bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of epitaxial GaxIn1−xAs/InP and AlyIn1−yAs/InP interface region by high resolution x-ray diffraction
Authors: Giannini C. , Tapfer L. , Tournié E. , Zhang Y. H. , Ploog K. H.
Published: July 2002

A detailed strain analysis of high resolution x-ray diffraction patterns taken from GaxIn1−xAs and AlyIn1−yAs layers grown lattice matched on (001) InP substrates shows the existence of distinct interface regions. Our dynamical diffraction model allows us to detect buried interface layers with submonolayer resolution and differences in composition of less than 1% from the rest of the epilayer. Comparison between experimental and theoretical rocking curves reveals that the interface region between epilayer and substrate comprises 2 monolayers InAs plus GaxIn1−xAs (or AlyIn1−yAs) interlayer with different group III element ratios.
Epitaxy enhancement of β-FeSi2 grown by ion beam assisted deposition
Authors: Terrasi A. , Ravesi S. , Grimaldi M. G. , Spinella C.
Published: July 2002

Iron silicide layers have been grown by 200 eV Ar+ beam assisted deposition of Fe onto 〈001〉 Si single crystals maintained at high temperatures. Polycrystalline FeSi has been formed at T=400 °C while partially epitaxial β-FeSi2 was obtained after deposition at 500 °C. Samples have been analyzed by Rutherford backscattering spectroscopy and transmission electron microscopy. The main effects of the ion beam have been the reduction of the average grain size of the polycrystallites for both phases and the enhancement of the epitaxial fraction for the β-FeSi2 phase. The role of the ion beam during the formation of the iron silicides has been discussed.
Negative absolute conductivity in quantum wires
Authors: Mickevic̆ius R. , Mitin V. , Stroscio Michael A. , Dutta M.
Published: July 2002

The effect of negative absolute conductivity in quasi-one-dimensional quantum wire structures is obtained by the Monte Carlo simulation. This negative conductivity is associated with inelastic optical phonon scattering leading to an asymmetric electron distribution function established under conditions of intensive electron photoinjection. Simulation results suggest that quantum wires are ideal for the experimental observation of negative absolute conductivity. The oscillations of photoconductivity as a function of injection energy can reveal the spectrum of optical phonons in quantum wires which differs considerably from that in bulk materials.
Rare earth doped silica waveguides on Si fabricated by flame hydrolysis deposition and aerosol doping
Authors: Bebbington J. A. , Barbarossa G. , Bonar J. R. , Aitchison J. S.
Published: July 2002

High silica waveguides on Si fabricated by flame hydrolysis deposition were doped with rate earth ions using an aerosol doping technique. Rare earth doping levels in the glass were dependent on the nebulized solution strength and the delivery rate of the aerosol to the burner, giving the possibility of selective area doping.
Anisotropy of the electron Hall mobility in 4H, 6H, and 15R silicon carbide
Authors: Schadt M. , Pensl G. , Devaty R. P. , Choyke W. J. , Stein R. , Stephani D.
Published: July 2002

Hall effect measurements in a Hall-bar configuration are performed on nitrogen-doped n-type bulk 4H, 6H, and 15R SiC single crystals cut into small parallelepipeds with their longest edges either parallel or perpendicular to the ĉ axis. In the temperature range investigated (40–700 K), an anisotropy of the electron Hall mobility is observed in all three polytypes. While the mobility perpendicular to the ĉ axis—with magnetic field perpendicular or parallel to the ĉ axis—is greater than the mobility parallel to the ĉ axis for 6H and 15R SiC, 4H SiC shows the opposite behavior.
Liquid metal ion source for cluster ions of metals and alloys: design and characteristics
Authors: Bhaskar N. D. , Klimcak C. M. , Frueholz R. P.
Published: July 2002

Currently liquid metal ion sources (LMISs) are of great interest for a wide variety of applications—ion implantation, ion microlithography, thrusters for electric space propulsion, etc. A novel application of the LMIS is for the production of metallic cluster ions. In our laboratory we have designed and optimized the performance of a LMIS for the production of cluster ions of alkali metals. Using liquid rubidium (Rb) we have observed copious production of singly charged cluster ions (Rb+N, N=1–100). As expected the largest fraction of the emission consists of atomic ions. For low source current (<5 μA) about 80% of the total emission current is that of Rb+. The remaining 20% consists of Rb+2 and Rb+3. However, for large emission currents (>80 μA) we observe cluster ions as large as Rb+100. We study the mass distribution using the time-of-flight technique.
Acceleration of 100 mA of H− in a single channel electrostatic quadrupole accelerator
Authors: Kwan J. W. , Ackerman G. D. , Chan C. F. , Cooper W. S. , de Vries G. J. , Steele W. F. , Stuart M. E. , Vella M. C. , Wells R. P. , Inoue T. , Okumura Y. , Mizuno M.
Published: July 2002

