bluuebunny/crossref_metadata_2025 huggingface dataset:
Recovery of microindents in a nickel–titanium shape-memory alloy: A “self-healing” effect
Authors: Ni Wangyang , Cheng Yang-Tse , Grummon David S.
Published: July 2002

The thermally induced recovery of microscopic deformation in a nickel–titanium shape-memory alloy was examined. Surface deformation was simulated by indenting the alloy in the martensite phase at room temperature using both spherical and pyramidal indenters. We show that deformation in spherical microindents can be almost completely reversed by moderate heating. Partial recovery was observed for pyramidal impressions formed by a Vickers indenter and the recovery ratio was independent of the indentation depth. The observations were rationalized using the concept of representative strain and maximum stress under the spherical and pyramidal indenters.
Water vapor as an active scalar in tropical atmospheric dynamics
Authors: Sobel Adam H.
Published: July 2002

Water vapor is a constituent of the tropical atmosphere which, though to a significant extent locally controlled by vertical advection, precipitation, and surface evaporation, is also affected by horizontal advection. Water vapor affects the flow in turn, because a humid atmosphere supports deep, precipitating convection more readily than a dry atmosphere. Precipitation heats the atmosphere, and this heating drives the flow. Water vapor is thus a dynamically active constituent. Simplifications to the primitive equations of dynamical meteorology, based on the so-called weak temperature gradient approximation, are presented which highlight this behavior. The weak temperature gradient approximation is valid on large scales near the equator. It eliminates gravity waves, leaving only balanced dynamics, though the fundamental balance occurs in the temperature rather than the momentum equation (as is customary in most balance models of geophysical fluid dynamics). The dynamical role of water vapor is examined in a couple of idealized contexts, where either the vertical or horizontal structure of the flow is severely simplified.
Transport properties of CrO2 (110) films grown on TiO2 buffered Si substrates by chemical vapor deposition
Authors: Liu S. J. , Juang J. Y. , Wu K. H. , Uen T. M. , Gou Y. S. , Lin J.-Y.
Published: July 2002

Epitaxial CrO2 (110)-oriented films were fabricated on Si (100) substrates buffered by rutile TiO2 derived from oxidation of a pulsed-laser-deposited TiN layer. The epitaxial films of CrO2 were prepared by chemical vapor deposition in a two-zone furnace with oxygen flow from a CrO3 precursor. The transport measurements show that the CrO2 films are metallic with a Curie temperature of about 380 K. The temperature dependence of resistivity was best described by a phenomenological expression ρ(T)=ρ0+AT2e(−Δ/T) over the range of 5–350 K with Δ=94 K. The magnetic measurements show the in-plane coercive fields are about 30 and 60 Oe at 300 and 5 K, respectively. The temperature dependent spontaneous magnetization follows Bloch’s T3/2 law and the slope suggests a critical wavelength of λΔ∼30.6 Å beyond which spin-flip scattering becomes important.
Asymmetric evolution of eddies in rotating shallow water
Authors: Arai Masazumi , Yamagata Toshio
Published: July 2002

It is demonstrated that cyclones evolve in a way different from that of anticyclones in rotating shallow water. The anticyclones merge and eventually take circular coherent forms, but cyclones are elongated with active enstrophy cascading. This asymmetric evolution is strengthened with increasing surface displacements. When the initial surface displacement exceeds a certain critical value, the initial elongation of a cyclonic ellipse ends up with splitting in two cyclones. This splitting of the cyclonic ellipse is always associated with the first appearance of a saddle point inside the core, due to irrotational, ageostrophic motion. The appearance of the saddle point inside the core seems to be a necessary condition for splitting of the core of the cyclonic ellipse with surface displacements. The linear stability analysis of the elliptical vortex is consistent qualitatively with both of the simulation results and the kinematic axisymmetrization/elongation principle.
Modeling spatial patterns in Dictyostelium
Authors: Levine Herbert
Published: July 2002

The life cycle of Dictyostelium discoideum provides a striking example of the transition from single cell behavior to multicellular cooperativity. In this paper the status of the attempts at making semiquantitative models of the aggregation phase of this cycle is reviewed. Specifically, it is discussed how the propagation of cAMP waves is a typical example of excitable signaling, which is then rendered unstable by coupling to cell chemotaxis. To investigate the streaming pattern that emerges from this clumping instability, we next turn to a new simulation strategy, which couples dynamical cell-like entities (‘‘bions’’) to continuum chemical concentration fields. Finally, we discuss two directions for further research: One is the study of the robustness with respect to the variation of system parameters (such as the cell density) exhibited by the biological system, but not by any simple model. The other concerns going beyond the aggregation phase to tackle the three-dimensional problem of slug formation and motion.
Nonlinear dynamics of the voice: Signal analysis and biomechanical modeling
Authors: Herzel Hanspeter , Berry David , Titze Ingo , Steinecke Ina
Published: July 2002

Irregularities in voiced speech are often observed as a consequence of vocal fold lesions, paralyses, and other pathological conditions. Many of these instabilities are related to the intrinsic nonlinearities in the vibrations of the vocal folds. In this paper, bifurcations in voice signals are analyzed using narrow-band spectrograms. We study sustained phonation of patients with laryngeal paralysis and data from an excised larynx experiment. These spectrograms are compared with computer simulations of an asymmetric 2-mass model of the vocal folds.
Characterization of high-efficiency silicon solar cells
Authors: Green M. A. , Blakers A. W. , Osterwald C. R.
Published: July 2002

