bluuebunny/crossref_metadata_2025 huggingface dataset:
Attenuation of 5.9-keV Photons by Helium and Hydrogen
Authors: McCrary J. H. , Looney L. D. , Atwater H. F.
Published: February 2004

The narrow-beam mass-attenuation coefficients of helium and hydrogen were measured at 5.895 keV. A Kβ-filtered 55Fe source of Mn x rays, a set of collimators, and a low-energy sodium iodide counter were used with a high-pressure gas chamber. Photon intensities were measured with and without gas in the chamber. The measured coefficients are 0.416±0.017 cm2/g for He and 0.400±0.016 cm2/g for H2. These numbers are compared with theoretical predictions of this parameter.
Diffraction from Planar Faults in the β-Brass-Type Ordered Structures
Authors: Mohanty G. P. , Schmidt R. A.
Published: February 2004

Diffraction effects due to a/8[110](11̄0)- and a/6[111](1̄1̄2)-type stacking faults and a/2[111](11̄0)- and a/2[111](1̄1̄2)-type antiphase boundaries in β-brass structures have been considered. The stacking faults broaden both the fundamental and the superstructure peaks, while only the latter class is affected by the antiphase boundaries. The magnitude of broadening, for the two stacking faults, varies from one reflection to another as well as among different orders of the same reflection leading to strong anisotropy in the particle sizes derived from the broadened peaks. When different orders of the same reflection are broadened differently by faulting, the measured particle size anisotropy is found to depend on the fault type as well as on the manner in which strain contributions are removed from the total broadening. Several strain correction procedures have been considered and expression for particle size due to faulting for each case is given. The effects of the two stacking faults differ in several ways. For example, while the (211) fault broadens all fundamental orders of a reflection to the same extent, the broadening due to (110) generally increases with the order of the reflection. All powder reflections along [111], however, are unaffected by the (110) faulting. These and other differences in the broadening behavior lead to quite dissimilar particle size anisotropies for the two stacking faults. Among other stacking faults that may also occur in β-brass structure, the a/6[111](11̄0)- and a/6[111](3̄21)-type faults predict particle size anisotropies identical to that for the a/6[111](1̄1̄2) fault. The a/3[1̄1̄1̄](1̄1̄2)-type fault, on the other hand, broadens the fundamental reflections to the same extent as the a/6[111](1̄1̄2) fault, but it affects the superstructure peaks differently; on this basis it is possible to distinguish between these two fault types from their diffraction effects. In addition to broadening, the stacking faults also lead to small peak shifts of varying magnitudes and directions which, in the case of powder peaks, occur only when there are spacing changes at the faults, but the relative shifts of the various powder peaks due to the (110) and (211) faults are similar; the same is also true of the antiphase boundaries
Direct Dislocation Velocity Measurement in Silicon by X-Ray Topography
Authors: Kannan V. C. , Washburn J.
Published: February 2004

Glide velocity measurements were made on isolated screw and 60° dislocations in silicon for the temperature range 775°–925°C. The x-ray topographs which were used to reveal dislocation displacements also gave qualitative information concerning the early stages of dislocation multiplication in highly perfect silicon crystals. Freshly generated dislocations were more mobile than aged dislocations. Pinning points which were tentatively attributed to thermal jogs developed along the lines. The pinning point spacing decreased with increasing temperature as would be expected for a jog formation energy of 1.2 eV. Heating to above 1000°C effectively immobilized all the dislocations present in the crystal. On subsequent loading at 825°C, no motion took place until the stress was high enough to cause catastrophic multiplication when a segment of dislocation did break away. This resulted in the formation of heavy bands of slip. For fresh dislocations the temperature dependence of velocity was analyzed on the basis of a kink pair nucleation and kink propagation model. The measured activation energy for motion of both 60° and screw dislocations was 1.8±0.3 eV.
Dynamics of Arc Ignition and Cathode Spot Movement of Thermionically Emitting Cathode Surfaces
Authors: Cheng Dah Yu
Published: February 2004

If a thermionically emitting cathode is used in an arc discharge the heat feedback from the gaseous plasma to the solid surface provides the continuity of the arc process. The response time of the temperature fluctuation of the solid-surface material is found to be at least an order-of-magnitude longer than that of the plasma, therefore, the rate process is necessarily controlled by the slowest branch in the process, namely, the solid material. Based on this logic, the initial stages of electric arc ignition and cathode spot growth are analyzed for a thermionically emitting cathode. A feedback heat-transfer process which couples the plasma and the solid surface is hypothesized to govern these phenomena. While the model chosen has been simplified to the extent that quantitative significance may be lost, however, several previously observed qualitative features of arcs are explained, one of which is the need for a minimum current density for the persistence of a thermionic arc. The cathode spot growth can also be accounted for by this hypothetical model.
Low-Temperature Thermal Conductivity of a Suspension of Copper Particles
Authors: Anderson A. C. , Rauch R. B.
Published: February 2004

Measurements obtained in the temperature range 0.1°–1°K illustrate the influence of the thermal boundary resistance on the thermal conductance of a suspension of copper particles in an amorphous dielectric.
Model for Critical Current and Magnetization of Type II Superconductors in Longitudinal Magnetic Fields
Authors: Sugahara Masanori
Published: February 2004