Neutral beams for the next generation tokamaks will be based on multiampere negative ion beams with a beam energy of about 1.0 MeV and pulse lengths of a thousand seconds. High intensity dc beams at these levels of beam energy will require extensive development in electrostatic accelerators. At Lawrence Berkeley Laboratory, a two-module electrostatic quadrupole (ESQ) accelerator was built to accelerate ions to 200 keV. In this experiment, up to 100 mA of H− beam current was obtained from a Japan Atomic Energy Research Institute cesiated volume source using a multiaperture preaccelerator which merged 19 beamlets into a single circular beam at the entrance to the ESQ accelerator. The H− beam was accelerated by the ESQ to accelerate 200 keV without any significant beam loss or emittance growth.
Development of electron-ion coincidence spectroscopy for the study of surface dynamics combined with synchrotron radiation
Authors: Mase Kazuhiko , Nagasono Mitsuru , Tanaka Shin-ichiro , Kamada Masao , Urisu Tsuneo , Murata Yoshitada
Published: July 2002

Energy-selected electron-ion coincidence spectroscopy for the study of surface dynamics combined with synchrotron radiation (SR) was developed. The equipment consists of an electron gun, a cylindrical mirror analyzer (CMA), and a time-of-flight (TOF) ion mass spectrometer. A sample surface was excited by SR, and energy of the emitted electron was analyzed by the CMA. The TOF spectrum of the desorbed ions was measured taking the energy-analyzed electron signal as the starting trigger. The ions coincidently desorbed with the electron gave a characteristic peak in the TOF spectrum. The apparatus was evaluated on the basis of photoelectron–photoion coincidence (PEPICO) and Auger electron–photoion coincidence (AEPICO) measurements of H2O condensed on gold foil. The results demonstrate that PEPICO and AEPICO combined with SR are powerful methods for investigating the ion desorption induced by core-level excitations.
Energetic neutral atom imaging of space plasmas
Authors: Gruntman Mike
Published: July 2002

Experimental techniques and instrumentation for space plasma imaging in fluxes of energetic neutral atoms (ENAs) are reviewed. ENAs are born in charge exchange collisions between space plasma energetic ions and background neutral gas. ENAs are ubiquitous in the space environment and their energies are in the range from a few eV up to >100 keV.Contrary to charged particles, ENAs can travel large distances through space with minimal disturbance, and by recording ENA fluxes as a function of observational direction, one can reconstruct a global image of a planetary magnetosphere or the heliosphere. Plasma ion energy distribution and ion composition can be remotely established by measuring ENA energies and masses. ENA imaging opens a new window on various phenomena in space plasmas with a promise to qualitatively improve our understanding of global magnetospheric and heliospheric processes. At first we review ENA fluxes in space and their properties, and present a brief history of ENA experimental studies and the evolution of experimental approaches. The concepts of ENA imaging and particle identification are considered and followed by comparison with corpuscular diagnostics of fusion plasmas. Basic ENA techniques and instrument components are then described in detail and critically evaluated; performance characteristics, limitations, and requirements to key instrumental elements are discussed. And finally, representative ENA instruments are shown, and promising instrumental approaches are identified.
Fabrication of binary phase surface relief optical elements by selective deposition of dielectric layers
Authors: Davidson N. , Ozeri R. , Baron R.
Published: July 2002

We propose and demonstrate a new scheme to fabricate surface relief binary phase elements by using selective deposition of dielectric layers through a contact mask. By using in situ optical thickness monitoring, accurate (∼1%), repeatable, and robust layer thicknesses are readily obtained, leading to accurate phases. We demonstrate our scheme by forming a circular π phase element that is used to form a dark optical trap for atoms.
Absolute calibration of a multichannel plate detector for low energy O, O−, and O+
Authors: Stephen T. M. , Peko B. L.
Published: July 2002

Absolute detection efficiencies of a commercial multichannel plate detector have been measured for O, O+, and O−, impacting at normal incidence for energies ranging from 30–1000 eV. In addition, the detection efficiencies for O relative to its ions are presented, as they may have a more universal application. The absolute detection efficiencies are strongly energy dependent and significant differences are observed for the various charge states at lower energies. The detection efficiencies for the different charge states appear to converge at higher energies. The strongest energy dependence is for O+; the detection efficiency varies by three orders of magnitude across the energy range studied. The weakest dependence is for O−, which varies less than one order of magnitude.
Etching nanometer sized holes of variable depth from carbon cluster impact induced defects on graphite surfaces
Authors: Bräuchle Götz , Richard-Schneider Sabine , Illig Dieter , Rockenberger Jörg , Beck Rainer D. , Kappes Manfred M.
Published: July 2002

Carbon cluster ion impact induced surface damage (500, 1850, and 4000 eV) of highly oriented pyrolytic graphite (HOPG) was imaged on an atomic scale with scanning tunneling microscopy. Penetration depth was accessed by oxygen etching of ion impact induced defects. Depending on ion energy, etched holes up to 6 monolayers deep were observed. Surface morphologies of HOPG crystals can be tailored by the described method promising many applications for surface science studies and nanostructuring.
Planar optical waveguides fabricated by molecular beam epitaxy of Pb-doped CaF2 layers
Authors: Lifante G. , Balaji T. , Muñoz-Yagüe A.
Published: July 2002

