bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo update for chain molecules: Biased Gaussian steps in torsional space
Authors: Favrin Giorgio , Irbäck Anders , Sjunnesson Fredrik
Published: July 2002

We develop a new elementary move for simulations of polymer chains in torsion angle space. The method is flexible and easy to implement. Tentative updates are drawn from a (conformation-dependent) Gaussian distribution that favors approximately local deformations of the chain. The degree of bias is controlled by a parameter b. The method is tested on a reduced model protein with 54 amino acids and the Ramachandran torsion angles as its only degrees of freedom, for different b. Without excessive fine tuning, we find that the effective step size can be increased by a factor of 3 compared to the unbiased b=0 case. The method may be useful for kinetic studies, too.
Theoretical study for partial molar volume of amino acids and polypeptides by the three-dimensional reference interaction site model
Authors: Harano Yuichi , Imai Takashi , Kovalenko Andriy , Kinoshita Masahiro , Hirata Fumio
Published: July 2002

We calculate the partial molar volume (PMV) of 20 amino acids in aqueous solution at infinite dilution by using the Kirkwood–Buff equation and the three-dimensional reference interaction site model (3D-RISM) integral equation theory for molecular liquids. As compared to the conventional, one-dimensional (1D-RISM) approach, the results exhibit drastic improvement for the quantitative agreement with experiments. The deviation from the experimental data seen for the relatively large amino acids is discussed in terms of the “ideal fluctuation volume” introduced in the previous study based on the 1D-RISM. Robustness of the new approach is further demonstrated by applying it to the PMV of polyglutamic acids in aqueous solution. The method provides reasonable account for the PMV increase with the chain length, both in α-helical and extended structures, whereas the 1D-RISM approach gives an unnatural decrease of the PMV for the α helix with a complete turn of the backbone.
Single molecule spectroscopy of disordered circular aggregates: A perturbation analysis
Authors: Dempster Sara E. , Jang Seogjoo , Silbey Robert J.
Published: July 2002

Analytical expressions for various spectroscopic observables are derived for statically disordered circular aggregates using first order perturbation theory. Quantities relevant to single molecule spectroscopy experiments on circular aggregates such as the distribution of energy splittings between levels that are degenerate in the absence of disorder, and the distribution of intensity of levels that are forbidden in the absence of disorder are derived. For simplicity only diagonal disorder is considered here. The approach, however, can be extended to include other types of disorder. Expressions are written in terms of standard deviations of Gaussian disorder, the symmetry number of the aggregate, and in some cases intermolecular couplings. The approach is compared to numerical simulations and shows excellent agreement over a large range of parameters. The expressions, however, provide a more direct framework to examine and understand the effects of disorder on spectroscopy of aggregates than do the simulations. The potential utility of the expressions for quantifying disorder and intermolecular couplings from single molecule experiments is also demonstrated.
Curing difficult cases in magnetic properties prediction with self-interaction corrected density functional theory
Authors: Patchkovskii S. , Autschbach J. , Ziegler T.
Published: July 2002

The Perdew–Zunger self-interaction correction (SIC) was implemented self-consistently within a molecular density functional theory (DFT) program, using the Krieger–Li–Iafrate approximation to the optimized effective potential, and the Vosko–Wilk–Nusair (VWN) functional. The computationally efficient implementation relies on the fitting of orbital densities for the evaluation of orbital Coulomb potentials, and allows for routine applications to large molecules. Due to the use of the effective potential approach, the evaluation of the energy derivatives can be handled by standard Kohn–Sham DFT techniques in a straightforward way. The SIC-VWN technique is applied to the calculation of nuclear magnetic resonance (NMR) parameters in representative small molecules, containing C, H, N, O, and F. Removal of self-interaction leads to a substantial improvement in the calculated isotropic chemical shifts for N, O, and F, where SIC-VWN holds an advantage over both local (VWN), and gradient-corrected functionals (Becke 88-Perdew 86, BP86). For C and H isotropic chemical shifts, which are well described by the gradient-corrected functionals, SIC-VWN performs as well as BP86. SIC-VWN also improves the description of the absolute chemical shielding, and of the principal components of the NMR shielding tensors. The changes arise mainly from adjustments in the Kohn–Sham orbital energies, leading to a better description of the paramagnetic contribution to the shielding tensor. For spin–spin coupling constants, SIC-VWN improves the description of the paramagnetic contribution. At the same time, the magnitude of the Fermi contact term is underestimated, yielding mixed overall results. Slow convergence of the spin-spin coupling results with the basis set size prevents a conclusive statistical evaluation for this property. The clear physical origin of the SIC-VWN effect in the prediction of magnetic properties opens the tantalizing possibility that this technique may be effective in solving problems often encountered in the calculations of NMR parameters of heavier nuclei.
Adsorption of acenes on flat and vicinal Cu(111) surfaces: Step induced formation of lateral order
Authors: Lukas S. , Vollmer S. , Witte G. , Wöll Ch.
Published: July 2002

