bluuebunny/crossref_metadata_2025 huggingface dataset:
Puddle-skimming: An efficient sampling of multidimensional configuration space
Authors: Rahman Jay A. , Tully John C.
Published: July 2002

We examine the effectiveness of a simple method for surmounting energy barriers and enhancing the exploration of configuration space in Monte Carlo (MC) and molecular dynamics (MD) simulations. Proposed previously for treating surface diffusion [M. M. Steiner, P.-A. Genilloud, and J. W. Wilkins, Phys. Rev. B 57, 10236 (1998)], the method has widespread applicability and is particularly advantageous for systems with potential energy landscapes whose features are not known a priori. The algorithm requires selection of a single parameter, a “boost energy” EB. The MC or MD simulation is carried out on an effective potential energy function that is equal to the true potential energy when it is greater than EB, but is equal to EB otherwise. Since the effective potential energy is, therefore, never less than EB, deep energy minima are removed analogous to a rough landscape that has been flooded with water. The bias introduced by altering the potential energy function in this way is easily and rigorously removed “on-the-fly.” We test the method with a MD simulation of the equilibrium populations of conformations of n-pentane. The method recovers the canonical equilibrium distribution with dramatically increased sampling efficiency and modest additional computational overhead, over a range of temperatures. In cases for which the potential energy function can be written as a sum of terms, the energy boost can be applied to the selected terms rather than to the entire potential energy function. We illustrate this by application to the dihedral angle term only of the empirical n-pentane potential energy function and show that this further enhances sampling efficiency. The simple nature of this algorithm allows it to be readily scaled to high-dimensional systems. We discuss the prognosis for applying this method to more complex systems such as liquids and macromolecules.
A multiquadrupole tandem mass spectrometer for the study of ion/surface collision processes
Authors: Ouyang Zheng , Grill Verena , Alvarez Jormarie , Doerge Christopher H. , Gianelli Luca , Thomas Peter , Rohrs Henry W. , Cooks R. Graham
Published: July 2002

A compact tandem mass spectrometer containing two quadrupole mass analyzers (Q), two octapole ion guides (o) and a static quadrupole ion beam-bending lens (q) (QoqoQ configuration) has been built for the study of ion/surface collisions. Positive or negative ions are generated by either electron impact ionization or chemical ionization, selected by mass-to-charge ratio using the first quadrupole mass analyzer, guided by the first octapole ion guide, bent by a static quadrupole lens through 90°, and then focused onto a surface at an angle of 45° with reference to the surface normal. The secondary ions generated from collisions with the target surface are collected at an emergent angle of 45°, guided by the second octapole ion guide, mass-analyzed by the second quadrupole mass analyzer, and finally detected. The collision energy can be varied from 0 to 500 eV. The primary ions can also be made to pass straight through the quadrupole bending lens and then examined by an electron multiplier prior to surface collisions. The surface can be introduced into the ion/surface collision region without venting the main chamber. The capabilities of the QoqoQ instrument are demonstrated by a variety of ion/surface collisional processes in the 10s of eV laboratory translational energy range. These include surface-induced dissociation, charge permutation, surface analysis via chemical sputtering, and selective surface modification via ion/surface reactions.
What is the shape of pores in natural rocks?
Authors: Chen Quan , Song Y.-Q.
Published: July 2002

The geometry of the internal space of porous media determines transport properties at all scales. We demonstrate an experimental scheme to quantify the body-to-throat ratio and the shape of the pores through direct measurements of geometrical quantities. We used a constant-rate mercury injection measurement to obtain the throat size and the pore-body volume, and an NMR method (DDIF) to obtain the pore size distribution in a Berea sandstone. This approach uniquely identifies the pore throats and bodies and enables a quantification of their sizes. Our results show that the pore bodies in Berea sandstone is likely to be described by a very much elongated cavity.
Light-triggered molecular devices based on photochromic oligothiophene substituted chromenes
Authors: Yassar A. , Garnier F. , Jaafari H. , Rebière-Galy N. , Frigoli M. , Moustrou C. , Samat A. , Guglielmetti R.
Published: July 2002

An original concept for the realization of a molecular photoswitch is proposed, based on photochrochromic oligothiophene-substituted chromenes. When optically excited, these compounds undergo a structural change passing from a neutral state (closed form) to a strongly polarized one (open form). This photochromism process is also accompanied by a large increase in the electrical conductivity.
Chaos and irreversibility in simple model systems
Authors: Hoover Wm. G. , Posch Harald A.
Published: July 2002

The multifractal link between chaotic time-reversible mechanics and thermodynamic irreversibility is illustrated for three simple chaotic model systems: the Baker Map, the Galton Board, and many-body color conductivity. By scaling time, or the momenta, or the driving forces, it can be shown that the dissipative nature of the three thermostated model systems has analogs in conservative Hamiltonian and Lagrangian mechanics. Links between the microscopic nonequilibrium Lyapunov spectra and macroscopic thermodynamic dissipation are also pointed out.
Texture and morphology of sputtered Cr thin films
Authors: Lee Hyung J.
Published: July 2002

