bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermoelectric power of thin copper films
Authors: Leonard William F. , Yu Ho-Yuan
Published: February 2004

The thermoelectric power and resistivity of annealed copper films have been measured. From the thermoelectric power values at the minimum in resistivity and the corresponding temperature coefficient of resistance, the calculated values of the energy dependence of the mean free path of conduction electrons and the Fermi surface area are −0.21 and −1.43, respectively.
Determination of mobility by 8-mm microwave magnetoreflectivity in semiconductors
Authors: Fortini André , Madelon Roger , Lande Richard , Hairie Alain
Published: February 2004

The simple Drude-Zener theory has been used to ascertain the behavior of microwave reflectivity of a semiconductor in a magnetic field. Magnetoreflectivity of the circular mode TE11 at 34 GHz has been measured with an experimental setup which is described. The Hall mobility results are given for Ge and Si samples. The interest and potentiality of the method are discussed.
Photoluminescence probing of heterojunctions
Authors: Daniels E. , Steinvall O.
Published: February 2004

Investigation of photoluminescence induced by a focused laser beam is shown to be a valuable method for studying composition changes at heterojunctions. The method is applied to Ga1-xAlxAs–GaAs heterojunctions grown by liquid phase epitaxy. The He–Ne laser used in these experiments excites near-band-edge recombination radiation, whose spectral distribution is analyzed. From the peak wavelength the Al concentration can be deduced. When the laser spot is moved in regions of the sample where the Al concentration changes with position, the spectral distribution of the photoluminescence is shifted in a very pronounced way. This results in a geometrical resolution much better than would be inferred from the laser spot diameter alone. The effect has been analyzed starting from a simple model of the process. It is shown that the Al concentration could be obtained with an accuracy of 1–2% and an effective geometrical resolution of a few microns all depending on the concentration gradient at the measuring point.
Parametrization of low-temperature deformation characteristics in single crystals of molybdenum
Authors: Amin Kamal E. , Murty K. Linga , Dorn John E.
Published: February 2004

The flow stress as a function of strain was measured in the temperature range from 77 to 400°K at strain rates between 10−1 and 10−6 sec−1 for single crystals of molybdenum. The flow stress is characterized by a rate-temperature parameter, defined through a general rate equation for low temperature thermally activated deformation processes in bcc structures. This parameter was utilized for testing the mechanical equation of state and in explaining the failure of such an equation in molybdenum. Whereas the rate of work hardening was found to be uniquely defined by strain and the parameter, the flow stress depended in addition on the thermal-mechanical history of the material.
Impurity centers in silicon films on sapphire
Authors: Ipri A. C. , Zemel J. N.
Published: February 2004

The MOS Hall technique has been used to measure the temperature dependence of the average electron and hole mobilities at various depths into the film. Electron and hole concentrations were also determined as a function of temperature and distance from the film surface. It has been found that a second ionization level, in addition to the intentionally introduced arsenic impurity, exists in the n-type films at approximately 0.1 eV below the conduction band. This level has been associated with the SiO4+ complex known to be present in oxygen-rich silicon. The density of this added impurity increases with depth into the film and has, therefore, been associated with the mobility gradient. The effect of this complex on the temperature dependence of the mobility is examined and the appearance of a temperature-independent neutral impurity scattering mechanism is evident. Neutral impurities in p-type films also are suggested to explain the hole-mobility-vs-temperature data and they may be related to the B-SiO4+ molecule known to exist in oxygen-rich boron-doped silicon. The effect of stress on the mobility has been suggested as a possible cause for the reduction in electron and hole mobilities at elevated temperatures where phonon scattering appears dominant.
Stark effect and line broadening in three-dimensional stochastic fields
Authors: Kim S. H. , Wilhelm H. E.
Published: February 2004

A theory is given for (i) the linear Stark effect of the substates of the first excited level of the hydrogen atom and (ii) the linear Stark broadening of the Lyman α line, in a three-dimensional stochastic electric field. The latter is represented in the form of a three-dimensional Fourier series with different random amplitudes and random phases for each vector component of the wave modes, which simulates a perturbing vector field varying with respect to intensity and direction in a random manner during an atomic transition. The line-shape function, which is a moment of the distribution function of the random amplitudes and random phases of the stochastic field, is evaluated by statistical methods. It is shown that the Lyman α line consists of a sequence of discrete spectra (Fourier analysis) at the frequencies ω=nωp, n=0,±1,±2, etc., where ωp is the plasma frequency. The line intensity decreases exponentially with the square of the order number n. The half-width of the spectral line is proportional to the square root of the mean square stochastic electric field. In the case of electron acoustic turbulence, the half-width is related to the unperturbed electron pressure. These results permit a quantitative determination of the intensity of the stochastic electric field and the average electron pressure in turbulent plasmas.
Large-angle L x-ray production by electrons
Authors: Dick C. E. , Lucas A. C. , Motz J. M. , Placious R. C. , Sparrow J. H.
Published: February 2004

Experimental values are given for the yields and spectral energy purities of K x-ray beams emitted at 120° and 180° from various targets when bombared by 0.01- to 3.0-MeV electron beams. Yields and purities are determined for beryllium, carbon, aluminum, titanium, copper, silver, and gold targets as a function of the target thickness and target inclination angle.
Electromagnetic flight stability by model impedance simulation
Authors: Iwasa Y.
Published: February 2004

