2 on the Si sputtering mechanism is much larger for low ion energies than for high ion energies. Whereas the Si sputtering yield is enhanced by the presence of Cl2, the SiO2 sputtering yield is hardly affected. Therefore, large differences in the etch rate (high selectivities) between Si and SiO2 are obtained at low ion energies.
InGaAs/InP multiple quantum well waveguide phase modulator
Authors: Koren U. , Koch T. L. , Presting H. , Miller B. I.
Published: July 2002

A double heterostructure ridge waveguide with multiple quantum well InGaAs/InP core and InP cladding layers has been operated as a phase modulator at 1.52 μm wavelength. The observed phase shift coefficient was 12°/V mm. With a 1-mm-long device we have achieved a half wavelength shift at 15 V bias and a maximum phase shift of 420° at 35 V. Quantum confined Stark effect has been observed in the shorter 1.49–1.52 μm wavelength region. The ability to obtain λ/2 modulation with a short device and relatively low voltage makes this device very attractive for practical applications.
Highly efficient Nd:YVO4 diode-laser end-pumped laser
Authors: Fields R. A. , Birnbaum M. , Fincher C. L.
Published: July 2002

We report on the first diode-laser (nominal 200-mW 10-stripe laser diode) end-pump lasing of Nd:YVO4. The lowest threshold (30 mW) and highest output power (120 mW) were observed from the Nd:YVO4 laser as compared to a similar Nd:YAG laser. Over 50% optical slope efficiency was obtained, the highest yet reported for diode pumping. This device performed at a 10% overall efficiency. Measurements over a 21-nm range show the advantages of the Nd:YVO4 pump absorption band.
Supervillin associates with androgen receptor and modulates its transcriptional activity
Authors: Ting Huei-Ju , Yeh Shuyuan , Nishimura Kazuo , Chang Chawnshang
Published: July 2002

Activation of androgen receptor (AR) via androgen in muscle cells has been closely linked to their growth and differentiation. Here, we report the cloning and characterization of supervillin (SV), a 205-kDa actin-binding protein, as an AR coregulator from the skeletal muscle cDNA library. Mammalian two-hybrid and glutathione
S
-transferase pull-down assays indicate a domain within SV (amino acids 594-1268) can interact with AR N terminus and DNA-binding domain–ligand-binding domain in a ligand-enhanced manner. Subcellular colocalization studies with fluorescence staining indicate SV can colocalize with AR in the presence of 5α-dihydrotestosterone in COS-1 cells. The functional reporter assays showed full-length SV and the SV peptide (amino acids 831-1281) within the interaction domain can enhance AR transactivation. Furthermore, SV can enhance the endogenous AR target gene, p27
KIP1
, expression in prostate PC-3(AR2) cells. SV preferentially enhanced AR rather than other tested nuclear receptors and could be induced by natural androgens better than other steroids. SV can also cooperate with other AR coregulators, such as ARA55 or ARA70, to enhance AR transactivation further. Unlike SRC-1 that can enhance the interaction between AR N terminus and AR C terminus, SV shows a mild suppressive effect on N–C interactions, suggesting SV may go through a different mechanism to enhance AR transactivation. Together, our data demonstrate that SV is an AR coregulator that can enhance AR transactivation in muscle and other cells.
Polycystic kidneys and chronic inflammatory lesions are the delayed consequences of loss of the suppressor of cytokine signaling-1 (SOCS-1)
Authors: Metcalf Donald , Mifsud Sandra , Di Rago Ladina , Nicola Nicos A. , Hilton Douglas J. , Alexander Warren S.
Published: July 2002

