bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray photoelectron spectroscopy and x-ray absorption near edge structure study of copper sites hosted at the internal surface of ZSM-5 zeolite: A comparison with quantitative and energetic data on the CO and NH3 adsorption
Authors: Bolis V. , Maggiorini S. , Meda L. , D’Acapito F. , Palomino G. Turnes , Bordiga S. , Lamberti C.
Published: July 2002

The oxidation state of Cu species dispersed in a Cu-ZSM-5 zeolite obtained by a nonconventional gas-phase CuCl exchange, and nominally containing only Cu(I) species, was studied by x-ray photoelectron spectroscopy (XPS) and x-ray absorption near edge structure (XANES) analyses. The oxidation of Cu(I) species to Cu(II) by simple exposure to the atmosphere and subsequent reduction by thermal activation in vacuo was monitored. The quantitative and energetic aspects of the formation of carbonyl-like and amino-complexes at the metallic sites was studied by means of adsorption microcalorimetry. CO and NH3 were used as probe molecules in order to assess the coordinative unsaturation of the Cu(I) cations. Adsorption heats comprised in the 130–40 kJ mol−1 interval were obtained for the formation of both type of complexes. The perturbation induced on the Cu centers and/or on the zeolite matrix by the adsorption of the probe molecules was monitored by parallel experiments of XPS, IR, and XANES. A significant fraction of CO and NH3 molecules are irreversibly held on Cu(I) sites even after outgassing at room temperature (RT) at a final dynamic vacuum of 10−5 Torr. On the contrary, no evidence of Cu(I)/CO or of Cu(I)/NH3 complexes was observed by XPS, indicating that such adducts are totally destroyed upon outgassing at 10−9 Torr. This fact implies a reconsideration of what was previously considered as a “stable adduct.” XPS allowed to reveal the existence of ammonia adsorbed on defective Al(III) species, and to explain the chemical nature of species formed at the earliest stages of NH3 dosage and characterized by a heat of adsorption as high as 180 kJ mol−1. By comparing the quantitative XPS and volumetric-calorimetric data it was inferred that a significant gradient of defects amount is present in the system. Finally, from the whole set of XPS measurements here reported and from parallel blank experiments on the ZSM-5 zeolite before Cu-exchange, a calibration scale for the N(1s) peak of various nitrogen species in the different zeolite samples is proposed.
Low-order scaling local electron correlation methods. III. Linear scaling local perturbative triples correction (T)
Authors: Schütz Martin
Published: July 2002

A new method for the perturbative calculation of the correlation energy due to connected triple excitations (T) in the framework of local coupled cluster theory is presented, for which all computational resources scale linearly with molecular size. One notable complication in the formalism for connected triples introduced by the local approach is the nondiagonality of the Fock matrix in the localized MO (LMO) and projected AO (PAO) basis, which leads to couplings between individual triples amplitudes via the internal–internal and external–external blocks of the Fock matrix, respectively. Further complications and couplings arise due to the nonorthogonality of the PAOs. While the couplings via the external–external block can easily be dealt with, this is more difficult for the internal–internal couplings. In a previous paper we already published preliminary results of an approximation of the method, which neglects these internal–internal couplings entirely and recovers about 97% of the total local triples correlation energy. In the present work we implemented the “full” local (T) method, which involves the iterative solution of a system of linear equations for the triples amplitudes to take the internal–internal couplings fully into account. Moreover, a further variant of the method was implemented, which approximates the internal–internal couplings at the level of first-order perturbation theory with respect to the off-diagonal elements of the Fock matrix in LMO basis, thus avoiding the need for an iterative solution of the triples equations and storage of the triples amplitudes. The latter variant reliably recovers more than 99% of the full local triples energy. Test calculations with more than 1000 basis functions and over 300 correlated electrons are presented, showing a speedup of about 106 relative to the estimated time of a corresponding conventional (T) calculation.
A modular, low-cost, digital signal processor-based lock-in card for measuring optical attenuation
Authors: Barragán L. A. , Artigas J. I. , Alonso R. , Villuendas F.
Published: July 2002

A modular, low-cost, digital signal processor (DSP)-based lock-in card is described for measuring optical attenuation. By transferring the lock-in operation from the analog domain to the digital domain, the nonlinearities gain, and offset errors and drifts are virtually eliminated. The dual phase lock-in operation has been implemented on the low-cost DSP56002 evaluation module (DSP56002EVM) of Motorola that is widely used in audio signal processing. This evaluation board contains a 24 bit DSP56002 DSP and a stereo CD-quality audio codec that makes the board ideal for implementing signal processing algorithms. Due to the maximum sampling rate of the codec embedded on the DSP56002EVM, the frequencies of the processed signals must be below 20 kHz. This specification is enough for the most common applications in the field of optics, where low or very low frequency (<1 kHz) references are frequent. The software algorithm implementing the lock-in amplifier can be particularized by the user on the basis of the needed performances. The effects of finite word length in the digital filter implementation are analyzed. This analysis reveals that a 24 bit word length is not enough to ensure the filter stability and the required frequency response. To overcome this problem, the double precision multiply mode must be used. When the DSP56002 enters this mode, double precision 48 bit by 48 bit multiplication can be performed. The lock-in performance has been tested. The measured amplitude variations of the reference sine signal are about 0.003%, which do not affect the signal measurement. The lock-in behaves like a band-pass filter centered on the reference frequency whose bandwidth is related to the low-pass filter cutoff frequency. The measured frequency response shows that the lock-in performs as theoretically predicted. The DSP56002EVM can be used as a lock-in for electrical signals in stand-alone operation. Besides, we have designed a card that interconnects to the DSP56002EVM and allows the ensemble to act as an optical attenuation detector that measures optical losses over 70 dB. This range is similar to that achievable by commercially available optical loss testers and makes it suitable for optical return loss measurements of all kinds of commercially available optical connectors.
Effects of very thin strain layers on dielectric properties of epitaxial Ba0.6Sr0.4TiO3 films
Authors: Park B. H. , Peterson E. J. , Jia Q. X. , Lee J. , Zeng X. , Si W. , Xi X. X.
Published: July 2002