Impedance measurements on scale models can be an extraordinarily simple and effective technique for comparing the lift and drag forces and the static and dynamic stabilities of various vehicle-guideway configurations for levitation by motion-induced eddy-current repulsion. The real and imaginary components of the input impedance of an inductive circuit are a measure of its power loss and magnetic energy storage, and the gradient of that energy is magnetic force. As a demonstration of the technique, we have applied it to variations of the magneplane system being developed by the MIT-Avco-Raytheon group.
Optimal stabilization of the Rayleigh-Taylor instability in the multiarm fluid pendulum
Authors: Zrnić Dušan S.
Published: February 2004

This work poses the control of an instability in the multiarm fluid pendulum as an optimal regulator problem. A linear set of equations describing the motion of fluid in the pendulum is developed and the analytical solution of the control problem is presented. An experiment of optimal stabilization on a two-arm pendulum is discussed and compared with theoretical predictions. The optimal control requires only a slightly more complex feedback circuit, but it reduces considerably the energy put into the system.
Reply to ``Comments on `Grain boundary contrast in field_ion microscope images' ''
Authors: French R. D. , Bishop G. H.
Published: February 2004

This communication is a reply to certain questions raised on the use of computer image simulation to study grain boundary contrast in field-ion microscope (FIM) images. The validity of the rigid lattice assumption is assessed for low- and high-angle grain boundaries. Resolution of grain boundary dislocations, boundary contrast as a function of boundary position, the difference in contrast in the FIM between coincidence and off-coincidence grain boundaries, and the priority of contrast events are discussed. A plane matching method used in transmission electron microscopy is introduced to clarify the relation of bulk boundary structure to surface contrast. The ring-match model of Page, Howell, and Ralph as it has been applied to our work is evaluated. Suggestions are made as to the proper use of computer image simulation and it is noted that a better approximation to simulation of grain boundary contrast might be achieved by using computer relaxed grain boundary structures as an input for our image simulation program.
Parametric heating of a dense arc plasma with 0.337 mm laser radiation
Authors: Jassby D. L.
Published: February 2004

The threshold intensity of 0.337 mm laser radiation for excitation of the parametric decay instability in a helium arc plasma at 4.0 eV is computed. The plasma density gradient reduces the intensity required from the laser by a factor of 16.
Anisotropic elastic solutions for line defects in high-symmetry cases
Authors: Hirth J.P. , Lothe J.
Published: February 2004

The elastic fields of line-force couples have been shown to be applicable to the description of dislocation core fields. The conventional anisotropic elastic theory of line defects is extended to include such couples. Explicit results are presented for cases of high-crystallographic symmetry for which the results can be expressed in simple analytical form.
Production of secondary electrons in nitrogen by fast electrons and simultaneous excitation of N2 bands
Authors: Borst Walter L. , Imami Mahmood
Published: February 2004

Band emissions from the second positive (2PG) and first negative (1NG) systems of nitrogen were excited by secondary electrons in the pressure range 10−4–1 Torr. The secondary electrons were produced in ionizing collisions between N2 molecules and fast primary electrons. Band intensities were monitored inside and outside the interaction volume of the primary electron beam, enabling one to separate the contributions from secondary and primary electrons to the total excitation. 2PG bands were produced solely by secondary electrons whereas 1NG bands were excited by secondary and/or primary electrons depending on the interaction volume viewed. The intensity ratio of the 2PG(0,0) band to the 1NG(0,0) band excited by slow secondary electrons was found to be about 0.5 in the pressure range 10−3–1 Torr at a primary electron energy of 1000 eV. The pronounced difference in the energy dependence of the excitation cross section of the 2PG and 1NG bands was utilized to infer the characteristics of the energy spectrum of secondary electrons φ(E). Assuming an analytic form φ(E)αE−n for the energy spectrum, a ``spectral index'' n≃1.9 was found using the measured intensity ratio. The energy dependence φ(E)αE−1.9 applies to secondary electron energies between 11 and approximately 200 eV and was found to be in good agreement with binary encounter theory. The present results were compared with atmospheric measurements of auroral emissions and electron spectra.
Excitation and ionization of neutral atoms in a dense discharge
Authors: Deutsch C.
Published: February 2004

Closed forms for the electron-impact cross sections are used to evaluate excitation and ionization rates of the ground state of some atoms currently used in discharge physics. A Maxwellian and a Druyvesten distribution are considered respectively for the electron distribution velocity. Excitation processes are seen to be much more dependent on the atomic structure and the electron distribution than on ionization. Conversely, the latter process is much more sensitive to the electron temperature. Estimates of the electron density for the existence of complete local thermodynamic equilibrium are also discussed.
Calorimetric investigation of Tl5Te3 superconductivity
Authors: Haemmerle W.H. , Reed W.A. , Juodakis A. , Kannewurf C.R.
Published: February 2004

