bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteadiness of an axisymmetric separating-reattaching flow: Numerical investigation
Authors: Deck Sébastien , Thorigny Pascal
Published: June 2007

The separated flow over a cylinder elongated by another cylinder of a smaller diameter is investigated numerically at the high subsonic regime using zonal detached eddy simulation (ZDES) and compared with the experimental data of Deprés, Reijasse, and Dussauge [AIAA J. 42, 2541 (2004)]. First, it is shown that this axisymmetric step flow has much in common with the two-dimensional facing step flows as regards the shear layer instability process. Second, the statistical and spectral properties of the pressure fluctuations are scrutinized. Close to the step, the surface pressure signature is characterized by low frequencies f.Lr∕U∞=O(0.08) (where Lr and U∞ denote, respectively, the mean reattachment length and free-stream velocity) and an upstream velocity of 0.26U∞ while in the second half-part of the recirculation higher frequencies fluctuations at f.Lr∕U∞≈0.6 and a downstream convection velocity 0.6U∞ are the dominant features. The current calculation shows that the separated bubble dynamics depends on very complex interactions of large eddies formed in the upstream free shear layer with the wall in the reattachment region. These structures are shed with a nondimensional frequency of about 0.2. Besides, it has been observed that the secondary corner vortex experiences a cycle of growth and decay. The correspondence between the frequencies of this secondary corner vortex dynamics and the flapping motion (f.Lr∕U∞≈0.08) suggests that there should be different aspects of the same motion. These results show that there is an ordered structure in this axisymmetric separating/reattaching flow which is dominated by large scale coherent motion. This is confirmed by a two-point correlation analysis of the pressure signals showing that the flow is dominated by highly coherent antisymmetric modes at the flapping and vortex shedding frequencies whose signatures are evidenced in the spectrum of the computed buffet loads. Possible onsets of a large-scale self-sustained motion of the separated area are finally discussed and the existence of an absolute instability of the axisymmetric recirculation bubble originating from a region located near the middle of the recirculating zone is conjectured.
Hash function based on chaotic map lattices
Authors: Wang Shihong , Hu Gang
Published: June 2007

A new hash function system, based on coupled chaotic map dynamics, is suggested. By combining floating point computation of chaos and some simple algebraic operations, the system reaches very high bit confusion and diffusion rates, and this enables the system to have desired statistical properties and strong collision resistance. The chaos-based hash function has its advantages for high security and fast performance, and it serves as one of the most highly competitive candidates for practical applications of hash function for software realization and secure information communications in computer networks.
On the existence of chaotic circumferential waves in spinning disks
Authors: Angoshtari Arzhang , Jalali Mir Abbas
Published: June 2007

We use a third-order perturbation theory and Melnikov’s method to prove the existence of chaos in spinning circular disks subject to a lateral point load. We show that the emergence of transverse homoclinic and heteroclinic points lead, respectively, to a random reversal in the traveling direction of circumferential waves and a random phase shift of magnitude π for both forward and backward wave components. These long-term phenomena occur in imperfect low-speed disks sufficiently far from fundamental resonances.
Observation of Co/CoO nanoparticles below the critical size for exchange bias
Authors: Dobrynin A. N. , Temst K. , Lievens P. , Margueritat J. , Gonzalo J. , Afonso C. N. , Piscopiello E. , Van Tendeloo G.
Published: June 2007

We compare the magnetic properties of pure and oxidized Co nanoparticles embedded in an amorphous Al2O3 matrix. Nanoparticles with diameters of 2 or 3 nm were prepared by alternate pulsed laser deposition in high vacuum conditions, and some of them were exposed to O2 after production and before being embedded. The nanoparticles are organized in layers, the effective edge-to-edge in-depth separation being 5 or 10 nm. The lower saturation magnetizations per Co atom for the samples containing oxidized nanoparticles provide evidence for the formation of antiferromagnetic CoO shells in the nanoparticles. None of the samples with Co/CoO nanoparticles show exchange bias, while vertical hysteresis loop shifts and enhanced coercivities (as compared to samples with pure Co nanoparticles) are observed. This constitutes evidence for the nanoparticles size being in all cases smaller than the critical size for exchange bias. The difference in coercivity versus temperature dependences for the samples with pure and oxidized Co nanoparticles shows that the exchange anisotropy in Co/CoO nanoparticles appears at temperatures lower than 50 K.
Impedance matching of a coaxial Marx generator with a relativistic field emission limited diode
Authors: Lin Ming-Chieh , Peng Kuan-Lin
Published: June 2007

