bluuebunny/crossref_metadata_2025 huggingface dataset:
Discrete solitons in inhomogeneous waveguide arrays
Authors: Pertsch T. , Peschel U. , Lederer F.
Published: May 2003

The existence and dynamical properties of discrete solitons in inhomogeneous waveguide arrays with a Kerr nonlinearity are studied in two different configurations. First we investigate the effect of a longitudinal periodic modulation of the coupling strength on the dynamics of discrete solitons. It is shown that resonances of internal modes of the soliton with the longitudinal structure may lead to soliton oscillations and decay. Second we study the existence and stability of discrete solitons in arrays exhibiting a linear variation of the waveguide effective index in the transverse direction. We find that resonant coupling between conventional discrete solitons and linear Wannier–Stark states leads to the formation of so-called hybrid discrete solitons.
Resonant breather states in Josephson coupled systems
Authors: Fistul M. V.
Published: May 2003

A review of diverse resonant effects appearing in weakly dissipative Josephson coupled systems in the presence of inhomogeneous dynamic localized state (discrete breather) is given. As particular examples I discuss the resonant interaction of breather states with linear electromagnetic excitations (EEs) in dc driven Josephson junction ladders and a single plaquette containing three Josephson junctions. Such resonant interaction manifests itself by resonant steps and various sharp switchings (voltage jumps) in the current-voltage characteristics. Moreover, the resonant interaction leads to an increase of breather dynamical complexity, e.g., enlargement of the breather core, low symmetry or quasiperiodic breather states. I show that the application of an external magnetic field allows to tune the resonant interaction, and correspondingly to increase (or decrease) the height of the resonant steps, to change the stability of the breather states.
Imaging of discrete breathers
Authors: Ustinov A. V.
Published: May 2003

In this paper I review our experiments on visualization of discrete breathers (intrinsic localized modes) in nonlinear lattices made of Josephson junctions. Properties of Josephson junctions and arrays made of such junctions are discussed in the Introduction. The visualization technique based on low temperature laser scanning microscopy (LSM) is described in detail. Images of discrete breathers in Josephson junction arrays of various geometries are presented. Possible further experiments that can be done using LSM technique are envisioned.
The method of variation of constants and multiple time scales in orbital mechanics
Authors: Newman William I. , Efroimsky Michael
Published: May 2003

The method of variation of constants is an important tool used to solve systems of ordinary differential equations, and was invented by Euler and Lagrange to solve a problem in orbital mechanics. This methodology assumes that certain “constants” associated with a homogeneous problem will vary in time in response to an external force. It also introduces one or more constraint equations. We show that these constraints can be generalized in analogy to gauge theories in physics, and that different constraints can offer conceptual advances and methodological benefits to the solution of the underlying problem. Examples are given from linear ordinary differential equation theory and from orbital mechanics. However, a slow driving force in the presence of multiple time scales contained in the underlying (homogeneous) problem nevertheless requires special care, and this has strong implications to the analytic and numerical solutions of problems ranging from celestial mechanics to molecular dynamics.
Selected topics in lattice dynamics: A critical review (including breathers)
Authors: Horton George K. , Cowley E. Roger
Published: May 2003

We present a review of lattice dynamics to provide the underpinnings for the study of nonlinear localized modes, the so-called breathers. After a historical survey we address the following topics: harmonic theories, anharmonic perturbation theory, self-consistent theories, classical simulation techniques, path-integral theories, realistic crystal potentials, and intrinsic localized modes. We discuss both static and dynamic properties of crystals, e.g., neutron and x-ray scattering. We do not consider transport properties. Throughout, our emphasis is on discussing the major advances in the field and citing the appropriate references. Our aim is to achieve clarity and simplicity for readers who wish to move on to the study of breathers. We have made a special effort to set up the language and notation that is generally accepted in the field. In order to acquaint the reader with the techniques used in lattice dynamics we have analyzed a number of key problems in detail including a comparison with the available experimental data.
Control of degenerate Hopf bifurcations in three-dimensional maps
Authors: Wen GuiLin , Xu Daolin , Xie JianHua
Published: May 2003

A feedback control method is proposed to create a degenerate Hopf bifurcation in three-dimensional maps at a desired parameter point. The particularity of this bifurcation is that the system admits a stable fixed point inside a stable Hopf circle, between which an unstable Hopf circle resides. The interest of this solution structure is that the asymptotic behavior of the system can be switched between stationary and quasi-periodic motions by only tuning the initial state conditions. A set of critical and stability conditions for the degenerate Hopf bifurcation are discussed. The washout-filter-based controller with a polynomial control law is utilized. The control gains are derived from the theory of Chenciner’s degenerate Hopf bifurcation with the aid of the center manifold reduction and the normal form evolution.
A secure communication scheme based on the phase synchronization of chaotic systems
Authors: Chen J. Y. , Wong K. W. , Cheng L. M. , Shuai J. W.
Published: May 2003