Molecular beam epitaxy of rare-earth-doped CaF2 layers has already been demonstrated. Optical waveguiding can be obtained if the refractive index variation due to the rare-earth doping is high enough. To tailor separately the refractive index profile and the active ion profile in the layer, the former should be modified by a passive impurity. This letter demonstrates that passive waveguides can be obtained in CaF2 epitaxial layers by the introduction of moderate concentrations of Pb. Monomode planar waveguides with a steplike index variation ranging from 0.2% to 1.2% (relative to CaF2) have been grown and characterized, paving the way for the independent engineering of refractive index and active center distributions in optical active waveguide devices made of this kind of material.
The dissociation energy and the charge state of a copper-pair center in silicon
Authors: Istratov A. A. , Hieslmair H. , Heiser T. , Flink C. , Weber E. R.
Published: July 2002

Thermal dissociation of Cu pairs was studied in p-type silicon. The dissociation energy of the Cu pair was found to be 1.02±0.07 eV, twice as high as the binding energy of a Coulombically bound donor-acceptor pair placed on nearest neighbor 〈111〉 sites. This implies that the pair is either covalently bonded, or it consists of an ionically bonded doubly negatively charged acceptor and a singly charged donor. To distinguish between these two models, the dependence of the hole emission rate on the electric field in the depletion region was studied. The absence of the Pool-Frenkel emission enhancement ruled out the acceptor nature of the center and the purely ionic type of bonding. On the other hand, the polarization potential describing emission from a neutral impurity gave a satisfactory fit to the experimental data. It is concluded that the Cu pair is a donor with either covalent or mixed type of bonding.
Characterization of optically dense, doped semiconductors by reflection THz time domain spectroscopy
Authors: Jeon Tae-In , Grischkowsky D.
Published: July 2002

We present reflection THz-time domain spectroscopy measurements of the complex conductivity of n-type, 0.038 Ω cm GaAs and n-type, 0.22 Ω cm Si wafers. These measurements clearly demonstrate the efficacy of the reflection technique on highly conductive, optically dense samples and approach the precision of THz–TDS transmission measurements. Because the THz-bandwidth, reflection measurements extend beyond the carrier collision frequency, we obtain direct measures of the mobility and the carrier number density.
Formation of V-shaped pits in InGaN/GaN multiquantum wells and bulk InGaN films
Authors: Kim Ig-Hyeon , Park Hyeong-Soo , Park Yong-Jo , Kim Taeil
Published: July 2002

InGaN/GaN multiquantum well structures and bulk InGaN films grown on (0001) sapphire substrate were investigated by cross-sectional transmission electron microscopy and double crystal x-ray diffraction. Highly strained InGaN layers with a high In mole fraction were found to contain V-shaped surface pits with (10–11) facet planes on pure or mixed screw treading dislocations. Phase separation was also found on thick InGaN/GaN superlattices and bulk InGaN layers. The mechanism of the surface pit formation was discussed in terms of strain energy and surface mobility of InGaN.
High quantum efficiency II–VI photodetectors for the blue and blue-violet spectral range
Authors: Ehinger M. , Koch C. , Korn M. , Albert D. , Nürnberger J. , Hock V. , Faschinger W. , Landwehr G.
Published: July 2002

We have fabricated II–VI wide gap hetero PIN photodiodes made of ZnMgSSe with excellent structural and interface quality and with a high external quantum efficiency of about 60%, which is close to the theoretical limit. The internal quantum efficiency reaches peak values of more than 80%. The onset of the sensitivity is very sharp, leading to a quantum efficiency less than 10−4 at wavelength 60 nm below the band gap. Using the wide range of energy gaps from 2.68 to 3.1 eV, it is possible to fabricate wavelength selective detectors if the energy gap of the top p-type layer is chosen slightly higher than that of the intrinsic zone. The dark current is below 0.1 pA/mm2. In the low signal limit, the noise equivalent power can be estimated to be below 10−15 WHz/mm2 at the peak external quantum efficiency, surpassing ultraviolet optimized Si detectors.
Dissociation of the 1.014 eV photoluminescence copper center in silicon crystal
Authors: Nakamura M.
Published: July 2002

In order to determine dissociation energy of the 1.014 eV photoluminescence (PL) Cu center in silicon crystal, decay of the PL intensity of the center by annealing samples at various temperatures was measured. The samples were prepared by contamination of Cu from Cu solution and heat treatment at 700 °C followed by rapid cooling to room temperature. From the temperature dependence of the time constant of the decay of the PL center, activation energy of dissociation of the center was obtained. The value was 0.47+±0.05 eV, which was much smaller than that (1.02 eV) obtained by the decay of the deep level transient spectroscopy Cu center peak at Ev+0.09 eV. The present value could reasonably explain the rapid thermalization of rearrangement of the centers after the release of stress which had been observed earlier.
Influence of quantum-well structural parameters on capacitance–voltage characteristics
Authors: Moon C. R. , Lim H.
Published: July 2002

The effects of quantum well (QW) parameters on the apparent carrier distribution (ACD) are investigated using a self-consistent simulation and the capacitance–voltage (C–V) profiling techniques. The simulation results show that the change of position expectation value of two-dimensional electrons (Δx̄2D) completely determines the full width at half maximum (FWHM) of 100 K ACD peak of In0.23Ga0.77As/GaAs single QW for the well width tw⩾125 Å. The value of Δx̄2D is insensitive to the variation of conduction band offset ΔEc for ΔEc⩾120 meV. The influence of the Debye averaging effects on the ACDs increases as the values of tw and ΔEc are decreased. In multi-QW structure, the Debye averaging effects influence the FWHM of ACD peaks mostly in the first QW. Extraction of QW parameters directly from the measured C–V profile is finally discussed.
The nitrogen transport in austenitic stainless steel at moderate temperatures
Authors: Parascandola S. , Möller W. , Williamson D. L.
Published: July 2002