We have epitaxially grown Ba0.6Sr0.4TiO3 (BST-0.4) thin films on MgO(001) substrates. By inserting a very thin Ba1−xSrxTiO3 (x=0.1–0.7) interlayer between the MgO substrate and the main layer of BST-0.4, we are able to manipulate the degree of the stress in BST-0.4 films. We have controlled the stress states, i.e., the lattice distortion ratio (D=in-plane lattice constant/out-of-plane lattice constant) of the BST-0.4 films by varying the chemical composition of the interlayers. We have found that small variations of D value can result in significantly large changes of dielectric properties. A BST-0.4 film under small tensile stress, which has a D value of 1.0023, shows the largest dielectric permittivity and tunability.
Electronic excitation and ionization spectra of azabenzenes: Pyridine revisited by the symmetry-adapted cluster configuration interaction method
Authors: Wan Jian , Hada Masahiko , Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

Electronic excited and ionized states of pyridine were reinvestigated by the symmetry-adapted cluster configuration interaction (SAC-CI) method using an extended basis set and a wide active space. The present SAC-CI results for the singlet and triplet excited states are greatly improved and agree well with the experimental observations, providing a firm assignment of all low-lying n→π* and π→π* valence excited states observed in the vacuum ultraviolet spectrum and electron energy-loss spectrum. The ionization potentials were reexamined by the SAC-CI general-R (R represents excitation operator) method. The first four ionization potentials are greatly improved compared with our previous results obtained by the SAC-CI single- and double-R (SD-R) method. The present theoretical ionization potentials are in good agreement with the experimental values in high-resolution synchrotron photoelectron spectrum for energy regions up to 25 eV (which contain outer- and inner-valence regions), and give a detailed theoretical assignment for the photoelectron spectra.
Parametric investigations of a nonconventional Hall thruster
Authors: Raitses Y. , Fisch N. J.
Published: July 2002

Hall thrusters might better scale to low power with nonconventional geometry. A 9 cm cylindrical, ceramic-channel, Hall thruster with a cusp-type magnetic field distribution has been investigated. It exhibits discharge characteristics similar to conventional coaxial Hall thrusters, but does not expose as much channel surface. Significantly, its operation is not accompanied by large amplitude discharge low frequency oscillations.
Interface-controlled Au/GaAs Schottky contact with surface sulfidation and interfacial hydrogenation
Authors: Kang Min-Gu , Park Hyung-Ho
Published: July 2002

We report a GaAs passivation method using sulfidation and hydrogenation to achieve the Au/GaAs interface free of defective interfacial compounds, through which improves the electrical properties of the Schottky contact. A sulfur-passivated GaAs Schottky diode exhibited improved contact properties, for example an enhanced barrier height and the lower reverse leakage current compared to the diode with conventional HCl-cleaned GaAs. The combination of the H-plasma treatment and the predeposition of an ultrathin Au overlayer enable to control the defective interfacial state of metallization-induced excess As: the Au overlayer seems to effectively protect GaAs from plasma-induced damage and attenuate the energy of penetrating hydrogen then the hydrogenated interface became defect-free since interfacial excess As effectively sublimated as volatile As hydrides. The reverse leakage current was reduced by an order and photoluminescence efficiency was greatly enhanced while there was no change in the dopant profile of GaAs substrate and none of Si–H. We describe a mechanism of the evolution of interfacial bonds during the processes to correlate to the improved electrical properties, which are systematically characterized by the surface/interface analysis tools such as x-ray photoelectron spectroscopy and attenuated-total-reflection Fourier-transform infrared spectroscopy and particularly the role of excess As is discussed in detail.
Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals
Authors: Schlegel H. Bernhard , Millam John M. , Iyengar Srinivasan S. , Voth Gregory A. , Daniels Andrew D. , Scuseria Gustavo E. , Frisch Michael J.
Published: July 2002

We propose and implement an alternative approach to the original Car–Parrinello method where the density matrix elements (instead of the molecular orbitals) are propagated together with the nuclear degrees of freedom. Our new approach has the advantage of leading to an O(N) computational scheme in the large system limit. Our implementation is based on atom-centered Gaussian orbitals, which are especially suited to deal effectively with general molecular systems. The methodology is illustrated by applications to the three-body dissociation of triazine and to the dynamics of a cluster of a chloride ion with 25 water molecules.
Continuous-time histories: Observables, probabilities, phase space structure and the classical limit
Authors: Anastopoulos Charis
Published: July 2002

