bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of extremely narrow metal wires
Authors: Natelson D. , Willett R. L. , West K. W. , Pfeiffer L. N.
Published: July 2002

A robust technique for fabrication of metal wires with controlled widths substantially below 10 nm is presented. By etching a cleaved molecular-beam epitaxy grown substrate, a mechanical template is produced with surface relief of atomic lateral definition. Using metal deposition and directional ion etching of such a substrate, electrically continuous wires are formed from AuPd alloy with diameters as small as 3 nm and lengths greater than 1 μm. This technique can be used with a variety of materials and makes metallic nanostructures on a previously inaccessible size scale.
Rovibrational transitions and nuclear spin conversion of methane in parahydrogen crystals
Authors: Miki M. , Momose T.
Published: July 2002

Solid parahydrogen is an excellent matrix for matrix-isolation spectroscopy because of its high spectral resolution. Here we describe the rovibrational structure and nuclear spin conversion of CH4 embedded in parahydrogen crystals studied by infrared absorption spectroscopy. The vibration–rotation absorptions of CH4 exhibit time-dependent intensity changes at 4.8 K. These changes are interpreted to be a result of the I=1→I=2 nuclear spin conversion that accompanies the J=1→J=0 rotational relaxation. The half-lifetime of the upper J=1 rotational state is unchanged by the addition of up to 2% orthohydrogen molecules but decreases with more than 10% orthohydrogen molecules. The increase of the decay rate at higher orthohydrogen concentration indicates that the magnetic field gradient across CH4 due to the orthohydrogen molecules mixes the nuclear spin states, which accelerates the conversion.
Energy estimates for the von Kármán model of thin-film blistering
Authors: Jin Weimin , Sternberg Peter
Published: July 2002

We consider the behavior of buckling driven thin-film blisterings using von Kármán’s plate theory. Our focus is on the setting where the blistered region is the unit square with clamped boundary conditions at the vertical sides and periodic ones along the horizontal sides. In this setting, we prove rigorous upper and lower bounds for the elastic energy which are of the same order as the film thickness. We also present a convincing argument for the necessity of branching of folds near the boundary as has been observed in experiments.
Modeling of nitrogen depth profiles in iron after nitriding with a homogenized laser beam
Authors: Carpene E. , Landry F. , Schaaf P.
Published: July 2002

In this letter we propose a phenomenological model to explain the nitrogen depth profile in iron after laser nitriding. The model is based on the one-dimensional diffusion equation and two sets of functions are use to fit the experimental profiles: complementary error function (erfc) and Gaussian. The different nature of these profiles reflects the presence of two stages in the process: the nitrogen is supplied in the sample as an erfc, while the diffusion to larger depths takes place as Gaussians.
Integral encounter theories of multistage reactions. I. Kinetic equations
Authors: Ivanov K. L. , Lukzen N. N. , Doktorov A. B. , Burshtein A. I.
Published: July 2002

The matrix kinetic equations for multi-stage reactions in liquid solutions are derived using a newly developed original method based on a many-particle master equation. The method leads to an infinite hierarchy for vector correlation patterns that can be truncated two different ways. The simplest one reproduces the conventional Integral Encounter Theory (IET), while the other allows a general modification of the kernel, resulting in the matrix formulation of so called Modified Encounter Theory (MET). Unlike IET, MET accounts for all binary contributions and correctly restores the long-time asymptotics of bimolecular reactions. The matrix MET, applied in Part II to reversible reactions of inter-molecular energy transfer, significantly improves the results obtained with other methods.
Calculation of the refractive indices in the optically anomalous crystal of 1,5-dichloro-2,3-dinitrobenzene
Authors: Munn R. W.
Published: July 2002

The title compound forms crystals composed of sectors that comprise planes of molecules disordered between two orientations, giving triclinic symmetry in an apparently orthorhombic unit cell. Various theoretical approaches to take account of the disorder and calculate the refractive indices of a sector are explored. Optical interactions between planes prove to be essential. Input zero-frequency molecular polarizabilities calculated at the Hartree–Fock level are little affected by molecular orientation or permanent electric fields in the crystal environment. From calculations treating the crystal as a sequence of planes that reproduces the observed average orientation, with each plane containing molecules ordered in a single orientation, the principal refractive indices at long wavelength (zero frequency) are estimated to be n1=1.500±0.01, n2=1.495±0.005, n3=1.420±0.005. These correspond to principal axes that lie within 5° of the orthorhombic a, b, and c axes, reflecting the true triclinic symmetry. The true values at optical frequencies should be larger and somewhat more anisotropic.
Heat pulse propagation studies on DIII-D and the Tokamak Fusion Test Reactor
Authors: Fredrickson E. D. , Austin M. E. , Groebner R. , Manickam J. , Rice B. , Schmidt G. , Snider R.
Published: July 2002

