bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-dimensional simulations of hydrodynamic instability in liquid bridges: Influence of temperature-dependent viscosity
Authors: Shevtsova V. M. , Melnikov D. E. , Legros J. C.
Published: July 2002

The development of thermocapillary convection inside a cylindrical liquid bridge is investigated by using a direct numerical simulation of the three-dimensional (3D), time-dependent problem for a wide range of Prandtl numbers, Pr=1,3,4,5 and Pr=35. Above the critical value of temperature difference between the supporting disks, two counterpropagating hydrothermal waves bifurcate from the two-dimensional (2D) steady state. The existence of standing and traveling waves is discussed. The dependence of viscosity upon temperature is taken into account. The critical Reynolds number and critical frequency at which the system undergoes a transition from a 2D steady state to a 3D oscillatory flow decreases if the viscosity diminishes with temperature. The stability boundary is determined for Pr=3–5 with a viscosity contrast (νmax/νmin) up to a factor 10. Near the threshold of instability the flow organization is similar for the constant and variable viscosity cases despite the large difference in critical Reynolds numbers. The influence of variable viscosity on the flow pattern is increased when going into the supercritical region. The study of spatial-temporal behavior of oscillatory convection for the high Prandtl number, Pr=35, demonstrates a good agreement with previously published experimental results. For this high Prandtl number liquids instability begins as a standing wave with an azimuthal wave number m=1 which then switches to an oblique traveling wave ≈4%–5% above the onset of instability.
Selective field ionization of high Rydberg states: Application to zero-kinetic-energy photoelectron spectroscopy
Authors: Hollenstein U. , Seiler R. , Schmutz H. , Andrist M. , Merkt F.
Published: July 2002

Sequences of pulsed electric fields have been designed and tested that enable a higher selectivity in the pulsed field ionization of high Rydberg states (n⩾100) than has so far been possible. The enhanced selectivity originates from the permutation of the parabolic quantum numbers n1 and n2 that is induced by a sufficiently rapid inversion of the electric field polarity during a pulse sequence. A reliable procedure, based on numerical simulations of the outcome of pulse field ionization sequences, has been developed to detect and control changes in the parabolic quantum numbers that can occur during a pulse sequence. The procedure can be used to assess under which conditions a clean permutation of the parabolic quantum numbers can be achieved. Unwanted randomization of m, n1 and n2, which reduces the selectivity of the field ionization process, can be avoided by minimizing the time intervals during which the electric field in the pulse sequence is almost zero. The high selectivity reached in the pulsed field ionization of high Rydberg states has been used to record pulsed-field-ionization zero-kinetic-energy photoelectron spectra of argon and nitrogen at an unprecedented resolution of 0.06 cm−1. This resolution opens new perspectives in photoelectron spectroscopy.
Lagrangian structures and the rate of strain in a partition of two-dimensional turbulence
Authors: Haller G.
Published: July 2002

We derive analytic criteria for the existence of hyperbolic (attracting or repelling), elliptic, and parabolic material lines in two-dimensional turbulence. The criteria use a frame-independent Eulerian partition of the physical space that is based on the sign definiteness of the strain acceleration tensor over directions of zero strain. For Navier–Stokes flows, our hyperbolicity criterion can be reformulated in terms of strain, vorticity, pressure, viscous and body forces. The special material lines we identify allow us to locate different kinds of material structures that enhance or suppress finite-time turbulent mixing: stretching and folding lines, Lagrangian vortex cores, and shear jets. We illustrate the use of our criteria on simulations of two-dimensional barotropic turbulence.
Polarized-photoreflectance characterization of an InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor structure
Authors: Lin C. J. , Huang Y. S. , Li N. Y. , Li P. W. , Tiong K. K.
Published: July 2002

We have characterized an InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor structure using polarized photoreflectance (PR) spectroscopy. The ordering parameter of the InGaP is deduced from the polarization {[110] and [11̄0]} dependence of the PR signals from the emitter region. The ordering related piezoelectric field is also found to influence the electric field, as evaluated from observed Franz–Keldysh oscillations, in the InGaP emitter region. The field in the emitter region is found to be about 25 kV/cm smaller than the theoretical value that does not take into account the possible ordering induced screening effect, while the field in the collector region agrees well with the theoretical value. In addition, the InGaAsN band gap is also determined by analyzing the PR spectrum of the base region.
Ultrasonic measurement of elastic constants of anisotropic materials with laser source and laser receiver focused on the same interface
Authors: Reverdy F. , Audoin B.
Published: July 2002

A noncontact laser–ultrasonic technique is described that allows determination of material properties of anisotropic platelike samples. The method requires access to only one side of the sample. Plane waves are synthesized by scanning the surface with a laser line source at judiciously chosen intervals. By doing so, the intricacies associated with the reflection of an acoustic ray off an interface of an anisotropic medium are avoided. The plane-wave reflections off the opposite free surface of the plate are simply governed by Snell's law. The identification of the stiffness-tensor components is based on the inversion of the phase velocities of these reflected plane waves. The procedure developed is presented and applied to both simulated and experimental signals. The method is easy to implement, and provides a means for noncontact measurement of elastic properties where there is limited access to test specimens.
Dispersed fluorescence spectroscopy of jet-cooled AgAu and Pt2
Authors: Fabbi Jacqueline C. , Langenberg Jon D. , Costello Quinton D. , Morse Michael D. , Karlsson Lars
Published: July 2002

