bluuebunny/crossref_metadata_2025 huggingface dataset:
Large magnetic entropy changes in Gd–Co amorphous ribbons
Authors: Zhang C. L. , Wang D. H. , Han Z. D. , Xuan H. C. , Gu B. X. , Du Y. W.
Published: January 2009

A series of magnetic amorphous ribbons with different Gd/Co ratios was prepared by melt-spun method. With the decrease in Gd/Co ratio, Curie temperatures increase gradually from 166 to 193 K. The maximum values of magnetic entropy changes under a magnetic field of 10 kOe are −3.1, −3.0, −2.9, and −2.8 J/kg K for Gd71Co29, Gd68Co32, Gd65Co35, and Gd62Co38 samples, respectively. The approximately constant peak values of ΔSM at different working temperatures indicate that they are advantageous for an Ericsson refrigeration cycle. In addition, these samples have large resistivity and greatly reduced magnetic hysteresis losses, which could increase the refrigeration efficiency. These advantages make the Gd–Co amorphous ribbons good candidates for the practical magnetic refrigeration.
Dynamic simulation of void nucleation during electromigration in narrow integrated circuit interconnects
Authors: Li Wei , Tan Cher Ming , Raghavan Nagarajan
Published: January 2009

Electromigration (EM) is an important failure mechanism in integrated circuit interconnections. Various models have been proposed to study the interconnect degradation due to EM from different perspectives. As the interconnect linewidth shrinks to submicrometer and below, a small growth in void size after void nucleation can sever the conduction path, and hence void nucleation time becomes the dominant part of the time to failure of an interconnect and the primary damage mechanism in EM failure. In this work, an alternative concept of EM modeling is proposed, and the EM lifetime of an interconnect during void nucleation is derived theoretically. A physics-based predictive Monte Carlo simulation methodology is used to model the void nucleation process during EM. To demonstrate the modeling concept and the simulation methodology developed in the present study, Al interconnect test structure is chosen as an illustration and it is shown that the model can predict the voiding location in the interconnect and estimate the median time to failure as verified experimentally.
Well-posedness and uniform decay rates for the Klein–Gordon equation with damping term and acoustic boundary conditions
Authors: Park Jong Yeoul , Ha Tae Gab
Published: January 2009

In this paper, we consider the Klein–Gordon equation u″+−αΔu+β|u′|pu′+γ|u|qu=0 with damping term and acoustic boundary conditions. This work is devoted to prove the existence of global solutions and uniform decay rates of solutions of the Klein–Gordon equation with damping term and acoustic boundary conditions.
Sensitivity of ignition scale backlit thin-shell implosions to hohlraum symmetry in the foot of the drive pulse
Authors: Kirkwood R. K. , Milovich J. , Bradley D. K. , Schmitt M. , Goldman S. R. , Kalantar D. H. , Meeker D. , Jones O. S. , Pollaine S. M. , Amendt P. A. , Dewald E. , Edwards J. , Landen O. L. , Nikroo A.
Published: January 2009

A necessary condition for igniting indirectly driven inertial confinement fusion spherical capsules on the National Ignition Facility (NIF) is controlling drive flux asymmetry to the 1% level time-integrated over the pulse and with <10%∕ns swings during the pulse [J. D. Lindl, P. Amendt, R. L. Berger et al., Phys. Plasmas 11, 339 (2003)]. While drive symmetry during the first 2ns of the pulse can be inferred by using the re-emission pattern from a surrogate high Z sphere and symmetry during the last 5ns inferred from the shape of fully imploded capsules, the midportion (≈2–10ns) has been shown to be amenable to detection by the in-flight shape of x-ray backlit thin-shell capsules. In this paper, we present sensitivity studies conducted on the University of Rochester’s OMEGA laser [J. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasmas 3, 2108 (1996)] of the thin-shell symmetry measurement technique at near NIF-scale for two candidate capsule ablator materials: Ge-doped CH and Cu-doped Be. These experiments use both point and area backlighting to cast 4.7keV radiographs of thin 1.4mm initial-diameter Ge-doped CH and Cu-doped Be shells when converged by a factor of ≈0.5 in radius. Distortions in the position of the transmission limb of the shells resulting from drive asymmetries are measured to an accuracy of a few micrometers, meeting requirements. The promising results to date allow us to compare measured and predicted distortions and by inference drive asymmetries for the first four asymmetry modes as a function of hohlraum illumination conditions.
Octonic representation of electromagnetic field equations
Authors: Mironov Victor L. , Mironov Sergey V.
Published: January 2009

In this paper we represent eight-component values “octons,” generating associative noncommutative algebra. It is shown that the electromagnetic field in a vacuum can be described by a generalized octonic equation, which leads both to the wave equations for potentials and fields and to the system of Maxwell’s equations. The octonic algebra allows one to perform compact combined calculations simultaneously with scalars, vectors, pseudoscalars, and pseudovectors. Examples of such calculations are demonstrated by deriving the relations for energy, momentum, and Lorentz invariants of the electromagnetic field.
Bistable control of ferromagnetic resonance frequencies in ferromagnetic trilayered dots
Authors: Nozaki Yukio , Tateishi Kentaro , Taharazako Shu-ichi , Yoshimura Satoru , Matsuyama Kimihide
Published: January 2009

