bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonant Raman scattering and luminescence in CuInS2 crystals
Authors: Wakita Kazuki , Hirooka Hiroyuki , Yasuda Shusaku , Fujita Futoshi , Yamamoto Nobuyuki
Published: July 2002

Resonant Raman scattering and luminescence have been examined for CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT). Resonant Raman spectra of CuInS2 have been observed, and the spectra show seven single-phonon peaks and one two-phonon peak. Among them, three single-phonon modes have been found in the low-Raman-shift region because of resonant enhancement of phonon modes. The enhancement of these phonon modes is caused by incoming resonance mediated by bound excitons on the THM crystal, while it is attributed to outgoing resonance due to intermediate states of free excitons on the IT crystal.
Nondestructive measurement of thickness and carrier concentration of GaAs epitaxial layer using infrared spectroscopic ellipsometry
Authors: Nakano H. , Sakamoto T. , Taniguchi K.
Published: July 2002

This article reports a method for evaluating thickness and the carrier concentration of n type GaAs epitaxial layers simultaneously by employing nondestructive, contactless infrared spectroscopic ellipsometry. The carrier concentration Nc and layer thickness d were correctly measured for carrier concentration over 5×1017 cm−3 with the method by simply assuming the plasma oscillation of free carriers and a step function carrier profile.
Two-dimensional simulation of an alternating current matrix plasma display cell: Cross-talk and other geometric effects
Authors: Punset C. , Boeuf J.-P. , Pitchford L. C.
Published: July 2002

A two-dimensional, user-friendly model of the discharge occurring in a plasma display panel cell was developed. This model was used to study the transient discharges in an alternating current plasma display with a matrix electrode configuration. The space and time variations of the charge particle densities, excitation rates, electric potential, and surface charge densities are described. The model is also used to study the conditions of existence of electrical interaction between adjacent cells and the effects of electrode misalignment.
Oriented diamond growth on silicon (111) using a solid carbon source
Authors: Woo H. K. , Lee C. S. , Bello I. , Lee S. T. , Wong K. W. , Wong N. B.
Published: July 2002

Textured diamond films have been grown on silicon (111) substrate by using hot filament chemical vapor deposition. A graphite plate immersed in hydrogen was used as the carbon source rather than the conventional gaseous methane source. During the nucleation period, a negative bias relative to the filaments was applied to the substrate. An epitaxial β-SiC layer was deposited during the bias treatment. Textured diamond film was subsequently grown on the β-SiC layer from the mixture of hydrogen and hydrocarbon species etched from the graphite.
Optical characterization of GaAs/AlGaAs quantum well wires fabricated using arsenic implantation induced intermixing
Authors: Ooi B. S. , Tang Y. S. , Helmy A. Saher , Bryce A. C. , Marsh J. H. , Paquette M. , Beauvais J.
Published: July 2002

We report the fabrication of GaAs/AlGaAs quantum well wires using implantation of As at 45 keV to induce quantum well intermixing. The intermixing process was first characterized giving optimized annealing parameters of 875 °C for 30 s and an implantation dose of 1×1013 cm−2. Wire widths from 35 to 1000 nm were defined using e-beam lithography followed by lift-off. Photoluminescence spectra from the lateral wells and barriers were observed from samples with wires as narrow as 50 nm. The energies of the lateral wells were found to remain constant for wire widths between 1000 and 150 nm, and start to shift significantly towards high energy for 80 nm wires, the signal from the lateral well eventually merging with that from the lateral barrier for 35 nm wires. An intermixing radius of about 17 nm was estimated for the process. Photoreflectance measurements were also carried out on these wire samples, showing that the wires appear to have a parabolic lateral potential and clear interwire coupling was observed from samples with barriers narrower than 50 nm.
Reply to “Comment on ‘Amorphization and defect recombination in ion implanted silicon carbide’ ” [J. Appl. Phys. 83, 3935 (1998)]
Authors: Grimaldi M. G. , Calcagno L.
Published: July 2002

The arguments supporting our determination of the atomic displacement energy in SiC by channeling analysis of the damage produced by a 30 keV He beam implanted at low temperature are discussed.
An alternative model for neutralization of radon decay products
Authors: Maddern T. M. , Hobbs D. A. , Mitchell L. W.
Published: July 2002

The widely held view that the majority of radon decay products carry a positive charge at birth, independent of the electric field at the site of production, is challenged. Following Wellisch [Philos. Mag. Suppl. 26, 623 (1913)] we propose that the fraction born charged is in fact a strong function of the electric field, and that neutralization of the decay products must be treated in two distinct parts corresponding to recombination in the initial ionization column and neutralization in the volume of the gas. We illustrate our argument with measurements relating to the birth (by momentum recoil at a surface) of Tl208 ions, and their subsequent transport through argon and air.
High-coercivity Sm2(Fe,Al,Mo)17C1.5 ribbons with addition of Mo
Authors: Tang W. , Jin Z. Q. , Yin J. H. , Zhang J. R. , Zhang S. Y. , Du Y. W.
Published: July 2002