Phase synchronization of chaotic systems with both weak and strong couplings has recently been investigated extensively. Similar to complete synchronization, this type of synchronization can also be applied in secure communications. We develop a digital secure communication scheme that utilizes the instantaneous phase as the signal transmitted from the drive to the response subsystems. Simulation results show that the scheme is difficult to be broken by some traditional attacks. Moreover, it operates with a weak positive conditional Lyapunov exponent in the response subsystem.
Space–time complexity in Hamiltonian dynamics
Authors: Afraimovich V. , Zaslavsky G. M.
Published: May 2003

New notions of the complexity function C(ε;t,s) and entropy function S(ε;t,s) are introduced to describe systems with nonzero or zero Lyapunov exponents or systems that exhibit strong intermittent behavior with “flights,” trappings, weak mixing, etc. The important part of the new notions is the first appearance of ε-separation of initially close trajectories. The complexity function is similar to the propagator p(t0,x0;t,x) with a replacement of x by the natural lengths s of trajectories, and its introduction does not assume of the space–time independence in the process of evolution of the system. A special stress is done on the choice of variables and the replacement t→η=ln t, s→ξ=ln s makes it possible to consider time-algebraic and space-algebraic complexity and some mixed cases. It is shown that for typical cases the entropy function S(ε;ξ,η) possesses invariants (α,β) that describe the fractal dimensions of the space–time structures of trajectories. The invariants (α,β) can be linked to the transport properties of the system, from one side, and to the Riemann invariants for simple waves, from the other side. This analog provides a new meaning for the transport exponent μ that can be considered as the speed of a Riemann wave in the log-phase space of the log-space–time variables. Some other applications of new notions are considered and numerical examples are presented.
Complete synchronizability of chaotic systems: A geometric approach
Authors: Solı́s-Perales G. , Ayala V. , Kliemann W. , Femat R.
Published: May 2003

Synchronizability of chaotic systems is studied in this contribution. Geometrical tools are used to understand the properties of vector fields in affine systems. The discussion is focused on synchronizability of chaotic systems with equal order. The analysis is based on the synchronous behavior of all states of the master/slave system (complete synchronization). We state sufficient and necessary conditions for complete synchronizability which are based on controllability and observability of nonlinear affine systems. In this sense, the synchronizability is studied for complete synchronization via state feedback control.
Localization in physical systems described by discrete nonlinear Schrödinger-type equations
Authors: Bishop A. R. , Kalosakas G. , Rasmussen K. O/. , Kevrekidis P. G.
Published: May 2003

Following a short introduction on localized modes in a model system, namely the discrete nonlinear Schrödinger equation, we present explicit results pertaining to three different physical systems described by similar equations. The applications range from the Raman scattering spectra of a complex electronic material through intrinsic localized vibrational modes, to the manifestation of an abrupt and irreversible delocalizing transition of Bose–Einstein condensates trapped in two-dimensional optical lattices, and to the instabilities of localized modes in coupled arrays of optical waveguides.
Increased phase synchronization of spontaneous calcium oscillations in epileptic human versus normal rat astrocyte cultures
Authors: Balázsi Gábor , Cornell-Bell Ann H. , Moss Frank
Published: May 2003

Stochastic synchronization analysis is applied to intracellular calcium oscillations in astrocyte cultures prepared from epileptic human temporal lobe. The same methods are applied to astrocyte cultures prepared from normal rat hippocampus. Our results indicate that phase-repulsive coupling in epileptic human astrocyte cultures is stronger, leading to an increased synchronization in epileptic human compared to normal rat astrocyte cultures.
Intrinsic localized modes in photonic crystal circuits
Authors: McGurn Arthur R.
Published: May 2003

A discussion of intrinsic localized modes (ILMs) in photonic crystals is given. Emphasis is placed on ILMs in photonic crystal waveguides formed from Kerr nonlinear materials and photonic crystal circuits formed by joining nonlinear photonic crystal waveguides into networks. Work on stationary and propagating ILMs in nonlinear waveguides and waveguide junctions is reviewed. In addition, some discussion is given on the optical bistability of the barrier and junction geometries and on recent results on two-dimensional ILMs in Kerr nonlinear photonic crystals.
The spatial logistic map as a simple prototype for spatiotemporal chaos
Authors: Willeboordse Frederick H.
Published: May 2003

A spatial extension of the logistic map—termed spatial logistic map—is found to display the same basic universality classes as the commonly studied diffusively coupled logistic lattice despite being vastly simpler. By analyzing the escape rates and the Lyapunov spectra it is shown that the main attractors of the spatial logistic map are stable and hence that it is a good candidate for serving as a prototype for the class of coupled map lattices which it is a part of. The spatial logistic map is then employed to provide an analytical derivation for the recently discovered linear scaling of the wavelength under increasing coupling ranges.
Chaotic mixing in a torus map
Authors: Thiffeault Jean-Luc , Childress Stephen
Published: May 2003

The advection and diffusion of a passive scalar is investigated for a map of the 2-torus. The map is chaotic, and the limit of almost-uniform stretching is considered. This allows an analytic understanding of the transition from a phase of constant scalar variance (for short times) to exponential decay (for long times). This transition is embodied in a short superexponential phase of decay. The asymptotic state in the exponential phase is an eigenfunction of the advection–diffusion operator, in which most of the scalar variance is concentrated at small scales, even though a large-scale mode sets the decay rate. The duration of the superexponential phase is proportional to the logarithm of the exponential decay rate; if the decay is slow enough then there is no superexponential phase at all.
Ab initio modeling of B and N in C29 and C29H24 nanodiamond
Authors: Barnard A. S. , Russo S. P. , Snook I. K.
Published: May 2003