The adsorption of benzene, naphthalene, and anthracene on Cu(111) and its vicinal Cu(221) and Cu(443) surfaces has been studied by means of thermal desorption spectra (TDS) and low energy electron diffraction. For each of the studied acenes (C4n+2H2n+4, n=1,2,3) two distinct binding states were observed in the TDS corresponding to adsorption on terraces and at step edges. The binding energies of both states are found to increase linearly with the number of aromatic rings n. In contrast to the Cu(111) surface where these acenes form disordered adlayers only, ordered films of naphthalene and anthracene with an alignment of the molecular axis along the steps could be prepared on the vicinal surfaces.
Measurement of surface charge densities on Brownian particles using total internal reflection microscopy
Authors: von Grünberg H. H. , Helden L. , Leiderer P. , Bechinger C.
Published: July 2002

Due to double-layer forces a charged colloid suspended in an electrolyte is repelled from a like-charged planar wall. We demonstrate that and how a precise measurement of these double-layer forces acting on a colloid near a glass surface can be used to determine surface charge densities. The effective wall–colloid potentials are measured using the total internal reflection microscopy technique, and a whole series of such potentials, taken for various different salt concentrations, are then analyzed in terms of a given theoretical interaction potential, where the surface charge densities are the only unknown parameters. We find reasonable values for the surface charge densities of silica and polystyrene spheres in water, and compare the proposed method with other more established techniques to measure surface charge densities on single particles.
Microstamping of freestanding bipolymer features
Authors: Moran P. M. , Robert C.
Published: July 2002

Microscale bipolymer features have been fabricated by microstamping. Rigid silicon stamps with microwells as small as 40 fl in volume were used. Bipolymer columns, 2 μm by 2 μm in cross section and 10 μm tall, were stamped from these small microwells. The top 1.5 μm of each column was made of polyetherimide with the remaining 8.5 μm being made from an ethyl cyanoacrylate. The ability to accurately fabricate microscale features consisting of more than one polymer has many potential uses. As an example, the potential use of bipolymer features in nerve guides is discussed.
Chaotic transition in a three-coupled phase-locked loop system
Authors: Tsuruda Hidekatsu , Shirahama Hiroyuki , Fukushima Kazuhiro , Nagadome Masakazu , Inoue Masayoshi
Published: July 2002

The chaotic transition is observed in a three-coupled phase-locked loop (PLL) system in both experiments and numerical simulations. In this system, three PLL oscillators are connected with the periodic boundary condition. Intermittency is found in partially synchronized phase, in which two of three oscillators synchronize with each other and form a pair, and the chaotic transition occurs due to the recombination of synchronized pairs so that different pair is re-formed. In this phase, on–off intermittency is also observed and statistical analyses are carried out for on–off intermittent time series. This intermittency is considered as a hybrid type of intermittency with both on–off intermittency and intermittency due to the recombination of synchronized pairs present in the same time series. We also show the chaotic transition phenomena in a three-coupled logistic map system.
2+1
gravity, chaos and time machines
Authors: Bengtsson Ingemar , Brännlund Johan
Published: July 2002

2+1 gravity for space–times with topology R×T2 has been much studied. We add a description of how to extend these space–times across a Cauchy horizon into a region where the torus becomes Lorentzian. The result is a one parameter family of tori given by a geodesic in the “Teichmüller space” of Lorentzian tori. We describe this in detail. We also point out that if the modular group is regarded as part of the gauge group then these space–times offer a nice toy model for the dynamics of Bianchi IX models; in the region where the tori are spacelike the dynamics is described exactly by a hyperbolic billiard. On the other hand the modular group acts ergodically on the Teichmüller space of Lorentzian tori.
Similarity reduction, generalized symmetries and integrability of Belov–Chaltikian and Blaszak–Marciniak lattice equations
Authors: Sahadevan R. , Khousalya S.
Published: July 2002

The Lie point symmetries of Belov–Chaltikian (BC) and Blaszak–Marciniak (BM) lattice equations is derived. Using the symmetries similarity reduction for both BC and BM lattice equations is obtained and show that each of the reduced equation possesses Lax representation and satisfies the singularity confinement criteria (a discrete version of the Painlevé property) indicating their integrability. Two particular solutions of both the reduced equations are given explicitly. A systematic investigation for nonclassical symmetries of BC and BM lattice equations is carried out. Further, a sequence of conserved densities and generalized symmetries of both BC and BM lattice equation are derived explicitly. The existence of a sequence of such symmetries is a predictor for integrability.
Modeling quantum measurement probability as a classical stochastic process
Authors: Gillespie Daniel T. , Alltop William O. , Martin Jorge M.
Published: July 2002

The time-dependent measurement probabilities for the simple two-state quantum oscillator seem to invite description as a classical two-state stochastic process. It has been shown that such a description cannot be achieved using a Markov process. Constructing a more general non-Markov process is a challenging task, requiring as it does the proper generalizations of the Markovian Chapman–Kolmogorov and master equations. Here we describe those non-Markovian generalizations in some detail, and we then apply them to the two-state quantum oscillator. We devise two non-Markovian processes that correctly model the measurement statistics of the oscillator, we clarify a third modeling process that was proposed earlier by others, and we exhibit numerical simulations of all three processes. Our results illuminate some interesting though widely unappreciated points in the theory of non-Markovian stochastic processes. But since quantum theory does not tell us which one of these quite different modeling processes “really” describes the behavior of the oscillator, and also since none of these processes says anything about the dynamics of other (noncommuting) oscillator observables, we can see no justification for regarding any of these processes as being fundamentally descriptive of quantum dynamics.
“Green” noise in quasistationary stochastic systems
Authors: Guz S. A. , Sviridov M. V.
Published: July 2002