The continuous-time histories program stems from the consistent histories approach to quantum theory and aims to provide a fully covariant formalism for quantum mechanics. In this paper we examine some structural points of the formalism. We demonstrate a general construction of history Hilbert spaces and identify a large class of time-averaged observables. We pay particular attention to the construction of the decoherence functional (the object that encodes probability information) in the continuous-time limit and its relation to the temporal structure of the theory. Phase space observables are introduced, through the study of general representations of the history group, which is the analog of the canonical group in the formalism. We can also define a closed-time-path (CTP) generating functional for each observable, which encodes the information of its correlation functions. The phase space version of the CTP generating functional leads to the implementation of Wigner–Weyl transforms, that gives a description of quantum theory solely in terms of phase space histories. These results allow the identification of an algorithm for going to the classical (stochastic) limit for a generic quantum system.
A local fluctuation theorem
Authors: Ayton Gary , Evans Denis J. , Searles Debra J.
Published: July 2002

The fluctuation theorem gives an analytic expression for the probability, in a nonequilibrium system of finite size observed for a finite time, that the dissipative flux will flow in the reverse direction to that required by the second law of thermodynamics. In the present paper a local version of the fluctuation theorem (LFT) is derived heuristically. We find that in the case of planar Poiseuille flow of a Newtonian fluid between thermostated walls, nonequilibrium molecular dynamics simulation results support our proposed LFT.
The importance of the forces acting on particles in turbulent flows
Authors: Armenio Vincenzo , Fiorotto Virgilio
Published: July 2002

The analysis of the importance of the forces that act over an ensemble of particles in a turbulent field has been carried out by using direct numerical simulation for a wide range of density ratios (2.65<ρ<2650). It has been observed that, compared to the Stokes drag, the added mass is always negligible, the pressure drag is relevant for density ratios O(1), and the Basset force is appreciable for the whole range investigated. However, the effect of these forces on the particle dispersion is about 1% for ρ∼1 as well as for large density ratios.
Relation between orientational correlation time and the self-diffusion coefficient of tagged probes in viscous liquids: A density functional theory analysis
Authors: Bagchi Biman
Published: July 2002

The usual explanation for the observed inverse relation between the orientational correlation time (τR) and the self-diffusion (DS) of a tagged solute probe in a viscous liquid is in terms of the hydrodynamic relations which are known to have dubious conceptual validity for small molecules. Here, we present a microscopic derivation of the relation between τR and DS. This derivation is based on the general ideas of the mode coupling theory, but uses the time-dependent density functional theory to obtain the torque–torque and force–force time correlation functions on the solute probe. Our analysis shows that the orientational correlation time (τR) is inversely proportional to the translational diffusion coefficient (D0) of the solvent molecules. Thus, the viscosity dependence of orientational correlation time enters through the viscosity dependence of the translational diffusion (D0). The same theoretical analysis also shows that the translational diffusion coefficient of the solute probe (DS) is also proportional to the translational diffusion coefficient, D0, of the solvent molecules. This result is in agreement with the recent computer simulation results which show that the product of τR and DS is a weak function of the density (hence of the viscosity) of the liquid. The microscopic expressions provide explanation, in terms of the solute–solvent direct correlation functions, the reason for the sensitivity of orientational diffusion to solute–solvent interaction potential.
Picosecond superconducting single-photon optical detector
Authors: Gol’tsman G. N. , Okunev O. , Chulkova G. , Lipatov A. , Semenov A. , Smirnov K. , Voronov B. , Dzardanov A. , Williams C. , Sobolewski Roman
Published: July 2002

We experimentally demonstrate a supercurrent-assisted, hotspot-formation mechanism for ultrafast detection and counting of visible and infrared photons. A photon-induced hotspot leads to a temporary formation of a resistive barrier across the superconducting sensor strip and results in an easily measurable voltage pulse. Subsequent hotspot healing in ∼30 ps time frame, restores the superconductivity (zero-voltage state), and the detector is ready to register another photon. Our device consists of an ultrathin, very narrow NbN strip, maintained at 4.2 K and current-biased close to the critical current. It exhibits an experimentally measured quantum efficiency of ∼20% for 0.81 μm wavelength photons and negligible dark counts.
Ultrafast impulsive excitation of coherent longitudinal acoustic phonon oscillations in highly photoexcited InSb
Authors: Cummings M. D. , Elezzabi A. Y.
Published: July 2002

We report on the direct time-resolved measurement of high-frequency coherent longitudinal acoustic (LA) phonon oscillations in InSb. Two LA-phonon modes are impulsively generated in a highly photoexcited InSb sample and detected through reflectivity modulation with a temporal resolution of 15 fs. The measured frequencies, νLA(L1)=4.34 THz and νLA(X3)=3.75 THz, are in excellent agreement with those obtained through time-independent techniques. A frequency redshift in the LA(L1)-phonon frequency, as the energy fluence is increased beyond 10% of the damage threshold fluence, is attributed to an anisotropic impulsive softening of the InSb lattice.
Efficient localized Hartree–Fock methods as effective exact-exchange Kohn–Sham methods for molecules
Authors: Della Sala Fabio , Görling Andreas
Published: July 2002