The effects of Mo additions on the structure and magnetic properties of Sm2Fe(15.5−x)MoxAl1.5C1.5 alloys have been investigated by means of x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and magnetic measurements. The XRD analyses show that as-cast alloys consist mainly of a 2/17-type carbide and a considerable amount of α-Fe. Rapidly quenched, the Mo-containing alloys form a nanocrystalline structure with an average grain size of 35–50 nm, while the amount of the α-Fe phase is greatly decreased. The magnetic hardening of the Mo-containing alloys can be achieved by direct quenching. A coercivity exceeding 1.35 T is obtained for the Sm2Fe14.9Mo0.6Al1.5C1.5 ribbon spun at 40 m/s. These results reveal that the addition of Mo is effective in improving the coercivity.
High resolution x-ray diffraction study of Bragg peak width in strained InGaAs/InAlAs/InP heterostructures
Authors: Yang Quankui , Li Aizhen , Chen Jianxin
Published: July 2002

Many different reflections have been investigated by high resolution x-ray diffraction on gas source molecular beam epitaxy grown strained InGaAs/InAlAs/InP heterostructures to study the Bragg peak widths. Results show that even deducting the influence of the intrinsic peak width and the influence of the instrument broadening, for either InGaAs or InAlAs epilayers, a linear relationship between the square of the Bragg peak width and tan2 θB exists with θB the Bragg angle, for either grazing incidence or grazing emergence geometry. Different slopes for the linear relationship have been measured in the two different geometries. Systematically, a much bigger peak width is detected under a grazing incidence geometry than that under a grazing emergence geometry for the same asymmetric reflection. The phenomenon is explained by the existence of a normal strain gradient along the growth direction of the epilayer’s thickness. This normal strain gradient has been confirmed by x-ray triple-axis two-dimensional mapping. Moreover, a broader peak width of the (117) reflection than that of the (155) reflection is consistent with the strain gradient.
Theoretical foundation of the optical current-controlled oscillator and its applications for subcarrier optical communications
Authors: Li Guifang , Tian Feng
Published: July 2002

The optical current-controlled oscillator (CCO) is an all-optical device capable of producing frequency-tunable intensity oscillations (subcarriers) on an optical carrier without any active radio frequency/microwave components. From a dynamic point of view, the CCO is a nonlinear optical system supporting the Hopf bifurcation and operating in the periodic orbit regime. This article presents the theoretical foundation for the optical CCO and its applications in optical communications involving subcarriers. Using one embodiment of the optical CCO employing a two-section Fabry-Pérot laser diode, we present the static and modulation characteristics of the periodic-orbit subcarrier of the optical CCO. The performance of CCO-based all-optical subcarrier optical communication system is estimated using representative device parameters. The relationship between the capacity of CCO-based communication networks and the frequency-tuning range of the CCO is also established.
Validation of actinometry for estimating relative hydrogen atom densities and electron energy evolution in plasma assisted diamond deposition reactors
Authors: Gicquel A. , Chenevier M. , Hassouni Kh. , Tserepi A. , Dubus M.
Published: July 2002

The validity of the actinometry method applied to H-atom mole fraction measurements has been analyzed. First, a theoretical approach allowed us to determine boundary conditions for which the validity of actinometry may be critical. For these specific conditions, corresponding to an upper limit of electron temperature of 20 000 K and a lower limit of H-atom mole fraction of 2%–4%, spatial distributions of the ground state H-atom relative densities provided either by two photon allowed transition or by optical emission spectroscopy (OES) were compared and seen to be proportional. This proves that the H atoms excited in the level of quantum number n=3 (level used for OES experiments) are produced directly from the ground electronic state during collisions with electrons. Actinometry can then be applied under these experimental conditions. Second, the emission intensity ratio of two lines issued from excited states of argon was demonstrated to be indirectly related to the “electron temperature” of the hot electrons of the plasma. This allowed us to predict the way of evolution of the plasma electrons’ energy as a function of the operating conditions. Thus, experiments (which have been confirmed by calculations) showed that the electron energy decreases as a function of the microwave power density and remains constant as a function of the methane percentage introduced in the feed gas at least up to 6%. The consequence is that the domain of diamond deposition discharge conditions for which actinometry is valid is quite wide. Once the microwave volumetric power density is more than 9 W cm−3, and the percentage of methane less than 6%, actinometry can be applied. However, the estimation of variations of H-atom mole fractions as a function of the operating conditions implies the use of correcting factors, which are discussed. They are mainly due to the large influence of the quenching processes under these experimental conditions. An experimental estimate of the quenching cross section of the H(n=3) atoms by ground state molecular hydrogen, which was unknown and involved in the correcting factors, is presented. Finally, relative variations of H-atom mole fraction in space and as a function of the methane percentage are shown.
Anomalous high-temperature coercivities in hard nanocomposite alloys
Authors: Lewis L. H. , Wang J.-Y. , Welch D. O. , Panchanathan V.
Published: July 2002