Mice with inactivation of the gene encoding the suppressor of cytokine signaling-1 (SOCS-1) die in neonatal life with an IFN-γ-dependent inflammatory disease dominated by fatty degeneration and necrosis of the liver. To establish the long-term pathological consequences of loss of SOCS-1 in mice, where initial survival was made possible by also deleting the IFN-γ gene, a comparison was made of the lifespan of groups of SOCS-1−/−IFN-γ−/−, SOCS-1+/+IFN-γ−/−and SOCS-1+/+IFN-γ+/+mice. Mice lacking the genes for both SOCS-1 and IFN-γ exhibited an accelerated death rate compared with control groups. Disease states developing selectively in SOCS-1−/−IFN-γ−/−mice were polycystic kidneys, pneumonia, chronic skin ulcers, and chronic granulomas in the gut and various other organs. Mice of all three groups developed cataracts, but disease development was accelerated in the groups lacking IFN-γ. SOCS-1−/−IFN-γ−/−mice exhibited a slightly increased predisposition to the development of T lymphoid leukemia, either spontaneous or radiation-induced. The development of polycystic kidneys may be caused by a developmental defect in renal-tubule organization noted in neonatal SOCS-1−/−mice. The chronic infections and granulomas of SOCS-1−/−IFN-γ−/−mice may be based on autoaggression of SOCS-1−/−T lymphoid and related cells or a functional deficiency of these cells when lacking SOCS-1.
Generation of an activating Zn
2+
switch in the dopamine transporter: Mutation of an intracellular tyrosine constitutively alters the conformational equilibrium of the transport cycle
Authors: Loland Claus Juul , Norregaard Lene , Litman Thomas , Gether Ulrik
Published: July 2002

Binding of Zn
2+
to the endogenous Zn
2+
binding site in the human dopamine transporter leads to potent inhibition of [
3
H]dopamine uptake. Here we show that mutation of an intracellular tyrosine to alanine (Y335A) converts this inhibitory Zn
2+
switch into an activating Zn
2+
switch, allowing Zn
2+
-dependent activation of the transporter. The tyrosine is part of a conserved YXXΦ trafficking motif (X is any residue and Φ is a residue with a bulky hydrophobic group), but Y335A did not show alterations in surface targeting or protein kinase C-mediated internalization. Despite wild-type levels of surface expression, Y335A displayed a dramatic decrease in [
3
H]dopamine uptake velocity (
V
max
) to less than 1% of the wild type. In addition, Y335A showed up to 150-fold decreases in the apparent affinity for cocaine, mazindol, and related inhibitors whereas the apparent affinity for several substrates was increased. However, the presence of Zn
2+
in micromolar concentrations increased the
V
max
up to 24-fold and partially restored the apparent affinities. The capability of Zn
2+
to restore transport is consistent with a reversible, constitutive shift in the distribution of conformational states in the transport cycle upon mutation of Tyr-335. We propose that this shift is caused by disruption of intramolecular interactions important for stabilizing the transporter in a conformation in which extracellular substrate can bind and initiate transport, and accordingly that Tyr-335 is critical for regulating isomerization between discrete states in the transport cycle.
Photoactivation of the flavin cofactor in
Xenopus laevis
(6–4) photolyase: Observation of a transient tyrosyl radical by time-resolved electron paramagnetic resonance
Authors: Weber Stefan , Kay Christopher W. M. , Mögling Heike , Möbius Klaus , Hitomi Kenichi , Todo Takeshi
Published: July 2002

The light-induced electron transfer reaction of flavin cofactor photoactivation in
Xenopus
laevis
(6–4) photolyase has been studied by continuous-wave and time-resolved electron paramagnetic resonance spectroscopy. When the photoactivation is initiated from the fully oxidized form of the flavin, a neutral flavin radical is observed as a long-lived paramagnetic intermediate of two consecutive single-electron reductions under participation of redox-active amino acid residues. By time-resolved electron paramagnetic resonance, a spin-polarized transient radical-pair signal was detected that shows remarkable differences to the signals observed in the related cyclobutane pyrimidine dimer photolyase enzyme. In (6–4) photolyase, a neutral tyrosine radical has been identified as the final electron donor, on the basis of the characteristic line width, hyperfine splitting pattern, and resonance magnetic field position of the tyrosine resonances of the transient radical pair.
Formation of a GNRA tetraloop in P5abc can disrupt an interdomain interaction in the
Tetrahymena
group I ribozyme
Authors: Zheng Minxue , Wu Ming , Tinoco Ignacio
Published: July 2002

