bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo study of the nucleation process during zeolite synthesis
Authors: Wu Minghong G. , Deem Michael W.
Published: July 2002

An atomic-scale model for silicate solutions is introduced for investigation of the nucleation process during zeolite synthesis in the absence of a structure directing agent. Monte Carlo schemes are developed to determine the equilibrium distribution of silicate cluster sizes within the context of this model. How the nucleation barrier and critical cluster size change with Si monomer concentration is discussed. Distance and angle histograms as well as ring size distributions are calculated and compared with known zeolite structures. The free energies of critical clusters are compared with those for small clusters of α-quartz.
Current-perpendicular-to-plane-magnetoresistance properties of Ru and Co/Ru interfaces
Authors: Eid K. , Fonck R. , Darwish M. AlHaj , Pratt W. P. , Bass J.
Published: July 2002

Recent interest in Ru and Co/Ru multilayers stimulated us to measure their properties in the current-perpendicular-to-plane (CPP) geometry at 4.2 K. For sputtered samples, we present results for the resistivity of Ru, the interface specific resistances of Cu/Ru and Co/Ru, the spin-memory-loss length lsfRu in Ru, the spin-memory-loss probability at Cu/Ru interfaces, and the spin-scattering anisotropy parameter γ at Co/Ru interfaces.
Diffuse x-ray scattering from 311 defects in Si
Authors: Nordlund K.
Published: July 2002

311 defects are extended, rodlike defects that play a central role in the processing of Si during integrated circuit manufacturing. Diffuse x-ray scattering techniques provide a nondestructive means to detect defects in solids. However, to date there has been no knowledge of what the x-ray scattering pattern from 311 defects looks like. Using a recently introduced fully atomistic modeling scheme, the diffuse x-ray scattering patterns were calculated from 311 defects. The results demonstrate how 311 defects can be detected, how the main varieties of 311 defect can be distinguished, and how both the defect width and length can be derived from the scattering.
Dynamical heterogeneities below the glass transition
Authors: Vollmayr-Lee K. , Kob W. , Binder K. , Zippelius A.
Published: July 2002

We present molecular dynamics simulations of a binary Lennard-Jones mixture at temperatures below the kinetic glass transition. The “mobility” of a particle is characterized by the amplitude of its fluctuation around its average position. The 5% particles with the largest/smallest mean amplitude are then defined as the relatively most mobile/immobile particles. We investigate for these 5% particles their spatial distribution and find them to be distributed very heterogeneously in that mobile as well as immobile particles form clusters. We suggest that this dynamic heterogeneity may be due to the fact that mobile/immobile particles are surrounded by fewer/more neighbors which form an effectively wider/narrower cage. The dependence of our results on the length of the simulation run indicates that individual particles have a characteristic mobility time scale which can be approximated via the non-Gaussian parameter.
High-temperature superconducting thick films with enhanced supercurrent carrying capability
Authors: Jia Q. X. , Foltyn S. R. , Arendt P. N. , Smith J. F.
Published: July 2002

We have developed a multilayer architecture to enhance the supercurrent carrying capability of thick (>1 μm) YBa2Cu3O7−x (YBCO) films. An interlayer of CeO2, which functions as a starting template for growth of subsequent high-quality YBCO layers, is used to prohibit the generation of microstructural defects that otherwise develop in the top region of thick YBCO films. Improved surface quality for a 3.3 μm thick YBCO film has been confirmed by the channeling data of Rutherford backscattering spectroscopy and by scanning electron microscopy. Compared to our routinely prepared single-layer YBCO thick film in which the supercurrent drops rapidly with increasing film thickness, this multilayer makes it possible to maintain critical current density over 1 MA/cm2 with less restrictions on the thickness of coated conductors.
Ion and electron acceleration in the field-reversed configuration with an odd-parity rotating magnetic field
Authors: Glasser A. H. , Cohen S. A.
Published: July 2002

The method for accelerating ions and electrons in the field-reversed configuration using odd-parity rotating magnetic fields (RMFs) in the ion-cyclotron range-of-frequencies (ICRF) is studied. The approach is based on long, accurate numerical integration of Hamilton’s equations for single-particle orbits. Rapid ion heating to thermonuclear conditions occurs in <0.1 ms in a modest-sized FRC. Strong variation of the magnetic-field strength over the confinement region prevents a true cyclotron resonance, resulting in stochastic though effective heating. Lyapunov exponents are computed to demonstrate chaotic orbits. Electrons are also effectively heated in this frequency range, primarily by a mechanism involving trapping in the wells of the azimuthal electric field. Odd-parity RMF promotes oppositely directed ion and electron motion near the minor axis, appropriate for supporting the plasma current.
Effects of pulse-length and emitter area on virtual cathode formation in electron guns
Authors: Valfells Ágúst , Feldman D. W. , Virgo M. , O’Shea P. G. , Lau Y. Y.
Published: July 2002

