bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport and magnetic properties of Ca3−xSrxCo4O9
Authors: Wang L. B. , Maignan A. , Pelloquin D. , Hébert S. , Raveau B.
Published: July 2002

Ca 3−x Sr x Co 4 O 9  (x=0,0.5,1.0) samples were prepared by solid state reaction. As expected from the radii of Sr2+ and Ca2+, the cell parameters increase with Sr content x. Simultaneously, the resistivity and the carrier localization at low temperature are enhanced. The thermoelectric power is not sensitive to the Sr content. Furthermore, the ferromagnetism at low temperature becomes weaker with the increase of Sr content, which shows that antiferromagnetism and ferromagnetism compete in these cobaltites. Large Sr content enhances antiferromagnetic interactions, which can be related to the extending plateau of magnetoresistance ratio of ρ(H)/ρ(0) in the low magnetic field region.
Cluster nucleation effects in CO(Ar)n: A stochastic analysis
Authors: Paesani F. , Gianturco F. A.
Published: July 2002

A previously discussed potential energy surface for the CO molecule interacting with one Ar atom is extended to evaluate its dependence on molecular vibrations. The |v=0〉 and the |v=1〉 adiabatic potential energy surfaces are employed within a stochastic treatment to obtain the vibrational transition energy changes as the number of rare gas atoms around the CO molecule is increased. The present calculations yield shifts of the excitation energy for the (v=0→v=1) transition in Ar cluster of variable size which are in fair agreement with experimental expectations for the limiting case of solid argon. The details of the preferential structuring and clustering of the adatoms around the dopant molecule are obtained from diffusion Monte Carlo calculations and are extensively analyzed. The specifics of the system behavior are discussed and a driving microscopic mechanism is suggested on the basis of the energy balance between the interaction potentials that are present in the title system.
Algebraic effective resonance Hamiltonian approach to highly excited SO2(X̃ 1A1): Effect of bending excitation on local-mode bifurcation
Authors: Sako Tokuei , Yamanouchi Kaoru , Iachello Francesco
Published: July 2002

Vibrational dynamics of highly excited SO2(X̃ 1A1) is studied based on an algebraically expanded effective Hamiltonian constructed from the generators of U(2) Lie algebras. A total of 512 experimental vibrational-level energies of SO2 are fitted to the full three-dimensional model Hamiltonian with a root-mean-square deviation of 2.34 cm−1. The vibrational wave functions are constructed from the eigenvectors of the best-fit Hamiltonian. An examination of the probability distribution of the wave functions reveals that the normal-to-local transition is promoted first, and then suppressed as the bend quantum number increases. The origin of these two competing effects of the bend excitation is discussed on the basis of the structure of the algebraic Hamiltonian.
Quasi-single helicity spectra in the Madison Symmetric Torus
Authors: Marrelli L. , Martin P. , Spizzo G. , Franz P. , Chapman B. E. , Craig D. , Sarff J. S. , Biewer T. M. , Prager S. C. , Reardon J. C.
Published: July 2002

Evidence of a self-organized collapse towards a narrow spectrum of magnetic instabilities in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, 131 (1991)] reversed field pinch device is presented. In this collapsed state, dubbed quasi-single helicity (QSH), the spectrum of magnetic modes condenses spontaneously to one dominant mode more completely than ever before observed. The amplitudes of all but the largest of the m=1 modes decrease in QSH states. New results about thermal features of QSH spectra and the identification of global control parameters for their onset are also discussed.
Crossover between strong- and weak-field critical adsorption and the determination of the universal exponent η⊥
Authors: Nickel B. , Schlesener F. , Donner W. , Dosch H. , Detlefs C.
Published: July 2002

We report a temperature-dependent x-ray diffraction study of critical behavior of sublattice order in thin epitaxial FeCo(001) films grown on MgO(001). The quantitative analysis of the diffraction profiles reveals a crossover phenomenon between strong-field and weak-field critical adsorption which occurs at a reduced temperature t1=1.3×10−3. We demonstrate that this scenario gives a first experimental access to the universal critical exponent η⊥ which governs the spatial decay of critical fluctuations perpendicular to the surface. We find η⊥=0.86(5).
Ab initio molecular dynamics simulation of the H/InP(100)–water interface
Authors: Gayathri Narayanan , Izvekov Sergei , Voth Gregory A.
Published: July 2002

A first principles simulation study of the H-terminated InP(100)–water interface is presented with an aim to understand the electronic structure of the interface. The simulation has been carried out using the ab initio Car–Parrinello molecular dynamics method within a pseudopotential formalism and the Becke–Lee–Yang–Parr generalized gradient approximation to the exchange-correlation potential. Dissociative adsorption of H2O molecules onto H/InP(100) surfaces, leading to formation of In–OH and In–H bonds on the (100) surface, occurs at the interface, in a manner similar to the experimentally demonstrated dissociative adsorption of H2O onto n-InP(110) surface. This process indicates a very strong coupling between the semiconductor and the water states. Also, simulation carried out for two H/InP(100) surfaces reveal that more H2O dissociations occur near the rougher atomically corrugated surface, in accordance with observations from experimental studies designed to determine the morphological influences on H2O dissociation near semiconductor surfaces. An analysis of the electronic structure of the interface further reveals the charge density profile of the H/InP(100) surface states to be strongly influenced by the water states, especially those arising from the first overlayer. Additionally, the net charge of the solvated H/InP(100) slab is found to be positive and the net atomic charges on the chemisorbed H atoms are found to be negative, indicating a charge transfer, particularly, from the surface-In atoms to the chemisorbed H atoms.
Quasielastic neutron scattering of propylene glycol and its 7-mer confined in clay
Authors: Swenson J. , Howells W. S.
Published: July 2002

