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Journal of Applied Physics

Issue 13 • Date Dec 1968

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Displaying Results 1 - 25 of 73
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Dispersion of Surface Elastic Waves Produced by a Conducting Grating on a Piezoelectric Crystal

    Page(s): 5819 - 5827
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    The dispersion relation for surface elastic waves propagating under a thin metallic grating on the basal plane of a hexagonal piezoelectric crystal is investigated theoretically. Purely reactive loads are assumed connected between adjacent electrodes of the grating. The effect of mass loading due to the grating is neglected. The presence of the grating imposes periodic boundary conditions on the piezoelectric fields associated with the surface wave. The resulting boundary‐condition problem is solved to obtain the dispersion relation for surface elastic waves. For a material with a large electromechanical coupling coefficient an inductance connected between adjacent electrodes can cause a large reduction in the phase velocity of the surface wave. (For example, in PZT‐4, this reduction is about 10%.) View full abstract»

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  • Dipole Modes and the One‐Wave Approximation for Trapped Modes in Magnetoplasma Cylinders

    Page(s): 5828 - 5833
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    An analysis of trapped mode wave propagation is given for a cold, uniform magnetoplasma cylinder embedded in free space. The full set of Maxwell's equations is used and an arbitrary magnetic field strength is allowed. New simplified forms are obtained for the dispersion relation and the fields. The simplified expressions are used to study the dipole modes in detail. Computer solutions are given for the dispersion relation and for the longitudinal fields when the cyclotron frequency is above and below the plasma frequency and for two different column diameters. The fields solutions are sums of terms with two different propagation constants. For the circularly symmetric and dipole modes, it is found that one wave dominates. This has led to a new approximation, the ``one‐wave'' approximation, which gives dispersion curves accurate to within 1% except for the lowest‐order dipole mode. View full abstract»

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  • Decay of Resonance Radiation in a Planar Afterglow with Reflecting Walls

    Page(s): 5834 - 5837
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    The decay time of Doppler‐broadened resonance radiation in a planar afterglow is calculated numerically and compared with the analytic approximation of Holstein. This comparison shows that Holstein's results are higher than the numerical results by as much as 18%, depending on the optical thickness. In addition, the effect of diffusely reflecting walls on the decay time is calculated numerically and compared with the analytic approximation of Weinstein where possible. The numerical results are rigorously valid in the optically thick limit and approximately valid in the optically thin limit. View full abstract»

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  • Electron‐Spin‐Echo Envelope Modulation Induced by a Very Small Alternating Magnetic Field

    Page(s): 5837 - 5841
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    The effect of an additional very small alternating magnetic field h¯ on the electron spin echoes is reported. When single crystals of K3Co (CN)6 containing Cr3+, and CaWO4 containing Ce3+, as substitutional impurities were subjected to the extra field h¯, the echo envelope was modulated at the frequency of the extra field ranging from 100 to 2000 kHz. The magnitude of the additional field h¯ ranged from 0.1 to 0.85 G and the depth of the periodic modulation of the echo envelope was directly dependent on the magnitude of h¯. The maxima and minima in the envelope occurred when the period T of the additional field h¯ was related to the time interval between the microwave pulses by τ=nT and τ=(n+½)T, respectively. The results are discussed in view of the paper by M. Dupont and G. A. Woonton [Can. J. Phys. 46, 87 (1968)] which shows that the additional magnetic field h¯ contributes to these effects only at those instants when microwave pulse is also present, provided the field h¯ is homogeneous over the sample. A classical treatment of the problem is presented. The effect of additional field h¯ on the echo amplitude is decided by the magnitude of h¯ relative to microwave field in the cavity and not relative to steady magnetic field H0. View full abstract»

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  • Radiation Annealing and Modification of the 35° and 65°K Defects in Germanium at Low Temperature

    Page(s): 5842 - 5845
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    Damage introduced in n‐type germanium at ∼7°K with 1.0 MeV electrons undergoes radiation annealing and conversion when subjected to 0.5 MeV electron irradiation at ∼7°K. The radiation annealing with only 65°K and permanent defects present appears to be a first‐order process with an estimated cross section of 10-15 cm2, while radiation annealing with 35°, 65°K, and permanent defects present appears to be a second‐order process. It appears that only the 35° and 65°K defects are recovered and 65°K defects are converted to 35°K defects as part of the radiation‐annealing process. The radiation anneal of converted damage seems to be the rate‐limiting process in the radiation anneal of 65°K defects. Our results show evidence for another type of 65°K defect as suggested by Zizine and by Whan's work. If this is the case, then our results imply that this second type is not subject to radiation annealing. View full abstract»

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  • Theory of the DeBlois Experiment. I. The Effect of the Small Coil

