bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbative analytical solutions of the magnetic forward problem for realistic volume conductors
Authors: Nolte Guido , Fieseler Thomas , Curio Gabriel
Published: July 2002

The magnetic field induced by a current dipole situated in a realistic volume conductor cannot be computed exactly. Here, we derive approximate analytical solutions based on the fact that in magnetoencephalography the deviation of the volume conductor (i.e., the head) from a spherical approximation is small. We present an explicit integral form which allows to calculate the nth order Taylor expansion of the magnetic field with respect to this deviation from the corresponding solution of the electric problem of order n−1. Especially, for a first order solution of the magnetic problem only the well-known electric solution for a spherical volume conductor is needed. The evaluation of this integral by a series of spherical harmonics results in a fast algorithm for the computation of the external magnetic field which is an excellent approximation of the true field for smooth volume conductor deformations of realistic magnitude. Since the approximation of the magnetic field is exactly curl-free it is equally good for all components. We estimate the performance for a realistic magnitude of deformations by comparing the results to the exact solution for a prolate spheroid. We found a relevant improvement over corresponding solutions given by the boundary element method for superficial sources while the performance is in the same order for deep sources.
Self-repair of ordered pattern of nanometer dimensions based on self-compensation properties of anodic porous alumina
Authors: Masuda Hideki , Yotsuya Masato , Asano Mari , Nishio Kazuyuki , Nakao Masashi , Yokoo Atsushi , Tamamura Toshiaki
Published: July 2002

The self-repair of an ordered pattern of nanometer dimensions based on the self-compensation properties of anodic porous alumina is demonstrated. In a pretextured pattern formed on Al using the nanoindentation process with an array of convexes, the deficiency sites of the pattern were found to be compensated automatically during the anodization. Combining the self-compensation properties of the pore configuration of the anodic porous alumina with the preparation of a replica of the compensated porous structure allowed us to develop a process which has the capability of self-repairing the imperfections in the starting pattern. It was confirmed that deficiencies in the starting pattern could be repaired automatically in the Ni pattern regenerated using the self-compensated anodic porous alumina as a template.
Diffraction gratings in polymer-dispersed liquid crystals recorded by means of polarization holographic technique
Authors: Cipparrone G. , Mazzulla A. , Russo G.
Published: July 2002

We report an observation of holographic gratings recorded exposing a homogeneous mixture of pre-polymer and liquid crystal to a polarization light pattern. This polarization pattern is obtained by a superposition of two waves with orthogonal linear polarization. The idea to use this technique to realize holographic gratings come from the possibility of controlling the liquid crystal alignment inside the droplets, through the photoalignment, during the photoinduced polymerization process. On the contrary, the usual methods of writing diffraction gratings in polymer-dispersed liquid crystals are based on intensity holographic technique that controls the polymerization process. Diffraction gratings, composed of liquid crystal droplets having a particular internal alignment, are observed.
Transient electrical discharges in small devices
Authors: Soto Leopoldo , Esaulov Andrey , Moreno José , Silva Patricio , Sylvester Gustavo , Zambra Marcelo , Nazarenko Andrey , Clausse Alejandro
Published: July 2002

Fundamental and applied research on plasmas with high energy density that are unstable and radiate can be done at a relatively low cost with small plasma pinches. In this paper we discuss three experiments using small pinch devices: a capillary discharge, a Z-pinch driven by a small generator, and a low energy plasma focus. The experiments were complemented by magnetohydrodynamics numerical calculations in order to assist the design and physical interpretation of the experimental data. The diagnostics used in the experiments include current and voltage monitors, multipinhole camera, holographic interferometry, and vacuum ultraviolet spectroscopy.
Design and characterization of thin film microcoolers
Authors: LaBounty Chris , Shakouri Ali , Bowers John E.
Published: July 2002

Thin film coolers can provide large cooling power densities compared to bulk thermoelectrics due to the close spacing of hot and cold junctions. Important parameters in the design of such coolers are investigated theoretically and experimentally. A three-dimensional (3D) finite element simulator (ANSYS) is used to model self-consistently thermal and electrical properties of a complete device structure. The dominant three-dimensional thermal and electrical spreading resistances acquired from the 3D simulation are also used in a one-dimensional model (MATLAB) to obtain faster, less rigorous results. Heat conduction, Joule heating, thermoelectric and thermionic cooling are included in these models as well as nonideal effects such as contact resistance, finite thermal resistance of the substrate and the heat sink, and heat generation in the wire bonds. Simulations exhibit good agreement with experimental results from InGaAsP-based thin film thermionic emission coolers which have demonstrated maximum cooling of 1.15 °C at room temperature. With the nonideal effects minimized, simulations predict that single stage thin film coolers can provide up to 20–30 °C degrees centigrade cooling with cooling power densities of several 1000 W/cm2.
Recent developments in atomic physics for the simulation of hot plasmas
Authors: Klapisch M. , Bar-Shalom A. , Oreg J. , Colombant D.
Published: July 2002