We consider the behavior of stochastic systems driven by noise with a zero value of spectral density at zero frequency (“green” noise). For this purpose we propose the version of the Krylov–Bogoliubov averaging method to study the systems which are not stationary in the case of an external white noise. We use the ergodicity of a nonlinear random function in the method, and obtain equations for any approximation of the theory. In particular, it is shown in the first approximation that there is an effective potential to describe the averaged motion of the system. We consider a phase-locked loop as an example and show that metastable states are possible. The lifetime of these states essentially increases if the form of a green noise spectrum becomes sharper in the low-frequency region. The high stability of the system driven by green noise is confirmed by numerical simulation. It is important that the theoretical result obtained by the averaging method and the one obtained in the simulation coincide with sufficient accuracy. In conclusion, we discuss some of the unsolved green noise problems.
Cliffordons
Authors: Finkelstein David R. , Galiautdinov Andrei A.
Published: July 2002

At higher energies the present complex quantum theory with its unitary group might expand into a real quantum theory with an orthogonal group, broken by an approximate i operator at lower energies. Implementing this possibility requires a real quantum double-valued statistics. A Clifford statistics, representing a swap (12) by a difference γ1−γ2 of Clifford units, is uniquely appropriate. Unlike the Maxwell–Boltzmann, Fermi–Dirac, Bose–Einstein, and para-statistics, which are tensorial and single-valued, and unlike anyons, which are confined to two dimensions, Clifford statistics are multivalued and work for any dimensionality. Nayak and Wilczek such Clifford statistics for the fractional quantum Hall effect. We apply them to toy quanta here. A complex-Clifford example has the energy spectrum of a system of spin-1/2 particles in an external magnetic field. This supports the proposal that the double-valued rotations—spin—seen at current energies might arise from double-valued permutations—swap—to be seen at higher energies. Another toy with real Clifford statistics illustrates how an effective imaginary unit i can arise naturally within a real quantum theory.
Quantum Chern–Simons vortices on a sphere
Authors: Romão Nuno M.
Published: July 2002

The quantization of the reduced first-order dynamics of the nonrelativistic model for Chern–Simons vortices introduced by Manton is studied on a sphere of given radius. We perform geometric quantization on the moduli space of static solutions, using a Kähler polarization, to construct the quantum Hilbert space. Its dimension is related to the volume of the moduli space in the usual classical limit. The angular momenta associated with the rotational SO(3) symmetry of the model are determined for both the classical and the quantum systems. The results obtained are consistent with the interpretation of the solitons in the model as interacting bosonic particles.
Variance analysis of the Coulomb blockade parameters in nanometer-size disordered arrays
Authors: Leroy Y. , Cordan A. S. , Goltzené A.
Published: July 2002

Recently, Coulomb blockade could be observed up to room temperature on devices with arrays. As they are highly disordered, an important dispersion of the electrical characteristics is expected. We calculate the dispersion of the Coulomb blockade threshold voltage Vth, for disordered arrays. At very low temperature, Vth increases with the array size, as well for one-dimensional (1D) as for two-dimensional (2D) arrays, and the relative dispersion remains smaller than 10%. Such a promising behavior does not hold at higher temperatures. On one hand, a larger gain in Vth with the array size is only obtained on 1D arrays. On the other one, the dispersion rapidly becomes catastrophic for both 1D and 2D cases. We propose a way to reduce it to a range of 10%–15%, almost compatible with very large scale integration applications.
Percolation model of the temperature dependence of resistance in doped manganese perovskites
Authors: Yuan S. L. , Li Z. Y. , Peng G. , Xiong C. S. , Xiong Y. H. , Tang C. Q.
Published: July 2002

Assuming the sample to consist of ferromagnetic metallic (FMM) particles with a volume fraction (f ) randomly distributed over the paramagnetic insulating background, Monte Carlo simulations of electrical conductivity show excellent fits to the data measured in (La1−xYx)2/3Ca1/3MnO3 (x=0, 0.1 and 0.2). We find that the transition to metallic state occurs as the f reaches a percolation threshold, suggesting that the percolation of FMM domains is responsible for the observed insulator–metal transition.
Comparison between phase space structures in coupled Morse systems and in various su(2) approximations
Authors: Jung C. , Ziemniak E. , Carvajal M. , Frank A. , Lemus R.
Published: July 2002

While Hamiltonians written in terms of position and momentum provide a transparent picture of the motion of a system, Hamiltonians written in terms of Lie algebras are easier to handle quantum mechanically. Therefore we are interested to know how to transform one into the other. Since the exact transformation often leads to complicated expressions, we look for approximations which preserve the essential features. As basic criterion we look for the degree of equality of the classical phase space structures. We illustrate our ideas for the case of two coupled Morse systems and its approximation in terms of the Lie algebra su(2), which is relevant to anharmonic models of molecular spectroscopy.
Magnetic field dependence of magnetization reversal behavior in Co/Pt multilayers
Authors: Cho Yoon-Chul , Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