    Page(s): 5846 - 5850
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    The nucleation field, as measured in the DeBlois experiment, should in principle be corrected for demagnetization, and for the field falloff. The correction for the demagnetization has been shown before to be negligibly small in practical cases and this is proved again here. The falloff, on the other hand, implies a correction of about 20% to the field at the center of the reversing coil. This correcting factor is calculated as a function of the coil dimensions, assuming, for the meantime, a perfectly smooth whisker. View full abstract»

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  • Theory of the DeBlois Experiment. II. Surface Imperfection in an Infinite Coil

    Page(s): 5850 - 5854
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    The demagnetizing field caused by surface roughness is approximated by the field of a straight‐cut, cylindrically symmetric, scratch or protrusion, in an infinite circular cylinder. Only the values of this field along the axis of symmetry, r=0, are taken into account, and are assumed to be the same for every value of r. For this simplified model, the nucleation field can be calculated by a one‐dimensional integration, which is done numerically. The results are of the same order of magnitude as the experimental values, and the general shape of the theoretical curve is rather similar to the experimental one. View full abstract»

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  • Low‐Energy Electron Scattering from Clean and Hydrogen‐Covered Nb (110) Surfaces

    Page(s): 5854 - 5858
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    Low‐energy electron diffraction (LEED) and inelastic electron scattering (IES) measurements are used to characterize the chemisorption of hydrogen onto the Nb (110) surface. It is found that hydrogen chemisorbs readily at room temperature, causing no rearrangement or faceting of the surface, and with only small changes in the work function. Distinctive changes in the intensity curves indicate that the hydrogen is chemisorbed in a crystalline layer with the same lattice structure as the Nb (110). Removal of the hydrogen is easily accomplished by heating to 400°C, and residual gas measurements show a higher mass 1 peak than usual, indicating that the hydrogen is adsorbed as atoms. Changes in both the LEED intensity curves and in the IES measurements both suggest that the beam penetrates more deeply into the crystal when hydrogen is chemisorbed than for the clean surface. View full abstract»

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  • Continuously Tunable Picosecond‐Pulse Organic‐Dye Laser

    Page(s): 5859 - 5860
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    Picosecond‐pulse outputs, tunable over a broad spectral range, have been demonstrated in an organic‐dye laser. Evidence is presented that no more than a small fraction of the available bandwidth cooperates in mode locking to produce individual short pulses. View full abstract»

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  • A High‐Resolution Scanning Transmission Electron Microscope

    Page(s): 5861 - 5868
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    A simple scanning transmission electron microscope has been built using a field‐emission electron source, a new electron gun, and one lens to produce a high‐contrast picture with 30 Å resolution. The final spot size is limited by the properties of the lens, in the same manner as a conventional transmission microscope. The field‐emission tip requires a pressure below 10-9 Torr for stable operation and can have a lifetime of several months. The intensity of the source is such that high‐quality pictures can be obtained in 10 sec. Specimen contamination or damage is small, as would be expected in view of the good vacuum conditions. The theory and design of the instrument are discussed, and experimental results are shown. View full abstract»

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  • Factors Influencing the Acoustic Properties of Vitreous Silica

    Page(s): 5868 - 5878
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    The acoustic properties of various types of commercial vitreous silica have been studied as functions of thermal treatment and impurity‐ion concentrations. The properties studied include density, shear‐wave attenuation, shear and dilatational wave velocities, and elastic constants. The measurements of acoustic properties are made on small sphere‐shaped samples using primarily a technique that sets the sphere into a resonant mode of vibration and then allows the vibration to decay freely. Both frequency and the decrement of the decay are measured. Use of this resonant sphere technique is particularly appropriate to the present study because it permits precise measurements on samples small enough to conveniently undergo laboratory modification, and it permits the study of velocity variations in a single block of material by measurements on many localized samples. The major findings of this study are the following: (1) An increase of one degree in fictive temperature (a parameter assumed to be related to the equilibrium internal configuration of the glass) causes a decrease in the shear velocity of 13–16 ppm. (2) Gross differences of OH content result in differences in shear velocity. Although shear velocity differences could be induced by changing fictive temperature and the gross OH content, shear velocity fluctuations (of the order ± 125 ppm) observed in single large pieces of commercial vitreous silica were not correlated with variation of either fictive temperature or OH content. Attempts to relate fluctuations of shear velocity to differences of Na content were inconclusive. View full abstract»

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  • Measurements of Demagnetizing Factors and Their Application to Magnetic Thermometry

    Page(s): 5878 - 5883
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    Shape corrections for several cerous magnesium nitrate magnetic thermometers have been measured, both for powders and for composites of oriented single crystals, in an attempt to establish a suitable geometry for a magnetic thermometer accurate to ∼1% at temperatures down to 0.02°K. The results indicate that this accuracy is available with either a spherical composite of oriented single crystals. or with a powder in the form of a right circular cylinder with ratio (length/diameter)=0.92. In either case it is imperative that the density be uniform within the sample. The results are compared with theoretical computations, where applicable, and with the results of other techniques. View full abstract»