Simulations of plasmas in which atoms are not completely stripped require atomic data, like average charge, ionization energies, and radiative properties (emissivity, opacity). These depend on populations of energy levels. The basic framework for obtaining the latter is the collisional radiative model (CRM), which bridges the gap between the low-density Corona Equilibrium (CE) and Local Thermodynamic Equilibrium (LTE). However, for nearly all but the simplest ions, the number of relevant bound states and cross sections is prohibitive. In this review we summarize some recent methods for handling complex ions: By focusing on an exact evaluation of relevant information and ignoring unobservable features, unresolved transition arrays (UTA) are obtained. The supertransition arrays (STA) model combines many UTAs in LTE. The STA code was recently extended to a non-LTE CRM called SCROLL. Using these models could improve radiation simulation in hot plasmas, even for simple spectra.
Multimass ion imaging detection: Application to photodissociation
Authors: Tsai Shang-Ting , Lin Chih-Kai , Lee Yuan T. , Ni Chi-Kung
Published: July 2002

A constant momentum mass spectrometer with a two-dimensional ion detector in conjunction with a pulsed vacuum ultraviolet laser is used for the simultaneous measurement of the translational energy distributions of many different fragments. A description of the apparatus and its performance are presented. Preliminary experimental results on the photodissociation of toluene and benzene are given.
Onset of ferromagnetism in Fe epitaxially grown on GaAs(001) (4×2) and (2×6)
Authors: Bensch F. , Garreau G. , Moosbühler R. , Bayreuther G. , Beaurepaire E.
Published: July 2002

Ultrathin Fe films were epitaxially grown at room temperature on GaAs(001) with either predominant (4×2) or (2×6) surface reconstruction. At nominal Fe coverages of tFe⩾2.8 monolayers (ML), a ferromagnetic state is observed below a certain critical temperature, TC. Surprisingly, the magnetic phase transition at TC appears even sharper than for Fe films on metallic single-crystal substrates, which were believed to be an excellent representation of two-dimensional (2D) ferromagnets. This may be due to the extremely short lateral length scale of film inhomogeneities. The critical exponent β=0.26 is close to the value expected for 2D XY systems of finite size. For tFe=3.6 ML, TC is close to room temperature. TC decreases steeply with decreasing Fe coverage, with an average slope of 270 K/ML. From a power law extrapolation, TC seems to vanish at tFe=2.5 ML. The onset of ferromagnetism at tFe=2.5 ML is interpreted as a percolation phenomenon during the coalescence process of Fe islands.
Photosensitivity in phosphate glass doped with Ag+ upon exposure to near-ultraviolet femtosecond laser pulses
Authors: Watanabe Yuichi , Namikawa Gaku , Onuki Teppei , Nishio Keishi , Tsuchiya Toshio
Published: July 2002

We report on a photosensitivity in soda-alumino-phosphate glass doped with silver ions Ag+ upon exposure to near-ultraviolet femtosecond laser pulses. The photosensitivity, i.e., formation of color centers such as Ag0 and Ag2+ in the glass is found to be associated with intensity-dependent nonlinear optical process induced by extremely high irradiance up to ∼TW/cm2 of the femtosecond pulses. We demonstrate a bright orange fluorescence from the induced color centers inside the glass, that should be applicable to a functional optical device.
Quantum optimal control of multiple targets: Development of a monotonically convergent algorithm and application to intramolecular vibrational energy redistribution control
Authors: Ohtsuki Yukiyoshi , Nakagami Kazuyuki , Fujimura Yuichi , Zhu Wusheng , Rabitz Herschel
Published: July 2002

An optimal control procedure is presented to design a field that transfers a molecule into an objective state that is specified by the expectation values of multiple target operators. This procedure explicitly includes constraints on the time behavior of specified operators during the control period. To calculate the optimal control field, we develop a new monotonically and quadratically convergent algorithm by introducing a quadruple space that consists of a direct product of the double (Liouville) space. In the absence of the time-dependent constraints, the algorithm represented in the quadruple-space notation reduces to that of the double-space notation. This simplified formulation is applied to a two dimensional system which models intramolecular vibrational energy redistribution (IVR) processes in polyatomic molecules. An optimal pulse is calculated that exploits IVR to transfer a specific amount of population to an optically inactive state, while the other portion of the population remains in the initial state at a control time. Using trajectory plots in quantum-number space, we numerically analyze how the control pathway changes depending on the amount of the excited population.
Growth rate of the Richtmyer–Meshkov instability when a rarefaction is reflected
Authors: Wouchuk J. G.
Published: July 2002

A model is presented that calculates the asymptotic growth rate of the linear Richtmyer–Meshkov instability when a rarefaction is reflected at the contact surface. The result is valid for any value of the incident shock Mach number and initial fluids parameters. There is very good agreement with previous numerical simulations and experiments done at high compressions. The technique developed in the model is seen to be highly accurate and allows us a fast evaluation of the asymptotic normal velocity at the interface.
Ab initio calculation of predissociation linewidths in the Schumann–Runge bands of the oxygen molecule
Authors: Li Yan , Liebermann H.-P. , Buenker R. J.
Published: July 2002