The flux-density distribution and its stability in the mixed state of type II superconductors in the presence of a stationary longitudinal magnetic field are considered in the connection with the critical conduction current and the longitudinal magnetization. It is believed that the theory similates reasonably well the behavior of the superconductors. It is found that the flux pinning by imperfections in the superconductors becomes ineffective in the longitudinal field because of the induced helical vibrations of pinned flux lines.
Time Decay and Temperature Dependence of Radiative Recombination in (Zn, O)-Doped GaP
Authors: Bhargava R. N.
Published: February 2004

To understand the kinetics of the radiative emission in (Zn, O)-doped GaP, primarily at room temperature, we have measured luminescence lifetimes using pulsed photoexcitation from argon and He–Ne lasers. Measurements of time decay and temperature dependences of the radiative emission have enabled us to identify the emission in the near infrared as resulting from transitions involving a bound electron at an oxygen site to a free hole in the valence band. Measurements of the temperature dependence of the decay times and the luminescence intensities of the exciton (red) and bound-electron to free-hole (infrared) emissions also substantiate the conclusions that nonradiative recombination paths exist out of the Zn–O complex and provide evidence for the existence of a nonradiative ``shunt path'' out of the conduction band. The bound electron to free-hole recombination out of oxygen is believed to be primarily radiative. For an annealed sample, the decay times of the exciton and bound-to-free transitions are measured to be 170 nsec and 11.2 μsec at 300°K. From the relative variation of the excitonic and bound electron to free-hole intensities and their decay times in annealed and quenched samples it is reaffirmed that on annealing the quantum efficiency of the exciton emission increases and, furthermore, concluded that on annealing the quantum efficiency of bound-electron to free-hole emission also increases. A study of these effects increases our understanding of the process of annealing. Finally a weak correlation between the external quantum efficiency of excitonic (red) emission and its decay times is established for optimally doped, solution-grown GaP.
Analysis of a Radio-Frequency Structure Used for Plasma Production and Heating
Authors: Orefice A. , Pozzoli R.
Published: February 2004

A theoretical study is presented on the characteristics and electromagnetic fields, in vacuum, of a widely used slow-wave line. The main interest of such a structure is connected with its performances as a plasma producing, heating, and ejecting device.
Transport Phenomena in Partially Ionized Cesium
Authors: Andersen Rodney J. , Nighan William L.
Published: February 2004

An analysis of electron and ion transport phenomena in cesium plasma is developed in a form consistent with experimental conditions typical of those in thermionic and magnetohydrodynamic energy converters and other cesium discharges. Particular attention is given to electron collision processes which are shown to influence significantly both electron and ion transport phenomena. Using available electron cross-section data, electron and ion transport coefficients are evaluated as a function of electron temperature and fractional ionization. Results are then applied to the plasma of a low-pressure nonuniform thermionic arc. Experimentally determined spatial variations of electron density and electron temperature are analyzed numerically and quantitative estimates are made of the spatial variation of electric field intensity, ion current density, and net charged particle production rate. The inferred production rate is compared with that evaluated using a nonequilibrium kinetic analysis developed by Norcross and Stone.
Surface Barriers on Zinc Oxide
Authors: Neville R. C. , Mead C. A.
Published: February 2004

The surface barrier systems consisting of gold and palladium on chemically prepared zinc oxide have been investigated in detail. Surface barrier energies have been determined by photoresponse, forward current versus voltage, thermal activation energy, and capacitance-voltage methods. Agreement in barrier energies obtained by the four methods is excellent. The barrier energy for gold is 0.66 eV and for palladium is 0.60 eV. Forward current-voltage characteristics were in quantitative agreement with simple Bethe diode theory as modified by the presence of image force lowering. The reverse current-voltage characteristic is in quantitative agreement with that expected from the simple image force lowering of the barrier, over a bias range of from 0.1 to 3 V. Carrier concentration derived from resistivity and Hall measurements agreed with that obtained from capacitance-voltage measurements. We believe this represents the first comprehensive study where such quantitative consistency has been demonstrated on a compound semiconductor barrier system. Existence of a deep level trap is indicated via the effects on capacitance measurements.
Accommodation of Misfit Across the Interface Between Crystals of Semiconducting Elements or Compounds
Authors: Matthews J. W. , Mader S. , Light T. B.
Published: February 2004

Misfit dislocations are often transported to the interface between crystals by glide. In materials with the diamond or sphalerite structures, glide is inhibited by the Peierls stress. The aim of this paper is to present a theory for misfit accommodation which includes the effect of the Peierls stress. It is used to explain the elastic strains present in films of germanium on gallium asrenide and to explain the reduction of these strains by a high-temperature anneal. The strains in germanium on gallium arsenide, and the effect of temperature upon them, are not explained by earlier theories for the accommodation of misfit between one crystal and another.
Effects of Deep Centers on n-Type GaP Schottky Barriers
Authors: Wronski C. R.
Published: February 2004