Sawtooth phenomena have been studied on DIII-D and the Tokamak Fusion Test Reactor (TFTR) [D. Meade and the TFTR Group, in Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, DC, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, pp. 9–24]. In the experiments the sawtooth characteristics were studied with fast electron temperature (ECE) and soft x-ray diagnostics. For the first time, measurements of a strong ballistic electron heat pulse were made in a shaped tokamak (DIII-D) [J. Luxon and DIII-D Group, in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] and the “ballistic effect” was stronger than was previously reported on TFTR. Evidence is presented in this paper that the ballistic effect is related to the fast growth phase of the sawtooth precursor. Fast, 2 ms interval, measurements on DIII-D were made of the ion temperature evolution following sawteeth and partial sawteeth to document the ion heat pulse characteristics. It is found that the ion heat pulse does not exhibit the very fast, “ballistic” behavior seen for the electrons. Further, for the first time it is shown that the electron heat pulses from partial sawtooth crashes (on DIII-D and TFTR) are seen to propagate at speeds close to those expected from the power balance calculations of the thermal diffusivities whereas heat pulses from fishbones propagate at rates more consistent with sawtooth induced heat pulses. These results suggest that the fast propagation of sawtooth-induced heat pulses is not a feature of nonlinear transport models, but that magnetohydrodynamic events can have a strong effect on electron thermal transport.
Rotationally resolved infrared spectrum of the Cl−–H2 anion complex
Authors: Wild D. A. , Wilson R. L. , Weiser P. S. , Bieske E. J.
Published: July 2002

The mid-infrared spectrum of the Cl37−–H2 anion complex has been measured over the 3990–4050 cm−1 range (H–H stretch region) using infrared vibrational predissociation spectroscopy. The spectrum features a well resolved Σ–Σ transition red shifted by 156 cm−1 from the free H2 molecule stretch. Analysis of the P and R branch line positions using a linear molecule energy level expression yields ν0=4004.77±0.08 cm−1, B″=0.853±0.002 cm−1, D″=(9.3±1.0)×10−5cm−1, B′=0.919±0.002 cm−1, and D′=(9.0±1.0)×10−5 cm−1. The Cl−–H2 complex appears to have a linear equilibrium structure, with a vibrationally averaged separation of 3.19 Å between the Cl− and the H2 center-of-mass. Vibrational excitation of the H–H stretch induces a 0.12 Å contraction in the intermolecular bond.
Hysteresis loops of ferroelectric bilayers and superlattices
Authors: Chew Khian-Hooi , Ong Lye-Hock , Osman Junaidah , Tilley D. R.
Published: July 2002

A ferroelectric superlattice with an antiferroelectric interfacial coupling is considered; the same model describes a bilayer with antiferroelectric coupling. By mapping minimum points in the Landau free energy expression and plotting them against the applied electric field, a triple hysteresis loop pattern is obtained. The loop patterns vary between typically ferroelectric and typically antiferroelectric depending on the layer thicknesses and the magnitude of the interfacial-coupling constant. This work suggests the possibility of designing multilayer elements for computer memories with four or more different storage states.
Local transport in Joint European Tokamak edge-localized, high-confinement mode plasmas with H, D, DT, and T isotopes
Authors: Budny R. V. , Ernst D. R. , Hahm T. S. , McCune D. C. , Christiansen J. P. , Cordey J. G. , Gowers C. G. , Guenther K. , Hawkes N. , Jarvis O. N. , Stubberfield P. M. , Zastrow K.-D. , Horton L. D. , Saibene G. , Sartori R. , Thomsen K. , von Hellermann M. G.
Published: July 2002

The edge-localized, high-confinement mode regime is of interest for future Tokamak reactors since high performance has been sustained for long durations. Experiments in the Joint European Tokamak [M. Keilhacker et al., Nuclear Fusion 39, 209 (1999)] have studied this regime using scans with the toroidal field and plasma current varied together in H, D, DT, and T isotopes. The local energy transport in more than fifty of these plasmas is analyzed, and empirical scaling relations are derived for energy transport coefficients during quasi-steady state conditions using dimensionless parameters. Neither the Bohm nor gyro-Bohm expressions give the shapes of the profiles. The scalings with β and ν* are in qualitative agreement with Ion Temperature Gradient theory.
Brownian dynamics simulation of the motion of a rigid sphere in a viscous fluid very near a wall
Authors: Sholl David S. , Fenwick Michael K. , Atman Edward , Prieve Dennis C.
Published: July 2002

Total Internal Reflection Microscopy (TIRM) is an experimental technique that allows the potential energy profile of a colloidal particle above a plate to be determined from the particle’s observed Brownian motion. We have used Brownian Dynamics (BD) simulations to numerically simulate TIRM experiments. We show that a careful treatment of the position dependence of the colloidal particle’s mobility near the plate is vital to performing realistic simulations. These simulations enable us to systematically study the effect of experiment duration, data sampling methods, and physical parameters of the suspended colloid on the accuracy of potential energy profiles determined via TIRM. We also present an analytical theory to predict the range of energies reliably probed by TIRM experiments. This theory provides simple guidelines for the design and optimization of future TIRM experiments. Finally, we have used BD simulations to investigate the use of radiation pressure and the effects of experimental noise in TIRM experiments.
Diagnostics of inductively coupled chlorine plasmas: Measurement of Cl2 and Cl number densities
Authors: Malyshev M. V. , Donnelly V. M.
Published: July 2002