The secondary structure of a truncated P5abc subdomain (tP5abc, a
56-nucleotide RNA) of the
Tetrahymena thermophila
group
I intron ribozyme changes when its tertiary structure forms. We have
now used heteronuclear NMR spectroscopy to determine its conformation
in solution. The tP5abc RNA that contains only secondary structure is
extended compared with the tertiary folded form; both forms coexist in
slow chemical exchange (the interconversion rate constant is slower
than 1 s
−1
) in the presence of magnesium. Kinetic
experiments have shown that tertiary folding of the P5abc subdomain is
one of the earliest folding transitions in the group I intron ribozyme,
and that it leads to a metastable misfolded intermediate. Previous
mutagenesis studies suggest that formation of the extended P5abc
structure described here destabilize a misfolded intermediate. This
study shows that the P5abc RNA subdomain containing a GNRA tetraloop in
P5c (in contrast to the five-nucleotide loop P5c in the tertiary folded
ribozyme) can disrupt the base-paired interdomain (P14) interaction
between P5c and P2.
Community-wide distribution of predator–prey interaction strength in kelp forests
Authors: Sala Enric , Graham Michael H.
Published: July 2002

The strength of interactions between predators and their prey (interaction strength) varies enormously among species within ecological communities. Understanding the community-wide distribution of interaction strengths is vital, given that communities dominated by weak interactions may be more stable and resistant to invasion. In the oceans, previous studies have reported log-normal distributions of per capita interaction strength. We estimated the distribution of predator–prey interaction strengths within a subtidal speciose herbivore community (45 species). Laboratory experiments were used to determine maximum per capita interaction strengths for eight species of herbivores (including amphipods, isopods, gastropods, and sea urchins) that graze on giant kelp (
Macrocystis pyrifera
) microscopic stages. We found that maximum per capita interaction strength saturated as a function of individual herbivore biomass, likely caused by predator/prey size thresholds. Incorporating this nonlinearity, we predicted maximum per capita interaction strength for the remaining herbivore species. The resulting distribution of per capita interaction strengths was bimodal, in striking contrast to previous reports from other communities. Although small herbivores often had per capita interaction strengths similar to larger herbivores, their tendency to have greater densities in the field increased their potential impact as grazers. These results indicate that previous conclusions about the distributions of interaction strength in natural communities are not general, and that intermediate-sized predators can under realistic circumstances represent the most effective consumers in natural communities.
RNA–protein hybrid ribozymes that efficiently cleave any mRNA independently of the structure of the target RNA
Authors: Warashina Masaki , Kuwabara Tomoko , Kato Yoshio , Sano Masayuki , Taira Kazunari
Published: July 2002

Ribozyme activity
in vivo
depends on achieving
high-level expression, intracellular stability
,
target
colocalization, and cleavage site access. At present, target site
selection is problematic because of unforeseeable secondary and
tertiary RNA structures that prevent cleavage. To overcome this design
obstacle, we wished to engineer a ribozyme that could access any chosen
site. To create this ribozyme, the constitutive transport element
(CTE), an RNA motif that has the ability to interact with intracellular
RNA helicases, was attached to our ribozymes so that the
helicase-bound, hybrid ribozymes would be produced in cells. This
modification significantly enhanced ribozyme activity
in
vivo
, permitting cleavage of sites previously found to be
inaccessible. To confer cleavage enhancement, the CTE must retain
helicase-binding activity. Binding experiments demonstrated the likely
involvement of RNA helicase(s). We found that attachment of the RNA
motif to our tRNA ribozymes leads to cleavage
in vivo
at
the chosen target site regardless of the local RNA secondary or
tertiary structure.
The HMG-domain protein BAP111 is important for the function of the BRM chromatin-remodeling complex
in vivo
Authors: Papoulas Ophelia , Daubresse Gary , Armstrong Jennifer A. , Jin Janet , Scott Matthew P. , Tamkun John W.
Published: July 2002