Preliminary results on silicon solar cells of improved performance recently have been described. The present paper describes the results of a more detailed optical and electrical characterization of these devices. The high performance of the cells is shown to be due to a high optical efficiency and near ideal junction characteristics combined with low parasitic resistance losses.
Free-carrier absorption from electrons in confined systems
Authors: Adamska Halina , Spector Harold N.
Published: July 2002

We present a calculation of the free-carrier absorption for ultrathin wires fabricated from III-V compound semiconductor materials, where the carriers are scattered either by polar optical phonons or acoustic phonons. The free-carrier absorption coefficient is found to be an oscillatory function of the photon frequency when the carriers are confined in quasi-one-dimensional structures. We also found that the absorption in quasi-one-dimensional structures is enhanced by going to wires of smaller transverse dimensions over its value in the bulk III-V compound semiconducting materials. In addition, it was found that in quasi-one-dimensional structures made from these materials the electron–polar-optical-phonon interaction gives greater contribution to the absorption than the electron–acoustic-phonon interaction. Moreover, the absorption is found to depend upon the polarization of the electromagnetic radiation relative to the direction of the confinement of the carriers. The oscillatory behavior of the free-carrier absorption is explained in terms of phonon-assisted transitions between the quantized subbands which arise from the confinement of the carriers in quasi-low-dimensional structures.
Electron recombination rates at the gold acceptor level in high-resistivity silicon
Authors: Lu Luke Su , Sah Chih-Tang
Published: July 2002

Electron emission rates and capture cross sections at the gold acceptor level have been obtained in silicon p+/n diodes with phosphorus concentrations of 1.2×1012 cm−3 and 5×1014 cm−3, showing no phosphorus concentration dependencies. This indicates negligible electric field dependence of the electron emission rate in the range 102–105 V/cm and supports the contention that the capture cross section is independent of the phosphorus dopant impurity concentration. The magnitude of the capture cross-section data supports a neutral potential model of the gold acceptor center.
Plasma-enhanced growth and composition of silicon oxynitride films
Authors: Denisse C. M. M. , Troost K. Z. , Oude Elferink J. B. , Habraken F. H. P. M. , van der Weg W. F. , Hendriks M.
Published: July 2002

Silicon oxynitride films with varying oxygen/nitrogen ratio were grown from SiH4, N2O, and NH3 by means of a plasma-enchanced chemical vapor deposition process. The elemental composition of the deposited films was measured by a variety of high-energy ion beam techniques. To determine the chemical structure we used Fourier transform infrared absorption spectroscopy and electron-spin resonance. Ellipsometric data and values for mechanical stress are also reported. We show that the entire range of compositions from silicon oxide to silicon nitride can be covered by applying two different processes and by adjusting the N2O/NH3 gas flow ratio of the respective processes. It is suggested that the N2O/SiH4 gas flow ratio is the major deposition characterization parameter, which also controls the chemical structure as far as the hydrogen bonding configuration is concerned. We found that the films contain significant amounts of excess silicon and that the mechanical stress in the oxynitrides is lower than in plasma nitride. The electron-spin density is low (∼1017/cm3) in all samples.
Temperature dependence of the thermal expansion of semiconductors CdIn2S4 and CdInGaS4
Authors: Toyoda T. , Irie T. , Endo S. , Nakanishi H.
Published: July 2002

The thermal expansion of the compound semiconductors CdIn2S4 and CdInGaS4 has been investigated on single crystals between 120 and 570 K. At high temperatures the anharmonic effects in lattice vibrations of the ∥c-axis direction in CdInGaS4 is similar to that in the ⊥c-axis direction, and that in CdIn2S4 is similar to that in CdInGaS4 in spite of the diverse crystal structures. The characteristic temperature (corresponding to Debye temperatures) for CdIn2S4 is 175±10 K, and for CdInGaS4 it is 200±10 K (∥c axis) and 161±10 K (⊥c axis), respectively.
Calculation of x-ray intensity from a rough sample based on a statistical model
Authors: Hwang Bing , Houska C. R.
Published: July 2002

An x-ray intensity correction is developed which begins with a roughness model that is often used to describe real surfaces. This is based upon a normal distribution of surface asperities relative to a mean plane. Pair correlation between absorbing elements along x-ray paths either entering or leaving the sample with respect to the signal producing element is accomplished by means of an exponential autocorrelation function. This allows the degree of roughness to be varied on a local scale to fit specific surfaces using statistical data. Equations are developed to describe x-ray fluorescence and diffraction signals for symmetric and asymmetric beam optics. Theory is compared with experiment using a roughened, fully stabilized zirconia sample.
Superconducting properties of amorphous MoX (X=Si, Ge) alloy films for Abrikosov vortex memory
Authors: Kubo Shugo
Published: July 2002