Magnetic field dependence of magnetization reversal in Co/Pt multilayers has been quantitatively investigated. Serial samples of Co/Pt multilayers have been prepared by dc magnetron sputtering under various Ar pressures. Magnetization reversal was monitored by magnetization viscosity measurement and direct domain observation using a magneto-optical microscope system. The wall-motion speed V and the nucleation rate R were determined using a domain reversal model based on time-resolved domain reversal patterns. Both V and R were found to be exponentially dependent on the reversing applied field. From the exponential dependence, the activation volumes of the wall-motion and nucleation could be determined based on a thermally activated relaxation model. The wall-motion activation volume was revealed to be slightly larger than the nucleation activation volume.
Space-like hypersurfaces in the de Sitter spaces
Authors: Ximin Liu
Published: July 2002

By using Cheng–Yau’s self-adjoint operator □, we study the space-like hypersurfaces in the de Sitter spaces and obtain some general rigidity results.
Surface anisotropy from photo-induced bond breaking at polymer surfaces: A sum-frequency vibrational spectroscopic study of polyimide
Authors: Oh-e Masahito , Kim Doseok , Shen Y. R.
Published: July 2002

Sum-frequency vibrational spectroscopy was used to study how linearly polarized UV irradiation could cause bond breaking and alter the surface structure of a polyimide. The spectroscopic results allowed the determination of an approximate orientational distribution of the polymer backbones at a rubbed surface. They also permitted deduction of rates of bond breaking of the backbones at the surface by UV dosage. The surface anisotropy resulting from bond breaking by linearly polarized UV irradiation was found to be relatively small, suggesting a weak azimuthal anchoring energy for liquid crystal films deposited on such a surface.
Thermodynamic energy exchange in a moving plate capacitor
Authors: Davis B. R. , Abbott D. , Parrondo J. M. R.
Published: July 2002

In this paper we describe an apparent paradox concerning a moving plate capacitor driven by thermal noise from a resistor. The plates are attracted together, but a demon restores the plates of the capacitor to their original position when the voltage across the capacitor is small—hence only small forces are present for the demon to work against. The demon has to work harder than this to avoid the situation of perpetual motion, but the open question is how? This is unsolved, however we explore the concept of a moving plate capacitor by examining the case where it is still excited by thermal noise, but where the restoring force on the capacitor plates is provided by a simple spring rather than some unknown demon. We display simulation results with interesting behavior, particularly where the capacitor plates collide with each other.
Shaping carbon nanostructures by controlling the synthesis process
Authors: Merkulov Vladimir I. , Guillorn Michael A. , Lowndes Douglas H. , Simpson Michael L. , Voelkl Edgar
Published: July 2002

The ability to control the nanoscale shape of nanostructures in a large-scale synthesis process is an essential and elusive goal of nanotechnology research. Here, we report significant progress toward that goal. We have developed a technique that enables controlled synthesis of nanoscale carbon structures with conical and cylinder-on-cone shapes and provides the capability to dynamically change the nanostructure shape during the synthesis process. In addition, we present a phenomenological model that explains the formation of these nanostructures and provides insight into methods for precisely engineering their shape. Since the growth process we report is highly deterministic in allowing large-scale synthesis of precisely engineered nanoscale components at defined locations, our approach provides an important tool for a practical nanotechnology.
Resonant Raman scattering by breathing modes of metal nanoparticles
Authors: Portales H. , Saviot L. , Duval E. , Fujii M. , Hayashi S. , Del Fatti N. , Vallée F.
Published: July 2002

Low-frequency Raman scattering experiments have been performed on metal nanoparticles embedded in two different thermally treated matrices. In addition to the well-known Raman scattering by the nanoparticle quadrupolar vibrational mode, the spectra measured in the 3–40 cm−1 frequency range exhibit several new bands. They are ascribed to resonant scattering by the nanoparticle breathing mode and its harmonics, in very good agreement with time-resolved measurements.
Synthesis and characterization of highly-conducting nitrogen-doped ultrananocrystalline diamond films
Authors: Bhattacharyya S. , Auciello O. , Birrell J. , Carlisle J. A. , Curtiss L. A. , Goyette A. N. , Gruen D. M. , Krauss A. R. , Schlueter J. , Sumant A. , Zapol P.
Published: July 2002

Ultrananocrystalline diamond (UNCD) films with up to 0.2% total nitrogen content were synthesized by a microwave plasma-enhanced chemical-vapor-deposition method using a CH4(1%)/Ar gas mixture and 1%–20% nitrogen gas added. The electrical conductivity of the nitrogen-doped UNCD films increases by five orders of magnitude (up to 143 Ω−1 cm−1) with increasing nitrogen content. Conductivity and Hall measurements made as a function of film temperature down to 4.2 K indicate that these films have the highest n-type conductivity and carrier concentration demonstrated for phase-pure diamond thin films. Grain-boundary conduction is proposed to explain the remarkable transport properties of these films.
Complexation of DNA with positive spheres: Phase diagram of charge inversion and reentrant condensation
Authors: Nguyen T. T. , Shklovskii B. I.
Published: July 2002

