bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocation-free and lattice-matched Si/GaP1−xNx/Si structure for photo-electronic integrated systems
Authors: Momose Kenji , Yonezu Hiroo , Fujimoto Yasuhiro , Furukawa Yuzo , Motomura Yoshifumi , Aiki Kunio
Published: July 2002

We proposed a Si/III–V–N compound semiconductors/Si structure, which is applicable to optoelectronic integrated circuits (OEICs). The feature of this structure is that optoelectronic devices and Si electronic devices could be fabricated by low-temperature planar process at the same time. A dislocation-free and lattice-matched Si/GaP1−xNx/Si (x=2.9%) structure, which is a basic structure for OEICs, was grown by molecular-beam epitaxy. The images of transmission electron microscopy revealed that there were no threading dislocations and misfit dislocations in the epitaxial layers. It was clarified that the Si and GaP1−xNx layers were lattice-matched to Si and had structural high crystalline quality comparable to Si substrates.
Injection-seeded terahertz-wave parametric generator with wide tunability
Authors: Kawase Kodo , Minamide Hiroaki , Imai Kazuhiro , Shikata Jun-ichi , Ito Hiromasa
Published: July 2002

We report on the development of a widely tunable (frequency: 0.7–2.4 THz and wavelength: 125–430 μm), injection-seeded THz-wave parametric generator (IS-TPG), which operates at room temperature. The spectral resolution (<100 MHz and 0.003 cm−1) is the Fourier transform limit of the nanosecond THz-wave pulse. The continuous scanning and narrow spectral bandwidth of the IS-TPG were verified in the absorption spectrum of low-pressure water vapor. The high peak power (>200 mW) of the output wave and the small beam divergence are suited to a variety of applications.
Influence of strain relaxation on structural and optical characteristics of InGaN/GaN multiple quantum wells with high indium composition
Authors: Cho H. K. , Lee J. Y. , Kim C. S. , Yang G. M.
Published: July 2002

Influence of strain relaxation on structural and optical properties of the InGaN/GaN multiple quantum wells (MQWs) with high indium composition grown by metalorganic chemical vapor deposition was investigated. From photoluminescence and transmission electron microscopy (TEM), we found that within the MQWs, the formation of misfit dislocation affects the degradation of optical properties more than the formation of stacking faults. For the MQWs with indium composition above the critical indium composition on the formation of misfit dislocation, the position of the main emission peak is significantly affected by the increase of quantum well numbers compared to samples with indium composition below the critical indium composition. The origins of redshift by the increase of quantum well numbers is believed to be caused by the increase of indium segregation in the MQWs using high-resolution TEM and energy dispersive x-ray spectroscopy.
Nonlinear instabilities in plasma with highly correlated dust grains
Authors: Wang Nian-Chun , Xie Bai-Song
Published: July 2002

Instabilities of ion waves modulated by low-frequency short-wavelength dust motion with highly correlated dust grains are studied. The effect of dust charging is also taken into account. It is found that the modulational behavior depends strongly on the dust correlation, charging and viscoelasticity.
Spontaneous chemical ordering and exchange bias in epitaxial Mn0.52Pd0.48/Fe(001) bilayers prepared at room temperature
Authors: Farrow R. F. C. , Marks R. F. , Toney M. F. , David S. , Kellock A. J. , Borchers J. A. , O’Donovan K. V. , Smith David J.
Published: July 2002

We report spontaneous chemical ordering of Mn0.52Pd0.48 films, grown by molecular-beam epitaxy on body-centered-cubic Fe(001) films. The bilayers were grown at room temperature onto a seeding structure of Ag(001)/Fe(001)/GaAs(001). X-ray, neutron, and electron diffraction studies confirm extensive chemical ordering of the MnPd to the L10 antiferromagnetic phase which grows as a twinned film with the tetragonal c axis in the film plane. The Fe film exhibits a unidirectional exchange bias and we find no pronounced difference in magnetic structure of the biased Fe layer at the ascending and descending crossing fields where the net moment along the applied field axis is zero.
X-ray diffraction order selection with a prism in DEI (abstract)
Authors: Zhong Z. , Chapman D. , Hasnah M. , Johnston E. , Kiss M. Z. , Oltulu O. , Rigon L. , Zhong N. , Pisano E. , Sayers D. , Thomlinson W.
Published: July 2002

The refraction angle of x rays as they pass through a refractive prism is proportional to the square of the x-ray wavelength. This was used to discriminate between different harmonic orders, which have different energies, created by a two-crystal 111 monochromator [Z. Zhong, J. Appl. Crystallogr 33, 1082 (2000)]. Refractive prisms inserted between the two monochromator crystals allowed selection, among the 111, 333, 444, and 555 diffractions, in our diffraction enhanced imaging program at National Synchrotron Light Source’s X15A beamline, and facilitated convenient change of sensitivity in our efforts to optimize DEI system for mammography and other radiographic applications. The selection of 111 and 333 refractions were straightforward since the necessary angle of refraction is small. The challenge of selecting 444 and 555 reflections was met by designing an acrylic prism that offered an angle of refraction much larger than 10 μrad and was compact enough to be manipulated in the small space between the crystals. Examples are given on use of the prism for studying the effects of diffraction orders on DEI’s refraction and extinction contrasts. Its modes of operation and limitations are discussed. The principle of harmonics selection through prism and the specific design of the device could be relevant for other synchrotron monochromators that have a harmonics concern or can benefit from change of energy, through selection of harmonic orders, without changing crystals.
Theoretical equations of state for temperature and electromagnetic field dependence of fluid systems, based on the quasi-Gaussian entropy theory
Authors: Amadei A. , Apol M. E. F. , Brancato G. , Di Nola A.
Published: July 2002