Recent experiments at the University of Maryland using photoemission from a dispenser cathode have yielded some interesting results regarding the effects of the area of emission and of the ratio between the pulse length and the gap transit time on the amount of current that may be drawn from an electron gun before a virtual cathode forms. The experiments show that a much higher current density may be drawn from a short pulse or limited emitter area than is anticipated by the Child–Langmuir limiting current. There is also evidence that the current may be increased even after virtual cathode formation, which leads a distinction between a limiting current density and a current density critical for virtual cathode formation. The experiments have also yielded some interesting results on the longitudinal structure of the current pulse passed through the anode. Some empirical and theoretical scaling laws regarding the formation of virtual cathodes in an electron gun will be presented. This work was motivated by the needs of the University of Maryland Electron Ring (UMER) [P. G. O’Shea, M. Reiser, R. A. Kishek et al., Nucl. Instrum. Methods Phys. Res. A 464, 646 (2001)] where the goal is to generate pulses that are well-localized in time and space.
Calculations of rotation–vibration states with the z axis perpendicular to the plane: High accuracy results for H3+
Authors: Kostin Maxim A. , Polyansky Oleg L. , Tennyson Jonathan
Published: July 2002

A method of calculation of rotation–vibration states for a general triatomic that places the body-fixed z axis perpendicular to the plane of the molecule is implemented within a discrete variable representation (DVR) for the vibrational motion. Calculations are presented for water and H3+. For H3+ the new method improves on previous high accuracy ab initio treatments of the rotation–vibration energies of the molecule both in accuracy and the range of rotational states that can be treated. Reliable treatment of quasilinear geometries means that the method is also promising for treating very highly excited states.
Theoretical study of the unusual potential energy curve of the A 1Σ+ state of AgH
Authors: Witek Henryk A. , Fedorov Dmitri G. , Hirao Kimihiko , Viel Alexandra , Widmark Per-Olof
Published: July 2002

The A 1Σ+ potential energy curve of AgH is studied by means of the second-order multistate multireference perturbation theory including the spin–orbit and relativistic effects. The anomalous behavior of the vibrational energy levels observed in experiment is reproduced well by theory. An analysis of the A 1Σ+ wave function shows that at the internuclear distance of 4–6 Å, a partial electron transfer from the 5pz orbital of silver to the 1s orbital of hydrogen occurs. This admixture of the ionic-like [core] 4d101sH2 configuration occurring in A 1Σ+ due to two avoided crossings, namely X 1Σ+ with A 1Σ+ and A 1Σ+ with C 1Σ+, is found to be responsible for the unusual shape of the A 1Σ+ potential energy curve: the effective potential is a superposition of a Morse-like covalent interaction between Ag and H, and the electrostatic ionic-like interaction between Ag+ and H−. We present spectroscopic parameters, vibrational levels, and rotational constants computed for a large number of vibrational levels and observe good agreement with available experimental data. The equilibrium distance agrees within 0.01 Å and the vibrational frequency within 60 cm−1 for the state-specific calculations. Larger relative discrepancy is observed for ωexe, about 30 cm−1, however the non-Morse-like nature of the energy curve makes it impossible to describe the levels only with ω and ωexe, so that direct comparison is not well defined.
Memory kernels and effective Hamiltonians from time-dependent methods. II. Vibrational predissociation
Authors: Brems Vincent , Desouter-Lecomte Michèle
Published: July 2002

The overlapping resonance regime is studied from a temporal viewpoint for a model vibrational predissociation in the framework of the Feshbach partitioning method. The memory kernels that are related by Fourier transformation to the energy dependent effective Hamiltonians are computed by wave packet propagation. The characteristic time τmemory of the memory kernel is compared with the time scale of the dynamics in the bound subspace. An approximate expression for τmemory is derived. τmemory depends on the bound-free couplings on a wide energy range. It is shown that the nonoscillatory shape of these couplings as a function of the dissociation energy is a typical feature of the vibrational predissociation.
Functional design of a pulsed two-frequency capacitively coupled plasma in CF4/Ar for SiO2 etching
Authors: Maeshige Kazunobu , Washio Gentaro , Yagisawa Takashi , Makabe Toshiaki
Published: July 2002

A capacitively coupled plasma (CCP) with a different frequency source at each of two parallel plates is a powerful tool for SiO2 etching. A time modulation of two-frequency CCP by a pulsed-power operation will be one of the practical solutions in the next generation of etchers, and will allow charging-free plasma processes for high-aspect-ratio holes or trench etching. We numerically predict the structure and functions of a pulsed two-frequency CCP in CF4(5%)/Ar, and discuss its ability to generate charge-free plasma processes. We also investigate the functional separation between plasma production by very high frequency (100 MHz) and bias voltage application by low frequency (1 MHz). Alternate injections of high-energy positive and negative ions are predicted during the off-phase of a pulsed two-frequency CCP.
Specific volume of glass-forming liquid Pd43Cu27Ni10P20 and related thermodynamic aspects of the glass transition
Authors: Lu I.-R. , Görler G. P. , Willnecker R.
Published: July 2002

The volume change of the easy glass-forming alloy Pd43Cu27Ni10P20 was investigated continuously from the liquid to the vitreous state upon cooling at 10 K/min by means of the sessile drop technique. From the data obtained on density and thermal expansion, the specific volume as a function of temperature was deduced in the complete temperature range of the stable and the undercooled liquid as well as in the vitreous state for this alloy. Thus, the temperature dependence of the specific volume V(T) could be compared over the entire liquid range of a metallic glass former to that of the enthalpy H(T) obtained from preceding calorimetric measurements. A strong correlation between both thermodynamic functions becomes evident from this comparison and is interpreted on the basis of free-volume theory. The increase of the heat capacity at the glass transition can be explicitly explained by the creation and annihilation of free volume.
Quantitative optical tomography of chemical waves and their organizing centers
Authors: Winfree A. T. , Caudle S. , Chen G. , McGuire P. , Szilagyi Z.
Published: July 2002