The specific heat of several samples in the γ phase of the Tl–Te system were measured between 1.8 and 4.2 °K to determine if the previously reported superconductivity in this phase was a true bulk effect or due to filaments of a second phase. These measurements show a change in the specific heat at Tc of the stoichiometric composition, Tl5Te3, which is 60% of the value predicted from the BCS theory. The only second phase present in some samples, as determined by x-ray diffraction, was TlTe; but even as large an amount present as 10% was found to make a very small contribution to the specific heat. Thus we conclude that Tl5Te3 is a bulk superconductor, characterized by an electronic specific heat of 5.7×10−3 cal/mole deg2 and a Debye ΘD of 96 °K. Some possible explanations are proposed to account for the low value of the measured specific heat.
Dielectrically induced surface wave on a YIG substrate
Authors: Parekh J. P. , Ponamgi S. R.
Published: February 2004

Exchange-free surface waves on a tangentially magnetized YIG substrate are treated for the case of propagation at right angles to the bias field. In addition to a modified magnetostatic solution, a new dielectrically induced surface wave is found to exist.
Dependence of threshold current density and beam spread on potential barrier height in close confinement lasers
Authors: Ahn B. H.
Published: February 2004

We have studied the effect of the potential barrier height of single-heterojunction GaAs lasers on the threshold current density and the beam spread. We have found that the minimum for the threshold current density occurs when the AlAs mole% is about 40% in the p+ region, and the beam spread remains approximately the same when the AlAs mole% is 40% or less in the p+ region. But the beam spread increases rapidly when the AlAs mole% is more than 58%. We explained these phenomena in terms of the change in the effective waveguide width in the optical waveguide model.
Low-temperature resistivity of two-phase Al–Al3Ni alloys
Authors: Simoneau R. , Bégin G.
Published: February 2004

The electrical resistivity of Al–Ni alloys in the composition range 2–28 wt% Ni was investigated. The eddy-current decay method was used and the measurements were made at three temperatures: 297, 77, and 4.2 K. The results are explained in term of a series and parallel resistor model which takes into account the existence of different regions in the alloy (i.e., the continuous eutectic mixture, and the dispersed proeutectic constituents). This model permitted the resistivity curve to be fitted at the three temperatures and resistivity value for Al3Ni to be computed. The sharp maximum of resistivity at the eutectic composition, appearing at low temperature, has been attributed to the scattering of electrons on precipitate boundaries and to peculiar morphological features.
Effect of a metallic thin film on the propagation of Rayleigh waves
Authors: Pouliquen Jean , Vaesken Guy
Published: February 2004

An oscillator built with a thermostated Rayleigh-wave delay line can be shifted in frequency by a metallic (Au, Cu, Al) thin film evaporated in a vacuum on the ultrasonic wave path. For very thin films (<100 Å) there is a rapid decrease in frequency of the oscillator and electrical resistance of the metal. This phenomenon is due to the short circuiting of the electric field to the interface and gives a good method for measuring the value of electromechanical coupling constant. For thicker film (from 200 to 3000 Å) the variations are much slower. The slope of the frequency-vs thickness curve is propotional to the mass of deposited metal.
Instabilities of electrically exploded wires
Authors: Vlastós Antonios E.
Published: February 2004

The experimental results summarized in this paper, which have been obtained with thin tungsten, copper, and constantan wires, give further evidence that electromagnetic macroinstabilities may develop near the melting point of thin wires which are heated by the sudden release of electrical energy and lead to their disintegration. At low-energy-input rates, screw-type instabilities develop in all the wire materials used. On the other hand, at high-energy-input rates, the copper and constantan wires show striations after the explosion, while tungsten wires are not striated but are split into tiny fibers which emerge along the whole wire. At the beginning the restrike channel of copper and constanstan wires has an helicoidal form. Annealed wires show the same results. Helical channels were not observed in tungsten wires. By twisting the copper and constantan wires in the opposite direction to the channel helix, it was possible to change the direction of the helix. Thus, the helical shape of the channel may be an effect produced by the twisting of the wire in mounting it betwen the electrodes. The possibility that the helicoidal shape of the channel is due to some other effect is also discussed.
Nonlinear mechanisms for self-focusing of microwaves in semiconductors
Authors: Sodha M. S. , Tewari D. P. , Kamal Jyoti , Pandey H. D. , Agarwal A. K. , Tripathi V. K.
Published: February 2004

A detailed investigation of various possible mechanisms of nonlinear self-focusing of electromagnetic waves in semiconductors has been made. In uncompensated semiconductors focusing is possible only due to mass modulation. However, in a semiconductor in which electron- to hole-concentration ratio is equal to the square of the mass ratio, free-carrier diffusion provides an additional effective mechanism for self-focusing. In both mechanisms thermal effects are much more important than the drift effects. When the duration of the signal is comparable with the free-carrier energy-relaxation time (which is ∼ 10−8 sec in InSb at low temperatures) its shape becomes distorted to a considerable extent, and the distortion provides a direct method for the detailed investigation of energy-relaxation mechanisms.
Dynamics of flux motion and ac losses in the superconducting sheath state
Authors: Wagner J. F. , Rollins R. W.
Published: February 2004

