bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma modeling for a nonsymmetric capacitive discharge driven by a nonsinusoidal radio frequency current
Authors: Dewan M. N. A. , McNally P. J. , Herbert P. A. F.
Published: July 2002

An analytical solution for the sheath dynamics of an asymmetrically driven capacitively coupled plasma is obtained under the assumptions of time-independent, collisionless ion motion, inertialess electrons, and uniform current density. Modeling is performed considering that the plasma is driven by a nonsinusoidal radio frequency (rf) current which can be resolved into a finite number of harmonic components. Together with different sheath parameters the equation for the bulk plasma impedance is also obtained to calculate the overall plasma impedance and the overall rf voltage. Assuming equal plate areas the solution for a symmetric discharge is also obtainable from this model. We have found that the even harmonic components of rf voltage and impedance are always present, even in a symmetric discharge. Experimental results are shown to be in qualitative agreement with the theoretical model. The values of normalized rf voltage and impedance harmonics assume lower values as the asymmetry of the plasma chamber decreases.
General parametric analysis of the linear two-stream instability
Authors: Lapuerta V. , Ahedo E.
Published: July 2002

A complete and systematic picture of the different forms of the linear two-stream instability is presented. It is based in an asymptotic study of the general solution of the dispersion relation, in terms of the three dimensionless parameters characterizing the steady state of a two-species plasma. In a zero temperature-ratio limit, the parametric regions of dominance of different acoustic, Langmuir, and reactive types of the instability are determined, and analytical expressions of the maximum growth rate for all types are derived. The continuous transition between different types and the main topological changes of the unstable branch, as the parametric space is covered, are discussed. For the small temperature-ratio case, new analytical expressions for the instability threshold are derived.
Maximum entropy approximation for Lyapunov exponents of chaotic maps
Authors: Ding Jiu , Mead Lawrence R.
Published: July 2002

Let S:[0,1]→[0,1] be a nonsingular chaotic map that preserves an integrable density f* that describes the statistics of the orbits. In this article we use the maximum entropy approach to approximate the density f* and the corresponding Lyapunov exponent.
Resonant electron beam interaction with several lower hybrid waves
Authors: Krafft C. , Volokitin A. S.
Published: July 2002

The processes governing the resonant interaction of lower hybrid waves with a radially bounded electron beam spiraling in a magnetized plasma are investigated for various physical parameters and beam-waves resonance conditions. It is shown that the nonlinear behavior of the beam-waves system strongly depends on three main parameters, that is, the growth rates of the waves’ instabilities in the presence of the spiraling beam, the waves’ amplitude maxima achieved at the stage of the first trapping process as well as the frequency and the intensity of the initial beam current modulation. The dependence of these factors on the system’s nonlinear evolution is examined in detail with the help of numerical simulations. Results obtained when studying the interaction of the spiral beam with two waves indicate that the control of the initial stage of the system’s evolution should be of great importance when one considers the interaction of the beam with several waves. Moreover, it is found that the waves’ instability growth rates as well as the radial flux of beam energy nonmonotonically depend on the beam injection pitch angle. In actual space experiments involving electron beam injection by a gun located on a rotating satellite, one should expect that the beam emission, even for a fixed pitch angle, will reveal a nonmonotonic behavior with fields’ amplitude maxima and minima depending on the time of registration.
Effects of CrAl underlayer and intermediate layer on the properties of CoCrPtTa thin film media
Authors: Pang S. I. , Wang J. P. , Chong T. C.
Published: July 2002

This study investigates the effects of CrAl underlayer and intermediate layer on the magnetic properties, crystallographic texture, and thermal stability of CoCrPtTa thin film media. Enhancement of coercivity and thermal stability factor of CoCrPtTa/CrAlx thin film media were observed when the Al concentration in the CrAl underlayer was varied from 0 to 14 at. %. We also observed an improvement in the in-plane crystallographic texture and a reduction in the magnetic switching volume and intergrain interaction. Results from the interdiffusion study using Auger depth profiling analysis, and ΔM measurement showed that the decrease of switching volume and intergrain interaction may be attributed to Al diffusion and/or Al-enhanced Cr diffusion into the grain boundary of the magnetic layer. Compared to CoCrPtTa/Cr media, we found that high coercivity, good in-plane crystallographic texture, and magnetic isolation for CoCrPtTa/CrAl media could be achieved at lower substrate temperature. This study has also shown that a CrAl intermediate layer could be used to further enhance the magnetic, crystallographic texture, and thermal stability properties of the CoCrPtTa thin film media.
Biased switching in an interacting pair of magnetic particles
Authors: Xu C. , Hui P. M. , Zhou J. H. , Li Z. Y.
Published: July 2002

The switching dynamics in a pair of coupled magnetic particles is studied numerically via the Landau-Liftshitz-Gilbert form of a set of damped gyromagnetic equations. The effects of dipolar interaction between the particles, anisotropy energy, switching and a small bias field are included. Initial conditions in which the moments are parallel and antiparallel to each other are considered. The presence of a small transverse bias field and inter-particle coupling assists in the switching of a moment. Due to the complicated interplay among different factors, the switching in one particle may lead to a corresponding precession of moment in a neighboring particle. We show that different final configurations may be attained depending on the inter-particle separation and the initial configuration.
Direct nano-wiring carbon nanotube using growth barrier: A possible mechanism of selective lateral growth
Authors: Lee Yun-Hi , Jang Yoon-Taek , Choi Chang-Hoon , Kim Eun-Kyu , Ju Byeong-Kwon , Kim Dong-Ho , Lee Chang-Woo , Yoon Sang-Soo
Published: July 2002