Interference from topological, chemical and biological analogies led to the guess that a wide variety of homogeneous three-dimensional materials characterized by ‘‘excitability’’ might support persistent particle-like ‘‘organizing centers.’’ These are vortex filaments, typically rings, around which excitation fronts circulate in the uniform medium. Robust organizing centers were recently discovered numerically in several cases, motivating a search for them in nature. But if a candidate were observed there would still be no way to examine it for the expected topological intricacies. To solve this problem we designed and constructed a hybrid chemical/optical/computational instrument using the familiar principles of tomography by filtered backprojection. We demonstrate here that it can quantitatively resolve chemical vortex filaments in a new excitable medium fashioned for the purpose. The next step, not described here, is to use the light sensitivity of this medium to contrive initial conditions from which topologically exotic organizing centers would arise and possibly persist.
Stable periodic motions in the problem on passage through a separatrix
Authors: Neishtadt A. I. , Sidorenko V. V. , Treschev D. V.
Published: July 2002

A Hamiltonian system with one degree of freedom depending on a slowly periodically varying in time parameter is considered. For every fixed value of the parameter there are separatrices on the phase portrait of the system. When parameter is changing in time, these separatrices are pulsing slowly periodically, and phase points of the system cross them repeatedly. In numeric experiments region swept by pulsing separatrices looks like a region of chaotic motion. However, it is shown in the present paper that if the system possesses some additional symmetry (like a pendulum in a slowly varying gravitational field), then typically in the region in question there are many periodic solutions surrounded by stability islands; total measure of these islands does not vanish and does not tend to 0 as rate of changing of the parameter tends to 0.
Effect of annealing process parameters on the properties of AuGe ohmic contacts to GaAs
Authors: Kulkarni A. K. , Lukowski J. T.
Published: July 2002

The control of the electrical and the structural properties of ohmic contacts to GaAs is extremely important in the fabrication of GaAs devices. The annealing process parameters such as anneal ramp rate, anneal dwell temperature, dwell duration, and anneal atmosphere need to be precisely controlled to obtain low contact resistivities. The importance of the changes in the structural properties as a function of annealing process parameters, which in turn determine the contact resistance, is illustrated here. The extent of Ge in-diffusion to the metal-GaAs interface and the Ga out-diffusion from the substrate determined from Auger depth profiles seem to dictate even the small changes in the specific contact resistances. The lowest values of contact resistivity are obtained when the ratio of Ge in-diffusion to Ga out-diffusion approached one.
Electron heating in silicon nitride and silicon oxynitride films
Authors: DiMaria D. J. , Abernathey J. R.
Published: July 2002

The vacuum emission technique has been used to study electron transport and heating in silicon nitride and silicon oxynitride. The experimental results are compared to data for silicon dioxide in which all the conduction-band electrons can gain several eV of energy at electric fields greater than 2 MV/cm. Although average electron energy as a function of electric field curves are very similar to silicon dioxide, the total number of electrons which can be heated to energies greater than 2 eV is greatly reduced because of the increased trapping in these films. Reduction in hot electrons due to increased trapping is correlated to increasing nitrogen content through the oxynitride phases to silicon nitride. Trapping/detrapping on energetically shallow sites in the forbidden gap controls the bulk limited conduction in these films, and very few electrons are allowed to move freely in the conduction band.
Isotope separation in a vacuum-arc centrifuge
Authors: Prasad Rahul R. , Krishnan Mahadevan
Published: July 2002

Measurements of isotope enrichment in collected samples of magnesium from a vacuum-arc centrifuge are described. The vacuum-arc centrifuge is a rigidly rotating, magnetized, highly ionized, quasineutral column of plasma. Rigid body rotation causes radial, centrifugal separation of isotopes. A multifluid model has been developed that predicts the observed isotope separation. Using this model and the observed scaling of rotation and plasma size with magnetic field, enrichments of 48Ca and 203Tl are predicted as functions of magnetic field. It is found that the yield (defined as the ratio of the fraction of the desired isotope collected to the maximum possible collected fraction at the desired enrichment) of both these isotopes increases with increasing magnetic field. The typical cost of separation in such a vacuum-arc centrifuge is calculated to be ∼70 keV/separated atom.
Magnetic properties of (C6H5CH2C4H11N2Cl)2CuCl4 and (CH2OHCH2NH3)2CuCl4
Authors: Landee Christopher P. , Halvorson Kris E. , Willett Roger D.
Published: July 2002

The two title compounds are two-dimensional magnetic systems with ferromagnetic interlayer exchange coupling. Magnetic susceptibility measurements ferromagnetic interlayer exchange coupling. Magnetic susceptibility measurements yield J/k=4.59 and 13.0 K for the two compounds, respectively. A smooth correlation between J and the intralayer Cu-Cu distance is observed. The latter compound orders at TN∼8 K, while no evidence for long-range order in the former is observed down to 1.4 K. The electron paramagnetic resonance linewidths of (C6H5CH2C4H11N2Cl)2CuCl4 are abnormally large with a dominant cos2 θ angular dependence. The assumption that the linewidths are dominated by spin anisotropies leads to an estimate of ‖dz/J‖≂0.14.
An investigation of the optical constants and band gap of chromium disilicide
Authors: Bost M. C. , Mahan John E.
Published: July 2002