The phase diagram of a water solution of DNA and oppositely charged spherical macroions is studied. DNA winds around spheres to form beads-on-a-string complexes resembling the chromatin 10 nm fiber. At small enough concentration of spheres these “artificial chromatin” complexes are negative, while at large enough concentrations of spheres the charge of DNA is inverted by the adsorbed spheres. Charges of complexes stabilize their solutions. In the plane of concentrations of DNA and spheres the phases with positive and negative complexes are separated by another phase, which contains the condensate of neutral DNA–spheres complexes. Thus, when the concentration of spheres grows, DNA–spheres complexes experience condensation and resolubilization (or reentrant condensation). Phenomenological theory of the phase diagram of reentrant condensation and charge inversion is suggested. Parameters of this theory are calculated by microscopic theory. It is shown that an important part of the effect of a monovalent salt on the phase diagram can be described by the nontrivial renormalization of the effective linear charge density of DNA wound around a sphere, due to the Onsager–Manning condensation. We argue that our phenomenological phase diagram or reentrant condensation is generic to a large class of strongly asymmetric electrolytes. Possible implications of these results for the natural chromatin are discussed.
Optical properties of Ag and Au nanowire gratings
Authors: Schider G. , Krenn J. R. , Gotschy W. , Lamprecht B. , Ditlbacher H. , Leitner A. , Aussenegg F. R.
Published: July 2002

The optical response of regularly arranged noble metal wires with nanoscopic cross sections (nanowire gratings) strongly depends on the polarization direction of the incident light. We use silver and gold nanowire gratings produced by electron beam lithography to study this effect by optical extinction spectroscopy. For a polarization direction perpendicular to the wire axis, the excitation of a dipolar plasmon mode dominates the extinction spectrum. The spectral position of the plasmon resonance can be tuned by an appropriate choice of nanowire geometry and material. For a polarization direction parallel to the wire axis, the profile of the extinction spectrum varies mainly as a function of the grating constant. In particular, a transmission maximum for small grating constants is found. By combining the surface plasmon excitation and grating effect for orthogonal polarization directions, a spectrally selective polarizer with an extinction ratio of 26 is demonstrated.
Application of time resolved area normalized emission spectroscopy to multicomponent systems
Authors: Koti A. S. R. , Periasamy N.
Published: July 2002

Time resolved emission spectroscopy (TRES) provides information on the excited state kinetics and heterogeneity of emissive species in a system. Time resolved area normalized emission spectroscopy (TRANES), an extension to TRES, is a novel, model-free method for the analysis of intrinsic or extrinsic fluorescence probes in complex chemical and biophysical systems [Koti, Krishna, and Periasamy, J. Phys. Chem. A 105, 1767 (2001)]. Observation of a single isoemissive point in TRANES analysis of fluorescence is an unambiguous indication of the presence of two emissive species in the system. The presence of multiple isoemissive points in TRANES spectra is confirmed using simulation and experimental data of multicomponent systems.
Switchable superconducting quantum interferometers
Authors: Eom J. , Johnson Mark
Published: July 2002

In a simple bilayer device structure, a ferromagnetic film spans a lithographically patterned superconducting loop. When the magnetization orientation is along the loop axis, magnetoquenched weak links are formed in each arm of the loop. Quantum interference effects are studied by applying a weak magnetic field perpendicular to the sample plane. Switching the magnetization between stable orientations parallel or perpendicular to the loop axis turns the interference ON or OFF, demonstrating a switchable superconducting quantum interference device.
Quadrupolar-shielding cross-correlations in solid state nuclear magnetic resonance: Detecting antisymmetric components in chemical shift tensors
Authors: Wi Sungsool , Frydman Lucio
Published: July 2002

This work investigates the nature of second order effects resulting in solid state nuclear magnetic resonance (NMR), from cross-correlations between the quadrupolar and shielding couplings. Using an average Hamiltonian theory, it is shown that these effects can bring the nonsecular terms of the shielding interaction into the realm of conventional detection. Such terms include the antisymmetric components of the chemical shift tensor, which, although postulated to exist, have hitherto eluded direct experimental measurement. As numerical calculations supported these analytical derivations, an experimental study was undertaken to measure such components from the single-crystal rotation plot of a half-integer quadrupolar nucleus. A custom-made probehead was built, a data analysis procedure developed, and together these were used to analyze the satellite transition spectra arising from a Co59 single crystal of cobalt(III)tris(acetylacetonate). The results of repetitive studies on such sample are reported.
An iterative procedure for the inversion of two-dimensional ion/photoelectron imaging experiments
Authors: Vrakking Marc J. J.
Published: July 2002

We present an iterative method for the extraction of velocity and angular distributions from two-dimensional (2D) ion/photoelectron imaging experiments. This method is based on the close relationship which exists between the initial 3D angular and velocity distribution and the measured 2D angular and radial distributions, and gives significantly better results than other inversion procedures which are commonly used today. Particularly, the procedure gets rid of the center-line noise which is one of the main artifacts in many current ion/photoelectron imaging experiments.
Fabrication of mesoscopic devices from graphite microdisks
Authors: Dujardin Erik , Thio Tineke , Lezec Henri , Ebbesen Thomas W.
Published: July 2002