The crystal texture and microstructure of sputtered Cr thin films deposited on various substrates using a dc magnetron were investigated. The characterization of Cr films was carried out by an x-ray technique for texture, scanning electron microscopy and transmission electron microscopy for structure and morphology, and scanning Auger microprobe for chemical distribution of sputtered films. The sputtered Cr thin films were found to have very strong [110] preferred orientation which was independent of the substrate. Very straight columnar structures were observed with domed tops which increased in width with deposition thickness. The columnar width near the surface was found to range from 300 to 2000 Å. The crystal grain size also increased with film thickness. The correlation between the physical and crystal structure and the texture development is discussed.
Enhanced ion stopping powers in high-temperature targets
Authors: Olsen J. N. , Mehlhorn T. A. , Maenchen J. , Johnson D. J.
Published: July 2002

Light ions deposit their energy in target materials by interaction with bound and free electrons. As the target heats toward inertial confinement fusion temperatures a progression of ionization states will be encountered. The stopping power of each ion created in this process will depend upon details of the respective bound electron states. In general, the net ion stopping power will increase compared to cold matter due to the free electron contribution. We report an experimental and theoretical study of enhanced ion stopping powers in targets heated by 0.5–1.4 TW/cm2 proton beams. The experiments were performed on the Proto-I accelerator with aluminum and nickel foil targets. The theoretical effort incorporated free and bound electron stopping terms in hydrocode simulations of the target response. At these intensities we observe and calculate stopping power enhancements of 100% for aluminum and 50% for nickel.
Secondary grain growth in thin films of semiconductors: Theoretical aspects
Authors: Thompson C. V.
Published: July 2002

Secondary grain growth in thin films can lead to grain sizes much greater than the film thickness. Surface energy anisotropy often provides an important fraction of the driving force for secondary grain growth, especially in the early stages of growth. Surface-energy-driven secondary grain growth leads to the development of large grains with restricted crystallographic textures. A model is presented for growth of secondary grains into a uniform matrix of columnar normal grains. The model indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temperature. While final secondary grain sizes will decrease with film thickness, their growth rates will increase. The final secondary grain sizes and orientations will be strongly affected by grain sizes and orientations in the initial film. The models presented here provide analytical tools for experimental study of secondary grain growth in thin films. They will be used in forthcoming publications describing experimental study of secondary grain growth in silicon and germanium.
Gas heating effects in hydrogen thyratron discharges
Authors: Penetrante B. M. , Kunhardt E. E.
Published: July 2002

A theoretical study of the effects of gas heating in hydrogen thyratron discharges is presented. Coulomb collisions between electrons and ions are identified to contribute predominantly to gas heating. Time-dependent numerical solutions for the gas temperature, as well as the electron and chemical kinetics are obtained. It is shown that the electron temperature can be significantly higher than that calculated using a cold gas assumption. A mechanism is proposed by which quenching can occur due to the onset of a gas temperature maximum for which the required current during the conduction phase cannot be maintained in the grid aperture region. The time it takes for the gas temperature to reach the quenching point is presented as a function of the current density.
Cluster-beam deposition of thin films: A molecular dynamics simulation
Authors: Müller Karl-Heinz
Published: July 2002

The time evolution of thin-film growth by ion cluster-beam deposition is studied using a two-dimensional molecular dynamics simulation in which it is assumed that the atomic interactions are of the Lennard–Jones form. The time-dependent local heating, atomic rearrangements, relaxation, and recrystallization of a cluster, impinging on a perfect film surface, is investigated at different cluster kinetic energies where the cluster size corresponds to 700 atoms in three dimensions. The evolving film microstructure which results when several clusters successively approach the surface is modeled. Neutral clusters which arrive with nozzle ejection velocity are found to remain almost intact during impact, producing a porous polycrystalline film. For ion cluster kinetic energy per atom in the order of the bond strength, atomic rearrangements, local heating, local melting, and succeeding recrystallization produces almost defect-free, homoepitaxial films which are densely packed.
Buried transverse-junction stripe laser for optoelectronic-integrated circuits
Authors: Ohta Jun , Kuroda Ken’ichi , Mitsunaga Kazumasa , Kyuma Kazuo , Hamanaka Kouichi , Nakayama Takashi
Published: July 2002

A buried transverse-junction stripe (TJS) laser, a suitable laser diode for integration, has been fabricated using molecular-beam epitaxial growth. The fundamental characteristics of the laser were as good as those of a conventional TJS laser. We have also demonstrated a fabrication process for the laser that is compatible with that for metal-semiconductor field-effect transistors (MESFETs) to be monolithically integrated. The characteristics of the MESFETs fabricated on substrates processed for the laser were the same as those fabricated on virgin substrates when such processed substrates were etched 2 μm deep.
An investigation of the thermal stability of the interfacial oxide in polycrystalline silicon emitter bipolar transistors by comparing device results with high-resolution electron microscopy observations
Authors: Wolstenholme G. R. , Jorgensen N. , Ashburn P. , Booker G. R.
Published: July 2002