This article presents measurements of absolute Cl2 and Cl number densities in a chlorine transformer-coupled plasma. It is part of a series of reports on measurements of densities and energy distributions of all charged and neutral species in the same plasma system over an extensive range of pressure and power. Cl2 and Cl number densities were determined from optical emission spectroscopy and advanced actinometry. Number densities relative to the Xe actinometry gas are reported as a function of pressure (1–20 mTorr) and power (10–1000 W) during slow etching of SiO2-covered Si wafers. A detailed treatment of the effects of gas temperature on the conversion of these ratios into absolute number densities is also included. Cl2 is largely (∼90%) dissociated at the highest powers, with a somewhat higher degree of dissociation at low pressure. The Cl number density becomes nearly independent of power at high powers (especially at lower pressure) due to the combination of a higher degree of dissociation of Cl2 and an overall drop in number density due to heating of the gas. A zero-dimensional (global) model is used to compute Cl2 and Cl number densities. It gives a Cl wall recombination coefficient of 0.04 on the plasma-seasoned stainless steel walls.
Donor–acceptor pair recombination in gallium sulfide
Authors: Aydinli A. , Gasanly N. M. , Gökşen K.
Published: July 2002

Low temperature photoluminescence of GaS single crystals shows three broad emission bands below 2.4 eV. Temperature and excitation light intensity dependencies of these bands reveal that all of them originate from close donor–acceptor pair recombination processes. Temperature dependence of the peak energies of two of these bands in the visible range follow, as expected, the band gap energy shift of GaS. However, the temperature dependence of the peak energy of the third band in the near infrared shows complex behavior by blueshifting at low temperatures followed by a redshift at intermediate temperatures and a second blueshift close to room temperature, which could only be explained via a configuration coordinate model. A simple model calculation indicates that the recombination centers are most likely located at the nearest neighbor lattice or interstitial sites.
Charge separation effects in electron-magnetohydrodynamic reconnection
Authors: Attico N. , Califano F. , Pegoraro F.
Published: July 2002

Charge separation effects in a strongly magnetized plasma are shown to reduce the rate of magnetic field line reconnection in the whistler frequency range. A system of two eigenmode equations describing the linear growth of the reconnection instability is derived and solved numerically, using the compressible electron-magnetohydrodynamic theory first developed by Kuvshinov et al. [Phys. Lett. A 241, 287 (1998)]. Charge separation introduces a “renormalized skin depth” λe, larger than the electron skin depth de, which results in the broadening of the eigenmode structure.
Photonic quantum-well structures: Multiple channeled filtering phenomena
Authors: Qiao Feng , Zhang Chun , Wan Jun , Zi Jian
Published: July 2002

One can construct photonic quantum-well structures by stacking different photonic crystals in a way similar to semiconductor quantum wells, provided that the photonic band gaps of the constituent photonic crystals are properly aligned. If the photonic band of the photonic crystal in the well region is just located into the photonic band gap of the photonic crystal in the barrier region, quantized confined photonic states will appear owing to the photonic confinement effects. It is found that the number of the confined states can be tuned by adjusting the number of period of the well region, leading to the phenomena of multiple channeled filtering. A huge amplification of the field intensity is found in the well region for the confined photonic states.
Solvation free energies of polar molecular solutes: Application of the two-sphere Born radius in continuum models of solvation
Authors: Babu C. Satheesan , Lim Carmay
Published: July 2002

A two-sphere description of the effective Born radius for spherical ions was found in previous work to yield accurate free energies for spherical ions. This effective Born radius (Reff) was identified as the mean of the ionic radius (Rion) and the distance to the first peak of the ion–oxygen/hydrogen radial charge or number density distribution function (Rgmax); i.e., Reff=(Rion+Rgmax)/2. To see whether this prescription also applies to the solvation of nonspherical polar molecules, it was used in finite-difference Poisson methods as well as in Kirkwood and generalized Born models to compute solvation free energies of model diatomic molecules of varying interatomic bond distances. Hydration free energies for the same model systems were also derived from free energy simulations in the presence of explicit water molecules. The good agreement between explicit solvent results and continuum solvent results with the two-sphere Born radius indicates that the latter description provides the required solute–solvent boundary in continuum solvent models. In contrast, using Rgmax alone to define the dielectric boundary in the three continuum solvent models yielded solvation free energies that deviated significantly from the respective simulation values.
Fabrication of highly ordered porous structures
Authors: Meng Q.-B. , Gu Z.-Z. , Sato O. , Fujishima A.
Published: July 2002

We describe a simple method for the fabrication of highly ordered porous structures. That is, utilizing capillary forces, ultrafine particles are directly used to assemble themselves in the voids of template while the template is being assembled. The highly ordered porous structures with larger area and uniform orientation can be produced after removing the template. The majority of the single domains with uniform orientation prepared can reach 0.1 mm2 in area. Scanning electron microscope images and transmission spectrum of these samples have confirmed the success of this simple method.
Statistics of the bleaching number and the bleaching time in single-molecule fluorescence spectroscopy
Authors: Molski Andrzej
Published: July 2002