The quasi-Gaussian entropy (QGE) theory employs the fact that a free-energy change can be written as the moment-generating function of the appropriate probability distribution function of macroscopic fluctuations of an extensive property. By modeling this distribution, one obtains a model of free energy and resulting thermodynamics as a function of one state variable. In this paper the QGE theory has been extended towards theoretical models or equations of state (EOS’s) of the thermodynamics of semiclassical systems as a function of two state variables. Two “monovariate” QGE models are combined in the canonical ensemble: one based on fluctuations of the excess energy (the confined gamma state giving the temperature dependence) and the other based on fluctuations of the reduced electromagnetic moment [various models as derived in the preceding paper [Apol, Amadei, and Di Nola, J. Chem. Phys. 116, 4426 (2002)], giving the external field dependence]. This provides theoretical EOS’s for fluid systems as a function of both temperature and electromagnetic field. Special limits of these EOS’s are considered: the general weak-field EOS and the limit to a Curie’s law behavior. Based on experimental data of water and simulation data using the extended simple point charge (SPC/E) water model at 45.0 and 55.51 mol/dm3, the specific EOS based on a relatively simple combination of the confined gamma state model with a discrete uniform state field model accurately reproduces the dielectric properties of water at constant density, as the temperature dependence of the weak-field dielectric constant for gases and liquids, and the field dependence of the dielectric constant of liquids.
Atomic scale heating in cathodic arc plasma deposition
Authors: Anders André
Published: July 2002

Energetic deposition using a filtered cathodic arc plasma is known to lead to very adherent and dense films. Interface mixing, subplantation depth, texture, and stress of the growing film are often studied as a function of the kinetic energy of condensing ions. Ions also have potential energy contributing to atomic scale heating, secondary electron emission, and potential sputtering, thereby affecting all film properties. We will show kinetic and potential energies of ions in cathodic arc plasmas. These energies are greater than the binding energy, surface binding energy, and activation energy of surface diffusion. The role of potential energy on film growth is not limited to the cathodic arc plasma deposition process.
Mean-field resonating-valence-bond theory for unpaired π-electrons in benzenoid carbon species
Authors: Ivanciuc O. , Bytautas L. , Klein D. J.
Published: July 2002

A qualitative resonance-theoretic view is presented for the description of a variety of conjugated π-network species identified with “subgraphs” (either finite or infinite) of the graphite network. Within the framework of this resonance theory, simple rules are described to provide qualitative information: On ground-state spin multiplicities; on patterns of ground-state spin density; and on exchange splittings to low-lying “spin-flipped” excited states. Beyond ordinary benzenoid molecules, illustrative applications are noted to a diversity of extended species, including: Differently structured edges on semi-infinite graphite; corner structures where edges along different directions meet; conjugated polymer-strip ends; and local defect vacancy structures in extended graphite. The variety of simple resonance-theoretic predictions are compared against a semiempirical unrestricted Hartree–Fock view of some quantitative tight-binding molecular-orbital-theoretic computations. Agreement in predictions from the resonance- and band-theoretic viewpoints is taken to engender reliability of the so coincident predictions. A traditional organic chemical resonance-theoretic view is thence conveniently reformulated and brought to bear on several extended nano-structured systems to reveal systematic patterns of π-electronic behavior.
Exchange coupling and magnetoresistance in nano-oxide layer built-in specular spin valves
Authors: Diao Zhitao , Huai Yiming , Chen Lifan
Published: July 2002

Spin valves with nano-oxide layers (ΔR/R=16% and ΔR∼3.7 Ω/sq) are developed based on plasma and natural oxidation processes, rendering stable giant magnetoresistance (GMR) reader with ultrahigh density recording of more than 50 Gbit/in2. A simple model is used to discuss the main features of the plasma and natural oxidation process and the magnetic coupling between two subpinned layers through the oxide layers. At full-film level, large enhancement of GMR combined with a strong exchange coupling field and thermally stable magnetic performance of these nano-oxide specular layers spin valves is demonstrated.
Magnetic force microscope study of antiferromagnet–ferromagnet exchange coupled films
Authors: Pokhil Taras , Song Dian , Linville Eric
Published: July 2002

Magnetic microstructure in micron and submicron size elements made of bilayer antiferromagnet–ferromagnet (AFM/FM) (AFM: NiMn, PtMn, and IrMn; FM: NiFe and CoFe) exchange coupled polycrystalline films have been studied using a magnetic force microscope. AFM/FM elements with various thickness of FM layer (50–500 A) have been examined and compared with nonexchange biased FM elements of the same size, shape, and thickness. Micromagnetic structures observed in AFM/FM elements with thick (>200 A) FM layer indicated that, in addition to unidirectional anisotropy, the AFM layer induces uniaxial anisotropy in a FM layer. Bilayers with a NiMn or PtMn AFM layer exhibited higher induced uniaxial anisotropy than ones with IrMn. In the elements with a thin (<100 A) FM layer and NiMn or PtMn as an AFM layer, a local switching of the magnetization direction under an external applied field has been observed. The size of the “switched” areas depends on the material and thickness of the FM and AFM layers. No local switching, just slight rippling of magnetization in the FM, was observed in the samples with an IrMn AFM layer. The results can be explained using either the model of thermally activated switching of AFM grains or the model of induced uniaxial anisotropy at the AFM/FM interface suggesting local variations of induced uniaxial anisotropy. In both models, the in-plane exchange in the FM layer has to be taken into account.
Transient dynamic analysis of brushless direct current motors by coupling electromagnetic and hydrodynamic forces
Authors: Kim Tae-Jong , Hwang Sang-Moon , Hwang Geun-Bae , Cho Jin-Rae , Kang Sung-Su , Moon Young-Hoon
Published: July 2002