The dynamics of propylene glycol (PG) and its 7-mer confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. Experiments were carried out in the temperature range 300–420 K and in two different scattering geometries in order to make the elastic Q-vector parallel and perpendicular to the clay platelets for a scattering angle of 90° (Q≈1.33 Å−1). The results indicate that at least three dynamical processes are present in the experimental time window of approximately 3–80 ps, although only two processes could be resolved at a given temperature. The fast process is independent of the chain length as well as the confinement, and in accordance with previous studies of the bulk liquids it is assigned to a rapid local motion of hydrogens in the polymeric backbone. The slow process has mainly a rotational character at T=300 K and it is assigned to the methyl group rotation. At higher temperatures this rotational motion is hidden behind a Q-dependent jump diffusion process. The calculated diffusion constant is roughly the same for our confined PG and 7-PG, and also similar to the corresponding bulk liquids (except for PG at high temperatures where the bulk liquid shows a considerably faster diffusion). However, the diffusion seems to increase slightly faster for confined 7-PG with increasing temperature. This is an interesting finding since a clear opposite trend was observed for the bulk liquids. The methyl group rotation at low temperatures (≈300 K) is significantly faster in the confined liquids. Both the methyl group rotation and the jump diffusion process seem to be present also in the direction perpendicular to the clay platelets for the 7-mers, but not for the monomers where only the fast local motion of hydrogens is observed in that direction. Furthermore, the pure translational diffusion (with a quasielastic broadening proportional to Q2) observed for the bulk liquids was never obtained for our confined liquids. The different results for the confined and bulk liquids are discussed in terms of confinement and free volume effects and surface interactions.
Relationship of resonant energies and Bloch wave vectors in resonant tunneling in superlattices
Authors: Khan Arif , Mohammad S. Noor
Published: July 2002

A relationship among the resonance energies in superlattices of different numbers of well and barrier layers has been established. It has been shown that the resonance wave vectors can be related to each other in such systems. The calculated dependence of resonance energies and resonance wave vectors on the mole fraction, and of the thickness of barrier layers of superlattices, throws new light on the quantum effects of electron transport through the superlattices.
Renormalization analysis of correlation properties in a quasiperiodically forced two-level system
Authors: Mestel B. D. , Osbaldestin A. H.
Published: July 2002

We give a rigorous renormalization analysis of the self-similarity of correlation functions in a quasiperiodically forced two-level system. More precisely, the system considered is a quantum two-level system in a time-dependent field consisting of periodic kicks with amplitude given by a discontinuous modulation function driven in a quasiperiodic manner at golden mean frequency. Mathematically, our analysis consists of a description of all piecewise-constant periodic orbits of an additive functional recurrence. We further establish a criterion for such orbits to be globally bounded functions. In a particular example, previously only treated numerically, we further calculate explicitly the asymptotic height of the main peaks in the correlation function.
Statistical mechanics of strong and weak point vortices in a cylinder
Authors: Bühler Oliver
Published: July 2002

The motion of 100 point vortices in a circular cylinder is simulated numerically and compared with theoretical predictions based on statistical mechanics. The novel aspect considered here is that the vortices have greatly different circulation strengths. Specifically, there are 4 strong vortices and 96 weak vortices, the net circulation in either group is zero, and the strong circulations are five times larger than the weak circulations. As envisaged by Onsager [Nuovo Cimento, Suppl. 6, 279 (1949)], such an arrangement leads to a substantial amplification of statistical trends such as the preferred clustering of the strong vortices in either same-signed or oppositely signed pairs, depending on the overall energy level. To prepare the ground, this behavior is illustrated here first by a simple toy model with exactly solvable statistics. A microcanonical ensemble based on the conserved total energy E and angular momentum M for the whole vortex system is then used, in which the few strong vortices are treated as a subsystem in contact with a reservoir composed of the many weak vortices. It is shown that allowing for the finite size of this reservoir is essential in order to predict the statistics of the strong vortices accurately. Notably, this goes beyond the standard canonical ensemble with positive or negative temperature. A certain approximation is then shown to allow a single random sample of uniformly distributed vortex configurations to be used to predict the strong vortex statistics for all possible values of E and M. Detailed predictions for the energy, two-vortex, and radial distribution functions of the strong vortices are then made for comparison with three simulated cases of near-zero M and low, neutral, or high E. It is found that the statistical mechanics predictions compare remarkably well with the numerical results, including a prediction of vortex accumulation at the cylinder wall for low values of E.
Field line resonances in a cylindrical plasma
Authors: Mitchell C. C. , Maggs J. E. , Gekelman W.
Published: July 2002