The kinetics of bleaching in single-molecule fluorescence spectroscopy (SMS) is studied using renewal theory. A five-state model of a dye molecule is considered where bleaching occurs from the excited triplet states. An exact formalism is developed to calculate the distributions of the bleaching number (i.e., the number of photon counts) and bleaching time (i.e., the time before photobleaching). For photostable dyes those distributions are well approximated by exponential distributions determined by the average bleaching number ν and the average bleaching time τ respectively. Exact formulation is developed to calculate ν and τ in terms of the transition rate constants. For photostable dyes the exact ν and τ are well approximated by expressions derived from a steady-state solution to the kinetic rate equations describing the molecule. The theory implies that experimental multiexponential fits to the distributions of the bleaching number and bleaching time are an indication that the SMS system is heterogeneous.
Expanded viewing-angle reflection from diffuse holographic-polymer dispersed liquid crystal films
Authors: Escuti M. J. , Kossyrev P. , Crawford G. P. , Fiske T. G. , Colegrove J. , Silverstein L. D.
Published: July 2002

A switchable diffuse reflective film with high color purity is demonstrated using holographic-polymer dispersed liquid crystals (HPDLC). By recording a diffuse hologram directly into the LC/polymer film, the diffuse mode HPDLC exhibit viewing angles an order of magnitude larger than the conventional mode. A simple phenomenological model based on coupled-wave theory is developed to describe our observations.
Quasimetallic silicon micromachined photonic crystals
Authors: Temelkuran B. , Bayindir Mehmet , Ozbay E. , Kavanaugh J. P. , Sigalas M. M. , Tuttle G.
Published: July 2002

We report on fabrication of a layer-by-layer photonic crystal using highly doped silicon wafers processed by semiconductor micromachining techniques. The crystals, built using (100) silicon wafers, resulted in an upper stop band edge at 100 GHz. The transmission and defect characteristics of these structures were found to be analogous to metallic photonic crystals. We also investigated the effect of doping concentration on the defect characteristics. The experimental results agree well with predictions of the transfer matrix method simulations.
Turbulent transport and equilibrium profiles in two-dimensional magnetohydrodynamics with background shear
Authors: Kim Eun-jin , Dubrulle Bérengère
Published: July 2002

Turbulent transport and equilibrium profile are studied in two-dimensional magnetohydrodynamics (2D MHD) in the presence of a background shear flow and a large-scale magnetic field; the latter quantities are assumed to be parallel and to vary in the perpendicular direction. The nonuniformity of the background is incorporated, to first order, by using the Gabor transform. The magnetic vector potential and momentum fluxes (or total stress) are calculated in both kinematic and dynamic cases in the case of unit magnetic Prandtl number, which then determines turbulent diffusivity and viscosity and equilibrium profile of the mean shear flow. The turbulent diffusion is found to be suppressed for a strong (large-scale) magnetic field. The Lorentz force changes the sign of the total stress resulting in the turbulent viscosity with an opposite sign compared to that in the hydrodynamical case. The former reduces the amplitude of the total stress for a fixed shear due to the cancellation between Reynolds and Maxwell stresses, therefore leading to the reduction in momentum transport. Since the divergence of momentum flux acts as an effective force on the background shear, the presence of the magnetic field can lead to an equilibrium shear profile which is different from that of the pure hydrodynamic case. In particular, the Lorentz force is shown to laminarize the mean shear flow.
Shear flow generation by drift waves revisited
Authors: Guzdar P. N. , Kleva R. G. , Chen Liu
Published: July 2002

The generation of shear flow by drift waves is an important area of investigation. In a recent paper [Chen et al., Phys. Plasmas 7, 3129 (2000)] an elegant formalism was developed for the generation of shear/zonal flows by drift waves in a toroidal plasma. The study of shear flow generation in fluids and plasmas is put into perspective. In this paper it is shown how a simple slab geometry analysis can lead to a similar dispersion relation and highlight the subtle differences between the slab and toroidal geometries.
Control of magnetization reversal process by light illumination in ferromagnetic semiconductor heterostructure p-(In, Mn)As/GaSb
Authors: Oiwa A. , Słupinski T. , Munekata H.
Published: July 2002

The reduction in coercive force by light illumination has been found in ferromagnetic semiconductor heterostructure p-(In, Mn)As/GaSb prepared by molecular-beam epitaxy. Enhanced ferromagnetic coupling between Mn ions, arising from excess photogenerated holes, reduces the domain wall energy and changes the magnetization hysteresis characteristics. The value of coercive force returns to the original value when excess holes recombine with trapped electrons.
Potentiometry of an operating organic semiconductor field-effect transistor
Authors: Seshadri Kannan , Frisbie C. Daniel
Published: July 2002

The potential profile across the channel of an operating sexithiophene-based field-effect transistor (FET) was investigated using an atomic force microscope with a conducting probe. A high impedance electrometer recorded the probe potential when it was placed in contact at fixed points with the channel surface. Tapping mode images taken with the same probe before and after individual point contact measurements verified that no damage was done to the device and allowed correlation of the potential profile with the device architecture. For any given source-drain bias, most of the potential drop occurred at the source and drain contacts, meaning the FET was contact limited. Moreover, the potential drop was not fixed but depended on the applied drain and gate voltages. This study demonstrates the utility of potential profiling for identifying high resistance bottlenecks to charge transport in organic-based devices.
Mapping of continuum and lattice models for describing the adsorption of an ideal chain anchored to a planar surface
Authors: Gorbunov A. A. , Skvortsov A. M. , van Male J. , Fleer G. J.
Published: July 2002