Metal-n-type GaP barriers from 1.5 to 1.8 eV were obtained with Ag and Pt evaporated films. The barrier heights were measured using three different methods and found to be in agreement. The characteristics of the barriers were studied and the effect of deep centers on them investigated. Changes in capacitance in the dark and under illumination were studied and densities of centers present in the GaP crystals were measured. A deep center present in all of the crystals was detected and its cross section for electron capture was estimated. Hole-electron recombination currents were observed and investigated. The centers detected in these barriers could explain the characteristics and the measured hole-electron lifetime of ∼5×10−11 sec.
Steady-State Junction-Current Distributions in Thin Resistive Films on Semiconductor Junctions (Solutions of ▿2v = ±ev)
Authors: Joyce W. B. , Wemple S. H.
Published: February 2004

A collection of exact and approximate solutions to the nonlinear equation ▿2v = ±ev is presented. This equation appears in an extremely wide variety of physical contexts, which are summarized. In particular, the equation is shown to describe the nonuniform junction voltage v resulting from current-spreading resistance when the p and/or n layer of a p-n-junction device is thin and only partly covered by a contact. Outlines are given for the application of the solutions to the design of electroluminescent diodes, lasers, transistors, and metal-insulator-metal or Schottky-barrier structures.
Contact-Limited Currents in Metal-Insulator-Metal Structures
Authors: McGill T. C. , Kurtin S. , Fishbone L. , Mead C. A.
Published: February 2004

The physical mechanisms underlying current flow in solid-state MIM structures are reviewed with emphasis on criteria for determining the dominant conduction mechanism in a given experimental situation. Measurements of the bias and temperature dependence of currents through structures incorporating a thin film of single-crystal gallium selenide are reported, and are shown to be in excellent agreement with the predictions of a simple physical model of contact-limited emission. Independently measured properties of bulk single-crystal gallium selenide are used in all calculations; no adjustable parameters are employed. We believe that this study presents unequivocal evidence for contact-limited thermionic currents in solid-state MIM structures.
Bulk Negative Differential Conductance in High-Purity n-Type Germanium
Authors: de Biasi Ronaldo S. , Yee S. S.
Published: February 2004

A theoretical study is made of hot-electron transfer from three light-mass valleys to one heavy-mass valley in n-type Ge, for a constant electric field in the [111] direction. The treatment is based on Boltzmann's equation, taking account of acoustic phonon scattering and impurity scattering between the different valleys. The electron distribution function is approximated by a drifted Maxwellian. The number of electrons, the velocity, and the electron temperature are determined by the exact numerical solution of the conservation equations of electron number, wave vector, and energy for every valley. The velocity-field characteristic is found to exhibit a negative differential conductance for lattice temperatures below 40°K. Uniaxial compressive stress along the field direction reduces the velocity at the threshold. These results are in good agreement with the experimental data reported by Kastalskii.
Equation for the Shock Adiabat
Authors: Prieto F. E. , Renero C.
Published: February 2004

The main difficulty in using a linear relationship to fit the experimental data on shock and particle velocities is the fact that the Hugoniot pressure tends to infinity as the relative compression tends to a maximum whose value depends on the material. It is shown in this paper that this difficulty is overcome by the use of a quadratic fit, since this leads to an expression free from infinity points for the Hugoniot as a function of volume. It is demonstrated, by the introduction of complex variable methods, that in the quadratic approximation the shock adiabat is a two branch function, with the branch point located precisely at the infinity point resulting from the linear approximation. Series expansions of the Hugoniot suggest the introduction of a quasilinear approximation as a limiting case. Numerical results obtained using the quadratic approximation for iron and cadmium are in very good agreement with the experimental data. The point of highest pressure reported for cadmium confirms the existence of the two branches for the Hugoniot.
Alternating Field Losses in Superconducting Strip Conductors
Authors: Linford R. M. F. , Rhodes R. G.
Published: February 2004

A wattmeter technique is described which advances the measurement of alternating field losses in superconducting materials. In particular, it allows realistic experiments on strip conductors, the geometric form most promising for application to electrical power engineering. Uniform current distribution is achieved, contact problems are eliminated, and sample preparation is minimal. It is shown how the alternating field measurements can be interpreted to yield values for several properties of the material studied. Critical fields, the surface shielding currents, and the bulk critical currents can all be determined. When these quantities are inserted into expressions derived from a modified critical state model, a good match to the experimental results is obtained. Comparison of the measured alternating field dissipation with the area enclosed by magnetization curves, derived in slowly varying fields, confirms the validity of the technique and the applicability of critical state models to results obtained at audiofrequencies.
Flux Flow and a New Critical-Current Formula
Authors: Urban Eugene W.
Published: February 2004

An experimental study has been conducted to measure the flow of magnetic flux through the wall of a specimen of Nb-25% Zr, and thereby to infer the magnetic induction and currents in the material. It was found that if at some magnetic field the specimen were heated to above Tc and recooled, then if the applied field were subsequently changed at a constant rate, the rate of flux flow into or out of the sample would increase linearly with time. If measured at a series of magnetic fields, the rates of these increases, or the flow rate slopes were found to be a smoothly increasing function of average applied field. Calculations based on eight published critical-current density models did not satisfactorily explain these observations. It was found, however, that the empirical current-density model Jc = αc[(μ0Hc2−B) / (B0+B)] did, in fact, give excellent agreement with the measured flow-rate behavior. In this paper the experimental and analytical results are described and the new critical-current density model is compared with earlier ones.
Noise Thermometry with the Josephson Effect
Authors: Kamper R. A. , Zimmerman J. E.
Published: February 2004