An ab initio density functional theory study of the structure and stability of a select group of dopants in a diamond nanocrystal is presented. The dopants studied here are boron and nitrogen. These atoms have been included substitutionally in the center of a 29 atom nanodiamond crystal, and the entire structure relaxed using the Vienna Ab Initio Simulation Package. The relaxed structures are then described by the relaxation energy, the structural (total) energy and a qualitative examination of the overall change in structure and bonding, considered via examination of the electron charge density. It is anticipated that these results will provide a better understanding of the crystal stability of doped nanodiamonds for use in diamondoid nanodevices.
Photo-induced reactions in the ion–molecule complex Mg+–OCNC2H5
Authors: Sun Ju-Long , Liu Haichuan , Han Ke-Li , Yang Shihe
Published: May 2003

Ion–molecule complexes of magnesium cation with ethyl isocyanate were produced in a laser-ablation supersonic expansion nozzle source. Photo-induced reactions in the 1:1 complexes have been studied in the spectral range of 230–410 nm. Photodissociation mass spectrometry revealed the persistent product Mg+ from nonreactive quenching throughout the entire wavelength range. As for the reactive channels, the photoproducts, Mg+OCN and C2H5+, were produced only in the blue absorption band of the complex with low yields. The action spectrum of Mg+(OCNC2H5) consists of two pronounced peaks on the red and blue sides of the Mg+ 32P←32S atomic transition. The ground state geometry of Mg+–OCNC2H5 was fully optimized at B3LYP/6-31+G** level by using GAUSSIAN 98 package. The calculated absorption spectrum of the complex using the optimized structure of its ground state agrees well with the observed action spectrum. Photofragment branching fractions of the products are almost independent of the photolysis photon energy for the 3Px,y,z excitations. The very low branching ratio of reactive products to nonreactive fragment suggests that evaporation is the main relaxation pathway in the photo-induced reactions of Mg+(OCNC2H5).
Quartz fibers as active elements in detectors for particle physics
Authors: Akchurin Nural , Wigmans Richard
Published: May 2003

Quartz fibers were first applied in particle physics experiments for detectors that operated at extremely high radiation levels. The superior radiation hardness of high-purity quartz was the driving consideration at that time. However, it turned out that these fibers have many other interesting and beneficial aspects, which derive from the fact that the Čerenkov effect forms the basis of the signals in quartz-based detectors. These aspects are reviewed in this paper. We also discuss some details of the radiation damage characteristics of quartz.
An alternating current superconductor susceptometric system to evaluate liver iron overload
Authors: Carneiro A. A. O. , Fernandes J. P. , Zago M. A. , Covas D. T. , Ângulo I. L. , Baffa O.
Published: May 2003

An ac susceptometric system to quantify liver iron overload composed of a second order axial gradiometer coil coupled to a rf superconducting quantum interference device detector and a large field coil array is presented. A homogeneous ac magnetizing field with low frequency (7.7 Hz) and low intensity (114 μT) is used. Preliminary measurements over a group of 34 normal individuals and 20 patients with iron overload show the ability of the instrument to perform the measurement and to distinguish normal and pathological individuals. The diamagnetic signature of the surrounding tissues is minimized using a special water bag on the torso. In summary it was shown that with a relatively simple instrumentation it was possible to build a superconducting susceptometer dedicated to quantify in vivo iron concentrations, which is clinically important information in the assessment and management of patients with liver iron overload, mainly those who regularly receive blood transfusion.
An integrated platform for surface forces measurements and fluorescence correlation spectroscopy
Authors: Mukhopadhyay Ashis , Zhao Jiang , Bae Sung Chul , Granick Steve
Published: May 2003

We describe an apparatus to measure the diffusion of dilute fluorophores in molecularly thin liquid films within a surface forces apparatus (SFA). The design is a significant modification of the traditional SFA in that it allows one to combine nanorheology with the single-molecule sensitive technique of fluorescence correlation spectroscopy. The primary enabling idea was to place a miniaturized SFA onto the stage of an optical microscope equipped with a long working distance objective and illuminated by a femtosecond laser. A secondary enabling idea was that the silver coating on the backside of mica, normally used in the traditional SFA design for interferometric measurements of the film thickness, was replaced by multilayer dielectric coatings that allowed simultaneous interferometry and fluorescence measurements in different regions of the optical spectrum. To illustrate the utility of this instrument, we contrast the translational diffusion of rhodamine dye molecules (in the solvent, 1,2-propane diol), in the unconfined bulk state and confined between mica sheets to the thickness 2.5 nm. The diffusion coefficient is found to decrease by 2 orders of magnitude under confinement.
The deformation-DIA: A new apparatus for high temperature triaxial deformation to pressures up to 15 GPa
Authors: Wang Yanbin , Durham William B. , Getting Ivan C. , Weidner Donald J.
Published: May 2003