In this work we report the direct nano-bridging of carbon nanotubes (CNT) between micro-sized islands using conventional photolithography technique necessary for the nanomachining and the molecular device applications compatible with the Si-based process. The most distinct feature in this work is to use a growth barrier of Nb metal or insulating layer on the top of the catalytic metal to prevent the growth of CNT from the vertical direction to the substrate. As a result, CNTs of either “straight line” or a perfect “Y shape” were selectively grown between lateral sides of the catalytic metals or pre-defined electrodes without any trace of vertical growth. The length of the CNTs was 500–1000 nm and the diameter thinner than about 20 nm. We suggest that magnetic and crystallographic characteristics due to the unique interaction between the Nb overlayer and ferromagnetic Ni catalysts and nano-granulation of Ni layer during the growth process are important for the lateral (i.e., parallel to the substrate) CNTs growth. These results clearly indicate that this method would be one of the most feasible fabrication techniques for the nanomachines or the electronic applications with a high integration level.
Interaction of As impurities with 30° partial dislocations in Si: An ab initio investigation
Authors: Antonelli A. , Justo J. F. , Fazzio A.
Published: July 2002

We investigated through ab initio total energy calculations the interaction of arsenic impurities with the core of a 30° partial dislocation in silicon. It was found that when an arsenic atom sits in a crystalline position near the dislocation core, there is charge transfer from the arsenic towards the dislocation core. As a result, the arsenic becomes positively charged and the core negatively charged. The results indicate that the structural changes around the impurity are very small in both environments, namely, the crystal and the dislocation core. In this scenario, the interaction between arsenic and the core is essentially electrostatic, which eventually leads to arsenic segregation. The segregation energy was found to be as large as 0.5 eV/atom. Additionally, it was found that arsenic pairing inside the core is not energetically favorable.
Piezoelectricity and Piezomagnetism: Duality in two-dimensional checkerboards
Authors: Fel Leonid G.
Published: July 2002

The duality approach in two-dimensional two-component regular checkerboards is extended to piezoelectricity and piezomagnetism. The relation between the effective piezoelectric and piezomagnetic moduli is found for a checkerboard with the p6′mm′-plane symmetry group (dichromatic triangle).
Global prescribed mean curvature foliations in cosmological space–times. I
Authors: Henkel Oliver
Published: July 2002

This work investigates some global questions about cosmological space–times with two-dimensional spherical, plane, and hyperbolic symmetry containing “well-behaved” matter. The result is that these space–times admit a global foliation by prescribed mean curvature surfaces, which extends at least toward a crushing singularity. The time function of the foliation is geometrically defined and unique up to the choice of an initial Cauchy surface. This work generalizes a similar analysis on constant mean curvature foliations and avoids the topological obstructions arising from the existence problem.
Elementary reaction schemes for physical and chemical vapor deposition of transition metal oxides on silicon for high-k gate dielectric applications
Authors: Niu D. , Ashcraft R. W. , Kelly M. J. , Chambers J. J. , Klein T. M. , Parsons G. N.
Published: July 2002

This article describes the kinetics of reactions that result in substrate consumption during formation of ultrathin transition metal oxides on silicon. Yttrium silicate films (∼40 Å) with an equivalent silicon dioxide thickness of ∼11 Å are demonstrated by physical vapor deposition (PVD) routes. Interface reactions that occur during deposition and during postdeposition treatment are observed and compared for PVD and chemical vapor deposition (CVD) yttrium oxides and CVD aluminum-oxide systems. Silicon diffusion, metal-silicon bond formation, and reactions involving hydroxides are proposed as critical processes in interface layer formation. For PVD of yttrium silicate, oxidation is thermally activated with an effective barrier of 0.3 eV, consistent with the oxidation of silicide being the rate-limited step. For CVD aluminum oxide, interface oxidation is consistent with a process limited by silicon diffusion into the deposited oxide layer.
A simple, high power, compact, intracavity frequency-doubled, Q-switched Nd:Y3Al5O12 laser
Authors: Christensen Scott , Kapteyn Henry C. , Murnane Margaret M. , Backus Sterling
Published: July 2002

We have developed a simple, compact, high power, diode-pumped, intracavity frequency-doubled, Nd:Y3Al5O12 laser capable of generating output powers of up to 70 W at 10 kHz, and 16.5 W at 1 kHz. The output beam quality is highly multimode, with an M2∼30. This, combined with the short output pulse duration of 36–60 ns, and the high average power, makes this laser ideal for pumping ultrafast Ti:sapphire laser amplifier systems.
Tandem time-of-flight mass spectrometer (TOF-TOF) with a quadratic-field ion mirror
Authors: Giannakopulos Anastassios E. , Thomas Benjamin , Colburn Alex W. , Reynolds David J. , Raptakis Emmanuel N. , Makarov Alexander A. , Derrick Peter J.
Published: July 2002