Structural and superconducting properties of amorphous MoSi (∼55%Si) and MoGe (∼30% Ge) films (200 and 50 nm in thickness) prepared by rf magnetron sputtering were examined for the purpose of application to Abrikosov vortex memory devices. Amorphous single-phase films were obtained at concentrations of 20% Si and above, while a crystalline phase was detected even at a concentration of 30% Ge. The highest critical temperature Tc was 7.2–7.3 K in Mo-20% Si films. Magnetic penetration depth λ at 4.2 K and upper critical field dependence on temperature Bc2(T) were measured. The Bc2(T) behavior in the vicinity of Tc showed a tail-like shape, which most likely is the result of structural inhomogeneities in the prepared films. The λ dependence on Si concentration was satisfactorily explained through the Ginzburg–Landau–Abrikosov–Gorkov (GLAG) dirty limit theory using the corrected critical temperature T*c which is defined by the Bc2(T) results. Other superconducting parameters (GL parameter κGL and coherence length ξGL ) and bare electronic density of states N(0) were derived from the GLAG relationships using the temperature slope of Bc2(T). Flux pinning was examined through the critical current measurements at 4.2 K in the perpendicular applied field to the film surface. Preliminary results on edge pinning estimation were also presented. It was concluded that the most suitable film for application to a vortex storage region (VSR) in an Abrikosov vortex memory cell is a 30% Si film 50 nm in thickness, considering both Tc and flux pinning force. Write current behavior of a memory cell with an amorphous Mo-30% Si VSR was discussed on the basis of measured superconducting properties.
Structural origin of surface morphological modifications developed on poly(ethylene terephthalate) by excimer laser photoablation
Authors: Novis Y. , Pireaux J. J. , Brezini A. , Petit E. , Caudano R. , Lutgen P. , Feyder G. , Lazare S.
Published: July 2002

The ablation of poly(ethylene terephthalate) (PET) by Ar-F laser radiation has been studied at different fluences for amorphous and semicrystalline samples by scanning electron microscopy (SEM) and by etch-depth measurements. The surface structure created on PET has been interpreted as resulting mainly from a difference in the etch rate—and not the ablation threshold—between amorphous and semicrystalline PET. SEM pictures clearly show the melting of the PET surface at high fluence (≥80 to 100 mJ/cm2) and not below that value. UV laser etching is proposed as a quick and easy method for observing the crystalline subsurface structure of aromatic semicrystalline polymers, as long as low fluence and a low pulse repetition rate are used.
Diffusion characteristics and waveguiding properties of proton-exchanged and annealed LiNbO3 channel waveguides
Authors: Vohra Sandeep T. , Mickelson Alan R. , Asher Sally E.
Published: July 2002

A discussion of the modeling of the proton exchange fabrication process is given, including a brief review of results of analytical measurements of concentration and concentration/index relation, as well as some presentation of the speculations made as to mechanisms of resulting index profiles. Discussion is then given to modeling of the exchange process itself via the ion exchange equations. The concentration profiles of protons and lithium in proton-exchanged LiNbO3 crystals measured by secondary ion mass spectroscopy are then presented. The proton concentration profiles are found to be nearly rectangular in shape. The diffusion characteristics of the ion exchange process are empirically modeled by solving the ion exchange equations with concentration dependent self-diffusion coefficients. Secondary ion mass spectroscopy (SIMS) measurements on annealed H+:LiNbO3 samples show proton and lithium concentration profiles to be Gaussian in nature. The proton and lithium concentration profiles of annealed H+:LiNbO3 samples are fit to the solutions of diffusion equations obtained from standard thermal diffusion theory. The fit has resulted in values of diffusion coefficient for protons during annealing (Dp=0.77 μm2/h). Index profiles are then calculated from these equations and are found to agree with the profiles reported elsewhere. Optical mode measurements on channel waveguides fabricated under various diffusion conditions and annealed for various times, are carried out. The mode-tailoring ability of the proton exchange process is demonstrated by fabricating low-loss single-mode channel guides (λ=0.83 μm) having channel widths as large as 6 and as small as 2.5 μm, simply by changing the diffusion conditions. Some anomalous results obtained in waveguides fabricated under extreme conditions are discussed.
Chemical and physical properties of laser-treated poly(ethyleneterephthalate)
Authors: Bahners Thomas , Knittel Dierk , Hillenkamp Franz , Bahr Ute , Benndorf Carsten , Schollmeyer Eckhard
Published: July 2002

The effects on chemical and physical properties due to microstructuring of poly(ethyleneterephthalate) (PETP) fiber surfaces by uv-laser treatment are investigated. Data are given about a slight color change at the irradiated sites of laser-treated fabrics in relation to the amount of debris deposited under ablating conditions. The debris formed under irradiation in air consists of carbonlike particles. Evidence is given for the pure pyrolytic nature of uv-laser ablation of PETP at 248 nm by using laser desorption mass spectroscopy. Chemical modification of the surface, yielding some carboxylic groups, an increased O-to-C ratio, and a small change of the crystallinity of the fiber surface is detected using x-ray-induced photoelectron spectroscopic and Fourier-transform infrared analysis.
Envelope solitons of highly dispersive and low dispersive spin waves in magnetic films (invited)
Authors: Kalinikos B. A. , Kovshikov N. G. , Slavin A. N.
Published: July 2002