The ferromagnetic resonance (FMR) properties of submicron-scale Ni81Fe19/Cu/Ni81Fe19 trilayered dots were investigated for application in programable microwave absorbers for broadband filter devices. The trilayered dot shows binary FMR frequencies, depending on the relative orientation of magnetization in the top and the bottom magnetic layers. A parallel configuration of magnetization, which exhibits a lower FMR frequency than that for an antiparallel configuration, can be metastable as the lateral aspect ratio of the dot is larger than the threshold value governed by the Cu layer thickness. The difference in the FMR frequencies between the parallel and antiparallel configurations is gradually decreased with elongation of the dot. This is associated with the suppression of the magnetostatic coupling energy that stabilizes the antiparallel configuration. It was also found that the magnetic configurations can be programed by the selective application of easy- and hard-axis magnetic fields used to saturate the magnetization. This is an appropriate manner to realize the antiparallel configuration of magnetization in the case of a dot with a plural stack of magnetic layers with the same coercive field.
Room-temperature electro-optic properties of strained SrTiO3 films grown on DyScO3
Authors: Ma Hongzhou , Levy Jeremy , Biegalski Michael D. , Trolier-McKinstry Susan , Schlom Darrell G.
Published: January 2009

The electro-optic response of epitaxially strained SrTiO3 grown on bulk DyScO3 substrates is measured as a function of applied in-plane bias (both magnitude and direction) and light polarization. The effective electro-optic coefficients are bias-field dependent. Hysteresis is observed at room temperature, indicative of residual polarity, which is believed to be due to long-lived alignment of nanopolar regions possibly due to defects. A simple model incorporating non-180°-nanoscale domains can account for most of the experimental observations.
Techniques for studying the separatrix of tokamak plasmas
Authors: Webster Anthony J.
Published: January 2009

This paper describes (physically and mathematically) how the plasma-vacuum boundary of a tokamak plasma equilibrium can be perturbed to form a separatrix with an X-point, while having an otherwise negligible affect on the plasma equilibrium. A deliberate consequence of the technique is that the radial and poloidal extent of the perturbed region may be arbitrarily localized. This has useful theoretical and physical consequences, namely (1) it is possible to take any plasma equilibrium and modify the outermost flux surface to form a separatrix with one or more additional x-points in a rigorous way, (2) subsequent studies will be able to separate the effects of shaping from those due to topological changes associated with a separatrix, for example, a circular cross-section plasma may be modified to form a separatrix that is circular everywhere except for an arbitrarily localized region that is perturbed to form an x-point, (3) because the perturbation is arbitrarily localized, there is the possibility for modifying the stability of the edge, without affecting the bulk plasma properties (or stability). Therefore the ideas presented here provide analytical and conceptual tools to study how a separatrix can affect plasma stability, and a potential experimental technique to study the stability of the plasma edge. The paper also investigates how the magnetic shear and the Mercier coefficient behave as a separatrix is approached, showing that for a nonzero toroidal current at the x-point, the Mercier coefficient always asymptotes to zero.
Experimental system to evaluate the effective diffusion coefficient of radon
Authors: Hosoda Masahiro , Tokonami Shinji , Sorimachi Atsuyuki , Janik Miroslaw , Ishikawa Tetsuo , Yatabe Yoshinori , Yamada Junya , Uchida Shigeo
Published: January 2009

The effective diffusion coefficient of radon is a very important factor in estimating the rate of radon exhalation from the ground surface. In this study, we developed an experimental system that overcomes technical problems in previous studies to accurately evaluate the effective diffusion coefficient. The radon source used for this system was the National Institute of Radiological Sciences radon chamber. This chamber is a calibrated international standard facility that can produce stable radon concentrations for long periods of time. Our tests showed that leakage of radon from the system was negligible. After the leakage test, we evaluated the effective diffusion coefficient in free-space and in dry porous materials at porosities of 35% and 45%. To ensure that the porous material in the column was as homogeneous as possible, we filled the column with an artificial soil with controlled grain size and grain composition. The measured values and theoretical calculations agreed well, which indicate that the proposed system can be used to accurately and quickly evaluate the effective diffusion coefficient.
Strongly intermittent edge plasma transport: Issues with modeling and interpretation of experimental data
Authors: Krasheninnikov S. I. , Pigarov A. Yu. , Soboleva T. K. , Rudakov D. L.
Published: January 2009

An impact of blobby transport on plasma dynamics along the magnetic field lines, cross-field plasma transport including E×B drift, and the rates of atomic processes is discussed. It is shown that nonlinearities of plasma transport processes and strong intermittency associated with blobs result in the problems with both interpretation of experimental data based on averaged plasma parameters and with edge plasma modeling with two-dimensional plasma transport codes.
Millimeter wave power sensing using micro- and nanoelectromechanical systems
Authors: Dragoman D. , Dragoman M. , Plana R.
Published: January 2009

This paper deals with the sensing of millimeter and submillimeter wave power using self-actuation of micro- and nano electromechanical systems at certain power thresholds. Since the sensing element and the device to be measured are integrated on the same substrate and fabricated in the same way, a very high degree of miniaturization and accuracy is achieved.
Surface radicals in silane/hydrogen discharges
Authors: Horvath Peter , Gallagher Alan
Published: January 2009