A tandem time-of-flight (TOF-TOF) mass spectrometer comprised of two ion mirrors is described. The first ion mirror, which is a linear-field, single-stage mirror (MS1) with an intermediate collision cell, has been designed to provide the temporal focus necessary for the second, quadratic-field ion mirror (MS2) to function effectively. Due to the wide energy-range focusing capabilities of the quadratic field employed in the second ion mirror all the fragment ions can be collected in one spectrum without the need to step the reflecting working voltage of the MS2. The size of the active area of the microchannel plate detector used in the preliminary experiments was the limiting factor governing the collection efficiently of fragment ions. The use of the first ion mirror to provide temporal focusing of the precursor ion packet at the first focal point of the quadratic mirror used as the MS2 requires no alteration of the focusing conditions for different masses, in contrast to delayed extraction or postsource pulsed focusing. Precursor ions formed by matrix-assisted laser desorption/ionization were mass-selected with an ion gate located before the collision cell and the fragment ions were mass analyzed using the quadratic-field ion mirror. Experimental results demonstrating effective high-energy collision-induced dissociation of polymer and fullerene molecule-ions are presented.
Direct measurement of laser power through a high numerical aperture oil immersion objective lens using a solid immersion lens
Authors: Matsuo Shigeki , Misawa Hiroaki
Published: July 2002

For many laser applications, information on irradiated laser power is important. However, direct measurement of laser power through a high numerical aperture objective lens is difficult in a laser microscope. In this article, we propose a method which use a solid immersion lens (SIL) for such measurements. A laser beam focused by an objective lens is introduced to the flat surface of a SIL, emitted through the spherical surface, and then detected. In this way the divergence of the laser beam is reduced, and as a result the detection efficiency of the laser power increases. From theoretical analysis, a Weierstrass-sphere type SIL was found to be an appropriate thickness for this propose. Transmittance of the SIL is evaluated for several refractive indeces. The validity of this method is confirmed experimentally.
Adhesives for highly polished surfaces and low temperature: A simple test and results
Authors: Silvera Isaac F. , Abate A. , Chijioke A. , Pashin A.
Published: July 2002

A simple procedure for testing the bonding strength of adhesives to surfaces is presented. Tests are carried out on highly polished bonded glass slides for a number of adhesives so the adhesion strength can be characterized. Samples are also cycled to liquid nitrogen temperature to test the effect of thermal cycling on the adhesion strength. A few tests were carried out at liquid helium temperatures. Results are presented for a number of commercially available adhesives.
Self-dual Chern–Simons vortices on Riemann surfaces
Authors: Kim Seongtag , Kim Yoonbai
Published: July 2002

We study self-dual multivortex solutions of Chern–Simons Higgs theory in a background curved space–time. The existence and decaying property of a solution are demonstrated.
Quantum-phase dynamics of dimer systems interacting with a two-mode squeezed coherent field
Authors: Nakano Masayoshi , Yamaguchi Kizashi
Published: July 2002

It is well-known that the interaction among atoms/molecules and quantized electromagnetic fields with a small number of photons provides a peculiar quantum nature, i.e., collapses and revivals in the Rabi oscillations. In this study, we investigate the intermolecular interaction (dipole–dipole interaction) effect on the collapse-revival behavior using several dimer models (composed of two kinds of two-state monomers with slightly different excitation energies) with different intermolecular distances in the presence of a two-mode squeezed coherent field, in which each mode is initially correlated. It is found that although the collapse-revival behavior is fairly overlapped and indistinct in the case of a noninteracting dimer under the present two-mode squeezed coherent field, the decrease in the intermolecular distance (the increase in the intermolecular interaction) resurrects relatively distinct collapse-revival behavior with longer collapse and revival times. By analyzing the quantum behavior from the viewpoint of the dynamics of two-mode Pegg–Barnett photon-phase distributions and off-diagonal dimer density matrices, this feature is found to closely relate to a significant change in the degree of contribution between one- and two-photon processes caused by the variation in the intermolecular interaction.
Nature of the interaction of paramagnetic atoms (A=4N,4P,3O,3S) with π systems and C60: A theoretical investigation of A⋅⋅⋅C6H6 and endohedral fullerenes A@C60
Authors: Park Jung Mee , Tarakeshwar P. , Kim Kwang S. , Clark Tim
Published: July 2002

The nature of the interaction of paramagnetic atoms A (=4N,4P,3O,3S) with π systems and C60 in the A⋅⋅⋅C6H6 complexes and endohedral fullerenes (A@C60) has been investigated employing second-order Møller–Plesset perturbation level of theory and density functional theory calculations. The coupled cluster theory with single and double excitations, and with perturbed triplet excitations have also been carried out for the A⋅⋅⋅C6H6 complexes. The calculated geometries indicate that the paramagnetic N and P atoms lie on the C6 axis of benzene in the A⋅⋅⋅C6H6 complex and at the center of the C60 cage in A@C60. On the other hand, the O and S atoms are slightly shifted towards the C–C bond of benzene in the A⋅⋅⋅C6H6 complex. A comparison of the calculated binding energies (BEs) of these paramagnetic complexes and the corresponding rare gas complexes like He⋅⋅⋅C6H6 and He@C60 indicate that the BEs of the former are much larger than those of the latter. For both the rare gas and paramagnetic atom complexes dispersive interactions dominate the BEs. The energy barriers of the penetration of the O atom through a [6,6] bond are lower than the penetration of the helium atom and higher than the penetration of a N atom.
Effects of ion beams on the early stages of MgO growth
Authors: Zepeda-Ruiz Luis A. , Srolovitz David J.
Published: July 2002