Vibration of a crankshaft-journal bearing system driven by an electric motor is a coupled phenomenon between hydrodynamic characteristics and magnetic origins through the motor air gap. With the advent of new high-energy magnets together with variable speed motor applications, magnetic sources of vibration are becoming more serious. This article investigates the transient whirl responses of a rotor system with purely hydrodynamic origins and compares it with that of magnetically coupled origins. Results show that magnetically coupled sources significantly affect the vibration of the rotor-motor system.
Magnetic domains and magnetostatic interactions of self-assembled Co dots
Authors: Yu Chengtao , Pearson J. , Li Dongqi
Published: July 2002

Submicron quasihexagonal Co dots and dot chains have been grown epitaxially on Ru(0001) substrate with molecular beam epitaxy and investigated with magnetic force microscopy. All the dots exhibit in-plane single domains. The magnetization of the dots in a chain prefers to align along the chain direction due to inter-dot magnetostatic interactions. Micromagnetic calculations suggest that the observed single domains are metastable energetically and both vortex and single domain can exist in zero field. The inter-dot magnetostatic coupling, modeled with a dot pair, introduces a uniaxial anisotropy with easy axis along the pair/chain direction. The induced anisotropy field decreases with increasing inter-dot distance and approaches the dipolar limit.
Structure and magnetic properties of nanostructured PrCo7−xTix(x=0–0.4) prepared by mechanical milling and subsequent annealing
Authors: Zhang Jian , Shen Bao-gen , Zhang Shao-ying , Wang Y. Q. , Duan X. F.
Published: July 2002

The Pr(Co,Ti)7 phase alloy, which can not be prepared directly by arc melting, has been synthesized by mechanical milling and subsequent annealing. A coercivity of 7.3 kOe, remanence ratio of 0.69 and maximum energy product of 10.6 MGOe have been achieved for the PrCo6.7Ti0.3 powder milled for 3 h and annealed at 800 °C for 3 min. The high coercivity obtained indicates that the addition of Ti causes the anisotropy of PrCo7 phase to change from planar to uniaxial. Transmission electron microscopy reveals that a uniform Pr(Co,Ti)7 microstructure with an average grain size of about 20 nm is developed in the powders. The electron energy-loss spectrum acquired from one grain proves that the Ti enters the Pr(Co,Ti)7 grain. The high coercivity obtained apparently arises from the high uniaxial anisotropy of Pr(Co,Ti)7 phase and the uniform nanoscale microstructure developed by mechanical milling and subsequent annealing.
Low temperature magnetoresistance in ferromagnetic nanoring system
Authors: Kasai S. , Niiyama T. , Saitoh E. , Miyajima H.
Published: July 2002

Quantum interference effect of conduction electrons in a ferromagnetic mesoscopic sized ring has been investigated. With decreasing temperature below 4.2 K, fine structures appear in the magnetoresistance. The observed aperiodic fluctuation of resistance with changing magnetic field can be attributed to the interference effect in the diffusive metals.
Domain control in magnetic shields using patterned permanent magnet underlayer
Authors: Pust Ladislav , Mack Anthony M. , Rea Christopher J. T. , Gangopadhyay Sunita , Ryan Patrick J. , Stankiewicz Andrzej
Published: July 2002

The domain state of a magnetic shield in a recording head can be controlled by an adjacent patterned permanent magnet layer. A 1.1-μm-thick electroplated Ni80Fe20 (NF) film with slight uniaxial magnetic anisotropy was patterned into rectangular magnetic shields with various dimensions over patterned thin film made from a 0.1-μm-thick CoCrPt permanent magnet (PM). The shape of the adjacent biasing PM layer should be the shape of a desired final domain in NF. Domain structure in the NF layer and the process of magnetic saturation were imaged using wide-field Kerr microscopy. The NF and PM layers are magnetically coupled and, therefore, a magnetic state with parallel magnetization is preferred. The PM direction of magnetization is set in high magnetic field and the final NF domain state is controlled by the shape of PM features. The simplest stable domain structure in a rectangular thin shield is of an “envelope” type. Using a PM underlayer, either clockwise or counterclockwise domain structure is preferred. Various resulting domain structures were studied after saturation in various magnetic fields and a range of directions. While the resulting domain structures in NF are regular and well defined after saturation in a large magnetic field, more complex and irregular structures are generated in NF after magnetization in a smaller field. Domain structures for various field treatments are discussed.
Ground state tunneling due to a distribution of tunnel splittings in Mn12-acetate
Authors: Mertes K. M. , Suzuki Yoko , Sarachik M. P. , Paltiel Y. , Shtrikman H. , Zeldov E. , Rumberger E. M. , Hendrickson D. N. , Christou G.
Published: July 2002