The impedance matching between the coaxial Marx generator and the relativistic field emission limited diode (RFELD) is investigated by using PSpice simulations. The PSpice model of the coaxial Marx generator has been built and verified by comparing the output voltage wave forms with the results of Kubota et al. [Y. Kubota, J. Kodaira, and A. Miyahara, Jpn. J. Appl. Phys. 20, 2397 (1981)]. By varying the passive resistive loads, the impedance of the coaxial Marx generator can be determined. In the PSpice simulation, we employ the analog behavioral modeling to describe the current-voltage characteristics of the RFELDs. The output characteristics including the voltage, current, and output power wave forms have been obtained. It is found that the output characteristics of the RFELDs with an effective work function of around 0.4 eV are in good matching with the coaxial Marx generator. The simulation results show a degradation in pulse duration or beam power for the cases of the RFELDs with higher or lower effective work functions. With our method, the impedance matching of the RFELDs with the coaxial Marx generator can be achieved and the corresponding output characteristics can be determined.
A numerical study of passive scalar evolution in peripheral regions
Authors: Salman H. , Haynes P. H.
Published: June 2007

We study the effect of slip and no-slip wall boundaries on the decay rate of a passive scalar in a spatially smooth and random in time velocity field. Numerical simulations are carried out to verify the effect of the peripheral (near-wall) regions on the decay of the scalar variance. Using two kinematic flow models with simple velocity fields, we show that, in the case of slip boundaries, the passive scalar is characterized by an initial rapid stirring followed by an exponential decay of the scalar variance. In stark contrast, results for the case with no-slip boundaries show that, following an initial rapid stirring of the scalar within the bulk, there is an intermediate-time regime where the variance follows a power-law decay. This intermediate regime is established as a result of the trapping of the scalar in the peripheral regions near the no-slip walls. Finally, the behavior of the scalar variance switches to a final regime that is characterized by an exponential decay rate. The results presented here indicate that the recent ensemble-based theories regarding the evolution of a passive scalar in the peripheral regions correctly predict the main stages of the scalar evolution that arise in a single flow realization.
Double-injection current transients as a way of measuring transport in insulating organic films
Authors: Juška G. , Genevičius K. , Sliaužys G. , Pivrikas A. , Scharber M. , Österbacka R.
Published: June 2007

We propose a double-injection current transient technique for the study of charge-carrier transport in thin insulating films of low-mobility materials with reduced carrier bimolecular recombination compared to the Langevin type. This experimentally simple technique, allows us to estimate the sum of the faster carrier and the slower carrier mobility’s (μf+μs) and the slower carrier mobility (μs). Furthermore, in thin films when the RC current overlaps the injection current transients we propose to estimate these transport parameters using the extracted charge as a function of injection pulse duration. The method is applied on bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene. We have experimentally verified the technique by measuring the charge carrier mobility’s and compared them with results obtained using standard time-of-flight and carrier extraction using linearly increasing voltage techniques.
Gain curves and hydrodynamic simulations of ignition and burn for direct-drive fast-ignition fusion targets
Authors: Solodov A. A. , Betti R. , Delettrez J. A. , Zhou C. D.
Published: June 2007

Hydrodynamic simulations of realistic high-gain fast-ignition targets are performed, including one-dimensional simulations of the implosion and two-dimensional simulations of ignition by a collimated electron beam and burn propagation. These simulations are used to generate gain curves for fast-ignition direct-drive inertial confinement fusion. The minimum energy required for ignition is computed for fast-electron beams with a monoenergetic or Maxwellian distribution, generated by a constant or Gaussian laser pulse. It is found that realistic fast-ignition targets can be ignited by monoenergetic collimated electron beams with a radius of 20μm, duration of 10ps, and energy of 15kJ. Simulations using ponderomotive temperature scaling for fast electrons and Gaussian laser pulses predict a minimum laser energy for ignition of 235kJ (105kJ) for the energy conversion efficiency from the laser to fast electrons 0.3 (0.5) and the wavelength of 1.054μm. Such large energies are required because ultra-intense lasers are predicted to generate very energetic (multi-MeV) electrons with stopping distance exceeding the target size. The fast-electron energy, the stopping distance and the minimum energy required for ignition can be reduced using frequency-doubled laser pulses. Simulations of idealized cone targets are also performed in order to determine a lower bound of the gain deterioration due to the cone.
Core-shell nanocrystalline structures in oxidized iron thin films prepared by sputtering at very low temperatures
Authors: Jiménez-Villacorta F. , Huttel Y. , Muñoz-Martín A. , Ballesteros C. , Román E. , Prieto C.
Published: June 2007

We report on the effect of preparation temperature in the magnetic properties of oxidized iron thin films deposited by dc-magnetron sputtering below room temperature. Films prepared at 300 K show a typical thin film magnetic behavior, whereas samples prepared at 200 K present visible features of granular core-shell system formed by an oxide shell surrounding a ferromagnetic core and displaying exchange anisotropy. These differences are directly linked to the film microstructure and composition. We present results of microstructure, composition, and chemical analysis in order to discuss the observed magnetic behavior. Size and shape of iron crystallographic grains were characterized by transmission electron microscopy. Depth-resolved compositional characterization, obtained by ion-beam analysis techniques, has probed the penetration of oxygen along the thickness of the films and x-ray photoelectron spectroscopy was used to identify the different iron oxide species present in the structures.
Vibrational modes in partially optimized molecular systems
Authors: Ghysels A. , Van Neck D. , Van Speybroeck V. , Verstraelen T. , Waroquier M.
Published: June 2007