A new deformation apparatus has been developed, based on the widely used cubic-anvil apparatus known as the DIA. Two differential rams, introduced in the upper and lower guide blocks, allow independent control of the differential strain and stress field under high confining pressure. Testing experiments with synchrotron x rays have demonstrated that this deformation DIA (D-DIA) is capable of generating up to 30% axial strain on a 1–2 mm long sample under confining pressures up to 15 GPa at simultaneous high temperatures. Various compressional strain rates from 10−3 to about 5×10−6 s−1 have been achieved. Extensional experiments have also been carried out successfully. Strains are measured by x-ray imaging of the sample which has a length measurement precision of ∼0.1 μm; pressures are monitored using standard materials with well established equations of state. X-ray transparent anvils made of sintered polycrystalline cubic boron nitride have been successfully tested, with a two-dimensional x-ray charge coupled device detector. Distortions in the diffraction lines due to differential stress can be measured with a precision of about 20 MPa.
A compact, lightweight gas standards generator for permeation tubes
Authors: Washenfelder R. A. , Roehl C. M. , McKinney K. A. , Julian R. R. , Wennberg P. O.
Published: May 2003

A lightweight, compact gas standards generator for permeation tubes is described. This system provides reliable temperature control during periods of intermittent power and is ideal for field measurements. A eutectic alloy with a high heat of fusion is used as a phase change material. This thermal source allows the system to maintain a constant permeation tube temperature of 46.6 °C for over 5 h in the absence of external power. This permeation system is currently being used in an aircraft chemical ionization mass spectrometer to provide HNO3 calibration.
Breaking of space–time symmetries in modulated traveling waves
Authors: Aubry Nadine , Lima Ricardo , Rahibe Mohammed
Published: May 2003

In this paper, we investigate the structure of traveling waves whose amplitude is spatially and/or temporally modulated. In particular, in the case of short wavelength modulations, we completely describe the breaking of symmetry induced by such modulations and their dynamical consequences.
The Cotton–Mouton effect of furan and its homologues in the gas phase, for the pure liquids and in solution
Authors: Cappelli Chiara , Rizzo Antonio , Mennucci Benedetta , Tomasi Jacopo , Cammi Roberto , Rikken Geert L. J. A. , Mathevet Renaud , Rizzo Carlo
Published: May 2003

The tensor components of the electric dipole polarizability at a wavelength of 632.8 nm, those of the magnetizability and the anisotropy of the static hypermagnetizability of furan, thiophene, and selenophene are computed using density functional theory (DFT). The polarizable continuum model (PCM) is employed to describe the system in the condensed phase. We can thus compare the temperature dependence of the Cotton–Mouton constant for the three molecules, both in the gas and in the condensed phase, pure liquids, and solutions, with the results of experiment performed using a 17 T radial access Bitter magnet at the Grenoble High Magnetic Field Laboratory. This allows to analyze, in a direct interaction of theory and experiment, the performance of DFT and PCM in describing high order nonlinear mixed electric and magnetic effects in condensed phase.
Apparatus for pulsed laser deposition of semiconductor thin films
Authors: Oszwaldowski M. , Berus T. , Sydorczuk P. , Rzeszutek J.
Published: May 2003

This article is devoted to description of an apparatus for pulsed laser deposition of thin films of semiconducting compounds. The apparatus is based on a typical vacuum plant for thin films deposition. The most important feature of this setup is an optical deflector that switches the laser beam between three rotating targets situated side by side. The switching program is controlled by a PC. Owing to a short switching time, below 100 ms, the program can consist many switching in a short time. This allows growing thin films with composed cross-sectional chemical compositions. Sequential repetition of the program allows obtaining composed superlattices.
Response compensation of temperature sensors: Frequency-domain estimation of thermal time constants
Authors: Tagawa M. , Kato K. , Ohta Y.
Published: May 2003

A technique for estimating in situ thermal time-constant values of temperature sensors in the frequency domain and for compensating the sensor outputs for the response delays is proposed. The scheme developed is complementary to the time-domain response-compensation ones previously reported by Tagawa et al. [Rev. Sci. Instrum. 69, 3370 (1998)]. The scheme provides fast, simple, and flexible data processing based on the fast Fourier transform algorithm, and is potentially applicable to the response compensation of various physical and chemical sensors.
Charge coupled device based phase resolved stroboscopic photometry of pulsars
Authors: Kotar Jurij , Vidrih Simon , Čadež Andrej
Published: May 2003

A stroboscope designed to observe pulsars in the optical spectrum is presented. The absolute phase of the stroboscope is synchronized to better than 2.5 μs with the known radio ephemerides for a given pulsar. The absolute timing is provided by the Global Positioning System clock. With such a device resolved photometry of pulsars can be performed. We demonstrated the instrument’s capabilities with the results of a set of observations of the Crab pulsar, the brightest of the known optical pulsars, with a visual magnitude of 16.5, and a rotational frequency of ∼29 Hz.
Digital control of an iodine stabilized He–Ne laser by using a personal computer and a simple electronic system
Authors: Pantelić Dejan V. , Panić Bratimir M. , Kovac̆ević Aleksander G.
Published: May 2003

