bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement Techniques for Superconducting Coils
Authors: Mains E. F. , Graham C. D.
Published: February 2004

This paper describes some of the experimental methods we have used in building high-field superconducting coils. Field-level measurements have been made using Hall probes, magnetoresistance probes, and search coils. Copper magnetoresistance probes are inexpensive, simple, and reproducible enough for routine testing; such probes can be built into a coil. Reproducibility is better than the normal limits of panel meters or X-Y recorders. The search coil and fluxmeter (or ballistic galvanometer) is used as a primary standard for calibrating other probes. Absolute accuracies of about ½% are attainable by calibration against an NMR probe. We have determined field uniformity by direct measurement of the field gradient. A small coil is oscillated ±1 mm at 10 Hz in the magnetic field. The ac voltage from the oscillating coil, which is directly proportional to the field gradient, is amplified and detected with a lock-in amplifier. Numerical integration of the gradients gives high-resolution field plots. This straightforward apparatus can detect a field gradient of 1 G/cm in a 100 kG field. Our coils are constructed in modules, with each module consisting of a pair of disks or pancakes wound from superconducting tape. It is obviously useful to detect which module in the coil is limiting. For this purpose we use a set of shunt resistors across the modules, and record the voltage across the shunts on an oscillograph as the transition from superconducting to normal state occurs. In this way, one can deduce where the normal region originates in the coil, and can tell something about the way in which it propagates.
The Transient Stabilization of Nb3Sn Composite Ribbon Magnets
Authors: Hale J. R. , Williams J. E. C.
Published: February 2004

The transient thermal behavior of a section of composite superconducting ribbon which has suffered a sudden rise in temperature has been investigated in detail by computer simulation. Three equations describing a one-dimensional flow of heat from the composite, through an insulating film and the subsequent diffusion of heat into a metal foil, have been programmed for simultaneous numerical solution. The effectiveness of the interleaving in stabilizing the composite has been found to depend strongly on the thermal conductivity and thickness of the insulating film, as well as the thermal properties of the metal foil. Also, values of the initial sudden temperature rise due to a collapse of magnetization have been calculated. Estimations, based on the numerical results, are made of the effectiveness of various interleaving materials.
Performance of Superconducting Oscillators and Filters
Authors: Stone J. L. , Hartwig W. H.
Published: February 2004

With improved understanding of loss mechanisms in superconducting resonant circuits (SRC), it is possible to design a variety of devices which take advantage of the high Q. Superconducting surface resistance, trapped-flux loss, and residual losses which are annealable can all be related to the superconductor itself. Dielectric dissipation is well enough understood to permit this source of loss to be minimized. Coupling and radiation losses can be made negligible. SRC's are described with a variety of geometries to fill the range from 10 MHz to 10 GHz. Type-I superconductors provide the highest Q's and are readily fabricated by several techniques. The superconducting resonator can be tuned accurately in frequency by temperature control of the surface reactance as well as the dielectric constant of liquid helium. An external magnetic field can be used to tune the bandwidth as well as the resonant frequency. A quarter-wave reentrant cavity is ideally suited for use as an optically tuned frequency-control element. This is accomplished using the photodielectric effect in high-resistivity semiconductor wafers which terminate the stub. Wide-bandwidth optical communication detectors are described using semiconductors with short recombination lifetime. Oscillators have been built which are optically tuned, and others which have frequency stabilities as good as commercial frequency counters. Use of SRC's to study material properties at low temperature is emphasized.
Magnetostatic Mode Propagation in Axially Magnetized YIG Rods Containing a Turning Point
Authors: Kedzie R. W.
Published: February 2004

The properties of a magnetostatic spin wave propagating in an axially magnetized YIG rod containing a turning point are described. Experimental results at X-band frequencies are presented which show an almost linear delay time vs applied-field characteristic. A theoretical model is discussed based on magnetostatic effects which includes spin-wave losses and gives a finite value of the wavevector k at a turning point. It is shown that theory deviates from experiment at high field because the Sommerfeld relation used in the theoretical calculation does not accurately specify the internal field. The departure of the delay time from linear behavior at low frequencies is shown to be due to differences in slope of the internal field at the turning point near the center of the rod as the applied field is reduced.
Dependence of Yield Stress on Diameter and Density of Prismatic Dislocation Loops
Authors: Crivelli-Visconti I. , Greenfield I. G.
Published: February 2004

An alloy of aluminum with 0.5 at.% magnesium was quenched from 600° to −10°C and aged at various temperatures and times. The supersaturation of vacancies generated by these treatments produced a large number of prismatic dislocation loops, which appreciably affected the mechanical properties of the alloy. In particular, the yield stress was raised to as much as three times the value of the fully annealed alloy. The loop density and loop diameter depended upon the treatment, and both showed influence on the yield stress. By means of electron microscopy, the structure of quenched and aged alloy specimens was observed before and after deformation, and related to the treatments. The mechanism of interaction between prismatic dislocation loops and glide dislocations has been analyzed, and the best fit for the experimental results is given by the expression for the increase in flow stress, Δτ=μb(dN)1/2/ω, where μ, b, d, N are the shear modulus, Burgers vector, loop diameter, and loop density, respectively, and ω is a constant found experimentally to be 2.5. This model accounts for the experimentally measured values of the yield stress for loop diameter up to about 500 Å, whereas for larger loops, the type of interaction presumably changes.
Injection-Laser Far-Field Patterns with Gaussian Profiles in the Junction Plane
Authors: Dyment J. C. , Zachos T. H.
Published: February 2004