A model for the transport of nitrogen in austenitic stainless steel at temperatures around 400 °C is presented and discussed. The model considers the diffusion of nitrogen under the influence of trapping and detrapping at trap sites formed by local chromium. Nitrogen depth profiles simulated on the basis of the model with diffusion and detrapping activation energies of 1.1 and 1.45 eV, respectively, are in good agreement with experimental nitrogen profiles.
Symmetry of quantum phase space in a degenerate Hamiltonian system
Authors: Berman G. P. , Demikhovskii V. Ya. , Kamenev D. I.
Published: July 2002

The structure of the global “quantum phase space” is analyzed for the harmonic oscillator perturbed by a monochromatic wave in the limit when the perturbation amplitude is small. Usually, the phenomenon of quantum resonance was studied in nondegenerate [G. M. Zaslavsky, Chaos in Dynamic Systems (Harwood Academic, Chur, 1985)] and degenerate [Demikhovskii, Kamenev, and Luna-Acosta, Phys. Rev. E 52, 3351 (1995)] classically chaotic systems only in the particular regions of the classical phase space, such as the center of the resonance or near the separatrix. The system under consideration is degenerate, and even an infinitely small perturbation generates in the classical phase space an infinite number of the resonant cells which are arranged in the pattern with the axial symmetry of the order 2μ (where μ is the resonance number). We show analytically that the Husimi functions of all Floquet states (the quantum phase space) have the same symmetry as the classical phase space. This correspondence is demonstrated numerically for the Husimi functions of the Floquet states corresponding to the motion near the elliptic stable points (centers of the classical resonance cells). The derived results are valid in the resonance approximation when the perturbation amplitude is small enough, and the stochastic layers in the classical phase space are exponentially thin. The developed approach can be used for studying a global symmetry of more complicated quantum systems with chaotic behavior.
Nuclear magnetic resonance spin–spin coupling constants from coupled perturbed density functional theory
Authors: Sychrovský Vladimı́r , Gräfenstein Jürgen , Cremer Dieter
Published: July 2002

For the first time, a complete implementation of coupled perturbed density functional theory (CPDFT) for the calculation of NMR spin–spin coupling constants (SSCCs) with pure and hybrid DFT is presented. By applying this method to several hydrides, hydrocarbons, and molecules with multiple bonds, the performance of DFT for the calculation of SSCCs is analyzed in dependence of the XC functional used. The importance of electron correlation effects is demonstrated and it is shown that the hybrid functional B3LYP leads to the best accuracy of calculated SSCCs. Also, CPDFT is compared with sum-over-states (SOS) DFT where it turns out that the former method is superior to the latter because it explicitly considers the dependence of the Kohn–Sham operator on the perturbed orbitals in DFT when calculating SSCCs. The four different coupling mechanisms contributing to the SSCC are discussed in connection with the electronic structure of the molecule.
Adhesion at a granular surface
Authors: Zhou J. N. , Rar A. , Otte D. , Barnard J. A.
Published: July 2002

The adhesion of an ultrathin carbon nitride (CN) coating to the surface of a two-phase CoPt–SiO2 granular film heterogeneous at the 10 nm scale has been studied using nanoscratch techniques. The nanoscratch resistance was found to depend sensitively on the volume fraction of the two phases. Both nanoscratch experiments and complementary electron microscopy observations indicate that CN adheres much more strongly to the SiO2 matrix than to the CoPt granules resulting in enhanced tribological performance in SiO2-rich films. The relative weakness of the CN/CoPt interface is correlated to the absence of interfacial metal nitride formation. The adhesion of the CN coating to the granular surface, the intrinsic mechanical properties of the underlying granular film (nanoindentation hardness and modulus), and the vertical rms surface roughness of the granular layer are all fundamentally changed as the CoPt content reaches the percolation threshold.
Anomalous behavior of longitudinally polarized sound waves in non-Heisenberg ferromagnets
Authors: Fridman Yu. A. , Spirin D. V.
Published: July 2002

The spectra of coupled magnetoelastic waves in two-dimensional and three-dimensional non-Heisenberg magnets with biaxial anisotropy are investigated. It is shown that for a certain relationship of the material constants, three different phase states can be realized in the system. The phase transitions between these states occur by “quantum spin reduction,” and the unstable branch of elementary excitations at the phase transition point is a linearly polarized quasiphonon mode.
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In+, and Tl+ impurities in alkali halide crystals: Distortion trends
Authors: Aguado Andrés
Published: July 2002

