bluuebunny/crossref_metadata_2025 huggingface dataset:
An assumption-violating application of the Lawrance–Knight deconvolution procedure: A retrieval of electronic coupling mechanisms underlying complex spectra
Authors: Altunata Selen , Field Robert W.
Published: July 2002

The Lawrance–Knight (L–K) deconvolution method is a spectral inversion scheme which allows one to relate an absorption or a fluorescence spectrum to the energies and couplings of the zero-order states via analytical expressions. In order to obtain accurate results, the L–K method can only be applied to spectra that arise from one precisely defined zero-order picture. Namely, a single bright state must be coupled directly to a background of noninteracting dark states, the “direct coupling model.” In most situations, the zero-order picture that gives rise to a particular absorption or a fluorescence spectrum is not known a priori. Nonetheless, it is typically assumed that the zero-order circumstances governing the spectral intensities are as described above, and the L–K method has been applied indiscriminantly to the spectra of a variety of systems such as pyrazine, acetylene, and naphthalene to extract zero-order parameters. We show here that if the L–K algorithm is applied to spectra where the underlying zero-order picture departs from the direct coupling limit, the resulting output has characteristic qualitative and quantitative features that reflect this situation. By applying the L–K method to a series of simulated spectra, we recover from a pattern of patterns new information about the couplings among the dark states. We have specifically considered the alternative picture of a single bright state coupled to a background of non-interacting states by a doorway-mediated mechanism. We demonstrate here that the L–K algorithm can be employed to distinguish between the contrasting doorway-mediated and direct coupling schemes, and also to obtain coupling matrix elements.
Fast spheroidal multipole imaging of elementary magnetic sources on the axis
Authors: Kildishev Alexander V. , Nyenhuis John A.
Published: July 2002

The multipole image (MI) is the set of coefficients in a harmonic expansion of the scalar magnetic potential of a magnetic source. Compared to the common spherical harmonics, spheroidal harmonics provide an improved description of the field near an elongated source. The total (spherical or spheroidal) MI can be retrieved using a superposition of elementary source images. This article presents fast two-term recursive formulae for multipole imaging of an elementary dipolar source on the axis. These results are compared to one-term recursions for the spherical MI. Another useful result is formulae linking the spherical and spheroidal MI by use of addition theorems and expansions of the Green function. Although it is possible to obtain the spheroidal MI from the spherical MI, perfect accuracy for a complex elongated source is not possible. In contrast, the spherical MI, which is appropriate for the magnetic field at the remote region, is accurately generated from the spheroidal MI, which provides the most precise description at the near zone.
Observation of collision induced state-to-state energy transfer in electronically and highly rotationally excited NH2
Authors: Wilhelm Ralf , Lindner Jörg
Published: July 2002

State-to-state energy transfer of NH2 in its excited state Ã 2A1 is investigated with time-resolved Fourier transform emission spectroscopy. Originating from collisions with NH3, rovibrational energy transfer in NH2(Ã 2A1) with energy separations |ΔE|<260 cm−1 and in multiples of ΔE≈−1050 cm−1 is observed. Based on the experimental determination of relative transition probabilities, absolute state-to-state rate constants are derived. Collisional changes in the rotational quantum number range from −3 to +4. The analysis of the time dependence of the levels populated by rovibrational energy transfer shows that this variety is not the result of secondary collisions.
Measurement of the recoalescence flux into the rear of a Taylor bubble
Authors: Delfos R. , Rops C. M. , Kockx J. P. , Nieuwstadt F. T. M.
Published: July 2002

Most of the theoretical models on vertical slug flow assume the mass balance of a Taylor bubble to depend only on the incoming gas flux at the top of the Taylor bubble and on the outgoing entrainment flux at the bottom. This means that the recoalescence flux, which is defined as the fraction of the entrainment flux that coalesces back into the bubble, is neglected. Only in Fernandes et al. [AIChE J. 29, 981 (1983)] is a model proposed for this recoalescence flux but their model has never been verified by measurements. Therefore, we set out in the present research to measure and quantify the recoalescence flux. Our experiments have been carried out in a recirculating flow facility with a vertical cylindrical test section with inner diameter Di=100 mm. In this test section a Taylor bubble is kept at a fixed vertical position by a constant downward liquid flow ΨL. A continuous stream of small helium bubbles is injected into the wake of the Taylor bubble. The recoalescence flux is then determined by measuring the concentration of helium in the Taylor bubble. Our experiments show that there is a recoalescence flux and that in general it cannot be neglected in the mass balance of the Taylor bubble. The total gas loss from the Taylor bubble, Ψin, and the recoalescence flux Ψrec increase both strongly with the Taylor bubble length, LTB. The fraction of entrained bubbles that recoalesces back into the Taylor bubble increases from 10% of the entrainment flux at LTB=60 cm (λG=Ψin/ΨL=4%) to 45% at LTB=91 cm (λG=11%).
Self-aligned double-gate single-electron transistor derived from 0.12-μm-scale electron-beam lithography
Authors: Nishiguchi K. , Oda S.
Published: July 2002