An electronic system used for active control and optical frequency stabilization of a He–Ne laser is described. It is based on digital acquisition and signal processing, using minimum of analog electronics, and a personal computer (PC). Main functions of the system: phase sensitive detection, automatic control, and user interface, are performed in software. Simultaneous frequency and iodine cell temperature stabilization algorithm is described. Electronic system is compact (occupies only desktop PC case), inexpensive (commercial, of-the-shelf, components are mostly used), and can be easily upgraded and reprogrammed (new stabilization algorithms can be implemented). It is now a part of the primary length standard of Yugoslavia.
Design of a triple resonance magic angle sample spinning probe for high field solid state nuclear magnetic resonance
Authors: Martin Rachel W. , Paulson Eric K. , Zilm Kurt W.
Published: May 2003

Standard design and construction practices used in building nuclear magnetic resonance (NMR) probes for the study of solid state samples become difficult if not entirely impractical to implement as the 1H resonance frequency approaches the self resonance frequency of commercial capacitors. We describe an approach that utilizes short variable transmission line segments as tunable reactances. Such an approach effectively controls stray reactances and provides a higher Q alternative to ceramic chip capacitors. The particular probe described is built to accommodate a 2.5 mm magic angle spinning rotor system, and is triply tuned to 13C, 15N, and 1H frequencies for use at 18.8 T (200, 80, and 800 MHz, respectively). Isolation of the three radio frequency (rf) channels is achieved using both a rejection trap and a transmission line notch filter. The compact geometry of this design allows three channels with high power handling capability to fit in a medium bore (63 mm) magnet. Extended time variable temperature operation is integral to the mechanical design, enabling the temperature control necessary for investigation of biological macromolecules. Accurate measurement of the air temperature near the sample rotor is achieved using a fiber optic thermometer, which does not interfere with the rf electronics. We also demonstrate that acceptable line shapes are only readily achieved using zero magnetic susceptibility wire in construction of the sample coil. Computer simulation of the circuit aided in the physical design of the probe. Representative data illustrating the efficiency, rf homogeneity, and signal to noise factor of the probe are presented.
Image correction in magneto-optical microscopy
Authors: Paturi P , Larsen B. Hvolbæk , Jacobsen B. A. , Andersen N. H.
Published: May 2003

An image-processing procedure that assures correct determination of the magnetic field distribution of magneto-optical images is presented. The method remedies image faults resulting from sources that are proportional to the incident light intensity, such as different types of defects in the indicator film and unevenness of light, as well as additive signals from detector bias, external light sources, etc. When properly corrected a better measurement of the local magnetic field can be made, even in the case of heavily damaged films. For superconductors the magnetic field distributions may be used for accurate determination of the current distributions without the spurious current loops associated with defects in the films.
Performance of a synthetic diamond backing-plate for the diamond-anvil cell at ultrahigh pressures
Authors: Kenichi Takemura , Satoshi Nakano
Published: May 2003

Synthetic diamond has been successfully used as a backing-plate in the diamond-anvil cell up to 223 GPa. Use of diamond for the backing-plate has a great advantage for powder x-ray diffraction experiments at ultrahigh pressures, for it makes it possible to measure full Debye–Scherrer rings to a maximum scattering angle of 34°. The mechanical strength of the diamond backing-plate should contribute to the extension of the current pressure limit of the diamond-anvil cell. Problems and future improvements of the design are discussed.
Near-field microwave microscope with improved sensitivity and spatial resolution
Authors: Tselev Alexander , Anlage Steven M. , Christen Hans M. , Moreland Robert L. , Talanov Vladimir V. , Schwartz Andrew R.
Published: May 2003

The near-field scanning microwave microscope has become a popular instrument to quantitatively image high-frequency properties of metals and dielectrics on length scales far shorter than the wavelength of the radiation. We have developed several new ways to operate this microscope to dramatically improve its spatial resolution and material property sensitivity. These include a novel distance-following method that takes advantage of the stability of a synthesized microwave source to improve the signal-to-noise ratio of our earlier frequency-following imaging technique. We also discuss novel height-modulated imaging techniques, culminating in a new tapping-mode method, which makes a 14 dB improvement in sensitivity, a 17.5 dB improvement in signal-to-noise ratio, and a factor of 2.3 improvement in spatial resolution compared to distance-following imaging.
Comparison of time corrections using charge amounts, peak values, slew rates, and signal widths in leading-edge discriminators
Authors: Lim Hansang , Parka) Jaehong
Published: May 2003

Leading-edge discriminators (LEDs) are one of the prominent discriminator technologies used in time-of-flight systems due to their simplicity, but they have large time walks such that the measured time should be corrected using amplitude information. In this article, we analyze off-line time corrections using four kinds of amplitude parameters, namely, peak values, slew rates, and signal widths, as well as charge amounts. For these corrections, we developed the front-end electronics (FEE) that can measure these amplitude parameters and performed time measurements with the developed FEE in an amplitude range of about 20 dB for various threshold conditions, which are the only adjustment in LEDs. Then, we off-line corrected the measured time using the amplitude parameters mentioned above and compared the corrected efficiencies by the used parameters and threshold conditions.
Noninvasive observation of magnetization reversal in micron-sized Co bars
Authors: Bekaert J. , Van Bael M. J. , Buntinx D. , Temst K. , Van Haesendonck C. , Moshchalkov V. V. , Lagae L. , De Boeck J. , Borghs G.
Published: May 2003