A computational study of the doping of alkali halide crystals (AX: A=Na and K; X=Cl and Br) by ns2 cations (Ga+, In+, and Tl+) is presented. Active clusters of increasing size (from 33 to 177 ions) are considered in order to deal with the large scale distortions induced by the substitutional impurities. Those clusters are embedded in accurate quantum environments representing the surrounding crystalline lattice. The convergence of the distortion results with the size of the active cluster is analyzed for some selected impurity systems. The most important conclusion from this study is that distortions along the (100) and (110) crystallographic directions are not independent. Once a reliable cluster model is found, distortion trends as a function of impurity, alkali cation, and halide anion are identified and discussed. These trends may be useful when analyzing other cation impurities in similar host lattices.
Modeling and characterization of polycrystalline-silicon thin-film transistors with a channel-length comparable to a grain size
Authors: Yamaguchi Ken
Published: July 2002

A spatially discrete grain-boundary model for characterizing polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) is developed. This model was formulated for an interface state localized at the grain boundary. Threshold voltage (Vth) variation was analyzed using the model by changing the trap density and the location and number of grain boundaries in the poly-Si channel. The Vth shifts were found to be linearly dependent on the trap density (NGB) at the grain boundary and almost independent of the boundary location. The dependence of Vth on NGB was 0.15 V per trap density of 1012 cm−2 in long-gate TFTs. Since grain formation in the poly-Si channel is not controllable (it tends to be random), the threshold-voltage shift (ΔVth) predicted by the simulation will appear as statistical fluctuation in device fabrication. Simulation of the Vth fluctuation ranges showed that ΔVth increases with a decrease in channel length and will exceed 0.2 V in TFTs with a channel length of 1 μm or less when there is one grain boundary in the channel region and the trap density is 1012 cm−2. However, adding a moderately doped p region near the source in an n-channel TFT will suppress threshold-voltage fluctuation, even when the grain formation is uncontrollable, as we have theoretically demonstrated through simulation.
Embedding dynamics for round-off errors near a periodic orbit
Authors: Lowenstein J. H. , Vivaldi F.
Published: July 2002

We study the propagation of round-off errors near the periodic orbits of a linear map conjugate to a planar rotation with rational rotation number. We embed the two-dimensional discrete phase space (a lattice) in a higher-dimensional torus, where points sharing the same round-off error are uniformly distributed within finitely many convex polyhedra. The embedding dynamics is linear and discontinuous, with algebraic integer coefficients. This representation affords efficient algorithms for classifying and computing the orbits and their exact densities, which we apply to the case of rational rotation number with denominator 7, corresponding to certain algebraic integers of degree three. We provide evidence that the hierarchical arrangement of orbits previously detected in quadratic cases [Lowenstein et al., Chaos 7, 49–66 (1997)] disappears, and that the growth of the number of orbits with the period is algebraic.
Electromagnetic energy transport along arrays of closely spaced metal rods as an analogue to plasmonic devices
Authors: Maier Stefan A. , Brongersma Mark L. , Atwater Harry A.
Published: July 2002

The transport of electromagnetic energy along structures consisting of arrays of closely spaced metal rods (spacing=0.2 cm) was investigated in the microwave regime at 8.0 GHz (λ=3.7 cm). The dispersion relation shows that information transport occurs at a group velocity of 0.6c. The electromagnetic energy is highly confined to the arrays (90% within a distance of 0.05λ from the array). The propagation loss in a straight array is 3 dB/8 cm. Routing of energy around 90° corners is possible with a power loss of 3–4 dB. Analogies to plasmon wires consisting of arrays of nm-size metal clusters are discussed.
Multistationarity, the basis of cell differentiation and memory. II. Logical analysis of regulatory networks in terms of feedback circuits
Authors: Thomas R. , Kaufman M.
Published: July 2002

Circuits and their involvement in complex dynamics are described in differential terms in Part I of this work. Here, we first explain why it may be appropriate to use a logical description, either by itself or in symbiosis with the differential description. The major problem of a logical description is to find an adequate way to involve time. The procedure we adopted differs radically from the classical one by its fully asynchronous character. In Sec. II we describe our “naive” logical approach, and use it to illustrate the major laws of circuitry (namely, the involvement of positive circuits in multistationarity and of negative circuits in periodicity) and in a biological example. Already in the naive description, the major steps of the logical description are to: (i) describe a model as a set of logical equations, (ii) derive the state table from the equations, (iii) derive the graph of the sequences of states from the state table, and (iv) determine which of the possible pathways will be actually followed in terms of time delays. In the following sections we consider multivalued variables where required, the introduction of logical parameters and of logical values ascribed to the thresholds, and the concept of characteristic state of a circuit. This generalized logical description provides an image whose qualitative fit with the differential description is quite remarkable. A major interest of the generalized logical description is that it implies a limited and often quite small number of possible combinations of values of the logical parameters. The space of the logical parameters is thus cut into a limited number of boxes, each of which is characterized by a defined qualitative behavior of the system. Our analysis tells which constraints on the logical parameters must be fulfilled in order for any circuit (or combination of circuits) to be functional. Functionality of a circuit will result in multistationarity (in the case of a positive circuit) or in a cycle (in the case of a negative circuit). The last sections deal with “more about time delays” and “reverse logic,” an approach that aims to proceed rationally from facts to models.
Chaotic synchronization of coupled ergodic maps
Authors: Sterling D. G.
Published: July 2002