Ab initio multireference single- and double-excitation configuration interaction calculations have been carried out for potential curves of the B 3Σu−, Πu1, Πu3, Πu5, and Σu+3 electronic states of the O2 molecule. Spin-orbit interaction matrix elements among the above states and rotational interaction matrix elements between the B 3Σu− and the Πu3 states have been evaluated in order to investigate the observed line broadening in the Schumann–Runge bands. Predissociation linewidths of rovibrational levels of the B 3Σu− state are determined by using the complex rotation method in conjunction with the Gauss–Hermite quadrature procedure. It has been found that the Πu5 and Πu3 states play dominant roles in the predissociation of the B 3Σu− state. The calculated linewidths for rovibrational levels of v=0–13, N=0–36 of the F2 fine-structure components of the B 3Σu− state are in very good agreement with experimental results. Isotopic effects in the predissociation of the B 3Σu− state are studied by means of calculations for O218 and O16O18 and the theoretical results are found to be in good agreement with the observed average predissociation linewidths in each case.
Passive external radio frequency filter for Langmuir probes
Authors: Wendt A. E.
Published: July 2002

A tunable passive circuit is introduced for radio frequency (rf) filtering of Langmuir probes used to measure plasma properties. The circuit produces a high impedance between the probe tip and ground so that the probe tip follows potential fluctuations in the plasma so that the probe bias voltage with respect to the plasma is constant on the time scale of rf fluctuations. Filtering is implemented at the fundamental frequency (13.56 MHz in this case) and the second and third harmonics. Representative probe traces and electron energy distribution functions from an inductively coupled plasma are presented to demonstrate filter performance.
Hydrogen-induced band gap tuning of (InGa)(AsN)/GaAs single quantum wells
Authors: Baldassarri H. v. H. G. , Bissiri M. , Polimeni A. , Capizzi M. , Fischer M. , Reinhardt M. , Forchel A.
Published: July 2002

The effect of atomic hydrogen on the electronic properties of (InGa)(AsN)/GaAs single quantum wells (QWs) has been investigated by photoluminescence (PL) spectroscopy. For increasing hydrogen dose, the band gap of the material increases until it reaches the value corresponding to a N-free reference QW. The band gap variation is accompanied by an increase of the line width of the PL spectra and a decrease of the PL efficiency. Annealing at 500 °C fully recovers the band gap and PL line width the sample had before hydrogenation. These results are accounted for by the formation of N–H complexes, which lowers the effective nitrogen content in the well.
Quantum dynamics using a discretized coherent state representation: An adaptive phase space method
Authors: Mauritz Andersson L.
Published: July 2002

We introduce a discretized coherent state representation (DCSR) for quantum dynamics. Expansion of a wave function in the nonorthogonal slightly overcomplete set is made with an identity operator computed using an iterative refinement method. Calculating the inverse of the overlap matrix is not necessary. The result is an accurate and efficient representation, where you only put basis functions in the region of phase space where the wave function is nonvanishing. Compared to traditional spatial grid methods, fewer grid points are needed. The DCSR can be viewed as an application of the Weyl–Heisenberg frame and extends it into a useful computational method. A scheme for fully quantum mechanical propagation is constructed and applied to the realistic problem of highly excited vibration in the heavy diatomic molecule Rb2. Compared to split-operator propagation in a conventional spatial grid, an order of magnitude longer time steps can be taken and fewer grid points are needed. The computational effort scales linearly with the number of basis functions. Nonreflecting boundary conditions are a natural property of the representation and is illustrated in a model of predissociation.
The performance of dynamic subgrid-scale models in the large eddy simulation of rotating homogeneous turbulence
Authors: Kobayashi Hiromichi , Shimomura Yutaka
Published: July 2002

The performance of dynamic subgrid-scale (SGS) models is numerically examined in the large eddy simulation of rotating homogeneous turbulences in comparison with the corresponding filtered data of the direct numerical simulation (DNS). The examined dynamic SGS models are: the dynamic Smagorinsky model (DSM), the dynamic mixed model (DMM), the dynamic Clark model (DCM), and the dynamic two-parameter Clark model (DTCM). All models are mathematically reformulated in a rotating frame from the corresponding expressions in an inertial frame. It is shown that the DSM and the DMM are not consistent with the constraint of asymptotic material frame indifference, but the DCM and the DTCM are consistent. All models except the DSM show similar decays of the grid-scale turbulent energies both in nonrotating and in rotating frames; they agree well with the DNS in the nonrotating case, but they are slightly less dissipative than the DNS in the rotating case. The DSM underestimated the grid-scale energy dissipation in the nonrotating case, though there is no major difference from other models in the rotating case. However, the DSM in a rotating frame, which takes a different form from that in an inertial frame, leads to an unphysical fluctuating decay for a homogeneous turbulence suddenly submitted to a rotation.
Regularity in chaotic reaction paths III: Ar6 local invariances at the reaction bottleneck
Authors: Komatsuzaki Tamiki , Berry R. Stephen
Published: July 2002