A comparison is made between the results of high-resolution electron microscope observations and the electrical characteristics of polycrystalline silicon emitter bipolar transistors. Devices are fabricated with and without a deliberately grown interfacial oxide layer, and the thermal stability of this oxide layer is investigated by carrying out a preanneal at temperatures between 800 and 1100 °C after polysilicon deposition, but prior to emitter implant and 900 °C drive-in. The electron microscope observations show that the deliberately grown interfacial oxide is of uniform thickness ∼14 Å, but breaks up when annealed at ∼950 °C and above, with ‘‘balling-up’’ occurring at ∼1100 °C. This correlates with a transistor gain that decreases from ∼1400 to ∼40. The electron microscopy also shows that a thin interfacial oxide layer is present even when not deliberately grown. This oxide breaks up when annealed at ∼900 °C and above, with ‘‘balling-up’’ occurring at ∼1000 °C. This correlates with a transistor gain that decreases from ∼240 to ∼50. Calculations of the effect that such interfacial oxide layers will have on the characteristics of polysilicon emitter bipolar transistors are made, and these predictions correlate well with the measured characteristics.
The inverse blackbody radiation problem: A regularization solution
Authors: Sun Xiaoguang , Jaggard Dwight L.
Published: July 2002

The inverse blackbody radiation problem is the problem of determining the temperature distribution of a thermal radiator given its total radiated power spectum. Although several inversion methods have been developed, they have problems in practical implementation because of the necessity of Laplace inversion or due to their inherent ill-posedness. Here a numerical implementation which addresses these problems using Tikhonov’s regularization technique is demonstrated. This method overcomes the problem of ill-posedness in practical implementations and yet yields accurate results. Examples shown here include a robust reconstruction in the presence of noise corrupted data.
An alternating-gradient magnetometer (invited)
Authors: Flanders P. J.
Published: July 2002

This paper describes a type of vibrating-sample magnetometer capable of sensitivity exceeding 10−8 emu. The instrument is 1000 times more sensitive than a conventional VSM with comparable working space, and is much quicker to use than a SQUID magnetometer, which generates point-by-point data. The magnetic sample is mounted on the end of a cantilevered rod that incorporates a piezoelectric element. The sample is magnetized by a dc field (variable in magnitude), and is simultaneously subjected to a small alternating field gradient. The alternating field gradient exerts an alternating force on the sample, proportional to the magnitude of the field gradient and to the magnetic moment of the sample. The resulting deflection of the cantilever rod is measured by the voltage output of the piezoelectric element. By operating at or near a mechanical resonance frequency of the cantilever, the output signal is greatly amplified. In practice, the operating frequency is 100–1000 Hz, with mechanical Q values of 25–250. Mechanical and acoustic noise in the environment limits the sensitivity. Measurements have been made with a signal-to-noise ratio of about 500 on a 25-μm sphere with a moment of 3.7×10−6 emu; this corresponds to a sensitivity of at least 10−8 emu. A complete hysteresis loop over ±10 kOe can be made in about 100 s. Measurements have been made over a temperature range from 77 to 400 K.
A general method for flux confinement in permanent-magnet structures
Authors: Abele M. G. , Leupold H. A.
Published: July 2002

This paper presents a general method for the design of permanent-magnet structures that provide uniform magnetic fields of up to several teslas in cavities of arbitrary shape. The mathematical criteria that govern confinement of flux within both yoked and yokeless magnetic structures are defined. The design procedure affords an exact determination of the distribution and magnetization of the magnetic material required for generation and confinement of the desired field within a cavity of given geometry. The method is applicable to magnets composed of ferrites and high-energy product materials for which the analysis can be based on a linear approximation to the demagnetization curve.
Application of nuclear magnetic resonance pore structure analysis to porous silica glass
Authors: D’Orazio F. , Tarczon J.C. , Halperin W. P. , Eguchi K. , Mizusaki T.
Published: July 2002

The internal pore structure of a series of porous silica glasses has been investigated using nuclear magnetic resonance (NMR). Longitudinal and transverse relaxation of protons in deionized water confined to the pores of the samples has been measured and interpreted to provide pore size distributions. Two sets of leached, phase-separated porous glasses were used: the first nominally bimodal, the second with a more uniform pore space with pore sizes varying from 32 to 1350 Å. Our results provide a stringent test of NMR relaxation as a method for pore size analysis. NMR relaxation experiments were performed on one sample as a function of the amount of water imbibed confirming the validity of the theoretical model used to interpret the magnetic resonance data.
Damage analysis and optical properties of Ge-implanted silicon
Authors: Borghesi A. , Piaggi A. , Stella A. , Guizzetti G. , Bisero D. , Queirolo G.
Published: July 2002

The damage produced in crystalline silicon by Ge-ion implantation at 130 keV is studied as a function of fluence. Rutherford backscattering measurements in channeling configuration and reflectivity measurements in the 1.5–6.5 eV energy range are presented and discussed. The results obtained by both techniques are in agreement on the determination of the amorphization threshold. The dielectric functions, obtained by the Kramers–Kronig analysis and checked through ellipsometric measurements at one selected wavelength, are well reproduced by finite sets of classical harmonic oscillators. It is possible to describe the behavior of the oscillator parameters through analytical functions of a single variable related to the amount of damage. In a representative case, the damage depth profile is obtained after the removal of controlled amounts of silicon by anodic oxidation, and good agreement is found with the Rutherford backscattering results.
Preparation of alternating MoS2 and WS2 single layers to form a new MoWS4 structure
Authors: Miremadi Bijan K. , Morrison S. Roy
Published: July 2002