The temperature and rate dependence of the penetration of flux through the superconducting sheath is studied in the Pb–In alloy system. The waveform of the response of the sheath to a small sinusoidal magnetic field is investigated over temperature and frequency domains from 0.15Tc to Tc and 50 Hz to 1 kHz, respectively. The measured waveform is discussed in terms of a simple flux-creep model, a viscous flow model, and a combined flux-creep—flux-flow model. The flow model assumes a distribution of pinning forces and an effective viscosity coefficient η, both of which are taken to be independent of frequency. No distribution of pinning forces is considered in either the flux-creep or combined models. Computer analysis is employed to fit the models to the observed waveforms and to study the variation in the model parameters as a function of frequency and temperature. The simple flux-creep model does not lead to the observed frequency dependence of the sheath response, while the flow model and the combined creep-flow model are more satisfactory. However, the viscosity coefficient, η, of the latter models is found to decrease with increasing frequency.
Magnetic and magneto-optical properties of sputtered MnGaGe films
Authors: Sawatzky E. , Street G. Bryan
Published: February 2004

The new compound manganese gallium germanide has recently been shown to be a uniaxial ferromagnet with a Curie temperature of 185°C. The easy axis of magnetization coincides with the tetragonal crystallographic c axis. Nearly completely c-axis-oriented films with hysteresis loops showing essentially 100% remanence normal to the film plane have been prepared by sputtering. The Curie temperature depends on the film composition. Curie temperatures as low as 110°C have been observed in nonstoichiometric films. These films have a room-temperature coercivity of 950 Oe compared to ≥2000 Oe for stoichiometric films. At a wavelength of 6328 Å the absorption coefficient α (corrected for reflectivity) is 5.8×105 cm−1 and the room-temperature polar Faraday rotation F is 80 000°/cm. The reflectivity R at normal incidence is 0.47. MnGaGe undergoes no phase changes before melting peritectically at 616°C and the films are stable to oxidation at temperatures well above the Curie point. The low Curie temperature combined with high remanence normal to the film plane make this material a potential candidate for magneto-optic file applications.
Variation of superconducting transition temperatures of transition-metal thin films deposited with the noble gases
Authors: Schmidt P. H. , Castellano R. N. , Barz H. , Cooper A. S. , Spencer E. G.
Published: February 2004

Thin films of Cr, Mo, Ti, W, and Zr metals deposited by dc ion-beam sputtering in noble-gas atmospheres were found to be superconducting with transition temperatures substantially higher than those observed for the respective bulk metals. Mo films deposited with a Xe ion beam were found to be superconducting at a temperature of 7.2°K. A variation of Tc was observed for films deposited using different noble gases with highest transition temperatures obtained for deposition with Xe gas. Ion-beam-sputtered thin films of Nb, Ru, Ta, and V metals were similarly deposited with a resulting decrease in Tc as compared with their bulk metals. Film deposition with noble gases of higher mass produced higher Tc's than those obtained with lower-mass noble gases. The mechanism responsible for the variation of the superconducting transition temperatures of these metal films is not known at this time but may be associated with the formation of a dilute solution/mixture of the noble gas and the transition metal, a change in the d electron density of states at the Fermi level and a change in the Debye temperature.
Domain walls in bubble films. II. Static properties of thick films
Authors: Schlömann Ernst
Published: February 2004

The structure of static walls is analyzed assuming that the film thickness is much larger than the wall width. It is shown that several different types of wall structure can exist. For the normal wall type the twist angle (angle between the wall magnetization at the midplane of the film and at the film surface) is approximately π/2. For the first anomalous type this angle is approximately −3π/2, for the second type approximately 5π/2, etc. The width and energy of such walls are calculated using (i) a Bloch-line approximation and (ii) a sinusoidal variational trial function (as in Paper I of this series). The ratio of the energy of a twisted wall (E) to the energy of an untwisted wall (E0) can in all cases be represented as E/E0=(1−βσ)1/2, where σ=4πM0/Ha [M0 is the saturation magnetization, Ha is the anisotropy field]. According to the first method β=0.692 for the normal wall type and β=0.692− (π/c)(D/2πM)1/2 for the first anomalous type. Here 2c is the film thickness, D=2A/M0, and A is the exchange stiffness constant. According to the second method β=0.654 for the normal wall type, and β=0.319 for the first anomalous type.
Magnetocrystalline anisotropy of rare-earth manganites
Authors: Schieber M. , Grill A. , Nowik I. , Wanklyn B. M. Y. , Sherwood R. C. , Van Uitert L. G.
Published: February 2004

Magnetic susceptibilities of R Mn2O5 where R=Sm, Tb, Ho, and Lu have been measured between 1.4 and 300°K in magnetic fields up to 15.3 kOe. A high magnetic anisotropy has been found in TbMn2O5 and HoMn2O5. The anisotropy of TbMn2O5 was explained in terms of two closely spaced singlets of the ground state of Tb3+.
Forces on null-flux magnetic levitation systems
Authors: Urankar L. , Miericke J.
Published: February 2004

Forces on rectangular current-carrying coils, above and below an infinite conducting sheet of arbitrary thickness and moving parallel to it, are investigated. Expressions are given for the lift and drag forces on the coils as a function of speed. Numerical calculations are made and compared with experimental results.
Magnetic levitation forces—finite conductor size
Authors: Atherton David L. , Castel B. , Puhach P. A.
Published: February 2004