Dispersed fluorescence spectroscopy has been used to study jet-cooled AgAu and Pt2. Fluorescence resulting from the excitation of five bands of the A←X 1Σ+ system of AgAu was dispersed, and 51 measured ground state vibrational levels were fit to provide ground state vibrational constants of ωe″=198.22±0.11 cm−1 and ωe″xe″=0.512±0.002 cm−1. A Franck–Condon calculation was performed using the experimental values of the ground and excited state vibrational frequencies and anharmonicities, providing an estimate of the change in bond length upon excitation of the A←X system of Δre=0.214±0.005 Å. Fluorescence resulting from four different excitations of Pt2 was dispersed, providing vibrational constants for the ground and two low-lying excited states. Ground state vibrational constants of ωe=222.3 cm−1 and ωexe=0.62 cm−1 were obtained, based on the analysis of 16 measured ground state vibrational levels. In addition, a low-lying excited state was located at T0=2877 cm−1, with ωe=197 cm−1. This state perturbed the ground state, from which it was deduced that it has the same symmetry as the ground state. A comparison to theoretical calculations suggests that both states have 0g+ symmetry. Finally, a metastable state of Pt2 lying at an unknown energy was determined to have ωe=211 cm−1, ωexe=0.4 cm−1.
How to achieve zigzag free Sm-C * state with a low pretilt angle: A numerical calculation
Authors: Wang Chenhui , Kurihara Ryuji , Bos Philip J. , Kobayashi Shunsuke
Published: July 2002

One of the most notorious problems with ferroelectric liquid crystal is obtaining zigzag free bistable devices. By investigating the balance between surface azimuthal anchoring energy and bulk elastic energy within the confined chevron layer geometry of C1 and C2, we propose that it is possible to get a zigzag free C1 state by low azimuthal anchoring alignment at a low pretilt angle. The critical azimuthal anchoring coefficient for the defect free C1 state is calculated. Its relationships with elastic constant, chevron angle, as well as surface topography effect are also discussed.
Angular effects and beam quality in optical parametric amplification
Authors: Jovanovic Igor , Comaskey Brian J. , Pennington Deanna M.
Published: July 2002

Advances in optical parametric devices, in particular those requiring high conversion efficiency, rely on pump laser and gain medium properties. We describe and theoretically model the source of dephasing due to angular deviation from ideal phase matching in optical parametric amplification. Real laser beams have angular content, which is described by their spatial frequency spectrum. Such beams cannot be treated as single plane waves in nonlinear interactions. Our mathematical model is based on a plane wave decomposition of Gaussian and top-hat beams into their components in spatial frequencies. Several popular nonlinear materials (beta-barium borate, lithium borate, and potassium dihydrogen phosphate) are examined for phase matching angles and dephasing is rigorously calculated. The impact of the beam angular content on small signal gain and on conversion efficiency in the strongly depleted regime is evaluated numerically. In addition, a criterion is formulated for beam quality tolerance in optical parametric amplifiers, for critical and noncritical phase matching. The impact of initial conditions in optical parametric amplification is considered. Our calculations are intended primarily for devices pumped with long (nanosecond) pulses.
Cosmological implications of a nonseparable 5D solution of the vacuum Einstein field equations
Authors: Fukui Takao , Seahra Sanjeev S. , Wesson Paul S.
Published: July 2002

An exact class of solutions of the 5D vacuum Einstein field equations (EFEs) is obtained. The metric coefficients are found to be nonseparable functions of time and the extra coordinate l and the induced metric on l=const hypersurfaces has the form of a Friedmann–Robertson–Walker cosmology. The 5D manifold and 3D and 4D submanifolds are in general curved, which distinguishes this solution from previous ones in the literature. The singularity structure of the manifold is explored: some models in the class do not exhibit a big bang, while others exhibit a big bang and a big crunch. For the models with an initial singularity, the equation of state of the induced matter evolves from radiation-like at early epochs to Milne-like at late times and the big bang manifests itself as a singular hypersurface in 5D. The projection of comoving 5D null geodesics onto the 4D submanifold is shown to be compatible with standard 4D comoving trajectories, while the expansion of 5D null congruences is shown to be in line with conventional notions of the Hubble expansion.
Time-resolved two-dimensional vibrational spectroscopy of a short α-helix in water
Authors: Woutersen Sander , Hamm Peter
Published: July 2002

Nonlinear two-dimensional (2D) vibrational spectroscopy has been used to investigate the amide I band of an alanine-based 21-residue α-helical peptide in aqueous solution. Whereas the linear absorption spectrum consists of a single, broad amide I band, the 2D vibrational spectrum clearly reveals that this band is composed of two amide I transitions, which are assigned to the A and E1 modes. The A–E1 frequency splitting is found to be approximately 10 cm−1. We find that the amide I band is inhomogeneously broadened due to conformational disorder of the helix. The 2D line shapes can be well described using distributions of the dihedral angles (φ,ψ) around their average values with a width of 20°, confirming previous molecular-dynamics studies. Time-resolved 2D measurements show that the conformation fluctuates on a time scale of picoseconds.
Response to “Comment on ‘Energy band structures of the low-dimensional antiferromagnets Sr2CuO3 and Sr2CuO2Cl2’ ” [J. Appl. Phys. 90, 3708 (2001)]
Authors: Wu Hua , Zheng Qing-qi
Published: July 2002