With few exceptions, studies of chaotic synchronization have focused on dissipative chaos. Though less well known, chaotic systems that lack dissipation may also synchronize. Motivated by an application in communication systems, we couple a family of ergodic maps on the N-torus and study the global stability of the synchronous state. While most trajectories synchronize at some time, there is a measure zero set that never synchronizes. We give explicit examples of these asynchronous orbits in dimensions two and four. On more typical trajectories, the synchronization error reaches arbitrarily small values and, in practice, converges. In dimension two we derive bounds on the average synchronization time for trajectories resulting from randomly chosen initial conditions. Numerical experiments suggest similar bounds exist in higher dimensions as well. Adding noise to the coupling signal destroys the invariance of the synchronous state and causes typical trajectories to desynchronize. We propose a modification of the standard coupling scheme that corrects this problem resulting in robust synchronization in the presence of noise.
Exact solutions to chaotic and stochastic systems
Authors: González J. A. , Reyes L. I. , Guerrero L. E.
Published: July 2002

We investigate functions that are exact solutions to chaotic dynamical systems. A generalization of these functions can produce truly random numbers. For the first time, we present solutions to random maps. This allows us to check, analytically, some recent results about the complexity of random dynamical systems. We confirm the result that a negative Lyapunov exponent does not imply predictability in random systems. We test the effectiveness of forecasting methods in distinguishing between chaotic and random time series. Using the explicit random functions, we can give explicit analytical formulas for the output signal in some systems with stochastic resonance. We study the influence of chaos on the stochastic resonance. We show, theoretically, the existence of a new type of solitonic stochastic resonance, where the shape of the kink is crucial. Using our models we can predict specific patterns in the output signal of stochastic resonance systems.
Efficient noncausal noise reduction for deterministic time series
Authors: Bröcker Jochen , Parlitz Ulrich
Published: July 2002

We present a simple noncausal noise reduction algorithm for time series that consist of noisy measurements of the state vectors of a deterministic (chaotic) nonlinear system. The underlying dynamical system is assumed to be known and to operate in discrete time. The noise reduction algorithm is an iterative scheme for finding exact deterministic orbits close to the measured noisy orbits. Furthermore, we discuss cases where the solution is not the original orbit but homoclinic to it.
On the nonparallel stability of the injection induced two-dimensional Taylor flow
Authors: Griffond Jérôme , Casalis Grégoire
Published: July 2002

In this article, the nonparallel stability of the so-called Taylor flow induced by wall injection is investigated. This flow is unstable due to nonparallel effects which have not been rigorously treated yet. Only inconsistent methods have been used. They give good results in comparison to the experiments but they suffer from a lack of justification and from the dependency of the results on the formulation. The necessity of using inconsistent methods instead of the Orr–Sommerfeld approach is justified in this article and a much more rigorous asymptotic analysis is performed in the limit of the large streamwise distances.
Two models for the dynamics of boiling in a short capillary tube
Authors: Cordonet A. , Lima R. , Ramos E.
Published: July 2002

We investigate the dynamical origin of the statistical properties of boiling in a short capillary tube. Two different models are proposed (Markov model and rotational model). We show that the behavior of each model may be related to different properties of the physical phenomenon. We conclude with a suggestion of a new experimental measurement which can help to adjust several aspects of the models.
The benefits of multibit chaotic sigma delta modulation
Authors: Reiss J. D. , Sandler M. B.
Published: July 2002

Sigma delta modulation is a popular technique for high-resolution analog-to-digital conversion and digital-to-analog conversion. We investigate chaotic phenomena in multibit first-order sigma–delta modulators. Particular attention is placed on the occurrence of periodic orbits or limit cycles. These may result in idle tones audible to the listener when sigma-delta modulation is used for audio signal processing. One suggested method of eliminating idle tones is the operation of a sigma delta modulator in the chaotic regime. Unfortunately, chaotic modulation of a first order sigma delta modulator is a poor system for signal processing. We show that minor variations on a traditional first order sigma–delta modulator, together with a multibit implementation, may be used to produce an effective, stable chaotic modulator that accurately encodes the input and helps remove the presence of idle tones.
A novel angular motion vacuum feedthrough
Authors: Leneman D. , Gekelman W.
Published: July 2002

A novel angular motion feedthrough to vacuum has been designed and implemented at the Large Plasma Device at UCLA. The mechanism is easy and inexpensive to build. If linear motion capability is added, then one can arbitrarily and precisely position a probe within a specific volume in a vacuum chamber.
Characterization of patterned irregularity in locally interacting, spatially extended systems: Ventricular fibrillation
Authors: Biktashev V. N. , Holden A. V.
Published: July 2002

The re-entrant ventricular arrhythmias of monomorphic ventricular tachycardia and fibrillation are produced by abnormal spatio-temporal patterns of propagation in the ventricular myocardium. These behaviors can be described by solutions of reaction-diffusion equation excitable medium models. The direct comparison of such solutions with existing experimental observations is virtually impossible as there are too many factors to be taken into account, including not only the complicated dynamics of the re-entrant waves of excitation in the tissue, but also the way the appearance of these waves on the surface is modified by the inhomogeneity, anisotropy and three-dimensional nature of heart tissue. One way of indirect comparison is to compare characteristics of the complexity of the model and the real data, that are invariant under these modifications of the signal. Karhunen–Loéve decomposition is a standard tool for evaluating the complexity of multidimensional signals. A comparison of the separate and conjoint complexities of the signals on the opposite sides of the preparation can be considered as an indicator how much three-dimensional effects are essential in the preparation behavior.
Tunable omnidirectional reflection bands and defect modes of a one-dimensional photonic band gap structure with liquid crystals
Authors: Ha Young-Ki , Yang Y.-C. , Kim J.-E. , Park H. Y. , Kee C.-S. , Lim H. , Lee J.-C.
Published: July 2002