Far-field patterns from GaAs junction lasers with stripe contacts about 13 μ wide have been studied. At room temperature, more than 80% of the diodes have approximately Gaussian intensity profiles along the junction plane. Intensity measurements show that the profiles are stable for currents exceeding threshold by 60% in some diodes. These experimental results indicate that the lasers oscillate predominantly in the lowest-order mode. Previously, with 50 μ stripe widths it was impossible to predetermine the dominant mode or to insure its stability for current variations greater than about 5%. In contrast to the symmetric behavior along the junction plane the radiation patterns perpendicular to the plane are asymmetric. Infrared photographs show a major lobe along with several minor lobes, the latter always appearing on the n side of the junction plane. The main features of the asymmetry are essentially the same for similarly fabricated diodes. Differences are found in the intensity distributions or widths of the minor lobes.
Nature of Stacking-Fault Defects in Epitaxial Silicon Layers
Authors: Dionne G.
Published: February 2004

The nature of the ``triangular'' defects in (111) oriented epitaxial silicon layers is derived without resorting to the assumption that the stair-rod partial dislocations at the bends of the {111} faulted planes are of the lowest-energy type α/6〈110〉. In addition, it is shown that the stair-rod partial dislocations at the four bends of the ``square'' defects in (100) oriented layers cannot all be of the lowest energy type α/6〈110〉; instead, the two possible lowest energy forms of the ``square'' defects are those for which the stair-rod dislocations at all four bends are of the α/3〈100〉 type.
A Novel Vapor-Deposition Effect in Amorphous Substances, as an Artifact Source in Electron Microscopy
Authors: Cryer N. B. , Keller A. , Willmouth F. M.
Published: February 2004

It was found, in the course of the electron-microscope study of fibrous objects of several kinds, that standard replicating and shadowing agents, such as carbon, platinum-carbon, and silicon monoxide, produce a structured overgrowth effect consisting of fine-scale side growths of about 100 Å width and a few hundred angstroms length giving the fiber a fir-tree-like appearance. The same effect is observed along edges of films. The direction of the growth is related both to the shadowing direction and to the orientation of the fiber or edge, and it may give a deceptive impression of this feature being a genuine characteristic of the sample investigated. Misinterpretation of electron micrographs due to this source has already come to our attention. The effect is of interest also from the point of view of vapor deposition in general. It is likely to be a manifestation of structured deposition occurring on all levels including the macroscopic. While the effect remains unexplained, some of the relevant features are presented in this paper.
Electrical Conductivity and Superconductivity in the γ Phase of the Tl–Te System
Authors: Juodakis A. , Kannewurf C. R.
Published: February 2004

Two methods for the preparation of single crystals of the γ phase of the Tl–Te system, Tl5Te3, are described. An analysis of electrical resistivity data on both single-crystal and polycrystalline material over the temperature range 4.2° to 500°K and thermal conductivity data on polycrystalline material over the temperature range 77° to 360°K indicates metallic behavior. Typical electrical characteristics at 300°K are: resistivity 2.5×10−4Ω·cm, carrier concentration 1.5×1022/cm3, and mobility 2.0cm2/V·sec. A transition to the superconducting state was observed at 2.19°K; this transition was studied as a function of magnetic field strength. The existence of superconductivity in the γ phase is generally compatible with predictions for the occurrence of superconductivity in degenerate semiconductors and semimetals with positive Hall coefficients.
Millimeter Interferometry of Unstable Molecules: Zeeman Effect in Carbon Monosulfide
Authors: Bates H. E. , Gallagher J. J. , Derr V. E.
Published: February 2004

The use of a millimeter wave semiconfocal interferometer in the Zeeman measurement of 12C32S is described. The J=1→2 rotational transition has been observed at 97.981 GHz in applied magnetic fields up to approximately 15 000 G. The interferometer provides a convenient technique for yielding long effective absorption path lengths. The interferometer employed had Q=2700 and an effective path length d=1.3 m. The rotational gJ-value was measured to be ± (0.269) ±0.005).
A Reflection Technique for Plasma-Density Measurement
Authors: Bruce R. L. , Crawford F. W. , Harp R. S.
Published: February 2004

The paper describes a microwave bridge technique in which the reflection from a discharge column placed transversely across a waveguide is measured and used to derive the plasma electron density. A first-order perturbation theory for the reflection has been checked by measurements on calibrated dielectric rods and is used to obtain number density data for comparison with independent measurements made by cavity-perturbation and dipole-resonance techniques. The method seems to be capable of giving less than 30% error and has the advantage that it is particularly well suited to time-resolved electron-density measurements. As an example, typical data are presented for the decay of an afterglow plasma in a magnetic field.
Intensity Distribution from a Sinusoidal Grating and Application to Diffusion Measurements
Authors: Blakely J. M. , Olson D. L.
Published: February 2004