The form of the Kohn–Sham (KS) exchange potential, which arises from the approximation that the Hartree–Fock (HF) and the exchange-only KS determinant are equal, is derived. Two related procedures to determine the KS exchange potential follow from this approximation: a self-consistent localized HF procedure and a transformation localized HF procedure yielding the local KS exchange potential from HF orbitals. Both procedures can be considered as almost exact exchange KS methods which require only occupied orbitals and are invariant with respect to unitary transformations of the orbitals, i.e., depend only on the first order density matrix. The resulting local KS exchange potentials are free of Coulomb self-interactions and exhibit the correct long-range 1/r-behavior. The Krieger, Li, and Iafrate (KLI) procedure to determine the KS exchange potential can be considered as an approximation to the introduced localized HF procedures. Highly efficient methods to carry out the presented localized HF as well as KLI procedures are introduced. An efficient basis set approach to calculate the Slater potential is presented. The methods can easily be implemented in present standard quantum chemistry codes. Applications to small and medium size molecules and clusters are presented. The Hartree–Fock and the exchange-only KS determinant are found to be surprisingly close. Qualitatively correct, Coulomb self-interaction free KS orbitals and eigenvalue spectra are obtained.
Dispersive electron transport in an electroluminescent polyfluorene copolymer measured by the current integration time-of-flight method
Authors: Campbell Alasdair J. , Bradley Donal D. C. , Antoniadis Homer
Published: July 2002

Here, we report results of both traditional current mode and current integration mode time-of-flight (TOF) measurements on the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT). Current mode TOF shows a strong but dispersive electron transport signal. The mobility derived from the current integration mode transit time (tQ) increases with decreasing film thickness as expected for dispersive transport. The fastest carriers in the photogenerated carrier packet are estimated to have mobilities of order 10−3 cm2/V s at applied fields of 0.5 MV/cm. Holes are heavily trapped close to the interface at which they are photogenerated. The TOF signals decay with repeated measurement and tQ remains constant with applied field. The transport properties of BT are thus in complete contrast to those of other polyfluorenes which show high-mobility nondispersive hole transport but weak and highly dispersive electron transport.
Electronic structure, diffusion, and p-doping at the Au/F16CuPc interface
Authors: Shen Chongfei , Kahn Antoine
Published: July 2002

Interfaces formed by evaporating Au on copper hexadecafluorophthalocyanine (F16CuPc) and F16CuPc on Au are studied via ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy. The energy position of the molecular levels is found to depend on the deposition sequence. The Au-on-F16CuPc interface exhibits a lower hole injection barrier than the F16CuPc-on-Au interface. This behavior is attributed to the diffusion of Au into the organic matrix at the Au-on-top interface, in which they behave as acceptors. Band bending consistent with p doping, i.e., upward away from the metal interface, is observed in Au-doped F16CuPc films. Current–voltage measurements on Au/F16CuPc/Au sandwich structures show a higher electron injection barrier at the top Au electrode contact than at the bottom electrode contact, also consistent with p-type doping F16CuPc by Au.
Nonlinear simulation of resistive ballooning modes in the Large Helical Device
Authors: Miura Hideaki , Hayashi Takaya , Sato Tetsuya
Published: July 2002

Nonlinear simulations of a magnetohydrodynamic (MHD) plasma in full three-dimensional geometry of the Large Helical Device (LHD) [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] are conducted. A series of simulations shows growth of resistive ballooning instability, for which the growth rate is seen to be proportional to the one-third power of the resistivity. Nonlinear saturation of the excited mode and its slow decay are observed. Distinct ridge/valley structures in the pressure are formed in the course of the nonlinear evolution. The compressibility and the viscous heating, as well as the thermal conduction, are shown to be crucial to suppress the pressure deformations. Indication of a pressure-driven relaxation phenomenon that leads to an equilibrium with broader pressure profile is observed.
Study of plasma–surface interactions during overetch of polycrystalline silicon gate etching with high-density HBr/O2 plasmas
Authors: Tuda Mutumi , Shintani Kenji , Tanimura Junji
Published: July 2002

Plasma–surface interactions occurring during overetch of polycrystalline silicon (poly-Si) gate etching with high-density HBr/O2 plasmas have been investigated by x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The temporal variation of the gate oxide thickness measured by XPS indicates that both deposition of etch by-products SiBrxOy and oxidation of Si substrate underlying thin SiO2 occur during exposure to HBr/O2 plasmas. In particular, significant deposition of SiBrxOy, presumably coming from reactor walls, was observed at the beginning of the overetch step. Furthermore, TEM observations revealed that the profile evolution of oxide-masked poly-Si gates during overetch is limited by a sidewall deposition layer, which acts as an oxide mask for etching of poly-Si.
Current collapse and the role of carbon in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy
Authors: Klein P. B. , Binari S. C. , Ikossi K. , Wickenden A. E. , Koleske D. D. , Henry R. L.
Published: July 2002