Thermal noise causes a random frequency modulation of the self-oscillation of a Josephson junction, and the temperature of the noise source can be determined by analysis of the generated signal. We show that a thermometer based on this principle would be theoretically capable of measuring temperatures in the microkelvin range, and describe a prototype thermometer which has recorded noise temperatures down to 0.075 K.
Superconducting Transmission Lines for High-Capacity dc Circuits
Authors: Belanger Brian C.
Published: February 2004

The hysteresis losses in commercial type-II superconductors preclude the use of these materials for ac superconducting power cables. While recent work has shown that ac cables employing high-purity Nb may have acceptably low losses, it is clear that dc cables would better exploit existing superconductivity technology and make possible the use of commercial NbTi and Nb3Sn. Coincidentally, there has been a lively interest in dc transmission in the electric power industry due partly to advances in SCR converters and partly to increased emphasis on system stability. In this paper the relevance of these developments is discussed. Several alternative designs for superconducting dc cables utilizing commercial superconductors are discussed and design and cost calculations are presented. Areas where additional experimental work is needed are identified.
Low-Frequency Electromagnetic Losses at 4.2°K of Neutron-Irradiated Nb
Authors: Sekula S. T.
Published: February 2004

Low-frequency (350 Hz) permeability measurements at 4.2°K using phase-sensitive techniques were carried out on a Nb sample irradiated in the Oak Ridge Research Reactor at 40°C to several fast neutron doses up to 6×1019 neutrons/cm2 (E> 1 MeV). In the mixed-state region a signal proportional to the electromagnetic loss in the sample is observed which decreases with increasing fast neutron dose. Quantitative estimates of the power dissipation in irradiated Nb are given which are then related to the magnetic field dependence of the critical current density Jc(H).
Material-Properties Analyzers Using Superconducting Resonators
Authors: Hinds James J. , Hartwig William H.
Published: February 2004

A low-temperature material-properties analyzer, using a superconducting microwave resonant cavity, is discussed. Placing semiconductor or dielectric material samples in the cavity perturbs the resonant frequency, absorbed microwave power, and cavity Q. Additional perturbations occur when the sample complex dielectric constant is altered by a thermal, nuclear radiation, or optical stimulus. In samples such as Si, GaAs, CdS, and CdTe, these perturbations have been used to determine such material properties as relaxation time, lifetime, Fermi level, trap ionization energy, trap density, capture cross section, free-carrier density, and trap population. A contactless experimental technique similar to the thermally stimulated conductivity experiment is proposed. The contactless ac measurement system is shown to be sensitive, accurate, useful with randomly shaped or powdered samples, and applicable to many types of insulators and semiconductors.
Superconducting Microstrip High-Q Microwave Resonators
Authors: DiNardo A. J. , Smith J. G. , Arams F. R.
Published: February 2004

Superconducting miniature microstrip resonators, operating at X band, have been constructed using vacuum-deposited lead on low-loss sintered alumina substrate. Unloaded Q's as high as 200 000 and 500 000 have been measured at 14.3 GHz at 4.2 and 1.8 K, respectively.
Magnetoacoustic Amplification in Extrinsic Nondegenerate Semiconductors
Authors: Abe Y. , Mikoshiba N.
Published: February 2004

The expression of an effective conductivity tensor for the magnetoacoustic amplification in extrinsic, nondegenerate semiconductors has been derived by the Boltzmann equation taking proper account of the Hall effect. This expression can be applied to any values of the frequency of acoustic waves and of the magnetic field in the classical regime. Detailed discussion of the Doppler-shifted ultrasonic cyclotron resonance (DSUCR) is given by taking n-InSb as an example. It is shown that the effective mass and the relaxation time of electrons can be determined by DSUCR. A brief discussion is also given on the magneto-acoustoelectric instability in n-InSb.
Phase Measurements of Reflected Ultrasonic Waves near the Rayleigh Critical Angle
Authors: Becker F. L.
Published: February 2004

The phase shift with incident angle of reflected ultrasonic waves incident near the Rayleigh critical angle is reported. These measurements substantiate the predictions of our model. These predictions differ substantially from previous theories which neglect attenuation. For frequencies above 15 MHz the slope of the phase shift of an ultrasonic wave reflected from a water-stainless steel boundary reverses at the Rayleigh critical angle. For frequencies below 15 MHz the phase shifts from 0° to +360°; above 15 MHz it shifts from 0° to +60° then to −60° and back to zero again on passing through the Rayleigh critical angle. The sensitivity of this phenomenon suggests that it could be useful in determining or evaluating changes in material properties such as cold work, residual stress, or grain sizes.
Crystallization of Rochelle Salt in Electric Fields
Authors: Hanani M. , Schieber M.
Published: February 2004