An improvement is made on our previous band results of Sr2CuO3 and Sr2CuO2Cl2 by including the extended Sr 4p valence orbital in the present self-consistent calculations. The differences between our improved results and those of others are mainly due to different evaluation of the ionicity in these compounds.
Giant resistance changes in (Al,Ga)As contact layers of GaAs/AlAs superlattices due to deep donors
Authors: Rogozia M. , Krispin P. , Grahn H. T.
Published: July 2002

For transport studies on semiconductor superlattices under optical excitation, samples with highly Si-doped (Al,Ga)As contact layers are used due to their optical transparency. These samples exhibit a giant increase of the resistance at temperatures below 200 K. Likewise, the current plateau in the I–V characteristics, which contains the region of current self-oscillations, is shifted towards higher voltages. However, the oscillation frequencies remain unchanged. The current decreases during cooling by as much as seven orders of magnitude. The giant resistance can be compensated for at low temperatures by applying a high dc voltage or by weak illumination. The optically induced current transients are shown to be controlled by thermally activated processes. On the basis of the activation energies derived, it is suggested that the dramatic resistance changes are mainly due to deep donors, i.e., well-known DX centers, in the (Al,Ga)As contact layers. The effects are negligible, if GaAs contact layers are used. Possible DX centers at the AlAs/GaAs interfaces inside the superlattice structure can be therefore neglected.
Fundamental difference between the two Hall thruster variants
Authors: Choueiri E. Y.
Published: July 2002

The fundamental difference between the two variants of Hall thrusters, the stationary plasma thruster (SPT) and the thruster with anode layer (TAL), is illustrated quantitatively using an analytical model that accounts for the effects of secondary electron emission (SEE) from the walls. The model includes a prescription for the quenching of the temperature of the electrons on their way to the anode which results from the enhanced electron energy losses to the wall that occur at, and upstream of, an axial location where the wall potential reverses from electron attracting to electron repellent. For the higher SEE coefficient of an insulator wall (compared to that of a metallic one) this sign reversal occurs at a lower electron temperature and is shown to lead to a more extended acceleration region due to the resulting relaxation of the potential gradient required to balance the electron pressure gradient. By replacing the boron nitride walls (SPT) of an idealized Hall thruster with stainless steel (TAL), the acceleration zone is shown analytically to collapse to a region near the anode having an extent that is about eight times smaller than that for the SPT. The results are used to construct a detailed phenomenological picture of the fundamental difference between the two Hall thruster variants.
Overshoot effect in transient electroluminescence from single layer organic light-emitting diodes
Authors: Pommerehne J. , Nikolaenkov D. V. , Nikitenko V. R. , Bässler H.
Published: July 2002

We investigate the transient electroluminescence (EL) from organic light-emitting diodes based on tristilbeneamine dispersed in polysulfone. An EL overshoot is observed a few microsecond after reversing the driving voltage. The amplitude of the EL spike can considerably exceed the previous EL level. The maximal EL intensity is strongly influenced by temperature, time duration of voltage pulse, and magnitude of reverse voltage. The time integrated overshoot intensity of EL scales as a power law with the pulse length. We attribute the overshoot effect to (i) the accumulation of holes on the interface formed by a polymer and a thin insulating layer of aluminum oxide next to the aluminum cathode under application of positive bias and (ii) the drift of holes back to the anode when the external field is being reversed and recombination with electrons in the bulk of the material. It turns out that the surface roughness of the Al2O3 layer is important for the recombination kinetics under reverse bias.
Absolute intensity measurements of the optical second-harmonic response of metals from 0.9 to 2.5 eV
Authors: Matranga Christopher , Guyot-Sionnest Philippe
Published: July 2002

The absolute intensity of the optical second-harmonic response and its spectral (ωfund≈0.9–2.5 eV) dependence has been measured for Ag(111), polycrystalline Ag, 4-Aminothiophenol/Ag (4-ATP/Ag) and decanethiol/Ag (DT/Ag) surfaces in contact with a liquid electrolyte. Preliminary spectra are also reported for polycrystalline Au and Cu(111) samples. For second-harmonic energies below the plasmon resonance, the magnitude of the nonlinear optical response of clean Ag samples increases as electrode potentials are made more positive. This trend reverses itself for energies above the plasmon resonance. The adsorbate-covered surfaces show a weak or nonexistent potential dependence. A unique feature is found in the 4-ATP/Ag spectra which could possibly be due to a surface charge-transfer state. The Ag results are discussed in the context of a free-electron response from which the spectral and potential dependence of the complex microscopic parameter, a(ω), are extracted. The features in the Au and Cu(111) spectra are not adequately described by this free-electron model and must be related to the effects of interband transitions on the nonlinear optical response.
Pressure-induced structural phase transition of dense droplet microemulsions studied by small-angle x-ray scattering
Authors: Seto Hideki , Nagao Michihiro , Kawabata Youhei , Takeda Takayoshi
Published: July 2002