Using threshold ionization mass spectrometry, radical densities have been measured at the substrate surface of a radio frequency discharge in silane plus hydrogen vapor. The conditions are 100–300 Pa pressure and a ratio (R) of hydrogen flow/silane flow typical of discharges used to produce large area amorphous (R=20) and microcrystalline (R=40) silicon. For comparison, we include measurements in pure-silane vapor. The neutral radicals observed in the mixed gases are H, SiH3, Si2H2, and Si2H5, with decreasing flux in that order. (Si2H4 is also seen in pure silane and SiH2 for R=20.) The H flux is sufficient for major film etching and restructuring, particularly for R=40. The ion-bombardment species were also measured, establishing that SiyHn+ (y=1–3) ion flux is much larger than the Hn+ (n=1–3) flux. The ion flux provides 15±4% of the total Si flux to the film for R=20 and 37±10% for R=40. This is larger than in pure-silane discharges, and it may be very important to film properties due to the impact energy.
Geckolike high shear strength by carbon nanotube fiber adhesives
Authors: Maeno Y. , Nakayama Y.
Published: January 2009

Carbon nanotube adhesives can adhere strongly to surfaces as a gecko does. The number of carbon nanotube layers is an important determinant of the contact area for adhesion. Balancing the catalyst ratio and buffer layer used for chemical vapor deposition processing controls the number of carbon nanotube layers and their distribution. The features of carbon nanotubes determine the shear strength of adhesion. Carbon nanotubes with a broad distribution of layers exhibit enhanced shear strength with equivalent adhesive capability to that of a natural Tokay Gecko (Gekko gecko)
Hydrogen sensing properties of a metamorphic high electron mobility transistor
Authors: Tsai Tsung-Han , Chen Huey-Ing , Chang Chung-Fu , Chiu Po-Shun , Liu Yi-Chun , Chen Li-Yang , Chen Tzu-Pin , Liu Wen-Chau
Published: January 2009

Hydrogen sensing properties of a metamorphic high electron mobility transistor (MHEMT) are studied and presented. This MHEMT-based sensor exhibits good pinch-off characteristics upon exposing to hydrogen gases. Besides, the current variation and threshold voltage shift of the studied device reveal larger response under hydrogen-containing conditions. The studied device shows fast responses and exhibits a large current variation magnitude of the order of milliamperes and a relatively low sensitivity due to the high baseline current. Based on the Langmiur isotherm, experimental current responses are consistent with the simulated curve. This indicates that the surface reaction is the rate limited factor for this hydrogen adsorption reaction.
Precision optical trapping via a programmable direct-digital-synthesis-based controller for acousto-optic deflectors
Authors: Mack A. H. , Trías M. K. , Mochrie S. G. J.
Published: January 2009

We describe a simple-to-construct programmable direct-digital-synthesis-based controller for use with acousto-optic deflectors. Our controller corrects for nonlinear diffraction efficiency versus diffraction angle, provides superior stability, functionality, and configurability, and costs a fraction of commercially available systems. Using this instrument, we move a 1 μm diameter bead by 1-nm-sized steps and resolve these steps.
Derivation of asymptotic two-dimensional time-dependent equations for surface water wave propagation
Authors: Lannes David , Bonneton Philippe
Published: January 2009

A general method for the derivation of asymptotic nonlinear models in shallow and deep water is presented. Starting from a general dimensionless version of the water wave equations, we reduce the problem to a system of two equations on the surface elevation and the velocity potential at the free surface. These equations involve a Dirichlet–Neumann operator and we show that many asymptotic models can be recovered by a simple analysis of this operator. Based on this method, a new two-dimensional fully dispersive model for small wave steepness is also derived, which extends to an uneven bottom the approach developed by Matsuno [Phys. Rev. E 47, 4593 (1993)] and Choi [J. Fluid Mech. 295, 381 (1995)]. This model is still valid in shallow water but with less precision than what can be achieved with the Green–Naghdi model when fully nonlinear waves are considered. The combination, or the coupling, of the new fully dispersive equations with the fully nonlinear shallow water Green–Naghdi equations represents a relevant model for describing ocean wave propagation from deep to shallow waters.
Minimizing sample shape and size effects in all purpose magnetometers
Authors: Van der Noordaa J. T.
Published: January 2009

When measurements are made with a magnetometer, it is usually assumed that the sample produces a pure dipolar magnetic field. This is only true if the sample is a sphere. If the sample is not a sphere, the magnetic field produced by it will also contain higher order multipoles. These components of the magnetic field depend not only on the volume and magnetization but also on the sample’s shape and size. Ideally the measured moment of a sample is only proportional to the sample’s magnetization and volume. This paper explains why a sample’s shape and size affects the response of a magnetometer, shows results illustrating this effect, and discusses conditions that will minimize these effects.
Shape-changing bodies in fluid: Hovering, ratcheting, and bursting
Authors: Spagnolie Saverio E. , Shelley Michael J.
Published: January 2009

Motivated by recent experiments on the hovering of passive bodies, we demonstrate how a simple shape-changing body can hover or ascend in an oscillating background flow. We study this ratcheting effect through numerical simulations of the two-dimensional Navier–Stokes equations at intermediate Reynolds number. This effect could describe a viable means of locomotion or transport in such environments as a tidal pool with wave-driven sloshing. We also consider the velocity burst achieved by a body through a rapid increase in its aspect ratio, which may contribute to the escape dynamics of such organisms as jellyfish.
Reverse biased Pt/nanostructured MoO3/SiC Schottky diode based hydrogen gas sensors
Authors: Yu J. , Ippolito S. J. , Shafiei M. , Dhawan D. , Wlodarski W. , Kalantar-zadeh K.
Published: January 2009