We report on detailed measurements of the magnetization of a single crystal of Mn12-acetate in a swept magnetic field for a set of closely spaced temperatures. We show that under some circumstances relaxation that should proceed from the ground state appears to be missing under conditions where one would expect it to be present. We argue that this enigma implies there is a distribution of tunnel splittings, so that the (normalized) magnetization determines the fraction of distributed molecules that tunnel instead of the tunneling probability of an identical set of molecules.
Magnetic anisotropy of iron chains embedded in copper
Authors: Eisenbach Markus , Stocks G. Malcolm , Újfalussy Balazs , Györffy Balazs L.
Published: July 2002

We investigate the magnetic ground states of iron inclusions embedded in fcc copper using first principles calculations. Of main interest are linear monatomic chains of atoms aligned along different directions in a Cu matrix. The method of calculation we employ is the locally selfconsistent multiple scattering (LSMS) real space method for solving the LDA Kohn–Sham–Dirac equation. We have extended our code to perform fully relativistic calculations to enable us to investigate the spin–orbit coupling effects leading to anisotropic and potentially noncollinear ordering of magnetic moments in these systems of iron inclusions in copper. With this approach we find that the magnetocrystalline anisotropy in systems of magnetic iron chains embedded in copper strongly prefers an orientation of the magnetic iron moments perpendicular to the direction of the chain. This competes with the magnetostatic energy of the chains, which is lowest when the moments are aligned parallel to the chain. Our parameter free calculations suggest that, depending on whether the chains of magnetic atoms are embedded in copper along the 100 or 110 directions, the ground state orientation of the magnetic moments on the iron sites can be either parallel or perpendicular to the chain, due to the balance of these competing effects.
A high sensitivity refractometer based upon a long period grating Mach–Zehnder interferometer
Authors: Allsop T. , Reeves R. , Webb D. J. , Bennion I. , Neal R.
Published: July 2002

A low cost interrogation scheme is demonstrated for a refractometer based on an in-line fiber long period grating (LPG) Mach–Zehnder interferometer. Using this interrogation scheme the minimum detectable change in refractive index of Δn∼1.8×10−6 is obtained, which is the highest resolution achieved using a fiber LPG device, and is comparable to precision techniques used in the industry including high performance liquid chromatography and ultraviolet spectroscopy.
Dielectric breakdown in magnetic tunnel junctions having an ultrathin barrier
Authors: Oliver Bryan , He Qing , Tang Xuefei , Nowak J.
Published: July 2002

Magnetic tunnel junctions have been fabricated by magnetron sputtering and patterned by deep ultraviolet photolithography. The tunnel magnetoresistance was 15%–22% and resistance times area product (R×A) 7–22 Ω μm2 for junctions having 4.75–5.5-Å-thick Al layer oxidized naturally. Two types of breakdown were observed: abrupt dielectric breakdown at an effective field of 10 MV/cm determined by the thickness of the tunnel barrier, and a gradual breakdown related to defects in the tunnel barrier. After the breakdown a metallic pinhole is created, the size of which depends on the maximum current applied to the junction. The current flowing through the pinhole creates a strong circular magnetic field that curls the local magnetization in the free layer around the pinhole. The subsequent free-layer reversal is very sensitive to the pinhole location. The electric properties after breakdown can be well described by an Ohmic resistor and a tunnel magnetoresistor connected in parallel.
On the resolution of the optical rotatory power of chiral molecules into atomic terms. A study of hydrogen peroxide
Authors: Ligabue A. , Lazzeretti P. , Varela M. P. Béccar , Ferraro M. B.
Published: July 2002

An additive scheme for resolving average optical rotatory power of a molecule into atomic contributions, based on the acceleration gauge for the electric dipole, and/or the torque formalism, has been applied to hydrogen peroxide. Extended calculations have been carried out to test the reliability of the partition method. Gross atomic isotropic contributions to the average molecular property from oxygen and hydrogen atoms have been evaluated. The force and torque gauges provide different numerical values for atomic contributions.
Elastic moduli of polycrystalline LaAlxNi5−x
Authors: Bereznitsky M. , Ode A. , Hightower J. E. , Yeheskel O. , Jacob I. , Leisure R. G.
Published: July 2002

A hot isostatic pressing technique was used to prepare bulk polycrystalline samples of the hydrogen-absorbing materials LaAlxNi5−x. Ultrasonic methods were used to measure the elastic moduli of these alloys. Small corrections were made for porosity so that values expected for the theoretical density were found. The various moduli decrease somewhat with increasing x; the porosity-corrected bulk and shear moduli are 137.4 GPa and 59.2 GPa, respectively, at x=0 and decrease to 126.6 GPa and 54.1 GPa, respectively, at x=1. Poisson’s ratio is independent of x within experimental error at 0.31, a value typical for metals. Debye temperatures calculated from the porosity-corrected elastic constants ranged from 360 to 365 K, and were in good agreement with those reported from heat capacity measurements. The present measurements on LaAlxNi5−x were combined with earlier heat capacity measurements of the hydrides, LaAlxNi5−xHy, to estimate the elastic moduli of the latter materials. The estimated hydride moduli were substantially lower than those of the hydrogen-free metal. These estimated moduli were used to calculate the elastic energy associated with coherent, ellipsoidal hydride precipitates. The minimum elastic energy was found for low aspect ratio, i.e., a plate as opposed to a sphere or needle.
Interaction of electron and hole plasma with coherent longitudinal optical phonons in GaAs
Authors: Chang Y.-M.
Published: July 2002