The layer compounds WS2 and MoS2 were exfoliated into single layers suspended in solution, and conditions were adjusted to produce a new homogeneous structure that is believed to be built of alternating layers of WS2 and MoS2 recrystallized in a 1T structure. Evidence for this is provided by the observation of a new hexagonal structure with expanded lattice spacings relative to the identical lattice spacings of the original materials and the fact that if the Mo:W ratio differs from 1:1, the x-ray diffraction shows that some of the original WS2 or MoS2 phase is present. The catalytic activity of the alternating layer material is extremely high for the methanation reaction. It is observed and concluded that the alternating material is made up of fine particles, and a model of flocculation is presented to explain why in this material the particles are so fine.
The bistability of the surface-stabilized ferroelectric liquid-crystal effect in electrically reoriented chevron structures
Authors: Hartmann W. J. A. M. , Luyckx-Smolders A. M. M.
Published: July 2002

Low-frequency electric field treatment irreversibly changes the chevron structure of a surface-stabilized ferroelectric liquid-crystal sample: the angle between the smectic layers and the surface normal decreases. As a result different textures appear, and a pronounced effect on the switching behavior is observed. The switching angle and the necessary pulse amplitudes at constant pulse width in order to obtain switching between both stable states have increased. The required reorientation voltages decrease with rising spontaneous polarization. This reorientation process is strongly influenced by the rubbing treatment of the alignment layer. The anchoring properties of the alignment layer seem to be directly responsible for the appearance of defects, for the initial switching angle, for the electric field strength necessary to reorient the chevron structure, and for the pulse amplitudes required to switch between both stable states.
The accuracy of heavy-ion mass measurements using time of flight-ion cyclotron resonance in a Penning trap
Authors: Bollen G. , Moore R. B. , Savard G. , Stolzenberg H.
Published: July 2002

Ion motion in a Penning trap and the electrical signals it can produce have been analyzed for the purpose of identifying the important causes of uncertainty in high-accuracy mass measurements of heavy ions. The role of the azimuthal quadrupole electric field in signal pickup, and its effects on ion motion at the sum frequency of the cyclotron and magnetron motions, have been identified. A useful scheme for calculating the signal strength and strength of the interaction between an applied field and the ion motion has been developed. The important sources of uncertainty in using the sum frequency of the cyclotron and magnetron motions for determining the ion mass are discussed. Particular application is made to the case of cyclotron resonance detection by observation of the time of flight of ejected ions.
Thermal conductivity of random media and regular fractals
Authors: Thovert J. F. , Wary F. , Adler P. M.
Published: July 2002

The Laplace equation can be solved in any two- and three-dimensional porous medium by means of a vectorized numerical code. It is applied to several structures such as random media derived from site percolation; close to the percolation threshold, the critical exponents are found to be very close to the ones corresponding to networks; the results are usefully compared to previous variational upper bounds and to the prediction of an approximate space renormalization. Media with double porosity such as catalyst pellets are also addressed. Finally the conductivity of most fractals is shown to follow an Archie’s law in the limit of large generation numbers; the exponents of the power laws can be retrieved by various renormalization arguments.
Laser heating effects in the characterization of carbon fibers by Raman spectroscopy
Authors: Ager Joel W. , Veirs D. Kirk , Shamir Jacob , Rosenblatt Gerd M.
Published: July 2002

The first-order Raman spectra of individual 8-μm-diam PAN-derived carbon fibers which had been annealed at five temperatures from 1700 to 2800 °C are measured as a function of incident laser power from 1 to 140 mW. In all fibers studied, the Raman frequencies of the graphite G band at ca. 1580 cm−1 and the disordered-induced D band at ca. 1360 cm−1 shift to lower frequency with increasing laser power. The largest shifts observed before the fiber is physically damaged are about 13 cm−1 at a laser power of 30 mW. The band positions decrease further at higher laser power, up to a maximum of about 20 cm−1 at 40 mW, and at powers above 30 mW the linewidths and the ID/IG intensity ratio change irreversibly as the fiber begins to erode. The irreversible changes extend several hundred μm away from the 2-μm spot illuminated by the laser. The effects are attributed to laser heating. To quantify the degree of laser heating the temperature dependence of the Raman shift (G band) in pure bulk, highly oriented pyrolytic graphite (HOPG) is measured. To study the heating effect in more detail, a Raman imaging experiment is carried out in which a 0.1-mm spot on a fiber is heated by a focused laser while the fiber is illuminated for Raman characterization along a 1.6 mm length by a low power probe laser. The spatially resolved Raman shifts obtained in this way are combined with the HOPG frequency-temperature calibration to obtain the in situ temperature profile of the laser heated fiber. The measured temperature profile along the fiber is in excellent agreement with a simple convective heat-transfer model. Assuming that the temperature dependence of the G-band position is the same in the fibers as in bulk graphite, the present experiments show that a laser power of 30 mW heats an 8 μm fiber to 330 °C and that above 330 °C irreversible changes are produced by erosion of the fiber. Unperturbed room-temperature Raman frequencies for the five groups of fibers are obtained by extrapolating to zero laser power.
Temperature dependence of the dielectric function of Si
Authors: Aoki Toshiaki , Adachi Sadao
Published: July 2002