Pt/nanostructured molybdenum oxide (MoO3)/SiC Schottky diode based gas sensors were fabricated for hydrogen (H2) gas sensing. Due to the enhanced performance, which is ascribed to the application of MoO3 nanostructures, these devices were used in reversed bias. MoO3 characterization by scanning electron microscopy showed morphology of randomly orientated nanoplatelets with thicknesses between 50 and 500 nm. An α-β mixed phase crystallographic structure of MoO3 was characterized by x-ray diffraction. At 180 °C, 1.343 V voltage shift in the reverse I-V curve and a Pt/MoO3 barrier height change of 20 meV were obtained after exposure to 1% H2 gas in synthetic air.
Controllable vortex polarity switching by spin polarized current
Authors: Jin Wei , He Huan , Chen Yuguang , Liu Yaowen
Published: January 2009

Nanopillars containing a perpendicular polarizer and a vortex free layer have been considered as promising structures for the selective switching of the vortex core by changing the spin polarized current direction. In this paper, two distinct switching routes of the vortex cores are found to be classified according to the lateral dimension of samples. For a relatively large element, the core polarity is switched through a creation and annihilation process of vortex-antivortex pair and the vortex circulation remains unchanged; however, for a small element, the core switching is mediated by a transient C-state, and moreover, the circulation is dominated by the current-generated Oersted field.
Magnetic phase evolution in the LaMn1−xFexO3+y system
Authors: de Lima O. F. , Coaquira J. A. H. , de Almeida R. L. , de Carvalho L. B. , Malik S. K.
Published: January 2009

We have investigated the crystal structure and magnetic properties for polycrystalline samples of LaMn1−xFexO3+y, in the whole range 0.0≤x≤1.0, prepared by solid state reaction in air. All samples show the ORT-2 orthorhombic structure that suppresses the Jahn–Teller distortion, thus favoring a ferromagnetic (FM) superexchange interaction between Mn3+–O–Mn3+. For x=0.0 the oxygen excess (y≈0.09) produces vacancies in the La and Mn sites and generates a fraction around 18% of Mn4+ ions and 82% of the usual Mn3+ ions, with possible double-exchange interaction between them. The Fe doping in this system is known to produce only stable Fe3+ ions. We find an evolution from a fairly strong FM phase with a Curie temperature TC∼160 K, for x=0.0, to an antiferromagnetic (AFM) phase with TN=790 K, for x=1.0, accompanied by clear signatures of a cluster-glass behavior. For intermediate Fe contents a mixed-phase state occurs, with a gradual decrease (increase) in the FM (AFM) phase, accompanied by a systematic transition broadening for 0.2<x<0.7. A model based on the expected exchange interaction among the various magnetic-ion types accounts very well for the saturation magnetization (MS) dependence on Fe doping.
Effects of thermal and athermal processing on the formation of buried SiC layers
Authors: Katharria Y. S. , Kumar Sandeep , Kanjilal D. , Chauhan Devki , Ghatak J. , Bhatta U. , Satyam P. V.
Published: January 2009

In the present study, systematic investigations on 100 keV C ion implanted Si (100) substrates annealed subsequently at a temperature of 1000 °C for 2 h or athermally processed using 110 MeV Ni8+ ion irradiation have been performed. A detailed analysis using the techniques of x-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy (TEM) at high resolutions is performed. The observations suggest the formation of cubic silicon carbide (β-SiC) crystallites surrounded by an amorphous background in the samples thermally annealed at 1000 °C. However, ion irradiation did not influence the as-implanted layers to any significant extent. Various defects formed after annealing inside C implanted Si such as missing planes, edge dislocations, and grain boundaries during thermal crystallization are visualized through high resolution TEM.
Magnetoimpedance and magnetization processes of FeCoNi electroplated tubes
Authors: Jantaratana Pongsakorn , Bebenin N. G. , Kurlyandskaya G. V.
Published: January 2009

The CuBe/Fe20Co6Ni74 electroplated wires were prepared by a standard electrodeposition technique. Heat treatment in a dc magnetic field of 20 Oe applied along the axis of the wire at a temperature of 320 °C for 1 h resulted in a magnetic anisotropy with a longitudinal magnetic anisotropy component. The impedance of the field annealed CuBe/Fe20Co6Ni74 wire strongly depends on the external magnetic field. Very high magnetoimpedence (MI) (up to 600%) and MI sensitivity (up to 570%/Oe) were observed at a relatively low frequency of 1 MHz and a current intensity of 20 mA. Two MI maxima were observed for each branch of the MI responses. These maxima are different from each other: the value of the MI ratio, the MI sensitivity, and the value of the field in which each particular MI maximum appears. The angular dependence of a linear MI effect was studied for specific conditions: f=1 MHz and Irms=20 mA. Detailed analysis of the MI responses indicated the existence of two different types of spin-orientational phase transitions, strongly connected with high order magnetic anisotropy.
Bimagnetic nanoparticles with enhanced exchange coupling and energy products
Authors: Nandwana Vikas , Chaubey Girija S. , Yano Kazuaki , Rong Chuan-bing , Liu J. Ping
Published: January 2009