We demonstrate how room temperature scanning Hall probe microscopy (SHPM) can be applied to noninvasively probe the (meta-)stable magnetic domain states of individual mesoscopic Co bars as a function of applied field. These data are correlated with magneto-optical measurements of the hysteresis loop as well as higher spatial resolution magnetic force microscopy imaging. In particular, we show how a combination of visualization techniques is essential for fully understanding the mechanism of magnetization reversal in such microstructures. Micromagnetic calculations of the magnetic states and their corresponding stray fields are correlated with the observations. These experiments prove the usefulness of a noninvasive and quantitative technique such as room temperature SHPM in an applied magnetic field to study magnetic domain stability and switching field distributions.
Electrical system for measurement of breakdown voltage of vacuum and gas-filled tubes using a dynamic method
Authors: Pejović Milić M. , Milosavljević Čedomir S. , Pejović Momčilo M.
Published: May 2003

This article describes an electrical system aimed at measuring and data acquisition of breakdown voltages of vacuum and gas-filled tubes. The measurements were performed using a nitrogen-filled tube at 4 mbar pressure. Based on the measured breakdown voltage data as a function of the applied voltage increase rate, a static breakdown voltage is estimated for the applied voltage gradient ranging from 0.1 to 1 V s−1 and from 1 to 10 V s−1. The histograms of breakdown voltages versus applied voltage increase rates from 0.1 and 0.5 V s−1 are approximated by the probability density functions using a fitting procedure.
Negative chlorine ions from multicusp radio frequency ion source for heavy ion fusion applications
Authors: Hahto S. K. , Hahto S. T. , Kwan J. W. , Leung K. N. , Grisham L. R.
Published: May 2003

Use of high mass atomic neutral beams produced from negative ions as drivers for inertial confinement fusion has been suggested recently. Best candidates for the negative ions would be bromine and iodine with sufficiently high mass and electron affinity. These materials require a heated vapor ion source. Chlorine was selected for initial testing because it has similar electron affinity to those of bromine and iodine, and is available in gaseous form. An experiment was set up by the Plasma and Ion Source Technology Group in Lawrence Berkeley National Laboratory to measure achievable current densities and other beam parameters by using a rf driven multicusp ion source [K. N. Leung, Rev. Sci. Instrum. 65, 1165 (1994); Q. Ji et al., Rev. Sci. Instrum. 73, 822 (2002)]. Current density of 45 mA/cm2 was achieved with 99.5% of the beam as atomic negative chlorine at 2.2 kW of rf power. An electron to negative ion ratio as low as 7 to 1 was observed, while the ratio of positive and negative chlorine ion currents was 1.3. This in addition to the fact that the front plate biasing had almost no effect to the negative chlorine ion and electron currents indicates that a very high percentage of the negative charge in the extraction area of the ion source was in form of Cl− ions. A comparison of positive and negative chlorine ion temperatures was conducted with the pepper pot emittance measurement technique and very similar transverse temperature values were obtained for positive and negative chlorine ions.
Vacuum-ultraviolet photoabsorption imaging system for laser plasma plume diagnostics
Authors: Hirsch J. S. , Kennedy E. T. , Neogi A. , Costello J. T. , Nicolosi P. , Poletto L.
Published: May 2003

We describe a recently designed and constructed system based on a 1 m normal incidence vacuum monochromator with corrected (toroidal) optics that produces a wavelength tuneable and collimated vacuum-ultraviolet (VUV) (λ=30–100 nm) beam. The VUV continuum source is a laser-generated gold plasma. The primary function of the system is the measurement of time resolved “images” or spatial distributions of photoabsorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest (in our case a second synchronised plasma) and recording the “footprint” of the attenuated beam on a charge coupled device. Using this VUV photoabsorption imaging or “shadowgraphy” technique we track and extract column density distributions in expanding plasma plumes. We can also measure the plume front velocity. We have characterized the system, particularly in relation to spectral and spatial resolution and the experimental results meet very well the expectations from ray tracing done at the design phase. We present first photoabsorption images and column density distributions of laser produced Ca plumes from the system.
Surface following scanning in a scanning microwave microscope
Authors: Kim Ahram , Kim J. , Hyun S. , Kim S. S. , Kim T. , Char K.
Published: May 2003

In this Note we provide the basic principle of obtaining topological data with a scanning microwave microscope. When samples are sufficiently “metallic,” the resonant frequencies as a function of the gap between the sample surface and the tip all fall on a universal curve despite differences in sheet resistance. Based on this result, we have built a surface-following feedback circuit and succeeded in measuring the topological images of many metallic surfaces. To date, the best topological resolution was 50 nm. However, we believe it can be further improved by reducing the background vibration and using electronics with lower noise.
Numerical analysis for a discontinuous rotation of the torus
Authors: Bruin H. , Lambert A. , Poggiaspalla G. , Vaienti S.
Published: May 2003