A small-angle x-ray scattering (SAXS) study of dense water-in-oil droplet microemulsions composed of water, decane, and AOT [sodium bis(2-ethylhexyl) sulfosuccinate] was performed in order to clarify phase behavior with applied pressure and the corresponding structural phase transitions. SAXS spectra were collected for pressures between ambient pressure (0.1 MPa) and 80 MPa and droplet volume fraction, φ, from 0.40 to 0.70. With increasing φ, the mean radius of droplets decreased slightly and polydispersity increased. With increasing pressure, the droplet structure transformed to a two-phase system with coexistence of lamellar and droplet structures, independent of the droplet volume fraction. These results suggest that, with increasing pressure, the increasing inter-droplet attractive force controls the pressure variation of the structure.
Slow β process in simple organic glass formers studied by one and two-dimensional H2 nuclear magnetic resonance. II. Discussion of motional models
Authors: Vogel M. , Rössler E.
Published: July 2002

Applying random walk simulations we analyze results of one- (1D) and two-dimensional (2D) H2 nuclear magnetic resonance (NMR) experiments carried out to study the slow β process of toluene-d5 below the glass transition temperature Tg. In this way, we reveal amplitude and mechanism of rotational jumps involved in the secondary relaxation of a rigid molecule. It is shown that essentially all toluene molecules take part in the β-process. In the glass, nearly independent of temperature, the attributed reorientation is restricted to small solid angles of typically 4°–5° for the majority of particles. Only 10%–20% of the molecules exhibit amplitudes χ>10°. Concerning the mechanism of this highly restricted motion we demonstrate that the reorientation takes place step-by-step via several elementary rotational jumps one after the other. Assuming thermally activated jumps within energy landscapes which are, first, attributed to a small section of the unit sphere for each respective molecule and, second, chosen in accordance with the distribution of energy barriers g(Eβ) found in dielectric spectroscopy we succeed in reproducing the results of 1D and 2D H2 NMR experiments on toluene below Tg.
Optimization of hyperplanar transition states
Authors: Jóhannesson Gı́sli H. , Jónsson Hannes
Published: July 2002

A method for systematically finding the optimal orientation and location of a hyperplanar dividing surface during a transition state theory calculation of a transition rate is presented. The optimization can be carried out during a reversible work evaluation of the free energy barrier. An application to Al adatom diffusion on an Al(100) surface is described. There, the method can converge to give the free energy barrier for the optimal mechanism, a concerted displacement, even when the calculation is initially set up for the less optimal hop mechanism. This illustrates that the method can reveal the optimal mechanism of a transition even when the calculation is started with an incorrect guess.
Long time scale kinetic Monte Carlo simulations without lattice approximation and predefined event table
Authors: Henkelman Graeme , Jónsson Hannes
Published: July 2002

We present a method for carrying out long time scale dynamics simulations within the harmonic transition state theory approximation. For each state of the system, characterized by a local minimum on the potential energy surface, multiple searches for saddle points are carried out using random initial directions. The dimer method is used for the saddle point searches and the rate for each transition mechanism is estimated using harmonic transition state theory. Transitions are selected and the clock advanced according to the kinetic Monte Carlo algorithm. Unlike traditional applications of kinetic Monte Carlo, the atoms are not assumed to sit on lattice sites and a list of all possible transitions need not be specified beforehand. Rather, the relevant transitions are found on the fly during the simulation. A multiple time scale simulation of Al(100) crystal growth is presented where the deposition event, occurring on the time scale of picoseconds, is simulated by ordinary classical dynamics, but the time interval in between deposition events, on the order of milliseconds, is simulated by the long time scale algorithm. The Al(100) surface is found to grow remarkably smooth, even at 30 K because of concerted displacements of multiple atoms with significantly lower activation energy than adatom diffusion on the flat terrace.
The effect of current prepulse on wire array Z-pinch implosions
Authors: Beg F. N. , Lebedev S. V. , Bland S. N. , Chittenden J. P. , Dangor A. E. , Haines M. G.
Published: July 2002

The effect of a prepulse current on the behavior of wire array Z pinches is investigated. The experiments were performed on the MAGPIE generator (1 MA peak current at 240 ns) [I. Mitchell, J. M. Bayley, J. P. Chittenden et al., Rev. Sci. Instrum. 67, 1533 (1996)]. A linear ramp current, ∼500-ns long, was used as a prepulse. The array consisted of 32 15-μm aluminum wires, 23-mm long arranged in a 16-mm diameter circle. With a prepulse of 1 kA/wire, a low density precursor plasma column (ne∼2×1017 cm−3) is formed on the array axis before the start of the main current. Later, the soft x-ray emission shows the growth of an m=1 helical instability in the precursor plasma, which indicates the presence of a current. Without a prepulse current, the precursor plasma on axis is uniform and does not show any instability. The x-ray pulse at stagnation on axis is at least 30 times smaller with a prepulse current than without.
Fractional kinetics for relaxation and superdiffusion in a magnetic field
Authors: Chechkin A. V. , Gonchar V. Yu. , Szydl/owski M.
Published: July 2002