Bimagnetic FePt/Fe3O4 nanoparticles with core/shell or heterodimer structure have been prepared using a sequential synthetic method. The dimension of both FePt and Fe3O4 was tuned by varying the synthesis parameters. The as-synthesized bimagnetic nanoparticles were superparamagnetic at room temperature. After being annealed in a reducing atmosphere, the FePt/Fe3O4 bimagnetic nanoparticles were converted to a hard magnetic nanocomposite with enhanced energy products due to the exchange coupling between the hard and soft magnetic phases. It was found that the exchange coupling in nanocomposites made from the core/shell nanoparticles is stronger than that from the heterodimer nanoparticles. By tuning the dimensions of the FePt and Fe3O4 phases, the energy product up to 17.8 MGOe was achieved in the annealed nanocomposites, which is 36% higher than the isotropic single-phase FePt counterpart.
Suppressed conductance in a metallic graphene nanojunction
Authors: Li Haidong , Wang Lin , Zheng Yisong
Published: January 2009

The linear conductance spectrum of a metallic graphene junction formed by interconnecting two gapless graphene nanoribbons is calculated. A strong conductance suppression appears in the vicinity of the Dirac point. We found that such a conductance suppression arises from the antiresonance effect associated with an edge state localized at the zigzag-edged shoulder of the junction. The conductance valley due to the antiresonance is rather robust in the presence of the impurity and vacancy scattering. Also the center of the conductance valley can be readily tuned by an electric field exerted on the wider nanoribbon.
Multiple diffraction of plane compressional waves by two circular cylindrical holes with surface effects
Authors: Wang G. F.
Published: January 2009

In the present paper, the multiple diffraction of plane harmonic compressional waves (P-wave) by two nanosized circular cylindrical holes embedded in an elastic solid is investigated. The surface elasticity theory is adopted to account for the effect of surface energy at nanoscales. It is found that when the radii of holes reduce to nanometers, surface energy significantly affects the diffraction of elastic waves. The dynamic stresses around the holes under incident waves of different frequencies are examined to display the influence of surface energy and the interaction between holes in the multiple scattering phenomena.
Direct simulation Monte Carlo on thermal distribution of rarefied gas under heated atomic force microscope nanoprobe
Authors: Liu X. J. , Yang Y. W. , Yang J. P.
Published: January 2009

Heated atomic force microscope (AFM) nanoprobe is an attractive instrument for highly local thermal processing. The gases between the nanoprobe and the sample surface exhibit different behaviors from the macroscopic gases due to the nanoscale probe-sample distance. In this paper, the thermal conduction of rarefied gases heated by an AFM nanoprobe is investigated by means of the direct simulation Monte Carlo method. The heat reservoir of AFM nanoprobe consists of a heater platform and a nanotip. The effects of heater platform and nanotip on the gas heat transfer are analyzed. It is found that both the size of heater platform and the geometry of nanotip have noticeable influence on the heat flux density distribution and the spatial resolution on the sample surface. The results show that a spatial resolution of a few tens of nanometers can be achieved by the hot AFM nanoprobe and the power provided to the spatial scale can be at an order of 10−8 W. It is also found that a sample surface can be efficiently heated locally without the contact of the nanoprobe and sample, thus alleviating the wear between them and improving the system reliability. The work provides an insight for rational design and optimization of the heated nanoprobe/surface configuration-based systems for topography applications.
Effect of initial target surface roughness on the evolution of ripple topography induced by oxygen sputtering of Al films
Authors: Mishra P. , Ghose D.
Published: January 2009

The effect of pre-existing random roughness on the evolution of ripple structures in O2+ sputtered thin Al films has been investigated. The results show that there is a considerable reduction in initial roughness of the film surface at the early stages of sputtering. For large scale surface structures, angle-dependent first order sputtering is responsible for ion beam smoothening, while for smaller microscopic features, different relaxation mechanisms dominate for smoothing of the surface. At the later stages of sputtering, the curvature dependent erosion instability sets in leading to the development of either coherent ripples or faceted structures depending on the degree of virgin film roughness and bombarding angle. It is found that coating a flat Si surface with ultrathin Al film and subsequent removal of the Al layer by oblique O2+ sputtering leads to the formation of ripple pattern with moderate amplitude in the Si matrix at much lower effective fluence than that would be in bare Si without Al masking.
Broadband ferromagnetic resonance linewidth measurement of magnetic tunnel junction multilayers
Authors: Sierra J. F. , Aliev F. G. , Heindl R. , Russek S. E. , Rippard W. H.
Published: January 2009

The broadband ferromagnetic resonance (FMR) linewidth of the free layer of magnetic tunnel junctions is used as a simple diagnostic of the quality of the magnetic structure. The FMR linewidth increases near the field regions of free layer reversal and pinned layer reversal, and this increase correlates with an increase in magnetic hysteresis in unpatterned films, low-frequency noise in patterned devices, and previous observations of magnetic domain ripple by use of Lorentz microscopy. Postannealing changes the free layer FMR linewidth, indicating that considerable magnetic disorder, originating in the exchange-biased pinned layer, is transferred to the free layer.
On Thermostats: Isokinetic or Hamiltonian? Finite or infinite?
Authors: Gallavotti Giovanni
Published: January 2009