The two deep traps responsible for current collapse in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy have been studied by photoionization spectroscopy. Varying the growth pressure of the high resistivity GaN buffer layer results in a change in the deep trap incorporation that is reflected in the observed current collapse. Variations in the measured trap concentrations with growth pressure and carbon incorporation indicate that the deepest trap is a carbon-related defect, while the mid-gap trap may be associated with grain boundaries or dislocations.
Chair–chair conformational flexibility, pseudorotation, and exocyclic group isomerization of monosaccharides in water
Authors: Polacek R. , Stenger J. , Kaatze U.
Published: July 2002

Acoustical absorption spectra between 10 kHz and 2 GHz are reported for various monosaccharides in water. With the exception of solutions of methyl-β-D-arabinopyranoside (0.5 mol/l) the spectra reveal absorption with relaxation characteristics in excess to the asymptotic high frequency absorption term. Up to three relaxation terms per spectrum emerge within the measuring frequency range. Regression analysis of the measured spectra in terms of a suitable analytical spectral function yields five relaxation regimes with relaxation times on the order of 1 μs, 100 ns, 10 ns, 1 ns, or 100 ps, respectively. These relaxation regimes are assigned to the chair–chair ring inversion, two modes of pseudorotation, an exocyclic side group isomerization and a molecular association mechanism. Particular emphasis is given to the ring inversion which is additionally verified by time resolved measurements of nonequilibrium tautomer systems, utilizing the coupling of the inversion to the carbohydrate mutarotation. Further evidence is derived from measurements of solutions of D-fructose in mixtures of ethanol and water.
Ab initio density functional theory: OEP-MBPT(2). A new orbital-dependent correlation functional
Authors: Grabowski Ireneusz , Hirata So , Ivanov Stanislav , Bartlett Rodney J.
Published: July 2002

Using the optimized effective potential (OEP) method in conjunction with an orbital-dependent correlation functional developed on the basis of second-order many-body perturbation theory [MBPT(2)], we perform ab initio correlated density functional calculations. Unlike other density functional methods, this offers the first correlation approximation in a converging series of approximations analogous to that in wave function theory. The OEP-MBPT(2) correlation potentials of the helium isoelectronic series and the neon atom are in excellent agreement with prior quantum Monte Carlo (QMC) results, while the widely used conventional approximate density functional theory correlation potentials have qualitatively incorrect behavior. Furthermore, the correlation energies, total energies, and the highest occupied orbital energies calculated by the OEP-MBPT(2) method are also remarkably accurate. Unlike QMC, our procedure can be applied to general molecules.
Second-order theory for the deformation of a Newtonian drop in a stationary flow field
Authors: Greco Francesco
Published: July 2002

The classical perturbative theory for a single drop immersed in a flowing immiscible fluid is revisited, with the well-known capillary number Ca, ratio of viscous to interfacial stresses, as the expansion parameter. Although the analysis is here limited to the Newtonian case under steady-state conditions, the perturbation method is innovative, as it makes use of rotational invariance to obtain workable tensorial representations of the pressure and velocity fields, and of the drop shape, at any order in Ca. The method is much less cumbersome than the classical one, based on an expansion in spherical harmonics. The analytical second-order solution thus obtained differs somewhat from previously reported results.
Magnetic properties and loss separation in iron powder soft magnetic composite materials
Authors: De Wulf Marc , Anestiev Ljubomir , Dupré Luc , Froyen Ludo , Melkebeek Jan
Published: July 2002

New developments in powder metallurgical composites make soft magnetic composite (SMC) material interesting for application in electrical machines, when combined with new machine design rules and new production techniques. In order to establish these design rules, one must pay attention to electromagnetic loss characteristics of SMC material. In this work, five different series of iron based SMCs are produced and studied: (1) Pure iron powder with resin; (2) sintered iron based powders; (3) pure iron powder with additions of Zn-st and carbon; (4) iron based powder alloys (Fe,Nb,Si); (5) commercially available iron powder “Somaloy.” The specimens were shaped as rectangular rods and characterized on a miniature single sheet tester which was calibrated to Epstein. The measured energy losses are analyzed following the loss separation theory of Bertotti, in which the total energy loss is decomposed into hysteresis loss, classical Foucault loss, and an excess loss component.
Synthesis of nanosized (Li0.5xFe0.5xZn1−x)Fe2O4 particles and magnetic properties
Authors: Gee S. H. , Hong Y. K. , Park M. H. , Erickson D. W. , Lamb P. J. , Sur J. C.
Published: July 2002

In an attempt to synthesize nanosized (Li0.5xFe0.5xZn1−x)Fe2O4 (0⩽x⩽1) particles with high magnetic saturation and low coercivity, the energetic ball milling technique was employed. LiCO3, α-Fe2O3, and ZnO powders were used as starting materials. The ball milled, partially crystallized lithium zinc ferrite starts to crystallize at about 600 °C. This is much lower than the temperature of 1000 °C, which is used in conventional methods. Particle size of lithium zinc ferrite was in the range of 20 to 50 nm. Regardless of the annealing temperature, the saturation magnetization increases with increasing x and reaches the maximum (about 80 emu/g) at x=0.7 [(Li0.35Fe0.35Zn0.3)Fe2O4], followed by a decrease to 60 emu/g for x=1 [(Li0.5Fe0.5)Fe2O4]. On the other hand, the coercivity of x=0.7 composition decreases with increasing annealing temperatures. Saturation magnetization and low coercivity for x=0.7 annealed at various temperatures are discussed in terms of site occupation.
A dynamical correlation functional
Authors: Handy Nicholas C. , Cohen Aron J.
Published: July 2002