In this paper, we study a class of piecewise rotations on the square. While few theoretical results are known about them, we numerically compute box-counting dimensions, correlation dimensions and complexity of the symbolic language produced by the system. Our results seem to confirm a conjecture that the fractal dimension of the exceptional set is two, as well as indicate that the dynamics on it is not ergodic. We also explore a relationship between the piecewise rotations and discretized rotations on lattices Z2n.
The phase diagram of the two center Lennard-Jones model as obtained from computer simulation and Wertheim’s thermodynamic perturbation theory
Authors: Vega C. , McBride C. , de Miguel E. , Blas F. J. , Galindo A.
Published: May 2003

The global phase diagram (i.e., vapor–liquid and fluid–solid equilibrium) of two-center Lennard-Jones (2CLJ) model molecules of bond length L=σ has been determined by computer simulation. The vapor–liquid equilibrium conditions are obtained using the Gibbs ensemble Monte Carlo method and by performing isobaric-isothermal NPT calculations at zero pressure. In the case of the solid phase, two close-packed solid structures are considered: In the first structure, the molecules are located in layers and all molecular axes point in the same direction; and in the second structure, the atoms form a close-packed arrangement but the molecular axes of the diatomic molecules have random orientations. It is shown that at the vapor–liquid–solid triple-point temperature, the orientationally disordered solid is the stable structure for the solid phase of this model. The vapor–liquid-disordered solid triple-point temperature of the 2CLJ model, with bond length L=σ, is located at T*=0.650(4). This is very close to the triple-point temperature of the Lennard-Jones monomer system (T*=0.687). At very low temperatures, the ordered solid is the stable phase. The vapor-ordered solid-disordered solid triple point is situated at T*=0.282. The simulation data are compared to Wertheim’s first-order thermodynamic perturbation theory (TPT1) for the fluid and solid phases. It is found that Wertheim’s TPT1 not only provides an accurate description of the equation of state in both the fluid and solid phases, but also provides accurate values of the free energies. The prediction of Wertheim’s TPT1 for the global phase diagram of the 2CLJ model shows excellent agreement with the simulation results, illustrating the possibility of using Wertheim’s perturbation theory to determine not only the vapor–liquid equilibria but also the global phase diagram of simple chain model molecules.
Effect of ion hydration on the first-order transition in the sequential wetting of hexane on brine
Authors: Weiss Volker C. , Indekeu Joseph O.
Published: May 2003

In recent experiments, a sequence of changes in the wetting state (“wetting transitions”) has been observed upon increasing the temperature in systems consisting of pentane on pure water and of hexane on brine. In this sequential-wetting scenario, there occurs a first-order transition from a partial-wetting state, in which only a microscopically thin film of adsorbate is present on the substrate, to a “frustrated complete wetting state” characterized by a mesoscopically, but not yet macroscopically thick wetting film. At higher temperatures, one observes a continuous divergence of the film thickness and finally, at the critical-wetting temperature, the complete-wetting state, featuring a macroscopic film thickness, is reached. This sequence of two transitions is brought about by an interplay of short-range and long-range interactions between substrate and adsorbate. The critical wetting transition is controlled by the long-range forces and is, thus, found by determining where the Hamaker constant, as calculated from a Dzyaloshinskii–Lifshitz–Pitaevskii-type theory, changes sign. The first-order transition involves both short-range and long-range forces and is, therefore, more difficult to locate. While the pentane–water system is well understood in this respect by now, a detailed theoretical description of the hexane–brine system is hampered by the a priori unknown modification of the interactions between substrate and adsorbate upon the addition of salt. In this work, we argue that the short-range interaction (contact energy) between hexane and pure water remains unchanged due to the formation of a depletion layer (a thin “layer” of pure water which is completely devoid of ions) at the surface of the electrolyte and that the presence of the salt manifests itself only in a modification of the long-range interaction between substrate and adsorbate. In a five-layer calculation considering brine, water, the first layer of adsorbed hexane molecules, liquid hexane, and vapor, we determine the new long-range interaction of brine with the adsorbate across the water layer. According to the recent theory of the excess surface tension of an electrolyte by Levin and Flores–Mena, this water layer is of constant, i.e., salt-concentration independent, thickness δ, with δ being the hydrodynamic radius of the ions in water. Once this radius has been determined, the first-order transition temperatures can be calculated from the dielectric properties of the five media. Our results for these temperatures are in good agreement with the experimental ones.
Optical properties and orientation of arrays of polystyrene spheres deposited using convective self-assembly
Authors: Wostyn Kurt , Zhao Yuxia , Yee Brandon , Clays Koen , Persoons André , de Schaetzen Gaetan , Hellemans Louis
Published: May 2003

We have prepared face centered close packed arrays of polystyrene spheres using convective self-assembly. The optical properties of the arrays of spheres were studied and revealed high quality crystals that could be prepared using this method. Optical transmission measurements over a wide spectral range reveal the presence of multiple diffraction peaks. Their use for checking the quality of the arrays is proposed. The orientation of the arrays formed was studied using optical diffraction and atomic force microscopy. Both indicate the formation of oriented crystals. This was until now only reported for epitaxial growth of colloidal crystals. The oriented crystals were used to measure their photonic band structure.
Isomerism of the anion of the indole–water dimer. Ab initio study
Authors: Jalbout Abraham F. , Hall Caitlin S. , Adamowicz Ludwik
Published: May 2003