In this paper the authors develop a method to accurately calculate localized vibrational modes for partially optimized molecular structures or for structures containing link atoms. The method avoids artificially introduced imaginary frequencies and keeps track of the invariance under global translations and rotations. Only a subblock of the Hessian matrix has to be constructed and diagonalized, leading to a serious reduction of the computational time for the frequency analysis. The mobile block Hessian approach (MBH) proposed in this work can be regarded as an extension of the partial Hessian vibrational analysis approach proposed by Head [Int. J. Quantum Chem. 65, 827 (1997)]. Instead of giving the nonoptimized region of the system an infinite mass, it is allowed to move as a rigid body with respect to the optimized region of the system. The MBH approach is then extended to the case where several parts of the molecule can move as independent multiple rigid blocks in combination with single atoms. The merits of both models are extensively tested on ethanol and di-n-octyl-ether.
An exact formulation of hyperdynamics simulations
Authors: Chen L. Y. , Horing N. J. M.
Published: June 2007

We introduce a new formula for the acceleration weight factor in the hyperdynamics simulation method, the use of which correctly provides an exact simulation of the true dynamics of a system. This new form of hyperdynamics is valid and applicable where the transition state theory (TST) is applicable and also where the TST is not applicable. To illustrate this new formulation, we perform hyperdynamics simulations for four systems ranging from one degree of freedom to 591 degrees of freedom: (1) We first analyze free diffusion having one degree of freedom. This system does not have a transition state. The TST and the original form of hyperdynamics are not applicable. Using the new form of hyperdynamics, we compute mean square displacement for a range of time. The results obtained agree perfectly with the analytical formula. (2) Then we examine the classical Kramers escape rate problem. The rate computed is in perfect agreement with the Kramers formula over a broad range of temperature. (3) We also study another classical problem: Computing the rate of effusion out of a cubic box through a tiny hole. This problem does not involve an energy barrier. Thus, the original form of hyperdynamics excludes the possibility of using a nonzero bias and is inappropriate. However, with the new weight factor formula, our new form of hyperdynamics can be easily implemented and it produces the exact results. (4) To illustrate applicability to systems of many degrees of freedom, we analyze diffusion of an atom adsorbed on the (001) surface of an fcc crystal. The system is modeled by an atom on top of a slab of six atomic layers. Each layer has 49 atoms. With the bottom two layers of atoms fixed, this system has 591 degrees of freedom. With very modest computing effort, we are able to characterize its diffusion pathways in the exchange-with-the-substrate and hop-over-the-bridge mechanisms.
Alfvén surface modes in dusty spin 1/2 quantum magnetoplasmas
Authors: Misra Amar P.
Published: June 2007

The dispersion properties of electromagnetic surface modes propagating at the planar interface between two structured quantum magnetoplasmas composed of electrons, positrons, and charged dust grains of different densities and between a plasma and free space with spin quantum effects are studied. It is shown that even in the absence of charged dust impurities, the spin quantum effects can lead to a new low frequency (in comparison to the electron/positron gyrofrequency) Alfvén surface modes which otherwise do not exist. The important applications to plasmas in the vicinity of pulsars and magnetars as well as in the dense astrophysical plasma environments where the particle density is about 1020–1025m−3 are discussed.
Oscillations of magnetized dusts in electronegative radio-frequency plasma
Authors: Gan Bao-Xia , Chen Yin-Hua , Yu M. Y.
Published: June 2007

Oscillations of charged dust grains in the sheath region of a radio-frequency plasma containing negative ions are studied using a one-dimensional model for collisional sheaths. The forces on a dust grain in the sheath region are first analyzed and the force-balance conditions found. Oscillations of the levitated dusts are then investigated. Linear lattice waves, including one-dimensional longitudinal and two-dimensional hexagonal transverse lattice waves, as well as the effects of the negative ions and external magnetic field on the waves, are discussed.
Thermal evolution of boron irradiation induced defects in predoped Si revealed by positron annihilation experiments
Authors: Nambissan P. M. G. , Bhagwat P. V. , Kurup M. B.
Published: June 2007

The isochronal annealing behavior of high energy (25–72 MeV) boron ion irradiation induced defects in boron-doped silicon is monitored through measurements of positron lifetimes and three distinct defect-evolution stages are identified. The initial boron doping created a defect environment where positrons could sensitively annihilate with the boron electrons, suggesting boron-decorated Si monovacancies as potential trapping sites. The irradiation results in the dissolution of boron from these sites and positrons are then trapped by the empty divacancies of Si. Charge neutralization of divacancies through interaction with boron atoms leads to enhanced positron trapping in the initial stages of isochronal annealing. The divacancies start annealing above 673 K. However, a remarkable defect evolution stage due to the diffusion of the boron atoms beyond their initial depths of penetration is seen above 873 K and it leaves the sample with defects still present even at the highest annealing temperature 1273 K used in this work.
White light emission from p-doped quaternary (AlInGa)N-based superlattices: Theoretical calculations for the cubic phase
Authors: Rodrigues S. C. P. , d’Eurydice M. N. , Sipahi G. M. , Scolfaro L. M. R. , da Silva E. F.
Published: June 2007