To elucidate the interphase interactions inherent to nanocomposite magnetic alloys, measurements of remanence Br, and coercivity Hci were made on a series of four meltspun, remanence-enhanced nanocomposite alloys consisting solely of various amounts of Nd2Fe14B and α-Fe. The phase constitution and microstructural scale of the alloys were characterized with synchrotron x-ray diffraction. Magnetic measurements were made using superconducting quantum interference device (SQUID) magnetometry on evacuated and encapsulated samples in the temperature range of 300 K⩽T⩽750 K, in order to characterize the α-Fe component independently of the Nd2Fe14B component. The high-temperature coercivities of the samples increase with the amount of α-Fe present in the samples, ranging from an average value of approximately 75 Oe for the sample with 14 wt % excess Fe to over 400 Oe at 700 K for the sample with 27 wt % excess Fe. The relatively high coercivities of the samples found at elevated temperatures imply that a tabular morphology of the α-Fe grains is conferring anisotropy to the phase; this conclusion is supported by transmission electron microscopy. It is concluded that while the significant coercivity of the α-Fe phase likely reduces the room-temperature remanence enhancement of the alloy below its theoretical ideal, the favorable interphase interface orientation promotes exchange coupling.
Bifurcation phenomena and chaotic attractors in a six-dimensional nonlinear system
Authors: Sutani T. , Czaszejko T. , Nafalski A.
Published: July 2002

Some chaotic properties of a six-dimensional nonlinear dynamic system are investigated. Equations describing the system are based on the equivalent circuit of a magnetic frequency tripler. Numerical solutions are obtained using the fourth-order Runge–Kutta algorithm. Simulation results reveal the presence of Hopf and a period-doubling bifurcations. Phase space projections at points before and after bifurcations show the existence of three symmetrical and four asymmetric regions. Poincaré maps reveal six different chaotic attractors within the asymmetric regions. The correlation dimension of the sea horse attractor is found to be 2.37.
Modeling of domain structure and anisotropy in glass-covered amorphous wires
Authors: Ménard D. , Frankland D. , Ciureanu P. , Yelon A. , Rouabhi M. , Cochrane R. W. , Chiriac H. , Óvári T. A.
Published: July 2002

Giant magnetoimpedance (GMI) at 10 MHz and longitudinal magnetization curves are used to investigate the anisotropy of Co68.15Fe4.35Si12.5B15 wires, glass-covered and after glass removal. The high resolution GMI response to the field shows hysteresis and large Barkhausen jumps, in good agreement with those observed in the magnetization curves. These are modeled through superposition of the response of the inner core and outer shell of the wires. The GMI response is calculated using the differential susceptibility deduced from the model, thus relating the domain structure to the observed magnetoimpedance.
Bioluminescence under static magnetic fields
Authors: Iwasaka M. , Ueno S.
Published: July 2002

In the present study, the effect of magnetic fields on the emission of light by a living system was studied. The fireflies Hotaria parvula and Luciola cruciata were used as the bioluminescence systems. The firefly light organ was fixed at the edge of an optical fiber. The emitted light was introduced into a single-channel photon-counting system using an optical fiber. We measured both the spectrum of a constant light emission and, the time course of bioluminescence pulses. Two horizontal-type superconducting magnets, which produced 8 and 14 T magnetic fields at their center, were used as the magnetic-field generators. We also carried out an in vitro study of bioluminescence. The enzymatic activity of luciferase was measured under a 14 T magnetic field. We measured emission spectra of bioluminescence over the interval 500–600 nm at 25 °C in a stable emission state. It was observed that the peak wavelength around 550 nm shifted to 560 nm under a 14 T magnetic field. However, the effects of magnetic fields were not significant. Also, we measured the time course of emissions at 550 nm in a transient emission state. The rate in the light intensity under a 14 T magnetic field increased compared to the control. There is a possibility that the change in the emission intensities under a magnetic field is related to a change in the biochemical systems of the firefly, such as the enzymatic process of luciferase and the excited singlet state with subsequent light emission.
Estimation of multiple sources using a three-dimensional vector measurement of a magnetoencephalogram
Authors: Kobayashi Koichiro , Uchikawa Yoshinori
Published: July 2002

We developed a three-dimensional (3D) second-order gradiometer connected to three superconducting quantum interference devices (SQUIDs) for vector measurement of a magnetoencephalogram (MEG) that can detect magnetic-field components perpendicular and tangential to the scalp simultaneously. Each coil is orthogonally wound with Nb–Ti wire on a rectangular solid 3×3×6 cm. To assess discrimination and separation of multiple sources, we carried out a 3D vector measurement of the MEG with a mixed auditory evoked field (AEF) and a somatosensory evoked field (SEF) overlapping in time. The magnetic-field distribution perpendicular to the scalp was not helpful in estimating the location and number of sources, owing to the lack of a dipole pattern. But the magnetic-field distribution tangential to the scalp can provide information about constraint conditions by visual inspection. We estimated multiple sources of the mixed AEF and SEF from the MEG data of the magnetic field tangential to the scalp and confirmed by comparison with magnetic resonance imaging (MRI) of a subject’s head superimposed source locations. We conclude that this 3D second-order gradiometer can provide constraint conditions by visual inspection for calculating the inverse problem with multiple sources.
Thermopower studies of percolating magnetic metallic nanostructures
Authors: Jing X. N. , Yan X.
Published: July 2002