Structural and transport properties of individual micron-sized graphitic disks with less than one hundred graphene layers are measured. Mesoscopic devices of any arbitrary geometry can be fabricated at the center of such disks by focussed ion beam lithography, with the rest of the disk serving as low-resistance contacts. Our approach is exemplified by the fabrication and characterization of 30 to 60 nm wide point-contact devices. A size effect is revealed by the suppression of the magnetoresistance in constrictions narrower than the smallest attained cyclotron orbit for fields up to 10 T.
Collision of hydrogen atom with single-walled carbon nanotube: Adsorption, insertion, and healing
Authors: Ma Yuchen , Xia Yueyuan , Zhao Mingwen , Ying Minju , Liu Xiangdong , Liu Pijun
Published: July 2002

Interaction of hydrogen atom with (5, 5) single-walled carbon nanotube (SWNT) has been studied over the collision energy range from 1 to 30 eV using a molecular dynamics simulation method. In the energy interval of 1–3 eV, the hydrogen atom can be chemisorbed on the outer wall of the SWNT, provided the impact point is near a vertex carbon atom of a hexagon. The lowest incident energy needed for a hydrogen atom to pass through a hexagon ring on the SWNT is estimated to be 14 eV. Hydrogen atoms that enter into the SWNT would either be encapsulated in it to form endohedral H@tube complex, or escape out of it. The hole on the sidewall of the nanotube induced by the collision of hydrogen atom can be healed after relaxation for several picoseconds.
An excess electron bound to urea. I. Canonical and zwitterionic tautomers
Authors: Skurski Piotr , Simons Jack
Published: July 2002

The possibility of electron binding to urea was studied at the coupled cluster level of theory with single, double, and noniterative triple excitations. It was found that none of the urea isomers forms a valence anionic state although almost all of them can attach an excess electron and form a stable dipole-bound or Rydberg anion. Moreover, the canonical tautomers are the lowest energy structures of the neutral and anion. The zwitterionic isomer was found to be locally stable only when solvated with an “extra” electron and the corresponding anion is a Rydberg species perturbed by a neighboring negative charge.
Dynamics of bicontinuous microemulsion phases with and without amphiphilic block-copolymers
Authors: Mihailescu M. , Monkenbusch M. , Endo H. , Allgaier J. , Gompper G. , Stellbrink J. , Richter D. , Jakobs B. , Sottmann T. , Farago B.
Published: July 2002

Neutron Spin Echo and Dynamic Light Scattering techniques are used for an extensive investigation of the bicontinuous phase in water/decane microemulsions. The dynamical behavior of different surfactant systems, decyl polyglycol ether (C10E4), C10E4 mixed with polyethylenepropylene/polyethyleneoxide amphiphilic block-copolymers-(PEPx/PEOy), and sodium-bisethylhexylsulfosuccinate (AOT) is investigated under comparable conditions. At scattering wave numbers q large compared to the inverse of the structure length scale, q0=2π/d, always stretched exponential relaxations ∝e−(Γqt)β with Γq∝q3 are found, as predicted theoretically. The relaxation rate increases almost linearly as function of the bicontinuous structure correlation scale—ξ≃d/2. The apparent bare bending modulus κ determined by fitting theoretical predictions to the experimental high-q data yields values of about 1.3kBT—as inferred from previous small angle neutron scattering (SANS) studies and from other methods. The effect of increasing rigidity of the surfactant layers by anchoring amphiphilic block-copolymers, predicted theoretically and revealed experimentally in structural investigations, could not be clearly resolved due to its small influence on the dynamics. At structural length scales, the relaxation rate in water–oil contrast shows a minimum corresponding to the maximum of the static structure factor. At length scales much larger than the typical structure length the relaxation is single-exponential with a q2 dependent rate. In this regime we find indications of the additional membrane interaction due to the presence of block-copolymers.
Band gaps, effective masses and refractive indices of PbSrSe thin films: Key properties for mid-infrared optoelectronic device applications
Authors: Shen W. Z. , Yang H. F. , Jiang L. F. , Wang K. , Yu G. , Wu H. Z. , McCann P. J.
Published: July 2002

Key properties of PbSrSe thin films grown by molecular beam epitaxy have been studied for mid-infrared optoelectronic device applications. Detailed knowledge of the material parameters for the device design is required. The material parameters considered are: temperature-dependent band gaps, composition (or band gap)-dependent effective masses, and energy-dependent refractive indices. The study has been carried out by a combination of temperature-dependent photoluminescence and absorption measurements with the theoretical models on PbSrSe thin films of Sr compositions of as high as 0.276. The derived empirical equations for band gaps, effective masses, and refractive indices have been employed successfully in PbSe/Pb0.934Sr0.066Se multiple quantum well mid-infrared laser systems, for studying the band offsets and subband behavior. We have shown that the derived material parameters clearly promise of being applied to other PbSrSe thin films and PbSe/PbSrSe heterostructure systems for their optoelectronic applications.
Time-dependent energy absorption changes during ultrafast lattice deformation
Authors: Jeschke Harald O. , Garcia Martı́n E. , Bennemann K. H.
Published: July 2002