The band structure and light emission spectra from p-doped quaternary (AlXIn1−X−YGaY)N∕(AlxIn1−x−yGay)N superlattices (SLs) in the zinc blend (cubic) structure are investigated by means of self-consistent calculations which are performed within the k⋅p approach. Exchange and correlation effects, within the generated hole gas, are taken into account in the local density approximation. The calculated luminescence and absorption spectra show that light emission, due to recombination from confined states in the wells, is redshifted in the doped systems as compared to undoped SLs. It is demonstrated that p-doped strained (AlXIn1−X−YGaY)N∕(AlxIn1−x−yGay)N SLs give rise to the feasibility of achieving white light emission, through emissions covering all the visible region spectra, from violet to red. These findings provide important guidelines for the interpretation of forthcoming experiments in quaternary group-III nitride-based alloy systems, and for the design of advanced optoelectronic devices based on these alloys.
Thermal loading of laser induced plasma shockwaves on thin films in nanoparticle removal
Authors: Varghese Ivin , Zhou Dong , Peri M. D. Murthy , Cetinkaya Cetin
Published: June 2007

Damage concerns, such as substrate/film material alterations, damage, and delamination of thin films, have become a central problem in sub-100 nm particle removal applications. In the laser induced plasma (LIP) removal technique both LIP shockwave and radiation heating are potential sources of thermomechanical damage. The specific objective of current study is to conduct a computational investigation of the LIP shockwave effect on the thermoelastic response of a thin chromium (Cr) film deposited on a quartz substrate and to identify the conditions leading to the onset of plastic film deformations. The experimentally characterized shockwave pressure and temperature (approximated from gas dynamic relations) were prescribed as boundary conditions in the computational analysis. From the shockwave arrival times for different travel distances, the shockwave radius as well as the velocity were obtained as a function of the shockwave propagation time. Radial (and circumferential) stresses, caused by thermal expansion of the Cr film, were most dominant and, hence, of damage concern. It is determined that the resultant temperature rise utilizing a 1064 nm Nd:yttrium-aluminum-garnet (YAG) laser (450 mJ) due to the film-shockwave interactions was not sufficiently high to initiate film and/or substrate damage. No material alteration/damage of the Cr film is predicted due to the temperature and pressure of LIP shockwaves at the firing distance of 2 mm, with a high strain rate gain factor of two (minimum), though damage was observed experimentally for 1064 nm Nd:YAG laser at the pulse energy of 370 mJ. Reported results indicate that the leading cause of observed thin film damage during nanoparticle removal is almost certainly radiation heating from the LIP core.
Ferromagnetic resonance investigation in as-prepared NiFe/FeMn/NiFe trilayer
Authors: Yuan S. J. , Xu K. , Yu L. M. , Cao S. X. , Jing C. , Zhang J. C.
Published: June 2007

NiFe/FeMn/NiFe trilayer prepared by dc magnetron sputtering was systematically investigated by ferromagnetic resonance technique (FMR) at room temperature. For NiFe/FeMn/NiFe trilayer, there are two distinct resonance peaks both in in-plane and out-of-plane FMR spectra, which are attributed to the two NiFe layers, respectively. The isotropic in-plane resonance field shift is negative for the bottom NiFe layer, while positive for the top NiFe layer. And, such phenomena result from the negative interfacial perpendicular anisotropy at the bottom NiFe/FeMn interface and positive interfacial perpendicular anisotropy at the top FeMn/NiFe interface. The linewidth of the bottom NiFe layer is larger than that of the top NiFe layer, which might be related to the greater exchange coupling at the bottom NiFe/FeMn interface.
Compositional and valent state inhomogeneities and ordering of oxygen vacancies in terbium-doped ceria
Authors: Ye Fei , Mori Toshiyuki , Ou Ding Rong , Zou Jin , Auchterlonie Graeme , Drennan John
Published: June 2007

Intragranular distributions of composition and valent state in sintered Tb-doped ceria have been systematically investigated. Through detailed studies of electron energy loss spectroscopy and energy filtering transmission electron microscopy, both compositional and valent state inhomogeneities of Ce and Tb were confirmed, which are related to the existence of nanosized domains in Tb-doped ceria. Compared with their matrix, the domains have higher Tb concentration and Ce and Tb cations in the domains tend to be trivalent. Furthermore, ordering of oxygen vacancies in the domains, which increases with increasing doping concentration, has been determined by EELS.
Investigation of the local composition enhancement and related dynamics in supercritical CO2-cosolvent mixtures via computer simulation: The case of ethanol in CO2
Authors: Skarmoutsos Ioannis , Dellis Dimitris , Samios Jannis
Published: June 2007