The purpose of this work is to search for a justifiable form for a molecular dynamic correlation functional. A detailed examination of Colle and Salvetti’s derivation of the LYP functional is presented. It is argued that the leading term is all important, and furthermore that it should account for αβ correlation. This term only depends upon the densities, and it has a truncation factor which is obtained from the size of the correlation hole. It is −c∫ραρβ/(ρ(1+dρ−1/3))dr. It reproduces the αβ correlation energies of (He–Ar) to a very high accuracy. The correlation functional which represents σσ correlation is more complex, because the two particle Hartree–Fock density matrix is zero at electron coalescence. The functional must therefore depend upon (∇ρ)2. Using these and related arguments we have found a four parameter generalized gradient functional which appears to perform nearly as well as the LYP functional. However unlike the LYP functional, it has two identifiable terms for αβ correlation, and two identifiable terms for σσ correlation. Together with our previously derived exchange functional, we have therefore obtained an exchange-correlation functional for molecular studies, the form for which can be more understandably justified. The performance of this new Generalized Gradient Approximation functional for molecular predictions is reported. It is a considerable improvement on the BLYP functional, and is in the category of an optimum Generalized Gradient functional. Finally the present status of the science of searching for exchange-correlation functions is reviewed. It is suggested that it may not be possible to find a local functional which is significantly more accurate for chemistry than the presently used Generalized Gradient Approximation functionals.
Density of states of Pb1 Si/SiO2 interface trap centers
Authors: Campbell J. P. , Lenahan P. M.
Published: July 2002

The electronic properties of the (100) Si/SiO2 interfacial defect called Pb1 are quite controversial. We present electron spin resonance measurements that demonstrate: (1) that the Pb1 defects have levels in the silicon band gap, (2) that the Pb1 correlation energy is significantly smaller than that of the Pb1 defect, and (3) that the Pb1 levels are skewed toward the lower part of the silicon band gap.
Poincaré invariance for continuous-time histories
Authors: Savvidou Ntina
Published: July 2002

We show that the relativistic analog of the two types of time translation in a nonrelativistic history theory is the existence of two distinct Poincaré groups. The “internal” Poincaré group is analogous to the one that arises in the standard canonical quantization scheme; the “external” Poincaré group is similar to the group that arises in a Lagrangian description of the standard theory. In particular, it performs explicit changes of the space–time foliation that is implicitly assumed in standard canonical field theory.
Polymer absorbers containing magnetic particles: Effect of polymer permittivity on wave absorption in the quasi-microwave band
Authors: Matsumoto Morihiko , Miyata Yoshimori
Published: July 2002

The effect of polymer permittivity on wave absorption in the quasi-microwave band has been elucidated for polymer composites containing magnetic metal particles. Absorbers composed of these composites can be thin when their real permittivities are high and their imaginary parts are low. However, in general, for polymer composites the real part actually increases as the imaginary part increases in this band. This implies that in designing absorbers we must choose a polymer species on the crossing area of the real and imaginary parts of the permittivity. Furthermore, to obtain thin absorbers, we should choose a polymer species with high real permittivities and low imaginary parts.
Nonequilibrium dynamics in lattice ecosystems: Chaotic stability and dissipative structures
Authors: Solé Ricard V. , Bascompte Jordi , Valls Joaquim
Published: July 2002

A generalized coupled map lattice (CML) model of ecosystem dynamics is presented. We consider the spatiotemporal behavior of a prey–predator map, a model of host–parasitoid interactions, and two-species competition. The latter model can show phase separation of domains (Turing-like structures) even when chaos is present. We also use this CML model to explore the time evolution and structural properties of ecological networks built with a set of N competing species. The May–Wigner criterion is applied as a measure of stability, and some regularities in the stable networks observed are discussed.
Spreading of a surfactant monolayer on a thin liquid film: Onset and evolution of digitated structures
Authors: Matar Omar K. , Troian Sandra M.
Published: July 2002