Molecular dynamics simulations are performed to examine the effects of ion beams in selecting MgO island orientations during ion beam assisted deposition. Sputter yields are determined as a function of ion beam orientation for MgO islands of different sizes. While channeling does occur in small islands, the difference in sputter yields between channeling and nonchanneling orientations is much smaller in small islands than in larger islands or a continuous film. The ion beam is capable of disrupting the entire island structure at small island sizes, while channeling dependent sputtering occurs at larger island size. Since less sputtering occurs for channeling orientations, the ion beam produces an orientation-dependent island growth rate. This leads to island orientation-dependent nucleation rates. A new analytical model parameterized using the molecular dynamics sputter yield data is developed which predicts how growth conditions affect these island-orientation nucleation rates. Based upon these simulations and model results, we suggest a strategy to efficiently grow thin, highly textured MgO films.
Impediment for poloidal currents in tokamaks
Authors: Jensen Torkil H.
Published: July 2002

Many tokamak discharges have beta poloidal (βp) near unity, i.e., the poloidal currents are relatively small. It is shown that the assumptions of conservation of the total canonical angular momentum of electrons and that the plasma will attain a configuration of minimum magnetic field energy lead to βp=1 equilibria. This result is used for a possible interpretation of recently obtained tokamak discharges with current holes.
New correlation energy functionals with explicit dependence on the number of electrons
Authors: Pérez-Jiménez Ángel J. , Pérez-Jordá José M. , Pastor-Abia Luis , Sancho-Garcı́a Juan C.
Published: July 2002

Based upon the idea of effective number of electrons, we develop simple but accurate correlation energy functionals to be used for density functional theory calculations. We derive both a spin-independent and a spin-dependent functional. The spin-dependent one, used in conjunction with Becke’s exchange functional [A. D. Becke, Phys. Rev. A 38, 3098 (1988)], yields excellent results for thermochemistry calculations, giving an average absolute error of 2.9 kcal/mol for a test set comprised of the enthalpies of formation of the 148 molecules in the extended G2 set [L. A. Curtiss, K. Raghavachari, P. C. Redfern, and J. A. Pople, J. Chem. Phys. 106, 1063 (1997); L. A. Curtiss, P. C. Redfern, K. Raghavachari, and J. A. Pople, ibid. 109, 42 (1998)] plus the total energies of the atoms H through Ar. We also discuss the problem of fractional occupation number, and we show that the corresponding principle of integer preference can be fulfilled by the procedure that we propose to build correlation energy functionals.
Hysteresis and bistability in the direct transition from 1:1 to 2:1 rhythm in periodically driven single ventricular cells
Authors: Yehia Ali R. , Jeandupeux Dominique , Alonso Francisco , Guevara Michael R.
Published: July 2002

The transmembrane potential of a single quiescent cell isolated from rabbit ventricular muscle was recorded using a suction electrode in whole-cell recording mode. The cell was then driven with a periodic train of current pulses injected into the cell through the same recording electrode. When the interpulse interval or basic cycle length (BCL) was sufficiently long, 1:1 rhythm resulted, with each stimulus pulse producing an action potential. Gradual decrease in BCL invariably resulted in loss of 1:1 synchronization at some point. When the pulse amplitude was set to a fixed low level and BCL gradually decreased, N+1:N rhythms (N⩾2) reminiscent of clinically observed Wenckebach rhythms were seen. Further decrease in BCL then yielded a 2:1 rhythm. In contrast, when the pulse amplitude was set to a fixed high level, a period-doubled 2:2 rhythm resembling alternans rhythm was seen before a 2:1 rhythm occurred. With the pulse amplitude set to an intermediate level (i.e., to a level between those at which Wenckebach and alternans rhythms were seen), there was a direct transition from 1:1 to 2:1 rhythm as the BCL was decreased: Wenckebach and alternans rhythms were not seen. When at that point the BCL was increased, the transition back to 1:1 rhythm occurred at a longer BCL than that at which the {1:1→2:1} transition had initially occurred, demonstrating hysteresis. With the BCL set to a value within the hysteresis range, injection of a single well-timed extrastimulus converted 1:1 rhythm into 2:1 rhythm or vice versa, providing incontrovertible evidence of bistability (the coexistence of two different periodic rhythms at a fixed set of stimulation parameters). Hysteresis between 1:1 and 2:1 rhythms was also seen when the stimulus amplitude, rather than the BCL, was changed. Simulations using numerical integration of an ionic model of a single ventricular cell formulated as a nonlinear system of differential equations provided results that were very similar to those found in the experiments. The steady-state action potential duration restitution curve, which is a plot of the duration of the action potential during 1:1 rhythm as a function of the recovery time or diastolic interval immediately preceding that action potential, was determined. Iteration of a finite-difference equation derived using the restitution curve predicted the direct {1:1↔2:1} transition, as well as bistability, in both the experimental and modeling work. However, prediction of the action potential duration during 2:1 rhythm was not as accurate in the experiments as in the model. Finally, we point out a few implications of our findings for cardiac arrhythmias (e.g., Mobitz type II block, ischemic alternans).
Dielectric response function for a quasi-one-dimensional semiconducting system
Authors: Lee Johnson , Spector Harold N.
Published: July 2002