A single-electron transistor (SET) with two gates was fabricated via the self-aligned evaporation of Al into a trench structure comprised of Si and SiO2. The initial trench, which was comparable to 0.12 μm lines and defined by electron-beam lithography, was reduced to 0.05×0.02 μm by a slightly anisotropic etching characteristic. These processes allow for the production of SET devices using current silicon fabrication techniques. The simultaneous formation of two gates allows for one gate to be used to control the background charge of each device. The shift of Coulomb oscillation peaks was clearly shown by controlling the second gate bias. An inverter logic operation at a temperature of 5 K with a gain of 1.3 was obtained. These characteristics indicate that such SET logic devices, based on a combination of the good performance of the Al SET and the high level of control of the fabrication of Si technology, have considerable potential for future use.
Local investigation of thin insulating barriers incorporated in magnetic tunnel junctions
Authors: Dimopoulos T. , Da Costa V. , Tiusan C. , Ounadjela K. , van den Berg H. A. M.
Published: July 2002

In this work we study properties of very thin insulating Al oxide films, used as barriers in magnetic tunnel junctions. For the small barrier thicknesses required for technological applications (∼10 Å), the presence of pinholes (direct contact between the ferromagnetic metals through the barrier), or oxidation inhomogeneities, are the major factors for vanishing of the tunnel magnetoresistance effect. We have produced and characterized very thin, pinhole-free Al oxide layers, incorporated in magnetic tunnel junctions. The transport properties of the different barriers were analyzed by barrier impedance scanning microscopy and were correlated with the magnetotransport properties of the patterned microsized junctions.
Theory of time dependent optical linewidths in supercooled liquids
Authors: Richert Ranko
Published: July 2002

The time resolved inhomogeneous broadened line shape is derived from analytical theory for a liquid which exhibits spatial heterogeneity regarding the solvent response times. The results demonstrate that it is the heterogeneous nature of supercooled liquids which gives rise to asymmetric lines and to a maximum in the linewidth as a function of time while the average emission energy experiences a redshift. The time dependence of the calculated and of the observed linewidths agree quantitatively, but differ significantly from the behavior expected for systems with homogeneous dynamics.
Laser pumped He4 magnetometer
Authors: Gilles H. , Hamel J. , Chéron B.
Published: July 2002

We describe the realization of a He4 magnetometer based on optical pumping with an optical frequency modulated laser diode light. Performances (amplitude and frequency spatial isotropy and sensitivity) of our device are presented and compared with those of other scalar magnetometers.
Fluctuations and thermodynamics properties of the constant shear strain ensemble
Authors: Zhou Z.
Published: July 2002

We develop the statistical mechanics of a pair of new ensembles called the constant shear strain ensembles that include the uniform dilation ensemble used frequently in computer simulations. We present a direct calculation of fluctuation formulas for the elastic constants, the specific heat, and the thermal expansion tensor in these new ensembles.
Novel technique for low-jitter dual-laser synchronization in a thin film deposition system
Authors: Mukherjee Pritish , Cuff John B. , Witanachchi Sarath
Published: July 2002

The need for precise laser pulse synchronization in a dual-laser ablation system to optimize the quality of the deposited thin films has been previously demonstrated. We present, in this article, a novel technique for the synchronization of an excimer and a CO2 laser with synchronization having a temporal fluctuation (jitter) of less than ±14 ns. This is several times better than the best precision of temporal synchronization possible using traditional electronic techniques and is crucial for the application of dual-laser ablation in the manufacturing of thin films. Evidence for reproducibility in the ablation of targets using this system is presented by analyzing the initial stages of the ablated plasma using a time-gated charge coupled device imaging system.
Glasses and proteins: Similarities and differences in their spectral diffusion dynamics
Authors: Schlichter J. , Friedrich J.
Published: July 2002

We compare the spectral diffusion dynamics of resorufin doped glycerol/H2O- and glycerol/D2O-glass with the respective dynamics of a chromoprotein in the same glass at 4.2 K. Spectral diffusion broadening of photochemical holes is measured over almost four orders of magnitude in time. In all samples there are strong aging phenomena. Resorufin in deuterated water/glycerol is reasonably well-described by the two level system (TLS) model. In the protonated glass, the TLS model does not seem to describe the experiments reasonably well. In the protein sample it totally fails.
Raman modes of the two-dimensional tetragonal polymeric phase of C60 under high pressure
Authors: Arvanitidis J. , Meletov K. P. , Papagelis K. , Ves S. , Kourouklis G. A. , Soldatov A. , Prassides K.
Published: July 2002

The effect of high hydrostatic pressure, up to 12 GPa, on the phonon spectrum and the material stability of the two-dimensional (2D) tetragonal C60 polymer have been studied by means of Raman spectroscopy in the frequency range 100–2000 cm−1. A number of Raman modes appear in the spectrum for pressures above ∼1.4 GPa, whose intensities increase with pressure. The pressure coefficients of the majority of the phonon modes change gradually to lower values for pressures around 4.0 GPa. The deformation of the C60 molecular cage along with the change of the material to a more isotropic state (as far as its elastic properties are concerned) resulting from the application of high pressure may be causing the observed effects in the Raman spectra. These effects are reversible upon pressure release and therefore the material is stable in the pressure region investigated.
Stochastic pitch angle diffusion due to electron-whistler wave–particle interactions
Authors: Wykes W. J. , Chapman S. C. , Rowlands G.
Published: July 2002

