bluuebunny/crossref_metadata_2025 huggingface dataset:
Anomalous diffusion of water in a hydrogel of sucrose and diepoxide monomers
Authors: Azurmendi H. F. , Ramia M. E.
Published: July 2002

This article is concerned with the study of the diffusion of water molecules in a hydrogel, obtained by controlled hydration of a cross-linked polymer of sucrose and diepoxide monomers. The diffusion measurements were performed by means of H1(NMR) nuclear magnetic resonance techniques in the fringe field of a superconductor magnet with a field gradient G≃36.3 T m−1. This large field gradient and the stimulated echo pulse sequence allows to obtain time-dependent diffusion coefficients and to observe a non-Debye feature of the relaxation attenuation function. The behavior of the diffusion follows a stretched exponential function plus a Debye term, allowing the determination of different water domains. In the long time limit the diffusion follows a long tail Lévy function, whose asymptotic limit is related to the fractal dimension of the underlying polymer structure.
Properties of 2,2,2-trifluoroethanol and water mixtures
Authors: Chitra Rajappa , Smith Paul E.
Published: July 2002

2,2,2-trifluoroethanol (TFE) is a commonly used cosolvent in experimental studies of peptides and proteins. Although concentration-dependent TFE effects have been well studied experimentally, the exact mechanism by which TFE affects the solubility and stability of peptides is still unclear. Here we report molecular dynamics simulations of TFE/water mixtures of different composition in an attempt to improve our atomic level understanding of the properties of TFE/water mixtures. The trends in most properties—densities, diffusion constants, dielectric constants, and enthalpies of mixing—were well reproduced, although quantitative agreement with experiment was poor. Other thermodynamic properties of the solutions—partial molar volumes, derivatives of activity coefficients, and isothermal compressibilities—were also determined using the Kirkwood–Buff theory of mixtures. The Kirkwood–Buff analysis indicated significant aggregation of TFE molecules in the mixtures, especially at low mole fractions, in agreement with experiment. However, the aggregation of TFE molecules was exaggerated using the current TFE and water models. The results suggest that the combination of simulation data and Kirkwood–Buff theory provides a powerful tool for the investigation of the thermodynamic properties of hydrogen bonding mixtures.
The influence of control on proper orthogonal decomposition of wall-bounded turbulent flows
Authors: Prabhu R. D. , Collis S. Scott , Chang Yong
Published: July 2002

This paper explores the effects of several wall-based, turbulence control strategies on the structure of the basis functions determined using the proper orthogonal decomposition (POD). This research is motivated by the observation that the POD basis functions are only optimal for the flow for which they were created. Under the action of control, the POD basis may be significantly altered so that the common assumption that effective reduced-order models for predictive control can be constructed from the POD basis of an uncontrolled flow may be suspect. This issue is explored for plane, incompressible, turbulent channel flow at Reynolds number, Reτ=180. Based on well- resolved large eddy simulations, POD bases are constructed for three flows: no control; opposition control, which achieves a 25% drag reduction; and optimal control, which gives a 40% drag reduction. Both controlled flows use wall transpiration as the control mechanism and only differ in the technique used to predict the control. For both controlled flows, the POD basis is altered from that of the no-control flow by the introduction of a localized shear layer near the walls and a nearly impenetrable virtual wall that hinders momentum transfer in the wall-normal direction thereby leading to drag reduction. A major difference between the two controlled flows is that the shear layer and associated virtual wall are located farther away from the physical wall when using optimal compared to opposition control. From this investigation, it is concluded that a no-control POD basis used as a low-dimensional model will not capture the key features of these controlled flows. In particular, it is shown that such an approximation leads to grossly underpredicted Reynolds stresses. These results indicate that a no-control POD basis should be supplemented with features of a controlled flow before using it as a low-dimensional approximation for predictive control.
A gyrokinetic ion zero electron inertia fluid electron model for turbulence simulations
Authors: Chen Yang , Parker Scott
Published: July 2002

This paper describes the formulation of a hybrid model with fully gyrokinetic ions and a zero-inertia fluid model for the electrons. The electron fluid equations are derived from moments of the drift kinetic equation, taking the small mass ratio limit, but with finite electron temperature. This model eliminates the inertial Alfvén wave and any physics relating to electron transit motion, making it useful for studying low frequency, high β (β≫me/mi) electromagnetic turbulence as well as kinetic magnetohydradynamics (MHD) physics including kinetic ballooning and toroidal Alfvén eigenmodes. Electromagnetic effects (δB⊥) are included through the parallel ion and electron current. A predictor-corrector scheme for the fluid part that is consistent with the gyrokinetic ion part has been developed. Here we derive the model equations, derive the linear kinetic-fluid theory in a three-dimensional shearless slab, and compare the simulation results with the linear theory.
Giant magnetoimpedance in layered FeSiB/Cu/FeSiB films
Authors: Zhou Yong , Yu Jinqiang , Zhao Xiaolin , Cai Bingchu
Published: July 2002