The relation between finite isokinetic thermostats and infinite Hamiltonian thermostats is studied and their equivalence in the thermodynamic limit is heuristically discussed.
Band edge optical transitions in dilute-nitride GaNSb
Authors: Wang D. , Svensson S. P. , Shterengas L. , Belenky G. , Kim C. S. , Vurgaftman I. , Meyer J. R.
Published: January 2009

Dilute-nitride GaNSb bulk materials with up to 1.4% nitrogen were grown by molecular beam epitaxy on GaSb substrates. Hall measurements indicate residual hole concentrations of nearly 1019 cm−3 at room temperature, but a decrease to below 1016 cm−3 and a hole mobility of 1300 cm2/V s at 4.5 K for a sample with 0.6% nitrogen. Photoluminescence (PL) and optical absorption measurements demonstrate a bandgap reduction by up to 300 meV with increasing nitrogen incorporation. The experimental absorption spectra are well fit by a functional dependence corresponding to direct allowed optical transitions, and the PL spectra are also consistent with that interpretation. Room temperature carrier relaxation times in the picosecond range are measured using an ultrafast PL upconversion technique.
Simulation of electrophoretic stretching of DNA in a microcontraction using an obstacle array for conformational preconditioning
Authors: Trahan Daniel W. , Doyle Patrick S.
Published: January 2009

Recently our group has reported experiments using an obstacle array to precondition the conformations of DNA molecules to facilitate their stretch in a microcontraction. Based upon previous successes simulating electrophoretic stretching in microcontractions without obstacles, we use our simulation model to study the deformation of DNA chains in a microcontraction preceded by an array of cylindrical obstacles. We compare our data to the experimental results and find good qualitative, and even quantitative, agreement concerning the behavior of the chains in the array; however, the simulations overpredict the mean stretch of the chains as they leave the contraction. We examine the amount of stretch gained between leaving the array and reaching the end of the contraction and speculate that the differences seen are caused by nonlinear electrokinetic effects that become important in the contraction due to a combination of field gradients and high field strengths.
Low-frequency surface plasmon polaritons propagating along a metal film with periodic cut-through slits in symmetric or asymmetric environments
Authors: Zhang Xufeng , Shen Linfang , Ran Lixin
Published: January 2009

Surface plasmon polaritons (SPPs) propagating along a perfectly electric conducting film with an array of cut-through slits are studied analytically, and both the symmetric and asymmetric dielectric environments of the film are considered. Such a film structure, whether its substrate and superstrate are the same or different, may support a series of SPP modes, whose dispersion relations can be tailored by the film geometry. There exists a fundamental mode without any cutoff in the symmetric film structure, whereas all modes in the asymmetric structure have cutoffs, meaning that even the lowest-order mode can only exist when the film thickness is larger than a certain value. Moreover, for both types of symmetric and asymmetric film structures, the SPP bands do not overlap each other in the frequency domain, so the single-mode propagation is available for each mode. The fields of SPP modes are often highly confined to the structured film when the frequency approaches the asymptotic frequency.
Evolution of the current wedge and the generator current circuit by fast reconnection
Authors: Ugai M.
Published: January 2009

Temporal dynamics of the generator current circuit and the current wedge is studied on the basis of the spontaneous fast reconnection model. Once the fast reconnection jet causes magnetic field dipolarization, a magnetohydrodynamic (MHD) generator arises ahead of the magnetic loop top. The generator effect is so powerful that a current circuit is strongly driven in the generator region and evolves to approach the loop footpoint through field-aligned currents along the loop boundary; simultaneously, the sheet current directed to the generator region turns its direction toward the loop footpoint, giving rise to current wedge evolution outside the generator current circuit. Both these current circuits are eventually connected to a layer around the separatrix in the loop footpoint, where the current is concentrated and intensified, leading to drastic energy dissipations. It is concluded that the MHD generator ahead of the magnetic loop top, provided and sustained by the fast reconnection jet, plays a crucial role in the two-ribbon heating observed in geomagentic substorms and solar flares.
Phase separation and persistent magnetic memory effect in La0.625Ca0.375MnO3 and La0.375Pr0.25Ca0.375MnO3 films
Authors: Tsai Y. T. , Chang W. J. , Hsieh C. C. , Luo C. W. , Wu K. H. , Uen T. M. , Juang J. Y. , Lin J. Y.
Published: January 2009

Both La0.375Pr0.25Ca0.375MnO3 (LPCMO) and La0.625Ca0.375MnO3 (LCMO) were found to exhibit coexistence of competing orders (phase separation) over a wide temperature range. However, substantial hysteretic behaviors in both of the temperature-dependent resistance [R(T)] and magnetization [M(T)] [also known as the persistent magnetic memory effect (PMME)] are only displayed in LPCMO. The results indicate that, in LPCMO, the size distribution of the coexisting charge-ordered insulating and metallic ferromagnetic (FM) phases plays a determinant role in the PMME effects in different temperature regimes. Moreover, due to the direct competition between the two coexisting phases, the system is most susceptible to the external applied field in the hysteretic temperature region. On the other hand, in LCMO, the phase transition between paramagnetic and FM is more like an isomorphic transition in pure materials, and thus does not show significant hysteresis.
Analytical formulation for the resonant frequency of split rings
Authors: Sydoruk O. , Tatartschuk E. , Shamonina E. , Solymar L.
Published: January 2009