An experimental study of the response to an impulsive driver in a low beta helium plasma is presented. Field line resonance (FLR) spectra are recorded and compared to an ideal magnetohydrodynamic (MHD) theory with finite ion cyclotron frequency corrections. The agreement between observed and predicted values is generally within the frequency resolution of the measurements except for the lowest frequency harmonic. The spectrum of the lowest harmonic is compared to various ultra low-frequency (ULF) FLR dispersion relations. Dispersion effects due to nonzero perpendicular wave number are found to be important for the lowest frequency harmonic. Quality factors are measured and compared to theoretical estimates from a two-fluid theory with finite parallel electron temperature. Reflection coefficient values are obtained using measured and estimated Q values.
Large fluorocarbon ions can contribute to film growth during plasma etching of silicon
Authors: Fuoco Erick R. , Hanley Luke
Published: July 2002

The preferential etching of SiO2 over Si in fluorocarbon plasmas occurs in part through the growth of a fluorocarbon layer. Large CxFy+,0 (x>1) ions and radicals have been observed in etching fluorocarbon plasmas, but their role in the etching processes has not been clearly identified. Here we use mass-selected 5–200 eV C3F5+ and C2F4+ ion beams to form nanometer thick fluorocarbon and SixCyFz films on H–Si(100). Monochromatic x-ray photoelectron spectroscopy (XPS) shows that the average elemental and chemical content of the deposited film is nearly independent of ion identity and kinetic energy. The chemical nature of the fluorocarbon film instead is controlled largely by surface chemical and diffusion processes. However, ion energy and structure do control the fluorocarbon film morphology. Atomic force microscopy shows that 200 eV C3F5+ ion impact forms large oblong pits in the Si substrate that are ∼30 nm deep and ∼200 nm across. No large pits are observed in the Si substrate for 25 eV C3F5+ bombardment. XPS depth profiles of the films depend strongly upon ion energy and structure where the spatial distribution of the F depth profile increases with incident ion energy for both C3F5+ and C2F4+. Higher F content is found deeper in the surface and the SixCyFz buried interface is thickest at higher ion energies. These results are discussed in terms of the role of large ions and radicals in etching fluorocarbon plasmas.
Quantization of the 3×3 nonadiabatic coupling matrix for three coupled states of the C2H molecule
Authors: Mebel A. M. , Halász G. J. , Vibók Á. , Alijah A. , Baer M.
Published: July 2002

The three ab initio nonadiabatic coupling terms related to the three strongly coupled states of the C2H molecule, i.e., 2 2A′, 3 2A′, and 4 2A′, were studied applying the line integral technique [M. Baer, Chem. Phys. Lett. 35, 112 (1975)]. The following was verified: (1) Due to the close proximity of the conical intersections between these three states, two-state quantization cannot always be satisfied between two successive states. (2) It is shown that in those cases where the two-state quantization fails a three-state quantization is satisfied. This three-state quantization is achieved by applying the 3×3 nonadiabatic coupling matrix that contains the three relevant nonadiabatic coupling terms. The quantization is shown to be satisfied along four different contours (in positions and sizes) surrounding the relevant conical intersections.
Liquid xenon as an ideal probe for many-body effects in impulsive Raman scattering
Authors: Boeijenga Nienke H. , Pugzlys Audrius , Jansen Thomas l. C. , Snijders Jaap G. , Duppen Koos
Published: July 2002

The collision induced effects in the third-order Raman response of liquid xenon have been studied both experimentally and theoretically. The effect of electron cloud overlap on the polarizability of xenon dimers was studied using accurate time-dependent density functional theory calculations. The dimer polarizabilities were used to fit parameters in a direct reaction field model that can be generalized to condensed phase systems. This model was employed in molecular dynamics simulations in order to calculate the impulsive Raman response of liquid xenon. Excellent agreement is found between the shape of the calculated and the measured anisotropic part of the response. The shape of this response is little affected by the electron overlap effects, but the intensity is strongly influenced by it. The shape of the isotropic response is predicted to be strongly dependent on electron overlap effects.
Effects of subgrid-scale modeling on time correlations in large eddy simulation
Authors: He Guo-Wei , Rubinstein R. , Wang Lian-Ping
Published: July 2002

The effects of the unresolved subgrid-scale (SGS) motions on the energy balance of the resolved scales in large eddy simulation (LES) have been investigated actively because modeling the energy transfer between the resolved and unresolved scales is crucial to constructing accurate SGS models. But the subgrid scales not only modify the energy balance, they also contribute to temporal decorrelation of the resolved scales. The importance of this effect in applications including the predictability problem and the evaluation of sound radiation by turbulent flows motivates the present study of the effect of SGS modeling on turbulent time correlations. This paper compares the two-point, two-time Eulerian velocity correlation in isotropic homogeneous turbulence evaluated by direct numerical simulation (DNS) with the correlations evaluated by LES using a standard spectral eddy viscosity. It proves convenient to express the two-point correlations in terms of spatial Fourier decomposition of the velocity field. The LES fields are more coherent than the DNS fields: their time correlations decay more slowly at all resolved scales of motion and both their integral scales and microscales are larger than those of the DNS field. Filtering alone is not responsible for this effect: in the Fourier representation, the time correlations of the filtered DNS field are identical to those of the DNS field itself. The possibility of modeling the decorrelating effects of the unresolved scales of motion by including a random force in the model is briefly discussed. The results could have applications to the problem of computing sound sources in isotropic homogeneous turbulence by LES.
Size-dependent magnetic properties of Mn0.5Zn0.5Fe2O4 nanoparticles in SiO2 matrix
Authors: Mandal K. , Chakraverty S. , Pan Mandal S. , Agudo P. , Pal M. , Chakravorty D.
Published: July 2002