Polycrystalline thin films of chromium disilicide were grown on silicon substrates. An analysis of the absorption coefficient data indicates that CrSi2 is a semiconductor with an indirect forbidden energy gap of slightly less than 0.35 eV. Measurements of the activation energy of the intrinsic conductivity confirm this result.
Shock-wave effect on the electron-hole plasma in CdSe
Authors: Garuthara R. , Lu X. Z. , Lee S. , Alfano R. R.
Published: July 2002

Picosecond laser pulses were utilized to generate shock waves in a cadmium selenide (CdSe) semiconductor. A pump-and-probe technique was used to study the shock pressure effects on the carriers in CdSe by measuring the photoluminescence emission. A blue shift was observed in the photoluminescence peak position under shock-wave loading. By varying the delay time of the probe pulse with respect to the pump pulse, a shock pressure profile in time was monitored. By analyzing the high-energy tail of electron-hole plasma photoluminescence spectra, the average hot-electron temperatures were deduced for both shocked and unshocked emission. Under shock loading conditions, an increase in the average hot-electron temperature was observed to remain over 20 ns.
Time-dependent modeling of resonant-tunneling diodes from direct solution of the Schrödinger equation
Authors: Mains R. K. , Haddad G. I.
Published: July 2002

Numerical solution of the time-dependent Schrödinger equation for resonant-tunneling diodes has been impeded by the difficulty in handling open-system boundary conditions. This paper presents a boundary condition method to simulate the interaction with ideal particle reservoirs at the device boundaries. A switching transient is calculated where the device is switched from the peak current state to the valley current state. In addition, this method was used to develop a small-signal analysis of resonant-tunneling diodes. Results for the small-signal equivalent circuit of a particular device versus frequency are presented.
Temperature variation of silicon’s nonlinearity constant and acoustic attenuation
Authors: Joharapurkar D. N.
Published: July 2002

The temperature variation of the nonlinearity constant in silicon has been studied for longitudinal waves along the [110] axis and for shear waves along the [110] axis with polarizations along the [11̄0] and [001] axes in the range 73–273 K. The temperature dependence of the nonlinearity constant estimated from the second- and third-order elastic constants has been further used to evaluate the acoustic attenuation in the Akhieser region at 475 MHz. The estimated values of acoustic attenuation have been compared with the measured values available in the literature and are found to be in good agreement. The nonlinearity constant showed a positive temperature coefficient for the longitudinal waves, whereas it showed a negative temperature coefficient for the shear waves. These results resemble those obtained for germanium and copper.
New, Z-direction anisotropically conductive composites
Authors: Jin S. , Sherwood R. C. , Mottine J. J. , Tiefel T. H. , Opila R. L. , Fulton J. A.
Published: July 2002

This paper reports the development of a new class of anisotropically conductive materials based on magnetic alignment of conductive particles in nonconductive matrix materials such as elastomeric or adhesive polymers. These composites contain many vertically aligned but laterally isolated chains of ferromagnetic metal spheres, the ends of which protrude from the surfaces for better electrical contact. These novel composite structures exhibit Z-direction-only electrical conduction (>1012 to 1 anisotropy ratio), which, in combination with the compliant nature of the materials, can be exploited advantageously for a variety of electronic applications including fine-pitch, area-array, circuit interconnections, circuit-testing, heat sink interfacing, and sensor devices. The movement and alignment of magnetic particles in a viscous medium, and the stability of configurations in a magnetic field will be discussed. The properties of the composite materials as well as potential circuit and device applications will be discussed.
Numerical simulation of transient response and resonant-tunneling characteristics of double-barrier semiconductor structures as a function of experimental parameters
Authors: Jensen K. L. , Buot F. A.
Published: July 2002

The tunneling current characteristics and transient response of double-barrier semiconductor structures are simulated for different barrier and quantum-well widths, barrier heights, operating bias voltage, and ambient temperatures, using the equation for the Wigner distribution function. The numerical results suggest the following: (a) There is a particle buildup inside the quantum well prior to the resonant current peak as the applied bias is varied: (b) the number of resonant energy levels seen in the simulation agrees with its proportionality to the square root of the product of the barrier height and quantum-well width: (c) the resonant peak width is larger for higher resonant energy levels than for the lower resonant energy levels in agreement with the different degree of localization of these levels; (d) at T=77 K, the current slowly increases with bias at lower bias than for T=300 K, with higher peak-to-valley ratio at T=77 K, presumably due to a much sharper convolution of the tunneling density and resonant energy level width; and (e) a higher degree of localization and existence of numerically resolved resonant energy level, in the case of asymmetrical barrier widths, occurs when the thicker barrier is located in the side with lower electron potential or higher voltage bias; no negative differential resistance was observed when these barrier widths were interchanged in our simulation. The superior accuracy of an alternative finite difference scheme, coupled with the Cayley form for the time evolution operator, in this type of numerical simulation is briefly discussed.
Oxygen sensitivity of a Pt-Pd/p-type CaFe2O4 diode
Authors: Matsumoto Y. , Hombo J. , Nitta F.
Published: July 2002