The giant magnetoimpedance (GMI) effect has been studied in layered FeSiB/Cu/FeSiB films in the frequency range of 100 kHz–40 MHz. The field dependence of the GMI ratio shows that the GMI ratio increases with magnetic field Ha, reaching a positive maximum value at a certain field, and then gradually decreases to negative values. For the layered films with a Cu film width of 0.6 mm, the positive maximum GMI ratio is about 2%, which is smaller than the value of the negative GMI ratio (−6.6% at Ha=50 Oe) at a frequency of 5.2 MHz. With an increase of the Cu film width, a positive maximum GMI ratio of 4.5% and 13.5% is obtained for Ha=30 Oe and 5.2 MHz in the layered films with a Cu film widths of 1 and 1.3 mm respectively. In addition, all the layered films exhibit a large negative GMI ratio at a relatively large field and it is associated with the easy axis orientation of the layered films. The increase of the positive maximum GMI ratio with an increase of Cu film width at 5.2 MHz may be due not only to the effect by the increase of Cu film width, but also to the difference in the easy axis orientations of the layered FeSiB/Cu/FeSiB films.
Ion-modulated ambipolar electrical conduction in thin-film transistors based on amorphous conjugated polymers
Authors: Chen X. Linda , Bao Zhenan , Schön J. Hendrik , Lovinger Andrew J. , Lin Yen-Yi , Crone Brian , Dodabalapur Ananth , Batlogg Bertram
Published: July 2002

Through gate-modulated electrochemical doping, ambipolar operation in thin-film transistors (TFTs) can be realized in air with solution processable conjugated polymers. Unlike other typical organic TFTs, which rely on high crystallinity for better charge transport, these electrochemically-doped transistors operate under a different mechanism and show very high current output even with completely amorphous polymers.
Origin of the “up,” “down” hysteresis offsets observed from polarization-graded ferroelectric materials
Authors: Fellberg Whitney , Mantese Joseph , Schubring Norman , Micheli Adolph
Published: July 2002

“Up” and “down” hysteresis offsets were observed in polarization-graded ferroelectrics. The polarization gradients were achieved by imposing temperature gradients across a bulk ferroelectric material near its Curie temperature. In the absence of temperature gradients, no hysteresis offsets were observed, ruling out extraneous sources as the cause of the aberrant behavior, thereby establishing the origin of the offsets in the class of polarization-graded ferroelectric devices.
Infrared multispectral detection using Si/SixGe1−x quantum well infrared photodetectors
Authors: Krapf D. , Adoram B. , Shappir J. , Sa’ar A. , Thomas S. G. , Liu J. L. , Wang K. L.
Published: July 2002

A modified p-type Si/SiGe quantum well infrared photodetector for multispectral infrared imaging applications is demonstrated. In order to improve the detector’s performances we have used a SiGe emitter and a low-temperature wet passivation process that give rise to a reduced dark current, even at relatively high bias voltages. Multispectral photoresponse at the long, mid and short wavelength infrared atmospheric windows was observed. The response peaks are assigned to the various classes of intervalence band transitions in the quantum wells and in the SiGe emitter layers.
The electronic structure of ZrCl
Authors: Ram R. S. , Adam A. G. , Sha W. , Tsouli A. , Liévin J. , Bernath P. F.
Published: July 2002

The proposed electronic assignments of our previously reported near infrared transitions of ZrCl [J. Mol. Spectrosc. 186, 335 (1997); 196, 235 (1999)] have been revised following the suggestion of Sakai, Mogi, and Miyoshi [J. Chem. Phys. 111, 3989 (1999)]. The ground state is now assigned as the X 2Δ state followed by the a 4Φ state being the lowest in the quartet manifold. The previously reported transitions [7.3]2Δ–a 2Φ, [9.4]2Φ–a 2Φ, and C 4Δ–X 4Φ are now reassigned to C 2Φ–X 2Δ, E 2Φ–X 2Δ, and d 4Δ–a 4Φ, respectively. The new assignments are supported by our own ab initio calculations. Laser excitation spectra of the 414 nm band system have also been observed at low resolution and are attributed to a Γ–a4Φ4 transition.
Structure and magnetic properties of electrodeposited cobalt nanowires
Authors: Bantu A. Kazadi Mukenga , Rivas J. , Zaragoza G. , López-Quintela M. A. , Blanco M. C.
Published: July 2002

This work focuses on the structural and magnetic characterization of arrays of cobalt nanowires, with diameters in the range 100–400 nm and lengths of L⩽6 μm, produced by electrodeposition in polycarbonate membrane pores. Diffraction patterns of Co nanowires indicate that Co is stabilized in the hcp structure with a preferential (0001) texture oriented close to the perpendicular direction of the wires axes. Magnetic measurements at room temperature reveal that magnetization undergoes a change in the easy axis as the length of the wires increases. Clearly, the easy axis of the shortest wires is oriented along the axis of the wire. In the case of the longest wires, a crossover of the easy axis is observed, from the perpendicular to the parallel direction to the wire for the shortest. This magnetic behavior is explained through a simple magnetostatic model, which takes into account the dipolar interactions among wires, the demagnetizing field, and the magnetocrystalline anisotropy of Co.
Interlevel Ge/Si quantum dot infrared photodetector
Authors: Yakimov A. I. , Dvurechenskii A. V. , Nikiforov A. I. , Proskuryakov Yu. Yu.
Published: July 2002