Theoretical description of spin wave (SW) envelope solitons in nonlinear dispersive media with dissipation is given in the framework of a nonlinear Schrodinger equation model with relaxation term. The conditions necessary for envelope soliton observation in magnetic films (MF) and film structures are formulated. Different types of SW dispersion laws are discussed in the context of envelope soliton formation. Formation of envelope solitons at microwave frequencies in periodic structures and in multilayered MF structures are predicted. Theoretically calculated parameters of envelope solitons of both highly dispersive and low dispersive SW in MF are compared to the results of experiments performed in thin yttrium-iron garnet (YIG) films. New experimental results showing the process of formation and propagation of envelope solitons of low dispersive dipolar SW in YIG films are presented.
Measurement of electron emission due to energetic ion bombardment in plasma source ion implantation
Authors: Shamim M. M. , Scheuer J. T. , Fetherston R. P. , Conrad J. R.
Published: July 2002

An experimental procedure has been developed to measure electron emission due to energetic ion bombardment during plasma source ion implantation. Spherical targets (radius=2 cm) of copper, stainless steel, graphite, titanium alloy (Ti-6Al-4V) and aluminum alloy (6061) were biased negatively to 20, 30, and 40 kV in argon and nitrogen plasmas. A Langmuir probe was used to detect the propagating sheath edge and a Rogowski transformer was used to measure the current to the target. The measurements of electron emission coefficients compare well with those measured under similar conditions [B. Szapiro and J. J. Rocca, J. Appl. Phys. 65, 3713 (1989)].
Preferred crystal orientation of cobalt ferrite thin films induced by ion bombardment during deposition
Authors: Okuno Shiho N. , Hashimoto Susumu , Inomata Koichiro
Published: July 2002

Thin films of CoxFe3−xO4−y (x=0.8, 1.0, 1.15, 1.25, 1.5, 1.8) were prepared under 100–500 eV Ar+-ion bombardment during film deposition. Strong crystallization enhancement and preferred crystallographic orientation were observed as the effects of ion bombardment. Crystal orientation normal to the substrate depended on the composition x; 〈110〉-axis orientation was obtained for x=0.8 and 1.0 and 〈111〉 orientation for x=1.15, 1.25, 1.5, and 1.8. The orientation was not affected significantly by the incident angle of the ions and did not vary with ion energy in the range 100–500 eV. The mechanism of crystal orientation is thought to have a close relationship with the preferential sputtering of oxygen by Ar+-ion impact.
Emission spectroscopy on a supersonically expanding argon/silane plasma
Authors: Meeusen G. J. , Ershov-Pavlov E. A. , Meulenbroeks R. F. G. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

Results from emission spectroscopy measurements on an Ar/SiH4 plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A, V=80 V, Ar flow=60 scc/s and pressure 4 × 104 Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4 is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018 m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+ particles. The emission spectrum of the SiH radical, the A 2Δ–X 2Π electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si+.
Carrier injection, transport, and their effects on photoinduced dielectric breakdown in single crystalline paraffin (n-C36H74)
Authors: Neff H. , Lange P.
Published: July 2002

Carrier injection and transport in single crystalline paraffin (n-C36H74) have been investigated on the basis of time-of-flight data and dc-photo injection experiments. The undoped, high-quality molecular crystal under study represents an isoelectronic analog to polyethylene. Charge-carrier mobilities in the range μ≤0.1 cm2 V−1 s−1 at room temperature have been recorded for both electrons and holes at moderate external electric fields. A clear correlation has been found between transport properties and the molecular structure. Regarding defect properties, significant differences have been observed for both types of carriers in the material. The presence of intrinsic deep traps is crucial to the transport properties of electrons and dominates the overall behavior of the material, but are virtually absent for injected holes. A decrease of the electron mobility with increasing external fields has been observed for electrons, while the opposite behavior has been found for injected holes. Photoinduced avalanching is largely due to a trap-enhanced field effect, as reported in the literature, where the massive injection of holes from the counter electrode is initiated above a critical external field.
Microscopic mechanics of fiber networks
Authors: Åström Jan , Saarinen Sami , Niskanen Kaarlo , Kurkijärvi Juhani
Published: July 2002

We report simulations of two-dimensional fiber networks of random geometry. The stress distribution along a fiber agrees with the mean-field Cox prediction, but the stress transfer factor is determined by the properties of the whole fiber and not by just the local segment stiffness as suggested by micromechanical models. This leads to a linear density dependence of the Young’s modulus of a network. The initial loss of stiffness at small strain can be explained with an exponential frequency distribution of microscopic stresses, and the asymptotic stiffness at large external strain agrees with mean-field predictions. The simulated behavior is independent of the microscopic fracture mechanism in both regions.
Tunneling and relaxation of photogenerated carriers in near-surface quantum wells
Authors: Emiliani V. , Bonanni B. , Frova A. , Capizzi M. , Martelli F. , Stone S.-S.
Published: July 2002

It is shown that photoluminescence emission from AlGaAs/GaAs and InGaAs/GaAs near-surface quantum wells can be a nonlinear function of the excitation-power density depending on the surface-barrier thickness and on the conditions of the barrier-oxide interface. By studying the rate-equation system, it is shown that this nonlinear effect is mainly due to the competition between tunneling to surface states and relaxation of photogenerated carriers within the quantum well. The information about surface states that one can obtain from this nonlinear behavior is also discussed.
Gain characteristics of a high concentration Er3+-doped phosphate glass waveguide
Authors: Ohtsuki T. , Peyghambarian N. , Honkanen S. , Najafi S. I.
Published: July 2002