We have theoretically investigated the tunability of the omnidirectional reflection band (ORB) of a one-dimensional (1D) photonic crystal (PC) consisting of alternating isotropic dielectric and nematic liquid crystal layers by an external electric field. The width of the ORB becomes wide as the external voltage increases, but the center frequency is changed little. The frequency of the defect mode created by inserting a layer of liquid crystals into a 1D dielectric PC is also found to be tunable by the application of external voltage. These properties can be applied to tunable optical filters or optical switches.
Random walk in an eddy and tube formation from fine particles
Authors: Kish L. B. , Vajtai R. , Ajayan P. M.
Published: July 2002

Tubular shape formation of an ensemble of ultrafine particles, captured by microscopic eddies in a fluid or gaseous medium, is investigated. In the circulation flow of the eddy, the small particles are driven by the deterministic hydrodynamical forces and the random forces of Brownian motion. The conditions for dynamically/statistically stable tube formation and the resulting tube parameters are obtained by analytic calculations and computer simulations, respectively. The model yields striking similarities to the characteristics of nanotube formation observed in turbulent media such as the carbon arc, and throws some light on tubular formation observed in fluid media.
Fokker–Planck model for nonlocal impact ionization in semiconductors
Authors: Jacob Biju , Robson P. N. , David J. P. R. , Rees G. J.
Published: July 2002

The probability distribution function (PDF) for impact ionization path length is a crucial quantity for understanding and modeling the low noise behavior of avalanche photodiodes with short multiplication regions. In such devices the ionization coefficient is no longer in equilibrium with the local electric field but depends on the carrier’s history. The high electric fields needed to produce avalanche gain narrow the PDF, thereby reducing the randomness in ionization position and hence the noise in the multiplication. In this article we present a method for calculating PDFs using a Fokker–Planck model. The results are compared with those obtained from an equivalent Monte Carlo simulation employing a parabolic energy band, deformation potential optical phonon scattering, and a hard energy threshold for impact ionization.
Folding in the Skyrme model
Authors: Houghton Conor J. , Krusch Steffen
Published: July 2002

There are only three stable singularities of a differentiable map between three-dimensional manifolds, namely folds, cusps and swallowtails. A Skyrme configuration is a map from space to SU2, and its singularities correspond to the points where the baryon density vanishes. In this article we consider the singularity structure of Skyrme configurations. The Skyrme model can only be solved numerically. However, there are good analytic ansätze. The simplest of these, the rational map ansatz, has a nongeneric singularity structure. This leads us to introduce a nonholomorphic ansatz as a generalization. For baryon numbers 2, 3, and 4, the approximate solutions derived from this ansatz are closer in energy to the true solutions than any other ansatz solution. We find that there is a tiny amount of negative baryon density for baryon number 3, but none for 2 or 4. We comment briefly on the relationship to Bogomolny–Prasad–Sommerfield monopoles.
Character of fluctuations in deterministically thermostatted nonequilibrium systems
Authors: Kumičák Juraj
Published: July 2002

The application of Nosé–Hoover equations of motion to analysis of stationary nonequilibrium systems—driven away from equilibrium by inherent thermostatting—is briefly discussed. The Galton board model, to which the analysis does apply, is described. Numerical simulations of this specific model suggest that the system exhibits 1/fk noise, with 1⩽k⩽2. Explanation of this property is based on the fact that the system approaches an ergodic strange attractor due to the action of inherent friction force. Analysis of generalized baker maps which also have strange attractors but exhibit white noise, is used to support the view that the presence of strange attractor alone is not sufficient for appearance of 1/f noise. It seems that the latter is found only in thermostatted systems which without thermostatting have 1/f2 noise. The paper concludes with the discussion of many open problems which still remain unsolved in this approach.
Nonlinear trans-resonant waves, vortices and patterns: From microresonators to the early Universe
Authors: Galiev Sh. U. , Galiyev T. Sh.
Published: July 2002

Perturbed wave equations are considered. Approximate general solutions of these equations are constructed, which describe wave phenomena in different physical and chemical systems. Analogies between surface waves, nonlinear and atom optics, field theories and acoustics of the early Universe can be seen in the similarities between the general solutions that govern each system. With the help of the general solutions and boundary conditions and/or resonant conditions we have derived the basic highly nonlinear ordinary differential equation or the basic algebraic equation for traveling waves. Then, approximate analytic resonant solutions are constructed, which describe the trans-resonant transformation of harmonic waves into traveling shock-, jet-, or mushroom-like waves. The mushroom-like waves can evolve into cloud-like and vortex-like structures. The motion and oscillations of these waves and structures can be very complex. Under parametric excitation these waves can vary their velocity, stop, and change the direction of their motion. Different dynamic patterns are yielded by these resonant traveling waves in the x–t and x–y planes. They simulate many patterns observed in liquid layers, optical systems, superconductors, Bose–Einstein condensates, micro- and electron resonators. The harmonic excitation may be compressed and transformed inside the resonant band into traveling or standing particle-like waves. The area of application of these solutions and results may possibly vary from the generation of nuclear particles, acoustical turbulence, and catastrophic seismic waves to the formation of galaxies and the Universe. In particular, the formation of galaxies and galaxy clusters may be connected with nonlinear and resonant phenomena in the early Universe.
Food chain chaos due to junction-fold point
Authors: Deng Bo
Published: July 2002