A self-assembled Ge/Si quantum dot interlevel infrared photodetector operating at room temperature and at normal incidence is demonstrated. The spectral response exhibits two peaks in the 58–82 and 132–147 meV energy regions with full width at half maximum linewidths as narrow as 25 meV. The two photocurrent maxima are ascribed to transitions from the hole ground state to the excited states in the dots. The peak detectivity and responsive quantum efficiency are 1.7×108 cm Hz1/2/W and 0.1% for the transition from the ground state to the first excited state and 7×107 cm Hz1/2/W and 0.08% for the transition from the ground state to the second excited state. At large dc bias, a redshift in the transition energies is observed. We argue that the resonance shifts are due to suppression of the depolarization field effect, representing the experimental manifestation of dynamic screening associated with collective electron–electron interaction in the dots.
Influence of temperature on the response of diamond radiation detectors
Authors: Tromson D. , Brambilla A. , Bergonzo P. , Mas A. , Hordequin C. , Mer C. , Foulon F.
Published: July 2002

The response of chemically vapor deposited diamond detectors under alpha particle irradiation was studied as a function of operating temperature in the 20–280 °C range. The detector characteristics were studied for two initial states: (i) the detector being either preirradiated under x-rays at a dose of 8 Gy (pumped state) or (ii) preannealed at 400 °C for 15 min (depumped state). Pumped detectors were found to exhibit a stable response (evaluated by measurement of the charge collection efficiency η) with temperature up to 120 °C, above which a decrease of η was observed. Preannealing at 400 °C was shown to induce, at room temperature, a drastic deterioration of the detector response, when compared to that of pumped devices, whereas above 150 °C a reversed trend was observed. Correlations between the response of the diamond detectors and the presence of traps were determined by comparing the detector response for α particles with the thermally stimulated current (TSC) spectrum. The temperature dependence of the detector response was shown to be related to the presence of a trapping level measured around 260 °C by TSC.
The effect of proton interactions on the conductivity behavior in systems with superionic phases
Authors: Pavlenko Natalie I. , Stasyuk Ihor V.
Published: July 2002

The proton conductivity in the hydrogen-bonded systems with strong correlations between protons is investigated theoretically in the context of the two-stage transport modeling approach. The distortions of the nearest ionic groups at the formation of the hydrogen bond are shown to be the reason for the strong proton–phonon coupling and polaronic effect. We evaluate as an example the conductivity for M3H(XO4)2-class of crystals with high proton mobility in (001) conducting planes. We analyze the temperature dependencies of the conductivity and its interbond and intrabond contributions in the vicinity of the transition from the disordered superionic to the ordered state. We investigate also the main peculiarities in the frequency dependencies of the conductivity appearing due to the proton interactions and compare our results with the previous conclusions obtained for the chains of noninteracting protons.
Six-dimensional quantum calculations of highly excited vibrational energy levels of hydrogen peroxide and its deuterated isotopomers
Authors: Chen Rongqing , Ma Guobin , Guo Hua
Published: July 2002

We report accurate calculations of vibrational energy levels of HOOH, DOOD, and HOOD up to 10 000 cm−1 above the zero-point energy levels on a high-quality ab initio potential energy surface. These energies were determined by the Lanczos algorithm based on repetitive matrix-vector multiplication. The six-dimensional vibrational Hamiltonian in the diatom–diatom Jacobi coordinate system was discretized in a mixed basis/grid representation. A direct product potential optimized discrete variable representation was used for the radial coordinates, while nondirect product spherical harmonics were employed for the angular degrees of freedom. The calculation and storage of the potential matrix in the angular finite basis representation were avoided by using a series of one-dimensional pseudo-spectral transformations to a direct product angular coordinate grid. The diatom–diatom exchange symmetry, when applicable, was incorporated into the basis, which significantly enhanced the efficiency for symmetric isotopomers. A few hundred low-lying vibrational levels of each isotopomer were assigned and compared with experimental data.
The Prague Asterix Laser System
Authors: Jungwirth K. , Cejnarova A. , Juha L. , Kralikova B. , Krasa J. , Krousky E. , Krupickova P. , Laska L. , Masek K. , Mocek T. , Pfeifer M. , Präg A. , Renner O. , Rohlena K. , Rus B. , Skala J. , Straka P. , Ullschmied J.
Published: July 2002

The Prague Asterix Laser System (PALS) is a new international laboratory where research teams are invited to compete for the beam time. The PALS Center runs an iodine photodissociation high-power laser system delivering up to 1.2 kJ of energy in ∼400 ps pulses at the wavelength of 1.315 μm. Optional doubling and tripling of the frequency is assured by large-diameter nonlinear crystals. The ASTERIX IV laser [H. Baumhacker et al., Appl. Phys. B 61, 325 (1995)], transferred from Garching into a new laser hall in Prague, was updated and put into operation on 8 June 2000. These upgrades include new beam delivery options and a twin interaction chamber, which is designed flexibly for a broad spectrum of applications. Results of the first series of experiments are presented and some planned upgrades are briefly described. These include implementation of adaptive optics, replacement of the iodine master oscillator by a more flexible solid state oscillator based on fiber optics, and a femtosecond extension of the laser output to reach the petawatt pulse power region.
Substituent-introduction of “hard” polarization characteristics in “soft” Pb(BIBII)O3–PbTiO3 ferroelectric ceramics
Authors: Chen Yun-Han , Uchino Kenji , Viehland Dwight
Published: July 2002