A simple approximate expression is derived for the resonant frequency of a singly split single ring that is among the first microwave resonators designed to be small relative to the wavelength. In addition to the usual gap capacitance the concept of surface capacitance is introduced. The surface capacitance is determined analytically by two different methods, first using analytical expressions for the electric field of a split cylinder, and second by using conformal mapping. Taking two practical examples the resonant frequency, found analytically, is shown to agree with that obtained by numerical simulations. The model could be used for studies of the resonant properties of split rings in the terahertz region.
Effect of electron irradiation dose on the performance of avalanche photodiode electron detectors
Authors: Kawauchi Taizo , Wilde Markus , Fukutani Katsuyuki , Okano Tatsuo , Kishimoto Shunji
Published: January 2009

Avalanche photodiodes (APDs) are efficient detectors for electrons with energies below 100 keV. The damaging effects of 8 keV electron beam irradiation on the dark current and the output signal of the APD detector were investigated in this study. The APD dark current increases after electron doses exceeding 1.4×1013 cm−2. Preirradiation by high doses of 8 keV electrons further causes a deformation of the pulse height distribution of the APD output in the subsequent detection of low-flux electrons. This effect is particularly prominent when the energy of the detected electrons is lower than that of the damaging electrons. By comparing the experimental data with results of a simulation based on an electron trapping model, we conclude that the degradation of the APD performance is attributable to an enhancement of secondary-electron trapping at irradiation induced defects.
Significant influence of film thickness on the percolation threshold of multiwall carbon nanotube/low density polyethylene composite films
Authors: Fu M. , Yu Y. , Xie J. J. , Wang L. P. , Fan M. Y. , Jiang S. L. , Zeng Y. K.
Published: January 2009

A series of multiwall carbon nanotube/low density polyethylene composite films with different thicknesses and different carbon nanotube volume fractions was prepared and studied. The result showed an interesting phenomenon which has not been reported before: when the thickness was reduced to the length range of the carbon nanotube filler, it could remarkably affect the percolation threshold of the composite films. Classical theories have been introduced to describe the phenomenon. This result could have significance in future applications of percolative composite thin/thick films.
Site-selective spectroscopy and crystallographic sites of Eu3+ ions doped in Gd2BaZnO5
Authors: Huang Yanlin , Shi Liang , Kim Eun Sik , Seo Hyo Jin
Published: January 2009

The influence of the cationic sites on the luminescent properties of Eu3+ in Gd2BaZnO5 has been investigated by the site-selective laser excitation and emission spectroscopy along with luminescence decays. It has been revealed that the Eu3+ ions occupy two cationic sites. The Stark energy levels of Eu3+ at two different sites in Gd2BaZnO5 were assigned from selectively excited emission spectra at 10 K. The energy transfer between two crystallographic Eu3+ centers in Gd2BaZnO5 is detected and discussed. Excitation spectrum in the UV region and UV-excited emission spectrum were also performed, which indicate that an efficient energy transfers from the host and directly Gd3+ to Eu3+ ions in Gd2BaZnO5.
Impact of germanium on vacancy clustering in germanium-doped silicon
Authors: Chroneos A. , Grimes R. W. , Bracht H.
Published: January 2009

Recent density functional theory calculations by Chen et al. [J. Appl. Phys. 103, 123519 (2008)] revealed that vacancies (V) tend to accumulate around germanium (Ge) atoms in Ge-doped silicon (Si) to form GeVn clusters. In the present study, we employ similar electronic structure calculations to predict the binding energies of GeVn and Vn clusters containing up to four V. It is verified that V are strongly attracted to pre-existing GeVn clusters. Nevertheless, by comparing with the stability of Vn clusters, we predict that the Ge contribution to the binding energy of the GeVn clusters is limited. We use mass action analysis to quantify the relative concentrations of GeVn and Vn clusters over a wide temperature range: Vn clusters dominate in Ge-doped Si under realistic conditions.
Effect of carbon incorporation on electrical properties of n-type GaN surfaces
Authors: Kimura Takeshi , Hashizume Tamotsu
Published: January 2009

We intentionally incorporated carbon into n-GaN by high-temperature annealing of a SiNx/CNx/GaN structure to study the effect of unintentional carbon incorporation on the electrical properties of n-type GaN surfaces. X-ray photoelectron spectroscopy results showed outdiffusion of Ga atoms from the GaN surface during high-temperature annealing even when the SiNx layer was present. The current-voltage characteristics showed a drastic increase in current in the forward and reverse directions of the Schottky diode in the carbon-incorporated sample. They also showed no temperature dependence from 150 to 300 K. The current-voltage curves of the carbon-incorporated samples in the forward and reverse directions could be almost completely reproduced by assuming an exponentially decaying distribution from the surface for shallow donors.
Coherent terahertz acoustic vibrations in polar and semipolar gallium nitride-based superlattices
Authors: Moss D. M. , Akimov A. V. , Kent A. J. , Glavin B. A. , Kappers M. J. , Hollander J. L. , Moram M. A. , Humphreys C. J.
Published: January 2009

The femtosecond optical pump-probe technique is used to generate and detect the coherent acoustic vibrations with a frequency 0.83×1012Hz in polar (0001) and semipolar (112¯2) InGaN∕GaN superlattices. The measured amplitude of the vibrations in the semipolar sample is about five times smaller that in the polar one. Analysis of the experimental data and theoretical estimates suggest that, in addition to piezoelectric, deformation potential provides essential contribution to both generation and detection of vibrations, especially in the semipolar superlattice.
Automatic beam alignment system for a pulsed infrared laser
Authors: Kral L.
Published: January 2009