Magnetic levitation is attracting attention as a practical noncontact suspension system for high-speed guided ground transport. Detailed mathematical analysis of magnetic lift and drag forces for nonidealized practical configurations is, however, difficult. This analysis examines only the effects of finite cross-sectional area of electromagnetic windings. A simple model of the magnetic lift and drag forces acting on a long current-carrying wire moving above and parallel to a thin infinite conducting plane is used. Corrections derived for finite conductor size, in terms of width and height, are shown to be small. Consequently, in practice, coils with finite winding cross sections can justifiably be represented by thin wires at their geometric centers.
Low-energy ion bombardment of silicon dioxide films on silicon
Authors: McCaughan Daniel V. , Murphy Vincent T.
Published: February 2004

Exposure of SiO2 layers on silicon to a plasma environment during sputtering or backsputtering leads to degradation of the electrical properties of the interface. In this paper the effects of the ion bombardment occurring during such exposure are detailed. A low-energy ion-bombardment apparatus, capable of giving ion beams ranging from 100 nA at 10 eV to 5 μA at 3.0 keV of mass-analyzed gas ions, constructed for this study, is described. Bombardment of SiO2 layers by Ar+, N+, or N2+ ions of any energy from 10 to 2800 eV leads to increase in interface state density and oxide interface charge. The oxide charge after bombardment is located both in the region of the Si/SiO2 interface and in the outside 50 Å of the SiO2. Both induced oxide interface charge and interface state density increase linearly with dose to a saturation value, typically to 8×1012 charges/cm2 and 1013 interface states/cm2 eV at midgap after a dose of 8×1013 ions/cm2 at 600 eV, on 1000Å of SiO2. At constant dose the induced charge and interface state density increase linearly with energy to approximately 500 eV, then are independent of energy to the maximum energy studied. After ion bombardment to a dose of ions of 5×1013/cm2 or greater, SiO2 films exhibit a significant reduction in dielectric strength. This reduction is not completely recoverable by annealing to temperatures up to 600°C. Annealing to 600°C does, however, remove all interface charge, interface states, and room-temperature trapping instabilities produced by the bombardment, as measured at room temperature after annealing. These experiments show that ion bombardment is capable of producing MOS degradation similar to some of the MOS degradation observed after sputter deposition of metals over oxide films. The increase in charge and interface states is a function of dose, so adjustment of plasma parameters during sputtering to give minimum ion bombardment should minimize this buildup of charge.
Diffusion-limited Si precipitation in evaporated Al/Si films
Authors: van Gurp G. J.
Published: February 2004

The precipitation behavior of evaporated Al/Si films with up to 1.8% Si is studied by microscopy as well as by resistance measurements after isochronal and isothermal aging. After deposition of the film most of the Si is precipitated. Temperature treatments above 300°C cause the solution of Si into Al. Aging of a supersaturated solution results in precipitation on grain boundaries at a rate that is much greater than in bulk Al/Si. The precipitation is described as a two-dimensional diffusion-limited process. A theoretical expression for the rate constant permits the determination of the diffusion coefficient of Si in Al. This is much greater than in bulk Al/Si and has an activation energy of about 0.85 eV. In view of the small vacancy concentration in the film, these results are explained by diffusion along dislocations, in agreement with electron micrographs revealing a dislocation density of about 1010 cm−2.
Driving circuits for energy-dependent loads
Authors: Shaw J. F. , Hug W. F. , Marlett C. J. , Hains C.
Published: February 2004

The behavior of a single-mesh pulse-forming network for driving flashlamps in both steady and nonsteady time response regimes is discussed. The resistance of the flashlamp is modeled as dependent only on the stored energy in the flashlamp discharge Ep. This energy in turn depends only on the electrical energy input and a characteristic decay time τd. It is shown that this formulation applies for any input pulse width and allows analysis of the case where the desired input pulse is shorter than τd. Numerical solutions are presented for a wide range of parameters and critical damping parameters are presented for response conditions ranging from steady to very nonsteady. Finally, a procedure for obtaining these parameters experimentally is presented so that the previously developed solutions can be applied to specific situtations of interest.
Emission characteristics of a tube-shaped laser oscillator
Authors: Milam David , Schlossberg Howard
Published: February 2004

Long-pulse emissions characteristics of an oscillator which uses a thin-walled tube of Nd: glass as the lasing medium are described. Slope efficiency in the single-shot mode was 0.9%, with 70% of the energy being contained in a full divergence angle of 1.7 mrad. Single-shot characteristics were maintained at repetition rates up to 0.2 pps in close agreement with the calculated thermal time constant. Peak output of 4.1 times the single-shot value was obtained at 2–3 pps.
Measurement of critical bulk and surface current densities in type-II superconductors in the mixed state
Authors: Kroeger D. M. , Koch C. C. , Coghlan W. A.
Published: February 2004