The general vector analysis of Stroke has been applied to the calculation of the intensity distribution among the real diffraction orders of a sinusoidal grating at normal incidence. The results are presented for gratings of period 10 and 20 μ and for various grating amplitudes up to 1 μ. The application of the results to surface-transport studies is discussed.
The Nature of Thermally Induced Stresses in Silicon and Their Relation to Observed X-Ray Diffraction and Birefringence Phenomena
Authors: Carron Gene J. , Walford L. K.
Published: February 2004

The purpose of this paper is to describe qualitatively the residual stress field induced in single-crystal silicon by electron-beam melting and subsequent resolidification in microspot areas. The stress distribution is derived by vector relationship from previously reported experimental observations by x-ray topography and polarized ir radiation techniques. The stresses are distributed in a sixfold pseudohyperbolic form around the microspots. This distribution correlates with the four-lobed x-ray rosettes recorded from various x-ray reflections. The stresses are concentrated normal to the {111} planes of the parent lattice and, in projection, all stresses in the 〈110〉 directions are tangential to the microspot. Infrared birefringence results from stresses along the 〈111〉 directions either within the microspot or at its interface with the parent lattice. These directions thus become uniaxial optic axes coincident with the stress direction. Recrystallization mechanisms are discussed in relation to the macro stacking fault observed by x-ray topography.
Pressure Dependence of the Mobility of Dislocations in Lithium Fluoride Crystals
Authors: Haworth W. L. , Davis L. A. , Gordon R. B.
Published: February 2004

Recently published measurements of the pressure dependence of dislocation mobility and of the macroscopic yield stress in LiF are discordant. To resolve this divergence of results, the effect of hydrostatic pressure on the yield strength for glide on the primary slip system of LiF single crystals has been measured by two methods: observation of dislocation etch pits and by stress-strain curves. As predicted by the microdynamical theory of plasticity, the same results are obtained by the two methods. The change in mobility of screw dislocations in LiF due to hydrostatic pressure is found to be unusually small as compared to that in other ionic crystals, and the formation of point defects as the rate-limiting step in controlling the velocity of screw dislocations is ruled out.
Electrical Effects of the Dissolution of n-Type Zinc Oxide
Authors: Williams Richard
Published: February 2004

When n-type zinc oxide dissolves in an acid there is a flow of current. The magnitude of the current depends on the rate of solution. Capacitance measurements show that there is a Schottky barrier at the interface between crystal and electrolyte. It is shown that the current flow is probably due to ionized donor centers in the Schottky barrier which are carried into the solution as the crystal dissolves. Fluctuations in the magnitude of the current with time are related to individual steps in the dissolution process.
Investigation of Photoconductivity in ZnS and CdS Phosphors with Microwave Methods
Authors: Kalikstein Kalman , Kramer Bernard , Gelfman Samuel
Published: February 2004

Microwave techniques are applied to measure the photoconductivity of activated CdS and ZnS phosphor powders. Polarization effects, which are a source of error in dc and low-frequency ac measurements, and surface effects introduced by electrodes are eliminated by the use of microwaves. The conduction-band electron density is determined by a waveguide perturbation calculation from the measurement of wave-propagation characteristics in the phosphors. For radiation at 5150 Å and incident light intensity of 170 μW/cm2, there is a density of 1014 conduction electrons per unit volume (cm3) in a CdS (Cl, Cu) powder. In ZnS (Cl, Cu), the electron density is 3×1013 at 8700 μW/cm2 intensity in the 3600 to 6000 Å wavelength band. When long-wavelength ir radiation is added to the exciting light the photoconductivity is decreased.
Diffusion of Tin into Silicon
Authors: Yeh T. H. , Hu S. M. , Kastl R. H.
Published: February 2004

Diffusion of a neutral impurity (tin) into silicon in the presence or absence of acceptor- or donor-type impurities, such as boron or phosphorus, has been investigated through neutron-activation analysis. The diffusivity of tin alone in near-intrinsic silicon is found to be D0=32 exp(−98 000/RT)cm2·sec−1. In the simultaneous diffusion of tin and phosphorus, the diffusivity of tin is enhanced. It is found to agree well with the calculated increase of diffusivity, based on the model of increase of total equilibrium lattice vacancy concentration with donor concentration. However, in the simultaneous diffusion of tin and boron, the diffusivity of tin (due to the presence of boron) is found to be approximately the same as that without the boron. Although the acceptor doping is expected to reduce the tin diffusivity slightly, this effect cannot be ascertained with experimental accuracy.
Cathodoluminescent Properties of the Rare Earths in Yttrium Oxide
Authors: Buchanan R. A. , Wickersheim K. A. , Weaver J. L. , Anderson E. E.
Published: February 2004

The cathodoluminescent properties of eleven trivalent rare earths in yttrium oxide (Y2O3) have been studied. Total intensities and selected line intensities were measured as a function of activator concentration. Emission spectra from 2000–30 000 Å, decay times, and energy efficiency are reported.
An Electron-Diffraction Anomaly Resulting from Secondary Electron Emission
Authors: Piggott M. R.
Published: February 2004