The supercritical mixture ethanol-carbon dioxide (EtOH–CO2) with mole fraction of ethanol XEtOH≅0.1 was investigated at 348K, by employing the molecular dynamics simulation technique in the canonical ensemble. The local intermolecular structure of the fluid was studied in terms of the calculated appropriate pair radial distribution functions. The estimated average local coordination numbers and mole fractions around the species in the mixture reveal the existence of local composition enhancement of ethanol around the ethanol molecules. This finding indicates the nonideal mixing behavior of the mixture due to the existence of aggregation between the ethanol molecules. Furthermore, the local environment redistribution dynamics have been explored by analyzing the time correlation functions (TCFs) of the total local coordination number (solvent, cosolvent) around the cosolvent molecules in appropriate parts. The analysis of these total TCFs in the auto-(solvent-solvent, cosolvent-cosolvent) and cross-(solvent-cosolvent, cosolvent-solvent) TCFs has shown that the time dependent redistribution process of the first solvation shell of ethanol is mainly determined by the redistribution of the CO2 solvent molecules. These results might be explained on the basis of the CO2–CO2 and EtOH–CO2 intermolecular forces, which are sufficiently weaker in comparison to the EtOH–EtOH hydrogen bonding interactions, creating in this way a significantly faster redistribution of the CO2 molecules in comparison with EtOH. Finally, the self-diffusion coefficients and the single reorientational dynamics of both the cosolvent and solvent species in the mixture have been predicted and discussed in relationship with the local environment around the species, which in the case of the EtOH molecules seem to be strongly affected.
Characteristics of two-dimensional flow around a rotating circular cylinder near a plane wall
Authors: Cheng Ming , Luo Li-Shi
Published: June 2007

We simulate a two-dimensional incompressible flow around a rotating circular cylinder near a plane wall at the Reynolds number Re=200 by using the lattice Boltzmann equation with multiple relaxation times. We investigate the flow pattern in the parameter space of the rotational rate γ≔ωa∕U and the normalized gap h≔H∕D, where ω is the angular velocity of the cylinder, a and D are the cylinder radius and diameter, respectively, U is the inflow velocity, and H is the gap between the cylinder and the wall. We quantify the effects of γ and h on the hydrodynamic forces and the frequency of vortex shedding from the cylinder. Our results indicate that two critical values of h, hdown and hup, exist, which depend on γ. The flow is steady when hhup. When hdown<h0, C¯D is no longer a monotonic function of h. The mean drag coefficient C¯D varies significantly in the range hdown<hhup, the wall effect diminishes.
Photoinduced evaporation of mass-selected aniline+(water)n (n=4–20) clusters
Authors: Nam Sang Hwan , Park Hye Sun , Lee Mi Ae , Cheong Nu Ri , Song Jae Kyu , Park Seung Min
Published: June 2007

Photofragmentation of mass-selected aniline+(water)n (An+Wn, n=4–20) clusters is investigated over photon energies ranging from 1.65to4.66eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An+Wn clusters and binding energies. The authors find that the An+W6 cluster has a highly symmetric structure and its binding energy in An+W6→An+W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An+W6 is relatively stable in metastable decay.
Invited Article: Simultaneous mapping of temperature and stress in microdevices using micro-Raman spectroscopy
Authors: Beechem Thomas , Graham Samuel , Kearney Sean P. , Phinney Leslie M. , Serrano Justin R.
Published: June 2007

Analysis of the Raman Stokes peak position and its shift has been frequently used to estimate either temperature or stress in microelectronics and microelectromechanical system devices. However, if both fields are evolving simultaneously, the Stokes shift represents a convolution of these effects, making it difficult to measure either quantity accurately. By using the relative independence of the Stokes linewidth to applied stress, it is possible to deconvolve the signal into an estimation of both temperature and stress. Using this property, a method is presented whereby the temperature and stress were simultaneously measured in doped polysilicon microheaters. A data collection and analysis method was developed to reduce the uncertainty in the measured stresses resulting in an accuracy of ±40MPa for an average applied stress of −325MPa and temperature of 520°C. Measurement results were compared to three-dimensional finite-element analysis of the microheaters and were shown to be in excellent agreement. This analysis shows that Raman spectroscopy has the potential to measure both evolving temperature and stress fields in devices using a single optical measurement.
Magnetotransport properties in (11¯00)- and (0001)-oriented MnAs films on GaAs substrates
Authors: Takagaki Y. , Friedland K.-J.
Published: June 2007

Magnetic-field dependence of the resistivities in MnAs(11¯00) films on GaAs(001) and MnAs(0001) films on GaAs(111)B is investigated at low temperatures. Correspondence of the features in the longitudinal and transverse resistivities under reorientations of the magnetization enables us to distinguish the anisotropic magnetoresistance effect from the effects originating from the band structure of MnAs. Simultaneous contributions of holes and electrons are evidenced for the (11¯00)-oriented films, whereas the transport is almost completely dominated by holes for the (0001)-oriented films. The magnetization flip by an in-plane magnetic field applied along the magnetic easy axis generates jumps in the longitudinal resistivity owing to the magnetoresistance originating from the band-structure effect.
High-frequency surface acoustic wave devices based on AlN/diamond layered structure realized using e-beam lithography
Authors: Assouar M. B. , Elmazria O. , Kirsch P. , Alnot P. , Mortet V. , Tiusan C.
Published: June 2007