In the Earth’s magnetosphere, electron–whistler mode wave–particle interactions are a candidate mechanism for auroral precipitation via electron phase space diffusion. Of particular interest are stochastic interactions between relativistic electrons and (as often observed) waves of more than one wave number. It can be shown that the interaction between electrons and two oppositely directed monochromatic whistlers is stochastic. Once a threshold is exceeded, stochastic trajectories exist in addition to regular orbits (Kolmogorov–Arnold–Moser, or KAM, surfaces) near resonance, and here their corresponding pitch angle diffusion is estimated. The treatment is extended to consider broad band whistler wave packets and it is shown that the stochastic diffusion mechanism is again present for interactions with one or two wave packets. The pitch angle diffusion coefficient is estimated from the dynamics of stochastic electrons. For wave amplitudes consistent with planetary magnetospheres, such as at the Earth and Jupiter, pitch angle diffusion due to stochastic interactions occurs on fast (millisecond) time scales resulting in significant increases in the pitch angle diffusion coefficient.
Improved electrical properties of (Pb, La)TiO3 thin films using compositionally and structurally compatible LaNiO3 thin films as bottom electrodes
Authors: Bao Dinghua , Wakiya Naoki , Shinozaki Kazuo , Mizutani Nobuyasu , Yao Xi
Published: July 2002

Homogeneous LaNiO3 thin films were grown on thermally oxidized silicon (SiO2/Si) substrates by a sol-gel technique, for the subsequent sol-gel deposition of (Pb, La)TiO3 thin films, under the assumption that the structural and compositional compatibility between ferroelectric films and bottom electrodes could lead to enhanced electrical properties of ferroelectric thin films. In this work, the LaNiO3 films served three functions: the first was used as bottom electrodes for the fabrication of integrated ferroelectric devices on Si due to their low resistivity; the second was used as a seeding layer, promoting perovskite phase formation due to their structural compatibility with ferroelectric films; and the third was to suppress the composition diffusion between ferroelectric films and bottom electrodes due to their composition compatibility. The experimental results demonstrated that (Pb, La)TiO3 films prepared on the LaNiO3/SiO2/Si substrates had excellent electrical properties. The dielectric constant and dielectric loss were 1468 and 0.033, respectively. The dielectric constant is rather high among the values reported for (Pb, La)TiO3 thin films. The remanent polarization and coercive field were 4.24 μC/cm2 and 23.2 kV/cm, respectively.
Femtosecond carrier dynamics in an asymmetrically excited GaAs photoconductive switch
Authors: Cummings M. D. , Holzman J. F. , Elezzabi A. Y.
Published: July 2002

We present a detailed analysis of the field screening and carrier dynamics which exist in the high-field region of an asymmetrically excited coplanar transmission line. Through time-resolved reflectivity measurements, it is found that the ballistic acceleration of carriers, and subsequent field screening, dominate the ultrashort electrical pulse generation. In addition, the formation of an instantaneous macroscopic polarization and the creation of coherently coupled plasmon–phonon modes are found to effect the electric field screening response. The results are in agreement with the field screening picture of edge-illuminated photoconduction and suggest that pulses as short as 110 fs can be generated.
Sticking of HCl to ice at hyperthermal energies: Dependence on incidence energy, incidence angle, and surface temperature
Authors: Al-Halabi A. , Kleyn A. W. , Kroes G. J.
Published: July 2002

We present calculations on the sticking of hyperthermal HCl to the basal plane (0001) face of ice Ih at normal and off-normal incidence. The dependence of the sticking probability on the incidence energy (Ei), the angle of incidence (θi), and the surface temperature (Ts) is discussed. Two sticking mechanisms are observed. For θi⩽30°, penetration of the (0001) face is possible at an energy of about 100 kJ/mol, which is an order of magnitude lower than energies for which the penetration of metallic or covalently bonded crystals by atoms becomes possible. This possibility is due to the open structure of single-crystalline ice Ih, in which the water molecules are arranged in superimposed hexagons, forming shafts running perpendicular to the ice surface. The penetration mechanism is operative for the entire range of Ts studied (110–190 K). The second sticking mechanism, i.e., adsorption, occurs for all Ei, θi, and Ti. For θi<45°, the adsorption probability increases with θi as would be expected, because the normal component of Ei that needs to be transferred to the surface for sticking to occur scales with cos2 θi. However, for θi⩾45°, the adsorption probability decreases with θi. The energy transfer from HCl to the ice surface and the energy dissipation within the surface are found to be fast and efficient at normal incidence.
Memory effect in the current–voltage characteristic of a low-band gap conjugated polymer
Authors: Taylor D. M. , Mills C. A.
Published: July 2002

Diodes formed by electrodeposition of the low-band gap polymer poly(4-dicyano methylene-4H-cyclopenta[2,1-b:3,4-b′]dithiophene), onto glass slides coated with indium tin oxide (ITO) and furnished with evaporated aluminum counterelectrodes exhibit a reversible bistability in their current–voltage (I–V) characteristics. Applying +5 V to the ITO electrode induces a “high” conductance state while applying −5 V induces a “low” conductance state. The effect is identical in most respects to recent observations in diodes formed from thin films of chromium-doped SrZrO3 sandwiched between SrRuO3 and gold electrodes. A number of mechanisms are discussed but the evidence points to the controlling influence of an interfacial depletion layer at the ITO–polymer interface. It is also shown that the high capacitances associated with such layers can lead to higher than expected displacement currents being generated during the automated collection of I–V data. The presence of such currents distorts the I–V characteristics in the low-bias low-current regime.
Experimental studies of the propagation of electrostatic ion perturbations by time-resolved laser-induced fluorescence
Authors: Bachet G. , Skiff F. , Doveil F. , Stern R. A.
Published: July 2002