The ultrafast time dependence of the energy absorption of covalent solids upon excitation with femtosecond laser pulses is theoretically analyzed. We use a microscopic theory to describe laser induced structural changes and their influence on the electronic properties. We show that from the time evolution of the energy absorbed by the system important information on the electronic and atomic structure during ultrafast phase transitions can be gained. Our results reflect how structural changes affect the capability of the system to absorb external energy.
Synthesis of ammonium and organic compounds from N2,H2O vapor, and CO2 gas mixtures by synchrotron radiation induced photochemical reactions at atmospheric pressure and at room temperature
Authors: Utsumi Yuichi , Hattori Tadashi
Published: July 2002

Several compound materials were synthesized from N2,H2O vapor, and CO2 gas mixtures using synchrotron radiation induced photochemical reactions at atmospheric pressure and at room temperature. Ammonium and nitrogen oxides were successfully synthesized from N2,H2O vapor, and O2 gas mixtures. It was also found that organic compounds such as carboxyl acids and amino acids were formed from a N2,H2O vapor, and CO2 gas mixture.
Formation of various types of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt, Ni, La, Y, and Rh)
Authors: Kong Qingyu , Shen Yifeng , Zhao Li , Zhuang Jun , Qian Shixiong , Li Yufen , Lin Yanghui , Cai Ruifang
Published: July 2002

A photofragmentation study of metal fullerides C60Mx (M=Sm, Pt, Ni, La, Y, and Rh) by excimer laser ablation-TOF mass spectrometry shows that many kinds of metallofullerenes have been observed in both the positive and negative ionic modes. For C60Mx (M=Sm, Pt, and Ni), the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage forming substitutional metallofullerene. While in the case of metal fullerides C60Mx (M=La, Y), evidence of the encapsulation of La and Y atoms in fullerene cages forming endohedral fullerenes has been observed. Different from the above two cases, the laser ablation of rhodium fulleride C60Rhx(x≈1) produces two sequences of rhodium-containing fullerene clusters C2nRh and C2n+1Rh(2n=50–62). Odd-numbered all-carbon clusters in the fullerene regime are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The structures of metallofullerenes C2n+1M and C2nM with an even and odd number of total atoms, respectively, are discussed. Formation mechanisms with the participation of odd-numbered all-carbon fullerene clusters as intermediates are supposed.
Imaging of voids in dielectric materials by x-ray induced partial discharge
Authors: Filho V. Swinka , Robert R.
Published: July 2002

A nondestructive testing technique for imaging of voids and inclusions in dielectric materials for high-voltage application is described. Void partial discharge occurrence is induced by a narrow pulsed x-ray beam scanning the bulk of epoxy sample. Phase resolved partial discharge analysis is recorded as a function of the beam position. The data matrix is used to construct digital gray scale images, which gives information about location, dimensions, and partial discharge activity in each defect in dielectric materials. This nondestructive technique can be an important tool to evaluate the degradation state of the material.
Hole injection with limited charge relaxation, lateral nonuniform hole trapping, and transient stress-induced leakage current in impulse-stressed thin (<5 nm) nitrided oxides
Authors: Chim W. K. , Lim P. S.
Published: July 2002

An anomalously high density of positive trapped charges was observed in thin (4.3-nm thickness) nitrided gate oxides subjected to high-field impulse stressing. Additionally, the transient stress-induced leakage current (AC-SILC) was found to be larger than the steady-state SILC (DC-SILC) in these impulse-stressed thin oxides, contrary to observations in dc-stressed thin oxides. The large AC-SILC was found to be related to the high density of positive trapped holes in the oxide. The hot-hole generation occurs via a regenerative feedback mechanism, with minimal charge relaxation due to the short duration of the impulse stress. This gives rise to an extremely high density of oxide trapped holes that were not observed under dc stress conditions. The trapped holes can be easily annealed electrically at room temperature and the annihilation of the positive oxide trapped charges is accompanied by a reduction in the AC-SILC and a higher number of interface states being created. The trapped holes can either be uniformly or nonuniformly distributed, depending on the polarity of the applied stressing impulse in relation to the substrate doping type. A better understanding of thin oxide degradation under impulse stressing can help in the choice of a suitable write/erase pulse amplitude and duration for use in endurance testing of nonvolatile semiconductor memories to ensure long-term reliable operation.
Magnetic pulsed extraction of highly charged ions from a plateau ECRIS
Authors: Müller L. , Heinen A. , Ortjohann H. W. , Andrä H. J.
Published: July 2002

The afterglow is known to produce pulses of highly charged ions suited for injection to accelerator facilities. We tested a new technique of magnetic pulsed extraction (PUMAEX) in our plateau-ECR ion source PECRIS III. This source has a large resonance volume due to a homogeneous magnetic field in its center. Compared to the afterglow of PECRIS III we reached higher ion currents. The length of the PUMAEX pulses depends on the electric current pulse in the PUMAEX coil, we used pulse lengths of 0.25–1 ms. The PUMAEX coil is a small coil which is placed around the extraction hole in the plasma electrode. The absolute value of the static magnetic field near the extraction hole can be reduced below the value in the center of the source by a current pulse in the PUMAEX coil. Thus the magnetic confinement of the ECR plasma is opened specifically in the direction of the extraction hole. The plasma therefore escapes from the source more directed to the extraction hole than in the afterglow. Since the coil is very near to the location where the magnetic field is changed, it was possible to use a small coil, which could be placed in the plasma electrode of PECRIS III without major modifications. Currently we are building a new multifrequency plateau ECRIS. Using eight frequencies between 17 and 18 GHz we intend to improve the electron heating in the large resonance volume. The highest frequency will be set to the resonance frequency of cold electrons at the center of the source. The lower frequencies will match the resonance frequencies of more energetic electrons as those electrons have a greater relativistic mass. In the near future we will test the PUMAEX technique with this new source.
Hot-wire plasma assisted chemical vapor deposition: A deposition technique to obtain silicon thin films
Authors: Ferreira I. , Fortunato E. , Martins R. , Vilarinho P.
Published: July 2002