Fractional Fokker–Planck equation is proposed for the kinetic description of relaxation and superdiffusion processes in constant magnetic and random electric fields. It is assumed that the random electric field acting on a test charged particle is isotropic and possesses non-Gaussian Levy stable statistics. These assumptions provide one with a straightforward possibility to consider formation of anomalous stationary states and superdiffusion processes, both properties are inherent to strongly nonequilibrium plasmas of solar systems and thermonuclear devices. The fractional kinetic equation is solved, the properties of the solution are studied, and analytical results are compared with those of numerical simulation based on the solution of the Langevin equations with a noise source having Levy stable probability density. It is found, in particular, that the stationary states are essentially non-Maxwellian ones and, at the diffusion stage of relaxation, the characteristic displacement of a particle grows superdiffusively with time and is inversely proportional to the magnetic field.
The dynamics of chain closure in semiflexible polymers
Authors: Dua Arti , Cherayil Binny J.
Published: July 2002

The mean first passage time of cyclization τ of a semiflexible polymer with reactive ends is calculated using the diffusion-reaction formalism of Wilemski and Fixman [J. Chem. Phys. 60, 866 (1974)]. The approach is based on a Smoluchowski-type equation for the time evolution, in the presence of a sink, of a many-body probability distribution function. In the present calculations, which are an extension of work carried out by Pastor et al. [J. Chem. Phys. 105, 3878 (1996)] on completely flexible Gaussian chains, the polymer is modeled as a continuous curve with a nonzero energy of bending. Inextensibility is enforced on average through chain-end contributions that suppress the excess fluctuations that lead to departures from the Kratky–Porod result for the mean-square end-to-end distance. The sink term in the generalized diffusion equation that describes the dynamics of the chain is modeled as a modified step function along the lines suggested by Pastor et al. Detailed calculations of τ as a function of the chain length N, the reaction distance a, and the stiffness parameter z are presented. Among other results, τ is found to be a power law in N, with a z-dependent scaling exponent that ranges between about 2.2–2.4.
Lifetime spectroscopy for defect characterization: Systematic analysis of the possibilities and restrictions
Authors: Rein S. , Rehrl T. , Warta W. , Glunz S. W.
Published: July 2002

Carrier lifetime is very sensitive to electrically active defects. Apart from detecting the presence of recombination active defects, lifetime measurements allow for a direct identification of defects if the temperature and injection dependence of carrier lifetime is analyzed. In the present work, the Shockley–Read–Hall (SRH) equation is analyzed from a theoretical point of view in order to demonstrate the possibilities and basic restrictions of i_njection- and t_emperature-d_ependent l_ifetime s_pectroscopy (IDLS and TDLS), respectively. To show the variety of attainable curve shapes and to evaluate their suitability for a spectroscopic determination of defect parameters, the general impact of the defect and material parameters on the injection and temperature dependence of SRH lifetime is investigated systematically. From this the basic requirements for an unambiguous defect characterization are derived. The experimental applicability of lifetime spectroscopy is demonstrated by TDLS and IDLS measurements performed on an intentionally molybdenum contaminated Si sample using the microwave-detected photoconductance decay technique and the quasi-steady-state photoconductance technique, respectively. The energy level of 0.335 eV below the conduction band determined from TDLS corresponds well with deep level transient spectroscopy-values reported in literature and could also be determined from the corresponding IDLS curve though not unambiguously.
Incorporation-related structural issues for beryllium doping during growth of GaN by rf-plasma molecular-beam epitaxy
Authors: Ptak A. J. , Wang Lijun , Giles N. C. , Myers T. H. , Romano L. T. , Tian C. , Hockett R. A. , Mitha S. , Van Lierde P.
Published: July 2002

Beryllium incorporation was studied for both Ga-polarity and N-polarity GaN using a series of Be step-doped epitaxial layers. Dopant concentration profiles indicated that surface polarity-related incorporation differences are not pronounced for Be. Significant surface accumulation of Be occurs during growth with surface accumulations approaching a monolayer for heavier doping levels. Transmission electron microscopy studies indicate the surface layer of Be has a significant effect on the microstructure, particularly for near monolayer coverage.
Effect of annealing of polythiophene derivative for polymer light-emitting diodes
Authors: Ahn Taekyung , Lee Haiwon , Han Sien-Ho
Published: July 2002

The nature of the interface between the light-emitting layer and the metal electrode is of importance in determining device performance. The heat treatment of the electroluminescence device can enhance the adhesion of the polymer on the electrode by reducing free volume or pores in the interface. The current–voltage–luminance characteristic of the device with heat treatment at various temperatures was investigated. The annealed device above glass transition temperature (Tg) showed the most efficient characteristics. The current density was increased about 20 times over that of the unannealed device. The light intensity was also increased dramatically by 40 times.
Ion-induced formation of regular nanostructures on amorphous GaSb surfaces
Authors: Facsko S. , Bobek T. , Kurz H. , Dekorsy T. , Kyrsta S. , Cremer R.
Published: July 2002

Crystalline and amorphous GaSb surfaces are compared concerning their response to sputter erosion with low energy Ar+ ions under normal incidence. We show that the formation of regular nanostructures on GaSb is basically independent of whether the initial material is crystalline or amorphous. The similarity in the temporal and spatial evolution demonstrates that the dynamics of the morphology evolution is entirely controlled by a thin amorphous surface layer.
Optical properties of 4H–SiC
Authors: Ahuja R. , Ferreira da Silva A. , Persson C. , Osorio-Guillén J. M. , Pepe I. , Järrendahl K. , Lindquist O. P. A. , Edwards N. V. , Wahab Q. , Johansson B.
Published: July 2002