Effects induced by the propagation of several kinds of electrostatic perturbation in a low-density collisionless argon plasma are observed with space, time, and velocity-resolved laser-induced fluorescence (LIF). The propagation of strong self-organized ion structures is observed and the associated electric field is determined. Snap shots of the ion phase space with a time resolution of 2 μs can be reconstructed from the experimental data. All the terms of the kinetic equation can also be determined from the data. A one-dimensional (1D) numerical simulation reproduces qualitatively the experimentally observed ion phase space behavior.
Compaction wave profiles: Simulations of gas gun experiments
Authors: Menikoff Ralph
Published: July 2002

Mesoscale simulations of a compaction wave in a granular bed of HMX have been performed. The grains are fully resolved in order that the compaction, i.e., the porosity behind the wave front, is determined by the elastic-plastic response of the grains rather than by an empirical law for the porosity as a function of pressure. Numerical wave profiles of the pressure and velocity are compared with data from a gas gun experiment. The experiment used an initial porosity of 36%, and the wave had a pressure comparable to the yield strength of the grains. The profiles are measured at the front and back of the granular bed. The transit time for the compaction wave to propagate between the gauges determines the wave speed. The wave speed depends on the porosity behind the wave and is affected by the strength model. The yield strength needed to match the experimental wave speed is discussed. Analysis of the lead wave through the granular bed, based on impedance matches using the Hugoniot loci, indicates that the compaction wave triggers a small amount of burn, less than 1% mass fraction, on the microsecond time scale of the experiment.
Highly oriented lead zirconium titanate thin films: Growth, control of texture, and its effect on dielectric properties
Authors: Kalpat S. , Uchino K.
Published: July 2002

Highly oriented lead–zirconium–titanate (PZT) thin films have been grown by reactive rf-magnetron sputtering using multielemental metallic targets. The effects of deposition parameters on the film composition and texture evolution have been established. The partial pressure of oxygen (O2) and the heating rates during postdeposition anneal have been identified as critical parameters for the growth of highly oriented (111) and (100) PZT films. In order to control the growth of (111) and (100) orientation it was important to understand the interplay between these two parameters. Texture evolution mechanisms for the growth of oriented films have been proposed based on the effect of O2/argon flow and heating rates on the crystallization kinetics. The effects of crystal orientation on the dielectric and ferroelectric properties of PZT films have also been investigated.
Facility to measure solar cell ac parameters using an impedance spectroscopy technique
Authors: Kumar R. Anil , Suresh M. S. , Nagaraju J.
Published: July 2002

An experimental setup has been developed to measure the solar cell ac parameters using an impedance spectroscopy technique. It consists of an electrochemical interface to set the dc bias voltage and to apply the signal voltage across the solar cell and a frequency response analyzer to generate the excitation signal with varying frequency and analyze the result. The setup is calibrated with a standard RC network and measured an error of ±4% (maximum). Solar cell capacitance, parallel resistance and series resistance are calculated from the measured data.
Effects of singlet and triplet energy transfer to molecular dopants in polymer light-emitting diodes and their usefulness in chromaticity tuning
Authors: Higgins R. W. T. , Monkman A. P. , Nothofer H.-G. , Scherf U.
Published: July 2002

Efficient, white emission with Commission Internationale de L’Eclairage coordinates of 0.33, 0.42 is demonstrated from polymer light-emitting diodes operating at high brightness. An amino endcapped polyfluorene derivative doped with rubrene was used as the active layer, in combination with a poly(3,4-ethylenedioxythiophene) hole transport layer. Luminescence reaches 14950 Cd/m2 at a current density of 150 mA/cm2, and the maximum external quantum efficiency measured 1.05% for a bias voltage of 9.0 V. We observe a fall in external quantum efficiency relative to the undoped device, which we ascribe to an increased singlet–triplet annihilation rate at the dopant sites.
Traveling wave electro-optic phase modulators based on intrinsically polar self-assembled chromophoric superlattices
Authors: Zhao Y.-G. , Wu A. , Lu H.-L. , Chang S. , Lu W.-K. , Ho S. T. , van der Boom M. E. , Marks T. J.
Published: July 2002

Traveling-wave electro-optic modulators based on chromophoric self-assembled superlattices (SASs) possessing intrinsically polar microstructures have been designed and fabricated. Although the thickness of the SAS layer is only ∼150 nm, a π-phase shift is clearly observed. From the measured Vπ value, the effective electro-optic coefficient of the SAS film is determined to be ∼21.8 pm/V at an input wavelength of 1064 nm.
Electron beam induced current and cathodoluminescence study of proton irradiated InAsxP1−x/InP quantum-well solar cells
Authors: Walters Robert J. , Summers G. P. , Messenger S. R. , Romero M. J. , Al-Jassim M. M. , Garcia R. , Araujo D. , Freundlich A. , Newman F. , Vilela M. F.
Published: July 2002