We have obtained the dielectric response function for a quasi-one-dimensional electron gas (Q1D). In the static limit (ω=0), we find that the dielectric function has a logarithmic singularity as a function of wave vector when q=2kf when the electron gas is degenerate, but that this singularity no longer occurs when the electron gas is nondegenerate. This singularity is considered to be indicative of the Peierls transition which has been predicted to occur in a one-dimensional electron gas. As the radius of our quantum well wire goes to zero, the dielectric response function is found to diverge logarithmically. This divergence is independent of the degeneracy of the electron gas or the value of q or ω, but is indicative of the divergence of the one-dimensional Fourier transform of the Coulomb potential. We compare our results to other calculations of the dielectric function for one- and quasi-one-dimensional systems, and discuss the reasons for the differences in the various results. The momentum relaxation rates for scattering of carriers by background and remote ionized impurities in a quantum well wire are investigated.
Interfacial reactions of iron thin films on silicon
Authors: Cheng H. C. , Yew T. R. , Chen L. J.
Published: July 2002

Interfacial reactions of iron thin films on silicon have been investigated by transmission electron microscopy. FeSi was found to form after 400 °C annealing. Small amount of Fe3Si was detected in samples annealed at 450 to 500 °C. β-FeSi2 grains were predominant with a few FeSi grains remained in samples annealed at 600 °C. β-FeSi2 was found to be the dominant phase, whereas α-FeSi2 was predominant in samples annealed at 900–1100 °C in N2 ambient and in vacuum, respectively. Two-step annealings were effective in promoting the growth and improving the quality of epitaxial FeSi2. More uniform growth of epitaxial FeSi2 was observed for samples annealed in vacuum than those heat treated in N2 ambient. The mechanisms of epitaxial growth and the influence of impurities on the interfacial reactions are discussed.
Edge autopreionization effect on power enhancement of a transversely excited atmospheric nitrogen laser
Authors: Kong Hong Jin , Park Dae Yoon
Published: July 2002

We report a power-enhanced nitrogen laser system which utilizes an autopreionization effect at the sharp edges of the Blumlein capacitor’s electrode. The power enhancing effect of the laser is investigated at various autopreionizer gap distance and the main electrode gap distance. A 43% increase and a stability within 2% in the laser output power are achieved at the optimal configuration. The present autopreionization technique is found to be simple yet more effective.
Formation and bonding structure of silicon nitride by 20-keV N+ ion implantation
Authors: Hasegawa S. , Zalm P. C.
Published: July 2002

The nitridation process and the annealing effects for Si-nitride layers prepared by 20-keV nitrogen ion implantation have been investigated by means of x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The surface layer of samples implanted at a low dose is composed of a mixture of elemental Si regions and Si-rich nitride regions. A continuous near-stoichiometric Si-nitride is formed without annealing as the implantation dose is increased. Shifts of the XPS spectra (Si2p and N1s lines) toward higher binding energy with the implantation dose are observed. Similar shifts are also observed in AES spectra. The peak energy of the Si2p line is connected with a weighted average coordination number at a Si site in the SiNn (n=0,...,4) configuration, and the chemical shift per Si-N bond is 1.0 eV. The shift of the N1s line is interpreted in terms of a change in the short-range order at an N site.
Measurement of optical absorption in epitaxial semiconductor layers by a photovoltage method
Authors: Blood P.
Published: July 2002

A method has been developed for measuring optical absorption in epitaxial semiconductors in which the layer structure is grown on a substrate of narrower band gap and opposite conductivity-type so that the intensity of light transmitted can be monitored by measuring the photovoltage generated at the buried junction using a transparent surface contact. This arrangement provides a quick method of locating the wavelengths of features in the absorption spectrum. However, it is also possible to obtain an absolute measurement of the transmitted fraction using a control sample from which the absorbing layer structure has been removed by etching, usually a matter of etching only a few microns. This result is independent of surface reflections and the linearity and spectral response of the junction. The method is described with particular reference to GaAs/AlGaAs multiple quantum well structures, though it is also applicable to single homogeneous layers, and the experimental results are in good agreement with those obtained by conventional transmission measurements on samples from which the substrate has been removed.
Singular point detection of discontinuous magnetization processes
Authors: Asti G. , Bolzoni F.
Published: July 2002

Ferromagnetic crystals can show discontinuous jumps in the magnetization curve for certain combinations of the anisotropy constants. These are called first-order magnetization processes (FOMP) and have been recently observed in a large variety of compounds. We analyze the problem of how this singularity in the magnetization curve is transformed in the case of a polycrystalline specimen. The problem is the same as that of the singular point detection theory (SPD) that was originally concerned with the singularities of the anisotropy fields and with the way of enhancing them. SPD is now a widely used method for measuring anisotropy fields. In the present work it is demonstrated that SPD can be extended to FOMP, thus allowing extremely clear and accurate measurements of critical fields using polycrystalline specimens. This provides in principle a powerful method for deep investigations in the anisotropy properties of ferromagnetic and ferrimagnetic materials. In Sec. II we examine how the critical parameters of FOMP are modified when the magnetizing field is rotated out of symmetry directions. It is demonstrated that an additional transition is possible within certain conditions of anisotropy constants and field orientations (extraordinary FOMP). Section III is an extension of the Stoner–Wohlfarth model including anisotropy constants up to the sixth order (K1, K2, and K3). In Sec. IV we determine by analytical methods the order of derivative dnM/dHn at which the singularity becomes apparent. For FOMP of type P a discontinuity Δχ appears in the differential susceptibility, while FOMP of type A give rise to a discontinuity in the second derivative of the magnetization d2M/dH2.
Immobilization mechanisms for ion-implanted deuterium in aluminum
Authors: Myers S. M. , Besenbacher F. , No/rskov J. K.
Published: July 2002