The polarization versus field (P–E) characteristics of modified Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) and Pb(Sc1/2Nb1/2)O3–PbTiO3 (PSN–PT) ceramics has been investigated for compositions close to the morphotropic phase boundary. Investigations were performed for various types of lower valent substituents on the B-site cation positions. It has been found possible to introduce “hard” P–E characteristics in “soft” PMN–PT and PSN–PT ferroelectric ceramics. In PMN–PT, only Mn was found to be effective in introducing such characteristics, however, in PSN–PT various lower valent substituents on higher valent sites were found to be effective. The results indicate that using a defect-engineering approach that it may be possible to develop materials with combinatory characteristics of hard and soft ferroelectrics.
Heteronuclear H1–13C multiple-spin correlation in solid-state nuclear magnetic resonance: Combining rotational-echo double-resonance recoupling and multiple-quantum spectroscopy
Authors: Saalwächter Kay , Spiess Hans W.
Published: July 2002

High-resolution magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy exploiting the dipole–dipole coupling between unlike spins is a powerful tool for the study of structure and dynamics. In particular, the rotational-echo double-resonance (REDOR) technique has established itself as a method for probing heteronuclear dipole–dipole couplings in isotopically dilute systems of low-γ nuclei. In organic substances it is, however, particularly advantageous to consider heteronuclear spin-pairs such as H1–13C, on account of the high natural abundance of H1 and thus a much wider range of possible applications, such as the determination of order parameters in liquid crystals and polymer melts. We describe the possibility of performing C13-observed REDOR in H1–13C systems, where very-fast MAS with spinning frequencies of up to 30 kHz is used to successfully suppress the perturbing homonuclear couplings among the protons, which would usually be expected to hamper a proper data analysis. Simple modifications of the REDOR experiment are presented which lead to a two-dimensional experiment in which heteronuclear multi-spin multiple-quantum modes are excited, the evolution of which is monitored in the indirect frequency dimension. The existence of higher quantum orders in the proton subspace of these heteronuclear coherences is proven by performing a phase-incremented spin-counting experiment, while a phase cycle can be implemented which allows the observation of specific selected coherence orders in the indirect dimension of two-dimensional shift correlation experiments. The significance of the heteronuclear approach to spin counting is discussed by comparison with well-known homonuclear spin-counting strategies. For the shift correlation, the high resolution of H1 chemical shifts in the indirect dimension is achieved by the use of high B0 fields (ωLH1/2π=700.13 MHz) combined with very-fast MAS, and dipolar coupling information can be extracted by analyzing either peak intensities or spinning-sideband patterns in the indirect frequency dimension. The method is termed dipolar heteronuclear multiple-spin correlation (DIP-HMSC).
Phase space optimization of quantum representations: Three-body systems and the bound states of HCO
Authors: Poirier Bill , Light J. C.
Published: July 2002

In an earlier paper [J. Chem. Phys. 111, 4869 (1999)] we introduced a quasiclassical phase space approach for generating a nearly optimal direct-product basis for representing an arbitrary quantum Hamiltonian within a given energy range of interest. From a few reduced-dimensional integrals, the method determines the optimal one-dimensional marginal Hamiltonians, whose eigenstates comprise the direct-product basis. In this paper the method is applied to three-body vibrational systems expressed in radial and angular coordinates. Numerical results are obtained for the bound state eigenenergies of the nonrotating HCO molecule, determined to ∼0.01 cm−1 accuracy using a phase space optimized direct-product basis of 1972 functions. This represents a computational reduction of several orders of magnitude, in comparison with previous calculations.
Atom–atom partitioning of intramolecular and intermolecular Coulomb energy
Authors: Popelier P. L. A. , Kosov D. S.
Published: July 2002

An atom–atom partitioning of the (super)molecular Coulomb energy is proposed on the basis of the topological partitioning of the electron density. Atom–atom contributions to the molecular intra- and intermolecular Coulomb energy are computed exactly, i.e., via a double integration over atomic basins, and by means of the spherical tensor multipole expansion, up to rank L=lA+lB+1=5. The convergence of the multipole expansion is able to reproduce the exact interaction energy with an accuracy of 0.1–2.3 kJ/mol at L=5 for atom pairs, each atom belonging to a different molecule constituting a van der Waals complex, and for nonbonded atom–atom interactions in single molecules. The atom–atom contributions do not show a significant basis set dependence (3%) provided electron correlation and polarization basis functions are included. The proposed atom–atom Coulomb interaction energy can be used both with post-Hartree–Fock wave functions and experimental charge densities in principle. The Coulomb interaction energy between two molecules in a van der Waals complex can be computed by summing the additive atom–atom contributions between the molecules. Our method is able to extract from the supermolecule wave function an estimate of the molecular interaction energy in a complex, without invoking the reference state of free noninteracting molecules. We provide computational details of this method and apply it to (C2H2)2; (HF)2; (H2O)2; butane; 1,3,5-hexatriene; acrolein and urocanic acid, thereby covering a cross section of hydrogen bonds, and covalent bonds with and without charge transfer.
Laser induced fluorescence of argon ions in a plasma presheath
Authors: Oksuz L. , Khedr M. Atta , Hershkowitz N.
Published: July 2002