The effects of proton irradiation on strained InAsxP1−x/InP-based quantum well solar cells (QWSCs) have been investigated by the electron beam induced current (EBIC) and cathodoluminescence (CL) techniques. From analysis of the EBIC data, capture rates within the quantum well region have been estimated, from which the open circuit voltages of the cells were calculated and shown to agree well with the measured values. Diffusion lengths have been estimated from analysis of both the EBIC and CL measurements. The location of the energy levels of proton-induced defects and their effectiveness as nonradiative recombination centers have been determined from Arrhenius plots of the total CL intensity emitted from the quantum wells following irradiation. The results suggest that deeper and narrower quantum wells increase the sensitivity of QWSCs to radiation damage.
Copolymer adsorption on planar substrates with a random distribution of chemical heterogeneities
Authors: Genzer Jan
Published: July 2002

We use a 3D SCF model of polymer adsorption to investigate the adsorption of A-B copolymers from A-B/homopolymer A mixtures onto planar substrates composed of two chemically distinct randomly distributed sites, one of which has a preferential affinity for the B segments of the copolymer. Our results show that when the chemically heterogeneous substrate motifs are recognized by the copolymer, the copolymers can transcript them with a relatively high fidelity into three dimensions. The way the surface motif is transferred is strongly dictated by the copolymer sequence. We show that block copolymers are capable of detecting small clusters of the substrate adsorption sites. The fidelity of the pattern shape and the distance from the substrate to which the pattern gets transferred increases with decreasing the length of the adsorbing block of the copolymer. Our results also indicate that increasing (i) the interactions between the copolymer adsorbing segments and the “sticky” points at the substrate, and/or (ii) the repulsion between the copolymer segments increases the total adsorbed amount of the copolymer at the mixture/substrate interface but it decreases the fidelity of the substrate chemical pattern transfer into the mixture. We show that, in contrast to the block copolymers, macromolecules with alternating sequence distributions adopt different conformations on random substrates in that they tend to localize at the boundaries between the C/D surface sites, where the substrate chemical pattern more closely matches the sequence distribution of the B stickers along the copolymer. We claim that this feature allows us to use alternating copolymers in situations where one needs to suppress the chemical pattern transfer on such random substrates.
Thermodynamic investigation of the Si7 and Si8 clusters by Knudsen cell mass spectrometry
Authors: Meloni G. , Gingerich K. A.
Published: July 2002

The Knudsen cell mass spectrometric method has been employed to measure the partial pressures of the Si7 and Si8 clusters under equilibrium conditions above liquid silicon, contained in a boron nitride liner inside a graphite Knudsen cell. Gaussian 2 (G2) theory and B3LYP density functional method were employed to determine the geometry, the vibrational frequencies, and the binding energy of the Si8 cluster. From the all-gas analyzed equilibria the following atomization enthalpies, ΔaH0o(Sin), and enthalpies of formation, ΔfH298.15o(Sin), in kJ mol−1, have been obtained: Si7, 2381±36 and 743±36; Si8, 2735±65 and 837±65. Experimental literature values for the electron affinities of Sin(n=3–8) have been combined with present and previous results to obtain the bonding energies for the Sin−(n=3–8) cluster anions. The experimental atomization energies are compared with available theoretical values.
Associating polyelectrolyte solutions: Normal and anomalous reversible gelation
Authors: Potemkin Igor I. , Andreenko Stepan A. , Khokhlov Alexei R.
Published: July 2002

An equilibrium mean-field theory for dilute polyelectrolyte solution with many associating groups per chain is presented. We have shown that in addition to physical gel formation, the aggregation of polyelectrolyte chains in dilute solution can lead to the formation of optimum size clusters. The physical reason for the stabilization of such clusters is discussed. Intramolecular optimum cluster formation (necklace conformation) becomes possible for long or charged enough chains. The study of influence of the number of stickers on phase behavior of associating polyelectrolyte solution enabled us to predict a rather unusual counterintuitive effect: The physical gel formation can be induced by the decrease of the number of stickers (anomalous physical gelation). This effect has pure polyelectrolyte nature and is connected with the translational entropy of mobile counterions.
Quantum correction factors for multiphonon processes in condensed phase vibrational relaxation
Authors: Cherayil Binny J.
Published: July 2002

An influence functional approach is used to determine the role of multiphonon processes in the rates of vibrational relaxation. Relaxation is considered to occur between a pair of coupled harmonic oscillators, representing an excited and a receiving mode on a single polyatomic solute, and a collection of independent harmonic oscillators, representing a solvent reservoir. The interaction between the oscillator pair in the solute is arbitrary and left unspecified, while interactions between solute and solvent are taken to be linear in the solute coordinates but quadratic or cubic in the solvent coordinates. The nonlinearities allow vibrational relaxation to occur through multiple excitations of phonons. Transitions rates for such multiphonon processes are derived, as are quantum corrections to the corresponding classical force correlation functions. The quantum correction factors are also shown to emerge directly from certain terms in the real part of the influence functional.
Ab initio A′1 ground potential energy surface and transition state theory kinetics study of the O(1D)+N2O→2NO, N2+O2(a 1Δg) reactions
Authors: González Miguel , Valero Rosendo , Anglada Josep Maria , Sayós R.
Published: July 2002