Aluminum was ion implanted with deuterium (D) and then subjected to linear temperature ramping, and the resulting D redistributions were monitored using the ion-induced nuclear reaction D(3He, p)4He. Data from such experiments were analyzed in terms of various immobilization processes, utilizing numerical solutions of the appropriate diffusion formalism. The identification of mechanisms was augmented by transmission electron microscopy. Irradiation defects believed to be of vacancy type were shown to trap the D with a binding enthalpy of 0.52±0.10 eV relative to solution sites, in excellent agreement with calculations based on effective medium theory. Stronger binding at the surface oxide was quantitatively described by assuming the formation of D2 molecules at the metal-oxide interface. At higher implanted concentrations the immobilization of D by precipitation of D2 bubbles was observed, and the subsequent release from these bubbles at more elevated temperatures was described by diffusion theory. Small, high-pressure He bubbles formed by ion implantation of He did not trap the D more strongly than the irradiation defects, in contrast to observations in a number of other metals, but consistent with predictions of effective medium theory for Al.
Production of relativistic, rotating electron beams by gyroresonant rf acceleration in a TE111 cavity
Authors: McDermott D. B. , Furuno D. S. , Luhmann N. C.
Published: July 2002

The rf acceleration of moderate current (≲1 A) electron beams to 500 keV in a TE111 cylindrical cavity resonator is described. Experimental results are compared with theory. rf to beam energy conversion efficiencies in excess of 50% have been observed. The resultant axis-encircling beam with a large ratio of perpendicular velocity to longitudinal velocity is ideal as a driver for a high-harmonic gyrotron.
Low ohmic contact to silicon with a magnesium/aluminum layered metallization
Authors: Akiya Masahiro , Nakamura Hiroaki
Published: July 2002

A low-contact-resistance, low-resistivity magnesium aluminum (Mg/Al) layered metal is described. Measurements according to the transmission line model showed a specific contact resistivity of 2.2 × 10−7 Ω cm2 to an n-type planar silicon test resistor. A barrier height of 0.46 eV for Mg-nSi contacts was confirmed for Schottky diodes.
Field emission from broad-area niobium cathodes: Effects of high-temperature treatment
Authors: Niedermann Ph. , Sankarraman N. , Noer R. J. , Fischer O/.
Published: July 2002

We present results made with a new apparatus for the study of localized field emission (FE) sites on broad-area cathodes. The apparatus is centered around a UHV field emission scanning microscope, consisting of a micromanipulator allowing precision cathode motion, a rotatable holder for broad and microtip anodes, and a fast high-voltage regulator allowing constant-current measurement of a distribution of both weakly and strongly emitting sites. Also included are an in situ scanning electron microscope, and a facility for in situ microfocus Auger analysis as well as scanning Auger microscopy. Samples can be high-temperature annealed without removal from UHV. These tools have been used to study the effects of heat treatment (HT) up to 2000 °C on the FE from nonanodized and anodized Nb cathodes. We find that HT at 800–900 °C increases the density of field emitters at a given electric field. HT at T>1000 °C reduces the density, while at T≥1400 °C a drastic decrease of the emission occurs. We have repeatedly obtained surfaces of cm2 size which do not emit at 100 MV/m. If such an emission-free surface is heat treated again at 800 °C, new emitters appear. Typically, the physical size of the field emitters is a few μm, although in some cases a larger particle was found and in others no feature was seen at 0.5-μm resolution. A large variety of elements is associated with these particles. We discuss in particular the nature and origin of three types of emitters: (1) sulfur containing particles, (2) carbon particles, both of which were identified as new emitters after a 800 °C HT, and (3) FE sites which are especially resistant against HT.
Optical ranging by wavelength multiplexed interferometry
Authors: Williams C. C. , Wickramasinghe H. K.
Published: July 2002

A new optical technique is described for measurement of absolute distance. The approach is based upon a wavelength multiplexed heterodyne interferometer with FM demodulation. By temporally multiplexing discrete wavelengths in a heterodyne interferometer, a complete elimination of interferometric range ambiguity can be achieved while maintaining the high range sensitivity and resolution of interferometry. The basic theory is presented and an algorithm is described for measurement of range over meter distances with submicrometer resolution. The experimental implementation of the wavelength multiplexed interferometer is described and ranging results with 2 μm resolution from 20 cm are presented. A scanned three-dimensional map of a surface contour with 3-mm topography is also presented.
Se-related deep levels in InGaAlP
Authors: Watanabe Miyoko O. , Ohba Yasuo
Published: July 2002