The interaction of photoexcited electron–hole plasma with impulsively excited coherent longitudinal optical phonons in GaAs was investigated via time-resolved second-harmonic generation. The dephasing time of coherent LO phonons reduces significantly as photoexcited electron–hole plasma is injected into the near-surface depletion region. The coherent LO-electron and LO-hole coupling modes can both be clearly observed and investigated in real time. The rapid dephasing of the coherent LO-hole coupling mode is attributed to the strong polar phonon-carrier interaction and the overdamped nature of hole plasma.
Quasiregular quantum-dot-like structure formation with postgrowth thermal annealing of InGaN/GaN quantum wells
Authors: Lin Yen-Sheng , Ma Kung-Jen , Hsu Cheng , Chung Yi-Yin , Liu Chih-Wen , Feng Shih-Wei , Cheng Yung-Chen , Yang C. C. , Mao Ming-Hua , Chuang Hui-Wen , Kuo Cheng-Ta , Tsang Jian-Shihn , Weirich Thomas E.
Published: July 2002

Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content (19%) was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence (PL) measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.
Resonant increase of x-ray emission in a microwave discharge at half-gyrofrequency
Authors: Vodopyanov A. V. , Golubev S. V. , Razin S. V. , Zorin V. G. , Ivanov A. A. , Rouillé C. , Bacal M.
Published: July 2002

Described in the present paper is the observation of a resonant increase of x-ray emission in a microwave discharge at half-gyrofrequency, where the equality ω=12ωBe is satisfied (here, ω is the microwave frequency and ωBe is the gyrofrequency of electrons). These experiments show, for the first time, that the x-ray flux produced at subharmonic resonance is five times higher than that produced at the cyclotron resonance, the photon energy being higher.
Stability analysis of a class of two-dimensional multipolar vortex equilibria
Authors: Crowdy Darren , Cloke Martin
Published: July 2002

The stability properties of a class of explicit multipolar vortex solutions of the two-dimensional Euler equations found in Crowdy [Phys. Fluids. 11, 2556 (1999)] are studied. While the tripole solutions are linearly unstable in all configurations, it is found that the exact quadrupolar vortices have distinguished linear stability properties revealing them to be neutrally stable in all configurations. This result is consistent with observations by previous investigators on the general robustness of quadrupolar vortex structures. Higher-order multipolar structures are linearly unstable when the satellites are too close together, but become neutrally stable when the satellites are far enough apart and the ambient vortex patch sufficiently distorted. The nonlinear evolution of perturbed solutions is investigated numerically using contour dynamics methods. Some new results concerning limiting states involving cusp singularities in the vortex patch boundaries are also presented.
Electron transport in the barriers of AlGaAs/GaAs quantum well structures observed by scanning-tunneling-microscope light-emission spectroscopy
Authors: Tsuruoka T. , Tanimoto R. , Ohizumi Y. , Arafune R. , Ushioda S.
Published: July 2002

Using scanning-tunneling-microscope light-emission (STM-LE) spectroscopy, we have investigated the transport properties of minority carriers in p-Al0.3Ga0.7As/p-GaAs quantum well (QW) structures. The optical measurements were performed on a cleaved (110) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the STM tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants.
Transfer ratio of the spin-valve transistor
Authors: van ’t Erve O. M. J. , Vlutters R. , Kumar P. S. Anil , Kim S. D. , Postma F. M. , Jansen R. , Lodder J. C.
Published: July 2002

We describe the factors that control the transfer ratio of the spin-valve transistor. An increase in transfer ratio is obtained by a systematic variation of the height of emitter and collector Schottky barrier, and of the nonmagnetic metals. Next, we found that in some cases, a thicker base leads to a higher transfer ratio. Finally, the thickness of the magnetic layers in the Ni80Fe20/Au/Co spin-valve base can be optimized for a maximum absolute change of collector current. An overall increase by a factor of 24 was achieved, without loss of the magnetocurrent.
Semiclassical Poincaré map for integrable systems
Authors: Lauritzen Bent
Published: July 2002

The semiclassical Poincaré map is applied to integrable systems and in particular to the rectangular billiard. The zeroes of the functional determinant are shown to give EBK quantization. The transfer operator is explicitly unitary and finite, resulting in a finite expansion of the Euler product over periodic orbits.
Nonlinear analysis of the pharmacological conversion of sustained atrial fibrillation in conscious goats by the class Ic drug cibenzoline
Authors: Hoekstra Bart P. T. , Diks Cees G. H. , Allessie Maurits A. , DeGoede Jacob
Published: July 2002