An ab initio study of the A′1 ground potential energy surface (PES) of the O(1D)+N2O(X 1∑+) system has been performed at the CASPT2//CASSCF (complete active space second-order perturbation theory//complete active space self-consistent field) level with Pople basis sets. The two reactions leading to 2 NO(X 2∏) [reaction (1)] and N2(X 1∑g+)+O2(a 1Δg) [reaction (2)] products have been investigated. In both reactions a trans-approach of the attacking oxygen to the N2O moiety is found to be preferred, more markedly in reaction (1). For this reaction also a cis-path is feasible and is possibly connected with the trans-path by a transition state placed below reactants. A thorough characterization of the entrance zone has been performed to allow for subsequent kinetics calculations. Fixed angle and minimum energy paths have been constructed and transition state geometries have been refined at the CASPT2 level, thus obtaining approximate structures and frequencies for the latter. From these calculations it can be inferred that both reactions proceed without an energy barrier. Rate constant calculations in the 100–1000 K temperature range based on CASPT2 structures and using the transition state theory yield values in good agreement with experiment for the two reactions, especially when a proper scaling of the energy barriers is performed. Also, for comparative purposes quasiclassical trajectory calculations were performed on reaction (1) in the same temperature range, using a previous pseudotriatomic analytical potential energy surface, obtaining good agreement with experiment.
On the generalized bulk viscosity behavior
Authors: Bertolini D. , Tani A.
Published: July 2002

We have calculated generalized longitudinal, ηL(k), shear, ηS(k), and bulk, ηB(k) viscosity of argon at two thermodynamic states by molecular-dynamics simulation. Bulk viscosity has been evaluated with two independent methods that give results in good agreement. We find that bulk viscosity stays positive at all wave numbers k, and show that negative values of ηB(k) are due to a straightforward (and incorrect) extension to finite k’s of the relation ηB(k)=ηL(k)−4ηS(k)/3, only valid at vanishing k’s.
Magnetic switching of single vortex permalloy elements
Authors: Schneider M. , Hoffmann H. , Zweck J.
Published: July 2002

Magnetic vortices play an important role in the switching behavior of micron- and submicron-sized ferromagnetic elements. We have prepared submicron permalloy elements by a combination of electron-beam lithography and liftoff technique on electron transparent membrane substrates. The magnetization reversal mechanism and the remanent magnetization configuration were observed by means of Lorentz transmission electron microscopy. In remanence, the investigated structures form a vortex configuration. In situ magnetizing experiments showed the possibility of adjusting the sense of magnetization rotation by introducing a slight geometric asymmetry to the otherwise circular nanostructures.
Diffusion-influenced reversible energy transfer reactions between polymers
Authors: Park Pyeong Jun , Lee Sangyoub
Published: July 2002

Diffusion-influenced reversible reactions of polymers are investigated by using the hierarchical reaction–diffusion equations for end-functionalized Rouse chains. It is found that distinct dynamic characteristics of reactive groups over different time scales lead to rich time-dependent behaviors in the reactant number densities, which are totally absent in the small molecular reactions. On time scales smaller than the Rouse relaxation time, the number densities of excited reactants are described by a scaling function, whereas for larger times, they approach the equilibrium values exponentially. A kinetic phase diagram locating regions of distinct behaviors of Stern–Volmer coefficients is presented which can be directly confirmed by experiments.
Inhomogeneous six-vertex model with domain wall boundary conditions and Bethe ansatz
Authors: Korepin V. , Zinn-Justin P.
Published: July 2002

In this note, we consider the six-vertex model with domain wall boundary conditions, defined on an M×M lattice, in the inhomogeneous case where the partition function depends on 2M inhomogeneities λj and μk. For a particular choice of the set of λj we find a new determinant representation for the partition function, which allows evaluation of the bulk free energy in the thermodynamic limit. This provides a new connection between two types of determinant formulas. We also show in a special case that spin correlations on the horizontal line going through the center coincide with the ones for periodic boundary conditions.
Monodisperse microbubbling: Absolute instabilities in coflowing gas–liquid jets
Authors: Gordillo José M. , Gañán-Calvo Alfonso M. , Pérez-Saborid Miguel
Published: July 2002

Contrary to liquid jets, gas jets in coflowing liquid streams are highly unstable. When these gas jets are sufficiently small, their breakup into microbubbles is strongly self-locked at a certain frequency. This provokes a continuous streaming of microbubbles with perfectly homogeneous and controllable size. In this work, we describe theoretically the absolute instability origin of the dynamical behavior of these gas jets surrounded by a coflowing liquid stream, and give the explanation to the extremely elusive nature of a gas ligament.
Bias-voltage-induced asymmetry in nanoelectronic Y-branches
Authors: Worschech L. , Xu H. Q. , Forchel A. , Samuelson L.
Published: July 2002

Pronounced asymmetries of electrical properties are observed in nanoelectronic, symmetric GaAs/AlGaAs Y-branches. Finite voltages Vl and Vr applied to the left- and right-hand side branch reservoir of a symmetric, ballistic Y-branch switching device in push–pull fashion (i.e., Vl=−Vr) lead to a negative output voltage Vs of the floating, central stem reservoir located between the two branches. We explain our observations exploiting the ballistic nature of the electron transport in the device.
Synthesis and magnetostriction of melt-spun Pr1−xTbx(Fe0.6Co0.4)2 alloys
Authors: Tang S. L. , Wang D. H. , Yang S. G. , Jin Z. Q. , Zhang J. R. , Zhang S. Y. , Du Y. W.
Published: July 2002