A fully automatic alignment system for a pulsed infrared laser beam (5 ns pulses, 10 Hz repetition rate, 1.3 μm wavelength) was developed and tested. It compensates for long-term fluctuations of the beam initial position and direction—the automatic realignment is performed every 10 min, and lasts typically 1–2 min. The system adjusts the beam initial position with a maximum error of 0.5 mm (10% of the beam diameter) and the beam direction with a maximum error of 50 μrad. The solution is based on two InGaAs quadrant photodiodes as the position detectors and two motorized mirrors controlled by a personal computer. The signals from the quadrant detectors are processed by a peak detector and digitized by an analog to digital converter, which is synchronized with the laser pulses.
Reducing the critical current for spin-transfer switching of perpendicularly magnetized nanomagnets
Authors: Mangin S. , Henry Y. , Ravelosona D. , Katine J. A. , Fullerton Eric E.
Published: January 2009

We describe nanopillar spin valves with perpendicular anisotropy designed to reduce the critical current needed for spin transfer magnetization reversal while maintaining thermal stability. By adjusting the perpendicular anisotropy and volume of the free element consisting of a [Co/Ni] multilayer, we observe that the critical current scales with the height of the anisotropy energy barrier and we achieve critical currents as low as 120 μA in quasistatic room-temperature measurements of a 45 nm diameter device. The field-current phase diagram of such a device is presented.
Superconductivity in S-substituted FeTe
Authors: Mizuguchi Yoshikazu , Tomioka Fumiaki , Tsuda Shunsuke , Yamaguchi Takahide , Takano Yoshihiko
Published: January 2009

We synthesized a superconducting system of FeTe1−xSx with a PbO-type structure. It has the simplest crystal structure among iron-based superconductors. The superconducting transition temperature is about 10 K when x is 0.2. The upper critical field Bc2(0) was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Since FeTe1−xSx is composed of nontoxic elements, this material is a candidate for applications that will further advance research on iron-based superconductors.
Towards efficient tin-doped indium oxide (ITO)-free inverted organic solar cells using metal cathodes
Authors: Meiss J. , Riede M. K. , Leo K.
Published: January 2009

We present zinc phthalocyanine (ZnPc):C60 bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%.
Effects of hydrogen implantation damage on the performance of InP/InGaAs/InP p-i-n photodiodes transferred on silicon
Authors: Chen Peng , Chen Winnie V. , Yu Paul K. L. , Tang Chak Wah , Lau Kei May , Mawst Luke , Paulson Charles , Kuech T. F. , Lau S. S.
Published: January 2009

Functioning InP/InGaAs/InP p-i-n photodiodes were integrated onto a Si substrate using hydrogen-induced layer transfer process (ion cut) combined with selective chemical etching. This device transfer process minimizes the hydrogen implantation-induced damage and simultaneously improves the transferred surface flatness for device processing. After transfer, the dark current under the reverse bias increased by ∼1.5 times over that of the as-grown photodiodes at −1.5 V, while the photoinduced current was comparable to that of the as-grown sample. These results were discussed in terms of interactions between minority carriers and the remaining implantation-induced damage.
Spatial-chirp compensation in dynamical holograms reconstructed with ultrafast lasers
Authors: Martínez-León Lluís , Clemente Pere , Tajahuerce Enrique , Mínguez-Vega Gladys , Mendoza-Yero Omel , Fernández-Alonso Mercedes , Lancis Jesús , Climent Vicent , Andrés Pedro
Published: January 2009

A computer generated hologram (CGH) reconstructed with a sub-100-fs laser pulse at the focal plane of a conventional refractive lens experiences a large amount of spatial chirp. We report the shaping of a 12 fs laser pulsed beam by means of a Fourier CGH implemented onto a spatial light modulator, using a hybrid diffractive-refractive lens triplet that provides spatial-chirp compensation. Experimental results demonstrate that parallel, arbitrary, and high-resolution patterning is possible with the proposed device.
Single electron charging in deterministically positioned InAs/InP quantum dots
Authors: Reimer M. E. , Dalacu D. , Lapointe J. , Poole P. J. , Kim D. , Aers G. C. , McKinnon W. R. , Williams R. L.
Published: January 2009

We demonstrate precise control of electron charging within a single deterministically positioned InAs/InP quantum dot emitting in the telecommunications band around 1500 nm. Photolumine-scence emission as a function of vertical electric field is used to monitor the electron number within the dot. From Stark shift spectroscopy, we extract strength and orientation for the built-in dipole moment that suggests a uniform InAs dot composition and a configuration in which the electron lies above the hole at zero electric field. The scalable gating technology we employ to electrically contact individual prepositioned quantum dots promises arrays of initialized single spins that can be used for fiber-based quantum information applications.
Dispersion in magnetostatic CoTaZr spin waveguides
Authors: Kozhanov A. , Ouellette D. , Griffith Z. , Rodwell M. , Jacob A. P. , Lee D. W. , Wang S. X. , Allen S. J.
Published: January 2009

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion and up to three different modes are identified. Attenuation lengths of the order of 3 μm are several orders of magnitude shorter than that predicted from eddy currents in these thin wires.