Mn 0.5 Zn 0.5 Fe 2 O 4 ferrite nanoparticles (<100 nm) in SiO2 matrix have been prepared by the sol-gel method. The particle size was varied by changing the duration of heat treatment above crystallization temperature. An x-ray diffraction study indicates the presence of single-phase spinel ferrite in the sample. The particle size was estimated by the x-ray diffraction method as well as from the micrograph taken by a transmission electron microscope. The magnetic properties of the samples were studied by a vibrating sample magnetometer. The samples show superparamagnetic behavior when the particle size is below 20 nm, which is confirmed by Mössbauer spectroscopy measurements. The average particle size in the superparamagnetic state was also estimated from the low-field magnetization measurement by considering the samples as consisting of noninteracting single domain particles.
Effect of hydrogen and oxygen on stainless steel nitriding
Authors: Figueroa C. A. , Wisnivesky D. , Alvarez F.
Published: July 2002

The influence of hydrogen and oxygen on stainless steel implanted by nitrogen low-energy ions is systematically studied. It is shown that hydrogen intervenes moderately in the process only when the oxygen partial pressure in the deposition chamber is relatively high. For very low-oxygen partial pressures, the energetic nitrogen molecules impinging on the substrate sputter the thin oxide layer formed on the substrate. This allows the growing of a rich nitrogen layer beneath the surface, improving the diffusing of the implanted atom deeper in the bulk material. For higher-oxygen partial pressures, the sputtering is ineffective, and an oxide layer partially covers the surface even in the presence of hydrogen. The maximum depth penetration of nitrogen depends on the degree of oxygen coverage, which is fairly well described by a Langmuir absorption isothermal. Hardness depth profiling is consistent with the existence of a diffusion barrier formed by the oxygen absorbed on the surface. In order to understand the role of hydrogen on the nitriding process, samples preimplanted with hydrogen were subsequently treated with nitrogen and the hardness depth profiling analyzed. These results may provide a clue about the practical consequences of oxygen and hydrogen on the nitriding process.
Penumbral imaging for measurement of the ablation density in laser-driven targets
Authors: Fujioka Shinsuke , Shiraga Hiroyuki , Nishikino Masaharu , Heya Manabu , Shigemori Keisuke , Nakai Mitsuo , Azechi Hiroshi , Nakai Sadao , Yamanaka Tatsuhiko
Published: July 2002

One-dimensional (1D) penumbral imaging technique with high spatial resolution has been developed, and applied to density profile measurements in laser-driven Rayleigh-Taylor instability experiments at a GEKKO XII-HIPER laser system. A laser-irradiated planar target was observed with side-on x-ray backlighting. A penumbral image of an x-ray radiograph was made by using a knife-edge imager. The x-ray radiograph was, then, reconstructed by differentiating the penumbral image with a proper Wiener filtering. A density profile was deduced from the reconstructed x-ray radiograph. In a proof-of-principle experiment, the density profile of a polystyrene (PS) target before laser irradiation was measured by using this method, and high spatial resolution of 3–4 μm was demonstrated. A laser-irradiated PS target in the acceleration phase was observed. The experimentally observed density profile was found to be consistent with the prediction by a 1D hydrodynamic simulation code. The x-ray penumbral imaging is a very simple and useful technique with high spatial resolution for research in hydrodynamics relevant to inertial fusion energy.
Effect of unintentionally introduced oxygen on the electron–cyclotron resonance chemical-vapor deposition of SiNX films
Authors: Cova P. , Masut R. A. , Grenier O. , Poulin S.
Published: July 2002

We establish the role of oxygen atoms on the structural, chemical, and mechanical properties of SiOXNY films grown on Si and InP substrates by electron–cyclotron resonance chemical-vapor deposition (ECR CVD) using a diluted SiH4 and N2 mixture in Ar, under controlled conditions. The mechanical and chemical properties of ECR-CVD SiNX films depend on the oxygen contamination even when this element is present in low concentrations. The compressive stress of SiNX films deposited with a low (and constant) content of oxygen (less than 12%) is shown to be in qualitative agreement with a model of repulsive Coulomb forces related mainly to polar N–H+− units in the SiNX network. We observe a decrease of the film compressive stress when the N2/SiH4 flow ratio increases, which is due to the increase of Si–N bonds in detriment of N–H bonds. Films deposited with high oxygen content in the plasma show a decrease of nitrogen incorporation. Oxygen radicals species compete with those of nitrogen in their reaction with silicon dangling bonds, which has as a consequence a decrease in the incorporation of nitrogen. Additional creation of oxygen radicals, with no hydrogen dilution, is more effective in decreasing the number of N–H bonds, or the compressive stress in the SiNX films, than the corresponding creation of nitrogen radicals. The mechanical properties of SiNX films contaminated with oxygen are controlled, in general, by the total number of both nitrogen plus oxygen atoms relative to silicon. The buffered HF (BHF) film etch rate is enhanced and thus is mainly controlled by the oxygen content. Low values of the compressive stress do not necessarily imply low values of BHF etch rate or a high N/Si ratio. We also present a discussion of the origin of the unintentional incorporation of oxygen in a ECR-CVD system designed for industrial production.
Evaluation of bulk trap density of tris-(8-hydroxyquinoline) aluminum
Authors: Mizuo Satoshi , Tabatake Shigeru , Naka Shigeki , Okada Hiroyuki , Onnagawa Hiroyoshi
Published: July 2002