We recently developed a new method to extract a many-body phase-space dividing surface, across which the transmission coefficient for the classical reaction path is unity. The example of isomerization of a 6-atom Lennard-Jones cluster showed that the action associated with the reaction coordinate is an approximate invariant of motion through the saddle regions, even at moderately high energies, at which most or all the other modes are chaotic [J. Chem. Phys. 105, 10838 (1999); Phys. Chem. Chem. Phys. 1, 1387 (1999)]. In the present article, we propose a new algorithm to analyze local invariances about the transition state of N-particle Hamiltonian systems. The approximate invariants of motion associated with a reaction coordinate in phase space densely distribute in the sea of chaotic modes in the region of the transition state. Using projections of distributions in only two principal coordinates, one can grasp and visualize the stable and unstable invariant manifolds to and from a hyperbolic point of a many-body nonlinear system, like those of the one-dimensional, integrable pendulum. This, in turn, reveals a new type of phase space bottleneck in the region of a transition state that emerges as the total energy increases, which may trap a reacting system in that region.
Effect of the electron thermal motion on the ponderomotive force in inductive plasma
Authors: Smolyakov A. I. , Godyak V. , Tyshetskiy Y.
Published: July 2002

Ponderomotive force in the inductively coupled discharge is analyzed in the kinetic approximation taking into account the effects of the electron thermal motion. It is shown that the ponderomotive force is significantly reduced in strongly nonlocal regimes when the characteristic transient frequency vth/δ far exceeds the frequency of the electromagnetic field oscillation ω,vth/δ>ω, where vth is the electron thermal velocity; δ is the characteristic length scale for field inhomogeneity. The developed theoretical model is in reasonable agreement with experimental data.
Algorithms to solve the (quantum) Sutherland model
Authors: Langmann Edwin
Published: July 2002

We give a self-contained presentation and comparison of two different algorithms to explicitly solve quantum many body models of indistinguishable particles moving on a circle and interacting with two-body potentials of 1/sin2-type. The first algorithm is due to Sutherland and well-known; the second one is a limiting case of a novel algorithm to solve the elliptic generalization of the Sutherland model. These two algorithms are different in several details. We show that they are equivalent, i.e., they yield the same solution and are equally simple.
Steady-state free precession experiments and exact treatment of diffusion in a uniform gradient
Authors: Freed D. E. , Scheven U. M. , Zielinski L. J. , Sen P. N. , Hürlimann M. D.
Published: July 2002

We derive an analytic solution for the magnetization of spins diffusing in a constant gradient field while applying a long stream of rf pulses, which is known as the steady-state free precession (SSFP) sequence. We calculate the diffusion-dependent amplitude of the free induction decay (FID) and higher order echoes for pulses with arbitrary flip angle α and pulse spacing TR. Stopped-SSFP experiments were performed in a permanent gradient field and the amplitudes of the first three higher order echoes were measured for a range of values of α and TR. Theoretical results are in excellent agreement with experimental results, using no adjustable parameters. We identify various diffusion regimes in a rather large parameter space of pulsing and relaxation times, diffusion coefficient, and flip angle and discuss the interplay of the relevant time scales present in the problem. This “phase diagram” provides a road map for designing experiments which enhance or suppress the sensitivity to diffusion. We delineate the limits of validity of the widely used ansatz put forth by Kaiser, Bartholdi, and Ernst in their seminal paper.
Superconducting gap and pair breaking in CeRu2 studied by point contacts
Authors: Moskalenko A. V. , Naidyuk Yu. G. , Yanson I. K. , Hedo M. , Inada Y. , Ōnuki Y. , Haga Y. , Yamamoto E.
Published: July 2002

The superconducting gap in a CeRu2 single crystal is investigated by point contacts. BCS-like behavior of the gap Δ in the temperature range below Tc* (Tc*<Tc, where Tc is the critical temperature) is established, indicating the presence of a gapless superconductivity region (between Tc* and Tc). The pair-breaking effect of paramagnetic impurities, supposedly Ce ions, is taken into consideration using the Scalski–Betbeder–Matibet–Weiss approach based on Abrikosov–Gorkov theory. It allows us to recalculate the superconducting order parameter Δα (in the presence of paramagnetic impurities) and the gap Δp (in the pure case) for the single crystal and for the previously studied polycrystalline CeRu2. The value 2Δp(0)≈2 meV, with 2Δp(0)/kBTc≈3.75, is found in both cases, indicating that CeRu2 is a “moderate” strong-coupling superconductor.
Quantal two-center Coulomb problem treated by means of the phase-integral method. II. Quantization conditions in the symmetric case expressed in terms of complete elliptic integrals. Numerical illustration
Authors: Athavan N. , Lakshmanan M. , Fröman N.
Published: July 2002

The contour integrals, occurring in the arbitrary-order phase-integral quantization conditions given in a previous paper, are in the first- and third-order approximations expressed in terms of complete elliptic integrals in the case that the charges of the Coulomb centers are equal. The evaluation of the integrals is facilitated by the knowledge of quasiclassical dynamics. The resulting quantization conditions involving complete elliptic integrals are solved numerically to obtain the energy eigenvalues and the separation constants of the 1sσ and 2pσ states of the hydrogen molecule ion for various values of the internuclear distance. The accuracy of the formulas obtained is illustrated by comparison with available numerically exact results.
Transparent magnetic composites of ZnFe2O4 nanoparticles in silica
Authors: Zhou Z. H. , Xue J. M. , Chan H. S. O. , Wang J.
Published: July 2002