A method is described for calculating the real (ε1) and imaginary (ε2) parts of the dielectric function of Si for temperatures between 30 and 793 K and for photon energies ranging from 1.7 to 5.7 eV. The model reveals distinct structures at energies of the E1 [two-dimensional (2D) M0 critical point (CP)], E2 [a mixture of damped harmonic oscillator (DHO) and 2D-M2 CP], E′0, and E′1 (DHO). The Kramers–Kronig transformation is employed in developing the 2D-M2 CP model. Since the excitonic effects profoundly modify the E1-gap CP structure at low temperatures, the model also accounts for the excitonic effects at this CP. Results are in satisfactory agreement with the experimental information over the entire range of photon energies. The temperature dependence of the strength and broadening parameters at each CP are also given and discussed.
Particle thermophoresis in low pressure glow discharges
Authors: Jellum G. M. , Daugherty J. E. , Graves D. B.
Published: July 2002

Particles in glow discharges are charged negatively and are therefore often suspended electrostatically since the plasma potential is usually more positive than the potential of surfaces bounding the plasma. However, in addition to responding to electrostatic and gravitational forces, particles are sensitive to forces associated with gradients in neutral gas temperature. A series of experiments were performed varying the temperature of water circulated through planar, parallel electrodes. When particles are present and are suspended in the discharge, they move away from a heated electrode and towards a cooled electrode. In the absence of particles, the discharge optical emission spatial profile and electrical characteristics did not change significantly for any combination of electrode heating or cooling. Particles remaining in the interelectrode gap after the discharge is extinguished appear to be uncharged. With particles present, the discharge electrical properties and time- and space-resolved optical emission take on characteristics associated with discharges in electron attaching gases. We report on a simple particle-scavenging arrangement based on thermophoresis that can be used to remove particles from a discharge.
Elastic scattering of electrons by free and bound zinc and cadmium atoms
Authors: McGarrah D. B. , Antolak A. J. , Williamson W.
Published: July 2002

Elastic differential and total cross sections are presented for electron scattering by zinc and cadmium using the optical model. The cross sections are determined for scattering by free atoms and by atoms bound in a solid. The phase shifts are obtained from solutions to the reduced radial wave equation based on an optical potential consisting of static, polarization, and exchange terms. Electron energies from 12.5 to 200 eV are considered. Differences in the total cross section from using two different forms of the exchange potential are examined. A simple parametric fit of the total cross section which is suitable for Monte Carlo applications is given.
High-energy spin waves in bcc iron
Authors: Perring T. G. , Boothroyd A. T. , Paul D. McK. , Taylor A. D. , Osborn R. , Newport R. J. , Blackman J. A. , Mook H. A.
Published: July 2002

The dispersion relation of the spin-wave excitations in bcc iron has been studied by neutron inelastic scattering at the spallation neutron source ISIS. Magnetic intensity was followed up to 550 meV along the [100] direction. The general form of the dispersion curve is in qualitative agreement with that calculated from a spin-polarized band model, and in particular the prediction of propagating modes extending above 300 meV has been confirmed.
On the temperature correction to the ruby pressure scale
Authors: Vos Willem L. , Schouten Jan A.
Published: July 2002

Since there is much confusion about the temperature correction to the ruby high-pressure scale, we have measured the temperature dependence of the ruby R1 and R2 lines for several ruby samples in the range 150–400 K. The results show a variation of 0.3 Å, corresponding to a pressure of 1 kbar. The results do not depend on chromium concentration. We find that the error in pressure can be reduced to about 0.4 kbar by measuring the temperature dependence of both the R1 and R2 lines of the ruby sample that are used in the high-pressure experiment.
Structural and electrical damage induced by high-energy heavy ions in SiO2/Si structures
Authors: Busch M. C. , Slaoui A. , Siffert P. , Dooryhee E. , Toulemonde M.
Published: July 2002

The structural and electrical properties of SiO2/Si structures irradiated by high-energy (≳0.5 GeV) Xe and Ni ions have been investigated. Structural analysis of the irradiated SiO2 films, performed with infrared spectroscopy, points to atomic displacements and broken and strained Si—O bonds induced by the irradiation. Using ir data, the damage cross section of the Xe and Ni ions has been deduced. The values are of about 8×10−13 and 6×10−14 cm2 for, respectively, 762 MeV Xe and 551 MeV Ni ions. Electrical measurements of irradiated SiO2/Si structures show an increase of the interface-state density Dit and of the oxide trapped-charge density N0t with the ion fluence. These results are compared with defects induced by heavy-ion irradiation in bulk silica and by light particle radiation in silicon dioxide. Electrically active point defects have been detected in irradiated silicon and are associated with vacancy complexes.
A Mössbauer effect study of the microscopic magnetic properties of Nd2Fe17 and Nd2Fe17N2.6
Authors: Long Gary J. , Pringle O. A. , Grandjean F. , Buschow K. H. J.
Published: July 2002