Methods from nonlinear dynamics were applied to test the hypothesis that the dynamics of sustained atrial fibrillation (AF) is modified by the class Ic drug cibenzoline during pharmacological conversion. The experiments were performed in conscious goats in which sustained AF was induced by continuous maintenance of AF via programmed electrical stimulation. Data were collected from electrophysiological experiments in five goats to terminate sustained AF by continuous infusion of cibenzoline. Sets of five unipolar epicardial electrograms of one minute duration were recorded from the left and right atrial free wall during sustained AF (control), and at three episodes during infusion of cibenzoline, when the mean AF interval had been prolonged to 25%, 50% and 85% with respect to control. Ventricular far-field potentials were removed from atrial electrograms by a coherent averaging procedure. Using the Grassberger-Procaccia method, the dynamics of the local atrial electrograms was investigated by estimating the (coarse-grained) correlation dimension and correlation entropy from the correlation integral. The results were related to a recently proposed classification (types I–III) of AF based on the degree of complexity of atrial activation patterns. The coarse-grained correlation dimension Dcg and entropy Kcg indicated that sustained AF corresponded to type II. During drug administration the coarse-grained parameters were not significantly different from control. Scaling regions in the correlation integral were observed after infusion of cibenzoline (3 out of 5 goats) suggesting that the drug introduced low-dimensional features (type I) in the dynamics of AF (correlation dimension D ranging from 2.8 to 4.4 and correlation entropy K from 1.6 to 6.2 nats/s). Sinus rhythm recorded shortly after cardioversion was very regular (D<2 and K<3 nats/s). The hypothesis that the electrograms during AF and sinus rhythm were generated by a static transformation of a linear Gaussian random process was rejected using a test for time reversibility. The nonlinear analysis revealed that cibenzoline does not significantly alter the dynamics of sustained AF during pharmacological conversion other than a slowing down of the atrial activation and a somewhat increasing global organization of the atrial activation pattern. The sudden change in the dynamical behavior at cardioversion suggests a mechanism that is reminiscent of a bifurcation.
The multifractal analysis of Gibbs measures: Motivation, mathematical foundation, and examples
Authors: Pesin Yakov , Weiss Howard
Published: July 2002

We first motivate the study of multifractals. We then present a rigorous mathematical foundation for the multifractal analysis of Gibbs measures invariant under dynamical systems. Finally we effect a complete multifractal analysis for several classes of hyperbolic dynamical systems.
Plume dynamics in quasi-2D turbulent convection
Authors: Bizon C. , Werne J. , Predtechensky A. A. , Julien K. , McCormick W. D. , Swift J. B. , Swinney Harry L.
Published: July 2002

We have studied turbulent convection in a vertical thin (Hele-Shaw) cell at very high Rayleigh numbers (up to 7×104 times the value for convective onset) through experiment, simulation, and analysis. Experimentally, convection is driven by an imposed concentration gradient in an isothermal cell. Model equations treat the fields in two dimensions, with the reduced dimension exerting its influence through a linear wall friction. Linear stability analysis of these equations demonstrates that as the thickness of the cell tends to zero, the critical Rayleigh number and wave number for convective onset do not depend on the velocity conditions at the top and bottom boundaries (i.e., no-slip or stress-free). At finite cell thickness δ, however, solutions with different boundary conditions behave differently. We simulate the model equations numerically for both types of boundary conditions. Time sequences of the full concentration fields from experiment and simulation display a large number of solutal plumes that are born in thin concentration boundary layers, merge to form vertical channels, and sometimes split at their tips via a Rayleigh-Taylor instability. Power spectra of the concentration field reveal scaling regions with slopes that depend on the Rayleigh number. We examine the scaling of nondimensional heat flux (the Nusselt number, Nu) and rms vertical velocity (the Péclet number, Pe) with the Rayleigh number (Ra*) for the simulations. Both no-slip and stress-free solutions exhibit the scaling NuRa*∼Pe2 that we develop from simple arguments involving dynamics in the interior, away from cell boundaries. In addition, for stress-free solutions a second relation, Nu∼nPe, is dictated by stagnation-point flows occurring at the horizontal boundaries; n is the number of plumes per unit length. No-slip solutions exhibit no such organization of the boundary flow and the results appear to agree with Priestley’s prediction of Nu∼Ra1/3.
Cosmic lacunarity
Authors: Provenzale A. , Spiegel E. A. , Thieberger R.
Published: July 2002

The present distribution of galaxies in space is a remnant of their formation and interaction. On a large enough scale, we may represent the galaxies as a set of points and quantify the structures in this set by its generalized dimensions [Beck and Schlögl, Thermodynamics of Chaotic Systems (Cambridge University Press, Cambridge, 1986); Paladin and Vulpiani, Phys. Rep. 156, 147 (1987)]. The results of such evaluation are often taken to be evidence of a fractal (or multifractal) distribution of galaxies. However, those results, for some scales, may also reveal the presence of singularities formed in the gravitational processes that produce structure in the galaxy distribution. To try to make some decision about this issue, we look for the more subtle galactic lacunarity. We believe that this quantity is discernible in the currently available data and that it provides important evidence on the galaxy formation process.
Influence of shank profile on laser heating of a field emitter
Authors: Hadley K. W. , Donders P. J. , Lee M. J. G.
Published: July 2002

Measurements of the temperature rise of a laser-illuminated field emitter show that a tip temperature in excess of 1000 °K can be obtained with a focused laser beam of modest power, and that the temperature rise per unit irradiance varies approximately in inverse proportion to the radius of the focal spot. By measuring the irradiance of the central fringe in the Fraunhofer diffraction pattern of a field emitter in a focused laser beam, the profile of the field emitter is mapped without removing it from the vacuum tube. A calculation based on the observed shank profile, and taking into account diffraction in a simple approximation, predicts a peak temperature rise in good agreement with experiment. These findings have important practical applications in photofield emission.
Self-consistent calculation of electronic states in AlGaAs/GaAs/AlGaAs selectively doped double-heterojunction systems under electric fields
Authors: Inoue Kaoru , Sakaki Hiroyuki , Yoshino Junji , Hotta Takashi
Published: July 2002