We have observed a small and temperature insensitive thermal electric power S as well as a large resistivity following ρ∼−log(T) dependence, in percolating magnetic metal-insulator nanocomposites, irrespective to the sign of the carrier. Upon annealing, however, a normal metallic behavior with a linear temperature dependence for both S and ρ was recovered. We propose that the large charging energy of the nanometer sized particles in the percolation conduction channels is responsible for this observation as well as the giant Hall effect or the loss of the effective carrier density in these nanostructures.
Numerical simulation of the magnetization structures in thin polycrystalline films with the random anisotropy and intergrain exchange
Authors: Berkov D. V. , Gorn N. L.
Published: July 2002

We have developed a new algorithm which enables a fast and exact evaluation of the dipolar interaction field for lattice systems of magnetic moments when periodic boundary conditions should be applied. The method uses the combination of (i) the fast Fourier transformation technique for the dipolar field evaluation and (ii) the modified version of the Ewald method known from the calculations of the Coulomb lattice sums. The algorithm enabled us to perform large-scale numerical simulations of the remagnetization processes in polycrystalline thin magnetic films with the random single-grain anisotropy and the intergrain exchange. The well known ripple-like structures forming during the remagnetization process were observed. The dependence of the ripple correlation lengths and the hysteresis loop parameters on the intergrain exchange coupling and the film thickness was studied.
Dependence of anti-Stokes/Stokes intensity ratios on substrate optical properties for Brillouin light scattering from ultrathin iron films
Authors: Cochran J. F. , From M. , Heinrich B.
Published: July 2002

Brillouin light scattering experiments have been used to investigate the intensity of 5145 Å laser light backscattered from spin waves in 20 monolayer thick Fe(001) films. The experiments have shown that the ratio of frequency upshifted light intensity to frequency downshifted light intensity depends upon the material of the substrate used to support the iron films. For a fixed magnetic field and for a fixed angle of incidence of the laser light this intensity ratio is much larger for an iron film deposited on a sulphur passivated GaAs(001) substrate than for an iron film deposited on a Ag(001) substrate. The data have been compared with a calculation that takes into account multiple scattering of the optical waves in the iron film and in a protective gold overlayer. The observations are in qualitative agreement with the theory, except for angles of incidence greater than 60°.
Phase formation and magnetic properties of Co-rare earth magnetic films
Authors: Liu Y. , Thomas Richard A. , Malhotra S. S. , Shan Z. S. , Liou S. H. , Sellmyer D. J.
Published: July 2002

Co–Sm and Co–Pr films were deposited by dc magnetron sputtering. Transmission electron microscopy and magnetic measurement were used to study the microstructure and magnetic property relationship. The nanostructure of the as-deposited Co 19 at. % Sm films consists of two phases: the amorphous phase and the crystallite phase. Upon annealing at 600 °C, the Co5Sm phase with the Cu5Ca structure, having grain size of about 20 nm, is obtained along with high coercivity (45 kOe). The as-deposited Co 22 at. % Sm films also have nanostructure similar to the Co 19 at. % films except the volume fraction of the crystallite is reduced. This is related to the concentration of Sm which promotes the formation of the amorphous phase. A new metastable phase Co3Sm is formed upon annealing of the Co 22 at. % Sm film at 500 °C. This phase has the DO19 structure in which the Sm atoms take ordered positions of a triangular pattern in the close-packed planes. A relatively high coercivity value of 29 kOe was obtained from this phase. The as-deposited Co–Pr films show mainly an amorphous phase. Upon annealing at 500 °C for 20 min, Co2Pr with the Mg2Cu-type structure was identified in the Co 35 at. % Pr film. Two phases were identified in the Co 16 at. % Pr films. Coercivities up to 3.1 kOe were achieved in these films.
Structural and magnetic properties of nanocomposite Co:C films
Authors: Yu M. , Liu Y. , Sellmyer D. J.
Published: July 2002

The structural and magnetic properties of Co:C nanocomposite films at room temperature were investigated as functions of annealing temperature, Co concentration, and film thickness. The as-deposited films, which were cosputtered from Co and C onto water-cooled glass substrates, are nonmagnetic amorphous Co–C alloys. Hexagonal-close-packed (hcp) Co grains are formed in 100 nm films annealed at 300 °C and most Co takes this structure when annealed at higher temperatures. The sizes of the Co grains range from 10 to 25 nm, with larger grain size resulting from higher annealing temperature and higher Co concentration. With the increase of annealing temperature and Co concentration, the magnetic activation or switching volumes increase faster than the physical grain volumes, suggesting the increased exchange coupling between neighboring Co grains. The films with high coercivities have optimal combinations of large grain size and weak intergrain exchange coupling before a network-like structure is formed and the percolation threshold is reached. Coercivities of about 800 Oe were obtained in 100 nm films with annealing temperature of 400 °C and Co concentration of 60 at. %. Higher coercivities, up to 1030 Oe, were obtained in films with reduced thickness and elevated annealing temperature.
Diffraction stress analysis of thin films: Modeling and experimental evaluation of elastic constants and grain interaction
Authors: van Leeuwen M. , Kamminga J.-D. , Mittemeijer E. J.
Published: July 2002