We describe the response of an insoluble surfactant monolayer spreading on the surface of a thin liquid film to small disturbances in the film thickness and surfactant concentration. The surface shear stress, which derives from variations in surfactant concentration at the air–liquid interface, rapidly drives liquid and surfactant from the source toward the distal region of higher surface tension. A previous linear stability analysis of a quasi-steady state solution describing the spreading of a finite strip of surfactant on a thin Newtonian film has predicted only stable modes. [Dynamics in Small Confining Systems III, Materials Research Society Symposium Proceedings, edited by J. M. Drake, J. Klafter, and E. R. Kopelman (Materials Research Society, Boston, 1996), Vol. 464, p. 237; Phys. Fluids A 9, 3645 (1997); O. K. Matar Ph.D. thesis, Princeton University, Princeton, NJ, 1998]. A perturbation analysis of the transient behavior, however, has revealed the possibility of significant amplification of disturbances in the film thickness within an order one shear time after the onset of flow [Phys. Fluids A 10, 1234 (1998); “Transient response of a surfactant monolayer spreading on a thin liquid film: Mechanism for amplification of disturbances,” submitted to Phys. Fluids]. In this paper we describe the linearized transient behavior and interpret which physical parameters most strongly affect the disturbance amplification ratio. We show how the disturbances localize behind the moving front and how the inclusion of van der Waals forces further enhances their growth and lifetime. We also present numerical solutions to the fully nonlinear 2D governing equations. As time evolves, the nonlinear system sustains disturbances of longer and longer wavelength, consistent with the quasi-steady state and transient linearized descriptions. In addition, for the parameter set investigated, disturbances consisting of several harmonics of a fundamental wavenumber do not couple significantly. The system eventually singles out the smallest wavenumber disturbance in the chosen set. The summary of results to date seems to suggest that the fingering process may be a transient response which nonetheless has a dramatic influence on the spreading process since the digitated structures redirect the flux of liquid and surfactant to produce nonuniform surface coverage.
Homoclinic bifurcation in a Hodgkin–Huxley model of thermally sensitive neurons
Authors: Feudel Ulrike , Neiman Alexander , Pei Xing , Wojtenek Winfried , Braun Hans , Huber Martin , Moss Frank
Published: July 2002

We study global bifurcations of the chaotic attractor in a modified Hodgkin–Huxley model of thermally sensitive neurons. The control parameter for this model is the temperature. The chaotic behavior is realized over a wide range of temperatures and is visualized using interspike intervals. We observe an abrupt increase of the interspike intervals in a certain temperature region. We identify this as a homoclinic bifurcation of a saddle-focus fixed point which is embedded in the chaotic attractors. The transition is accompanied by intermittency, which obeys a universal scaling law for the average length of trajectory segments exhibiting only short interspike intervals with the distance from the onset of intermittency. We also present experimental results of interspike interval measurements taken from the crayfish caudal photoreceptor, which qualitatively demonstrate the same bifurcation structure.
Molecular dynamics simulation of liquids on the Connection Machine
Authors: Mel’čuk Andrew I. , Giles Roscoe C. , Gould Harvey
Published: July 2002

A molecular dynamics algorithm for performing large-scale simulations on the Connection Machine, a massively parallel supercomputer, is discussed. The algorithm uses a cell data structure to obtain the near neighbors of each particle as the fluid evolves. The main features of the data structure are that one processor per particle is used to integrate the equations of motion of each particle and one processor per cell is assigned to compute the interparticle forces. The results for Lennard-Jones fluids of 15 000 to 500 000 particles indicate that the algorithm scales linearly with the number of particles.
Daubechies wavelets and Mathematica
Authors: Rowe Alistair C. H. , Abbott Paul C.
Published: July 2002

Analysis of time series data using wavelets provides both scale (frequency) and position information. In contrast, the Fourier transform provides frequency information only. We discuss the Daubechies formulation of wavelets, with reference to the WaveletTransform package that calculates the filter coefficients for any Daubechies basis to arbitrary precision. Examples of the wavelet transform applied to selected time series are presented to highlight the advantages of wavelets. We indicate an application of wavelets to sampled Barkhausen noise, a nonlinear phenomenon encountered in magnetic systems. The elements of the WaveletTransform package are discussed, with the emphasis being on the calculation of filter coefficients and their application to the discrete wavelet transform (DWT) and its inverse. With the construction of quadrature mirror filters, an efficient implementation of the DWT is possible and is similar in structure to the fast Fourier transform algorithm. © 1995 American Institute of Physics.
Effects of crystal structure on piezoelectric and ferroelectric properties of copoly(vinylidenefluoride-tetrafluoroethylene)
Authors: Tasaka Shigeru , Miyata Seizo
Published: July 2002

Piezoelectricity and ferroelectricity in 80 mol % vinylidene fluoride/20 mol % tetrafluoroethylene copolymers (VDF-TFE) were measured at various crystallinities. In order to control the crystallinity, the crystallization pressure was varied from 1 bar to 5 kbar. All the samples prepared by this method contain only form I crystals but with various crystallinity and crystal thickness. Especially, a band structure characterizing an extended chain crystal was observed in the sample crystallized under more than 4 kbar. Remanent polarization (Pr) obtained by the D-E hysteresis curve increases with the crystallinity, and the extrapolated Pr of 126 mC/m2 nearly coincides with the value of 140 mC/m2 calculated from the unit cell dimension and dipole moment. The coercive field (Ec) decreases with the crystallinity. This would be explained by the expansion of intermolecular distance and a decrease in the number of folds, which affect the rotation of dipoles under an electric field.
A sensitive new method for the determination of adhesive bonding between a particle and a substrate
Authors: Dybwad G. L.
Published: July 2002