The characteristics of presheaths near an electrically floating plate in weakly collisional argon multidipole plasmas are investigated with a combination of data from laser induced fluorescence using a diode laser, Mach probes, emissive probes, and Langmuir probes. It is shown that ion–neutral collisions result in an increase in ion temperature from approximately room temperature in the bulk plasma to 0.13 eV, 0.5 cm from the plate, the location of the closest measurement. In addition, at that point, the presheath plasma potential drop is greater than Te/2, and the drift velocity is equal to 0.5 cs, where cs is the ion sound velocity.
Micromagnetic calculation of the high frequency dynamics of nano-size rectangular ferromagnetic stripes
Authors: Gérardin O. , Le Gall H. , Donahue M. J. , Vukadinovic N.
Published: July 2002

Nano-size ferromagnetic dots, wires, and stripes are of great interest for future high speed magnetic sensors and ultrahigh density magnetic storage. High frequency dynamic excitation is one way to investigate the time scale of the magnetization reversal in submicron particles with lateral nanometer dimension. Macroscopic models like the Landau–Lifshitz (LL) model are often used to describe the switching process. However, these models do not take into account the nonuniformity of the magnetization structure. In this article dynamic micromagnetic calculations are used in determining the high frequency susceptibility of a 1 μm×50 nm×5 nm Permalloy stripe. The studied structure exhibits two resonance modes. The higher, primary peak is around 10 GHz and can be identified with the uniform resonance mode predicted by the macroscopic LL model. The low frequency peak is attributed to the splay of the magnetization distribution near the end of the stripe.
Exchange anisotropy and micromagnetic properties of PtMn/NiFe bilayers
Authors: Pokhil Taras , Linville Eric , Mao Sining
Published: July 2002

Magnetic microstructure, exchange induced uniaxial and unidirectional anisotropy and structural transformation have been studied in PtMn/NiFe bilayer films and small elements as a function of annealing time. The relationship between the fcc-fct ordering phase transformation in PtMn and the development of exchange induced magnetic properties in PtMn/NiFe bilayers is complicated by the fact that the transformation occurs throughout the entire volume of the PtMn film, while the exchange between the layers is predominantly an interface effect. Consequently, the development of the exchange anisotropy should depend primarily on the character of the structural transformation at the interface between PtMn and NiFe. The purpose of this article is to correlate the volume phase transformation in PtMn to the development of exchange anisotropy and micromagnetic behavior in PtMn/NiFe bilayers. The interface structure can be inferred from the anisotropy and micromagnetic measurements, leading to a model that explains the relationship between the volume and interface transformation structures in PtMn, and magnetic properties of the bilayers. The structure and magnetic properties were characterized by x-ray diffraction, vibrating sample magnetometry, and magnetic force microscopy.
The holonomy Lie algebras of neutral metrics in dimension four
Authors: Ghanam Ryad , Thompson G.
Published: July 2002

The Lie algebra isomorphism between su(1,1)×su(1,1) and o(2,2) is used to obtain a list of subalgebras of the latter. The resulting list of 32 subalgebras is then examined on a case by case basis to see if each can be the Lie algebra of the holonomy group of a neutral metric in four dimensions. The conclusions, taken in conjunction with previously known results, furnish a classification of such Lie subalgebras of o(2,2), with only one case remaining unresolved.
Dynamical heterogeneity in a dense quasi-two-dimensional colloidal liquid
Authors: Cui Bianxiao , Lin Binhua , Rice Stuart A.
Published: July 2002

This paper reports the results of experimental studies of the dynamics of particles in a dense quasi-two-dimensional colloidal liquid. We find that at high density, near close packing but still in the liquid phase, the spatial configurations of the particles in the colloidal liquid consist of small ordered domains separated by disordered boundaries. There are frequent exchanges of particles between the ordered and disordered domains, so the lifetime of a particular ordered domain is short and the state of the system is ergodic. The motion of a particle in an ordered domain is constrained but fully two dimensional. The motion of a particle in a disordered boundary has considerable one-dimensional file-server character. By virtue of exchanges of particles between the ordered and disordered domains, the time dependence of the particle displacement has mixed character. We find that the particle dynamics in the dense quasi-two-dimensional colloidal liquid can be characterized with three simultaneous competing relaxation processes, each of which contributes to the mean square displacement of a particle, W(t). For an interval that is shorter than the time between collisions, W(t) is linear in time; this behavior results from particle displacements within a cage of fluctuating neighbors. At somewhat longer time W(t) has a sublinear dependence on time. We associate this temporal behavior with a file-server-like contribution to W(t), arising from correlated motion in the disordered boundary regions. At very long time we find that there are contributions to W(t) from infrequent large displacements, of the order of a particle diameter in length. The large displacements are associated with density fluctuations that fluidize the surroundings of a particle. These large displacements are not ballistic; rather the particle diffuses rapidly enough to generate a large displacement per unit time. The common occurrence of structural heterogeneity in simulation studies of systems with different particle–particle interactions and different equations of motion suggests that fluctuations that generate local order in a dense two-dimensional liquid may be a ubiquitous feature of that class of systems. The results of the experiments reported in this paper suggest that current theories of relaxation processes in a dense quasi-two-dimensional liquid do not adequately take into account the class of fluctuations that generates transient ordered domains in the liquid; nor do these theories adequately account for the cooperative relaxation of those domains.
DNA microarray inspection by interference microscopy
Authors: Vabre L. , Dubois A. , Potier M. C. , Stehlé J. L. , Boccara A. C.
Published: July 2002