The kinetics of crystal growth of ferroelectric Rochelle salt have been studied by applying an electric field of 2–1000 V/cm between the upper face of a crystal and an electrode immersed in the water solution in contact with the lower part of the growing or dissolving crystal. It has been found that the rate of growth decreases while the rate of dissolution increases by about 10% above and below Tc of 24°C of the crystal at 900 V/cm. The results are discussed theoretically. Similar experiments under the application of a uniaxial pressure showed a less significant effect, while a vibrating alternating pressure of about 8 kg/cm2, and 50 cycles/sec showed a significant increase of the rate of growth by about 50%.
Alternating-Current Loss Measurements in Thin-Film Type II Superconductors
Authors: Deis D. W. , Gavaler J. R. , Jones C. K. , Patterson A.
Published: February 2004

An electrical technique is described for measuring the audio frequency power dissipation in superconducting thin films as a function of the ac current density, frequency, and applied dc magnetic field. This system has been used to study the ac losses in thin films of Nb and NbN, and also the ac and dc critical current densities in these materials. The results can be explained by assuming that both hysteresis and eddy current losses contribute.
X-Ray Topographic Study of Tensile Deformation in Nearly Perfect Copper Crystals
Authors: Young F. W. , Sherrill F. A.
Published: February 2004

Copper crystals of a cross section 0.5×0.1 cm and of an initial dislocation density N 50–5000 cm/cm3 were deformed in tension, and Borrmann x-ray topographs were taken after successive stress increments. All dislocations were observed and Burgers vectors determined. Stereo pairs were made. The grown-in dislocations were moved very little by the applied stresses. For crystals of lower initial N most of the dislocation generation could be related to localized stresses in the grip regions, while in the crystals with higher initial N the generation probably was related to grown-in dislocations. Slip bands were formed; neither the source nor the generation mechanism of dislocations in these bands was determined. Screw dislocations were left by edge segments running across the crystal. These screw dislocations cross slipped frequently at stresses of a few grams/mm2, and this process tended to fill the volume of crystal between the bands with dislocations. By counting all new dislocations the shear strain was computed and ``stress-strain'' curves were obtained.
Observations of Needle Crystals of Tungsten Oxide by Field Emission and Electron Microscopy
Authors: Okuyama Fumio
Published: February 2004

The needle crystals of tungsten oxide grown on the tungsten substrate from vapor phase were studied by field emission and electron microscopy. The observed FEM patterns of the needles were quite similar to the pattern of the oxidized W emitter and mostly had twofold symmetry. Less frequently, the FEM patterns having nearly threefold and fourfold symmetries could be observed. From the comparison of the electron micrographs as well as the FEM patterns of a needle and an oxidized W emitter, evidence was obtained that the surface of the oxidized emitter tip had been covered with thin layer of a crystallized tungsten oxide.
Dynamic Yield Strengths of B4C, BeO, and Al2O3 Ceramics
Authors: Gust W. H. , Royce E. B.
Published: February 2004

The dynamic yield strengths for several ceramics were measured under shock-wave compression. The Hugoniot elastic limit (HEL), taken as the amplitude of the observed elastic precursor traveling ahead of the compressional wave, was 154 and 82 kbar for B4C and BeO ceramics, respectively, and ranged from 61 to 134 kbar for several Al2O3 ceramics. These values apply to 6-mm-thick samples; in some cases there were variations with sample thickness. The offset of the Hugoniot from the isotropic compression curve in the stress-volume plane indicates a nonzero yield strength above the HEL for Al2O3. This yield strength is somewhat less than the yield strength at the HEL. The compaction of porous ceramics is found to follow a quadratic variation with stress, full compaction being reached at about four times the HEL.
University of Virginia Superconducting Wind-Tunnel Balance
Authors: Zapata R. N. , Parker H. M. , Moss F. E. , Hamlet I. L. , Kilgore R. S.
Published: February 2004

The design of an electromagnetic balance using superconducting coils is reported. Both dc and ac coils are used to support aerodynamic models in a supersonic (Mach 3) wind tunnel and to simultaneously measure the forces acting on them along 3 orthogonal axes. Major design characteristics include: adoption of symmetrical coil arrangement to provide maximum space for the wind tunnel; 3 gradient-coil pairs capable of being driven between 0 and 350 A at a frequency of 30 Hz by specially designed power supplies; a vertical wind tunnel with a 6-in. test section located in the axial room-temperature access of a 250-liter liquid-helium Dewar. Results on ac losses for prototype gradient coils wound of three different superconducting materials are reported.
Recent Developments in Superconducting Devices
Authors: Zimmerman J. E.
Published: February 2004

A simple model of the electrical characteristics of thin-film-bridge, point-contact, and tunnel junction Josephson devices is given, along with some comments on their relative performance at very low and at very high frequencies. A particular example is the dc IV characteristic of a point contact at the center of a parallel-disk microwave cavity. Some recent developments in devices and the application of a point-contact loop (SQUID) device to magnetocardiography is described.
Absorption Spectra by Magnetostatic Spin Waves in Axially Magnetized YIG Rods
Authors: Abe Kenji , Ikushima Ichiro
Published: February 2004