Anomalous electron-diffraction ring patterns have been obtained from metals evaporated quickly onto polished surfaces of insulators at high temperatures (500°–1000°C). The radii of the rings in any one pattern are all reduced by a fixed amount, so that for example the radius of the (222) ring is not twice that of the (111). Measurements are described which indicate that the anomaly is due to secondary-electron emission from the specimen producing predominantly cylindrically or spherically symmetric fields according to whether the current density of the beam at the specimen is low or high. The anomaly, which is only observed on discontinuous, or insulating films, makes the identification of the surface structure of such films difficult, but can be mitigated by using low current density in the incident electron beam.
Time-Domain Analysis and Measurement Techniques for Distributed RC Structures. II. Impulse Measurement Techniques
Authors: Peirson Robert C. , Bertnolli Edward C.
Published: February 2004

Transient analysis for uniform RC structures is considered in this paper. A method is presented for determining the parameters of such structures. The measurements are obtained using impulse excitations in open-circuit and short-circuit configurations. The theoretical results obtained predict fairly the experimental results.
Lateral Diffusion in Ag–Se Thin-Film Couples
Authors: Johnson D. B. , Brown L. C.
Published: February 2004

A study has been made of the diffusion of Ag along thin evaporated Se films using optical and transmission electron microscopy. The effect on the growth rate of varying both the Ag and Se film thicknesses and the temperature has been studied. Thin-film diffusion rates were considerably faster than in bulk diffusion couples, indicating some short-circuit diffusion mechanism. An important result of the investigation was the demonstration of a critical thickness ratio of Ag to Se which had to be exceeded in order for diffusion to occur. This critical ratio depended only on the stoichiometry of the Ag2Se phase formed during diffusion.
Correction of X-Ray Diffraction Profiles for Instrumental Broadening in Transmission Geometry
Authors: Ergun Sabri
Published: February 2004

When polychromatic radiation is used in x-ray diffraction, the instrumental broadening changes with the scattering angle. It is shown that when transmission geometry is used, the instrumental broadening can be expressed in terms of the profile of the primary beam and its wavelength distribution. Consequently, it becomes possible to correct the entire intensity profile for instrumental broadening. The correction is made by successive foldings of the observed profile, and is demonstrated using the intensity profiles of raw and heat-treated carbon black.
The Electric Strength of Nitrogen at Elevated Pressures and Small Gap Spacings
Authors: Watson P. Keith , Sharbaugh A. Harry
Published: February 2004

The dc sparking potential of pure nitrogen gas has been measured under uniform electric field conditions in the pressure range 15–3000 psi(1–200 atm) and at electrode separations ranging from 0.25–25 mil (6.3–635 μ). Departures from Paschen's law are found to occur at very high fields and to depend upon cathode conditions. A model is proposed in which current is field-emitted from the cathode under the combined effect of the applied field and a positive-ion space charge on an insulating layer at the cathode; at sufficiently high fields, the cathode current becomes unstable and breakdown results. The model predicts a relationship between an electron multiplication factor and the applied field, which is found to be consistent with the experimental results.
Microwave Measurements on Laser-Produced Blast Waves
Authors: Hall Richard B.
Published: February 2004

The expansion of a laser-produced blast wave is measured by creating a laser spark at the center of a microwave cavity. The radius measurement as a function of time is accomplished by measuring the cavities' resonant frequency shift. In addition, the blast-wave surface temperature and electron density can be deduced by measuring the change in Q of the cavity. The results are in general agreement with ideal blast-wave theory.
Striation Formation during Steady-State Glow Discharge
Authors: Palmer J. D.
Published: February 2004

The formation of striations in a normal glow discharge has been correlated with a relaxation-type phenomenon occurring at the cathode dark space-negative-glow interface and caused by cathode instabilities. A beamlike stream of electrons, in excess of that number necessary to maintain the normal glow brought about by cathode instabilities, is postulated to cross the cathode-fall region and to interact with neutrals in the negative glow causing a surplus of positive ions at the interface between the cathode dark space and the negative glow. This group of positive ions constitutes a positive striation which in turn is accelerated toward the cathode. The experimentally observed times for these striations to cross the cathode-fall region give a velocity of approximately 104 cm/sec, which is in good agreement with other observations of positiveions striation velocities.
Barrier Heights and Contact Properties of n-Type ZnSe Crystals
Authors: Swank R. K. , Aven M. , Devine J. Z.
Published: February 2004

Barrier-height measurements performed on ultrahigh-vacuum-cleaved and on chemically etched ZnSe crystals have shown that for most metals the barrier height increases linearly with its electronegativity. The highly reactive metals Mg, Ca, and Ba were found to be an exception, showing a decrease in barrier height with increasing electronegativity. Aluminum, when diffused into ZnSe from a film deposited onto a vacuum-cleaved surface, was found to produce a low-resistivity surface layer, followed by a highly compensated region in the crystal. This effect is suggested to arise from copious generation of Zn vacancies at the Al-doped crystal surface, and their influx, ahead of the Al, into the crystal.
``Dip Effects'' in the Flux-Flow Resistance in High-Purity Niobium
Authors: Slettenmark B. , Åström H. U.
Published: February 2004