DNA microarrays are important new objects for biologists, offering new methods for high speed genetic studies. Many of them are manufactured by depositing small spots, containing DNA molecules, on a coated glass slide. The quality of the spots (homogeneity, shape) is an important factor in the quality of the analysis. We have built a Linnik-type interference microscope providing three-dimensional (3D) topographic images of surfaces with a sensitivity of a few tens of picometers in height. This microscope has been applied to obtain 3D images of DNA microarray spots. We demonstrate the ability of the system to reveal the quality of DNA spots.
Random Taylor hypothesis and the behavior of local and convective accelerations in isotropic turbulence
Authors: Tsinober Arkady , Vedula Prakash , Yeung P. K.
Published: July 2002

The properties of acceleration fluctuations in isotropic turbulence are studied in direct numerical simulations (DNS) by decomposing the acceleration as the sum of local and convective contributions (aL=∂u/∂t and aC=u⋅∇u), or alternatively as the sum of irrotational and solenoidal contributions [aI=−∇(p/ρ) and aS=ν∇2u]. The main emphasis is on the nature of the mutual cancellation between aL and aC which must occur in order for the acceleration (a) to be small as predicted by the “random Taylor hypothesis” [Tennekes, J. Fluid Mech. 67, 561 (1975)] of small eddies in turbulent flow being passively “swept” past a stationary Eulerian observer. Results at Taylor-scale Reynolds number up to 240 show that the random-Taylor scenario 〈a2〉≪〈aC2〉≈〈aL2〉, accompanied by strong antialignment between the vectors aL and aC, is indeed increasingly valid at higher Reynolds number. Mutual cancellation between aL and aC also leads to the solenoidal part of a being small compared to its irrotational part. Results for spectra in wave number space indicate that, at a given Reynolds number, the random Taylor hypothesis has greater validity at decreasing scale sizes. Finally, comparisons with DNS data in Gaussian random fields show that the mutual cancellation between aL and aC is essentially a kinematic effect, although the Reynolds number trends are made stronger by the dynamics implied in the Navier–Stokes equations.
A multiple-quantum nuclear magnetic resonance study of interstitial Li clusters in LixC60
Authors: Tomaselli M. , Meier B. H. , Riccò M. , Shiroka T. , Sartori A.
Published: July 2002

We characterize the local Li structure in the low-temperature phase of LixC60 with 6⩽x⩽15 using single and multiple-quantum Li7 nuclear magnetic resonance spectroscopy. We find clear evidence for Li cluster intercalated structures in all compositions. The inferred cluster sizes are significantly smaller than those suggested by the stoichiometry and could be explained by a more disordered LixC60 low-temperature phase than expected from the current models.
Molecular dynamics study of polymer melt confined between walls
Authors: Aoyagi Takeshi , Takimoto Jun-ichi , Doi Masao
Published: July 2002

Coarse-grained molecular dynamics simulation of a bead–spring polymer model has been conducted for polymer melt confined between two solid walls. The wall effect was studied by changing the distance between the walls and the wall–polymer interaction. It was observed that the polymers near the walls are compressed towards the walls: the component of the radius of gyration tensor perpendicular to the wall surfaces decreases in a layer near the walls. The thickness of this surface layer, estimated from the analysis of the static polymer structure, is about 1.0–1.5 times the radius of gyration Rg in the bulk, and is independent of the distance between the walls and the wall–polymer interaction. The relaxation time of the polymers, obtained from the autocorrelation of normal modes, increases with increasing the strength of the wall–polymer interaction and with decreasing the distance between the walls. These wall effects are observed at a distance much larger than Rg. This result is in agreement with the recent dielectric measurements of cis-polyisoprene confined between mica surfaces reported by Cho, Watanabe, and Granick [J. Chem. Phys. 110, 9688 (1999)]. The thickness of the surface layer was also estimated by the position dependence of the static and dynamic properties, and was found to agree with that estimated by the viscoelastic measurements.
Peristaltically driven channel flows with applications toward micromixing
Authors: Selverov Kiril P. , Stone H. A.
Published: July 2002

Flows driven by a transverse, small amplitude traveling wave propagating along the boundary of a closed rectangular container are examined. High-frequency motions are the primary focus of interest, although low-frequency results are discussed also. Using asymptotic analysis appropriate for high frequencies, the steady, time-independent (streaming) flow is computed analytically and compared with results of the exact calculation. The boundary-layer structure is delineated and average Eulerian and Lagrangian flow characteristics are compared. Experiments confirming the major qualitative high-frequency findings are reported in an Appendix. The results could be useful for modeling peristaltically operated microelectromechanical systems devices where fluid motion needs to be produced without internal moving mechanical components.
Design of a high-pressure single pulse shock tube for chemical kinetic investigations
Authors: Tranter R. S. , Brezinsky K. , Fulle D.
Published: July 2002