An ideal polymer chain anchored to a planar surface is considered by using both lattice and continuum model approaches. A general equation relating the lattice and continuum model adsorption interaction parameters is derived in a consistent way by substituting the exact continuum solution for the free chain end distribution function into the lattice model boundary condition. This equation is not mathematically exact but provides excellent results. With the use of this relation the quantitative equivalence between lattice and continuum results was demonstrated for chains of both infinite and finite length and for all three regimes corresponding to attractive, repulsive and adsorption-threshold energy of polymer-surface interaction. The obtained equations are used to discuss the distribution functions describing the tail of an anchored macromolecule and its adsorbed parts. For the tail-related properties the results are independent of the microscopic details of the polymer chain and the adsorbing surface. One interesting result obtained in the vicinity of adsorption threshold point is a bimodal tail length distribution function, which manifests chain populations with either tail or loop dominance. The properties related to the number of surface contacts contain, apart from universal scaling terms, also a nonuniversal factor depending on microscopic details of polymer-surface interaction. We derived an equation for calculating this nonuniversal factor for different lattice models and demonstrated excellent agreement between the lattice results and the continuum model.
Optoelectronic device performance on reduced threading dislocation density GaAs/Si
Authors: Taylor P. J. , Jesser W. A. , Benson J. D. , Martinka M. , Dinan J. H. , Bradshaw J. , Lara-Taysing M. , Leavitt R. P. , Simonis G. , Chang W. , Clark W. W. , Bertness K. A.
Published: July 2002

A technique for the heteroepitaxy of GaAs/Si films having reduced threading dislocation density is presented. The important attribute of this technique is the suppression of three-dimensional Volmer–Weber island formation during initial deposition. This suppression is achieved by deposition of a stoichiometric GaAs buffer layer by a migration-enhanced epitaxy technique on silicon at 348 K to a thickness greater than the “monolithic thickness,” hm. Subsequent GaAs films deposited by conventional molecular beam epitaxy on buffer layers of thickness greater than hm possess structural and optical characteristics that exceed those for state-of-the-art GaAs/Si layers: an x-ray full width at half maximum (FWHM) of 110 arcsec with a dislocation density at the film surface of 3×106 cm−2 and a concomitant 4 K photoluminescence FWHM of 2.1 meV. The p-i-n structures suitable for use as light-emitting diodes (LEDs) that were grown on the reduced threading dislocation density GaAs/Si and by means of forward- and reverse-bias measurements, demonstrated an ideality factor of n=1.5, an increased reverse-bias breakdown electric field of 2.1×107 V/m, and an intrinsic region resistivity of 4×107 Ω cm for LEDs of increasingly smaller mesa size.
Collapse transition of isolated Lennard-Jones chain molecules: Exact results for short chains
Authors: Taylor Mark P.
Published: July 2002

In this work we study flexible Lennard-Jones (LJ) interaction-site chain molecules with fixed bond length L. We obtain “exact” numerical results for the equilibrium configurational and energetic properties of LJ n-mer chains with n=3, 4, and 5 for a range of L (0.8⩽L/σ⩽21/6). These results include intramolecular site–site distribution functions and fluctuations in site–site distances, radius of gyration, and average potential energy. We also compute the single-chain specific heat, chemical potential, and theta temperature. With decreasing temperature the chain molecules undergo a collapse transition from an extended disordered state to a compact highly ordered state, exhibiting an associated peak in the specific heat. For n=3 and 4 the collapse occurs in a single stage whereas for the n=5 chains a multistage collapse is possible. In particular, the specific heat peaks associated with the collapse of 5-mer chains with bond lengths in the range 0.85⩽L/σ⩽1.0 display either a low temperature shoulder or secondary maximum. We show that this complex specific heat behavior arises when the initial collapsed state consists of a set of distinct and nondegenerate structural isomers. The secondary peak or shoulder is a result of the “freezing out” of all but the ground state isomer. These results are discussed in the context of the similar anomalous specific heat behavior, and low temperature structural transitions observed in recent simulation studies of square-well and LJ chains and atomic clusters.
Nonlinear three-mode interactions in a developing mixing layer
Authors: Nikitopoulos D. E. , Liu J. T. C.
Published: July 2002

A formulation based on the integral energy method is presented for the nonlinear interaction problem between three large-scale coherent modes in a developing, laminar, free shear layer. Both binary and three-mode nonlinear, modal energy and phase interactions are included. The modal evolution and the development of the mean flow are very sensitive to the initial energy contents and phases of the participating modes. A strong nonlinear coupling of the modal phase variations with the variations of the modal energies is shown to exist and is responsible for the rapid reversal of the intermodal energy interactions in favor of the occasionally declining mode. This nonlinear mechanism preserves the higher frequency modes far downstream and is shown to contribute to the appearance of mean flow contraction observed in experiments through local collective return of energy to the mean by all modes. Depending on initial conditions, our results support the arguments of R. A. Petersen and R. C. Clough [“The influence of higher harmonics on vortex pairing in an axisymmetric mixing layer,” J. Fluid Mech. 239, 81 (1992)] and M. R. Hajj, R. W. Miksad, and E. J. Powers [“Subharmonic growth by parametric resonance,” ibid. 236, 35 (1992)] according to which three-mode resonances are more efficient than the binary ones. Two cases relevant to situations where the first and second subharmonics of the most amplified mode are forced involving the 3/2 harmonic and 2/3 subharmonic, respectively, are examined. Comparisons of the calculations with experimental results indicate good qualitative agreement.
Modified surface fluctuations by impurity binding in amphiphilic dispersions
Authors: Goldman Carla
Published: July 2002