Uniformly dispersed zinc ferrite nanoparticles were successfully synthesized in a silica matrix with both the magnetic properties and optical transparency designed in the visible wavelength region. The phases and morphologies of these zinc ferrite particles were examined using x-ray diffraction, Raman spectroscopy and transmission electron microscopy. A much lower formation temperature of ZnFe2O4 in nanocomposites was observed in this study compared to in conventional ceramic processing. The particle size of zinc ferrite was found to vary with the initial drying temperature, suggesting that a well-established silica network provides more confinement to the growth of ZnFe2O4 particles. The magnetic properties, measured at both 78 K and room temperature, exhibit strong dependence on the particle size, and are related to the unusual cation distribution and the surface effect of nanoparticles. The optical properties of the nanocomposites can be adjusted by varying the concentration of the zinc ferrite nanoparticles. Their absorption coefficient remains below 400 cm−1 in the red visible light region.
A reduced approach for modeling the influence of nanoclusters and {113} defects on transient enhanced diffusion
Authors: Stiebel D. , Pichler P. , Cowern N. E. B.
Published: July 2002

To simulate transient enhanced diffusion (TED) of dopants after ion implantation, a very accurate model for the interaction of self–interstitials with extended defects is indispensable. Recently, such a model has been published by Cowern including the formation of {113} defects via small self–interstitial clusters. Extracted from experimental results, this continuum model consists of a large set of coupled differential equations and, consequently, simulation times are rather high. In this letter, we present a model based on only seven differential equations leading to almost identical results in comparison to those of the original model. The reduction obtained will allow the application of the clustering model for the simulation of TED in commercial software tools.
Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition
Authors: Chhowalla M. , Teo K. B. K. , Ducati C. , Rupesinghe N. L. , Amaratunga G. A. J. , Ferrari A. C. , Roy D. , Robertson J. , Milne W. I.
Published: July 2002

The growth of vertically aligned carbon nanotubes using a direct current plasma enhanced chemical vapor deposition system is reported. The growth properties are studied as a function of the Ni catalyst layer thickness, bias voltage, deposition temperature, C2H2:NH3 ratio, and pressure. It was found that the diameter, growth rate, and areal density of the nanotubes are controlled by the initial thickness of the catalyst layer. The alignment of the nanotubes depends on the electric field. Our results indicate that the growth occurs by diffusion of carbon through the Ni catalyst particle, which rides on the top of the growing tube.
Multipactor suppression in amplitude modulated radio frequency fields
Authors: Semenov Vladimir , Kryazhev Alexey , Anderson Dan , Lisak Mietek
Published: July 2002

An analytical and numerical investigation is made of the multipactor phenomenon in a simple one-dimensional metallic gap exposed to a homogeneous rf field consisting of two carrier waves with close but separated frequencies. It is shown that in contrast to the monochromatic case (i.e., single carrier operation), the time varying envelope resulting from the beating of the two carrier waves leads to a suppression of multipactor breakdown, if appropriate combinations of carrier amplitudes and frequency separation are chosen. Furthermore, this combination depends neither on gap width nor on frequency range. Based on these results it is possible to draw the tentative conclusion that the regime of multicarrier operation is very promising from the point of view of avoiding breakdown. This is particularly so in systems of complicated geometry where single carrier operation may lead to multipactor breakdown for any high level of rf power.
Robust microfabricated field-effect sensor for monitoring molecular adsorption in liquids
Authors: Cooper E. B. , Fritz J. , Wiegand G. , Wagner P. , Manalis S. R.
Published: July 2002

We show that a microfabricated field-effect sensor located at the terminus of a freestanding cantilever can detect surface potential changes resulting from the adsorption of charged molecules in an aqueous environment. The charge sensitive region, defined by lightly doped silicon, is embedded within the heavily doped silicon cantilever. Since both the electrical trace and sensitive region are passivated with thermally diffused silicon dioxide, the entire cantilever can be immersed in buffer solutions and cleaned with strong acids without degrading its electrical response. As an example, we demonstrate that the device can reproducibly detect adsorption of positively charged poly-L-lysine (PLL) on silicon dioxide. We also demonstrate that PLL adsorption and pH can be measured in discrete solutions by scanning the cantilever through parallel, distinct streams within a microfluidic channel array.
Test of negative ion beams from a microwave ion source with a charge exchange canal for accelerator mass spectrometry applications
Authors: Kim S-W. , Schneider R. J. , von Reden K. F. , Hayes J. M. , Wills J. S. C.
Published: July 2002

A test facility has been constructed to evaluate negative ion beams from small gaseous samples for accelerator mass spectrometry applications. The positive ion beams from the microwave ion source are passed into a charge-exchange canal (CXC) where the ions exchange electrons with magnesium vapor and become negatively charged. Positive molecular ions were either neutralized or broken up into atomic ions and neutral atoms and molecules by collision processes. Most of the resulting particles were suspected to be neutrals. In studies with injections of CO2 pulses, the resulting positive and negative C12 current peaks gave a 0.09% yield of C− ions from CO2 molecules, which includes a charge-exchange efficiency of 10%. Since nitrogen does not form a stable negative ion, N14 background is virtually eliminated after the beam goes through the CXC, which is necessary for radiocarbon measurements.
Hydrogen in mechanically prepared nanostructured h-BN: a critical comparison with that in nanostructured graphite
Authors: Wang P. , Orimo S. , Matsushima T. , Fujii H. , Majer G.
Published: July 2002