Donor-related deep levels in Se-doped In0.5(Ga1−xAlx)0.5P (x=0.0–1.0) have been studied by DLTS, C–V measurement, and photocapacitance techniques. Two donor-related deep levels were found. The concentrations linearly increased with the donor concentration, and strongly depended on the alloy composition. These levels were found to be dominant donors in the composition range from x=0.3 to x=1.0. They had 0.20 and 0.29 eV activation energies for electron thermal emission, 0.08 and 0.11 eV energies for electron thermal capture, and they had 0.7 and 1.1 eV energies for electron optical emission, respectively. Internally consistent configuration coordinate diagrams were proposed for these levels, suggesting that both levels are similar to DX centers commonly found in n-AlGaAs.
Trapping of ion-implanted deuterium in molybdenum
Authors: Myers S. M. , Besenbacher F.
Published: July 2002

Trapping of ion-implanted deuterium (D) by irradiation defects and He bubbles in Mo was investigated through nuclear-reaction profiling of the D during linear ramping of temperature. Resulting data were analyzed using transport theory to obtain trap strengths. Nanometer-size He bubbles were found to trap the D with a binding enthalpy of 1.15±0.15 eV relative to the solution site, consistent with two independent predictions based, respectively, on effective-medium theory and thermodynamic analysis. Implantation damage trapped the D with three different binding enthalpies, 1.15, 1.03, and 0.80 eV, attributed, respectively, to vacancy clusters, monovacancies with low D occupancy, and monovacancies with high D occupancy. The two latter values are in good agreement with effective-medium theory.
Excimer laser initiated chemical vapor deposition of tungsten films on silicon dioxide
Authors: Shintani Akira , Tsuzuku Susumu , Nishitani Eisuke , Nakatani Mitsuo
Published: July 2002

Photochemical vapor deposition technique using an ArF excimer laser has been employed to deposit W films on SiO2 and Si from a WF6 and H2 system. Adhesion characteristics of the film to SiO2 are found to depend both on substrate temperature and on H2/WF6 gas flow ratio: good adhesion is obtained with an increase in the temperature or the ratio. Film formation has reaction orders of 1, 1/2 , and 1 with respect to deposition time, and WF6 and H2 partial pressures, respectively. An activation energy of 0.36 eV is estimated for this film formation on both SiO2 and Si; this energy is plausibly due to H atom diffusion on the W surface. These findings are different from conventional thermal chemical vapor deposition. Film resistivities as low as about 2× the value of bulk W have been observed in the substrate temperature range 250–500 °C. The crystalline structure of the film deposited in this temperature range is uniquely of the α phase. The crystal orientation of the film depends both on substrate temperature and on H2/WF6 gas flow ratio: at low ratios, the dominant crystal orientation varies from (110) to (200) with an increase in temperature. With an increase in H2/WF6 gas flow ratio, the dominant crystal orientation is changed from (200) to (110).
Propagation and generation of Josephson radiation in superconductor/insulator superlattices
Authors: Auvil P. R. , Ketterson J. B.
Published: July 2002

The wave propagation and generation characteristics of a metal-insulator superlattice are calculated in a low-field Landau–Ginzburg model, including Josephson coupling through the insulating layers. It is shown that a significant increase in the phase velocity of the electromagnetic waves propagating in the superlattice occurs when the thickness of the superconducting layers becomes much less than the London penetration depth, suggesting that increased output of Josephson radiation may be achieved from such structures. Wave generation via the ac Josephson effect (in the presence of applied dc electric and magnetic fields) is studied for both parallel and series driven multilayer structures.
Magnetic and structural properties of high rate dual ion-beam sputtered NiFe films (invited)
Authors: Lo Jerry , Hwang C. , Huang T. C. , Campbell R. , Allee David
Published: July 2002

NiFe films in the thickness range of 300 Å to 8 μm with excellent magnetic properties and thickness uniformity have been prepared using a newly developed high rate (up to 1000 Å/min) dual ion beam sputtering system. Deposition conditions such as the ion assist voltage, the target orientation and the partial pressure of nitrogen have been optimized to reduce coercivity Hc and stress as well as to improve film adhesion and film thermal stability. The annealing effects on Hc, the anisotropy field Hk, as well as the resistivity of the NiFe films were studied. X-ray diffraction and transmission electron microscopy techniques were used to characterize the microstructures of the films and were correlated with their magnetic properties. These ion-beam sputtered NiFe films have excellent magnetic, electrical, mechanical, and microstructural properties to make them suitable for thin-film head applications.
High-velocity laser-driven implosions with gold-coated microballoons
Authors: Juraszek D. , Billon D. , Bernard M. , Bocher J. L. , Decroisette M. , Garconnet J. P. , Le Breton J. P. , Meynial D. , Schneider Ph.
Published: July 2002

Velocities up to 108 cm s−1 have been obtained in laser implosion of gold-coated microballoons driven by irradiation at short wavelength. Despite a strongly nonuniform illumination, high neutron yield (i.e., high fuel temperature) has been obtained. Results analysis suggests that the glass shell is first decompressed by x-ray radiation from the gold coating and then accelerated by the ablation pressure.
Growth of ZnSe on Ge(100) substrates by molecular-beam epitaxy
Authors: Yamaguchi Eiji , Takayasu Ichiro , Minato Tetsuo , Kawashima Mitsuo
Published: July 2002