We report in this paper on the study and the realization of surface acoustic wave devices based on an AlN/diamond layered structure intended for the X band (8 GHz). Both x-ray diffraction and transmission electronic microscopy, used for characterization of the structural properties of the AlN/diamond structure, have shown (002) highly oriented sputtered AlN films on free-standing chemical vapor deposition diamond films. Surface roughness of the AlN/diamond structure was measured by atomic force microscopy and showed a very low surface roughness, less than 1 nm. Low surface roughness is very important to reduce the acoustic propagation losses. SAW devices operating in the range of 8 GHz were realized by the combination of the high velocity of the AlN/diamond layered structure and the high lateral resolution obtained using e-beam lithography (EBL). Due to high electrical resistivity of the AlN film, interdigital transducers with submicronic resolution were patterned by an adapted technological EBL process. The analysis of device performances in terms of electromechanical coupling coefficient and temperature stability was carried out and discussed. The dispersion of both parameters as a function of wavelength was experimentally determined, and showed the obtention of an electromechanical coupling coefficient up to 1.4% and a temperature coefficient of frequency varying between 9 and 20 ppm/°C. The dispersion curves of phase velocity were also analyzed and experimental results show a good agreement with theoretical calculations.
Grounding positions of superconducting layer for effective magnetic isolation in Josephson integrated circuits
Authors: Mizugaki Yoshinao , Kashiwa Ryuta , Moriya Masataka , Usami Kouichi , Kobayashi Tadayuki
Published: June 2007

Mutual inductances between two superconducting strip lines coupled through a grounded shield layer are evaluated by both experiments and numerical calculation. A conventional superconducting quantum interference device method on a Nb Josephson integrated circuit chip is employed for experiments. Four test circuits are designed to investigate the effects of ground contacts. Grounding the shield layer at one point or at two points located perpendicular to the line direction does not improve the shielding effect, whereas grounding at two points located parallel to the line direction reduced the mutual inductance by 67%. Mutual inductances calculated using an inductance extraction program, FASTHENRY, agree with the experimental results. Numerical results of current distributions in the shield layers demonstrate that the enhanced shielding current improves the magnetic isolation.
Topology optimization of waveguide bends with wide, flat bandwidth in air-bridge-type photonic crystal slabs
Authors: Watanabe Yoshinori , Ikeda Naoki , Sugimoto Yoshimasa , Takata Yoshiaki , Kitagawa Yoshinori , Mizutani Akio , Ozaki Nobuhiko , Asakawa Kiyoshi
Published: June 2007

We employed the topology optimization (TO) method to improve the transmission bandwidth of waveguide bends in air-bridge-type, two-dimensional photonic crystal slabs. We experimentally confirmed that bend loss at longer wavelengths in the vicinity of the band edge was suppressed by using TO. The optimized bends showed good performance, comparable to that of straight waveguides.
Strongly sublinear growth of the photorefractive effect for increasing pump intensities in doped lithium-niobate crystals
Authors: Minzioni Paolo , Cristiani Ilaria , Degiorgio Vittorio , Kokanyan Edvard P.
Published: June 2007

The pump intensity dependence of the birefringence change due to the photorefractive effect in Hf-doped and Mg-doped congruent lithium niobate (LN) crystals is reported. The birefringence change is found to be very weakly dependent on the light intensity. This is in strong contrast with the behavior of undoped congruent LN, and much more similar to what happens in stoichiometric LN. The data also indicate that the minimum HfO2 concentration necessary to strongly reduce photorefractivity is lower than the value of 4 mol % indicated by previous experiments.
Small amplitude current free double layers in a collisionless plasma
Authors: Saharia K. , Goswami K. S.
Published: June 2007

The existence of small amplitude double layers without relative electron and ion drift is reported. Solutions of these double layers exist only when the ratio of electron to ion temperature is less than unity. Electron densities and temperature profiles across the double layers are discussed.
Quadrupole moment function and absolute infrared quadrupolar intensities for N2
Authors: Li Hui , Le Roy Robert J.
Published: June 2007

High level ab initio methods have been used to calculate values of the quadrupole moment of the ground XΣg+1 state of N2 on a dense radial mesh spanning the interval of 0.8–12.1a.u. Detailed convergence tests indicate that the resulting equilibrium values of the quadrupole moment ϴe=−1.1273a.u. and its first radial derivative  ∣dϴ(R)∕dR∣e=0.9604a.u. have absolute uncertainties of 0.3% and 0.8%, respectively, and are more accurate than the best experimental values of these quantities. The calculated quadrupole moment function, together with a recently reported accurate analytic empirical potential energy function [Le Roy et al., J. Chem. Phys. 125, 164310 (2006)], is used to generate values of the radial matrix elements determining the absolute intensities of infrared vibration-rotation transitions of ground-state N2, which take full account of vibration-rotation interactions. These results should improve the reliability of the interpretations of N2 contributions to infrared atmospheric spectra.
Effects of open-loop and closed-loop control on subsonic cavity flows
Authors: Little J. , Debiasi M. , Caraballo E. , Samimy M.
Published: June 2007