A single pulse shock tube has been designed and constructed in order to achieve extremely high pressures and temperatures to facilitate gas-phase chemical kinetic experiments. Postshock pressures of greater than 1000 atmospheres have been obtained. Temperatures greater than 1400 K have been achieved and, in principle, temperatures greater than 2000 K are easily attainable. These high temperatures and pressures permit the investigation of hydrocarbon species pyrolysis and oxidation reactions. Since these reactions occur on the time scale of 0.5–2 ms the shock tube has been constructed with an adjustable length driven section that permits variation of reaction viewing times. For any given reaction viewing time, samples can be withdrawn through a specially constructed automated sampling apparatus for subsequent species analysis with gas chromatography and mass spectrometry. The details of the design and construction that have permitted the successful generation of very high-pressure shocks in this unique apparatus are described. Additional information is provided concerning the diaphragms used in the high-pressure shock tube.
Dielectric and electric properties of donor- and acceptor-doped ferroelectric SrBi2Ta2O9
Authors: Kumar M. Mahesh , Ye Z.-G.
Published: July 2002

Substitutions by aliovalent cations on the Sr2+ and Bi3+ sites of ferroelectric SrBi2Ta2O9 (SBT) have been carried out, which result in modified dielectric and electrical properties of SBT. The substitution of 10 mole % Fe3+ for Sr2+ shows an increase of 80 °C in TC, whereas Ca2+ substitution for Bi3+ gives rise to diffused and frequency dispersive dielectric maxima with a relaxor-like behavior. The presence of Ca2+ in the (Bi2O2)2+ layers appears to enhance the mobility of charge carriers thus increasing the bulk conduction of the sample. On the other hand, Fe3+ addition in the perovskite-like units results in a sharp dielectric anomaly at the ferroelectric phase transition, with a bulk conductance similar to that of pure SBT compound, but with a reduced space charge relaxation time. The low temperature conductivity mechanism shows a frequency dependence, which can be ascribed to the space charge mainly due to the oxygen vacancies. The dielectric and conductivity properties of the Ca2+-doped SBT make it a promising material for the fatigue resistance in device applications.
Universal phase diagram for high-piezoelectric perovskite systems
Authors: Cox D. E. , Noheda B. , Shirane G. , Uesu Y. , Fujishiro K. , Yamada Y.
Published: July 2002

Strong piezoelectricity in perovskite-type PbZn1−xTixO3 (PZT) and Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) systems is generally associated with the existence of a morphotropic phase boundary (MPB) separating regions with rhombohedral and tetragonal symmetry. An x-ray study of PZN–9% PT has revealed the presence of an orthorhombic phase at the MPB, and a near-vertical boundary between the rhombohedral and orthorhombic phases, similar to that found for PZT between the rhombohedral and monoclinic phases. We discuss the results in the light of a recent theoretical paper by Vanderbilt and Cohen [Phys. Rev. B 63, 94108 (2001)], which attributes these low-symmetry phases to the high anharmonicity in these oxide systems.
Low-temperature formation of silicon nitride gate dielectrics by atomic-layer deposition
Authors: Nakajima Anri , Yoshimoto Takashi , Kidera Toshirou , Yokoyama Shin
Published: July 2002

Thin (equivalent oxide thickness Teq of 2.4 nm) silicon nitride layers were deposited on Si substrates by an atomic-layer-deposition (ALD) technique at low temperatures (<550 °C). The interface state density at the ALD silicon nitride/Si-substrate interface was almost the same as that of the gate SiO2. No hysteresis was observed in the gate capacitance–gate voltage characteristics. The gate leakage current was the level comparable with that through SiO2 of the same Teq. The conduction mechanism of the leakage current was investigated and was found to be the direct tunneling. The ALD technique allows us to fabricate an extremely thin, very uniform silicon nitride layer with atomic-scale control for the near-future gate dielectrics.
A low-scaling method for second order Møller–Plesset calculations
Authors: Saebø Svein , Pulay Peter
Published: July 2002

A new low-order scaling algorithm for second-order Møller–Plesset perturbation theory (MP2) is described. The method employs localized orbitals. However, in contrast to other local MP2 schemes reported in the literature, all atomic orbitals with significant contribution to the correlation energy for a given pair or orbitals are retained. Our approach introduces no significant approximation if all significant pairs are taken into account. However, the method is intended for the description of correlation between spatially close (“strong”) pairs; for distant pairs, more economical alternative methods can be used. The algorithm, including a new implementation of the simultaneous two-index integral transformation scheme, is described in detail. Test calculations demonstrate asymptotic linear scaling for large systems. Strong-pair MP2 calculations with more that 1800 contracted basis functions performed on a single PC are reported. As a consequence of its lower scaling, the calculation of the MP2 energy is less expensive than the calculations of the Hartree–Fock energy for large systems. This has been demonstrated for strong pair correlation but is expected to hold also for full MP2 calculations.
Epitaxial-grade polycrystalline Pb(Zr,Ti)O3 film deposited at low temperature by pulsed-metalorganic chemical vapor deposition
Authors: Aratani Masanori , Oikawa Takahiro , Ozeki Tomohiko , Funakubo Hiroshi
Published: July 2002

Epitaxial-grade polycrystalline Pb(Zr,Ti)O3 (PZT) films were deposited at 415 °C by source- gas-pulsed-introduced metalorganic chemical vapor deposition. The polycrystalline PZT film with Zr/(Zr+Ti)=0.35 which was prepared on (111)Pt/Ti/SiO2/Si substrate showed highly (100)- and (001)-preferred orientations. Well-saturated ferroelectricity with a remanent polarization (Pr) and coercive field of 41.4 μC/cm2 and 78.5 kV/cm, respectively, was obtained. This Pr value is almost the same as that of epitaxially grown films at 580 °C with the same composition and orientations taking into account of the volume fraction of (100) and (001) orientations.
Photoelectron spectroscopy of mono-niobium carbide clusters NbCn−(n=2–7): Evidence for a cyclic to linear structural transition
Authors: Zhai Hua-Jin , Liu Shu-Rong , Li Xi , Wang Lai-Sheng
Published: July 2002