The bulk trap density of tris-(8-hydroxyquinoline) aluminum was evaluated by the differential method, and was found to be approximately 1016 cm−3 at the Fermi level of 0.45–0.60 eV below the lowest unoccupied molecular orbital (LUMO). This value increased with an increase in the Fermi energy. The activation energy of the current density changed with the applied voltage, i.e., traps of various energy levels contribute to conduction. The existence of a shallow trap at the Fermi level of 0.55 eV below the LUMO was confirmed.
Resistivity and Hall voltage study of phosphorus segregation in polycrystalline Si1−xGex films
Authors: Qin W. , Ast D. G. , Kamins T. I.
Published: July 2002

Hall and resistivity measurements as functions of temperature were used to study the segregation of phosphorus in atmospheric-pressure chemical-vapor deposited, ∼300-nm-thick, polycrystalline Si0.95Ge0.5 and Si0.9Ge0.1 thin films, implanted at 80 keV with 6×1013–5×1014 P/cm2, and annealed for 1 h at 800 °C. Hall measurements, feasible only in heavily doped films, indicated that 29% of the dopant in Si0.9Ge0.1 and 42% of the dopant in Si0.95Ge0.05 were electrically inactive. The loss is consistent with dopant segregation to grain boundaries. The density of grain-boundary carrier traps is 3.6×1012/cm2 in Si0.95Ge0.05 and 4.4×1012/cm2 in Si0.9Ge0.1, increasing sublinearly with Ge content.
Fluorescence dynamics of phenyl-substituted polyphenylenevinylene–trinitrofluorenone blend systems
Authors: Im C. , Lupton J. M. , Schouwink P. , Heun S. , Becker H. , Bässler H.
Published: July 2002

Steady state as well as time-resolved fluorescence of phenyl-substituted polyphenylenevinylene (PhPPV), both in solution and in form of spin-coated films has been measured. In solution the fluorescence decays exponentially with a 1/e decay time of ∼700 ps, whereas the 1/e decay in the film is ∼400 ps and the decay approaches a stretched exponential with an exponent of β=0.65. Doping the film with various amounts of trinitrofluorenone (TNF) shortens the decay time without changing the form of the Kohlrausch–Williams–Watts decay law. This is a signature of exciton dissociation in a random polymer matrix through electron transfer to TNF. The data analysis shows that “neat” PhPPV contains a concentration of ⩽ 0.04 wt. % of unidentified electron scavengers. A hypsochromic shift of the cw fluorescence spectra in doped films is interpreted in terms of increasingly frustrated spectral relaxation of singlet excitations within the inhomogeneously broadened distribution of hopping states.
Free radicals in superfluid liquid helium nanodroplets: A pyrolysis source for the production of propargyl radical
Authors: Küpper Jochen , Merritt Jeremy M. , Miller Roger E.
Published: July 2002

An effusive pyrolysis source is described for generating a continuous beam of radicals under conditions appropriate for the helium droplet pick-up method. Rotationally resolved spectra are reported for the ν1 vibrational mode of the propargyl radical in helium droplets at 3322.15 cm−1. Stark spectra are also recorded that allow for the first experimental determination of the permanent electric dipole moment of propargyl, namely, −0.150 and −0.148 D for ground and excited states, respectively, in good agreement with previously reported ab initio results of −0.14 D. The infrared spectrum of the ν1 mode of propargyl-bromide is also reported. The future application of these methods for the production of novel radical clusters is discussed.
π-hydrogen bonding between water and aromatic hydrocarbons at high temperatures and pressures
Authors: Furutaka Seiya , Ikawa Shun-ichi
Published: July 2002

Infrared OH stretching absorption of HDO isolated in aromatic hydrocarbons have been measured at temperatures of 473 and 523 K and at pressures in the 100–350 bar range. The peak frequencies are dependent on the solvents and their order, benzene>toluene>ethylbenzene>cumene>o-xylene∼m-xylene>mesitylene, is exactly the same as the order for the ionization potentials of the hydrocarbons. Shifts of the frequencies from that of HDO in hexane, which was measured as a reference at the same temperature and pressure, were analyzed using a charge transfer theory for hydrogen bonding. Distances between the water molecule and a solvent phenyl ring were estimated to be 2.8±0.1 and 2.9±0.1 Å at 473 and 523 K, respectively. These values are consistent with a structure of a water–benzene complex determined by a jet-cooled microwave spectroscopy. These facts suggest that the π-hydrogen bond between water and aromatic hydrocarbons exists even at the high temperatures under pressure.
Ion acceleration, magnetic field line reconnection, and multiple current filament coalescence of a relativistic electron beam in a plasma
Authors: Sakai Jun-ichi , Saito Shinji , Mae Hirokazu , Farina Daniela , Lontano Maurizio , Califano Francesco , Pegoraro Francesco , Bulanov Sergei V.
Published: July 2002