Pseudobinary Pr1−xTbx(Fe0.6Co0.4)2 (0⩽x⩽0.4) cubic Laves single phases have been synthesized by melt spinning and subsequent annealing. Their structure, magnetic properties and stability have been investigated. The composition, at which the anisotropy of Pr1−xTbx(Fe0.6Co0.4)2 is compensated, is close to x=0.1. The spontaneous magnetostrictions λ111 of Pr0.9Tb0.1(Fe0.6Co0.4)2 and Pr0.8Tb0.2(Fe0.6Co0.4)2 are larger than 1500×10−6 and 1900×10−6, respectively. Pr1−xTbx(Fe0.6Co0.4)2 (0.1⩽x⩽0.4) ribbon-based materials with 3% epoxy resin combine high magnetostriction with significant magnetic coercivity. Pr0.9Tb0.1(Fe0.6Co0.4)2 is a promising magnetostrictive material.
Interface properties and in-plane linear photoluminescence polarization in highly excited type-II ZnSe/BeTe heterostructures with equivalent and nonequivalent interfaces
Authors: Zaitsev S. V. , Maksimov A. A. , Kulakovskii V. D. , Tartakovskii I. I. , Yakovlev D. R. , Ossau W. , Hansen L. , Landwehr G. , Waag A.
Published: July 2002

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Structures with four possible configurations of normal and inverted interfaces have been investigated. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. The comparison of interfaces formed by the growth on anion (Se, Te) and cation (Zn, Be) terminated layers has shown that the latter demonstrate a significantly higher nonradiative recombination rate. In agreement with the quantum well (QW) symmetry, the photoluminescence (PL) of the structures with nonequivalent normal and inverted interfaces is highly linearly polarized both at low and high excitation densities. Unexpectedly, a similarly strong PL polarization has been found for structures with equivalent interfaces up to carrier densities of 1012 cm−2 per QW. The polarization is explained by a built-in electric field, it decreases with increasing carrier concentration due to screening of the electric field. Finally, we have found that the PL polarization degree at interfaces with Be–Se and Zn–Te bonds amounts to about 50% and ≈70%, respectively.
Improving efficiency by balancing carrier transport in poly(9,9-dioctylfluorene) light-emitting diodes using tetraphenylporphyrin as a hole-trapping, emissive dopant
Authors: Campbell Alasdair J. , Bradley Donal D. C. , Virgili Tersilla , Lidzey David G. , Antoniadis Homer
Published: July 2002

Unbalanced carrier transport is known to strongly affect the efficiency of polymer light-emitting diodes. Here, we report the results of time-of-flight (TOF), current density–voltage, and electroluminescence (EL) quantum efficiency measurements on single-layer poly(9,9-dioctylfluorene) (PFO) devices doped with the red-emitter tetraphenylporphyrin (TPP). TOF shows that PFO is a unipolar conductor, with hole transport much better than electron transport. At a field of 5×105 V/cm, a nondispersive hole mobility of 4×10−5–5×10−4 cm2/V s, dependent on sample morphology, is obtained. Upon the addition of 5% by weight TPP, hole transport becomes as highly dispersive as electron transport, having no measurable average mobility. This results in a decrease in the current for a given applied bias but an increase in the external EL quantum efficiency. TPP acts as a strong hole trap, reducing the dominant hole current and producing more balanced carrier transport. At TPP concentrations above 6%, the device characteristics start to revert to those found at lower TPP concentrations. This is due to the onset of efficient hole transport between the dopant molecules that reestablishes a transport imbalance.
How well do Car–Parrinello simulations reproduce the Born–Oppenheimer surface? Theory and examples
Authors: Tangney P. , Scandolo S.
Published: July 2002

We derive an analytic expression for the average difference between the forces on the ions in a Car–Parrinello simulation and the forces obtained at the same ionic positions when the electrons are at their ground state. We show that for common values of the fictitious electron mass, a systematic bias may affect the Car–Parrinello forces in systems where the electron–ion coupling is large. We show that in the limit where the electronic orbitals are rigidly dragged by the ions the difference between the two dynamics amounts to a rescaling of the ionic masses, thereby leaving the thermodynamics intact. We study the examples of crystalline magnesium oxide and crystalline and molten silicon. We find that for crystalline silicon the errors are very small. For crystalline MgO the errors are very large but the dynamics can be quite well corrected within the rigid-ion model. We conclude that it is important to control the effect of the electron mass parameter on the quantities extracted from Car–Parrinello simulations.
On the thickness expansion of the electric potential in the determination of two-dimensional equations for the vibration of electroded piezoelectric plates
Authors: Tiersten H. F.
Published: July 2002

In the derivation of two-dimensional equations for the vibration of piezoelectric plates from variational equations, expansions of the mechanical and electrical variables in the thickness coordinate are employed. If the major surfaces of the plate are electroded and the electric potential is expanded in functions of the thickness coordinate which do not vanish at the electrodes, the variations of the different orders of the expansion potentials are not independent because the electric potential must satisfy constraint conditions at the electrodes where it is independent of position. In this work, the electric potential is expanded in functions of the thickness coordinate which do not vanish at the surface electrodes and the constraint conditions are included by means of the method of Lagrange multipliers. The resulting piezoelectric plate equations are obtained along with an integral condition on the Lagrange multipliers over the electrodes, which results in the equation for the current through the electrodes. It is shown that the elimination of the Lagrange multipliers results in a reduced system of electrostatic plate equations and associated edge conditions, which is easier to use.
Continuous reel-to-reel measurement of critical currents of coated conductors
Authors: Grimaldi G. , Bauer M. , Kinder H.
Published: July 2002