The flux-flow resistance in the mixed state of single crystallites of high-purity niobium (RRR∼10 000) has been investigated by a sensitive ac method. A dip in the flux-flow resistance at a transverse magnetic field Ha close to Hc2, which is equivalent to the well-known ``peak effect,'' was observed. In addition to this a dip at a lower field Hb was observed in some samples. The dip at Hb appeared to be independent of temperature while the dip at Ha exhibited the same temperature dependence as Hc2. The dips had a width of 10–20 G. After a slight plastic deformation the dip at Hb appeared to vanish while the dip at Ha became stronger and broader. Different mechanisms for the dips are discussed, but since none completely describe the experimental results, it is concluded that two independent mechanisms for pinning of the flux-line lattice ought to be considered in order to explain the observations.
Amorphous versus Crystalline GeTe Films. I. Growth and Structural Behavior
Authors: Chopra K. L. , Bahl S. K.
Published: February 2004

As a part of our structural, optical, and electrical studies of amorphous versus crystalline GeTe films, this paper (Part I) reports on the growth and structural properties of the thin-film polymorphs (amorphous, rhombohedral, and NaCl) of GeTe. When deposited above ∼130°C, the GeTe films are polycrystalline. Epitaxial or partially oriented growth of the rhombohedral phase is obtained on NaCl and mica at deposition temperatures ∼250°C. An electric field ∼4000 V/cm applied laterally on the substrate during deposition lowers the epitaxial temperature. At temperatures above 250°C, the high-temperature NaCl structure grows epitaxially. Below a deposition temperature of ∼130°C, the films are amorphous. The amorphous phase transforms very rapidly via random nucleation centers to the crystalline phase at a well-defined temperature of 145±3°C accompanied by the release of heat ∼1±0.2 kcal/mole (∼½kTm, where k is Boltzmann's constant and Tm is the melting point of GeTe). The transformation temperature increases linearly with decreasing thickness below 350 Å. The amorphous→crystalline transformation induced thermally as well as by electron beam irradiation has been studied inside the electron microscope. The crystalline phase is found to grow in the form of spherulites and dendrites. If induced by electron beam irradiation, the spherulites are polycrystalline rhombohedral in structure while the dendrites exhibit oriented NaCl structure.
Morphology of Polysiloxanes Crystallized from the Melt
Authors: Haller M. N. , Magill J. H.
Published: February 2004

Poly(tetramethyl-p-silphenylene siloxanes) fractions ranging in molecular weight from below 104 to above 106, have been crystallized isothermally from the melt. Samples have also been prepared under a wide range of annealing conditions varying from minutes to hundreds of hours. The morphology of these fractured specimens has been studied by electron microscopy. The ease of fracture increases as the molecular weight decreases. The dimension of the lamellar structures ascertained from Pt shadowing and Au decoration techniques increase as the annealing or crystallization temperature is raised. The values are consistent with the crystallite sizes estimated from small-angle x-ray measurements made on these same samples, and with the single crystal thickness obtained under comparable crystallization temperatures. The topography of the fracture surfaces depends on the molecular weight and on the annealing conditions. The lower molecular weight samples (below 40 000) exhibit a much higher degree of regularity than the fractions with longer chain lengths.
Study of Structure and Crystallite Formation in the Glass System Fe2O3·Na2O·B2O3 by Use of the Mössbauer Effect
Authors: Bukrey R. R. , Kenealy P. F. , Beard G. B. , Hooper H. O.
Published: February 2004

Mössbauer techniques have been employed to identify crystal formation in the glass system Na2O·B2O3·Fe2O3 containing up to 35 wt% Fe2O3. Glasses in this system exhibit a broadened, quadrupole split doublet with a peak separation of 1.0 mm/sec, a full width at half-maximum of 0.7 mm/sec, and an isomer shift of 0.3–0.5 mm/sec with respect to Fe in Cr. The formation of crystalline material is correlated qualitatively with sample composition and preparation procedure. The sensitivity of the Mössbauer technique for identification of small amounts of crystalline material is compared with that of x-ray techniques.
Diffraction by Faulted Close-Packed Lattices: An Analytic Solution for Systems Without Long-Range Correlation of Stacking Symbols
Authors: Holloway H.
Published: February 2004

Wilson's method for analysis of diffraction by faulted close-packed lattices is generalized in a way which eliminates the need for solution of a difference equation. This leads to an analytic solution for the intensity distribution associated with a difference equation of any order. The solution is valid for all systems where there is not a long-range correlation between the stacking symbols of close-packed layers (this criterion is reexpressed in terms of properties of the difference equation). The method is used to derive analytic solutions for the intensity distributions which arise from Jagodzinski's two-parameter model for growth faulting and from fcc crystals with mixtures of intrinsic, extrinsic, and growth faults.
Effect of Preionization on Microwave Breakdown in Neon
Authors: Bandel H. W. , MacDonald A. D.
Published: February 2004

Microwave breakdown field strengths at 3.06 GHz have been measured in neon as a function of electron concentration present at the time of application of the field. Measurements were made for initial electron concentrations varying from about 106 to 109 cm−3, for three cavity diffusion lengths, and pressures varying from about 0.2 to 80 Torr. Decreases in breakdown field strength of more than 80% are observed as electron concentration increases enough to change the electron loss process from free-electron diffusion to ambipolar diffusion.
Analysis of Gain Interaction in a Multimode Gas Laser
Authors: Bambini A. , Burlamacchi P.
Published: February 2004