Nanostructured h-BN was prepared by mechanical milling under hydrogen atmosphere. The hydrogen concentration reaches up to 2.6 mass% after milling for 80 h, and this value corresponds to ca. 35% of that of nanostructured graphite as was previously reported. In addition to the hydrogen desorption starting at about 570 K, nitrogen desorption was also detected at about 700 K. There was no recrystallization phenomenon at least below 1173 K. The dissimilarities on the (de-)hydriding properties between nanostructured h-BN and graphite might be due to the different local electronic structure near the specific defects.
Mixed quantum classical rate theory for dissipative systems
Authors: Liao Jie-Lou , Pollak Eli
Published: July 2002

Numerically exact solutions for the quantum rate of potential barrier crossing in dissipative systems are only possible for highly idealized systems. It is, therefore, of interest to develop approximate theories of more general applicability. In this paper we formulate a mixed quantum classical thermodynamical rate theory for dissipative systems. The theory consists of two parts. The evaluation of a thermal flux and the computation of the classically evolved product projection operator. Since the dividing surface is perpendicular to the unstable normal mode of the dissipative system, we reformulate the theory in terms of the unstable normal mode and a collective bath mode. The influence functional for the thermal flux matrix elements in this representation is derived. The classical mechanics are reformulated in terms of the same two degrees of freedom. The one-dimensional Langevin equation for the system coordinate is replaced by a coupled set of Langevin equations for the unstable normal mode and the collective bath mode. The resulting rate expression is given in the continuum limit, so that computation of the rate does not necessitate a discretization of the bath modes. To overcome the necessity of computing a multidimensional Fourier transform of the matrix elements of the thermal flux operator, we adapt, as in previous studies, a method of Creswick [Mod. Phys. Lett. B 9, 693 (1995)], by which only a one-dimensional Fourier transform is needed. This transform is computed by quadrature. The resulting theory is tested against the landmark numerical results of Topaler and Makri [J. Chem. Phys. 101, 7500 (1994)] obtained for barrier crossing in a symmetric double well potential. We find that mixed quantum classical rate theory (MQCLT) provides a substantial improvement over our previous quantum transition state theory as well as centroid transition state theory computations and is in overall good agreement with the exact results.
Scallop formation and dissolution of Cu–Sn intermetallic compound during solder reflow
Authors: Ma D. , Wang W. D. , Lahiri S. K.
Published: July 2002

In an effort to elucidate the mechanism of scallop formation during reflow of solder/Cu joints in flip-chip and ball-grid array technologies, a planar intermetallic compound layer (mostly Cu6Sn5) was initially formed by solid-state annealing of SnPb/Cu joints at 150 °C for ten days. Upon subsequent reflow, dissolution of the intermetallics and formation of scallops were observed. Detailed investigation has indicated that a thermal grooving process due to dissolution of the intermetallic compound during the initial reflow cycles contributes to the formation of scalloped structures, the morphology of which is dictated by the force equilibrium condition. Upon subjecting the solder joints to several reflows, the average intermetallic layer thickness was found to first decrease and then increase, which is shown to be in good agreement with the prediction made by a model based on simultaneous dissolution and growth of the intermetallic compound during reflow. The kinetics of dissolution-growth process during reflow has also been shown to depend on the initial intermetallic thickness and the shape of the solder cap.
Predicting the Newtonian viscosity of complex fluids from high strain rate molecular simulations
Authors: McCabe Clare , Manke Charles W. , Cummings Peter T.
Published: July 2002

The prediction of viscosity by molecular simulation has been a goal of molecular modeling essentially since its inception. With today’s computing power, the Newtonian or zero shear viscosity of a low molecular weight fluid can easily be determined using equilibrium and nonequilibrium molecular dynamics simulation methods. However, both methods are constrained to systems with relatively short relaxation times that are accessible on the timescale of a molecular dynamics simulation. Here we demonstrate that using a simple scaling relation enables us to predict the Newtonian viscosity of a molecule at any state point for a small fraction of the time that it takes to obtain the same result through nonequilibrium or equilibrium molecular dynamics simulation.
Dependence of power losses on tensile stress for Fe–Si nonoriented steel up to destruction
Authors: Permiakov V. , Dupre L. , Makaveev D. , Melkebeek J.
Published: July 2002

The study of the influence of mechanical stress on magnetic properties of laminated steel is the subject of recent research. In this article the separation of losses is performed and the dependence of total, hysteresis, and excess losses on tensile stress up to destruction is investigated. With increasing of tensile stress, the total and hysteresis losses are first decreasing and then increasing at higher stress. The increase of total losses was found mainly due to the hysteresis losses while the classical and excess components influence the total power losses to a minor extent. For extensive deformation the increase of losses becomes dramatically high.
On the propagation of a two-dimensional viscous density current under surface waves
Authors: Ng Chiu-On , Fu Sau-Chung
Published: July 2002