We investigated a series of mono-niobium carbide clusters, NbCn− (n=2–7), using anion photoelectron spectroscopy. Vibrationally resolved photoelectron spectra were observed for NbC2− and NbC3−, which were both shown to have cyclic C2v structures. Two isomers were observed for NbC4− and NbC5−. The weak and low electron binding energy isomers were shown to be cyclic structures forming a series with NbC2− and NbC3−, and all have similar and low electron binding energies. The main isomers of NbC4− and NbC5−, which possess much higher electron binding energies, were shown to be due to linear structures, which form a series with NbC6− and NbC7−. All the linear NbCn− clusters were observed to have high electron binding energies and exhibit an even–odd alternation, similar to that observed for pure linear carbon clusters in the same size range. A cyclic to linear structural transition was thus observed for the NbCn− clusters from NbC3− to NbC4−, with the cyclic structures favored for the smaller clusters and the linear isomers favored for the larger clusters.
Primary and secondary effective charges for electrical double layer systems with asymmetric electrolytes
Authors: Ulander Johan , Greberg Hans , Kjellander Roland
Published: July 2002

We show that the screening of the electrostatic potential in electrolytes can in exact theory be expressed in terms of a generalized screened Colomb potential, analogous to the Yukawa potential from the Debye–Hückel approximation, provided the source charge of the potential is renormalized. The renormalized charge distribution is identical to that of a “dressed particle” in dressed ion theory, DIT, of Kjellander and Mitchell. Using DIT we analyze the leading terms of the decay of density profiles and electrostatic potential outside a charged planar wall in contact with 1:2 electrolytes. The formalism leads in a natural manner to the definition of a primary and a secondary effective charge of an object immersed in an electrolyte. These charges are associated with the leading and second leading decay modes of the potential, which have different decay lengths. It is found that both leading terms in the decay are important; together, they give in many cases a very good representation of the density profiles and the potential for distances larger than about a couple of ionic diameters from the wall. When varying the actual (bare) surface charge density σ of the wall, two points of zero effective surface charge density are found: one at low (but nonzero) and one at high value of σ. The former occurs when the counterions are monovalent and the latter when the counterions are divalent and the electrolyte concentration is sufficiently high. Both are associated with effective charge reversals where the surface appears to attain a charge of opposite sign compared to its bare charge. The double layer interaction between two equally charged particles is attractive at large separations at a point of zero effective surface charge.
Low frequency electrostatic instability in a helicon plasma
Authors: Light Max , Chen Francis F. , Colestock P. L.
Published: July 2002

Recent discoveries in a helicon plasma show a decrease in equilibrium plasma density as magnetic field strength is increased. This can be explained in the framework of a low frequency electrostatic instability. However, quiescent plasma behavior in helicon sources has been hitherto accepted. To verify the existence of an instability, extensive measurements of fluctuating quantities and losses as a function of magnetic field were implemented. Furthermore, a theoretical model was developed to compare to the measurements. Theory and measurement show very good agreement; both verifying the existence of a low frequency instability and showing that it is indeed responsible for the observed density characteristic.
Effects of excitation density on cathodoluminescence from GaN
Authors: Kucheyev S. O. , Toth M. , Phillips M. R. , Williams J. S. , Jagadish C.
Published: July 2002

Wurtzite GaN epilayers are studied by cathodoluminescence (CL) spectroscopy. Results show that the intensities of donor–acceptor pair (DAP) and yellow luminescence (YL) peaks sublinearly depend on excitation density, presumably, due to saturation effects. The intensity of near-gap emission, however, exhibits a superlinear dependence on electron-beam excitation. In contrast to photoluminescence measurements, CL studies of GaN are usually performed in a regime with a strongly nonlinear dependence of luminescence intensities on excitation due to a large difference in carrier generation rates for these two techniques. As a result, the ratios of near-gap to YL and DAP emission intensities strongly depend on electron-beam current. Moreover, electron-beam spot size (i.e., beam focusing) dramatically affects CL intensity. An understanding of such saturation effects is necessary for a correct interpretation of CL spectra from GaN.
Dc superconducting quantum interference device amplifier for gravitational wave detectors with a true noise temperature of 16 μK
Authors: Vinante Andrea , Mezzena Renato , Prodi Giovanni Andrea , Vitale Stefano , Cerdonio Massimo , Falferi Paolo , Bonaldi Michele
Published: July 2002

We report on the noise characterization of a two-stage dc superconducting quantum interference device (SQUID) amplifier developed for resonant gravitational wave detectors. The back action noise is estimated by coupling the SQUID to an electrical resonator at 1.6 kHz with Q=1.1×106. From measurements of back action and additive SQUID noise, performed in the temperature range 1.5–4.2 K, an upper limit is set on the noise temperature Tn of the device at the resonator frequency. The best value obtained at 1.5 K is Tn⩽16 μK and corresponds to 200 resonator quanta. The thermal component of the noise temperature is found in reasonable agreement with the predicted value.