The electronic states in AlGaAs/GaAs/AlGaAs selectively doped double-heterojunction (SD-DH) systems or single-quantum-well systems have been calculated self-consistently for the case where an external gate voltage is applied perpendicularly to the surface. The transition from symmetrical to asymmetrical distributions of electrons by the external field is predicted. The calculated variations of electron concentration with the gate voltage are found to be in good agreement with the experiment. The shoulder structures observed in the gate voltage dependence of mobility have been successfully accounted for and ascribed to the onset of electron population in the upper subbands. This analysis is expected to provide powerful means to evaluate the performance of SD-DH field-effect transistors and to optimize their design.
Excitation of magnetostatic surface waves: Effect of finite sample width
Authors: Bajpai S. N.
Published: July 2002

This paper investigates the excitation of magnetostatic surface waves in the multilayered structure consisting of a ferrimagnetic film sandwiched between two metal-backed dielectrics. The analysis takes into account the effect of finite width of the ferrimagnetic film. Numerical results have been presented for a wide range of parameters. The inclusion of the width explains the finite radiation resistance observed at lower cutoff frequency of the surface wave.
Characterization of implantation and annealing of Zn-implanted InP by Raman spectrometry
Authors: Bedel E. , Landa G. , Carles R. , Renucci J. B. , Roquais J. M. , Favennec P. N.
Published: July 2002

First- and second-order Raman scattering by Zn-implanted InP is investigated in order to determine critical fluences needed for a complete disturbance of the lattice and recovery temperature during thermal annealing. Disorder-activated first-order acoustical scattering and second-order optical scattering are shown to be highly sensitive probes.
Adhesion of evaporated titanium films to ion-bombarded polyethylene
Authors: Bodö P. , Sundgren J.-E.
Published: July 2002

Ti films were deposited onto high-density polyethylene (HDPE) samples by electron-beam evaporation. Prior to film deposition the samples were in situ pretreated by Ar ion bombardment using a sputter ion gun. The adhesion of the films, determined as the pull strength required for film failure, was measured as a function of ion dose. HDPE substrates processed at two different temperatures were examined. The adhesion of the Ti films to HDPE samples processed at ≊150 °C increased with the ion dose to a steady-state value corresponding to the cohesive strength of the HDPE substrate. The adhesion to the samples processed at ≊200 °C increased to a maximum and then decreased for further ion bombardment to a level of the same order as that for films deposited onto as-prepared samples. The effects of the ion bombardment upon the HDPE surface chemistry were examined by means of x-ray photoelectron spectroscopy (XPS). The ion bombardment resulted in dehydrogenation and cross linking of the surface region and for prolonged ion bombardment, a graphitelike surface was obtained. The film/substrate interface as well as the initial Ti film growth were examined by XPS analysis. A chemical interaction which resulted in Ti–C bonds was observed at the interface. The Ti film growth followed a pronounced three-dimensional growth mode on as-prepared surfaces whereas the ion bombardment resulted in a change toward a more two-dimensional growth mode. The difference in adhesion behavior for the two types of HDPE substrates was found to be due to a difference in the amounts of low molecular weight products present within the substrates. The HDPE substrates processed at ≊200 °C contained larger amounts of low molecular weight products and also had a lower degree of crystallinity and a less closely packed structure compared to those substrates processed at ≊150 °C. This resulted in a segregation of low molecular weight products towards the surface of substrates processed at ∼200 °C. This segregation in turn is suggested to lead to a weak boundary layer, reducing the adhesion to as-prepared samples and to substrates exposed to a high ion dose.
Influence of SiO2 at the Ta2O5/Si interface on dielectric characteristics of Ta2O5 capacitors
Authors: Nishioka Yasushiro , Shinriki Hiroshi , Mukai Kiichiro
Published: July 2002

The influence of SiO2 formed on a Si wafer during Ta2O5 deposition on the dielectric characteristics of Ta2O5 capacitors is investigated. Two types of capacitor were investigated and compared: W/Ta2O5 (27 nm)/W and Al/Ta2O5 (27 nm)/Si capacitors. The thickness of the SiO2 at the interface of Ta2O5/Si is estimated to be 2.4 nm. The W/Ta2O5 (27 nm)/W capacitor is shown to have a dielectric constant of 22 if we assume it is an ‘‘ideal’’ single layer dielectric. On the other hand, for the Al/Ta2O5/Si capacitor if we assume that an SiO2 layer is formed during Ta2O5 deposition we can account for the variation in apparent dielectric constant with thickness and the leakage current as a function of voltage with a model which assumes that when electrons are injected from the SiO2/Si interface, electron charge accumulates at the Ta2O5/SiO2 interface; this reduces the electric field in SiO2 before catastrophic breakdown of the SiO2 film occurs. This results in a relatively small voltage drop (less than 1 V) across the SiO2 film at the interface. Taking into account the voltage drop across SiO2 layer, the current-voltage characteristics of Al/Ta2O5 (27 nm)/SiO2 (2.4 nm)/Si and W/Ta2O5 (27 nm)/W show good agreement by assuming steady-state conduction and the existence of interface charge between Ta2O5 and SiO2.
Simultaneous observation of viscoelastic deformation and induced magnetic anisotropy in [Co1−x(FeNi)x]75Si15B10 metallic glasses
Authors: Fernández-Gubieda M. L. , Barandiarán J. M. , Nielsen O. V.
Published: July 2002