We have studied the oxygen sensitivity of Pt-Pd/p-type CaFe2O4 and Pt-Pd/Rh/p-type CaFe2O4 diodes, where the CaFe2O4 shows Fermi-level pinning. Reversible oxygen responses were observed for both diodes at 100 °C, where current under bias increased with increasing oxygen concentration. Response time was decreased by Rh doping. Thus, these diodes act as an oxygen sensor at a relatively low temperature. The Schottky barrier height φ, determined from the measured photocurrent-bias curve, decreased with an increase in oxygen concentration. The experimental value of dφ/d log PO2 was 0.11 eV/decade, and was much larger than the theoretical value of 0.018 eV/decade based on the surface redox reaction mechanism in the surface state. The difference in this value indicates that the bias is applied to the bulk as well as to the space-charge layer at the surface.
Acceleration instability in elastic-plastic solids. II. Analytical techniques
Authors: Robinson Allen C. , Swegle J. W.
Published: July 2002

Acceleration instability occurs when a body is accelerated by surface tractions. This situation resembles classic Rayleigh–Taylor instability, but differs due to the temporal and spatial variation of the stress field in the accelerated body caused by wave propagation and the time dependence of accelerating forces. We wish to investigate the extent to which the acceleration instability response of elastic-plastic solids can be approximately modeled using a modal or one-degree-of-freedom technique assuming incompressible flow. We find that neither the response equation derived assuming a linear elastic shear response nor the so-called ‘‘minimum amplitude’’ instability criterion derived from rigid plastic analyses is sufficient to describe overall stability characteristics at very large driving pressures. Several versions of the modal approximation technique for an elastic-plastic material response are derived and compared. The features which must be included in such an analysis in order to obtain qualitative agreement with instability results seen in numerical experiments are indicated. However, quantitative agreement with numerical results cannot be obtained in all cases. The sources of the disagreement and the regions of applicability of the analysis are discussed.
Electrical conductivity of sputtered films of strontium titanate
Authors: Gerblinger Josef , Meixner Hans
Published: July 2002

Among the most critical parameters in the fabrication of rf-sputtered SrTiO3 films is the argon/oxygen ratio in the sputter gas and the substrate temperature, both during sputtering and during postannealing. After sputtering at 500 °C, SrTiO3 layers on Al2O3 substrates are partially crystalline. In this case the SrTiO3 lattice is enlarged compared to the bulk material. With increasing temperatures of postannealing, the crystallites grow and the lattice shrinks. In particular, films that are sputtered with a sputter gas containing oxygen show an additional phase of TiO2 when they are annealed at temperatures of more than 1000 °C. This paper shows for the first time results of the investigation of sputtered SrTiO3 films on Al2 O3 substrates as a function of oxygen partial pressure P(O2 ). The measurements performed on the sputtered films indicate semiconducting properties in agreement with the bulk material: At temperatures between 700 and 1100 °C the electrical conductivity of sputtered layers changes from p-type to n-type as the oxygen partial pressure decreases from 1 to 10−15 bar. The enthalpy of oxidation in the SrTiO3 films is 1.24 eV, and the energy gap is 3.30 eV. These values are in agreement with those reported for bulk materials.
Magnetic properties of nanoscale iron particles
Authors: Kerznizan Carl F. , Klabunde Kenneth J. , Sorensen Christopher M. , Hadjipanayis George C.
Published: July 2002

Nanoscale iron particles have been prepared by iron atom clustering in cold pentane. Particle growth was terminated by adsorption of oleic acid, yielding air stable particles ranging in size from 2 to 12 nm, that were amorphous according to XRD analysis. TEM analysis showed that the larger particles were prolate spheroids while the smaller ones were spherical. The as-prepared sample had a room-temperature saturation magnetization of Ms=12.3 emu/g and a coercivity of Hc=60 Oe. Following heat treatment under argon, the XRD results showed the following reaction scheme; oxidation to Fe3O4 at 360 °C and reduction to α-Fe at 520 °C (processes apparently due to adsorbed oleic acid). For the 520 °C treated sample, the room-temperature saturation magnetization and coercivity were Ms=200 emu/g and Hc=20 Oe, respectively. By pentane washing of the as-prepared sample, the smaller particles could be separated by filtration leaving the larger 8–12 nm particles on the filter. This powdered sample trapped on the filter had Ms=54.9 emu/g and Hc=60 Oe. Heat treatment of this sample also caused oxidation to Fe3O4 at 360 °C and an incomplete reduction to α-Fe and FeO at 520 °C. For the 520 °C treated sample, the recorded room-temperature saturation magnetization and coercivity were Ms=123.6 emu/g and Hc=20 Oe, respectively.
Activation analysis of dislocation nucleation from crack tip in α-Fe
Authors: Cheung Kin S. , Argon Ali S. , Yip Sidney
Published: July 2002

Guided by a recent molecular dynamics simulation study of mode I crack-tip environment in α-Fe using an embedded atom method potential, an activation analysis of the nucleation of a half-square dislocation loop from the tip of a tensile crack has been carried out based on the Rice and Thomson approach [Philos. Mag. 29, 78 (1974)]. The effects of elastic softening due to stress and temperature as well as the nonlinear stress distribution around the crack are introduced by incorporating the simulation results self-consistently into the analysis, reducing the previously overestimated activation barrier by a factor of over 15.
Hysteresis in the high pressure transformation of bcc- to hcp-iron
Authors: Taylor R. D. , Pasternak M. P. , Jeanloz R.
Published: July 2002