The absorptions of microwave in axially magnetized YIG rods were measured and analyzed. The absorption peaks are interpreted to be caused by magnetostatic spin waves propagating along the rod axis. The mode numbers of absorption peaks are determined by the sample shape and the value of ωs/γ-Hi(0), where ωs is the microwave angular frequency, γ is the gyromagnetic ratio, and Hi(0) is the internal magnetic field at the rod center and depends upon the applied magnetic field. In calculations of mode number and mode separation, the demagnetizing field Hd in the rod is approximated by a quadratic equation of the square of normalized position ζ. The mode separations are independent of the microwave frequency. The experimental values of the applied magnetic field at which the absorption peaks are caused coincide with the theoretical values.
Magnetic and Structural Properties of Epitaxial and Polycrystalline GdIG Films Prepared by rf Sputtering
Authors: Sawatzky E. , Kay E.
Published: February 2004

Magnetic, magneto-optical, and structural properties of rf-sputtered gadolinium iron garnet films are presented. The dependence of the Faraday rotation on film structure is explored and the behavior of coercivity and hysteresis loop characteristics with temperature discussed in some detail. The coercive force was found to be independent of structural differences in the various films as monitored by x-ray and electron diffraction techniques. The high coercivities observed are attributed in part to a strain induced magnetoelastic interaction. Hysteresis loops with squareness ratios of Mr/Ms=1 are obtained from (111)-oriented films on (111) YAG substrates. Domain patterns are highly temperature stable in the epitaxial films. There is no evidence from magnetic and structural properties in these epitaxially grown films that cation diffusion from substrate to film is involved at crystallization temperatures below about 900°C. The films are considered quite adequate for magneto-optic memory applications.
Magnetic Contrast in the Scanning Electron Microscope
Authors: Wardly G. A.
Published: February 2004

An analysis of secondary electron magnetic contrast is presented. The analysis is based upon a simple cylindrically symmetric model for the fringing fields of materials. It is carried to the point of predicting ``optimum'' secondary electron collection and describes two quantitative field measurement techniques. An analyzer was constructed for observing magnetic contrast in the SEM. It performs both a secondary electron energy discrimination function and a spatial aperturing function using diametrically opposing apertures. This type of analyzer has a potential signal-to-noise ratio about half that of ``optimum'' electron collection. Observations are made in this analyzer of various magnetic materials. Pictorial, semiquantitative, and quantitative results are described. Good field measurement correlations exist between the two techniques described and also between them and the ``expected'' values.
Analog Computer Simulation of Weakly Connected Superconducting Rings
Authors: Simmonds M. B. , Parker W. H.
Published: February 2004

A system comprising a weakly connected superconducting ring inductively coupled to a resonant L-C circuit has been studied using an analog computer. The weak link was characterized either by a current-phase relation of the form I = Ic sinφ or by ``supercurrent breakdown,'' and the results were compared. The effects of a normal resistance shunting the junction were investigated, as well as the effects of thermal current noise in the ring. Some criteria for achieving optimum coupling to the superconducting ring are discussed.
Studies of the Effect of Ion Current on Instantaneous Triple-Probe Measurements
Authors: Chen Sin-Li
Published: February 2004

A dimensionless parameter η is suggested to express the variation of ion current with probe potential in a probe characteristics which can be used to evaluate the influence of ion current on the ``instantaneous'' measurement of plasma parameters by means of triple-probe technique. Theoretical values of η for a cylindrical probe are obtained as a function of dimensionless parameter ξe(=rp/he), where rp is probe radius and he is the electron Debye length, both from classical sheath model and the more modern unified probe theory. η is small (<0.01) when ξe is large (>100), increases as ξe and mi (the ion mass) decreases and has a saturation value approximately equal to 0.1–0.2 at small ξe(≲1). The experimental η agrees with the theoretical η within a factor of 2. The possible errors associated with the direct-display system of plasma parameters using the triple-probe method by assuming η=0 are discussed in connection with η, and the results show that the maximum errors are within 40% for electron temperature determination and within a factor of 3 for electron density determination, respectively.
Influence of p-n Junctions on the Mobile Charge Density in MOS Transistors
Authors: Schlegel Earl S.
Published: February 2004

Empirical data shows that the mobile charge density in the oxides of MOS and bipolar microcircuits depends on the proximity of a p-n junction. This data is consistent with a model in which positive mobile ions migrate from oxides over n-regions to those over p-regions. Several practical consequences of this phenomenon are discussed.
Interpretation of Quantum Interference Properties of Double Josephson Junctions
Authors: Fulton T. A. , Dynes R. C.
Published: February 2004

Experiments and analysis of the quantum interference of the critical current Ic in a magnetic field B for double Josephson (tunneling barrier) junctions are described. A recent treatment1 of this problem is employed which takes account of geometric asymmetries, loop self-inductance L, and possible nonsinusoidal supercurrent-phase relations Is (φ). The observed Ic (B) curves are found to agree with the predictions of the model. The Is (φ) are directly extracted from the Ic (B) and accurately conform to Josephson's original prediction of Is (φ) = I0 sinφ. For larger values of L, Ic is often observed to be multivalued for some or all B, or to display discrete, periodic jumps. This behavior is shown to be the combined result of multiple local maxima in an effective current-phase relation for the double junction and the particular dynamics of φ encountered in tunnel junction geometries.
Weak Link Point Contact Devices
Authors: Buhrman R. A. , Lukens J. E. , Strait S. F. , Webb W. W.
Published: February 2004