We have shown that the thermodynamics of a 2D multicomponent ideal gas model describes well the thermal behavior of light-scattering (LS) data in lipid dispersions of dimyristoyl phosphatidylglycerol (DMPG) [C. Goldman, K. A. Riske, and M. T. Lamy-Freund, Phys. Rev. E 60, 7349 (1999)]. The components (species) of the gas are distinguished by size and by their ability to display shape fluctuations. Here, the model is extended to include the presence of thermalized impurities bound to the fluctuating species. We show that with this modification the model accounts also for additional LS data from DMPG under variation of ionic strength.
Photonic-crystal distributed-feedback lasers
Authors: Vurgaftman I. , Meyer J. R.
Published: July 2002

The far-field emission characteristics and spectral purity of photonic-crystal distributed-feedback (PCDFB) lasers, in which the grating is defined on a two-dimensional lattice that is tilted with respect to the facets, are analyzed using a self-consistent time-domain simulation. It is shown that both conventional DFB and angled-grating DFB devices are special cases of the PCDFB laser, which combines and enhances the best features of each. The simulations project both high beam quality and high spectral purity out to pump stripes of width ≫100 μm.
40% tunneling magnetoresistance after anneal at 380 °C for tunnel junctions with iron–oxide interface layers
Authors: Zhang Zongzhi , Cardoso S. , Freitas P. P. , Batlle X. , Wei P. , Barradas N. , Soares J. C.
Published: July 2002

Spin tunnel junctions fabricated with one interposed Fe–FeOx layer between the Al2O3 barrier and the top CoFe pinned electrode show large tunneling magnetoresistance (TMR) (40%) for anneals up to 380 °C. The annealing temperature TTMR*, where maximum TMR occurs, increases with the inserted Fe–FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment (which usually starts to decay below 300 °C in the normal junctions) increases with annealing temperature up to 380 °C and remains at a maximum until 450 °C. The large TMR at high temperature is related with the diffusion of extra Fe (from the Fe–FeOx layer) into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe3O4, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoelectron spectra for the Fe 2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R×A values and barrier parameters versus annealing temperature for samples with 7 and 25 Å Fe–FeOx also reflects the above structural changes in the inserted layer.
Nonuniform magnetization structures in thin soft type ferromagnetic elements of elliptical shape
Authors: Usov Nickolai A. , Chang Ching-Ray , Wei Zung-Hang
Published: July 2002

Stable micromagnetic configurations are investigated numerically in thin soft type ferromagnetic elements of elliptical shape with typical thickness of Lz=30 nm and with aspect ratios 1⩽Lx/Ly⩽3.5. In this range of aspect ratios, a complete description of stable magnetization patterns is obtained including the critical sizes for stability of one-vortex and quasiuniform micromagnetic configurations. Due to the absence of sharp corners, the behavior of the quasiuniform state in an external magnetic field resembles that of a single-domain ellipsoidal particle of similar shape. Nevertheless, the calculated hysteresis loops of elliptical elements show a considerable reduction of the switching fields in comparison with the Stoner–Wohlfarth model due to the development of a new mode of instability. The magnetization curves for elliptical elements, initially in one-, two- or three- vortex states, show significant dependence on the direction of external magnetic field with respect to the longest particle’s axis.
Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg, Zn)O multiple quantum wells
Authors: Makino T. , Tuan N. T. , Sun H. D. , Chia C. H. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Suemoto T. , Akiyama H. , Baba M. , Saito S. , Tomita T. , Koinuma H.
Published: July 2002

We report on temperature dependence of excitonic photoluminescence (PL) from ZnO/(Mg, Zn)O multiple quantum wells (MQWs). Two kinds of MQWs having different barrier heights grown by laser molecular-beam epitaxy showed significantly different temperature dependences of PL spectra; in ZnO/Mg0.27Zn0.73O MQWs, the PL peak energy at 50–200 K was a monotonically increasing function of temperature, which was opposite to that ascribed by band gap shrinkage. Moreover, spectra taken at 95–200 K encompassed two peaks, both of which originated from recombination of localized excitons. The temperature-induced shift (redshift-blueshift-peak duplication-redshift) at 5–300 K is caused by a change in the exciton dynamics with increasing temperature due to inhomogeneity and the exciton localization effect. On the other hand, the corresponding dependence in ZnO/Mg0.12Zn0.88O MQWs (lower barrier height) was similar to that in bulk II–VI semiconductors.
Asymmetric forcing of a turbulent rectangular jet with a piezoelectric actuator
Authors: Pothos Stamatios , Longmire Ellen K.
Published: July 2002