The optical band gap energy and the dielectric functions of n-type 4H–SiC have been investigated experimentally by transmission spectroscopy and spectroscopic ellipsometry and theoretically by an ab initio full-potential linear muffin-tin-orbital method. We present the real and imaginary parts of the dielectric functions, resolved into the transverse and longitudinal photon moment a, and we show that the anisotropy is small in 4H–SiC. The measurements and the calculations fall closely together in a wide range of energies.
Application of positron annihilation lifetime technique to the study of deep level transients in semiconductors
Authors: Deng A. H. , Shan Y. Y. , Fung S. , Beling C. D.
Published: July 2002

Unlike its conventional applications in lattice defect characterization, positron annihilation lifetime technique was applied to study temperature-dependent deep level transients in semiconductors. Defect levels in the band gap can be determined as they are determined by conventional deep level transient spectroscopy (DLTS) studies. The promising advantage of this application of positron annihilation over the conventional DLTS is that it could further extract extra microstructure information of deep-level defects, such as whether a deep level defect is vacancy related or not. A demonstration of EL2 defect level transient study in GaAs was shown and the EL2 level of 0.82±0.02 eV was obtained by a standard Arrhenius analysis, similar to that in conventional DLTS studies.
Generalized effective medium theory and dielectric relaxation in particle-filled polymeric resins
Authors: Brosseau Christian
Published: July 2002

Dielectric relaxation in disordered solids continue to be in the focus due to the important technological applications in the context of microwave and optical remote sensing and communication. The pragmatic philosophy of the present article is to use a combination of Jonscher’s phenomenological equations with a generalized effective medium equation, due to McLachlan, to study the microwave relaxation dynamics in a technologically interesting system, i.e., a particle-filled polymeric resin. The introduction of a small number of parameters (critical exponents s and t, conductivity threshold φc) into the standard Bruggeman effective medium equation dramatically improves its predictive power. This approach, termed the McLachlan–Jonscher model, has the potential to be quite flexible and is very sensitive to the values of the critical exponents s, t and of the conductivity threshold φc. Furthermore, a comparison of the calculated complex effective permittivity for a series of carbon black-filled polymers with experimental results shows that it can accurately describe the microwave response over a broad range of volume fraction of carbon black. These considerations illustrate the potential for using this coarse grained model to help understand the dielectric relaxation of particle dispersions in polymeric matrixes.
Effects of grain size on the residual loss of Mn–Zn ferrites
Authors: Jeong Weon Hee , Han Young Ho , Song Byung Moo
Published: July 2002

The effect of grain sizes on the electromagnetic properties of Mn–Zn ferrites has been studied up to MHz range. The average grain sizes ranging between 2 and 5 μm were obtained by varying the sintering time from 2 to 12 h. The total power loss at 1 MHz, 25 mT, and 100 °C increased with grain size. The hysteresis loss decreased with increasing grain size, while the eddy current loss and the residual loss increased. The residual loss showed a drastic increase at the grain size around 4 μm, which confirms that the residual loss due to the domain wall resonance should be taken into account at a grain size greater than the monodomain dimension.
Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1
Authors: Hu Feng-xia , Shen Bao-gen , Sun Ji-rong , Wang Guang-jun , Cheng Zhao-hua
Published: July 2002

A very large magnetic entropy change ΔS has been observed in Fe-based cubic NaZn13-type compound LaFe11.2Co0.7Si1.1 near the Curie temperature TC of 274 K. The value of the entropy change is ∼20.3 J/kg K under a magnetic field of 5 T at TC=274 K. It markedly exceeds that of pure Gd at the corresponding temperature range [V. K. Pecharsky & K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1999)]. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The very large magnetic entropy change and low cost suggest that the compound LaFe11.2Co0.7Si1.1 has great potential for applications as magnetic refrigerants near room temperature.
Half-metallic ferromagnetism: Example of CrO2 (invited)
Authors: Coey J. M. D. , Venkatesan M.
Published: July 2002

A broad classification scheme is proposed for half-metallic ferromagnets which embraces the possibilities of itinerant and localized electrons, as well as semimetallic and semiconducting electronic structure. Examples of each type are given. The problems of defining and measuring spin polarization are discussed and some characteristics of half-metals are reviewed with reference to chromium dioxide.
Magnetic entropy change in LaFe13−xSix intermetallic compounds
Authors: Wen G. H. , Zheng R. K. , Zhang X. X. , Wang W. H. , Chen J. L. , Wu G. H.
Published: July 2002

LaFe 13−x Si x compounds with x=2.4, 2.6, and 2.8 have been carefully prepared and characterized. X-ray powder diffraction patterns confirm that the samples crystallize in a single NaZn13-type phase. Magnetic measurements were carried out in a SQUID magnetometer. The Curie temperature Tc of LaFe13−xSix samples decreases nearly 50 K with x increasing from 2.4 to 2.8. Around Tc the samples exhibit a typical second-order phase transition; and no trace of first-order itinerant electron metamagnetic transition is found. So it is reliable to characterize the magnetic entropy change around Tc using magnetic measurement. The maximum values of the magnetic entropy change of LaFe13−xSix samples are found to be, at their Curie temperatures, 5.85, 5.9, and 3.7 J/kg.K for x=2.4, 2.6, and 2.8 under a field change of 5 T (0–5 T). For the quite large magnetic entropy change over a wide temperature range, LaFe13−xSix compounds appear, therefore, to be a potential material candidate for magnetic cooling.
Excitonic line broadening in PbSrSe thin films grown by molecular beam epitaxy
Authors: Shen W. Z. , Wu H. Z. , McCann P. J.
Published: July 2002