A channel waveguide on an erbium-doped phosphate laser glass was fabricated by a dry silver-film ion exchange technique, and its gain properties were studied experimentally. The propagation loss of the fabricated waveguide was 0.63 dB/cm at 1.3 μm. Er3+ concentration of 1×1020 ions/cm3 was chosen so that no concentration quenching occurred. This was confirmed by measuring a fluorescence lifetime of 8.4 ms at 1.54 μm. Gain of the fabricated waveguide was measured by using a Ti:sapphire laser at a wavelength of 980 nm and a laser diode of 1.532 μm producing pump and signal beams, respectively. The signal wavelength used for the experiments was shorter than the emission peak, and the measured gain of the 1.8 cm waveguide was comparable to the total loss. However, the model showed that lasing is expected at the emission peak with a 3.6 cm long waveguide fabricated in a similar manner. Calculation results showed that the potential gain of 8.8 dB can be realized with 250 mW pump power, and a 7.2 cm long waveguide provided that mode overlap can be increased by 20% and propagation loss can be reduced by 50%. No significant upconversion effect was observed up to 1.1×106 W/cm2 pump intensity.
Free-carrier absorption in n-type gallium arsenide films for polar optical phonon scattering
Authors: Wu Chhi-Chong , Lin Chau-Jy
Published: July 2002

The free-carrier absorption in n-type GaAs films has been investigated for quantum well structures fabricated from III–V semiconducting materials where polar optical phonon scattering is predominant. Attention is given mainly to the case where the electromagnetic radiation is polarized in the layer plane, and the processes involving both emission and absorption of polar optical phonons. The energy band of electrons in semiconductors is assumed to be nonparabolic. Results are shown that the free-carrier absorption coefficient in n-type GaAs films depends upon the photon frequency, the width of the quantum wells, and temperature. However, in the small quantum well region such as the width of quantum wells d<30 Å, the free-carrier absorption coefficient will be independent of temperature. Moreover, the free-carrier absorption coefficient oscillates with the width of quantum wells for larger quantum wells.
Interfacial reactions and thermal stability of ultrahigh vacuum deposited multilayered Mo/Si structures
Authors: Liang J. M. , Chen L. J.
Published: July 2002

Interfacial reactions and thermal stability of ultrahigh vacuum deposited multilayered Mo/Si structures have been investigated by high resolution transmission electron microscopy in conjunction with fast Fourier transform and autocorrelation function analysis. For samples with nominal atomic ratios Mo:Si=1:2 and 3:1, well defined multilayered Mo/Si structures were obtained after annealing at 250 °C for 30 min. On the other hand, distinct multilayered MoSi2/Si structure was formed only for Mo:Si=1:2 samples after annealing at 650 °C for 1 h. Multiphases were observed to simultaneously form in samples annealed at 400–500 °C. After 600 °C annealing for 1 h, tetragonal MoSi2 was the only silicide phase observed for the Mo:Si=1:2 samples, whereas both tetragonal and hexagonal MoSi2 were present in Mo:Si-3:1 samples. The stability of the multilayered Mo/Si structures was found to depend critically on the atomic ratios of constituent elements, bilayer period, and annealing conditions. The results are interpreted in terms of the delicate balance among intermixing of constituent atoms, silicide formation, and crystallization of silicon.
A universal optimum work rate potential for continuous endoreversible Carnot heat engine cycles
Authors: Blank David A. , Bhattacharyya Souvik
Published: July 2002

The theory of finite-time thermodynamics for continuous endoreversible cycles gives rise to an optimum efficiency at maximum power output of η=1−(TL/TH)0.5 in contrast to the upper limit of η=1−(TL/TH) obtained from infinite-time thermodynamics. It is shown here that, additionally, for continuous Carnot cycles, the finite-time optimum rate of work output (Ẇopt), is exactly half of that obtained for infinite-time reversible cycles (Carnot work rate, Ẇrev) operating between the same temperature limits (i.e. Ẇopt=1/2Ẇrev). This expression is true for both unoptimized as well as optimized mass flow rate conditions. The formulation used in the analysis is universal for all working substances for cycles using linear heat transfer laws.
Eigenfrequencies of a rectangular atomic force microscope cantilever in a medium
Authors: Elmer Franz-Josef , Dreier Markus
Published: July 2002

We calculate the eigenfrequencies of a rectangular cantilever of an atomic force microscope immersed in a fluid or a gas. To do so, the problem of combined elastomechanical and hydrodynamical equations is solved approximatively. The results are compared with experimentally obtained frequencies. For water the difference between experiment and theory is less than 4% if the ratio of the height of the cantilever to the width is less than 1/20 and if the corresponding eigenmode has at least four nodes.
Contact boundary conditions and the Dyakonov–Shur instability in high electron mobility transistors
Authors: Crowne Frank J.
Published: July 2002