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  • Fluorescence Decay Measurements of Eu2+ in CaF2:Eu+Ho

    Page(s): 5883 - 5884
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    Measurements have been made of the submicrosecond fluorescence decay times of Eu2+ in the mixed dopant, Eu2++Ho3+ system. Significant decrease of Eu2+ decay time as a function of Ho concentration confirms effective Eu2+→Ho3+ nonradiative energy transfer. A system using analog techniques for direct measurement of short decay times is described. View full abstract»

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  • Electrical Resistivity of Iron‐Carbon Alloys

    Page(s): 5885 - 5889
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    The electrical resistivity of alpha iron‐carbon solid solutions prepared from iron sources of two different general impurity levels has been studied as a function of applied longitudinal magnetic field up to 60 kOe at 4.2°K. It is demonstrated that the presence of interstitial carbon increases the resistivity of iron and drastically alters the shape of the magnetoresistivity curves, particularly at low fields. The residual resistivity contribution per at.% carbon in solution in high‐purity iron is about 4.9 μΩ·cm. At 78°K the value is approximately 6.0 μΩ·cm/at.% carbon, indicating a positive deviation from the Matthiessen rule. In comparison, the residual resistivity of a lower‐purity iron is about 6.5 μΩ·cm/at.% carbon at both 4.2° and 78°K. The magnetoresistivity data for the two iron materials containing carbon in interstitial solid solution are shown to obey the Kohler rule to a moderate approximation. In addition, some data are presented which demonstrate that the magnetoresistivity is dependent on the dispersion of the carbon in the iron. View full abstract»

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  • Work‐Function Measurements on Monolayer Films of Uranium on (100), (110), and (113) Oriented Faces of Tungsten Single Crystals by Photoelectric and Contact Potential Difference Techniques

    Page(s): 5890 - 5896
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    Uranium was evaporated on to (100), (110), and (113) oriented faces of tungsten single crystals at pressures of less than 5×10-10 Torr. The work function of each of the tungsten surfaces was monitored by photoelectric and contact potential difference (Kelvin and electron‐beam) techniques as uranium was adsorbed on to it. The curves of work function versus coverage were fitted to the Gyftopoulos and Levine theory of monolayer adsorption. The constants of the curves gave a value for the density of uranium atoms in a monolayer on each tungsten surface: For the (100) surface one uranium atom is adsorbed for each unit cell in the two‐dimensional lattice of surface tungsten atoms, on the (110) surface there is one uranium atom for every two unit cells, and on the (113) surface there are four uranium atoms to each unit cell. For film thicknesses of about two monolayers, the photoelectric technique gave work function values in excellent agreement with those from the Kelvin method, the values being 3.73±0.02 eV for the U/W100 system, 3.90±0.03 eV for U/W110, and 3.67±0.03 eV for U/W113. The electron‐beam values were higher than those obtained by the other two methods, probably due to an increase in the electron reflection coefficient as uranium is adsorbed on to tungsten. View full abstract»

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  • Power Flow in Plasma‐Filled Waveguides. II

    Page(s): 5897 - 5902
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    Power flow in cyclotron and plasma modes propagating in a waveguide filled with a cold collisionless axially magnetized plasma studied in an earlier paper is reexamined. It is shown that proper evaluation of field expressions at special points on the Brillouin diagram removes difficulties contained in the earlier presentation. Comparison of suitably normalized total power flow as a function of the propagation constants for various representative examples of mode types is made. Interaction or coupling impedances relating transverse and longitudinal electric fields to power flow are calculated. View full abstract»

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  • Electrostatic Waves in Inhomogeneous and Magnetized Plasmas

    Page(s): 5902 - 5907
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    A formalism is presented for the study of electrostatic waves in inhomogeneous plasmas immersed in a finite magnetic field perpendicular to the direction of inhomogeneity. Based on the quasistatic approximation, the dispersion relation is found to depend on the geometrical factors through four quantities which possess certain general properties. The dispersion relation is solved explicitly for the case of cold plasma. It is found that, in stationary plasmas, the density inhomogeneity not only affects the frequencies of oscillation but also alters the propagation and evanescent bands of the two branches of electron plasma waves. For streaming plasmas, the inhomogeneity, the transverse boundary, and the finite magnetic field all contribute to the stabilization of the instability. It is shown that the concept of effective density, developed previously for the infinite magnetic field case, can be extended to the case of finite magnetic field. Various special cases studied previously are recovered by putting appropriate parameters to zero in this general formalism. View full abstract»

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  • Secondary Twinning in Electron‐Diffraction Patterns of Nickel Films Epitaxially Grown