We demonstrate that magnetization measurements by Hall probe arrays are well suited for the reel-to-reel determination of critical currents along long lengths of coated conductors. The technique is contact-free, fast, simple, and it has a high spatial resolution. A quantitative measure of the critical current as a function of position was obtained by fitting a simple critical state model to the field distribution. The technique is demonstrated for YBa2Cu3O7 films on single-crystal substrates and then applied to samples of coated conductors.
Stimulated emission, excited state absorption, and laser modeling of the Nd3+:Ca3Ga2Ge3O12 laser system
Authors: Romero J. J. , Jaque D. , Ramos-Lara F. , Boulon G. , Guyot Y. , Caldiño U. , Garcı́a Solé J.
Published: July 2002

We report on a full spectroscopic characterization of the Nd3+ ion doped Ca3Ga2Ge3O12 garnet laser host for different ion concentrations ranging from 2.5×10−2 up to 2 at. %. The Judd–Ofelt formalism has been applied, from which the stimulated emission cross section corresponding to the F3/24→I11/24 laser channel has been determined. Excited state absorption cross section in the 1020–1090 nm region has been measured by using two beam spectroscopy, demonstrating that the 1060 and 1065 nm laser emissions are not affected by excited state absorptions. Finally, once the basic parameters are known, we have applied the mode overlap formalism in order to determine the Nd3+ concentration and crystal length which optimize the laser performance of an end diode pumped laser system. We have concluded that the maximum output power is reached when the Nd3+ concentration and crystal length are around 0.75 at. % and 0.6 mm, respectively
Graphical peak analysis method for determining densities and emission rates of traps in dielectric film from transient discharge current
Authors: Matsuura Hideharu , Hase Takashi , Sekimoto Yasuhiro , Uchikura Masaharu , Simizu Masaru
Published: July 2002

The purpose of this study is to propose and test the graphical peak analysis method [discharge current transient spectroscopy (DCTS)] for determining the densities and emission rates of traps in a dielectric thin film from the transient discharge current Idis(t) in a capacitor at a constant temperature, different from thermally stimulated current (TSC). It is theoretically demonstrated that DCTS can distinguish among traps with close emission rates. Experimentally, the densities and emission rates of five traps in Pb(Zr, Ti)O3 thin films are determined using DCTS. Here, these five emission rates are between 1×10−3 s−1 and 7×10−2 s−1. One kind of trap, which was determined by TSC under the assumption of one emission rate, is found to be distinguished into five kinds of traps with close emission rates by DCTS.
Dynamic high-strain-rate mechanical behavior of microstructurally biased two-phase TIB2+AL2O3 ceramics
Authors: Kennedy Greg , Ferranti Louis , Russell Rod , Zhou Min , Thadhani Naresh
Published: July 2002

The dynamic high-strain-rate behavior of two-phase TiB2+Al2O3 ceramics with biased microstructures was investigated in this study. The microstructural bias includes differences in phase (grain) size and phase distribution such that in one case a continuous (interconnected) TiB2 network surrounds the Al2O3 phase (qualitatively termed “T@A”) and in another case the TiB2 and Al2O3 phases are interdispersed and uniformly intertwined with each other (qualitatively termed “TinA”). Quantitative microscopy was used to characterize the phase size and the integral curvature which is taken as a measure of TiB2 phase connectivity around Al2O3. Dynamic compression and tension (spall) properties were measured using plate impact experiments. The measurements used piezoelectric polyvinyldine fluoride stress gauges to obtain the loading profile and to determine the Hugoniot elastic limit. In addition, velocity interferometry system for any reflector interferometry was used to obtain the spall signal and the tensile dynamic strengths of the materials. Experimental results reveal that while the σHEL and the compressive strengths of TiB2+Al2O3 ceramics are dependent on the average grain (phase) size, the tensile (spall) strength scales with the TiB2-phase connectivity. This result suggests that the interconnected TiB2/Al2O3 microstructural morphology provides a stronger impediment to failure in tension compared with the morphology with simply interdispersed phases.
Electron-confined longitudinal optical phonon interaction and strong magnetic field effects on the binding energy in GaAs quantum wells
Authors: Es-Sbai N. , Sali A. , Fliyou M. , Abarkan E.
Published: July 2002

We study the ground state binding energy of a hydrogenic impurity subjected to an external strong magnetic field taking into account the interaction of the electron with confined longitudinal optical phonon in GaAs/Ga1−xAlxAs quantum wells. Calculations are performed within the effective mass approximation using the Lee–Low–Pines [Phys. Rev. 90, 297 (1953)] variational method and considering the infinite and finite character of the barrier potential. The binding energy is calculated as a function of the magnetic field and the well size. The results we have obtained show that the binding energies are very sensitive to the combined effect of the well size, the strong applied magnetic field, and the electron–phonon interaction.