Dyakonov and Shur have proposed a novel device structure based on dc biasing an ordinary high electron mobility transistor (HEMT) while subjecting it to unusual ac boundary conditions at its source and drain [M. Dyakonov and M. Shur, Phys. Rev. Lett. 71, 2465 (1993)]. Under these conditions, the drifting two-dimensional electron gas within the HEMT channel acts as a trapped one-component plasma which exhibits damped normal-mode oscillations similar to those of an organ pipe under zero dc bias, and an unexpected instability and gain at large dc biases. In this article, the work of Dyakonov and Shur is generalized by allowing the plasma more hydrodynamic degrees of freedom. In particular, it is found that the description used by Dyakonov and Shur must be generalized to incorporate a more complicated picture of the plasma modes.
Internal friction and dislocation collective pinning in disordered quenched solid solutions
Authors: D’Anna G. , Benoit W. , Vinokur V. M.
Published: July 2002

We introduce the collective pinning of dislocations in disordered quenched solid solutions and calculate the macroscopic mechanical response to a small dc or ac applied stress. This work is a generalization of the Granato–Lücke string model, able to describe self-consistently short and long range dislocation motion. Under dc applied stress the long distance dislocation creep has at the microscopic level avalanche features, which result in a macroscopic nonlinear “glassy” velocity-stress characteristic. Under ac conditions the model predicts, in addition to the anelastic internal friction relaxation in the high frequency regime, a linear internal friction background which remains amplitude-independent down to a crossover frequency to a strongly nonlinear internal friction regime.
Three-dimensional simulation of quantum cellular automata and the zero-dimensional approximation
Authors: Wu Hua , Sprung D. W. L.
Published: July 2002

We present a three-dimensional simulation of the quantum cellular automaton introduced by Lent et al. [P. D. Tougaw and C. S. Lent, J. Appl. Phys. 75, 1818 (1994)]. The Hamiltonian for their Hubbard model is derived as a zero-dimensional approximation to the Hamiltonian in our simulation. This provides a link between device properties and the phenomenological parameters such as tunneling coefficients.
Electro-optic characterization of (Pb, La)TiO3 thin films using prism-coupling technique
Authors: Boudrioua A. , Dogheche E. , Remiens D. , Loulergue J. C.
Published: July 2002

Transparent ferroelectric (Pb, La)TiO3 thin films were grown by rf magnetron sputtering on (0001) Al2O3 substrates. The prism coupling technique was used to determine the refractive indices of the obtained planar waveguides (n0=2.3745 and ne=2.2705 at 633 nm). Measurements by such a guided-waves technique have also been of considerable interest for studying the anisotropic nature of films. Furthermore, electro-optic effects were investigated in this study using the change of the resonant coupling angle. The latter is induced by the refractive index variation caused by the electro-optic effect when an electric field is applied to the film. The linear electro-optic coefficient r13, thus obtained, is about 55 pm/V. This high value illustrates the suitability of this material for optical applications such as optical switches and electro-optic modulations.
Simulation of dislocations on the mesoscopic scale. I. Methods and examples
Authors: Schwarz K. W.
Published: July 2002

Peach–Koehler theory is implemented to simulate the motion of arbitrarily configured interacting dislocations, located on arbitrary glide planes and having any allowed Burgers vector. The self-interaction is regularized by a modified Brown procedure, which remains stable and loses accuracy in a well-controlled manner as atomic dimensions are approached. The method is illustrated by applying it to several examples of single and interacting dislocations in an fcc slip system. The critical strain for the propagation of a dislocation in a capped layer is calculated and found to be in excellent agreement with theory. Dislocations in a layer with a free surface are studied to test simplified methods for modeling the dislocation–surface interaction. Frank–Read sources are simulated in an infinite medium and in a strained layer. The latter are seen to give rise to the characteristic pileup structures often observed experimentally. The interaction between two initially straight dislocations on intersecting glide planes is studied as a function of relative angle and initial separation. It is found that an attractive instability occurs for a well-defined range of relative angles, and that this range depends only weakly on the initial separation. While this suggests that the detailed calculation of such interactions could be replaced by a simple set of interaction rules specifying their outcome, a variety of factors limiting the usefulness of such rules can be identified. It is further determined that, when an attractive instability occurs, the configuration assumed by the dislocations as they near each other bears little resemblance to any simple starting configuration. This suggests that calculations of the type presented here could provide a useful starting point for atomistic calculations of interacting dislocations.
Predicted bond length variation in wurtzite and zinc-blende InGaN and AlGaN alloys
Authors: Mattila T. , Zunger Alex
Published: July 2002

Valence force field simulations utilizing large supercells are used to investigate the bond lengths in wurtzite and zinc-blende InxGa1−xN and AlxGa1−xN random alloys. We find that (i) while the first-neighbor cation–anion shell is split into two distinct values in both wurtzite and zinc-blende alloys (RGa−N1≠RIn−N1), the second-neighbor cation–anion bonds are equal (RGa−N2=RIn−N2). (ii) The second-neighbor cation–anion bonds exhibit a crucial difference between wurtzite and zinc-blende binary structures: in wurtzite we find two bond distances which differ in length by 13% while in the zinc-blende structure there is only one bond length. This splitting is preserved in the alloy, and acts as a fingerprint, distinguishing the wurtzite from the zinc-blende structure. (iii) The small splitting of the first-neighbor cation–anion bonds in the wurtzite structure due to nonideal c/a ratio is preserved in the alloy, but is obscured by the bond length broadening. (iv) The cation–cation bond lengths exhibit three distinct values in the alloy (Ga–Ga, Ga–In, and In–In), while the anion–anion bonds are split into two values corresponding to N–Ga–N and N–In–N. (v) The cation–related splitting of the bonds and alloy broadening are considerably larger in InGaN alloy than in AlGaN alloy due to larger mismatch between the binary compounds. (vi) The calculated first-neighbor cation–anion and cation–cation bond lengths in InxGa1−xN alloy are in good agreement with the available experimental data. The remaining bond lengths are provided as predictions. In particular, the predicted splitting for the second-neighbor cation–anion bonds in the wurtzite structure awaits experimental testing.
Dynamics of a coplanar-electrode plasma display panel cell. I. Basic operation
Authors: Rauf Shahid , Kushner Mark J.
Published: July 2002