Well-known grain interaction models for the description of macroscopic elastic behavior of polycrystalline specimens, as due to Voigt, Reuss, Neerfeld–Hill, and Eshelby–Kröner, may be successfully applied to bulk specimens, but are shown to be less suited for thin films. An elaboration of a proposal due to Vook and Witt for grain interaction is given. It is assumed that the strain parallel to the specimen surface is equal in all crystallites and that the stress perpendicular to the specimen surface is zero in all crystallites. It is shown that these assumptions give rise to elastic anisotropy of the specimen on the macroscopic scale. It is also shown that in this case the dependence of the measured lattice strain (in a diffraction experiment) on the squared sine of the specimen tilt angle ψ (cf. the sin2 ψ method), is nonlinear, contrary to what is predicted by the bulk grain interaction models. This is the first time that nonlinear sin2 ψ plots have been calculated using an elastic grain interaction model, in the absence of crystallographic texture. Experimental verification has been achieved by x-ray diffraction strain measurements performed on a vapor deposited nickel film. The experimental results are in good accordance with the Vook–Witt [J. Appl. Phys. 7, 2169 (1965)] grain interaction model. This is the first experimental evidence of direction dependent grain interaction in thin films.
Dynamical corrections to transition state theory for multistate systems: Surface self-diffusion in the rare-event regime
Authors: Voter Arthur F. , Doll Jimmie D.
Published: July 2002

We derive an expression for the classical rate constant between any two states of a multistate system. The rate is given as the transition state theory rate of escape from the originating state, multiplied by a dynamical correction factor in the form of a time-correlation function which is evaluated using molecular dynamics techniques. This method is desiged to treat cases in which reactive state-change events are so infrequent (e.g., at low temperature) that direct molecular dynamics calculations are unfeasible. In this regime where dynamical recrossings occur much more quickly than the average time between reactive state changes, the concept of a rate between two nonadjacent states becomes meaningful. We apply the method to the surface diffusion of Rh on Rh(100) at the temperatures employed in field ion microscope experiments.
Foreign gas relaxation of the J=0→1 transition of HC15N. A study of the temperature dependence by coherent transients
Authors: Rohart François , Derozier Dominique , Legrand Jean
Published: July 2002

The temperature dependence of the foreign gas relaxation of hydrogen cyanide, one of the best candidate for radioastronomical observations in planetary atmospheres, is investigated in the 135–300 K range. The measurement process exploits the delayed nutation phenomenon which leads to the determination of the population relaxation. This coherent transient technique avoids any deconvolution of the Doppler effect and a novel pulse sequence which alleviates the difficulties related to the finite saturation actually achieved is proposed. The great sensitivity of the method requires active gas pressures (≲10−4 Torr) much lower than in linewidth techniques, and thus results in an extension by about 60 K of the lower bound of attained temperatures. The millimeter Stark spectrometer used, driven by a microprocessor system, is temperature controlled by a cold gaseous nitrogen flow; its careful design allows a strong reduction of thermal transpiration effects, and thus leads to proper pressure measurements for working pressures in the 10−3–10−2 Torr range. Measurements are made on the J=0→1 transition of HC15N with N2, H2, D2, Ar, and He as buffer gases. The usual T−n temperature dependence of relaxation rates is deduced, but departures are observed at temperatures lower than 160 K for the diatomic perturbers. By comparison with previously published works, neither isotopic nor vibrational dependences are observed, and population and coherence relaxation rates are similar. Experimental results are fairly reproduced by numerical calculations made in the frame of the Anderson–Tsao–Curnutte theory, which allows theoretical predictions for other HCN rotational lines of planetary interest.
Dynamics of vibrationally excited ozone formed by three-body recombination. II. Kinetics and mechanism
Authors: Rawlins W. T. , Caledonia G. E. , Armstrong R. A.
Published: July 2002

Spectrally resolved infrared chemiluminescence from vibrationally excited ozone, O3(v), has been used to study the reaction kinetics of O3(v) in discharged O2/Ar mixtures at ∼1 Torr and 80–150 K. Dependences of the excited state number densities on temperature and O2 mole fraction indicate O3(v) is formed primarily by three-body recombination of O with O2 and is destroyed by rapid chemical reaction with O. Several secondary excitation reactions involving vibrationally and electronically excited O2 are also indicated. The data are treated with a detailed steady-state analysis of the discharge kinetics, to extract estimates for rate coefficients of the key elementary reactions. The effective ‘‘quasinascent’’ state distribution in recombination is also inferred; this distribution shows selective recombination into the asymmetric stretching mode, but an apparently statistical (i.e., collisionally scrambled) behavior among the vibrational states within that mode. The results are discussed in terms of the detailed dynamics of three-body recombination.
The size of a polymer in random media
Authors: Edwards S. F. , Muthukumar M.
Published: July 2002

Stimulated by the recent numerical simulation results of Baumgartner and Muthukumar, an analytic derivation is given of the size of a polymer in random media. It is shown that, for long Gaussian chains in three dimensions, the mean square end-to-end distance 〈∼(R2)〉 is proportional to (uρ0)−2, where uρ0 is the measure of the scattering power of the medium. For example, ρ0 is the number density of the scatterers and u is the strength of the pseudopotential between the chain segments. A simple extrapolation formula has been obtained using the replica theory for 〈∼(R2)〉 at intermediate values of (uρ0)2L, 〈∼(R2)〉 =(Ll/z) [1−exp(−z)], where L is the chain length, l is the Kuhn step length, and z=εu2ρ20Ll5 with ε being a numerical coefficient. This theory agrees well with the simulation results. The effects of space dimensionality and the replica breaking symmetry are also addressed.
Block copolymer self-assembly in selective solvents: Spherical micelles with segregated cores
Authors: Nagarajan R. , Ganesh K.
Published: July 2002