Pb 1−x Sr x Se thin films grown by molecular beam epitaxy have been investigated by x-ray diffraction and temperature-dependent photoluminescence measurements with the Sr composition as high as 0.276. Temperature and composition dependent excitonic line broadening effects in PbSrSe thin films have been studied on the basis of the proposed theoretical models and the experimentally obtained lattice constant, excitonic energy gap and effective mass as a function of the alloy composition. The exciton-longitudinal optical phonon coupling model has been employed successfully for PbSrSe with a coupling strength of 51.0 meV, which can be well explained by the proposed theoretical approach. The lattice deformation may have played a key role in the composition dependent broadening in PbSrSe at low temperature, rather than the normally observed alloy disorder effect in III–V and II–VI semiconductor materials.
Discovery of CO2 precursor in the reaction of CO and O on Pt(111)
Authors: Han S.-J. , Lee C.-W. , Yoon H. , Kang H.
Published: July 2002

Using the technique of Cs+ reactive ion scattering (Cs+ RIS), we show that reaction of CO and O on a Pt(111) surface produces a CO2 precursor species that remains stable on the surface up to 300 K. The CO2 species desorbs with an activation energy of 46±3 kJ mol−1, whereas the direct CO2 gas formation via Langmuir–Hinshelwood (LH) reaction between CO and O occurs with an activation energy of 75±4 kJ mol−1. The observation reveals a new pathway for CO oxidation reaction, which is different from the direct LH pathway.
Hydrogen detection on bare SiO2 between metal gates
Authors: Filippini D. , Lundström I.
Published: July 2002

Metal–oxide–semiconductor capacitors with gates of Pd and Au forming a wedge of bare SiO2 between them are described. It is observed that the sensitivity to H2 in the wedge is larger than on the outside of the metal gates due to the different distribution of potential determined by the geometrical constraints. The local response of the surface is obtained through the use of a scanning light pulse technique (SLPT), showing photocurrent versus voltage curves shifted to more negative voltages upon exposure to hydrogen. Within the wedge there is a two-dimensional photocurrent pattern composed of two splitting photocurrent peaks, whose separation depends on the distance between Au and Pd contacts, and on the composition of the ambient. The transient response in the wedge is comparable to that of Pd gates and the possibility of detecting hydrogen (or other molecules) by measuring on the bare insulator surface provides new possibilities for the use of field-effect structures for gas sensing. A semiempirical model describing the SLPT response for this particular geometry, conforms to experimental features, and highlights the sensitivity of the distribution of potential in the wedge to the density of charges induced on the semiconductor surface.
Stress-induced and electromigration voiding in aluminum interconnects passivated with silicon nitride
Authors: Lee Seok-Hee , Bravman John C. , Doan Jonathan C. , Lee Samantha , Flinn Paul A. , Marieb Thomas N.
Published: July 2002

Void nucleation in aluminum interconnects passivated with silicon nitride was studied using a high voltage scanning electron microscope. Extensive stress-induced voiding was observed in these interconnects independent of the passivation thickness. Some of the stress-induced and electromigration-induced voids formed away from the interconnect sidewall in contrast to results from other studies of void nucleation in passivated aluminum lines. Nuclear reaction analysis measured large amounts of hydrogen in the aluminum films passivated with silicon nitride. Transmission electron microscopy showed a high density of nanometer-sized bubbles in the aluminum. These bubbles, which are thought to have formed from hydrogen that evolved from the silicon nitride layer during processing, served as nucleation sites for stress- and electromigration-induced voids.
Anisotropic thermal expansion of BiB3O6
Authors: Teng Bing , Wang Zhengping , Jiang Huaidong , Cheng Xiufeng , Liu Hong , Hu Xiaobo , Dong Shengming , Wang Jiyang , Shao Zongshu
Published: July 2002

The anisotropic thermal expansion of BiB3O6 (BIBO) has been studied in the temperature range 25 °C to 299.5 °C. The three values of principal expansion coefficients α1, α2, and α3 and the orientation of the principal axes are −26.43×10−6 K−1, 50.4×10−6 K−1, 8.53×10−6 K−1, and 8.9°, respectively. Large anisotropic thermal expansion will cause thermal stress, leading to crystal fracture and the formation of parasitic crystallites along the direction of crystal grain growth. The results obtained from transmission electron microscopy support the evidence observed from the measurements of expansion. This may be useful for BIBO crystal growth.
Spin-dependent transport in multiwalled carbon nanotubes
Authors: Zhao B. , Mönch I. , Mühl T. , Vinzelberg H. , Schneider C. M.
Published: July 2002