In our previous paper [J. Chem. Phys. 112, 3726 (2000)], the experimental group of Schermann, who participated in the study, reported a Rydberg electron transfer spectroscopy (RET) experiments concerning anions of the indole–water clusters. For [indole–(water)1]− the RET spectrum showed two peaks. Based on the ab initio calculations one of the peaks was assigned to a dipole–bound (DB) anion with a diffuse excess electron and it was speculated that the second peak could correspond either to an excited electronic state or an excited vibrational state of the DB anion. In this paper we present calculations that describe a second anion of the indole–water dimer where the excess electron is suspended between the indole and water molecules and also occupies a diffuse orbital. The relation of this finding to the RET spectrum is discussed and we propose assigning the two RET peaks to the two anions described in the calculations.
The treacherous potential energy hypersurface of AgSiO
Authors: Guo Rui , Balasubramanian Krishnan , Schaefer Henry F.
Published: May 2003

The AgSiO system has been a source of puzzlement for more than a decade, with experimental and theoretical studies providing bewildering conclusions regarding the structure of the molecule. State-of-the-art coupled cluster and multireference configuration interaction methods have been applied in the present work, in conjunction with techniques designed to incorporate relativistic effects. With the coupled cluster single and double excitation with perturbatively applied triples method [CCSD(T)], the Ag–SiO dissociation energy is predicted to be 6.8 kcal/mole, with the equilibrium structure being a nearly isosceles triangle. However, a second minimum Ag–O–Si structure with bond angle ∼150° lies at less than 1 kcal/mole. With the multireference CI approach (up to 48 million configurations) the energetic order of these two minima is reversed. In contrast, both second-order perturbation theory and density functional theory predict an Ag–Si–O structure with bond angle ∼115° to be the global minimum. The present theoretical results represent a major step forward. However, it is clear that AgSiO will be the subject of experimental and theoretical studies for some time to come.
The hydrolysis process of the cis-dichloro(ethylenediamine)platinum(II): A theoretical study
Authors: Costa Luiz Antônio Sodré , Rocha Willian R. , De Almeida Wagner B. , Dos Santos Hélio F.
Published: May 2003

The hydrolysis process of the cisplatin analog cis-dichloro(ethylenediamine)platinum(II) (cis-DEP) was theoretically investigated at the Hartree–Fock, density functional theory and the second order Møller–Plesset perturbation theory levels of calculation. The stationary points on the gas phase potential energy surface for the first and second hydrolysis steps were fully optimized and characterized. For the first aquation process the gas phase results are in satisfactory agreement with the experimental data. However in order to reproduce the observed rate constant for the second hydrolysis step it is essential to include the solvent effect. The structures and energetic properties are similar to the values found for the parent compound cisplatin, showing that the cis-DEP analog should be considered as a potential drug concerning its hydrolysis process.
Sodium hydroxide formation in water clusters: The role of hydrated electrons and the influence of electric field
Authors: Ferro Y. , Allouche A.
Published: May 2003

The stability, structure and reactivity of Na(H2O)7, Na2(H2O)7, and Na2(H2O)10 clusters have been investigated by means of the density functional theory (DFT) method. In all cases, the 3s Na electrons are located far from their nuclei and hydrated. Particular emphasis has been placed on the influence of the water-generated electric field on sodium dimer polarization. The metal atoms hydrolysis reaction has been studied for the lone sodium atom as well as for the sodium pair; the calculated activation energies are found to be very similar in terms of magnitude. Reaction mechanisms are proposed that exhibit the role of the hydrogen bond cooperative effects in combination with proton tunneling.
Ion dissociation dynamics of the chlorine azide cation (ClN3+) investigated by velocity map imaging
Authors: Hansen N. , Wodtke A. M. , Komissarov A. V. , Morokuma K. , Heaven M. C.
Published: May 2003

The velocity map imaging technique was applied to study the unimolecular dissociation dynamics of the chlorine azide cation (ClN3+) fragmenting into N2 and NCl+ and the results have been compared to quantum chemical calculations. The ClN3+ ion was produced in a molecular beam by two-photon ionization of chlorine azide (ClN3) with laser light at λ≈202 nm. Rotationally resolved REMPI spectra and velocity map images of state selected N2(X 1Σg+,v,J) quantify the energy deposition into N2 and NCl+ products. Photoelectron velocity map images show near zero-energy electron production. Angular distributions of the photo-fragments suggest we are witnessing the influence of the ions first excited (A′) electronic state on its decomposition. The N2(X 1Σg+) dissociation products are mainly in the vibrational ground state, but rotationally excited with J up to ∼60. An unusual vibrational distribution in the NCl+ (X 2Π,v=1–12) product was observed, which is evidence of nonstatistical ion decomposition. This work also provides an accurate determination of the energetics for the reaction, ClN3+→N2(X 1Σg+)+NCl+(X 2Π)(ΔE<−0.2 eV), from which one may use prior photoionization threshold data to derive the energetics of the ClN3→N2(X 1Σg+)+NCl(a 1Δ)(ΔE<0.3 eV) dissociation.