This work presents an experimental investigation of the effects of open- and closed-loop control techniques on the flow structure and surface pressure signature in subsonic cavity flows. The cases include the uncontrolled (baseline) Mach 0.30 flow over a shallow cavity of aspect ratio 4 with Reynolds number based on the cavity depth of 105, and four actively controlled flows. The controlled cases include open-loop at two discrete frequencies and two closed-loop cases: parallel proportional with time delay and reduced-order model-based linear quadratic. Measurements and analyses include particle image velocimetry, spectra and spectrograms of surface pressure and velocity fluctuations, flow visualization, and proper orthogonal decomposition. Data are presented and analyzed in an effort to better understand the behavior of the cavity flow in response to a variety of actuation cases. Results show that both open- and closed-loop control have significant effects on the flow dynamics and surface pressure behavior. In addition, the results reveal substantial differences between the effects of each type of open-loop and closed-loop control.
Spatial resolution enhancement/smoothing of stereo–particle-image-velocimetry data using proper-orthogonal-decomposition–based and Kriging interpolation methods
Authors: Gunes Hasan , Rist Ulrich
Published: June 2007

Methods for data reconstruction and spatial enhancement of experimental data for a transitional boundary layer with laminar separation bubble are investigated. Particularly, proper orthogonal decomposition (POD) is applied to direct numerical simulation (DNS) data to extract the DNS-based POD modes, which are projected onto the experimental data (via a least-squares procedure) in order to obtain model coefficients. These model coefficients are then used to reconstruct, “interpolate,” and smooth the experimental data based on the DNS modes. In addition, in order to compare and assess the effectiveness of the present DNS-based procedure, Kriging interpolation is performed on the experimental (as well as numerical) data. These procedures are applied to time periodic (experimental) instantaneous spanwise vorticity (ωz) at a constant spanwise location. We have demonstrated that particle-image-velocimetry (PIV)-based POD modes can be smoothed by Kriging interpolation, thus a noise-free reconstruction of PIV data can be achieved. It is also found that for very low resolution experimental data, DNS-based interpolation is superior over Kriging interpolation. On the other hand, Kriging interpolation based on the Gaussian correlation model works very well for sufficiently high resolution experimental data. The correlation parameter can be used to control the degree of smoothness in the data reconstruction. Both procedures effectively eliminate the unwanted noise in the experimental data. One important difference between the two procedures is that, with quite some confidence, the DNS-based procedure can also be used for “extrapolation” since the model coefficients do not depend on spatial variation. In fact, we show that near-wall spanwise vorticity, which is not available from experimental data, can be recovered faithfully. Moreover, the enhancement (interpolation and smoothing) of full three-dimensional PIV data has been performed by Kriging interpolation employing a Gaussian correlation model.
Field emission and electron deposition profiles as a function of carbon nanotube tip geometries
Authors: Sanchez Jaime A. , Wong Basil T. , Pinar Mengüç M. , Albella P.
Published: June 2007

We present an analysis to explore the electron distribution within a workpiece subjected to field emission from the tip of a carbon nanotube. By calculating the field emission current density at sites on the periphery of the tip and by mapping this current density towards the surface using the trajectories followed by the electrons, we are able to determine the shape of the electron beam profile on the surface. Once this profile is obtained we can solve electron-beam transport equation by means of Monte Carlo simulation to determine the electron distribution inside the workpiece. We repeat these simulations for various applied voltages, gap distances, and for different tip shapes in order to understand the effects that these parameters may have on the distribution of the deposited electrons. These distributions are needed to investigate the field emission based nanomachining process.
Stabilities of multiply charged dimers and clusters of fullerenes
Authors: Zettergren Henning , Schmidt Henning T. , Reinhed Peter , Cederquist Henrik , Jensen Jens , Hvelplund Preben , Tomita Shigeo , Manil Bruno , Rangama Jimmy , Huber Bernd A.
Published: June 2007

The authors find even-odd variations as functions of r (⩽7) for multiple ionization of van der Waals dimers in slow Xe30++[C60]2([C60C70])→⋯+[C60]2r+([C60C70]r+) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2r+→C60r1++C60r2+ in the same (r1=r2) or nearby (r1=r2±1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]nr+ (n>3 and r=2–5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]nr+ are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60+ molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r−1) C60+–C60+ pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.
Evolution of a nonspontaneous, high piezoelectric coupling symmetry axis in relaxor-ferroelectric single crystals
Authors: Amin Ahmed , Cross Leslie E. , Lee Ho-Yong
Published: June 2007