Consideration is given to a basic food chain model satisfying the trophic time diversification hypothesis which translates the model into a singularly perturbed system of three time scales. It is demonstrated that in some realistic system parameter region, the model has a unimodal or logistic-like Poincaré return map when the singular parameter for the fastest variable is at the limiting value 0. It is also demonstrated that the unimodal map goes through a sequence of period-doubling bifurcations to chaos. The mechanism for the creation of the unimodal criticality is due to the existence of a junction-fold point [B. Deng, J. Math. Biol. 38, 21–78 (1999)]. The fact that junction-fold points are structurally stable and the limiting structures persist gives us a rigorous but dynamical explanation as to why basic food chain dynamics can be chaotic.
Large magnetocaloric effect in manganites with charge order
Authors: Sande P. , Hueso L. E. , Miguéns D. R. , Rivas J. , Rivadulla F. , López-Quintela M. A.
Published: July 2002

In this work, we report the magnetocaloric effect (|ΔSM|), around the charge/orbital ordering transition in the mixed valent manganite Nd0.5Sr0.5MnO3. The magnitude of |ΔSM| around this first-order transition is around three times larger than that obtained around the second-order transition (ferromagnetic-metallic-to-paramagnetic-insulator) in the same compound. Actually, the magnetocaloric response around the charge-order transition is comparable to pure Gd, the rare earth with the highest magnetocaloric effect. The possibility of an easy tuning of the charge-order transition temperatures in doped manganites opens a way of investigation materials usable in magnetic refrigerators.
An accurate analytic H4 potential energy surface
Authors: Boothroyd A. I. , Martin P. G. , Keogh W. J. , Peterson M. J.
Published: July 2002

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. The 6101 ab initio H4 energies reported in 1991 by Boothroyd et al. demonstrated large inaccuracies in analytic H4 surfaces available at that time. Some undesirable features remained in the more accurate H4 surfaces fitted to these energies by Keogh and by Aguado et al., due in part to the relatively sparse coverage of the six-dimensional H4 conformation space afforded by the 6101 ab initio energies. To improve the coverage, 42 079 new ab initio H4 energies were calculated, using Buenker’s multiple reference (single and) double excitation configuration interaction program. Here the lowest excited states were computed as well as the ground state, and energies for the original 6101 conformations were recomputed. The ab initio energies have an estimated rms “random” error of ∼0.5 millihartree and a systematic error of ∼1 millihartree (0.6 kcal/mol). A new analytical H4 PES was fitted to these 48 180 ab initio energies (and to an additional 13 367 points generated at large separations), yielding a significant improvement over previous H4 surfaces. This new PES has an rms error of 1.43 millihartree relative to these 48 180 ab initio energies (the fitting procedure used a reduced weight for high energies, yielding a weighted rms error of 1.15 millihartree for these 48 180 ab initio energies). For the 39 064 ab initio energies that lie below twice the H2 dissociation energy, the new PES has an rms error of 0.95 millihartree. These rms errors are comparable to the estimated error in the ab initio energies themselves. The new PES also fits the van der Waals well to an accuracy of about 5%. For relatively compact conformations (energies higher than the H2 dissociation energy), the conical intersection between the ground state and the first excited state is the largest source of error in the analytic surface. The position of this conical intersection forms a somewhat complicated three-dimensional hypersurface in the six-dimensional conformation space of H4. A large portion of the position of the conical intersection has been mapped out, but trying to include the conical intersection explicitly in an analytic surface is beyond the scope of the present paper.
Branching of 2D tori off an equilibrium of a cosymmetric system (codimension-1 bifurcation)
Authors: Kurakin L. G. , Yudovich V. I.
Published: July 2002

The standard object for vector fields with a nontrivial cosymmetry is a continuous one-parameter family of equilibria. Characteristically, the stability spectrum of equilibrium varies along such a family, though the spectrum always contains a zero point. Consequently, in the general position a family consists of stable and unstable arcs separated by boundary equilibria, which are neutrally stable in the linear approximation. In the present paper the central manifold method and the Lyapunov–Schmidt method are used to investigate the branching bifurcation of invariant two-dimensional tori in cosymmetric systems off a boundary equilibrium whose spectrum contains, besides the requisite point 0, two pairs of purely imaginary eigenvalues. A number of new effects, as compared with the classic case of an isolated equilibrium, are found: the bifurcation studied has codimension 1 (2 for an isolated equilibrium); it is accompanied by a branching bifurcation of a normal limit cycle; and, a stable arc can be created on an unstable arc.