We present an improved method for calculating from ac susceptibility measurements the critical current density Jc and the surface shielding parameter ΔH in bulk cylindrical type-II superconductors. As first developed by Bean, the method involves harmonic analysis of an expression for the voltage induced in a pickup coil around a specimen in an axial dc magnetic field with a small ac ripple field superimposed. Values of Jc obtained by Bean's analysis are suspect, in part because the analysis does not take account of surface shielding. Ullmaier modified Bean's calculation to remedy this defect. For simplicity he did not Fourier analyze the induced voltage, choosing instead to determine Jc and ΔH by comparing experimental waveforms with his calculated voltage. This procedure permits only a qualitative indication of whether the data are consistent with the model. For generality, we have modified Ullmaier's calculation by removing the approximation that the depth of penetration of the ac field is small compared to the sample radius. The resulting expression for the induced voltage V is a complicated function of Jc, ΔH, and the amplitude of the ripple field h0. A graphical procedure has been developed which not only extracts values of Jc and ΔH from plots of the amplitude of the third harmonic of V vs h0, but also provides a check for consistency of the data with the model.
Reply to ``Comments on `Behavior of simple screw dislocation arrays under an applied stress' and `Principles underlying the behavior of passing dislocations' ''
Authors: Sadananda K. , Marcinkowski M. J.
Published: February 2004

It is shown that the selection of an applied stress as a dependent or as an independent variable in dislocation equilibrium calculations limits their applicability to constant strain-rate tensile or creep tests, respectively. Furthermore, it is shown that equilibrium calculations based on force techniques may not represent realistic configurations since the force techniques cannot incorporate in their formulation the history dependence of the equilibrium configuration, whereas the energy minimization techniques can.
Electrical transport properties of the alloy (V0.3Nb0.7)O2
Authors: Kristensen I. K.
Published: February 2004

Measurements of the electrical conductivity and the Seebeck effect in (V0.3Nb0.3)O2 are reported. A semiconductor-to-metal transition is observed at about 1000°K and an evaluation of the activation energy for the electron mobility gives 0.05 eV.
Thermal and elastic properties of silicate oxyapatite crystals
Authors: Hopkins R. H. , de Klerk J. , Piotrowski P. , Walker M. S. , Mathur M. P.
Published: February 2004

The elastic constants, thermal expansion, thermal conductivity, and heat capacity are reported for a new class of laser hosts-the silicate oxyapatites. For Ca2La8(SiO4)6O2 the five independent elastic constants obtained from sound velocity measurements are C11=1.71, C12=0.62, C33=1.71, C44=0.52, and C13=0.39×1012 dyn/cm2, the thermal expansion coefficients at 300 K are 8.9×10−6 and 6.6×10−6 K−1 for the [101̄0] and [0001] directions, respectively, and the room-temperature conductivity is about 0.019 W cm−1 K−1. Sound velocity and thermal expansion data suggest a Debye temperature between 400 and 500 K.
Correlation of density periodicity to pressure-induced polymorphic transformations in solid elements
Authors: Liu Lin-gun
Published: February 2004

The densities of the elements have been found to be a periodic function of the atomic number in the same manner as the other properties of elements in the Periodic Table. The existence of pressure-induced polymorphic transformations in solid elements at room temperature is found to be closely correlated to the periodicity of density, hence, the atomic number. With only the exception of Fe, no phase transformations have been found for 28 solid elements having densities greater than ρ=2.8+0.12Z. For elements having densities less than the above value, there might be only two exceptions out of 50 solid elements that do undergo pressure-induced phase transformations within the range of presently available apparatus. This correlation seems to serve as a better indication for finding a high-pressure phase than is predicted on the basis of crystal structure, and has led the author to find a high-pressure phase in Se, Tm, and Lu.
Vapor epitaxy of CaF2 on NaCl
Authors: Koch F. A. , Vook R. W.
Published: February 2004

CaF2 films, 30–1500 Å thick, were vapor deposited at 25–380 °C onto air-cleaved and vacuum-cleaved NaCl in residual gas pressures of order 10−5, 10−7, and 10−9 Torr. The structures of the films were examined by in situ ultrahigh-vacuum reflection high-energy electron diffraction, conventional transmission electron microscopy, transmission electron diffraction, and surface replication techniques. The films consisted of many crystallites, the complex {110} and {001} orientations of which depended on the deposition parameters. The transition from the complex {110} orientation to the {001} orientation is described in detail in terms of its dependence on the deposition temperature, film thickness, vacuum, and substrate conditions. The {001} orientation was identified with two sets of thin needle-shaped crystallites oriented at 90° to each other. The {110} orientation was identified with two sets of larger crystals also oriented at 90° to each other. At higher deposition temperatures the larger crystals formed predominantly at cleavage steps on the NaCl substrate, while the needlelike crystals tended to form on the surface of the films.
Optical study of lithium-defect complexes in irradiated silicon
Authors: Chen C. S. , Corelli J. C.
Published: February 2004