Amorphous ribbons of composition [Co1−x(FeNi)x]75Si15B10 (0.1<x<0.7) have been annealed by means of short current pulses while applying a tensile stress. Current densities in the range 3–5 107 A m−2 have been used. Viscoelastic deformation and induced magnetic anisotropy were determined simultaneously after the treatments at room temperature. The induced anisotropy consists of two contributions, one irreversible and the other recoverable. The strength of the anisotropy and the deformation depends on the anneal temperature, the thermal history of the sample and the composition. The composition dependence of the plastic deformation and the induced anisotropy is similar and the influence of preannealing on magnetic anisotropy and linear deformation is considerable.
Pyroelectric linear array infrared sensors made of c-axis-oriented La-modified PbTiO3 thin films
Authors: Takayama Ryoichi , Tomita Yoshihiro , Iijima Kenji , Ueda Ichiro
Published: July 2002

High sensitive pyroelectric linear array infrared sensors have been fabricated by using highly c-axis-oriented and spontaneous polarized La-modified PbTiO3 (PLT) thin films. The PLT thin films were deposited on (100)-oriented Pt thin film on (100)-cleaved MgO single crystals by rf magnetron sputtering. The PLT thin films have high figures of merit for voltage responsivity (Rv) and specific detectivity (D*) without a poling treatment. Two types of linear array sensors, reticulated and nonreticulated ones, were fabricated. The reticulated and nonreticulated sensors have 64 elements with 200-μm pitch and 128 elements with 100-μm pitch, respectively. Ni-Cr electrodes of approximate area 1.9×10−4 or 3.0×10−4 cm2 were deposited by vacuum evaporation on the PLT thin films with a thickness of 3–4 μm. Pyroelectric coefficients and their uniformity increased upon decreasing the deposition area of an element. Therefore, the elements of reticulated sensors have considerably larger Rv and D* compared with the nonreticulated ones. The nonreticulated sensors have a very high resolution of 10 elements/mm, a large Rv of 900 V/W at 100 Hz, and a high D* (500 K, 100, 1) of 1.5×108 cm Hz1/2/W. Furthermore, the reticulated ones have a very large Rv of 2200 V/W, a remarkably high D* of 6×108 cm Hz1/2/W, and a very low thermal cross talk of <3% at 100 Hz.
Photomagnetism in Ca2+Fe4+-doped yttrium iron garnet
Authors: Pardavi-Horváth M. , Wigen P. E. , Vértesy G.
Published: July 2002

The temperature and time dependence of the initial susceptibility of the optical magnetization curve has been measured on epitaxial films of nominally pure yttrium iron garnet (YIG) and Ca: YIG, during illumination with white light after cooling in the dark. Depending on sample composition, the maximum temperature for the occurrence of the light-induced effects on the susceptibility is 100 K<T<150 K. The change to the low-susceptibility state by light excitation is a three-step process. The first process is characterized by time constants of the order of minutes, during the second process the susceptibility does not change, and finally, at 93 K after 4 h of illumination by 15 mW/cm2 light intensity the susceptibility of Ca0.11:YIG is observed to decrease to a saturation value in less than an hour. After turning off the light the susceptibility remains constant for about an hour, then increases with a similar time dependence. No light-induced effects were detected in YIG over the same range of temperature, time, or light intensity. The results are interpreted in terms of light excitation of holes from the tetrahedral Fe4+ sites nearest to the Ca2+ impurity first to orientationally inequivalent sites on the same coordination sphere, then to nonmagnetic centers with intermediate energies, and finally to the next-nearest tetrahedral Fe3+ sites of higher anisotropy energy. The energy difference between the two states is measured to be in the 3–10-meV range.
An exactly solvable model for calculating critical misfit and thickness in epitaxial superlattices. II. Layers of unequal elastic constants and thicknesses
Authors: Jesser W. A. , van der Merwe Jan H.
Published: July 2002

In this paper we have extended the exact predictions of critical misfit fc and critical thickness hc —based on equilibrium suppositions and a parabolic interfacial potential of shear strength μ—in superlattices of layer pairs A and B introducing the thicknesses ha=ηac and hb=ηbc, and moduli μa, μb, and μ as variables. The extension involved an explicit relation between the homogeneous misfit strains ēa and ēb on the one hand and the thicknesses and moduli on the other in order to ensure lateral force balance. Analytical expressions for the energies ED and Eē per unit area per interface for misfit dislocations and mistfit strains, respectively, have been derived and minimized to obtain the equilibrium value ēm needed to predict fc and hc. The expression for ED is rather complicated. However, a variety of simplifying approximations were feasible, which not only facilitated numerical evaluation but also made it possible to draw various conclusions. The case of equal elastic constants but variable layer thicknesses shows that a maximum value of fc occurred when the superlattice layers have equal thickness and that the layers of equal thickness produced the minimum value of ED. Scaling relations were found that could be used to calculate ED values simply by selecting the appropriate combinations of thicknesses and elastic constants. The effect of changing elastic constants or the values of critical thickness or critical misft could also be represented through scaling relations.