A nonspontaneous, high permittivity, “dielectrically soft” symmetry axis possessing large piezoelectric activity was found to exist for predominantly ferroelectric tetragonal FT compositions in the relaxor lead magnesium niobate–ferroelectric lead titanate, (1−x)Pb(Mg1∕3Nb2∕3)O3–(x)PbTiO3, single crystal system. The symmetry axis is not unique. It is a function of the lead titanate concentration x. It becomes the [011] for tetragonal compositions with x>0.35 and the [001] for rhombohedral compositions with 0.30⩽x⩽0.34. The two axial systems are degenerate for x∼0.35 and likely to converge at x∼0.29. Domain engineering for maximum piezoelectric response is easily accomplished for electric fields applied along this axis. The observations were confirmed by (1) isothermal resonance-antiresonance experiments on length extensional bars fabricated from multidomain (1−x)Pb(Mg1∕3Nb2∕3)O3–(x)PTiO3 single crystals and (2) numerical computation using Devonshire’s theory. The results are discussed in terms of recent field-temperature-composition phase diagram and lattice fluctuations along the four equivalent ⟨111⟩ directions. Implications for improved sound projector performance and the influence of dc fields on the domain states are discussed.
Millisecond radiative recombination in poly(phenylene vinylene)-based light-emitting diodes from transient electroluminescence
Authors: Garcia-Belmonte Germà , Montero José M. , Barea Eva M. , Bisquert Juan , Bolink Henk J.
Published: June 2007

The current and electroluminescence transient responses of standard poly(phenylene vinylene)-based light-emitting devices have been investigated. The electroluminescence time response is longer (milliseconds scale) than the current switch-off time by more than one order of magnitude, in the case of small area devices (<0.1cm2). For large area devices (∼6cm2) the electroluminescence decay time decreases from 1.45msto∼100μs with increasing bias voltage. The fast current decay limits the electroluminescence decay at higher voltages. Several approaches are discussed to interpret the observed slow decrease of electroluminescence after turning off the bias. One relies upon the Langevin-type bimolecular recombination kinetics which is governed by the minority carriers (electrons), and another focuses on the slow release of trapped electrons as possible explanations. Additionally, we show that the device current density is mainly determined by the transport of the fastest carriers (holes).
Effect of In additions on the thermoelectric properties of the type-I clathrate compound Ba8Ga16Ge30
Authors: Okamoto Norihiko L. , Kishida Kyosuke , Tanaka Katsushi , Inui Haruyuki
Published: June 2007

The thermoelectric properties of quaternary type-I clathrate compounds, Ba8Ga16−xInxGe30 (x=0–9), have been investigated as a function of In content and temperature. The substitution of In atoms for Ga atoms leads to a decrease in electrical resistivity, as well as a decrease in thermal conductivity. The decrease in electrical resisitivity is explained in terms of the In occupancy behavior in the 6c sites, whereas the decrease in thermal conductivity in terms of the increased extent of the rattling motion of Ba atoms due to the increased lattice constant. As a result, the value of thermoelectric dimensionless figure of merit (ZT) of Ba8Ga16Ge30 is improved by In substitutions from 0.49 to 1.03 at 670°C when x=6.
Theoretical and experimental study of elliptical Debye clusters
Authors: Sheridan T. E. , Wells K. D. , Garee M. J. , Herrick A. C.
Published: June 2007

An elliptical Debye cluster is a system of n identical charged particles interacting through a screened Coulomb potential and confined in a two-dimensional anisotropic parabolic well. A model for this system has been developed with three parameters: n, the well anisotropy α2, and the Debye shielding parameter κ. From numerical solutions of the model, it is shown that the breathing mode persists as the anisotropy increases, and that the normalized, squared breathing frequency increases linearly with α2. Elliptical clusters with n=49 and 15 particles were studied experimentally. The anisotropic potential well was created using a rectangular aperture (17.5×30.2 mm) placed on a flat electrode. The well anisotropy was determined by measuring the center-of-mass (c.m.) oscillation frequencies along the major and minor axes of the resulting elliptical clusters using both driven and thermal oscillations. The two methods give results that are in good agreement. For n=49 particles, the square of the ratios of the c.m. frequencies was α2=2.9±0.1. From a measurement of the breathing frequency, the shielding parameter was found to be κ=2.1±0.2. For n=15 particles, α2=2.8±0.1 and κ=1.9±0.2, demonstrating that an analysis using the model gives consistent results.
Spontaneous formation of one-dimensional ripples in transit to highly ordered two-dimensional herringbone structures through sequential and unequal biaxial mechanical stretching
Authors: Lin Pei-Chun , Yang Shu
Published: June 2007

The authors report the formation of various submicron wrinkle patterns and their transition from one-dimensional (1D) ripples to two-dimensional (2D) herringbone structures on poly(dimethylsiloxane) films. Using mechanical force they can separately control the amount and timing of strain applied to the substrate on both planar directions (either simultaneously or sequentially), which appears to be critical to maneuver the pattern formation in real time. They demonstrate reversible transitions from flat to 1D ripple, to ripple with bifurcation, to ripple/herringbone mixed features, and to well-controlled formation of a highly ordered zigzag-based 2D herringbone structures.