The α-Fe/ε-Fe phase transformation has been studied by the Mössbauer effect of 57Fe in a diamond anvil cell at hydrostatic pressures up to 24 GPa. The α/ε composition is easily derived as a function of intermediate pressure. For rising pressures the transition onset is about 9 GPa and half of the α-Fe is converted to ε-Fe by 15 GPa; for lowering pressures half of the ε-Fe is converted to α-Fe by 9 GPa. The transition pressures, widths, and hysteresis widths are compared to those obtained by other techniques.
Surface acoustic waves propagating along the grooves of a periodic grating
Authors: Mayer A. P. , Zierau W. , Maradudin A. A.
Published: July 2002

In this paper the propagation of surface acoustic waves along the grooves of a periodically corrugated surface is studied. It is shown that in addition to a surface acoustic wave of predominantly sagittal polarization, which corresponds to the Rayleigh wave, an additional surface wave exists which is of predominantly shear horizontal polarization. While the former remains localized at the surface for arbitrarily short wavelengths, the latter has a limited range of existence. The dispersion curves of these surface waves are calculated numerically for a shallow sawtooth grating.
Characteristics of silicon nitride deposited by plasma-enhanced chemical vapor deposition using a dual frequency radio-frequency source
Authors: Pearce C. W. , Fetcho R. F. , Gross M. D. , Koefer R. F. , Pudliner R. A.
Published: July 2002

We have studied the effect of plasma excitation frequency on the properties of plasma-enhanced chemical vapor deposition silicon nitride films. The use of two radio-frequency sources, one at 270 kHz and the other at 13.56 MHz, enabled us to vary film properties such as stress by altering the amount of power supplied by each source. The low-frequency excitation was seen to favor the formation of N—H bonds in the deposited film. The relative amounts of N—H and Si—H bonds in the film were found to determine many of the film properties such as stress, conduction and wet etch rate.
Barkhausen noise in ferromagnetic metallic glass Fe40Ni38Mo4B18
Authors: Kim H. C. , Lee K. H. , Kim C. G. , Hwang D. G.
Published: July 2002

The influence of heat treatment on conventional magnetic properties and Barkhausen noise were studied in ferromagnetic amorphous Fe40Ni38Mo4B18 during a half magnetization cycle for both amorphous and crystallized states. The coercivity, Hc, was low in the amorphous specimens and maximum as the crystallization starts, and decreased with further annealing. The differential permeability near to the Hc, μc, was high in the amorphous specimens and minimum at the onset of crystallization, then increased with annealing. The single peak behavior of the Barkhausen noise (BN) envelope was a common feature in both amorphous and crystallized specimens, and the variation of peak height with heat treatment was similar to that of μc. The magnetic anisotropy energy constant, Ku, was lowest when the crystallite started to nucleate in the amorphous specimen, and increased as the crystallite size grows. The difference between ribbon and transverse directions of BN characteristics such as BN energy, peak height of BN envelope, Hp, and corresponding field strength to Hp, respectively, was the lowest at the onset of crystallization, and increased with Ku. A model was proposed to predict the linear relationship between the magnetic anisotropy energy constant detected by torque measurement and the one evaluated by BN measurement.
A model that gives rise to effective dry friction in micromagnetics
Authors: Baltensperger W. , Helman J. S.
Published: July 2002

Since dissipation requires motion, the velocity-independent friction of macroscopic systems must result from quick motions of some of its components. A model of coupled spins, some of which have irregular anisotropy directions, presents velocity independent loss. During a collective motion of the spin ensemble, an irregular spin can fall into its potential minimum, from which it is subsequently dragged out again. The effect is pronounced for a range of the ratio between the anisotropy and interatomic exchange coupling constants which is typical for rare-earth ions.
Pyrite thin films: Improvements in their optical and electrical properties by annealing at different temperatures in a sulfur atmosphere
Authors: de las Heras C. , Ferrer I. J. , Sánchez C.
Published: July 2002

Iron pyrite thin films prepared by flash evaporation of pyrite powder have been annealed at different temperatures in a sulfur atmosphere. We present some results on the influence of the annealing temperature (from 250 to 450 °C) on the optical and electrical properties of three groups of samples with different thicknesses (≂0.3, 0.6, and 1 μm, respectively). Sulfuration temperature has a clear influence on the optical absorption and electrical resistivity of the films, with some differences in their behavior depending on the film thickness. In light of the available present knowledge of pyrite thin films, interpretation of the obtained results is difficult, it suggests that the shape of the optical absorption curves (and their absorption edge) at low photon energies is determined by the density of point defects, which decreases on increasing the annealing temperature. On the other hand, the electrical resistivity seems to be influenced by both the film grain size and point defect density.
Influence of irradiation conditions on plasma evolution in laser-surface interaction
Authors: Hermann J. , Boulmer-Leborgne C. , Dubreuil B. , Mihailescu I. N.
Published: July 2002