The
Drosophila
trithorax group gene
brahma
(
brm
) encodes the ATPase subunit
of a SWI/SNF-like chromatin-remodeling complex. A key question about
chromatin-remodeling complexes is how they interact with DNA,
particularly in the large genomes of higher eukaryotes. Here, we report
the characterization of BAP111, a BRM-associated protein that contains
a high mobility group (HMG) domain predicted to bind distorted or bent
DNA. The presence of an HMG domain in BAP111 suggests that it may
modulate interactions between the BRM complex and chromatin. BAP111 is
an abundant nuclear protein that is present in all cells throughout
development. By using gel filtration chromatography and
immunoprecipitation assays, we found that the majority of BAP111
protein in embryos is associated with the BRM complex. Furthermore,
heterozygosity for
BAP111
enhanced the phenotypes
resulting from a partial loss of
brm
function. These
data demonstrate that the BAP111 subunit is important for BRM complex
function
in vivo
.
Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: The tyrosine radical Y\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{_{D}^{{\bullet}}}}\end{equation*}\end{document}
Authors: Hofbauer W. , Zouni A. , Bittl R. , Kern J. , Orth P. , Lendzian F. , Fromme P. , Witt H. T. , Lubitz W.
Published: July 2002

Electron paramagnetic resonance (EPR) spectroscopy at 94 GHz is
used to study the dark-stable tyrosine radical
Y

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in single crystals of photosystem II
core complexes (cc) isolated from the thermophilic cyanobacterium
Synechococcus elongatus.
These complexes contain at least
17 subunits, including the water-oxidizing complex (WOC), and 32
chlorophyll
a
molecules/PS II; they are active in
light-induced electron transfer and water oxidation. The crystals
belong to the orthorhombic space group
P2
1
2
1
2
1
, with four PS II dimers per
unit cell. High-frequency EPR is used for enhancing the sensitivity of
experiments performed on small single crystals as well as for
increasing the spectral resolution of the g tensor components and of
the different crystal sites. Magnitude and orientation of the g tensor
of Y

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and related information on several
proton hyperfine tensors are deduced from analysis of angular-dependent
EPR spectra. The precise orientation of tyrosine
Y

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in PS II is obtained as a first step in
the EPR characterization of paramagnetic species in these single
crystals.
Corneal collagen fibril structure in three dimensions: Structural insights into fibril assembly, mechanical properties, and tissue organization
Authors: Holmes David F. , Gilpin Christopher J. , Baldock Clair , Ziese Ulrike , Koster Abraham J. , Kadler Karl E.
Published: July 2002

The ability of the cornea to transmit light while being mechanically resilient is directly attributable to the formation of an extracellular matrix containing orthogonal sheets of collagen fibrils. The detailed structure of the fibrils and how this structure underpins the mechanical properties and organization of the cornea is understood poorly. In this study, we used automated electron tomography to study the three-dimensional organization of molecules in corneal collagen fibrils. The reconstructions show that the collagen molecules in the 36-nm diameter collagen fibrils are organized into microfibrils (≈4-nm diameter) that are tilted by ≈15° to the fibril long axis in a right-handed helix. An unexpected finding was that the microfibrils exhibit a constant-tilt angle independent of radial position within the fibril. This feature suggests that microfibrils in concentric layers are not always parallel to each other and cannot retain the same neighbors between layers. Analysis of the lateral structure shows that the microfibrils exhibit regions of order and disorder within the 67-nm axial repeat of collagen fibrils. Furthermore, the microfibrils are ordered at three specific regions of the axial repeat of collagen fibrils that correspond to the N- and C-telopeptides and the d-band of the gap zone. The reconstructions also show macromolecules binding to the fibril surface at sites that correspond precisely to where the microfibrils are most orderly.