A theory of micellization of AB diblock copolymer molecules in a selective solvent S is developed here. The micelles are assumed to have a completely segregated core region consisting only of the A block and a shell region consisting of the solvent S and the solvent compatible B block. The theory allows one to predict the critical micelle concentration, the micelle size distribution, the average aggregation number, as well as the core radius and the shell thickness of the micelle. The novel outcome of the present theory, in contrast to the treatments of micellization pioneered by de Gennes, Leibler, Orland, and Wheeler, and Noolandi and Hong, is its prediction that the solvent compatible B block plays an important role in determining the micellization behavior. The influence of the B block becomes relatively more prominent in systems where the solvent S constitutes a very good solvent for the B block. Further, scaling relations that are not system specific have been developed relating explicitly the micellar size parameters to the characteristics of the block copolymer and the solvent.
Static properties of polymer chains in porous media
Authors: Honeycutt J. D. , Thirumalai D.
Published: July 2002

The static properties of a polymer molecule in a porous medium are investigated. The porous medium is simulated using a site percolation model in which the various sites are occupied (or unoccupied) randomly. A freely jointed chain is allowed to move in continuous space between the obstacles. Effects of excluded volume interactions between the links have also been studied. Using a generalized Flory theory, we have shown that, when the strength of disorder is large enough, the mean square end-to-end distance scales as N2ν, where N is the number of links in the chain, and ν takes on a value different from that for a free chain. Under these conditions, the polymer assumes a compact, globule-like conformation. For sufficiently large N, the Flory theory gives ν=1/(d+2) for freely jointed chains and ν=1/d for chains with excluded volume. Various correlation functions such as the distribution of the end-to-end distance and density profile of monomers with respect to the center of mass of the chain have been computed using Monte Carlo simulations. These results are interpreted using scaling concepts and an approximate variational theory based on replica methods. The limitations of the replica variational theory are assessed by an application to the directed polymer in a quenched random environment. We have also studied the shape fluctuations that the polymer molecule undergoes in the random environment. It is argued that these shape fluctuations are relevant to the transport mechanism of polymers in random media. The results obtained for the porous media are contrasted with those found for polymers in media where the obstacles are arranged in a regular manner.
Local volume fraction fluctuations in heterogeneous media
Authors: Lu Binglin , Torquato S.
Published: July 2002

The volume fractions of multiphase heterogeneous media fluctuate on a spatially local level even for statistically homogeneous materials. A general formulation is given to represent the standard deviation associated with the local volume fraction of statistically homogeneous but anisotropic D-dimensional two-phase media for arbitrary-shaped observation regions. The standard deviation divided by the macroscopic volume fraction, termed the coarseness, is computed for D-dimensional distributions of penetrable as well as impenetrable spheres, for a wide range of densities and observation-region sizes. The effect of impenetrability of the particles, for fixed observation-region size, is to reduce the coarseness relative to that of the penetrable-sphere model, especially at high densities. For either sphere model, increasing the dimensionality D decreases the coarseness.
Theoretical study of ion solvation at the water liquid–vapor interface
Authors: Benjamin Ilan
Published: July 2002

Molecular dynamics calculations are reported for several ions at the liquid–vapor interface of water. The transition from the bulk to the interface region is investigated from structural, energetic, and dynamical points of view by calculating ion–water geometries, radial distribution functions, solvent molecular reorientation times, solvent polarization fluctuations, and solvation free energy as a function of distance from the interface. It is shown that ions tend to keep most of the structural and dynamical properties of their first solvation shell intact as they are moved into the interface, and that the tendency for negative adsorption (positive free energy of adsorption) is associated with weaker and fewer long range interactions. A comparison of some of the molecular dynamics results to predictions of simple continuum models is discussed, showing generally poor quantitative agreement.
Formation and vibrational relaxation of OH (X 2Πi,v) by O2 and CO2
Authors: Dodd James A. , Lipson Steven J. , Blumberg William A. M.
Published: July 2002

Time-resolved OH(X 2Πi,v=1–9) populations have been measured and analyzed to determine parameters relating to formation mechanisms and vibrational relaxation. OH(v) was formed in electron-irradiated Ar/H2/O3 mixtures containing added O2 or CO2 as relaxer species. OH(v→v−1,v−2) emission was observed using time-resolved Fourier spectroscopy. Spectra were then fit to determine time-dependent populations. Population data were analyzed using a single-quantum relaxation model, but the possible effects of multiquantum relaxation were also considered. The model includes provision for OH(v) production via H+O3→OH(v)+O2 after e-beam termination, which has been found to have a significant effect on the results. The following relaxation rate constants are obtained: kv=1–6(O2)=1.3±0.4, 2.7±0.8, 5.2±1.5, 8.8±3.0, 17±7, 30±15 (10−13 cm3s−1) and kv=1–4(CO2)=1.8±0.5, 4.8±1.5, 14±5, 28±10 (10−13 cm3s−1), respectively. Two different exponential decay rates are necessary to characterize the time dependence of the inferred H atom concentration. The role of O(1D)+H2→OH+H is also discussed.
Experimental determination of the dielectric virial coefficients of atomic gases as a function of temperature
Authors: Huot J. , Bose T. K.
Published: July 2002