The Mössbauer spectra of Nd2Fe17 and Nd2Fe17N2.6 have been measured at various temperatures between 78 and 295 K and analyzed with a model that is based on the Wigner–Seitz cell environment of each iron site, the orientation of the magnetization, and the magnetic moments as determined from either neutron-diffraction measurements or band-structure calculations. Upon nitrogenation of Nd2Fe17, the weighted average isomer shift increases from 0.060 to 0.164 mm/s and further the isomer shifts of the four crystallographically distinct sites increase in agreement with the increase observed in their Wigner–Seitz cell volumes and the presence of a nitrogen near neighbor for two of the sites. Upon nitrogenation of Nd2Fe17, the weighted average hyperfine field increases from 292.3 to 333.8 kOe. However, the increases on the 6c and 18f sites are much smaller than those observed on the 9d and 18h sites; changes which are in agreement with calculated changes in the magnetic moments upon nitrogenation of Nd2Fe17, Gd2Fe17, and Y2Fe17. The temperature dependence of the isomer shifts indicates an increase in covalency upon the formation of the nitride.
A high-performance hysteresis loop tracer
Authors: Kulik Tadeusz , Savage Howard T. , Hernando Antonio
Published: July 2002

A high-performance and inexpensive hysteresis loop tracer has been developed to measure quasistatic (0.02 Hz or less) hysteresis loops of soft ferromagnetic materials. It was applied very successfully to measure straight pieces of amorphous and nanocrystalline ribbons and amorphous wires. Especially high-magnetic-field resolution is required when nanocrystalline ferromagnets and amorphous wires are measured. Nanocrystalline materials exhibit very low coercivity (Hc=0.1–0.5 A/m). The error of Hc measurement using this tracer does not exceed 0.05 A/m even though the amorphous wires have very small cross section (0.008 mm2). The examples of hysteresis loops measured at low (50 A/m) and high magnetic field (14 kA/m) are presented. The apparatus consists of an IBM-compatible computer equipped with 12 bit analog-to-digital and digital-to-analog converters, bipolar power supply, fluxmeter, solenoid and a pickup coil connected to a compensation coil. This equipment is free of 50 Hz noise, a significant problem in the performance of low-frequency loop tracers. The software was developed to enable measurement and immediate display of the M-H hysteresis loop. Previous loops also can be displayed and printed. Calibration of the setup is also possible. In the case of straight samples the measurements are followed by calculation of demagnetization factor Nd(calc.) using the ellipsoidal approximation of the sample shape. It was found that the experimental value of Nd is 30%–40% of the calculated value Nd(calc.) for the ribbons studied. Higher values of Nd correspond to the thicker ribbons where better agreement was obtained.
Phase and amplitude characteristics of nearly degenerate four-wave mixing in Fabry–Perot semiconductor lasers
Authors: Simpson T. B. , Liu J. M.
Published: July 2002

The phase and amplitude characteristics of nearly degenerate four-wave mixing in a semiconductor laser are studied. The experimental data can be quantitatively understood with a theoretical analysis in terms of the phase and amplitude modulations caused by mixing between the optical fields in the injected laser.
Thermodynamic properties of metastable Ag-Cu alloys
Authors: Najafabadi R. , Srolovitz D. J. , Ma E. , Atzmon M.
Published: July 2002

The enthalpies of formation of metastable fcc Ag-Cu solid solutions, produced by ball milling of elemental powders, were determined by differential scanning calorimetry. Experimental thermodynamic data for these metastable alloys and for the equilibrium phases are compared with both calculation of phase diagrams (CALPHAD) and atomistic simulation predictions. The atomistic simulations were performed using the free-energy minimization method (FEMM). The FEMM determination of the equilibrium Ag-Cu phase diagram and the enthalpy of formation and lattice parameters of the metastable solid solutions are in good agreement with the experimental measurements. CALPHAD calculations made in the same metastable regime, however, significantly overestimate the enthalpy of formation. Thus, the FEMM is a viable alternative approach for the calculation of thermodynamic properties of equilibrium and metastable phases, provided reliable interatomic potentials are available. The FEMM is also capable of determining such properties as the lattice parameter which are not available from CALPHAD calculations.
Detection of particulates in a rf plasma by laser evaporation and subsequent discharge formation
Authors: Stoffels W. W. , Stoffels E. , Kroesen G. M. W. , De Hoog F. J.
Published: July 2002

Nd:YAG-laser-induced evaporation of particulates formed in an Ar-CCl2F2 rf plasma and the subsequent discharge in the vapor have been investigated in situ by means of optical emission spectroscopy. The estimated threshold for discharge formation is 5×106 W/cm2. The maximum laser-induced emission intensity is observed when the laser is operated in the long-pulse mode (about 200 μs pulse duration) at the fundamental frequency. The wavelength integrated intensity of this continuum emission has been compared with light scattering intensity at the same laser energy. It has been found that the laser-induced emission intensity can be more than ten times higher than the scattering intensity, especially for particulates with a diameter much smaller than the wavelength of the laser. Therefore, this effect provides a sensitive particulate detection method.
Pressure dependence of Sm:YAG fluorescence to 50 GPa: A new calibration as a high pressure scale
Authors: Yusa Hitoshi , Yagi Takehiko , Arashi Haruo
Published: July 2002