The plasma plume induced by pulsed CO2 laser irradiation of a Ti target at power densities up to 4×108 W cm−2 was studied by emission spectroscopy. Time- and space-resolved measurements were performed by varying laser intensity, laser temporal pulse shape, ambient gas pressure, and the nature of the ambient gas. Experimental results are discussed by comparison with usual models. We show that shock wave and plasma propagation depend critically on the ratio Ivap/Ii, Ivap being the intensity threshold for surface vaporization and Ii the plasma ignition threshold of the ambient gas. Spectroscopic diagnostics of the helium breakdown plasma show maximum values of electron temperature and electron density in the order of kTe∼10 eV and ne=1018 cm−3, respectively. The plasma cannot be described by local thermodynamic equilibrium modeling. Nevertheless, excited metal atoms appear to be in equilibrium with electrons, hence, they can be used like a probe to measure the electron temperature. In order to get information on the role of the plasma in the laser-surface interaction, Ti surfaces were investigated by microscopy after irradiation. Thus an enhanced momentum transfer from the plasma to the target due to the recoil pressure of the breakdown plasma could be evidenced.
Mathematical modeling of a free-burning arc in the presence of metal vapor
Authors: Gonzalez J. J. , Gleizes A. , Proulx P. , Boulos M.
Published: July 2002

The transfer of energy between the plasma and the iron anode and the evaporation of metal were taken into account in modeling a short free-burning arc in argon at atmospheric pressure. The presence of metal vapor in the plasma modifies the electrical conductivity and the radiated power and leads to arc cooling in the anode region. In return, the arc cooling modifies the rate of vaporization of the anode and thus the calculated concentration of iron vapor in the arc.
Surface photovoltage in undoped semi-insulating GaAs
Authors: Liu Qiang , Chen Chao , Ruda Harry
Published: July 2002

A theoretical model for the surface photovoltage effect (SPV) in undoped semi-insulating (SI) GaAs is presented. This model accounts for the large measured SPV signals for SI material compared with n-type material. The SPV effect in undoped SI GaAs is shown to originate predominantly from the large difference in mobilities between electrons and holes. The low dark conductivity of SI GaAs also plays an important role in determining the large measured signal. In this article experimental SPV data for undoped SI GaAs are presented and explained using the proposed model, providing a value for the ambipolar diffusion length of 1.4 μm. The native surface field contribution to the measured SPV signal is estimated experimentally and found to be negligible in undoped SI GaAs compared with the aforementioned effects.
Effects of Si incorporation on the structural change of a-BxSi1−x alloy films
Authors: Ong C. W. , Chik K. P. , Wong H. K.
Published: July 2002

Amorphous BxSi1−x films can be easily prepared by low-pressure chemical vapor deposition over the whole range of x from 0 to 1. In this article, the structural change of BxSi1−x films (0≤x≤1) was studied by x-ray diffraction and infrared (IR) absorption experiments. It was found that these two methods are complementary to each other. X-ray results showed that when x is decreased, there is a gradual transition from the a-B structure, through the SiB4 structure, to the amorphous silicon structure. The transition to a-Si structure is complete at a surprisingly high boron concentration of around 40 at. %. Infrared data also revealed an unexpected result—that the presence of boron suppresses the formation of Si—H types of bonds in high silicon content films, in strong contrast to films prepared by the glow discharge method. Another interesting feature of the IR absorption spectra is the predominance of an absorption band, related to the presence of boronlike icosahedral clusters, in all films with boron concentration to as low as 17.6 at. %. Such clusters may be responsible for the easy formation of an impurity band lying about 0.2 eV above the valence mobility edge, as observed by transport measurements of boron-doped a-Si films prepared by low-pressure chemical vapor deposition.
One-dimensional antiferromagnetic behavior in AVOPO4 (A=NH4, Na) prepared from hydrothermal conditions
Authors: O’Connor Charles J. , Soghomonian Victoria , Haushalter Robert C. , Wang Zhanwen , Zubieta Jon
Published: July 2002

Two vanadyl phosphates have recently been prepared in superheated aqueous solutions. These materials crystallize as infinite chains of corner sharing VO6 octahedra. The magnetic susceptibility of NaVOPO4 and NH4VOPO4 has been recorded over the 2–300 K temperature region. At higher temperatures, both compounds exhibit Curie–Weiss paramagnetism, where C=0.416 emu K/mole, θ=−31 K; C=0.417 emu K/mole, θ=−4.7 K, for NaVOPO4 and NH4VOPO4, respectively. At lower temperatures, there is a maximum in the temperature-dependent magnetic susceptibility of each compound. The data for the two compounds have been successfully analyzed following the Bonner–Fisher numerical calculations that describe the magnetic behavior of antiferromagnetically coupled S=1/2 one-dimensional chains. The results of the Bonner–Fisher analysis of the magnetic data are g=2.00, J/k=−18.6 for NaVOPO4; and g=2.1, J/k=−3.26 K, for NH4VOPO4. The theoretical analysis of the NH4VOPO4 analog also required the addition of a molecular field correction (zJ′/k=−2.1 K) to the Bonner–Fisher calculation in order to adequately fit the data. The trans crosslinking via phosphate bridges of the magnetic chains in NH4VOPO4 gives rise to the onset of long-range antiferromagnetic ordering at TAF=3.71 K.
Atomic carbon vapor as a diamond growth precursor in thermal plasmas
Authors: Yu Benjamin W. , Girshick Steven L.
Published: July 2002