A moving wall section attached to a piezoelectric actuator was used to perturb the airflow exiting a fully developed turbulent channel. The Reynolds number based on the channel width and centerline velocity was 4240. The maximum velocity of the moving wall section was 9.5 cm/s (2.3% of the mean centerline velocity), and the maximum displacement was 120 microns, corresponding to 1.84 wall units. The actuator frequency and displacement amplitude were tuned independently to generate different effects on the flow, and both quantities were documented to provide precise boundary conditions for numerical codes. Hot-wire measurements showed that actuation affects both the mean and rms velocity profiles downstream of the channel exit. In all cases, forcing yields mean profiles that are symmetric with respect to the centerline. However, forcing at low frequencies (St⩽0.30) causes faster decay of the centerline velocity, higher spreading rates in the far field, and asymmetric rms profiles compared with unforced flow. The maximum rms value crosses from the actuator side to the nonactuator side as the flow moves downstream. This behavior is thought to be caused by staggered but asymmetric vortical structures developing in the opposing shear layers. Forcing from St=0.39 through 1.46 leads to altered but symmetric rms profiles and spectra compared with unforced flow. Forcing in the range St<0.50 yields centerline rms values that initially are larger than, but further downstream smaller than in the unforced flow.
Improvement of thermal stability of exchange coupling in FeMn/NiFe films by Pt addition
Authors: Jimbo Mutsuko , Yokochi Shingo , Yamagishi Koji , Kurita Junichi
Published: July 2002

An exchange coupling and thermal stability study in Fe(Pt)Mn/NiFe films has been investigated. When Pt atoms were added to an FeMn layer, most of the properties in the FeMn/NiFe films were improved. The resistivity increased with increasing Pt content. The exchange coupling field had a maximum value for a film with about 5–8 at. % Pt. Especially, it was found that the addition of Pt to the FeMn layer improved considerably the thermal stability of exchange coupling in the FeMn/NiFe films. In Fe39Pt6Mn55(7 nm)/NiFe(10 nm) films, an exchange coupling field of 100 Oe and a coercivity of 40 Oe were obtained after annealing at 300 °C. From the result of Auger analysis, it was found that diffusion of the interface was suppressed by Pt addition.
Hole drift mobility in μc-Si:H
Authors: Juška G. , Viliūnas M. , Arlauskas K. , Nekrašas N. , Wyrsch N. , Feitknecht L.
Published: July 2002

In microcrystalline hydrogenated silicon (μc-Si:H), the drift mobility dependencies of holes on electric field and temperature have been measured by using a method of equilibrium charge extraction by linearly increasing voltage. At room temperature the estimated value of the drift mobility of holes is much lower than in crystalline silicon and slightly higher than in amorphous hydrogenated silicon (a-Si:H). In the case of stochastic transport of charge carriers with energetically distributed localized states, the numerical model of this method gives insight into the mobility dependence on electric field. From the numerical modeling and experimental measurement results, it follows that the hole drift mobility dependence on electric field is predetermined by electric field stimulated release from localized states.
Temperature-dependent magnetoresistance of magnetic tunnel junctions with ultraviolet light-assisted oxidized barriers
Authors: May U. , Samm K. , Kittur H. , Hauch J. , Calarco R. , Rüdiger U. , Güntherodt G.
Published: July 2002

Co(10 nm)/AlO x ( nominally 2 nm)/Co(20 nm) tunnel junctions have been prepared under ultrahigh vacuum conditions applying a shadow mask technique. An ultraviolet light-assisted oxidation process of the AlOx barrier has been optimized by in situ x-ray photoelectron spectroscopy, in conjunction with temperature-dependent tunneling magnetoresistance measurements. Optimum-oxidized tunnel junctions show a magnetoresistance of 20% at 285 K, and up to 38% at 100 K. For under-oxidized samples, with a remaining Al layer between the Co bottom electrode and the AlOx barrier, the tunneling magnetoresistance decreases more rapidly with increasing temperature than observed for the over-oxidized samples. The resistance × area product of optimum-oxidized tunneling junctions exhibits a minimum, and increases for under- and over-oxidized samples.
Electron spin resonance study of defects in Si1−xGex alloy nanocrystals embedded in SiO2 matrices: Mechanism of luminescence quenching
Authors: Toshikiyo Kimiaki , Tokunaga Masakazu , Takeoka Shinji , Fujii Minoru , Hayashi Shinji
Published: July 2002

Dangling bond defects in Si1−xGex alloy nanocrystals (nc-Si1−xGex) as small as 4 nm in diameter embedded in SiO2 thin films were studied by electron spin resonance (ESR), and the effects of the defects on photoluminescence (PL) properties were discussed. It was found that the ESR spectrum is a superposition of signals from Si and Ge dangling bonds at the interfaces between nc-Si1−xGex and SiO2 matrices (Si and Ge Pb centers). As the Ge concentration increased, the signal from the Ge Pb centers increased, while that from the Si Pb centers was nearly independent of Ge concentration. The increase in the number of Ge Pb centers was accompanied by strong quenching of the PL. The observed correlation between the two measurements suggests that the Ge Pb centers act as efficient nonradiative recombination centers for photogenerated carriers, resulting in the quenching of the main PL.
High-resolution x-ray study of thin GaN film on SiC
Authors: Kazimirov A. , Faleev N. , Temkin H. , Bedzyk M. J. , Dmitriev V. , Melnik Yu.
Published: July 2002