The second dielectric virial coefficient of atomic gases has been measured precisely as a function of temperature in the range 77 to 407 K. The experimental values are compared with the ab initio calculations and the collision-induced polarized scattered spectra (CIS). Except for helium, the Bε values derived from ab initio calculations and CIS parameters do not agree with the experimental values.
Photon/fragment-ion coincidence investigation of the continuous ultraviolet emissions produced by fast ion impact on CF4 and CHF3
Authors: Müller U. , Lange M. , Haas W. , Brenn R.
Published: July 2002

A continuous or quasicontinuous band in the 200–500 nm wavelength range is known to be emitted following electron, fast ion, and vacuum ultraviolet photon impact on CF4 and CHF3. We performed a photon/fragment-ion coincidence experiment following 2 MeV H+, He+, and He++ impact on CF4 and CHF3 with the objective to clarify the nature of the emitter and to study the dynamics of the emission process. In the case of the CF4 molecule, we observed that the photons in the 200–500 nm range are quantitatively coincident with CF+3 fragments. From the structure of the coincidence peak, we derived the kinetic energy released during the dissociation process and compared it with the optical spectrum and the total available energy. It follows that the CF+3 ions are vibrationally and/or rotationally highly excited. The continuous emission is explained by a radiative decay of the CF+4(C̃ 2T2) state to the CF+4(X̃ 2A1) repulsive potential surface which dissociates quickly into CF+3 and a fluorine atom. In the case of the CHF3 molecule, we observed coincidences between photons in the 200–500 nm range and CHF+2 fragments. A process similar to that in the CF4 molecule seems to take place with the CHF+3(D̃) state as the precursor. In the same spectral range, a band of electronically excited CF*2 radicals seems also to be present. No coincidences between photons in the 200–500 nm range and CF+3 were observed which excludes the CF+*3 as a precursor of the continuous bands in the case of CHF3 and makes it very unlikely in the case of CF4.
The nature of monomer inversion in the ammonia dimer
Authors: Olthof E. H. T. , van der Avoird A. , Wormer P. E. S. , Loeser J. G. , Saykally R. J.
Published: July 2002

A model is presented for calculating the splittings due to umbrella inversion of the monomers in (NH3)2. Input to the model are the six-dimensional dimer bound state wave functions for rigid monomers, calculated previously [E. H. T. Olthof, A. van der Avoird, and P. E. S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. This model is based on first-order (quasi) degenerate perturbation theory and adaptation of the wave functions to the group chain G36⊆G72⊆G144. The umbrella inversion splittings depend sensitively on the intermolecular potential from which the bound state wave functions are obtained. A complete interpretation of the observed splitting pattern [J. G. Loeser, C. A. Schmuttenmaer, R. C. Cohen, M. J. Elrod, D. W. Steyert, R. J. Saykally, R. E. Bumgarner, and G. A. Blake, J. Chem. Phys. 97, 4727 (1992)] and quantitative agreement with the measured splittings, which range over three orders of magnitude, are obtained from the potential that reproduces the far-infrared spectrum of (NH3)2 and the dipole moment and nuclear quadrupole splittings of (NH3)2 and (ND3)2. The umbrella inversion splittings of (ND3)2 are predicted.
The reaction of carbon dioxide with an oxygen precovered Ag(110) surface
Authors: Stensgaard I. , Laegsgaard E. , Besenbacher F.
Published: July 2002

The reaction of CO2 with an oxygen precovered Ag(110) surface leading to the formation of carbonate has been studied by scanning tunneling microscopy (STM) for different oxygen precoverages. For coverages below 0.5 monolayers (ML), the reaction is initiated preferentially at steps and defects and spreads uniaxially along the [001] directed, added -Ag–O- rows. For initial oxygen coverages below 0.25 ML, complete transformation to the carbonate structure is observed. For higher initial oxygen coverage, the unreacted rows are compressed to a (2×1) structure. In all cases, the ratio of carbonate ions formed to oxygen atoms consumed from added rows is close to 1:2. The reaction leads to a new reconstruction of the surface in which the carbonate ions are seen to reside preferentially on top of [11̄0] directed triplet structures.
Crossover to potential energy landscape dominated dynamics in a model glass-forming liquid
Authors: Schrøder Thomas B. , Sastry Srikanth , Dyre Jeppe C. , Glotzer Sharon C.
Published: July 2002