Plasma display panels (PDPs) are a technology for large-area high-brightness flat panel displays. There is considerable interest in improving PDP efficiency by optimizing the cell design, input voltage characteristics, operating conditions and gas mixture. In this article, we report on a two-dimensional computer model for PDPs which has been used to investigate the operation of a coplanar-electrode PDP cell sustained in He/Ne/Xe gas mixtures. The plasma transport equations are implicitly integrated in time to enable simulation of complex gas mixtures and PDP cell designs. To resolve the details of the electron dynamics, the electron temperature is computed by solving the electron energy equation. A Monte Carlo simulation for secondary electrons and a radiation transport model for visible light emission are also employed. The basic operation of the PDP cell is described in this article. The first pulse was usually found to initiate a discharge between the top electrodes and the bottom address electrode, which was grounded. Only after a positive surface charge was formed on the bottom dielectric did the discharge shift to being between the coplanar electrodes. For our conditions, radiation from Xe2* made a larger contribution to exciting the phosphor for visible light emission than radiation from the resonance states of Xe since radiation from Xe2* is optically thin.
Electron diffraction from polycrystalline materials showing stress induced preferred orientation
Authors: McKenzie D. R. , Bilek M. M. M.
Published: July 2002

The Gibbs free energy as generalized by J. F. Nye [Physical Properties of Crystals (Clarendon Press, Oxford, 1957), p. 179] is minimized in thermodynamic systems held at constant temperature and constant stress. This function is orientation dependent in all crystal systems in stress fields which are not purely hydrostatic. There are situations in which preferred orientation arises as a result of the synthesis of materials under impressed stress conditions such as thin film growth under ion bombardment and the pressing of powders into solids. Here, we derive the orientational constraints for cubic crystals which result from growth under a general biaxial stress field. The sign of the expression δ=s11−s12−12s44 determines the behavior of a cubic crystal. Electron diffraction patterns of face-centered-cubic specimens with both positive and negative values of δ are calculated using a program in MATLAB and displayed in a form suitable for direct comparison with experiment. The use of a biaxial stress with unequal principal components for producing highly oriented polycrystalline material is discussed. In the case of δ positive, as occurs in silicon, the preferred orientation is simply an alignment of the 〈100〉 directions along the principal stresses. For δ negative, as occurs in titanium nitride, the preferred orientation depends on the ratio of the principal stresses and low index directions are aligned with the principal stresses only when the principal stresses are either equal or one of them is zero. In the general case, arc-like diffraction patterns are produced. The results of a calculation of a diffraction pattern from a cross-sectional TiN film are compared with diffraction patterns reported by L. Hultman et al. [J. Appl. Phys. 78, 5395 (1995)] and show good agreement.
Constructal-theory tree networks of “constant” thermal resistance
Authors: Neagu M. , Bejan A.
Published: July 2002

In this article we show that the global thermal resistance to flow between a volume and one point can be reduced to unprecedented levels by shaping the external boundary of each volume element. This degree of freedom is optimized, next to internal features such as the shape and volume fraction of the high-conductivity channels. The volume is covered in a sequence of optimization and assembly steps that proceeds toward larger sizes. The resulting architecture is a leaf-like tree structure with high-conductivity nerves and low-conductivity leaf material. The same constant resistance characterizes the flow from each point on the periphery of the structure to the common sink point. Nearly optimal structures in which the leaf shapes are replaced by needle-like (triangle-in-triangle) shapes are also developed. The fractal-like character of these designs and their relevance to the trend toward fractal-like properties in natural flow structures are discussed in the concluding section of the article.
Occasional “long-range” nonequilibrium body-centered-cubic structures in NiFe/Cu spin valves
Authors: Geng H. , Heckman J. W. , Pratt W. P. , Bass J. , Espinosa F. J. , Conradson S. D. , Lederman D. , Crimp M. A.
Published: July 2002

We describe conventional and high-resolution transmission electron microscopy (HRTEM) characterization of the microstructure of sputtered NiFe/Cu giant magnetoresistance spin valves (Cu/FeMn/NiFe/Cu/NiFe) sandwiched between thick Nb contact layers. Six spin valves, sputtered at different temperatures, three with thin (3 nm) and three with thick (24 and 30 nm) NiFe layers, were studied. All of the spin-valve layers were smooth and continuous, consisting of columnar grains generally 20–90 nm wide. In most cases, the grains had grown epitaxially from the bottom contact, through the entire multilayer, to the top contact layer. The columnar grains grew on the closest-packed planes (i.e., {110} planes for bcc Nb and {111} planes for fcc Cu, FeMn, and NiFe spin-valve components). This epitaxial growth yields an apparent Kurdjumov–Sachs {111}fcc∥{110}bcc; 〈110〉fcc∥〈111〉bcc orientation relationship. However, HRTEM imaging supported by fast Fourier transform analysis reveals that in some of the columnar grains the Cu, FeMn, and NiFe layers take up a nonequilibrium bcc structure. In these cases, the bcc Cu, FeMn, and NiFe layers grow on {110} planes and are epitaxial with the Nb contacts for the individual grain columns. While bcc Cu has been observed elsewhere, the length scale of the nonequilibrium bcc phases reported here is an order of magnitude greater than previously observed.
Device performance and polymer morphology in polymer light emitting diodes: The control of thin film morphology and device quantum efficiency
Authors: Shi Y. , Liu J. , Yang Y.
Published: July 2002