Recent progress in our laboratory on permanently adjusted niobium point-contact superconducting devices is reported. We describe an rf noise-biased magnetometer obtained by appropriate tuning and adjustment of the point contact in a superconducting ring that is coupled to an LC tank circuit which provides an rf noise-bias excitation at its resonant frequency. Thus, the rf oscillator normally used is eliminated although the magnetometer noise increases somewhat. In addition, mechanically stable, room-temperature-helium-temperature-cyclable insulated double and single point contacts have been obtained by use of an insulating glass with thermal expansion matching that of niobium. When two of these contacts are connected in parallel the resultant device works quite well as a cyclable dc-biased magnetometer. Study of the temperature dependence of the critical current is facilitated by the stable configuration. Preliminary measurements showed sharp rises of the critical current with decreasing temperature similar to those observed with S-N-S sandwiches. Properties of these devices are discussed.
Determination of Constitutive Relationships with Multiple Gauges in Nondivergent Waves
Authors: Cowperthwaite M. , Williams R. F.
Published: February 2004

Constitutive relationships are calculated from multiple Lagrangian gauge records obtained in one-dimensional wave experiments by integrating the flow equations expressing conservation of mass and momentum along a particle path. Specifically for flow adjacent to a constant state, particle velocity-stress relationships are calculated from multiple stress-time profiles by integrating the combined mass and momentum equations, and specific volume-particle velocity relationships are calculated from multiple particle velocity-time profiles by integrating the continuity equation; but these relationships are in general approximate because the material derivatives required to perform the integrations are generated from gauges separated by finite distances. Steady-state, simple isentropic, and simple nonisentropic waves are shown to be the only flows that allow constitutive relationships among stress, particle velocity, and specific volume to be determined exactly with either a pair of stress gauges or a pair of particle velocity gauges. A general procedure for the determination of constitutive relationships with three gauges is formulated and tested with an exact solution for a strong decaying shock wave to indicate errors resulting from the finite separation of gauges when constitutive relationships cannot be calculated exactly.
Grain-Boundary Grooving by Surface Diffusion for Finite Surface Slopes
Authors: Robertson W. M.
Published: February 2004

The nonlinear fourth-order differential equation which determines the shape of a grain-boundary groove forming by surface diffusion is solved numerically for groove root slopes ranging from 0 to 4. The groove width is within five percent of the small-slope groove width for all groove root slopes calculated. The groove depth departs by more than ten percent from the small-slope depth for groove root slopes greater than about 0.7.
Forces on Superconducting Coils by Induced Body Currents
Authors: Borcherts Robert H. , Reitz John R.
Published: February 2004

The lift and drag forces on a persistent-current superconducting coil suspended above the rim of a rotating aluminum wheel have been measured. Calculations have been made for various coil geometries above conducting roadbeds that are infinite in extent as well as of finite width. Experimental measurements carried out up to a speed of 300 mph over a solid as well as a ``hoop-like'' aluminum wheel give support to the existence of a magnetic Reynolds number, λ = μσνh; for the infinite roadbed case the theoretical lift-to-drag ratio is FL/FD ≈ 2(λ/π)1/2. Here σ is the conductivity of the roadbed, ν the relative velocity of the coil and roadbed, and h the height of the coil above the roadbed. Above 70–80 mph we measure a 0.55 dependence for the lift-to-drag ratio. This ratio reaches a value of 8 at 300 mph—approximately one-half that predicted by the above expression for the infinite geometry. The ``hoop-like'' wheel shows lift and drag forces to fall off more rapidly at low speeds than for the solid wheel—as might be expected from skin depth considerations.
Very High Critical Current and Field Characteristics of Niobium Nitride Thin Films
Authors: Gavaler J. R. , Janocko M. A. , Patterson A. , Jones C. K.
Published: February 2004

Previous results indicated that NbN thin films possess critical current and field characteristics significantly superior to that of bulk NbN having a similar Tc. Further measurements made on NbN films, with thicknesses between 50 Å and 8 μ, now show that, at 4.2°K, certain of these films exhibit the highest current densities of any presently known superconductor in all fields from zero up to the limit of our measurement capability (210 kOe). In addition, anomalously high current and field values have been measured in very thin (<300 Å) films. These thinner films show no depressions in Jc (measured at 4.2°K) or in Hc2 (O) values despite decreases in Tc from almost 16°K (in the thicker films) down to 11°K.
Preparation and Superconducting Properties of Ta–Sn and Nb–Ge Diffusion Layers
Authors: Otto G.
Published: February 2004

Intermetallic compounds of the binary systems Ta–Sn and Nb–Ge were formed on Ta or Nb wires by the method of vapor phase diffusion at elevated temperatures. Critical temperature Tc, critical magnetic field Hc2 at 4.2°K, and composition of these layers were measured as a function of heating conditions. Ta–Sn diffusion layers prepared at temperatures between 800° and 1200°C consisted predominantly of the A15 compound Ta3Sn with a lattice parameter of a = 5.278 Å. The observed layer thickness was always less than 1 μ. Simultaneous maxima in Tc (7.0°K) and Hc2 (50 kOe) were measured on wires heated at 950°C for 3 days. On the other hand, in the Nb–Ge system the A15 compound Nb3Ge did not form in the same temperature range. The layers, of thickness from 1 to 20 μ, consisted only of NbGe2 for a diffusion temperature of 800°C, Nb3Ge2 for 1200°C, and a mixture of the two at 1000°C. Consequently Hc2 at 4.2°K for these samples did not exceed 12 kOe.
Multifilament Nb3Sn Superconducting Wire
Authors: Kaufmann A. R. , Pickett J. J.
Published: February 2004