The pressure-induced wavelength shift of laser excited fluorescence from samarium doped yttrium aluminum garnet (Sm:YAG) was measured up to 50 GPa at room temperature against the R1 shift of ruby. The intense peaks between 618 and 611 nm were analyzed as triplet, Y1, Y2, and Y3, by curve fitting analysis using the Lorentzian function. The Y1 (λ0=618 nm) peak was the strongest peak up to 40 GPa, while the Y2 (λ0=616 nm) peak became the strongest above it. Based upon the pressure dependency of the Y1 and Y2 peaks, the following pressure scales are proposed: P=3.25×Δλ(Y1)–0.015×Δλ2(Y1); P=4.03×Δλ(Y2)–0.055×Δλ2(Y2). As the temperature dependence of the fluorescence shift of Sm:YAG is negligibly small, these pressure scales will be useful for high pressure and high temperature experiments using a diamond anvil cell.
Domain configurations due to multiple misfit relaxation mechanisms in epitaxial ferroelectric thin films. I. Theory
Authors: Speck J. S. , Pompe W.
Published: July 2002

Possible mechanisms for strain relaxation in ferroelectric thin films are developed. The models are applicable to tetragonal thin film ferroelectrics grown epitaxially on (001) cubic single crystal substrates. We assume growth at temperatures in excess of the Curie temperature (Tc). The extent of strain accommodation by misfit dislocations is considered at the growth temperature (Tg). On cooling to Tc, further misfit dislocation generation is possible due to differences in thermal expansion behavior of the film and substrate. During the paraelectric to ferroelectric transition (PE→FE) additional strains develop in the film. The total strain for the FE phase may be relieved either by further misfit generation or by domain formation. We have developed temperature dependent stability maps that predict the stable domain structure that forms during the PE→FE transition. The stability maps incorporate the role of the following parameters: (i) substrate lattice parameter, (ii) differential thermal expansion characteristics between the film and substrate, (iii) cooling rate, and (iv) depolarizing fields and electrode geometry. Further, the role of dislocation stabilization of domain patterns is discussed.
Domain configurations due to multiple misfit relaxation mechanisms in epitaxial ferroelectric thin films. II. Experimental verification and implications
Authors: Speck J. S. , Seifert A. , Pompe W. , Ramesh R.
Published: July 2002

In part I, the concept of a temperature dependent domain stability map was developed for tetragonal ferroelectrics grown on cubic substrates. In this paper, a range of experimental data on domain populations in heteroepitaxial ferroelectric films is placed in the context of the stability map. Experimental data shows that differential thermal expansion, cooling rate, and applied electric fields may be effectively used to control domain structure, particularly in the system PZT, PZT/YBCO/LaAlO3, and LSCO/PLZT/LSCO/LaAlO3. It will be shown that misfit dislocations generated during growth screen the majority of the lattice mismatch with the substrate. Thus, the variety of domain patterns that develop during the Curie transition depend on processing parameters and can be successfully explained by applying the temperature dependent coherent domain stability map developed in part I.
Texture in multilayer metallization structures
Authors: Tracy D. P. , Knorr D. B. , Rodbell K. P.
Published: July 2002

The effects of thin Ti, TiN, or Ti/TiN underlayers on the development of the crystallographic texture and the grain structure are explored. Metal layers ∼0.5 μm in thickness of Al-0.5Cu or of Cu are deposited on these underlayers and on amorphous SiO2 as a reference. A strongly textured underlayer such as Ti〈0002〉 or Ti〈0002〉/TiN〈111〉 induces a similarly strong 〈111〉 texture in the AlCu. In copper with 〈111〉, 〈200〉, and random texture components, an underlayer induces a stronger 〈111〉 component compared to an analogous film deposited on SiO2. A nearly random texture in TiN significantly weakens the texture in subsequent metal films. Grain size distributions in all AlCu films are monomodal reflecting a process of normal grain growth. The grain size distribution for Cu sometimes deviates from lognormal. The bimodal distribution implies that grain growth is abnormal even though the median grain size does not exceed a low multiple of the film thickness.
Temperature-dependent pseudodielectric functions of GaAs determined by spectroscopic ellipsometry
Authors: Maracas G. N. , Kuo C. H. , Anand S. , Droopad R.
Published: July 2002

We present the pseudodielectric functions of GaAs in the temperature range between 30 and 650 °C. Data in the spectral range of 1.24<E<5.00 eV was obtained by spectroscopic ellipsometry in a molecular beam epitaxy system specially designed for this purpose. All measurements were performed in situ to avoid the presence of surface adsorbates, oxides, roughness and to ensure a group V stabilized surface is maintained. A simple two-phase model could thus be used to extract the dielectric functions. Temperature dependence of the critical point energies E1, E1+Δ1, E0′ and E2 are also determined.
Chemical and physical sputtering of fluorinated silicon
Authors: Barone M. E. , Graves D. B.
Published: July 2002