An equilibrated model glass-forming liquid is studied by mapping successive configurations produced by molecular dynamics simulation onto a time series of inherent structures (local minima in the potential energy). Using this “inherent dynamics” approach we find direct numerical evidence for the long held view that below a crossover temperature, Tx, the liquid’s dynamics can be separated into (i) vibrations around inherent structures and (ii) transitions between inherent structures [M. Goldstein, J. Chem. Phys. 51, 3728 (1969)], i.e., the dynamics become “dominated” by the potential energy landscape. In agreement with previous proposals, we find that Tx is within the vicinity of the mode-coupling critical temperature Tc. We further find that near Tx, transitions between inherent structures occur via cooperative, stringlike rearrangements of groups of particles moving distances substantially smaller than the average interparticle distance.
A possible explanation for the probabilities of quantum mechanics
Authors: Aerts Dirk
Published: July 2002

It is shown that a lack of knowledge about the measurements of a physical system gives rise to a nonclassical probability calculus for this physical system. It is also shown that the nonclassical probability calculus of quantum mechanics can be interpreted as being the result of a lack of knowledge about the measurements. Examples are given of macroscopic real systems that have a nonclassical probability calculus. A macroscopic real system that has a quantum probability calculus is also given, and more specifically a model for the spin of a spin- 1/2 particle is constructed. These results are analyzed in light of the old hidden variable problem.
On supersymmetries in nonrelativistic quantum mechanics
Authors: Beckers J. , Debergh N. , Nikitin A. G.
Published: July 2002

One-dimensional nonrelativistic systems are studied when time-independent potential interactions are involved. Their supersymmetries are determined and their closed subsets generating kinematical invariance Lie superalgebras are pointed out. The study of even supersymmetries is particularly enlightened through the already known symmetries of the corresponding Schrödinger equation. Three tables collect the even, odd, and total supersymmetries as well as the invariance (super)algebras.
Error bounds in maximum entropy approximations
Authors: Leaseburg Michael J. , Mead Lawrence R.
Published: July 2002

A useful technique in underdetermined inverse problems is that of maximum entropy. A simple error bound for averages over a distribution approximated by the maximum entropy method in the case of the undetermined Hausdorff moment problem was devised. Under the conditions specified, the error bound for averages over such an approximate distribution can be very tight. Numerical examples to illustrate are presented.
Exotic smoothness and physics
Authors: Brans Carl H.
Published: July 2002

The essential role played by differentiable structures in physics is reviewed in light of recent mathematical discoveries that topologically trivial space–time models, especially the simplest one, R4, possess a rich multiplicity of such structures, no two of which are diffeomorphic to each other and thus to the standard one. This means that physics has available to it a new panoply of structures for space–time models. These can be thought of as a source of new global, but not properly topological, features. This article reviews some background differential topology together with a discussion of the role which a differentiable structure necessarily plays in the statement of any physical theory, recalling that diffeomorphisms are at the heart of the principle of general relativity. Some of the history of the discovery of exotic, i.e., nonstandard, differentiable structures is reviewed. Some new results suggesting the spatial localization of such exotic structures are described and speculations are made on the possible opportunities that such structures present for the further development of physical theories.
Direct numerical simulation of a three-dimensional turbulent boundary layer
Authors: Moin P. , Shih T.-H. , Driver D. , Mansour N. N.
Published: July 2002

The effects of transverse strain on an initially two-dimensional turbulent boundary layer are studied in a direct numerical simulation of a planar channel flow with impulsively started transverse pressure gradient. Consistent with experiments in three-dimensional boundary layers, the simulation shows a decrease in the Reynolds shear stress with increasing transverse strain. Also, the directions of the Reynolds shear stress vector and the mean velocity gradient vector were found to differ. In addition, the simulation shows a drop in the turbulent kinetic energy. Terms in the Reynolds stress transport equations were computed. The balances indicate that the decrease in turbulent kinetic energy is a result of a decrease in turbulence production, along with an increase in turbulent dissipation. Intuitive reasoning and current turbulence models would predict an increase in kinetic energy along with increases in production and dissipation rates as a result of increased mean-flow strain rate. Later in the evolution of the flow, both turbulence production and dissipation increase.
On the influence of the Basset history force on the motion of a particle through a fluid
Authors: Thomas P. J.
Published: July 2002

A parameter study on the development of the Basset integral appearing in the equation of motion of a particle moving through a fluid is presented. The particle motion is investigated numerically for the flow across an aerodynamic shock. It is found that, for this type of flow, the Basset history force acting upon the particle described by the Basset integral can be many times larger than the viscous drag in the immediate shock region. It is demonstrated how this force affects the particle motion across the shock for different particles. Nevertheless, the results obtained show that it is justified to neglect the Basset integral for the theoretical description of the motion of the types of particles commonly used in flow measurement tracer techniques for the type of flow considered here.
A nonequilibrium representation of the turbulent viscosity based on a two-scale turbulence theory
Authors: Yoshizawa Akira , Nisizima Shoiti
Published: July 2002

A nonequilibrium expression of the turbulent viscosity in the Reynolds stress is proposed with the aid of the results from a two-scale direct-interaction approximation. The usual turbulent-viscosity model based on the turbulent kinetic energy K and its dissipation rate ε is extended to include their nonequilibrium effects DK/Dt and Dε/Dt (D/Dt is the Lagrange derivative based on the mean velocity). The validity of the proposed model is confirmed through the test in homogeneous shear turbulence that is in a state of nonequilibrium in time. The relationship with the similar model presented by Taulbee [Phys. Fluids A 4, 2555 (1992)] is also discussed.