The x-ray standing wave method (XSW) and high-resolution x-ray diffraction were used to study the structural perfection and polarity of GaN epitaxial thin film grown by hydride vapor phase epitaxy on the Si-face SiC substrate. The x-ray standing wave was generated inside the 300 nm thin film under the condition of Bragg diffraction from the film. Excellent crystalline quality of the GaN film was revealed by both x-ray techniques. The XSW analysis of the angular dependencies of the Ga–K fluorescence yield measured while scanning through the GaN(0002) diffraction peak unambiguously showed the Ga polarity of the film. Correlation between the mosaic structure and the static Debye–Waller factor of the GaN lattice was also studied.
Molecular fluorescence in the vicinity of a nanoscopic probe
Authors: Hamann H. F. , Kuno M. , Gallagher A. , Nesbitt D. J.
Published: July 2002

The dramatic modifications of molecular fluorescence in the proximity of a sharp nanoscopic probe is investigated by an apertureless or antenna-based near-field scanning optical microscope, which exploits the interactions between a fluorescent sample and a laser illuminated Si atomic force microscope probe. Specifically, luminescence is monitored from evanescently excited, dye-doped polystyrene nanospheres (RS=20–80 nm) on a fused silica prism surface as a function of probe-sample geometry. The incident laser field is enhanced in the near-field of the probe tip, resulting in images with high sensitivity (σmin≈2 Å2 in a 1 Hz detection bandwidth) and strongly subdiffraction-limited spatial resolution. At probe-sample distances greater than ≈λ/2, the images are dominated by far-field interference between (i) direct fluorescence from the molecular sample and (ii) indirect fluorescence from image dipoles induced in the atomic force microscope probe. Near-field “shadowing” of the molecular fluorescence by the probe also occurs and is studied as a function of probe-sample-detector geometry. Finally, effects of probe-sample proximity on the fluorescence emission spectrum are investigated. In summary, the data elucidate several novel near- and far-field molecular fluorescence enhancement effects relevant to further development of molecular and nanostructural spectroscopic methods with spatial resolution well below the diffraction limit.
Exciton dissociation effects on time resolved photoluminescence measurements of an Al0.53In0.47P/Ga0.52In0.48P/Al0.53In0.47P-quantum well structure
Authors: Ishitani Yoshihiro , Fujita Toshiaki , Nakasa Keijiro , Harima Yutaka
Published: July 2002

Temporal developments of the photoluminescence (PL) intensity at temperatures of 7, 100, and 294 K are analyzed using the rate equations including the exciton dissociation and association terms for an Al0.53In0.47P/Ga0.52In0.48P/Al0.53In0.47P-quantum well structure. At 7 K, the nonexponential time dependence of the PL intensity is caused by the exciton dissociation process. At 7 and 100 K, PL intensity is dominated by the exciton recombination even if the exciton density is smaller than the dissociated carrier density. The thermally excited background carriers affect the recombination processes at 100 and 294 K. At 294 K, the rise part of the PL intensity is dominated by the exciton recombination, though the dissociated carrier density dominates.
On streamwise vortices in a turbulent wall jet that flows over a convex surface
Authors: Likhachev O. , Neuendorf R. , Wygnanski I.
Published: July 2002

Flow visualization and correlation measurements revealed the existence of large streamwise vortices in a turbulent wall jet that is attached to a circular cylinder. These coherent structures were not to be found near the nozzle, nor were they artificially triggered, thus the vortices could be a product of centrifugal instability. The existence and scale of this large-scale coherent motion were corroborated by stability analysis applied to the measured mean flow.
Rotational spectra of SiCN, SiNC, and the SiCnH (n=2, 4–6) radicals
Authors: McCarthy M. C. , Apponi A. J. , Gottlieb C. A. , Thaddeus P.
Published: July 2002

Three new silicon–carbon chains, SiC4H, SiC5H, and SiC6H, have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. A detailed spectroscopic characterization of these and the previously described chains SiCCH, SiCN, and SiNC is given here. All six radicals are linear chains with Π2 electronic ground states and all have resolvable hyperfine structure in their lower rotational transitions; all except SiC5H have resolved lambda-type doubling. Because transitions of SiCCH, SiCN, and SiNC were also detected by millimeter-wave absorption spectroscopy in both spin components, for these the rotational, centrifugal distortion, and fine structure and hyperfine coupling constants were determined to high precision using the standard Hamiltonian for a molecule in a Π2 state. For SiC4H, SiC5H, and SiC6H, at least seven transitions in the lowest-energy fine structure component were measured between 7 and 30 GHz, and, at most, five spectroscopic constants were required to reproduce their spectra to a few parts in 107. The hyperfine coupling constants of the SiCnH radicals are fairly close to those of isovalent Cn+1H, indicating that the chemical bonding may be similar. The missing radical in the present sequence, SiC3H, may soon be found along with cyclic isomers of SiCCH and SiC4H. If SiC5H possesses strong electronic transitions in the visible like isovalent C6H, its spectrum should be detectable by long path optical spectroscopy.