Results are presented from analytical theory and from two-dimensional particle-in-cell simulations of the relaxation of a relativistic electron beam in a plasma. When the electron beam enters the plasma, it generates a return current which is carried by the plasma electrons. In a collisionless plasma, the relaxation mechanism is related to the onset of an electromagnetic filamentation instability, which leads to the generation of a quasistatic magnetic field associated with many small-scale current filaments. Each filament consists of a direct and of a return electric current which repel each other. This produces a strong electric field which accelerates the ions in the radial direction. In the long time evolution, the successive coalescence of the small-scale current filaments is seen. This process is accompanied by the reconnection of the magnetic field lines, by the formation of current sheets, and by strong ion acceleration inside these sheets.
Effect of temperature and cubic anisotropy on the switching field of cylindrical Ni particles
Authors: Hwang M. , Shima M. , Ross C. A. , Seberino C. , Bertram H. N.
Published: July 2002

The switching fields of nickel cylinders with ∼100 nm dimensions have been compared to the predictions of a numerical micromagnetic model. The switching field decreases with increasing diameter and decreasing aspect ratio, and there is a good agreement between the measured and calculated values. The samples have a strong 〈111〉 preferred crystal orientation, and the large increase in switching field and switching field distribution with decreasing temperature can be explained by the large increase in magnetocrystalline anisotropy at low temperatures.
Ultrafast imaging of incoherent rotation magnetic switching with experimental and numerical micromagnetic dynamics
Authors: Hiebert W. K. , Ballentine G. E. , Lagae L. , Hunt R. W. , Freeman M. R.
Published: July 2002

The magnetic switching dynamics of a thin-film microstructured Permalloy element were directly observed undergoing incoherent rotation reversal processes. Experimental spatiotemporally and vectorially resolved micromagnetic data show qualitative agreement with numerically simulated micromagnetic data for the nonequilibrium states during reversal. The application of a longitudinal reversal field launches rotations in both clockwise and counterclockwise directions, governed by magnetostatic effects, at different locations within a shaped sample. Reversal proceeds by fast initial selection into these rotation directions, immediately leading to vortex formation throughout the sample, and is followed by slower expulsion of the vortices in the transverse-to-field direction through local coherent rotation. Apparent transient damping, magnetization vector length averages, and spatial frequency representations support energy transfer concepts for the reversal process.
Optical determination of the dopant concentration in the δ-doping layer
Authors: Drabińska Aneta , Babiński A. , Tomaszewicz T. , Bozek R. , Baranowski J. M.
Published: July 2002

Room temperature electroreflectance measurements on δ-doped low-dimensional structures are presented. Previously proposed electroreflectance bias-wavelength mapping is used for characterization of (1) a modulation Si δ-doped pseudomorphic InGaAs/GaAs quantum well and (2) a Sn δ-doped GaAs layer. An electric field above and below the δ-doping plane found from the Fourier transform applied to Franz–Keldysh oscillations was used to find the δ-dopant concentration in investigated structures. The position of the δ-doping plane and a Schottky barrier height are also determined.
High mobility AlGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy
Authors: Manfra M. J. , Weimann N. G. , Hsu J. W. P. , Pfeiffer L. N. , West K. W. , Syed S. , Stormer H. L. , Pan W. , Lang D. V. , Chu S. N. G. , Kowach G. , Sergent A. M. , Caissie J. , Molvar K. M. , Mahoney L. J. , Molnar R. J.
Published: July 2002

We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (∼20 μm) GaN templates are characterized by low threading dislocation densities (∼5×108 cm−2) and by room temperature resistivities of ∼108 Ω cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x⩽0.05) AlxGa1-xN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density ns⩽2×1012 cm−2. We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n type at room temperature and characterized an unusually low dislocation density of ∼2×108 cm−2, we have grown an Al0.05Ga0.95N/GaN heterostructure with a record mobility of 75 000 cm2/V s at sheet density of 1.5×1012 cm−2 and T=4.2 K. The same heterostructure design grown on a semi-insulating HVPE template yielded a peak mobility of 62 000 cm2/V s at a density of ns=1.7×1012 cm−2 and T=4.2 K. The observation of the fractional quantum Hall effect at filling factor ν=5/3 in the AlGaN/GaN system is reported. It is also demonstrated that thick semi-insulating GaN templates grown by HVPE are a viable substrate for the growth of high electron mobility transistors. Typical Al0.25Ga0.75N/GaN heterostructures exhibit room temperature density of 1.0×1013 cm−3 and mobility of ∼1500 cm2/V s. The dc and rf characteristics of transistors grown by MBE on a HVPE template are presented.
Hopping dynamics of ions and polarons in disordered materials: On the potential of nonlinear conductivity spectroscopy
Authors: Roling Bernhard
Published: July 2002

The potential of nonlinear conductivity spectroscopy for obtaining new information on the hopping dynamics of mobile charge carriers in disordered materials is analyzed from a theoretical as well as from an experimental point of view. The nonlinear conductivity spectra of simple hopping models are calculated by means of analytical methods and Monte Carlo simulations. It is shown that the nonlinearity of the high-frequency conductivity is strongly influenced by the local asymmetry of the potential landscape, while the nonlinearity of the low-frequency conductivity is sensitive to the structure of the long-range diffusion pathways. Furthermore, experimental results for the nonlinear conductivity of ion conducting glasses and polymers are reviewed.
A computational study of photoisomerization in Al3O3− clusters
Authors: Cui X.-Y. , Morrison I. , Han J.-G.
Published: July 2002