Optical spectroscopy has been used to study the interaction of lithium impurity atoms with neutron-produced defects in silicon. In addition to the divacancy-associated defect absorption bands at 1.8, 3.46, and 3.61 μ, several additional radiation-produced infrared absorption bands at 1.36, 1.50, 1.6, 1.94, 2.05, 2.09, 2.14, and 2.4 μ are observed in lithium-doped silicon, irrespective of the oxygen concentration. In high oxygen concentration Si, a decrease of the (LiO)+ vibrational band (9.85 μ) accompanied by an increase of the oxygen interstitial vibrational band (9 μ) is observed after neutron irradiation (at ∼ 300 ∘K) and heat treatment (to ∼ 150 ∘C). This fact rules out the possibility of the formation of the Li–O-vacancy defect complex. The presence of Li decreases the production of the oxygen-vacancy complex. The intensity and the annealing temperature of the divacancy-associated bands strongly depend upon the Li concentration, and the intensity is found to be substantially lower than that observed for Si not containing Li. The Li defect bands exhibit a saturation in their intensity for (Li) ≈6×1016 cm−3. The bands increase in intensity after T ≳80 ∘C heat treatment. This is attributed to the dissociation of other simple Li-associated defects. The results indicate that higher-order defect complexes are responsible for the bands and the divacancy must be directly involved in the formation of at least one of them. Detailed annealing studies are presented.
Absorption current, dielectric constant, and dielectric loss by the tunnelling mechanism
Authors: Wintle H. J.
Published: February 2004

A calculation is made of the absorption current, dielectric constant, and dielectric loss expected from electron tunnelling into a single trapping level in a dielectric. Plots are given of current versus time, and of dielectric constant, dielectric loss, and loss tangent against frequency. This model reproduces the approximate t−n dependence of absorption current (t =time, n≃1) and nearly constant tanδ observed experimentally in many insulators. Numerical comparison with experiment shows that this model may apply to the case of high polymers, but is unable to account for the higher absorption currents occurring in oxides and similar inorganic dielectrics.
High-energy oxygen jet propagation
Authors: Glenn H. David
Published: February 2004

This paper reports jet time of arrival, brightness-temperature profiles, and pressure histories over a greater distance and for longer observation times than have been previously reported. A Voitenko compressor was used to produce a high-energy jet of oxygen through a 2-cm-i.d. 350-cm-long steel pipe containing air initially at 2.5 μ of mercury. Velocities for the various identified jet components varied from 2.46 to 7.95 cm/μsec. Brightness temperature, and pressure profiles were obtained at specific locations along the pipe. A maximum brightness temperature of 136000°K was measured for the plasma jet as it started down the exit pipe. The results suggest that delayed entrainment of ablated wall material was the predominant factor in reducing pressure and temperature in the slower components of the jet.
The Ga–As–Ge–Sn system: 800 °C liquidus isotherm and electrical properties of Ge–Sn-doped GaAs
Authors: Panish M. B.
Published: February 2004

A large part of the 800 °C liquidus isotherm of the Ga–As–Ge–Sn system has been determined experimentally and it has been demonstrated that the compositions of the liquidus surface can be adequately represented by a ``simple solution'' thermodynamic model. GaAs crystals were grown from liquidus compositions along several cuts of the quaternary isotherm and the variation of the electrical properties as a function of the composition of the liquid from which they were grown was determined. An extensive range of net carrier concentrations with heavy compensation can be readily achieved in this system.
Effects of composition profile on characteristics of GaAs–Ga1−xAlxAs double-heterostructure lasers
Authors: Nakashima H. , Chinone N. , Taguchi Y. , Nakada O.
Published: February 2004

Electron probe x-ray microanalysis was used to measure the composition profile in the active layers of GaAs–Ga1−xAlxAs double-heterostructure wafers along the growth direction. The threshold current density and the lasing wavelength of the double-heterostructure laser were found to be consistently affected by the composition profile in the active layer.
Diagnostic rate monitoring of sputtered ZnO films
Authors: Denburg D. L. , Dybwad G. L.
Published: February 2004

Sputtered thin films of ZnO are useful in bulk wave and surface wave acoustic devices. The monitor described uses a multiple-beam interference technique which, in addition to thickness and deposition rate, yields information concerning the acoustic coupling factor of the films. The pattern of the resulting interference fringes is shown to be a sensitive on-line monitor for changes in the surface properties of the growing film. When a pattern change representative of a film ``going bad'' is recognized, the run can be aborted with a subsequent saving in time and material. Interference fringe patterns have been simulated on a computer using an optical theory which includes changes in surface reflectivity and incoherent surface scattering. The simulated patterns are found to occur experimentally. A set of experiments was done to correlate specific deviations from the ideal deposition process with a characteristic nonideal fringe pattern. A series of films was deposited under different conditions. Each was x rayed, tested acoustically, and examined in a scanning electron microscope. The results demonstrated that the rate monitor detected the changes in the normal growth process. This method has application in the deposition of other dielectric materials for which surface properties are a significant parameter. Once the user establishes the acceptable patterns for his film, deviations are easily detected.
ac breakdown studies of built-up barium stearate films
Authors: Agarwal V. K. , Srivastava V. K.
Published: February 2004

The potential importance of built-up films of barium stearate for making dielectric devices has led to the necessity of the study of their ac breakdown characteristics. In the present work, the thickness and frequency dependence of the ac breakdown field has been reported in the thickness range 100–1500 Å and in the frequency region 10–200 kHz. The breakdown field is found to vary with thickness as d–a (α=0.68) and the breakdown voltage is found to increase with increasing thickness and frequency in the specified ranges. Although the experimental results could not be interpreted at present, the results obtained may prove useful in ac device applications of the films.
Luminescence of GaP at high excitation
Authors: Tsu R.
Published: February 2004