We use an AT-cut quartz resonator with metal electrodes as the substrate. The material properties of the quartz are well characterized, and the resonant frequency can be determined accurately (1 part in 108 or better). When a particle was placed on the electrode, the resonant frequency increased, contrary to mass loading theory. If the particle-resonator system is modeled mechanically as a coupled oscillator system, indeed the resonant frequency should increase. The increase is determined by the bonding force constant between particle and substrate, which we calculate. Experimental data on the autohesion force constants of Au spheres on etched Au electrodes in air is presented.
Organic heterojunctions utilizing two conducting polymers: Poly(acetylene)/poly(N-methylpyrrole) junctions
Authors: Koezuka H. , Hyodo K. , MacDiarmid A. G.
Published: July 2002

The first solid-state heterojunction device involving two conducting organic polymers has been fabricated from polyacetylene, (CH)x and p-doped poly(N-methylpyrrole), (PNP). Its electronic properties can be modified by chemically partly reducing or partly oxidizing the PNP. The junction has been characterized by J-V studies and by preliminary photovoltaic measurements.
Mössbauer study of Ti-implanted 52100 steel
Authors: Williamson D. L. , Kustas F. M. , Fobare D. F. , Misra M. S.
Published: July 2002

Information on phase transformations, solid solution alloying, compound formation, and residual stresses resulting from Ti implantation of AISI 52100 steel is obtained using 57Fe Mössbauer spectroscopy. Results show that Ti implantation reduced the near-surface (0.1 μm) retained austenite content by about 70% or more and evidence is found for an implantation-induced strain field which extends much deeper than the Ti-implanted zone. Changes in the near-surface martensite Mössbauer resonance are attributed to Ti on substitutional sites. Neither of the intermetallic compounds Fe2Ti or FeTi, nor the amorphous Fe–Ti or Fe–Ti–C phases are unambiguously detected. However some features of the data are not explained by Ti and/or C solid solution alloy analysis and this leads to suggestions for a≲10-nm-thick amorphous phase layer and surface oxides/carbides. The microstructural modifications are correlated with tribological properties measured on the same specimens. Improvements are attributed to the retained austenite reduction, residual compressive stresses in the near-surface layer, and the surface compounds.
Time response of a nematic liquid-crystal cell in a switched dc electric field
Authors: Mada Hitoshi , Osajima Kyoichi
Published: July 2002

When a nematic liquid-crystal cell is driven by a dc electric field, the capacitance of the cell increases in the first several tens of milliseconds, but after that the capacitance tends to decrease with a rather long relaxation time of the order of 1 s. This phenomenon can be explained by impurity ions which form the electrical double layer at the surface. The results calculated using this simple model qualitatively agreed with the exprimental results. The amount of charge in the double layer and the ion mobility are derived as 2×1011 ions/cm2 and 2.7×10−7 cm2/V s, respectively.
Mössbauer effect on mechanically alloyed Fe-Zr glasses
Authors: Michaelsen C. , Hellstern E.
Published: July 2002

Amorphous Fe-Zr powders have been prepared by mechanical alloying in a powder mill and studied by means of Mössbauer spectroscopy. It is shown, that, within the glass-forming range from about 30 to 78 at. % Fe, the local structure of the mechanically alloyed samples, detected by the Mössbauer effect, is the same as in the corresponding melt-spun or sputtered samples. The existence of a two-phase region above 78 at. % Fe between an amorphous phase and crystalline Fe confirms that amorphization by mechanical alloying is a metastable equilibrium process. There is evidence that mechanical alloying enhances the Zr solubility in Fe up to 5 at. %.
Magnetotransport: An ideal probe of anisotropy energies in epitaxial films (invited)
Authors: Dahlberg E. Dan , Riggs Kevin , Prinz G. A.
Published: July 2002

Magnetotransport measurements provide an ideal probe to determine the various anisotropy energies in epitaxial magnetic films. The extraordinary Hall effect (EHE) can be used to determine the perpendicular or surface anisotropy energy while the anisotropic magnetoresistance (AMR) can be used to investigate the in-plane anisotropy energies. The advantage of magnetotransport over more tranditional measurement techniques used to determine these anisotropy energies is the ease of the technique, the lack of a need for sophisticated equipment, and the insensitivity of the techniques to the magnetic properties of a semiconducting or insulating substrate. Both the EHE and the AMR have been used to study the magnetic properties of epitaxial iron films grown on GaAs substrates. The results of the EHE and the AMR study and how the various anisotropy energies compare with those determined by the more traditional techniques of ferromagnetic resonance and vibrating sample magnetometry will be discussed.
A transverse-wave ultrasonic oblique-incidence technique for interfacial weakness detection in adhesive bonds
Authors: Pilarski Aleksander , Rose Joseph L.
Published: July 2002

The possibility of using an ultrasonic oblique-incidence technique for improved sensitivity in interfacial weakness detection in adhesive bonds was considered. The analysis of the sensitivity of transverse waves to interface imperfections on an interface between two semi-infinite half-spaces with bond rigidities of various finite values was carried out. Excellent sensitivity is obtained for discriminating bond weaknesses from a perfect or welded situation to a poor or smooth situation compared to longitudinal wave impingement and reflection. The optimal angle of incidence for the inspection of an aluminum-to-aluminum adhesively bonded structure in an immersion technique is presented. Additionally, a signal feature in frequency domain for adhesive and cohesive weakness distinction is proposed.