Ab initio calculations are employed to understand the photoisomerization process in small Al3O3− clusters. This process is the first example of a photoinduced isomerization observed in an anion cluster gas-phase system. Potential energy surfaces for the ground state and the excited state (S1 and T1) are explored by means of B3LYP, MP2, CI-singles, and CASSCF methods. We demonstrate that the isomerization process occurs between the global minimum singlet state Book structure (C2v,1A1) and the triplet state Ring structure (C2v,3B2). The calculated vertical excitation energy is 3.62 eV at the CASSCF level of approximation, in good agreement with the experimental value (3.49 eV). A nonplanar conical intersection, which hosts the intersystem crossing between the S1 and T1 surfaces is identified at the region of around R(1,6)=2.4 Å. Beyond the experimental results, we predict, that this isomerization is reversible upon absorption of a phonon with energy of 1.92 eV. Our results describe a unique system, whose structure depends on its spin multiplicity; it exists as the Book structure on singlet states and as the Ring structure on triplet states.
Cyclic identities involving Jacobi elliptic functions
Authors: Khare Avinash , Sukhatme Uday
Published: July 2002

We state and discuss numerous new mathematical identities involving Jacobi elliptic functions sn(x,m), cn(x,m), and dn(x,m), where m is the elliptic modulus parameter. In all identities, the arguments of the Jacobi functions are separated by either 2K(m)/p or 4K(m)/p, where p is an integer and K(m) is the complete elliptic integral of the first kind. Each p-point identity of rank r involves a cyclic homogeneous polynomial of degree r (in Jacobi elliptic functions with p equally spaced arguments) related to other cyclic homogeneous polynomials of degree r−2 or smaller. We algebraically demonstrate the derivation of several of our identities for specific small values of p and r by using standard properties of Jacobi elliptic functions. Identities corresponding to higher values of p and r are verified numerically using advanced mathematical software packages.
Validation of variational transition state theory with multidimensional tunneling contributions against accurate quantum mechanical dynamics for H+CH4→H2+CH3 in an extended temperature interval
Authors: Pu Jingzhi , Truhlar Donald G.
Published: July 2002

Variational transition state theory with multidimensional tunneling contributions (VTST/MT) is tested against quantum mechanical rate constants for the reaction H+CH4→H2+CH3 at temperatures up to 1000 K. The VTST/MT method can be and has been applied to many reactions that cannot be treated by rigorous quantum dynamics methods. Studying the accuracy of VTST/MT by comparison with accurate quantal results that are becoming available for systems of increasing size is important for validating the theory. In the present study, covering a factor of five in temperature, the VTST/MT method is found to have a mean deviation from accurate quantal rate constants for a six-body reaction of only 13% and maximum deviation of only 23%.
Photofragment emission yield spectroscopy of acetylene in the D̃ 1Πu, Ẽ 1A, and F̃ 1Σu+ states by vacuum ultraviolet and infrared vacuum ultraviolet double-resonance laser excitations
Authors: Kono Mitsuhiko , Hoshina Kennosuke , Yamanouchi Kaoru
Published: July 2002

The excitation spectra of the D̃ 1Πu, Ẽ 1A, and F̃ 1Σu+ states of C2H2 in the 135.3–130.8 nm range are measured under jet-cooled conditions by detecting fluorescence emitted from C2H(Ã 2Π) or C2H(B̃ 2A′) photofragments. In the photofragment emission yield spectra, the origin bands of the D̃–X̃ and F̃–X̃ transitions are observed with Lorentzian profiles with bandwidth (Γ) of 58.9(4) and 66.7(2) cm−1, respectively. By identifying the bending progressions of the Ẽ–X̃ transition appearing with narrower Lorentzian profiles with, Γ∼40 cm−1, the band previously considered to be the origin band of the Ẽ–X̃ transition is assigned to the transition to the second overtone (v3=3) level in the near-cis bending (ν3) mode. The transitions to the C–H stretch excited levels in the D̃ and F̃ states are observed using the infrared-VUV double resonance excitation scheme. The D̃ 311, D̃ 111 311, F̃ 311, and F̃ 111 311 bands are identified at 74 334(3), 74 121(5), 74 522(3), and 74 388(3) cm−1, respectively, with much broader bandwidth (Γ>100 cm−1) than the D̃–X̃ and F̃–X̃ origin bands, indicating that the dissociation is accelerated significantly in both of the D̃ and F̃ states when the antisymmetric C–H stretch (ν3) mode in the D̃ and F̃ states is excited.
Nuclear spin relaxation in ligands outside of the first coordination sphere in a gadolinium (III) complex:  Effects of intermolecular forces
Authors: Kruk Danuta , Kowalewski Jozef
Published: July 2002