A detailed surface chemistry mechanism is proposed for chemical vapor deposition of diamond films, which extends the growth-by-methyl mechanism proposed by Harris to treat any CHm radical, m=0–3, as a growth monomer. Numerical computations were performed in which the mechanism was coupled to a model for the boundary layer above the substrate, for conditions typical of diamond deposition in an atmospheric-pressure thermal plasma. The predicted linear growth rate increases strongly as the boundary layer thickness δ is decreased, and the results indicate a strong dependence of the diamond growth chemistry on δ. For relatively thick boundary layers (modest velocities of the reactant jet) growth is dominated by CH3. For very thin boundary layers (high velocities) the model predicts that growth is dominated by C. For the transition region where C and CH3 each contribute about 40% to growth, CH2 also contributes about 17%. The carbon conversion efficiency is also predicted to peak in the transition region, and drops sharply for very thin boundary layers.
Impedance of porous Si
Authors: Shen Wu-Mian , Tomkiewicz Micha , Lévy-Clément Claude
Published: July 2002

The impedance of photoelectrochemically etched n-Si was measured in a liquid junction made of a methanolic solution of oxidized and reduced dimethylferrocene. The results show that both the equivalent circuit and each of the individual elements are almost identical with those of the smooth Si that was used as a substrate. These results were interpreted in terms of a depletion layer model in which the space-charge layer is present only at the bottom of the macropores and at the interface between the electrolyte and the smooth part of the substrate. The poles that separate the pores are completely depleted of majority carriers. The nanoporous layer that is deposited on top of the macropores and can be removed by KOH is completely transparent to the impedance measurements.
Epitaxial iridium silicide formation during deposition of Ir on Si(100) at high temperature under ultrahigh vacuum
Authors: Chung C. K. , Hwang J.
Published: July 2002

The iridium silicide formation has been examined by depositing Ir on Si(100) at different substrate temperatures under ultrahigh vacuum. An epitaxial Ir3Si4 film was formed when an Ir film of 100 Å was directly deposited on Si(100) at 475 °C and annealed at the same temperature for 1.5 h. The Ir3Si4 was not observed in the conventional annealing process at 475 °C. In contrast, a polycrystalline IrSi film was formed after deposition of an Ir film of 100 Å on Si(100) at room temperature and followed by annealing at 475 °C for 1.5 h under ultrahigh vacuum. Formation of the epitaxial Ir3Si4 on Si(100) is attributed to the interfacial energy of the Ir3Si4/Si(100) interface.
Aligned two-phase magnets: Permanent magnetism of the future? (invited)
Authors: Skomski R.
Published: July 2002

Micromagnetic calculations are used to investigate coercivity and energy products of magnets consisting of an aligned hard-magnetic skeleton phase and a soft-magnetic phase with high saturation magnetization. Compared to the present-day theoretical limit of 516 kJ/m3 for single-phase Nd2Fe14B, the energy product in suitable nanostructured Sm2Fe17N3/Fe65Co35 composites is predicted to be as high as 1090 kJ/m3. The influence of the skeleton’s texture and shape is discussed, and aligned nanocrystalline two-phase magnets are compared with remanence-enhanced isotropic magnets. In particular, it is shown how the nucleation-based analytical approach breaks down in the isotropic limit. Finally, we outline conceivable processing methods and discuss potential applications of ‘‘megajoule’’ magnets.
Dielectric relaxation in amorphous thin films of SrTiO3 at elevated temperatures
Authors: Morii K. , Kawano H. , Fujii I. , Matsui T. , Nakayama Y.
Published: July 2002

Temperature and frequency dispersion of dielectric permittivity was investigated on thin amorphous films of SrTiO3 prepared by a sputtering method using neutralized argon-ion beams. The amorphous SrTiO3 films deposited on glass substrates exhibited a marked dielectric relaxation at temperatures 500–800 K in a frequency range 0.1–50 kHz. This behavior was explained based on a dipolar relaxation of the Cole–Cole type with the static dielectric constant ε′s≂380, the constant at high frequency ε′∞≂35, and the distribution parameter of the relaxation time β≂0.8. The analysis of the temperature dependence of relaxation time gave the activation energy for the relaxation of about 1.08 eV and the characteristic relaxation time of the order of 10−12 s. In the temperature range where strong relaxation occurred, a semiconductor-type conduction having the activation energy of about 0.84 eV became dominant. A correlation between the mechanisms of the dielectric relaxation and the thermally activated motions of ionized defects in the amorphous structure is discussed.
Electrical characterization of Al/Si ohmic contacts to heavily boron doped polycrystalline diamond films
Authors: Werner M. , Johnston C. , Chalker P. R. , Romani S. , Buckley-Golder I. M.
Published: July 2002

Mesa etched transmission line model (TLM) test structures with different contact lengths have been fabricated on heavily boron doped polycrystalline diamond films. The behavior of the contact and contact end resistance measurements can be fully explained using the TLM. No influence of the grain size on the contact resistivity has been observed. High surface boron doping concentrations led to low contact resistivities, in agreement with numerical calculations. Annealing of Al/Si–diamond contacts at 450 °C in N2 leads to lower contact resistivities due the formation of SiC at the metal–diamond interface. The temperature dependence of the specific contact resistivity can be described well with a tunneling model before annealing. After annealing no useful fit is possible, indicative of the fact that the SiC interface layer acts as defect layer.