This study aims to develop an asymptotic theory for the slow spreading of a thin layer of viscous immiscible dense liquid on the bottom of a waterway under the combined effects of surface waves and density current. By virtue of the sharply different length and time scales (wave periodic excitation being effective at fast scales, while gravity and streaming currents at slow scales), a multiple-scale perturbation analysis is conducted. Evolution equations are deduced for the local and global profile distributions of the dense liquid layer as functions of the slow-time variables. When reflected waves are present, the balance between gravity and streaming will result, on a time scale one order of magnitude longer than the wave period, in an undulating water/liquid interface whose displacement amplitude is much smaller than the thickness of the dense liquid layer. On the global scale, the streaming current can predominate and drive the dense liquid to propagate with a distinct pattern in the direction of the surface waves.
The relationship between magnetic interactions and near neighbor interatomic distances in the transition metal sublattice of R(Mn/Fe)6A6 (R=Nd or Sm, A=Ge or Sn)
Authors: Marasinghe G. K. , Han J. , James W. J. , Yelon W. B. , Ali N.
Published: July 2002

The magnetic and crystallographic structures of R(Fe/Mn)6A6 (R=Nd or Sm and A=Ge or Sn) intermetallics have been investigated using x-ray and neutron diffraction techniques and superconducting quantum interference device magnetic measurements. For both stannides (A=Sn) and germanides (A=Ge), the lattice contracts with increasing iron content. In the case of the stannides, substitution of manganese by iron enhances the saturation magnetization and Curie temperature at low iron concentrations (x⩽2) suggesting the presence of an extremely rare occurrence, positive coupling between iron and manganese magnetic moments. In contrast, the magnetic properties of the germanides deteriorate rapidly as manganese is replaced by iron. This difference in the dependence of magnetic properties on the iron content between the germanides and stannides is explained using the Bethe–Slater relationship between near neighbor exchange interactions and interatomic distances. Based on the observations described in this article, it is concluded that the critical near neighbor interatomic distance above which manganese/iron moments couple positively in these intermetallics is ∼2.614 Å.
Magnetoelastic surface acoustic wave attenuation and anisotropic magnetoresistance in Ni3Fe thin films
Authors: Wiegert R. F.
Published: July 2002

The vector magnetic field, H, dependencies of 0.6 GHz magnetoelastic Rayleigh-mode surface acoustic wave (SAW) attenuation, A(H), and dc magnetoresistance of polycrystalline Ni3Fe thin films with thicknesses ranging from 5.3 nm to 179.1 nm were measured and correlated with the samples’ uniaxial anisotropy fields, HK. For all samples, low hysteresis, symmetric, resonance-type A(H) response peaks were observed for H⊥HK. The data indicate that a combination of film-substrate stresses and the very small magnetocrystalline anisotropy coefficients of Ni3Fe result in films with well-defined, low dispersion HK and potential applications for magnetically tunable microwave SAW devices.
A chemical Hamiltonian approach study of the basis set superposition error changes on electron densities and one- and two-center energy components
Authors: Salvador Pedro , Duran Miquel , Fradera Xavier
Published: July 2002

The basis set superposition error-corrected first-order electron densities of several hydrogen bonded complexes of increasing molecular size have been obtained with the Hartree–Fock and density-functional theory versions of the chemical Hamiltonian approach (CHA) methodology. A detailed analysis of the local basis set superposition error (BSSE) effects has been carried out by comparing the uncorrected electron densities and energy components with the CHA ones. Topological analysis of the electron density through the atoms in molecules theory is used in order to obtain a quantitative measure of the BSSE effects in terms of the characterization of the critical points of the electron density. Density difference isocontour maps are also depicted in order to show the local electron density redistributions induced by the BSSE-correction. We show that the effects of the BSSE are common for all the complexes studied, namely water dimer, formic acid dimer and uracil–water complex. The formic acid dimer and uracil–water density difference maps at frozen geometry reveal that the effects of the BSSE do not extend significantly beyond the atoms involved in the interaction and their first neighbors. The main redistribution effects are not strictly localized on the intermolecular region and mostly take place in the valence shells of the heavy atoms directly involved in the intermolecular interaction. These trends are also confirmed by means of an energy decomposition analysis performed at the Hartree–Fock level of theory with the recently proposed chemical energy component analysis (CECA) method. In agreement to previous results, we found that inclusion of diffuse functions is of utmost importance in order to minimize the magnitude of the BSSE. However, both the electron density difference maps and the CECA analysis confirm that the local effects of the BSSE are very different when diffuse functions are present in the calculation.
Resonant-cavity InGaN multiple-quantum-well green light-emitting diode grown by molecular-beam epitaxy
Authors: Naranjo F. B. , Fernández S. , Sánchez-Garcı́a M. A. , Calle F. , Calleja E.
Published: July 2002