We report on the observation of spin-polarized tunneling magnetoresistance (TMR) in Co-contacted multiwalled carbon nanotubes (MWCNT). At 4.2 K, we find an increase of the TMR with decreasing junction bias. When the junction bias current is 0.5 nA, the highest value of −36% for the TMR ratio is found. The enhanced TMR ratio is explained by the onset of co-tunneling effects. We also find an asymmetric switching behavior which is tentatively attributed to an additional exchange biasing in the electrode material. For different samples, the TMR can assume both negative and positive values, indicating that the sign of the spin polarization of the tunneling electrons can be reversed. Apparently, the spin polarization sign depends sensitively on the detailed electronic configuration of the contact between Co and MWCNT.
Synthesis of ferrite and nickel ferrite nanoparticles using radio-frequency thermal plasma torch
Authors: Son S. , Taheri M. , Carpenter E. , Harris V. G. , McHenry M. E.
Published: July 2002

Nanocrystalline (NC) ferrite powders have been synthesized using a 50 kW–3 MHz rf thermal plasma torch for high-frequency soft magnet applications. A mixed powder of Ni and Fe (Ni:Fe=1:2), a NiFe permalloy powder with additional Fe powder (Ni:Fe=1:2), and a NiFe permalloy powder (Ni:Fe=1:1) were used as precursors for synthesis. Airflow into the reactor chamber was the source of oxygen for oxide formation. XRD patterns clearly show that the precursor powders were transformed into NC ferrite particles with an average particle size of 20–30 nm. SEM and TEM studies indicated that NC ferrite particles had well-defined polygonal growth forms with some exhibiting (111) faceting and many with truncated octahedral and truncated cubic shapes. The Ni content in the ferrite particles was observed to increase in going from mixed Ni and Fe to mixed permalloy and iron and finally to only permalloy starting precursor. The plasma-torch synthesized ferrite materials using exclusively the NiFe permalloy precursor had 40%–48% Ni content in the Ni-ferrite particle, differing from the NiFe2O4 ideal stoichiometry. EXAFS was used to probe the cation coordination in low Ni magnetite species. The coercivity and Neel temperature of the high Ni content ferrite sample were 58 Oe and ∼590 °C, respectively.
Advances in full-wave modeling of radio frequency heated, multidimensional plasmas
Authors: Jaeger E. F. , Berry L. A. , D’Azevedo E. , Batchelor D. B. , Carter M. D. , White K. F. , Weitzner H.
Published: July 2002

Previous full-wave models for rf heating in multidimensional plasmas have relied on either cold-plasma or finite Larmor radius approximations. These models assume that the perpendicular wavelength of the rf field is much larger than the ion Larmor radius, and they are therefore limited to relatively long wavelengths and low cyclotron harmonics. Recently, alternate full-wave models have been developed that eliminate these restrictions. These “all orders spectral algorithms” take advantage of new computational techniques for massively parallel computers to solve the integral form of the wave equation in multiple dimensions without any restriction on wavelength relative to orbit size, and with no limit on the number of cyclotron harmonics retained. These new models give high-resolution, two-dimensional solutions for mode conversion and high harmonic fast wave heating in tokamak geometry. In addition, they have been extended to give fully three-dimensional solutions of the integral wave equation for minority ion cyclotron heating in stellarator geometry. By combining multiple periodic solutions for individual helical field periods, it is possible to obtain complete wave solutions valid over the entire volume of the stellarator for arbitrary antenna geometry.
Shear-free relativistic fluids and the absence of movable branch points
Authors: Halburd R. G.
Published: July 2002

The problem of determining the metric for a nonstatic shear-free spherically symmetric fluid (either charged or neutral) reduces to the problem of determining a one-parameter family of solutions to a second-order ordinary differential equation (ODE) containing two arbitrary functions f and g. Choices for f and g are determined such that this ODE admits a one-parameter family of solutions that have poles as their only movable singularities. This property is strictly weaker than the Painlevé property and it is used to identify classes of solvable models. It is shown that this procedure systematically generates many exact solutions including the Vaidya metric, which does not arise from the standard Painlevé analysis of the second-order ODE. Interior solutions are matched to exterior Reissner–Nordstrøm metrics. Some solutions given in terms of second Painlevé transcendents are described.
The fuzzy sphere ⋆-product and spin networks
Authors: Freidel Laurent , Krasnov Kirill
Published: July 2002

We analyze the expansion of the fuzzy sphere noncommutative product in powers of the noncommutativity parameter. To analyze this expansion we develop a graphical technique that uses spin networks. This technique is potentially interesting in its own right as introducing spin networks of Penrose into noncommutative geometry. Our analysis leads to a clarification of the link between the fuzzy sphere noncommutative product and the usual deformation quantization of the sphere in terms of the ⋆-product.
A continued fraction approach to cross diffusivity in a binary fluid mixture
Authors: Dhole Kajal , Samanta Alok , Ghosh Swapan K.
Published: July 2002

A microscopic approach to the cross diffusivity in a binary fluid mixture has been developed using the theoretical framework of continued fraction method. A suitable transformation of the velocities of distinct particles is used to formulate a continued fraction approach for the cross velocity correlation function. The self diffusivities needed for this calculation are obtained through mode coupling theory. The proposed theory is applied to a Lennard-Jones fluid mixture and the calculated cross diffusivities are found to be in good agreement with the available computer simulation results. The theory predicts the correct trend for the variation of cross diffusivity with mass and composition and also explains qualitatively the nature of the time dependence of the cross velocity correlation.