In this paper a theory of the gain interaction among n oscillating modes is developed starting from the rate-equations viewpoint. This approach permits one to evaluate the gain interaction terms among modes due to the well-known hole burning effect in laser gain with nonhomogeneous line shape. No account was taken of phase differences among modes and the nonlinear polarization of the medium, which give rise to combination tones. By this treatment, it is also possible to find the gain and saturation terms in single frequency operation, which are similar to those derived by Lamb. Some results on the time behavior of two modes and a set of n modes are given in the subsequent sections. Finally, formal results in second-order approximation are given, as well as an evaluation of the range of validity of first-order theory for one-mode operation at the center of Doppler curve of gain.
Graphical Representation of Plane Elastic Wave Propagation in Flat Plates
Authors: Schwartz Milton
Published: February 2004

A simplified representation of the propagation of plane elastic waves in an isotropic infinite plate has been developed by means of a graphical technique. The method employs oblique two-dimensional lattices which have been constructed for both the dilatational and distortional wave components. These lattices, which were obtained from a knowledge of the wave velocities and the plate thickness, have properties closely related to those of the Pascal triangle. The lattices have been used to study some of the geometric and kinematic characteristics of the wave propagation. Examples have been given for alumina.
Optical Constants of Thin Discontinuous Nickel Films
Authors: Marton J. P. , Schlesinger M.
Published: February 2004

The optical constants and the thickness of thin discontinuous nickel films were determined simultaneously from reflection and transmission measurements in the photon energy range 0.5–6 eV. From supplementary mass measurements, the fractional volume of metal in each film was established and the optical constants of the individual nickel islands were calculated using the Maxwell Garnett theory. The optical constants of islands are compared to those of bulk nickel and are found to give a qualitative support to the speculation that the free electron part of the optical conductivity in metal islands should suffer a size effect much stronger than the effects due to interband processes.
Influence of Finite Magnetic Field Strength on Electrostatic Waves in Bounded Plasmas
Authors: Lee Kai Fong
Published: February 2004

This paper presents some quantitative calculation showing the dependence of the effective density for electron waves on the confining magnetic field strength. The effect of a noninfinite magnetic field on ion wave propagation is also investigated.
Anomalous Effect of Temperature on Shock-Wave Propagation in Cu–Zn
Authors: Coleburn N. L. , Forbes J. W.
Published: February 2004

Shock-wave measurements at initial temperatures from 20° to 450°C reveal shock-wave propagation in (β+α) brass is appreciably affected by alloy preheating. The shock-velocity value measured in the alloy at 20°C is decreased ∼20% at 450°C for equal incident pressures. The anomaly is attributed to a disorder- order rearrangement occurring in Cu–Zn alloys of near stoichiometric composition.
Phonon Scattering by Cottrell Atmospheres. II. Time-Dependent Effects
Authors: Klemens P. G.
Published: February 2004

The formation of Cottrell atmospheres can change the scattering of phonons by dislocations and affect the lattice thermal resistivity due to dislocations. Since diffusion of solute atoms is required to form atmospheres, they are not formed instantaneously. Changes due to annealing are similarly time dependent. The diffusion equation is solved in its Fourier-inverted form, and changes in phonon scattering and lattice thermal conductivity are obtained in a simple form. Changes in lattice thermal conductivity at higher temperatures occur faster, since diffusion through shorter distances is involved. Diffusion coefficients can be determined by studying the time dependence of the lattice thermal conductivity on annealing, and for slow diffusion this method may have advantages.
Mössbauer Effect in the Ferroelectric PbTiO3–BiFeO3 Solid Solutions
Authors: Yagnik C. M. , Canner J. P. , Gerson R. , James W. J.
Published: February 2004

The Mössbauer spectra of the system (PbTiO3)0.95(BiFeO3)0.05 have been studied from room temperature to 585°C. From the isomer-shift of 0.491±0.02 mmsec−1 at room temperature, the ionicity is found to be 60%±20%. A discontinuity in the plot of isomer-shift versus temperature at Tc is observed. The quadrupole splitting at room temperature is 0.295±0.02 mm/sec−1. A dip in the Mössbauer fraction is observed at the Curie temperature.
Effect of Substrate Surface Finish on the X-Ray Reflectivity of Multilayer Soap Crystals
Authors: Charles M. W.
Published: February 2004

Optimized lead stearate multilayer crystals have been built, using a modified Langmuir-Blodgett technique, onto glass substrates of varying surface finish. The relative ultra-soft x-ray peak reflectivities of these crystals were measured. The results show that under normal conditions, where thick crystals are in use, a surface finish of very much better than 1 μin. CLA is required to achieve maximum peak reflectivity. Untreated float glass, highly polished steel, and selected mica were shown to provide suitable substrates.
Contrast from Twin Boundaries in Field-Ion Micrographs
Authors: Chandrasekharaiah M. N. , Ranganathan S.
Published: February 2004