A resonant-cavity InGaN/GaN multiple-quantum-well (MQW) light-emitting diode, incorporating a semitransparent Bragg reflector, has been grown by molecular-beam epitaxy. A 30% reflectivity AlGaN/GaN distributed Bragg reflector and a Ni coating were used as bottom and top mirrors, for backside emission. High-resolution x-ray diffraction data reveal high-quality interfaces in both the MQW and the Bragg mirror. Room-temperature electroluminescence is centered at 507 nm, with an estimated output power of 5 μW at 20 mA. The external efficiency is increased by a factor of 12 compared to similar structures without resonant cavity.
Analytical current–voltage model for polycrystalline-silicon thin-film transistors
Authors: Kimura Mutsumi , Takizawa Teruo , Inoue Satoshi , Shimoda Tatsuya
Published: July 2002

An analytical current–voltage model has been developed for polycrystalline-silicon thin-film transistors. This model is based on the Poisson equation and the carrier density equation on the condition that the active layer is intrinsic and fabricated on the insulator substrate. The surface state density was determined, and it was found that the conventional method underestimates this density.
Integral equation theory of Lennard-Jones fluids: A modified Verlet bridge function approach
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

An integral equation theory for the Lennard-Jones fluid is investigated using a proposed new closure based on a modification of the Verlet-modified bridge function, which has been known to be very successful for hard body fluids. The structural and thermodynamic properties of the Lennard-Jones fluid calculated from this integral equation theory over a wide range of temperature and density show very good agreement with those obtained from simulation or from equation of state derived from simulation data. The gas–liquid phase diagram predicted by the present integral equation theory is shown to agree quite well with the corresponding simulation results.
Classical dynamics of dissociative adsorption for a nonactivated system: The role of zero point energy
Authors: Busnengo H. F. , Crespos C. , Dong W. , Rayez J. C. , Salin A.
Published: July 2002

We present dissociative adsorption probabilities of H2 on Pd(111) computed with the classical trajectory method. We perform both classical (C) and quasiclassical (QC) calculations, the latter including, by contrast with the former, the initial zero point energy (ZPE) of H2. We analyze in detail the role played by the ZPE and demonstrate the strong and weak points of both C and QC calculations. We show that ZPE is crucial in accelerating the molecules toward the surface through vibrational softening. However, at low energies, dynamic trapping is quenched in QC calculations by processes of vibration to rotation energy transfer that would be associated with closed channels in a quantum approach. In this study we use a new representation of the H2/Pd(111) potential energy surface (obtained by interpolation of ab initio data) with a significantly better accuracy in the entrance channel region which plays a decisive role in the dissociation dynamics.
Thick inductive cross shaped metal meshes
Authors: Möller K. D. , Sternberg O. , Grebel H. , Lalanne Philippe
Published: July 2002

Experimental data for inductive cross shaped metal meshes with periodicity constant g=20 μm have been reproduced by the Micro-Stripes simulation program for various thicknesses. A similar calculation has been performed with the Fourier modal method for cross shaped meshes with periodicity constant g=1 μm. The transmittances of all these meshes show similar resonance peaks and the same dependence on thickness. A rudimentary coupled oscillator model describes very well the dependence on thickness of the peaks, suggesting that the transmittance of all metal meshes in the short and long wavelength region are excited surface waves coupled by waveguide modes of the openings.
Rotors produced and driven in laser tweezers with reversed direction of rotation
Authors: Galajda Péter , Ormos Pál
Published: July 2002

Micrometer size rotors are produced by light induced polymerization of light curing resin. The propeller shaped rotors trapped in laser tweezers are rotated by the trapping light. A rotor is built that changes the direction of rotation by changing its position relative to the focus. The design is based on the very high numerical aperture of the focused light in the tweezers: the light has a large average component of momentum pointing perpendicular to the optical axis, in the opposite direction before and behind the focus. This perpendicular component drives the rotation and the direction changes when the rotor is moving around the focus.
Image of second harmonic emission generated from ponderomotively excited plasma density gradient
Authors: Mori M. , Takahashi E. , Kondo K.
Published: July 2002

Second harmonic generation from the plasma density gradient induced by ponderomotive force was studied. Images of the second harmonic emission at the focal point, where the intensity was above 1017 W/cm2, were observed. These images strongly depended on the polarization of the pump pulse. The polarization of the second harmonic was the same as that of the pump pulse. Experimental results were well explained by the theoretical model.
Space degradation of multijunction solar cells: An electroluminescence study
Authors: Zazoui M. , Bourgoin J. C.
Published: July 2002

We propose the use of electroluminescence to measure irradiation-induced degradation of multijunction photovoltaic cells in order to probe the degradation of each individual junction. We illustrate the technique by studying the decrease of the luminescence of GaInP, GaAs and Ge junctions of a triple-junction solar cell produced by 1 MeV electrons for fluences corresponding to space conditions.
Steady states in the twin-well potential oscillator: Computer simulations and approximate analytical studies
Authors: Szemplińska-Stupnicka Wanda , Rudowski Jerzy
Published: July 2002

The paper is focused on the phenomena of various steady-state oscillations exhibited by the twin-well potential system. Regions of existence of different attractors in the system parameter domain are examined and a picture book of different steady states for fixed damping and forcing is presented: 20 different combinations of single or coexisting, small orbit or large orbit, periodic and chaotic attractors are displayed. Computer simulations are followed by an approximate analytical analysis: A study of various forms of instability of periodic solutions gives close form approximate criteria for occurrence of T-periodic small orbit and large orbit oscillations, and for cross-well chaos.