This article describes paramagnetic relaxation enhancement (PRE) in systems with high electron spin, S, where there is molecular interaction between a paramagnetic ion and a ligand outside of the first coordination sphere. The new feature of our treatment is an improved handling of the electron-spin relaxation, making use of the Redfield theory. Following a common approach, a well-defined second coordination sphere is assumed, and the PRE contribution from these more distant and shorter-lived ligands is treated in a way similar to that used for the first coordination sphere. This model is called “ordered second sphere,” OSS. In addition, we develop here a formalism similar to that of Hwang and Freed [J. Chem. Phys. 63, 4017 (1975)], but accounting for the electron-spin relaxation effects. We denote this formalism “diffuse second sphere,” DSS. The description of the dynamics of the intermolecular dipole–dipole interaction is based on the Smoluchowski equation, with a potential of mean force related to the radial distribution function. We have used a finite-difference method to calculate numerically a correlation function for translational motion, taking into account the intermolecular forces leading to an arbitrary radial distribution of the ligand protons. The OSS and DSS models, including the Redfield description of the electron-spin relaxation, were used to interpret the PRE in an aqueous solution of a slowly rotating gadolinium (III) complex (S=7/2) bound to a protein.
Closed loop learning control with reduced space quantum dynamics
Authors: Kim Young Sik , Rabitz Herschel
Published: July 2002

This paper investigates the ability of closed loop quantum learning control techniques to meet a posed physical objective while simultaneously steering the dynamics to lie in a specified subspace. Achievement of successful control with reduced space dynamics can have a number of benefits including a more easily understood control mechanism. Judicious choices for the cost functional may be introduced such that the closed loop optimal control experiments can steer the dynamics to lie within a subspace of the system eigenstates without requiring any prior detailed knowledge about the system Hamiltonian. Learning control with reduced space dynamics takes advantage of the expected existence of a multiplicity of fields that can all give acceptable quality control outcomes. The procedure eliminates the hard demands of following a specific dynamical path by only asking that the dynamics reside in a subspace. Additional measurements characterizing the subspace are necessary to monitor the system evolution during the control field learning process. This procedure is simulated for optimally controlled population transfer experiments in systems of one and two degrees of freedom. The results demonstrate that optimal control fields can be found that successfully derive the system to the target state while staying within the desired subspace.
Temperature structure functions for air flow over moderately heated ground
Authors: Aivalis K. G. , Sreenivasan K. R. , Tsuji Y. , Klewicki J. C. , Biltoft C. A.
Published: July 2002

We study temperature structure functions of second, fourth, and sixth orders at heights of up to 2 m above the ground in moderately heated atmosphere. Most of the data come from measurements over salt flats of the Utah desert, with well-defined wind direction and uniform temperature boundary conditions. As in high-Rayleigh-number convection in a closed container, a thermal boundary layer develops near the ground, its thickness here being of the order of 50 cm. We demonstrate the coexistence of two scaling ranges, one of which corresponds to the classical inertial range and the other to the buoyant range influenced by thermal convection. The determination of scaling exponents in the two ranges is facilitated by the use of a scaling function. We present the variations with height of scaling exponents in both ranges, as well as the crossover scales from one range to another.
Thermodynamic properties of the Si–SiO2 system
Authors: Capron N. , Boureau G. , Pasturel A. , Hafner J.
Published: July 2002

In this paper, using the density functional theory, we present a consistent study of the thermodynamics of the Si–SiO2 system. This includes the formation of oxygen interstitials in silicon and of oxygen vacancies in silicon dioxide (cristobalite). Relations between calculated and measured quantities are analyzed. The composition dependence of the Si–Si and Si–O bond energies are discussed, which has implications on the modeling of the Si–SiO2 interface.
Near-field Raman imaging of organic molecules by an apertureless metallic probe scanning optical microscope
Authors: Hayazawa Norikiko , Inouye Yasushi , Sekkat Zouheir , Kawata Satoshi
Published: July 2002

Near-field Raman imaging of organic molecules is demonstrated by an apertureless near-field scanning optical microscope, the tip of which is a silver-layer-coated cantilever of an atomic force microscope (AFM). The virtue of the enhanced electric field at the tip apex due to the surface plasmon polariton excitations enhances the Raman scattering cross sections. This phenomenon allows us to reveal from near-field Raman images the molecular vibrational distributions of Rhodamine6G and Crystal Violet molecules beyond the diffraction limit of a light. These molecular vibrations cannot be distinguished by AFM topographic images.
Improved alternating gradient transport and focusing of neutral molecules
Authors: Kalnins Juris , Lambertson Glen , Gould Harvey
Published: July 2002

Polar molecules, in strong-field seeking states, can be transported and focused by an alternating sequence of electric field gradients that focus in one transverse direction while defocusing in the other. We show, by calculation and numerical simulation, how one may greatly improve the alternating gradient transport and focusing of molecules. We use a new optimized multipole lens design, a FODO-lattice beam transport line, and lenses to match the beam transport line to the beam source and to the final focus. We derive analytic expressions for the potentials, fields, and gradients that may be used to design these lenses. We describe a simple lens optimization procedure and derive the equations of motion for tracking molecules through a beam transport line. As an example, we model a straight beamline that transports a 560 m/s jet-source beam of methyl fluoride 15 m from its source and focuses it to 2 mm diameter. We calculate the beam transport line acceptance and beam survival, for a beam with a velocity spread, and estimate the transmitted intensity for specified source conditions. Possible applications are discussed.