Molecular dynamics simulations were performed on low-energy argon-ion bombardment (200, 50, and 20 eV) of silicon layers with varying amounts of fluorine incorporated. At low fluorine incorporation in the layers (F/Si<0.5), only physical sputtering was observed, although the physical sputtering yield increased compared to pure amorphous silicon. At higher levels of fluorine incorporation into the silicon layer, ion impact resulted in the formation of weakly bound SiFx (x=1–3) species in the layer. This phenomenon appears to be similar to chemical sputtering as defined by Winters and Coburn [H. F. Winters and J. W. Coburn, Surf. Sci. Rep. 14, 164 (1992)]. The overall yield, due to both physical and chemical sputtering, was found to follow a square-root dependence on ion energy. The threshold ion impact energy for the formation of weakly bound species in heavily fluorinated silicon layers extrapolated to ≤4 eV, and for physical sputtering to about 20 eV. The simulations imply that the source of the ion-neutral synergism in ion-assisted etching occurs on the collision cascade time scale (∼10−12 s) with the creation of these weakly bound species. The overall rate determining step for ion-assisted etching, however, is often a much slower process involving thermal desorption, chemical reaction, or diffusion. This difference between the source of the ion-neutral synergy (creation of weakly bound species in 1 ps or less) and the rate determining step (often on much longer time scales) has probably contributed to the confusion that has surrounded discussions of the mechanisms of ion-assisted etching.
Molecular dynamics simulation of mass transfer in molten silicon
Authors: Kakimoto Koichi
Published: July 2002

The diffusion constant in a silicon melt is calculated using molecular dynamics simulation based on the modified Stillinger–Weber [Phys. Rev. B 31, 5262 (1985)] interatomic potential in the temperature range 1550 K<T<1900 K. Temperature dependence of the calculated diffusion constant can be expressed using the following equation: D=3.8×10−4 exp(−0.27 eV/kT)cm2/s. Viscosity of the silicon melt, which correlates to the diffusion constant, is also estimated using Eyring’s relation reported by S. Glasstone, K. J. Laidler, and H. Eyring [McGraw–Hill, New York (1941)]. The calculated viscosity correlates well with experimental data which were obtained by an oscillating cup method except in the temperature range from 1550 to 1750 K.
Generation of the slow wave to characterize air filled porous fabrics
Authors: Gómez Alvarez-Arenas T. E. , Elvira Segura L. , Riera Franco de Sarabia E.
Published: July 2002

Acoustical methods are a very important tool for the characterization of porous materials. In some cases, the appearance of two different longitudinal modes was reported which provides important information of the material properties. It was shown recently that for porous fabrics in water the appearance of these two modes is closely related to the properties of the fluid inside the pores. Here, we present a work on the transmission of airborne ultrasound through air-filled fabrics. The article confirms previously published results about slow wave generation and shows the use of the slow wave for characterization of porous samples.
Electrical explosion of segmented wires
Authors: Me-Bar Y. , Harel R.
Published: July 2002

The phenomena occurring during the electrical explosion of segmented wires are described. It was observed that with a wire of varying thickness, the smaller diameter parts explode first, ejecting metal vapor radially. Breakdown occurs through the vapor, creating current carrying channels which bypass the larger diameter parts of the wire. This may result, in some cases, in the larger diameter parts not exploding at all. Only slight deformations occur at their ends. On the boundary between the exploding and nonexploding sections of the wire, such deformations may cause the evolution of an accelerated slug, similar to that of a ‘‘self-forging fragment.’’ The phenomenon of the current bypassing metallic parts may offer an explanation to the way the ‘‘segmented diverters’’ work on a lightning stricken aircraft radome.
Dielectric property measurements of ZrO2-strengthened Al2O3
Authors: Link G. , Heidinger R.
Published: July 2002

Ceramic composites of alumina and zirconia with ZrO2 contents up to 20 wt % and negligible porosity were investigated at radio frequencies from 104 to 107 Hz, and in a temperature range from 20 to 550 °C. If unstabilized or partially stabilized ZrO2 is dispersed in pure alumina in order to improve the mechanical strength the dielectric properties are changed. The admixture of zirconia leads to enhanced dielectric losses and an increased temperature dependence of the permittivity. It is demonstrated that it is essential to distinguish between the contributions of the monoclinic and the tetragonal phase of zirconia. The presence of the tetragonal ZrO2 phase is correlated with the appearance of two peaks in the temperature dependence of the dielectric loss and two smoothed steps in the temperature dependence of the permittivity. This is well described by a Debye model for dipole relaxation. The presence of the monoclinic phase leads to a higher exponential temperature rise for the dielectric loss as well as for the permittivity, due to an increased ionic conductivity. At higher frequencies the influence of ionic conductivity is smaller and the dipole relaxation processes are shifted to higher temperatures. Therefore, for high-frequency applications low-loss composites with improved mechanical properties can be realized if monoclinic zirconia is kept at low levels.
Properties of a photovoltaic detector based on an n-type GaN Schottky barrier
Authors: Binet F. , Duboz J. Y. , Laurent N. , Rosencher E. , Briot O. , Aulombard R. L.
Published: July 2002

In this article, we report on the characterization of a photovoltaic detector based on an n-type GaN Schottky barrier. We first present the photovoltaic responsivity above the gap. Its spectrum is explained by the combined effects of absorption and diffusion. The hole diffusion length is estimated to be in the 0.1 μm range with a numerical model. The photoresponse below the gap is also investigated and it is shown that the current generated by the internal photoemission is the major contribution to the photocurrent at reverse biases at 80 K. At room temperature, an additional component to the photocurrent is clearly demonstrated and identified. This extra current stems from the existence of traps. Several spectroscopy techniques are used to characterize those traps. The supplementary current emitted from the traps in the depletion region accounts for the spectral and the temporal behavior of the Schottky photodetector at room temperature.