Apart from the usual bound-exciton luminescence due to Te and N centers, broader peaks which appear at high excitation may be explained by free-carrier screening of the free excitons, or by high-density excitonic effects. A weak-luminescence structure observed around 2.8 eV, which has not been reported before, may be due to conduction electrons at the L point with holes at Γ15 bound to the neutral donors.
Pulsed holography with a dye laser
Authors: Shajenko P.
Published: February 2004

An experimental setup of a pulsed dye laser for holographic applications is described. The capabilities of the dye laser with tunable frequency for recording holograms are demonstrated.
Infrared-to-visible conversion in CaF2:Er3+ — a sequential pair process
Authors: Verber C. M.
Published: February 2004

The upconversion of a 1.5-μ pump beam to 0.65-μ visible fluorescence has been studied in CaF2:Er3+ over a concentration ranging from 0.01% to 2.0%. The upconversion process has a cubic dependence upon pump intensity and ion concentration and has been shown to result from a sequence of two cooperative pair interactions.
Quasi-equilibrium approach to the zirconium-oxygen combustion phenomena
Authors: Gupta Suresh K. , Maloney Kenneth M.
Published: February 2004

It is found, for pressures ∼ 1 atm and above, that the residence time for molecular oxygen, in the gas-phase combustion region surrounding a 525-μ zirconium droplet, is of the same order of magnitude or greater than the mean dissociation time for molecular oxygen; therefore, the concentration of species are assumed to have their equilibrium values. The distribution of species over the reaction zone is calculated by means of free-energy minimization. Calculation of the existing equilibria, under known reaction conditions, illustrates the existence or nonexistence of condensed phases. The meanings of reactive collision probability, apparent over-all oxygen incorporation probability and minimum fractional surface oxygen incorporation, as related to droplet combustion, are discussed. Additionally, some methods by which the maximum combustion temperature of the Zr-oxygen system has been obtained are discussed.
Depolarization fields in thin ferroelectric films
Authors: Mehta R. R. , Silverman B. D. , Jacobs J. T.
Published: February 2004

A mechanism is proposed to explain depolarization phenomena that have been observed in thin ferroelectric films and related multilayer devices. It is shown that, for a short-circuited electrode-ferroelectric structure, incomplete compensation of the ferroelectric polarization charge results when the center of gravity of this charge and the free compensation charge are not coincident. Depolarization fields in the ferroelectric arising from such incomplete compensations are estimated. A simple switching calculation shows such fields to be of sufficient strength to account for the initial polarization decay rate observed in Pb0.92Bi0.07La0.01 (Fe0.405Nb0.325Zr0.27)O3 films. The results of measurements involving changes in film thickness, electron concentration in the electrodes, and contact materials will be discussed and shown to be consistent with the mechanism proposed.
Alpha-particle stopping cross section of silicon and germanium
Authors: Lin W. K. , Olson H. G. , Powers D.
Published: February 2004

Stopping cross sections of α particles for silicon and germanium have been measured for 0.3 ≤ Eα ≤ 2.0 MeV. The experimental method consists of the elastic scattering of α particles either from a thick Ta backing onto which the stopping element has been evaporated, or directly from the film of stopping element evaporated onto a thick Al backing. The results were analyzed using a three-parameter formula for stopping cross section recently suggested by Brice. With the parameters obtained from a least-squares fit, the formula was found to be useful in calculating the stopping cross sections for energies outside the measurements, at least up to 10 MeV.
Waveguide electro-optic modulation in II-VI compounds
Authors: Martin W. E.
Published: February 2004

Electro-optic intensity modulation is demonstrated in three-dimensional (channel) waveguides fabricated by diffusion techniques in ZnSe and CdS. Typical diffused guide dimensions are 1.6 μm × 19 μm × 2 mm. Two electrode configurations giving two field orientations are used. Waveguide modulators with Vπ (voltage for π radians phase shift) of 72 V with rise times less than 5 nsec are described. Waveguide modulation in epitaxial layers of ZnS and ZnSe on GaAs is also described.
Limitation on deep trapping of injected space charge in naphthalene and CdS monocrystals
Authors: Dresner J. , Campos M. , Moreno R. A.
Published: February 2004

Studies of space-charge-limited currents as a function of sample thickness L were made for CdS and naphthalene monocrystals. In naphthalene, where the current-voltage curves show the behavior expected for a discrete trap energy above the Fermi level, we find VTFL essentially independent of L. We attribute this result to the presence of few filamentary regions oriented along the c axis and differing only slightly from the bulk in distribution of defects. For CdS we find VTFL ∝L, consistent with a constant upper field limit of 1.6 × 104 V/cm on a one-carrier injection current. In neither material can the volume concentration of traps be determined from the voltage at which the current rises sharply.
Energy deposition of electrons in gas lasers
Authors: Bass J. N. , Green A. E. S.
Published: February 2004

Energy deposition techniques are applied to the analysis of gas lasers. Using the latest available electron impact cross sections for molecular hydrogen, efficiencies for depositing energy in various excited states are computed. These results, together with the laser rate equations, are used to predict population inversions and emissions in the Werner bands of molecular hydrogen for a laser pumped by 400-keV electrons. Reasonable qualitative agreement with experiment is obtained.