Epitaxial growth of ZnSe on Ge(100) substrates by molecular-beam epitaxy was investigated and compared with the simultaneous growth on GaAs(100). Single crystalline ZnSe on Ge was obtained with a flat and smooth surface at a temperature of higher than 370 °C and flux ratio JSe/JZn of around 2.0. The crystal quality, as revealed by double-crystal x-ray rocking curves, was not superior to that grown on GaAs, despite a better lattice parameter match with the substrate. This is due to the fact that the grown layer contains antiphase domains and inclines against the Ge substrate towards 〈011〉 by about 0.1°.
High-pressure thermal oxidation of n-GaAs in an atmosphere of oxygen and water vapor
Authors: Basu Nandita , Bhat K. N.
Published: July 2002

A low-temperature (∼250 °C) high-pressure oxidation technique is used for the thermal oxidation of gallium arsenide in an ambient of oxygen and water vapor. It is shown that a uniform and chemically stable oxide with high band-gap energy can be grown on GaAs by this process. The role of water vapor and oxygen is studied in detail to obtain information on the oxidation mechanism. The electrical characteristics and the composition of this oxide are presented to demonstrate its suitability for surface passivation and metal-oxide–semiconductor devices.
Refractive indices and the exponential optical absorption for Hg1−xCdxTe (0.2<x<0.3)
Authors: Toyoda Taro
Published: July 2002

For Hg1−xCdxTe, in the range of x≂0.2–0.3, the optical dispersion parameters (single-oscillator energy E0, dispersion energy Ed, and lattice-oscillator energy E1) and the fitting parameter of the exponential optical absorption (steepness factor σ) were analyzed using the data by Finkman and co-workers [J. Appl. Phys. 50, 4356 (1979); J. Appl. Phys. 56, 2896 (1984)]. For Hg1−xCdxTe the values of E0, Ed, and σ in this composition range are smaller than those for CdTe, and E1 is larger than that for CdTe. The results suggest that the chemical bonding of Hg1−xCdxTe in the range of x≂0.2–0.3 is smaller than that of CdTe.
Design applications of magnetic mirrors
Authors: Leupold H. A. , Potenziani II E.
Published: July 2002

Recent developments in permanent-magnet materials have made possible several novel high-field sources, with great potential application in mm/microwave/optical technology. However, even the crude approximations to such structures, which are needed to make their manufacture viable, entail as many as 72 pieces with eight distinct shapes. It has recently been shown that simpler equivalents of some of the structures can be made with half a structure, when the effect of the missing half is replaced with that of a reflected anti-image in a ferromagnetic plane. In this manner, the number of parts is decreased to 36. Further reductions are possible by image formation of still smaller segments in perfectly diamagnetic superconducting mirrors. For the ultimate reduction, only nine of the original 72 structural segments are needed. Access to the magnetic field for electrical leads and other conduits is also greatly facilitated. In this paper, we describe and analyze several examples of such design simplification with combinations of superconducting and ferromagnetic mirrors.
Microwave dielectric constants of silicon, gallium arsenide, and quartz
Authors: Seeger Karlheinz
Published: July 2002

For a determination of the dielectric constants ε of semiconductors, a microwave transmission interference method has been applied. For the first time, a calculation is presented which yields the full interference spectrum, not only the position of the extremal points. A comparison of the theoretical and experimental spectra results in a higher precision than previously obtained. A metal evaporation of the sample faces which are in contact with the waveguide walls turns out to be very important. Relative dielectric constants of 11.6 for silicon, 12.8 for gallium arsenide, and 4.6 for crystalline quartz, all ±0.05, have been obtained.
The frequency fluctuations of an electronically cooled rotor
Authors: Bernard B. E. , Ritter R. C.
Published: July 2002

Calculations show that even a modest-sized laboratory rotor can exhibit stability comparable to that of good frequency standards if it is sufficiently undisturbed and if its moment of inertia is constant enough. Further, they show that electronic cooling, by derivative feedback, can cause these properties to be exhibited in much more useful regimes of time and frequency. This paper describes theoretical and experimental efforts to evaluate the effects of such feedback on a practical rotor, presently far from ideal. The stability measurements are presented in the context of averaging or observing times and are given as Allan variances. Representative results for a small (few centimeters) rotor with a magnetic suspension bearing show noise 4–5 orders of magnitude above the ideal, i.e., noise caused only by molecular bombardment. These analyses are in the region of averaging times from 10 to 100 000 s, and the measurements were in a laboratory in which the temperature fluctuates a few degrees Centigrade. Measured and calculated transfer functions from temperature variations show that a significant part of the excess noise is caused by these variations. Electronic cooling by feedback behaves as expected in modifying the noise spectrum. The changes in the excess noise are the same as those calculated for molecular bombardment noise, thus indicating that the excess noise comes from frequency-independent driving torques.
Hopping conduction in undoped low-pressure chemically vapor deposited polycrystalline silicon films in relation to the film deposition conditions
Authors: Dimitriadis C. A. , Coxon P. A.
Published: July 2002

The conductivity of undoped low-pressure chemically vapor deposited polycrystalline silicon films has been investigated in the temperature range 77–300 K as a function of the deposition conditions of the film. At low temperatures hopping conductivity has been identified. Decrease of the deposition pressure of the polysilicon film shifts the hopping region to lower temperatures and reduces the density of localized trap states and, also, the degree of disorder.