We have produced amorphous intrinsic silicon thin films by hot-wire plasma assisted chemical vapor deposition, a process that combines the traditional rf plasma and the recent hot-wire techniques. In this work we have studied the influence of hydrogen gas dilution and rf power on the surface morphology, composition, structure and electro-optical properties of these films. The results show that by using this deposition technique it is possible to obtain at moderate rf power and filament temperature, compact i-type silicon films with ημτ of the order of 10−5 cm2 V−1, without hydrogen dilution.
Tunneling time of spin-polarized electrons in ferromagnet/insulator (semiconductor) double junctions under an applied electric field
Authors: Wang Bin , Guo Yong , Gu Bing-Lin
Published: July 2002

Based on the group velocity concept and the two-band model, we investigated tunneling properties of spin-polarized electrons traversing ferromagnetic/insulator (semiconductor) double junctions under the influence of an external electric field. The tunneling time and the transmission coefficient, as well as the spin polarization, were calculated and examined. Effects of the electric field and quantum size are also considered. The results indicate that the tunneling time strongly depends on the spin orientation of tunneling electrons. In a wide range of incident energy, spin-down electrons spend a longer time tunneling through the structure than spin-up ones, and the difference of the tunneling time Δτ between electrons with opposite spin orientation is very sensitive to the incident energy in the lower energy region. Moreover, the variation of Δτ with the increasing of the incident energy shows pronounced oscillations for certain applied bias and structural size. The results also indicate that spin polarization shows frequent wide-range oscillations and its magnitude decays slowly with the increasing of the incident energy, which reveal that the spin polarization in ferromagnetic/insulator (semiconductor) junctions possesses quite different properties from those exhibited in semiconductor electric-barrier or magnetic-barrier structures as well as semiconductor/semimagnetic semiconductor hybrid heterostructures.
Identification of silicon as the dominant hole trap in YVO4 crystals
Authors: Garces N. Y. , Halliburton L. E. , Stevens K. T. , Shone M. , Foundos G. K.
Published: July 2002

Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) have been used to characterize the dominant hole trap in undoped Czochralski-grown yttrium–orthovanadate (YVO4) crystals. A silicon impurity, present inadvertently, replaces a vanadium ion and allows a hole to be trapped on one of the four adjacent oxygen ions. The unpaired spin resides in an oxygen p orbital oriented perpendicular to the plane defined by the silicon ion, the electron-deficient oxygen, and the two yttrium ions nearest the oxygen. Principal values of the g matrix (2.0033, 2.0090, and 2.0771) were obtained from EPR data taken at 15 K. Direct verification of the participation of silicon was obtained from ENDOR data taken at 12 K. We have found that this trapped-hole center appeared in large concentrations in all of our Czochralski-grown YVO4 crystals that were exposed to ionizing radiation (i.e., x rays or an ultraviolet laser beam) while the crystal was at 77 K. Interestingly, a small concentration of this trapped-hole center was present in some as-grown YVO4 crystals before exposure to ionizing radiation.
Chemical defect explanation for the effect of postdeposition treatments on CuInSe2
Authors: Otte K. , Chassé T. , Lippold G. , Rauschenbach B. , Szargan R.
Published: July 2002

Hydrogenation and oxygenation of CuInSe2 have been studied by x-ray photoelectron spectroscopy in order to investigate the defect properties. Initially oxidized p-type material was type-converted by low-energy hydrogen ion implantation. A defect model has been proposed suggesting the reactivation of selenium vacancies (VSe) as well as the creation of additional indium copper antisites (InCu). For cleaved samples, a direct influence of hydrogen on the net-donor concentration has been assumed. The resulting Cu-depleted surface is the result of the downward band bending and the migration of Cu into the bulk by the built-in electric field. The subsequent oxidation leads to a passivation of VSe and InCu, and hence yields a reconversion of the conductivity type.
Measurement of electron saturation velocity in Ga0.52In0.48P in a double heterojunction bipolar transistor
Authors: Yee M. , Houston P. A. , David J. P. R.
Published: July 2002

The electron saturation velocity in Ga0.52In0.48P has been measured as a function of temperature, utilizing the Kirk effect in double heterojunction bipolar transistors. An AlGaAs base was used to eliminate the conduction band spike, and measurements were carefully performed using pulse biasing to minimize self-heating. Voltage drops across the base and collector series resistances were also taken into account. The measured room temperature saturation velocity of 5.0×106 cm/s decreased rapidly to 2.9×106 cm/s at 200 °C. These results provide useful data for modeling and design, and are particularly important for the prediction of frequency performance of Ga0.52In0.48P/(Al)GaAs/Ga0.52In0.48P double heterojunction bipolar transistors when operating at typical elevated temperatures in power applications.