Computer simulation and geometrical construction have been used to study the contrast from twin boundaries in field-ion micrographs. For specific orientations of the twin boundary, m rings of (hkl) planes in the matrix are expected to match with n rings of (h′k′l′) planes in the twin, where m and n are integers and (hkl) and (h′k′l′) are prominent planes in the matrix and the twin, respectively.
Microwave Nonlinear Properties of the Langmuir Probe
Authors: Weiner Maurice , True Robert M.
Published: February 2004

Microwave nonlinear effects have been observed in the plasma sheath surrounding a probe. Results were obtained for a PIG discharge operating at 2.0×10−2 Torr xenon. The nonlinear effects include mixing, harmonic generation, and current rectification. The probe is connected directly to a 50-Ω coaxial transmission line through which the microwave signal is coupled to the sheath. Strong nonlinear interaction is observed when the probe is dc biased at a point in the exponential region of the Langmuir curve. The explanation of this interaction is based on the usual interpretation of the sheath as a nonlinear capacitance.
Work Function of Ultrahigh-Vacuum-Deposited Beryllium Films
Authors: Dixon R. D. , Lott L. A.
Published: February 2004

The work function of ultrahigh-vacuum-deposited beryllium films was measured to be 5.08±0.08 eV. Upon exposure of freshly deposited films to pure oxygen at pressures of 5×10−7 Torr, the work function was observed to decrease rapidly to a stable value of 3.60±0.05 eV.
Free Molecule Flow Through a Long Rotating Tube
Authors: Berman Abraham S.
Published: February 2004

The free molecule flow through a long tube rotating about its axis is investigated. For a given density difference across the tube ends the mass flow through the rotating tube is less than that through a stationary tube. For low rotation speed the decrease in mass flow is proportional to the square of the angular velocity while for high speed the mass flow is proportional to the reciprocal of the square of the angular velocity. Radial molecular density and molecular flux distributions are also obtained.
Tensile Properties of Polymer-Filler Composites
Authors: Wang T. T. , Schonhorn Harold
Published: February 2004

The strain energy and stress concentrations in filled polyethylene are analyzed and the effect of fillers on the ultimate tensile strength of these composites is discussed. It is shown that the usual two-phase model is inadequate to describe filler-polyethylene composites when the filler has a high surface energy. Since a high surface energy substrate is known to generate a transcrystalline region, at the polymer matrix-filler interface, which differs mechanically from the bulk polymer, a three-phase model is proposed and analyzed. The agreement between theory and experiment lends credence to the three-phase model.
Electron Tunneling into Chromium Single Crystals
Authors: Shen L. Y. L.
Published: February 2004

The conductance of Cr (single crystal)-Cr oxide-Pb tunnel junctions was measured as a function of bias voltage for (100) and (110) crystalline directions. Although our results are complicated by the presence of a large background conductance, the data support the existence of an antiferromagnetic energy gap in Cr at 1°K.
Interaction of a Cylindrical Beam with a Plasma. II. Experiment and Comparison with Theory
Authors: Self S. A.
Published: February 2004

Measurements were made on a beam-plasma system under particularly simple and well-defined conditions. The system could be operated at will in three distinct regimes, namely, (i) a quiescent steady-state regime, (ii) a linear growth regime, and (iii) a nonlinear regime. The steady state is shown to agree with a static theory for a beam-generated plasma. Measurements of spatially growing waves in the linear regime are described and compared with the theory discussed in Pt. I. Reasonable agreement is found when the effects of finite beam radius and collisions are taken into account, but it is indicated that the effect of radial inhomogeneity is also important. Some preliminary measurements on the nonlinear regime are reported and discussed in terms of the available nonlinear and quasilinear theories. The latter are judged too idealized to account for the nonlinear behavior of real beam-plasma systems and suggestions are made concerning those respects in which they need generalizing.
Time-Decay Characteristics for the Green Emission from a GaP p-n Junction
Authors: Harper F. E. , Hakki B. W.
Published: February 2004

The time decay of the green emission, centered at 0.555 μ (2.23 eV) at 300°K, from Si–Zn-doped GaP p-n junctions was measured for temperatures in the range 100°–300°K and for current densities in the range 15–60 A/cm2. At T=102°K the green emission decayed like t−1 for long times indicating that the emission is due to pair recombination between electrons and holes bound to distant impurities. For 155°K≤ T≤300°K, the green emission decayed exponentially with a single exponential time constant which was independent of the current density and which decreased with increasing temperature as exp (−eφ/kT) with φ=0.005 eV. This exponential time decay shows that the green emission is no longer due to pair recombination; this emission is attributed to the decay of a bound exciton. The dependence of the emission intensity upon current density was found to be Jm where m=1.5 at T=300°K and decreased to 0.9 at T=102°K.
Ferromagnetic Resonance in Single-Crystal BaO·6Fe2O3
Authors: Kurtin Stephen , Foner S. , Lax B.
Published: February 2004