We present the results of a systematic study on how the processing conditions of spin casting affect the morphology of polymer thin films, and how the morphology affects polymer light-emitting diode (LED) performance. The absorption peaks of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) thin films, which reflects the conjugation of π electrons, are strongly correlated to the spin-casting conditions. At high spin speed, better conjugation is observed. In addition, the photoluminescence emission peak of MEH-PPV films at ∼630 nm has a strong correlation to polymer aggregation. By proper selection of organic solvents, polymer solution concentrations, and spin speeds, we are able to control the aggregation of the polymer chains. Subsequently, we are able to control the emission color and the quantum efficiency of the MEH-PPV LEDs by simply adjusting the spin-casting conditions. Although spin casting is the most commonly used technique for the preparation of polymer thin films, our finding suggests that the thin-film preparation, and thus the formation of polymer morphology, is a much more complicated process than previously assumed.
Solid–liquid phase changes in simulated isoenergetic Ar13
Authors: Jellinek Julius , Beck Thomas L. , Berry R. Stephen
Published: July 2002

Simulations by molecular dynamics of 13-particle clusters of argon display distinct nonrigid, liquid-like and near-rigid, solid-like ‘‘phases.’’ The simulations, conducted at constant total energy, display a low-energy region in which only the solid-like form appears, a high-energy region in which only the liquid-like form appears, and an intermediate band of energy—a ‘‘coexistence region’’— in which clusters exhibit both forms. The intervals of time spent in each phase in the two-form coexistence region are long compared with the intervals required to establish equilibrium-like properties distinctive of each form, such as mean square displacement and power spectrum, so that well-defined phases can be said to exist. The fraction of time spent in each phase is a function of the energy. When a long simulation is separated into regions of solid-like and liquid-like behavior, the curve of the derived caloric equation of state is double valued in the two-phase range of energy, forming two well-defined, smooth branches. When, instead, the caloric curve is constructed from averages over all of a long run, its form is smooth and monotonic showing no trace of the ‘‘loop’’ that had been reported for earlier treatments with much shorter molecular dynamics runs, and which we could also reproduce with short runs.
Broadband near-infrared luminescence of Cr+3 in the elpasolite lattices Cs2NaInCl6, Cs2NaYCl6, and Cs2NaYBr6
Authors: Knochenmuss Richard , Reber Christian , Rajasekharan Melath V. , Güdel Hans U.
Published: July 2002

The broadband luminescence of isolated Cr+3 ions in the title lattices is investigated. The 4T2g lowest excited state is found to undergo Jahn–Teller distortion in the eg coordinate, in addition to expanding along the a1g. Quantitative determination of these displacements is made by analysis of extremely rich fine structure, the Ham quenched spin-orbit splitting of the electronic origins, and the Stokes shifts. The Jahn–Teller effect is found to influence the temperature dependence of lifetimes at less than 50 K. In the yttrium containing hosts, nonradiative deactivation of the 4T2g is observed above 200 K. Analysis of lifetime data with four models for nonradiative decay indicates that the active vibrations are again the a1g and eg. The trends observed among the host materials are rationalized on the basis of the freedom of the 4T2g to relax along the a1g and eg directions.
Melting and freezing in isothermal Ar13 clusters
Authors: Davis Heidi L. , Jellinek Julius , Berry R. Stephen
Published: July 2002

Microcanonical simulations have shown that Ar13 clusters have sharp but unequal melting and freezing energies. Between these energies, a hot solid-like form and a cooler, liquid-like form coexist in dynamic equilibrium. Monte Carlo and isothermal molecular dynamics simulations confirm that this coexistence behavior persists under canonical conditions as well. Many properties demonstrate the solid and liquid character of the two coexisting ‘‘phases.’’ One previous result seemed to contradict this: Quirke and Sheng evaluated nearest neighbor angular distribution function P(θ); its nonzero value for θ=π/2 at 33 K was interpreted as that of a hot solid in a ‘‘premelting expansion.’’ Actually, that result is the average of a bimodal distribution, one mode for the solid and the other for the liquid. The average shifts smoothly with T, and each form’s P(θ) changes slightly with temperature. The solid has tiny nonzero probability for π/2. The liquid has a minimum probability there, but far above zero. Mean-square displacements and power spectra calculated at 33 K from the Nosé constant temperature molecular dynamics method exhibit properties which are clearly distinguishable and identifiable with two distinct phases, as they are under isoergic conditions. Hence our results can be added to the evidence supporting the picture for finite systems of two phases coexisting over a finite temperature and energy range.