A superconducting copper matrix wire containing many filaments of 0.4-mil-diam Nb3Sn has been prepared and tested. The critical current in transverse magnetic fields in the range of 50–100 kG is 3–5 times greater than that of the Nb–Ti alloy but not as high as that of the best Nb3Sn. The critical temperature of this material is about 17°K.
Hc2(4.2 K) of High-Temperature Superconducting Alloys
Authors: Foner S. , McNiff E. J. , Matthias B. T. , Geballe T. H. , Willens R. H. , Corenzwit E.
Published: February 2004

Previous measurements of Hc2 versus T in selected Nb3Al1−xGex and Nb3Al alloys for dc magnetic fields to 200 kG and for 14 K ≤ T ≤ Tc have now been extended to 4.2 K by means of pulsed magnetic fields and rf measurements. Long pulse (10 msec) multipalyer coils producing fields to 450 kG were employed for the measurements. At 4.2 K, Hc2≃410 kG and Hc2≃320 kG for the highest Tc, Nb3Al1−xGex and Nb3Al alloys, respectively. These values of Hc2 are the highest measured for any superconductor. Measurements of Hc2 in several related alloys with somewhat lower values of Tc and Hc2 will also be presented. The large values of Hc2 (4.2 K) for all of these alloys permit great latitude of engineering design for practical wire materials. A summary of the physical properties of these high-temperature superconductors, measurement techniques, present limits of high Hc2 materials, and a current appraisal of technical possibilities will be presented. A summary of some of the measurements has appeared.1
State of the Art of Superconducting Magnets
Authors: Stekly Z. J. J.
Published: February 2004

The superconducting magnet is a key component in various applications including high-energy physics, fusion research, magnetohydrodynamics, and electrical rotating machinery. This paper presents the general characteristics of state of the art of superconducting magnets as far as design and performance are concerned. The properties of superconductors are briefly discussed, and the basis for design of stable conductors is set forth. Examples of conductors and magnet systems are given and discussed. The current state of affairs is summarized in three plots showing size versus field achieved, current density in coils as a function of magnetic field, and specific weight versus stored magnetic energy.
Computation and Construction of a High-Field Superconductive Dipole
Authors: Westendorp W. F.
Published: February 2004

Superconductive dipoles show promise as bending magnets for the output beams of particle accelerators. For a specific case considered criteria are fields of 40 kG or higher, lengths of 20 ft or more, uniformity of field of 0.2%, within useful diameters of 4 in., and magnet currents below 500 A. Multilayer construction employing tape-type superconductors will be discussed. The 0.2% uniformity criterion can be met if careful attention is paid to the actual integral number of tapes per slot in the computer program. The curved ends of the winding also require support and some experimental work on this will be described. The transfer of the forces on the conductors outwards to the retaining rings will be discussed. The resulting bending moments are computed.
Prototype Superconducting Magnets for NAL Beam Lines
Authors: Fast R. W. , Strauss B. P. , Cohn C. M. , Heim J. R. , Scholefield E. H.
Published: February 2004

Two superconducting 20-kG dipole magnets have been designed and constructed at the National Accelerator Laboratory. They are of a type called ``superferric'' since they consist of an iron yoke and superconducting coil. A 76-cm-long model providing a 4×10-cm aperture operated successfully at 20 kG with a conductor current density of 36 kA/cm2 and a stored energy of 8 kJ.1 A 3-m-long prototype requiring 17 kA/cm2 to reach 20 kG in a 4×13-cm aperture will be tested soon. The iron yokes of these magnets are refrigerated to 4.2 K. Niobium-titanium multifilament conductor is used. The coils are short free and the magnets can be charged to full field in 5 min or less. A 60-cm-long, 4-kG/cm iron-free quadrupole with a 15-cm bore is being constructed. The field is shaped by coil sectors of 18° and 36°. A conductor current density of 21 kA/cm2 is required to achieve a gradient of 4 kG/cm. The stored energy will be about 300 kJ at this gradient.
Effect of Tin Additive on Indium Thin-Film Superconducting Transmission Lines
Authors: Mason P. V.
Published: February 2004

We report the effect of adding up to 23% tin to the indium film of a thin-film/tantalum-oxide/bulktantalum transmission line. Addition of tin reduces the velocity and increases the delay for fixed length by 1.4% for each percent of tin. Agreement with the predictions of Pippard's nonlocal theory when meanfree path is reduced is excellent. Pulse attenuation and shape degradation are not increased by addition of tin. Attenuation as low as 10 dB per microsecond of delay was observed at 1.25 K. The added tin reduces by 50% the sensitivity of velocity to temperature near Tc, and improves the reproducibility of velocity from line to line. Critical temperature is increased to 6 K for 23% tin, in good agreement with previous measurements.