The theory of magnetic resonance in a highly anisotropic, uniaxial ferrimagnet is summarized and curves for the resonance behavior of BaO·6Fe2O3 as a function of applied field, parametrized with respect to angular orientation, are presented. Microwave ferrimagnetic resonance experiments performed from 66 to 120 GHz in a broad-band, low-Q waveguide system with applied magnetic fields up to 60 kG are reported. Sample temperatures ranged from 4.2° to 300°K. Good agreement between theory and experiment is found for spherical, single-crystal samples. Samples of platelet geometry were also investigated. A brief discussion of observed magnetostatic modes is presented, along with evidence of nonlinear resonance response in BaO·Fe2O3 at 4.2°K for rf powers above 60 mW in the waveguide. The observed resonance linewidth was less than 10 G at 4.2°K.
Exchangestriction in CdCr2S4 and CdCr2Se4
Authors: Martin G. W. , Kellogg A. T. , White R. L. , White R. M. , Pinch H.
Published: February 2004

The ferromagnetic semiconductors CdCr2S4 and CdCr2Se4 both show large (∼0.2 eV) shifts in their absorption edges as the temperature decreases below their Curie temperatures. In CdCr2S4, the absorption edge increases with decreasing temperature; while in CdCrSe4, it decreases with decreasing temperature. It has been proposed that these shifts arise from a lattice expansion or contraction which is, in turn, produced by exchangestriction. Using x-ray diffraction, we have measured the lattice parameters of these two materials from room-temperature down to 4.2°K. CdCr2S4 is found to expand with decreasing temperature below its Curie point. The total fractional change is of the order of 10−3. CdCrSe4 contracts, but its fractional change is even smaller than that of the sulfide. Therefore, if this small expansivity is to explain the absorption-edge shift, it requires a deformation potential of the order of 100 eV, which is at least an order of magnitude larger than what one would reasonably expect. Therefore, it appears that exchangestriction is not responsible for the large absorption-edge shifts in these materials.
NMR of Cd in CdCr2Se4 and CdCr2S4; Transferred Spin Polarization in Chalcogenide Spinels
Authors: Stauss G. H.
Published: February 2004

Zero-field NMR in powdered samples yields sharp Cd lines appropriate to the symmetry of A sites in these normal cubic spinel ferromagnets. The hyperfine fields at 4.2°K are +136.2±0.2 and (+) 167.0±0.2 kOe, respectively, in CdCr2Se4 and CdCr2S4. The field strength in CdCr2Se4 at 77°K is consistent with the T3/2-only temperature dependence previously found for Cr, Se, and the magnetization. Hyperfine field values for all of the constituents of CdCr2Se4 are thus known.1 Large values at Cd and Se and a low isotropic field at Cr indicate sizeable covalent and overlap transfer of spin polarization away from the magnetic ions. The separate isotropic and axial field values found, with considerations of crystal structure, lead to semiphenomenological estimates of the amounts of unpaired spin transferred. The number of unpaired spins per nearest Cr neighbor in the 4s orbit of Se is about 0.004; in the ``empty'' Cd 5s, 0.002; per ligand-Cr bond, about 0.03 in the ``empty'' Cr 4s. The effective number per Cr–Se bond in the Se 4p is about 0.03; in the Cr 3d roughly 0.005; these represent weighted differences between occupations of different π bonding orbitals and also σ orbitals. A consistent model indicates that σ bonding orbitals mixing outer ligand p electrons with ``empty'' cation s orbits are particularly significant. This description helps to explain the magnetization properties and the role of the nonmagnetic cations. A full paper will appear in the Physical Review.
Ultrasonic Attenuation at Magnetic Phase Transitions
Authors: Luthi B. , Papon P. , Pollina R. J.
Published: February 2004

Recent quantitative measurements of frequency and temperature dependence of the ultrasonic attenuation in the vicinity of the magnetic phase transition for a number of materials (Gd, Tb, Dy, Ho, RbMnF3, MnFr2, Cr) enable us to compare these results critically with a number of recent theories. For each of these materials we know the spin-phonon coupling mechanism and the coupling strength fairly well. The comparison shows that the anisotropic case (materials with a rather large uniaxial anisotropy) is better understood than the isotropic one. The only case where the spin-phonon coupling is of the single ion magnetostrictive type, shear wave in Dy, the critical attenuation can be semiquantitatively accounted for by considering higher-order spin components in the magnetoelastic Hamiltonian.
Detection and Analysis of Magnetic Fields Produced by Bioelectric Currents in Humans
Authors: Cohen D.
Published: February 2004

Measurements of the ac and dc magnetic fields produced by natural internal ion currents in living systems are useful in principle because they can reduce or bypass the limitations of surface voltage measurements. The magnetic field from the normal human heart was first detected by two groups using gradient detectors and little or no shielding. The fields arose from the same ion currents seen on the ECG, and were ∼5×10−7 G at QRS maximum. Using a well shielded room and a detector consisting of a compact coil feeding a parametric amplifier, the following measurements have been made by the author: (1) mapping the heart's magnetic field around the torsos of one abnormal and six normal human subjects, verifying a previously proposed model; (2) mapping a coarse distribution of the magnetic field (∼10−9 G) around the human head produced by the same alpha-rhythm currents conventionally seen on the EEG; (3) exploring the dc field around the abdomen, which has a maximum of ∼10−6 G and is of undetermined origin. Further evaluation of data and experiments are necessary to see if the theoretical benefits of surface magnetic field measurements can be conveniently realized in practice.