    Page(s): 5908 - 5910
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    Nickel films epitaxially grown were obtained by high‐vacuum evaporation on rocksalt faces cleaved in air. They were partially oriented and their thickness was 800 Å. The electron‐diffraction pattern was interpreted assuming the presence of primary and secondary twins and the orientation (001) parallel to the substrate. The composite reciprocal lattice, containing points from the matrix and from primary and secondary twins, was calculated by means of a computer program. The symmetry elements of this lattice permit the deduction of a selection rule to determine the location of the secondary twinned points with respect to those of the matrix and a cubic supercell with nine unitary cells along each orthogonal axis. The double diffraction points, arising from beams rediffracted in secondary twins, explain the origin of the rings interior to the (111). View full abstract»

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  • Molecular Motion in 2‐Chlorostyrene‐Styrene Copolymers from Dielectric Measurements

    Page(s): 5910 - 5912
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    The complex dielectric constant of several poly(2‐chlorostyrene‐styrene) copolymers was measured in the range 0.1–105 Hz from 27° to 150°C. Using the WLF equation, the relaxation frequency at the glass temperature was found to be about 10-4 Hz for each copolymer, within the uncertainty in the dilatometric glass temperatures. Normalized loss peaks are compared and their shapes are found to be nearly independent of Cl concentration in the range of this study. From these curves it is concluded that in this case the observed non‐Debye behavior is not the result of dipole‐dipole interaction nor steric hindrance between chlorine atoms, but is related to the structure of the polystyrene molecule itself. Loss peaks below the glass temperature (associated with the hindered rotation of the phenyl group) were observed for the copolymer having the highest Cl concentration. View full abstract»

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  • Noise and Equivalent Circuit of Double Injection

    Page(s): 5913 - 5918
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    Measurements of the high-frequency noise of a silicon double-injection diode result in 2>=α·4kT(1/r)Δf with α=1.04 and in agreement with the literature. A new interpretation demands Nyquist noise with α≡1 in these devices at high frequencies. This is in accord with an equivalent circuit derived for the double-injection process. Speculations are made on the general validity of Nyquist noise in nonlinear devices at high frequencies. In addition, generation-recombination noise is suggested as the prime source of the low-frequency noise. View full abstract»

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  • Resonant Frequencies and Fields in a Cavity Containing a Magnetoplasma Dielectric

    Page(s): 5919 - 5927
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    The problem of a microwave cavity filled with a uniform, cold magnetoplasma is solved exactly. The results are a set of three nonlinear simultaneous equations for the frequency and two spatial parameters. Explict expressions for all field components for oscillations of a circular cylindrical cavity are given. Numerical results are given showing frequencies and spatial parameters of eight resonant modes in a particular cavity with the cyclotron frequency equal to the unloaded TE111 frequency. Four of the modes correspond to the TE111 resonance and four correspond to the TM111 resonance. In addition, there is a plasma type of resonance at a frequency below the plasma frequency and below the whistler resonance. View full abstract»

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  • Dynamic Behavior of a Passive Q‐Switched Ruby Laser

    Page(s): 5928 - 5931
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    The risetime, fall time, and peak power of the output light pulse of a single‐mode passive Q‐switched ruby laser have been experimentally measured vs the measured ratio nt/nai, where nt and nai are the threshold inversions, respectively, without and with the saturable absorber. The results are compared with the fastswitch theory. As far as pulse risetime and fall time, the agreement is fairly good (within 20%). The experimental output power has been found 60% smaller than that theoretically expected. View full abstract»

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  • Observations of the Flow in an Electromagnetic Shock Tube

    Page(s): 5932 - 5935
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    Observations of the flow of plasma in an electromagnetic shock tube are presented. Three regimes of flow are found in hydrogen. At high initial gas pressure (≫1 Torr) and low speeds (≪2 cm/μsec) a cold shock‐heated region of gas followed by hot turbulent expanding driver gas was found. At low initial gas pressure (≪0.5 Torr) and high speeds (≫4 cm/μsec) a single hot turbulent body of gas moved down the tube. An intermediate flow was found in the intermediate pressure‐speed regime. The onset of turbulence in the flow was found to be described by the hydrodynamic Reynolds number. View full abstract»

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  • Emission Processes Accompanying Megavolt Electron Irradiation of Dielectrics

    Page(s): 5935 - 5940
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    Experiments are described in which thick dielectric samples were irradiated with electrons of about 1 MeV energy. The temporal growth of the external electric field from the trapped charge was punctuated by either total or partial collapses associated with electron emission and followed by pauses before the process repeated itself. Secondary electrons are thought to be heated by the growing internal space charge field until thermionic emission induces a regenerative growth of positive surface charge sufficient to oppose it. Electron emission then ceases until recombination can restore the outside surface of the dielectric to a field‐free condition from which it grows again. Analysis of this model shows that it appropriately describes the observed phenomena. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory