bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study of Scattering from a Plasma Column
Authors: Butterworth J. E. , Moore Richard L.
Published: February 2004

Experiments in microwave scattering from the positive column of a low-pressure (1.2 mTorr) discharge in mercury under steady-state conditions are described. A free-space environment is simulated with an anechoic chamber and a unique cw cancellation method. Both amplitude and phase of the scattered electric field for both polarizations in the 2.6- to 5.85-GHz frequency range are measured, so that diffraction characteristics, radar cross sections, frequency ratio, and phase shifts are determined. A calibration procedure using silver-plated cylinders provides a higher degree of accuracy than has been attainable heretofore. Diffraction measurements show that, for plane-wave incidence, the scattering is purely dipolar with a 95% confidence interval, except for overdense scattering where a significant monopole term characteristic of conductors is observed. Theoretical comparisons yield an estimate of the electron density distribution function and the damping of the Tonks-Dattner resonances, as well as providing a test of the various theories. The results of some auxiliary experiments to find the average electron density in the column and the effects of wall and mercury reservoir temperatures upon the observed results are also given.
Statistical Properties of Light Scattered by Particles Suspended in a Turbulent Fluid
Authors: Di Porto P. , Crosignani B. , Bertolotti M.
Published: February 2004

Second- and fourth-order statistical properties of radiation scattered by particles in a turbulent fluid are theoretically investigated. They turn out to be of a non-Gaussian type so that the measurements of the statistics of the scattered field can furnish information not obtainable by means of a spectrum analysis. In particular, our results are related with the possibility of measuring a Lagrangian type of velocity correlation function, generalized to different fluid elements.
Ion Emission from Rhenium and Lanthanum Hexaboride Coated Rhenium Filaments
Authors: Ford R. R. , Lichtman David
Published: February 2004

When heated, rhenium, like many other metals emits positive ions of the alkali metals. However the emission properties of these ions from rhenium are found to be very different from those observed for tungsten. In addition, the ion emission from rhenium can be greatly reduced relative to that of tungsten by short term vacuum annealing at sufficiently high temperatures. Lanthanum hexaboride coated rhenium filaments also emit positive alkali ions in a manner very similar to that of bare rhenium. The implications, both practical and theoretical of these results are discussed.
Photoinjection into SiO2: Use of Optical Interference to Determine Electron and Hole Contributions
Authors: Powell R. J.
Published: February 2004

There is some ambiguity in the interpretation of internal photoemission experiments because of the possible contribution of hole emission from the anode in addition to the cathode electron emission. Furthermore, correction of the photocurrent spectra for optical intensity variations is essential to determination of photoelectric yields. Optical interference is useful as a tool in ascertaining current contributions from the two electrodes since wavelengths of intensity maxima at one electrode correspond approximately to minima at the other. In this work an analysis technique is presented whereby the absorbed intensities are accurately calculated, the absolute quantum yields are determined assuming no hole photocurrent contribution, and the resulting spectra are examined for evidence of such currents. Internal photoemission measurements have been made on Au–SiO2–Si structures, and the technique has been applied to obtain quantum yields and look for evidence of hole emission. It is shown that hole photocurrents (if present) are negligible compared to electron currents and in particular for photoemission from Si into SiO2 the quantum yield for holes must be at least two orders-of-magnitude smaller than the electron quantum yield. The first determination of absolute quantum yields for Si–SiO2 and Au–SiO2 emission is presented, and the barrier energies are found to be 4.30 and 4.15 eV respectively. The optical problem has been solved for a range of oxide thicknesses and it is shown that intensity variations are not negligible even for very thin (∼500 Å) films.
Electrohydrodynamic Ion Source
Authors: Mahoney John F. , Yahiku Arthur Y. , Daley Howard L. , Moore R. David , Perel Julius
Published: February 2004

An electrohydrodynamic (EHD) technique is used to generate ions from liquid metals. Liquid metal is fed to the tip of a capillary needle emitter with a voltage difference applied between the emitter and an extractor electrode to produce an intense electric field at the liquid surface. Electrostatic forces overcome surface tension forces to produce ions by field emission. When using liquid cesium, time-of-flight mass analysis showed the ion current to be primarily Cs+ with a small percentage of Cs2+ and Cs3+. Electron currents of over 1 mA have been produced by operating the emitter at 2 kV negative. Besides cesium, alkali ion beams have been generated using NaK/cesium alloy and sodium. Calculations show that liquid metals of low work functions appear more suitable for production of atomic ions while higher work functions metals may produce multiatomic ions and charged droplets in addition to atomic ions.
Critical Pressures of the Positive Corona Discharge between Concentric Cylinders in Air
Authors: Robinson Myron
Published: February 2004

If, in a positive corona discharge in air, the voltage is raised at constant gas pressure over a range of pressures beyond atmospheric, the sparkover voltage is found to exhibit a maximum and then decline, eventually to coincide with the corona-starting voltage. The pressure at this point of intersection, beyond which sparkover alone obtains, without antecedent corona, is termed the critical pressure, and the associated gas density the critical density. The critical characteristics of dry air at room temperature are here determined in terms of electrode dimensions for a coaxial wire-tube system. The critical relative density δcr (air density relative to atmospheric conditions) is shown to fit the empirical relation δcr=k[(1/r)+c],where r is the wire radius, k an increasing function of tube radius, and c is a constant. The sparkover voltage Vcr corresponding to the critical density is given empirically by Vcr=ln(R/r){Ar+B[k(1+cr)]1/2},where R is the tube radius and A and B are constants of the gas. The unattainability of positive corona above the critical density is shown not to be absolute as hitherto supposed. Substantial corona currents are possible behond δcr provided that a suitable path in voltage-current-pressure space is followed in circumventing the critical density.
High-Speed Impact between a Liquid Drop and a Solid Surface
Authors: Heymann F. J.
Published: February 2004

The dynamics of high-speed impact between a compressible liquid drop and a solid surface are reviewed. Previous estimates for the maximum impact pressure have been based on one-dimensional approximations. This paper presents a two-dimensional approximation, adapted from a closely related analysis of the oblique impact between two solid plates. This is valid only for the ``initial'' phase of the impact during which the expanding shock front generated by the impact still remains attached to the target surface, and no lateral outflow takes place. The derivations assume a linear relationship between shock velocity and particle velocity change across the shock front. Numerical results are presented for water and sodium, and can be generalized as follows: The contact pressure remains substantially equal to the one-dimensional pressure until the contact angle φ at the edge has reached about half of its critical value, at which the assumed model beaks down and lateral outflow must initiate. As this critical condition is further approached, the contact edge pressure increases progressively, and its critical value Pc is taken as the maximum impact pressure. The ratio Pc/ρ0C0V0 always exceeds about 2.75 exhibiting a minimum in the vicinity of V0/C0=0.2, where ρ0 and C0 are the density and acoustic velocity of the liquid, and V0 is the impact velocity. These pressures are considerably higher than have been heretofore supposed, but circumstantial experimental evidence supports the present results.
Cataphoresis in the Helium-Cadmium Laser Discharge Tube
Authors: Sosnowski T. P.
Published: February 2004

Side light measurements were made to determine the density distribution of cadmium in a discharge tube similar to that used in the helium-cadmium laser. The tube had an electrode at either end and a single cadmium source near the center. The discharge causes the cadmium vapor to flow toward the cathode, the flow being greater at higher currents. At currents greater than 20 mA almost no cadmium is found a few cm from the cadmium source in the discharge region between it and the anode. In contrast to this, large amounts of cadmium are found in the discharge region between the source and the cathode. It is concluded that an approximately uniform density of metal vapor may be maintained in a helium-cadmium laser by placing a single cadmium source near the anode and passing currents greater than 50 mA through the tube. A very simple theory gives a qualitative explanation of the gross features of the density distribution curves.
Temperature-Dependent Measurements of a Flashlamp-Pumped Dye Laser
Authors: Huth B. G. , Farmer G. I. , Kagan M. R.
Published: February 2004

A decreased laser threshold has been observed for a flashlamp-pumped Rhodamine B dye laser as the temperature of the solution is reduced. The change in threshold is attributed to an increase in the fluorescence quantum yield and can be characterized by an activation energy of 880 cm−1.
Thermodynamics of Type-I and Type-II Superconductors
Authors: Hake R. R.
Published: February 2004

After a brief description of the basic characteristics of type-I and type-II superconductors, we outline a unified classical thermodynamic treatment. Some applications of thermodynamic formulae to type-II behavior are then discussed, closing with a consideration of upper-critical-field limits and fluctuation super-conductivity in bulk extreme-type-II superconductors.
Theoretical Calculations on Defects in the Flux Line Lattice
Authors: Hill D. C. , Morrison D. D. , Rose R. M.
Published: February 2004

Computer-aided variational calculations were made to examine the stability of various lattice defects in the flux line lattice which exist in the mixed state of Type II superconductors. The stable configurations agreed with the direct experimental observations of Essmann and Träuble, if the dislocations and vacancies were assumed to be associated with pinning centers. The presence of pinning centers can also explain the frequencies with which the various defects are observed. The Peierls potential for dislocations was also calculated, as well as the theoretical shear stress for the perfect lattice.
Plastic Deformation and Superconductivity in Lead, Indium, and Thallium
Authors: Schenck J. F. , Shaw R. W.
Published: February 2004

Experimental results are presented for wire specimens of high-purity lead, indium, and thallium which have been plastically deformed at liquid-helium temperatures and subsequently annealed. The deformation is found to broaden the magnetic transition to the normal state in lead but not in indium or thallium. It is proposed that, for the deformed lead samples in fields larger than the bulk critical field, the superconductivity is confined to small filaments with high defect concentrations. These filaments are restricted to the surface of lightly deformed samples but exist throughout the bulk of heavily deformed samples. It is suggested that the Mendelssohn sponge model is a natural consequence of the Ginzburg-Landau theory for those inhomogeneous, Type I superconductors which, when homogeneous, have large kappa values. Significant recovery of the strain-induced resistivity increment is observed below liquid-nitrogen temperatures for all three metals; at higher temperatures a pronounced recrystallization step is observed in lead but not in indium and thallium. Dislocation-induced deviations from Kohler's rule were observed in the magneto-resistivity of lead.
Analysis of Magnetic Thin Film Structure for High Permeability
Authors: Hagedorn F. B.
Published: February 2004

An analysis is given which predicts that initial permeabilities of the order of a million could be obtained from typical permalloy thin films if these films were in the appropriate type of structure. A three-layered structure, where a permalloy layer is sandwiched between and exchange-coupled to a pair of highly anisotropic magnetic layers, can be arranged so that having the outer layers magnetized in opposite directions is a stable magnetic configuration. Because of the exchange coupling, this configuration creates a domain wall in the plane of the permalloy layer. The methods of micromagnetics are applied in this paper to obtain a mathematical analysis of such domain walls, giving the functional dependences of the wall shape and position on the external field, layer thicknesses and magnetic parameters. Numerical results show that typical permalloy layers of several microns in thickness could exhibit initial permeabilities of a million in external fields of the order of millioersteds. Inductor applications of this structure are briefly discussed, where it is concluded that such inductors would have a maximum operating frequency of 105 to 106 Hz.
Molecular Field Theory for Randomly Substituted Ferrimagnetic Garnet Systems with Canted Local Spins
Authors: Nowik Israel
Published: February 2004

The theory given in a previous paper is extended and refined. The restriction of collinear spins is lifted and the idea of local random canting as suggested by Geller is introduced. It is assumed that when a magnetic ion has only one magnetic nearest neighbor, its spin need not be collinear with the total magnetization. The angle by which these spins are canted is treated as an adjustable parameter. This refinement of the theory makes it possible to reproduce in addition to the substitution dependence of the magnetic moments at 0°K, also the substitution dependence of the transition temperatures for a large variety of ferrimagnetic-substituted garnet systems. All of this agreement is obtained with a single adjustable parameter. When substitution is in the octahedral site, canting occurs in the tetrahedral site, and vice versa. It was found that spins in the ``d'' site having only one nearest magnetic neighbor are canted by about 60° when substitution is performed in the ``a'' site. Very small canting, if any at all, occurs in the ``a'' site when the substitution is in the ``d'' site. In double-substituted systems there is also no canting in the ``a'' site and the canting angles in the ``d'' site are ∼0° in Mg-substituted systems, ∼50° in Sc-substituted systems, and ∼75° in Zr-substituted systems.
Viscosity Tensor Approach to the Damping of Rayleigh Waves
Authors: King P. J. , Sheard F. W.
Published: February 2004

An expression for the attenuation of a generalized Rayleigh wave has been obtained in terms of a viscosity tensor. The values of the viscosity components for Quartz obtained from measurements on bulk waves have been used to calculate the attenuation of several surface waves on that material. An attenuation of 7.0 dB/cm is predicted at 1 GHz and 300°K for the surface wave propagating in the X direction on the Y surface. This figure compares favorably with an experimental value of 8.6 dB/cm at the same temperature and frequency. An expression for the attenuation of Rayleigh waves on isotropic media is also discussed.
Gas-Heating Effect on Plasma Resistivity
Authors: Creedon John E. , Schneider Sol
Published: February 2004

Measurements of hydrogen-plasma resistivity were made under conditions where the dominant loss mechanism was electron-neutral collisions. Three-inch-diameter tubes with electrode spacings ranging from 2–22 in. were used. Measurements were made at 15 and 22 kA, 56-μsec pulse duration, and pressures of 0.5, 1.0, and 5.0 Torr. Electron-density and temperature measurements were obtained from Stark-broadening and line-intensity-ratio techniques. The predicted resistivity was calculated from electron-proton and electron-neutral collisions and compared with the absolute resistivity calculated from the voltage-current measurements. The gas-temperature rise was determined from the resistance difference obtained from the two calculations. The energy expended in gas heating was determined to be 2.4%–4.2% of the input energy.
Interaction of a Cylindrical Beam with a Plasma. I. Theory
Authors: Self S. A.
Published: February 2004

A linear analysis is given of the interaction of a homogeneous, monoenergetic cylindrical electron beam, of radius a, with a homogeneous, cold plasma for two cases: (A) the plasma is unbounded and (B) the plasma is bounded by a conductor at radius b(>a). The quasistatic approximation is compared with an exact treatment, and conditions for the validity of the former are stated. A stability analysis according to the method of Derfler and Briggs is carried out in terms of the mapping of ω into the complex k plane. It is shown that the system is always absolutely unstable and the dependence of the stability analysis on the system parameters is discussed. The absolute instability is, however, always sufficiently weak that in practice the stabilizing effect of collisions is sufficient to quench it, leaving a convective instability. The properties of the amplifying waves resulting from collisional stabilization are discussed for parameter values appropriate to the experimental conditions described in Pt. II.
Spectroscopic Investigations of High-Current Hollow-Cathode Discharges in Flowing Nitrogen at Low Pressures
Authors: Bleekrode R. , van Benthem W.
Published: February 2004

The spontaneous emission from hollow-cathode discharges in flowing nitrogen is investigated spectroscopically. The vibrational energy distributions in the C3Πu state of N2 and the B 2Σu+ state of N2+ are studied as a function of discharge current, magnetic-field strength, and gas flow. The rotational energy distributions in these electronic states of N2 have also been determined. The effective rotational and vibrational excitation temperatures are found to be low, TR≈2000°K and TV≈10 000°K respectively. The electronic excitation of N+, the dominant emitting atomic species, is found to correspond to much higher temperatures, Texc=35 000°K.
Gyromagnetic Effect on the Mobility of Charged Particles in Weakly Ionized Gases
Authors: Tsuda Takao , Fukao Shoichiro , Maeda Ken-ichi
Published: February 2004

In crossed electric and magnetic dc fields, gyrating electrons are subject to alternate acceleration and deceleration by the electric field. This speed modulation occurs with the gyrofrequency and periodically changes the colliding electron's velocity relative to the background neutral particles. Thus these orbiting electrons see the collision cross-sectional area, associated with target neutral particles, as if it were varying with the gyrofrequency. In very weakly ionized gases, this effect of time-varying collision probability can cause deviated random walk of guiding centers in the direction of the electric field applied perpendicular to the dc magnetic field. It is proposed and argued that the conventional Chapman-Cowling type formula giving the Pedersen mobility need be reexamined in the light of this new effect. Results of computer simulation indicate that the value of the Pedersen mobility of electrons differs considerably from that given by the conventional formula.
Electrical Studies of Low-Temperature Neutron- and Electron-Irradiated Epitaxial n-Type GaAs
Authors: Stein H. J.
Published: February 2004

An investigation of electron- and neutron-produced defects in epitaxially grown n-type (n≃2×1015 cm−3) GaAs has been performed using Hall effect and electrical-conductivity measurements. Electron irradiation (2 MeV) at 80°K decreases both the carrier concentration and the mobility. Below room temperature annealing of electrically active defects is observed near 250°K in agreement with thermal-conductivity and length-change measurements of electron-produced defect annealing in n-type bulk-grown GaAs. An electron-irradiation-produced carrier removal rate of 2.9 cm−1 at 80°K, an observed electronic energy level at ∼Ec − 0.14 eV, and a major annealing recovery stage near 500°K, are in reasonable accord with previous results from bulk-grown n-type GaAs. The accord between the present results from epitaxially grown GaAs and previous results from bulk-grown GaAs suggests that crystal growth method does not play a dominant role in electrically active defect formation under electron irradiation. Neutron irradiation (approximately fission spectrum) at 76°K also decreases both the carrier concentration and the mobility in n-type epitaxial GaAs. A light sensitivity of the carrier concentration and mobility is observed in GaAs after neutron irradiation. By analogy to Si and because disordered regions have been observed in electron microscope measurements on neutron-irradiated GaAs, the neutron-produced light sensitivity is ascribed to disordered regions. In contrast with electron produced defects in GaAs, neutron-produced defects in the epitaxial layer exhibit only small and diffuse annealing throughout the 76°−600°K temperature range investigated. The mobility degradation after 400°K annealing is measurement-temperature dependent and decreases with increasing measurement temperature between 76° and 300°K. The carrier removal rate, on the other hand, is nearly independent of measurement temperature and is 3.0 cm−1 at 296°K. This carrier removal rate is in reasonable agreement with previous results on bulk-grown n-type GaAs irradiated at ∼375°K. Gunn effect oscillators are critically dependent upon carrier concentration, therefore carrier removal is expected to be the dominant failure mechanism for these devices under irradiation.
Detection of Single Collisions of Fast Neutrons by Nucleation of Tyndall Flowers in Ice
Authors: Engelhardt H. , Müller-Krumbhaar H. , Bullemer B. , Riehl N.
Published: February 2004

Lattice disturbances have been created in otherwise ``perfect'' ice single crystals by high-energy neutron irradiation. These imperfections act as nucleation centers for internal melting when ir energy is absorbed. The interaction of a single neutron with the ice lattice is marked by one generated Tyndall flower which can easily be localized. A method is described to grow large single crystals of ice having the low concentration of imperfections necessary for the experiment. We also describe a Schlieren optical device with high resolution constructed for the observation of melting figures in the ice crystals.
Double Injection in Trap-Free Silicon at Low Operating Points
Authors: Hiramatsu M. , Okazaki S.
Published: February 2004

Double injection currents are measured over the temperature range from 340° to 200°K with plane parallel metal-semiconductor-metal structures. It is shown that, in the absence of traps, the injecting junction limits the current at low operating points. The current-voltage characteristics are analyzed considering the property of the injection contact.
Generalized Current and Conductance Extrema in Metal-Insulator-Semiconductor Tunnel Junctions
Authors: Chang L. L. , Moore J. S.
Published: February 2004

A unified analysis of the current and conductance in metal-insulator-degenerate semiconductor tunnel junctions has been made by taking into account the nonparabolic momentum in the insulator. The current may either increase monotonically with voltages or exhibit a maximum at a voltage less than the Fermi energy of the semiconductor. For the conductance, a minimum always occurs at a voltage equal to or less than the Fermi energy and an additional pair of extrema may exist. The criteria for the various cases are established in terms of a set of reduced parameters of the junction. The effect of nonequal electron masses in the semiconductor and the insulator is also considered and shown to be important in determining the different shapes of the characteristics.
Surface Illumination of Semiconductor Panels
Authors: Mavaddat R. , Levin B. J.
Published: February 2004

General expressions are derived for the excess carrier densities in the bulk of finite or infinitely extended panels of semiconductor materials under light excitation. The illumination is in the form of nonpenetrating rectangular or circular light spots partially covering the surface of the panel. Theoretical calculations reveal an appreciable change in excess carrier distribution as the illuminated region approaches the boundary of the material. Experimental results verify this effect and favorably compare with the theoretical calculations.
Coherent Emission from Indium Antimonide with Closely Spaced Coplanar Contacts
Authors: Swartz G. A.
Published: February 2004

A new versatile, coplanar-contact device configuration has been developed for generation of coherent emission from indium antimonide. The devices, which generate coherent emission at frequencies from 6.5–30 GHz, exhibit three types of operation. Emission is generated (1) in a thin-layer plasma formed by a strong transverse magnetic field, (2) in a thin-layer plasma formed as a result of the coplanar-contact configuration (zero magnetic field) and (3) in a thick-layer plasma formed by forcing carriers away from the contact surface with a strong transverse magnetic field. For a resonance condition between the coplanar-contacts, the emission frequency generated in a thin-layer plasma is related to the contact spacing in accordance with two-stream theory. The emission from the thin-layer plasma in zero magnetic field and the noise emission generated at room temperature by the coplanar-contact samples in a 10-kG transverse magnetic field give further confirmation of the two-stream theory. In the first and second types of operation, instantaneous power levels of 1 to 10 μW are emitted for input power levels of 0.2 to 1.5 W. In the third type of operation, with an input power level of 2 W, peak output power levels of 80 to 170 μW at X-band frequencies have been measured. The low input power level, which is required for emission from the coplanar-contact devices, permits cw operation in several samples. Low-frequency instabilities generated in a thick plasma layer are frequency-locked by a 160-MHz external signal with a −3 dBm power level.
Nonisothermal Conduction in Indium Antimonide with Orthogonal Electric and Magnetic Fields
Authors: King J. E.
Published: February 2004

A nonisothermal analysis of dc conduction in n- and p-type indium antimonide subjected to orthogonal electric and magnetic fields has been developed including the effects of energy-dependent acoustic phonon and polar optical phonon scattering as well as minority carrier drift. Also, included are carrier generation, based on an equilibrium between carrier densities and particle temperatures, magnetoresistive effects and carrier injection. Threshold microwave emission curves are calculated assuming that a minimum minority carrier density is required to observe shot noise. The threshold curves give general agreement with the experimental results. When the distortion of the field due to the Hall effect is included, excellent agreement between theoretical and experimental curves is obtained.
Structural and Electrical Properties of Flash-Evaporated Thin GaAs Films
Authors: Farukhi M. R. , Charlson E. J.
Published: February 2004

A flash-evaporation scheme modified to control the rate of the powder fed to the evaporator was used for depositing GaAs films on amorphous glass substrates. A bulk wurtzite form of GaAs was identified with x-ray diffraction and found to have an axial ratio of 1.684. Periodic faulting induced by the plunger movement of the powder feeder is believed to cause the formation of this phase. Structural analysis of the flash evaporated films was performed using a Fourier line broadening analysis and integral breadth measurements. A least-squares approximation of the ``Warren-Averbach'' method yielded particle sizes of 125 Å and rms microstrains of 0.0018. The films were doped during growth with germanium, indium and tin. Germanium doping produced p-type films while both indium and tin doping produced n-type. Mobilities greater than 0.5 cm2/V·sec were found for a narrow range of doping (1018−1019/cm3) for the tin doped films. The films grown with no auxiliary dopant were p-type with carrier concentrations exceeding 1019/cm3 and mobilities near 0.01 cm2/V·sec.
Numerical Integration of Plane Elastic-Relaxing Plastic Shock Waves by a Two-Step Method
Authors: Horie Y.
Published: February 2004

Hyperbolic equations with a differential constitutive relation of dynamic yielding are numerically integrated without the pseudoviscosity. The scheme is shown to be stable and the resulting shock profiles are smooth. The example of iron shows a good agreement with those of the viscosity method and suggests that a stable profile will emerge as mesh sizes decrease.
Epitaxial Poly- and Monocrystalline CaF2 Films
Authors: Bujor Marc , Vook R. W.
Published: February 2004

CaF2 films were deposited onto (100) cleavage faces of NaCl in a high-vacuum bell-jar system. The dependence of structure on substrate temperature and film thickness was studied by transmission electron microscopy and diffraction. When the substrate temperature was around 350°C during deposition, a simple (100)CaF2⧸⧸(100)NaCl with 〈110〉CaF2⧸⧸〈100〉NaCl epitaxial monocrystalline orientation was found. Complex (110)CaF2⧸⧸(100)NaCl with average 〈111〉CaF2⧸⧸〈100〉NaCl epitaxial polycrystalline orientations were found when the substrate was in the 150°–250°C range during deposition. Hard-sphere models of the CaF2 and NaCl faces were used to interpret these results qualitatively.
Optical Properties of Island Films of Metals at Long Wavelengths
Authors: Marton J. P.
Published: February 2004

In the Drude approximation, and for long wavelengths, the real part n of the complex index ñ=n−ik of island films of metals is shown to be independent of the electronic properties of the parent metal, and of the wavelength of radiation. The value of the index is found to depend on the Maxwell Garnett density parameter q only, and is always greater than one, in agreement with experimental results.
Shape of Modulation-Broadened, Unsaturated, Lorentzian Magnetic Resonance Transitions: A Theoretical and Experimental Study of the Fourier Coefficient aoL
Authors: Buckmaster H. A. , Skirrow J. D.
Published: February 2004

The effect of modulation broadening on the shape of the Fourier coefficient aoL describing an unsaturated, Lorentzian absorption line is examined theoretically and experimentally. An unusual feature, not previously observed, is the development of two equal maxima when the normalized modulation amplitude (1/β) exceeds (2)1/2. Expressions are derived for the position and amplitude of these maxima as a function of (1/β). Experimental evidence confirming these expressions is presented.
Averaging Fourth-Rank Tensors with Weight Functions
Authors: Morris Peter R.
Published: February 2004

The problem of averaging fourth-rank tensors with texture describing weight functions has been solved for orthotropic physical symmetry and for orthorhombic crystal symmetry. The results are presented in tabular form. The procedure for extending the tabular results to tetragonal, hexagonal, and cubic crystal symmetries is indicated. The solution requires the coefficients of the generalized spherical harmonic expansion of the weight function up to fourth order, and entails only those approximations required to obtain such coefficients from experimental data.
A Variational Formulation Specifying the Breakdown Criteria for Plasmas Subjected to Spatially Nonuniform Electric Fields
Authors: Mayhan Joseph T. , Fante Ronald L.
Published: February 2004

A variational formulation of the breakdown criteria for plasmas subjected to spatially nonuniform electric fields is developed and applied to several examples. The method uses a different variational function than that developed previously by Epstein. The method is valid for both pulse and cw breakdown. Several examples are presented which illustrate the theory. Application of the formulation to breakdown in front of a narrow infinite slot radiator on a ground plane demonstrates that an effective diffusion length can be defined which is a function of the aperture width.
Acoustic Attenuation and Amplification in Nondegenerate Semiconductors
Authors: Jacoboni Carlo , Prohofsky E. W.
Published: February 2004

A general transport theory of the electronic contribution to the acoustic attenuation and amplification in nondegenerate semiconductors is presented. External electric and magnetic fields are applied parallel to the acoustic wavevector. The piezoelectric and the deformation potential coupling mechanisms have been taken into account together with self-consistent electromagnetic fields. The classical theory predicts that there is no dependence of the attenuation on longitudinal magnetic field. A numerical example has been fully developed and the results are compared with the approximations for low (ql<1) and high (ql>1) frequencies. The values of both the frequency and amplification coefficient for the frequency of maximum amplification are found to be larger in the exact theory than those found by extrapolation of the ql<1 approximation.
Correlation of Scattering Cross Sections of Disturbed Index of Refraction and Acceleration Increment of a Conventional Aircraft due to Clear-Air Turbulence
Authors: Griffin C. R. , LaGrone A. H.
Published: February 2004

The theoretical relationship between the effective radar back-scattering cross section for regions of atmospheric turbulence and the mechanical energy of the turbulence is developed along the lines of existing theory. The cross-section formula is developed in terms of two parameters, the mean gradient of refractive index fluctuations in turbulence, and the mean squared velocity fluctuations. Next, the response of a conventional aircraft, in terms of load factor, to vertical components of velocity is determined, and with the substitution of related parameters, an expression is obtained for turbulence scattering cross section in terms of mean-squared load factors and refractive-index gradient. The mean-squared load factors and refractive-index gradient are measurable from an aircraft. A method is thus obtained for verifying the theory of homogeneous turbulence and associated scattering theory by simultaneous measurement of radar cross-section parameters from an aircraft in the turbulence and by a ground-based radar set.
Characteristics of a Low-Voltage Helium–Neon Laser
Authors: Herceg J. E. , Miley G. H.
Published: February 2004

Laser oscillations of wavelength 3.39 μ are reported for a low-voltage discharge in He–Ne (7:1 pressure ratio). This discharge, characterized by high current (1–2 A), low voltage (24–30 V anode bias), and negative resistance, used a triode laser tube design with plane-parallel electrodes, a heated oxide-coated cathode, a plasma stabilizing grid, and a movable anode. Laser outputs for a pressure range of 2.5–4.0 Torr, grid-anode separations of 4.0–0.8 mm, and a cathode temperature of ≈600°C are reported. A maximum laser output of about 0.3 mW was found at spacings of 5.0–6.0 mm and about 3.0 Torr. A visible shift in the discharge color from pink to orange as the voltage is reduced is tentatively explained in terms of an increasing importance of direct neon excitation at lower voltages. The effect of this on the laser signal and threshold is discussed.
Space-Charge Domains at Dislocation Sites
Authors: Mataré H. F. , Laakso C. W.
Published: February 2004

Germanium and silicon monocrystals with grown-in artificial medium-angle grain boundaries and isolated edge dislocations are subjected to a scanning electron beam (SEB) with simultaneous application of a bias field in a transverse direction. Amplification of the induced current signal yields oscillograms clearly revealing the polar character of defects and the junction behavior of a two-dimensional array of edge dislocations. These results can be explained on account of the dislocation model, as discussed earlier, and support studies and conclusions with respect to the influence of the space charge of edge dislocations on carrier transport in crystals.
Anharmonic Vibrations and Specific Heats at High Temperatures
Authors: Mazur Paul
Published: February 2004

It is well known that, when working in the harmonic approximation, the contribution of the lattice vibrations to the specific heat of solids approaches the Dulong-Petit value at high temperatures. Experimentally, considerable deviations from the Dulong-Petit value have been observed in certain materials. One question that arises in this connection is: Can anharmonic forces account for the observed differences? The answer to this question depends on the lattice spacing as well as on the harmonic and anharmonic force constants. For reasonable values of these parameters, our work seems to imply that if one demands that the rms amplitude of vibration of the atom be less than approximately one-half the lattice spacing, then ``large'' deviations from the Dulong-Petit value are not to be expected. The model we use is that of a linear chain with harmonic and anharmonic forces between nearest neighbors.
Electromigration in Thin Al Films
Authors: Blech I. A. , Meieran E. S.
Published: February 2004

The process of electromigration in thin Al films was studied directly by transmission electron microscopy during passage of current. Due to electromigration, the Al ions migrate along the test strip from cathode to anode, and the actual opening up of holes near the cathode was observed. The rate of hole formation was used to measure an activation energy. For the test strips, a value of 0.7 eV was obtained. In addition, some physical aspects of hole formation, as well as material buildup in the form of hillocks and single-crystal Al whiskers, were observed. Some of the whiskers, which were less than 1 μ in diameter, were seen to grow to unusual lengths, occasionally exceeding 50 μ.
Thickness-Dependent Oscillatory Behavior of Resistivity and Hall Coefficient in Thin Single-Crystal Bismuth Films
Authors: Duggal V. P. , Rup Raj
Published: February 2004

Measurements of electrical resistivity and Hall coefficient of single-crystal bismuth films evaporated on hot mica substrates, in the thickness range 250–1700 Å, were made at room temperature and 90°K. Resistivity and Hall coefficient of these films showed an oscillatory dependence on film thickness on account of the quantum size effect. The experimental period of the oscillations agrees with the theory of quantum size effect at these thicknesses. Detailed electron transmission studies were made to investigate the reported disagreement between experimental curves and theoretical predictions at small thicknesses. The role of structure on the behavior of resistivity and Hall coefficient of thinner films is discussed.
Electrical and Optical Properties of Ni–P Films
Authors: Schlesinger M. , Marton J. P.
Published: February 2004

Results of optical and electrical measurements on Ni–P films are presented, in both as-deposited and annealed films. The films range in thickness from 100 Å to 1 mm and vary in composition from 0%–16% P by weight. The results suggest a model for the structure of these films. For freshly deposited films, the model consists of a three-dimensionally aggregated liquid-like structure with nonmetallic properties. Heat treatment is shown to create a packing tendency followed by crystallization of the films at a relatively high temperature (around 350°C). This results in a nearly metallic film behavior.
Diffraction-Limited High-Radiance Nd-Glass Laser System
Authors: Hagen W. F.
Published: February 2004

A Nd-glass laser system is described which emits a 4 GW diffraction-limited laser pulse, at a radiance of 2×1017 W/cm2-sr. The device consists of a spatially mode-selected Q-switched oscillator, operating in the lowest-order HE11 transverse mode, followed by a series of amplifiers that are coupled with appropriate beam-expanding optics. A beam with a far-field pattern corresponding to Fraunhofer diffraction, is approached through use of the proper gain distribution in the first amplifier and a 50% over-expanded beam to allow uniform illumination of a physical aperture placed in front of the second amplifier. This system requires high optical-quality laser rods capable of transmitting a diffraction-limited beam. The induced optical distortions of the amplifiers during the pumping are minimized by appropriate Nd-doping concentration, surface treatment, flashtube arrangement, and filtering of the pump light, yielding uniformity of pumping over almost the full cross section. Some experimental results on long-path air breakdown, efficient second-harmonic generation, and durability of laser glasses are described.
Magnetoelastic Waves in Time-Varying Magnetic Fields. I. Theory
Authors: Rezende S. M. , Morgenthaler F. R.
Published: February 2004

The propagation of microwave magnetoelastic waves in a ferromagnet subjected to pulsed magnetic fields is investigated. In the first part of the theoretical analysis, which is restricted to spatially uniform bias fields, the interaction between the magnetic and elastic systems is neglected. Using a coupled-mode approach, it is found that plane spin waves propagate in magnetic field transients with negligible reflections, constant momentum and wavenumber, but variable frequency and power flow. These results are further elucidated by the study of a simple step transient case. In the presence of magnetoelastic interaction, the theory indicates that besides changes in frequency and in power flow, the field gradient causes group velocity modulation and changes in the time duration of wave packets. In addition there is an exchange of momentum between the different branches of the dispersion relation; this exchange is calculated in terms of critical field gradients, The analysis reveals that negligible coupling occurs for field gradients much smaller than the critical values, resulting in efficient magnon/phonon conversion. The critical gradients are given for both shear and longitudinal waves propagating along certain crystallographic directions.
Magnetoelastic Waves in Time-Varying Magnetic Fields. II. Experiments
Authors: Rezende S. M. , Morgenthaler F. R.
Published: February 2004

Experiments which confirm the theoretical predictions on the propagation of magnetoelastic waves in time varying magnetic fields are described. Frequency shifts of spin waves as high as 1 GHz have been observed at L-band microwave frequencies, in axially magnetized rods of yttrium iron garnet (YIG). Experiments with magnetoelastic pulses were performed in cases where the signal is injected either as a phonon (piezoelectric excitation) or as a magnon (fine-wire excitation). The first demonstrates frequency conversion and group velocity modulation. The latter scheme is suitable to observe pulse compression and expansion due to frequency modulation and time changes of the group velocity. It is also used to ``trap'' magnetoelastic pulses in the phonon state, by removal of the crossover point.
On the Calculation of Activation Energies and Frequency Factors from Glow Curves
Authors: Chen Reuven
Published: February 2004

A new method for calculating activation energies and frequency factors from thermoluminescence and thermally stimulated current peaks is described. The validity of this method, as well as of most of the other known ones, is examined for a broad range of energies and frequency factors by the use of a computer. A combination of theoretical and empirical-computational analysis is used to give corrected formulas for some of the previous methods, while only empirical corrections are given for some others. Apart from the maximum temperature, Tm, the new method uses the total half width ω=T2−T1, where T1 and T2 are the half-intensity temperatures on the low- and high-temperature side of the peak, respectively. For a first-order peak with a frequency factor independent of temperature, the activation energy is given by E=2kTm(1.26 Tm/ω−1) where k is Boltzmann's constant. The frequency factor is found for this case to be s=[2β(1.26 Tm/ω−1)/(e2Tm)] exp(2.52Tm/ω) where β is the (linear) heating rate and e is 2.718. For the case where pre-exponential factors depend on temperature as some power function and for second-order glow peaks, similar formulas are developed for the calculation of activation energies. The relative advantages of the various methods are discussed both from the theoretical and experimental points of view. The method for distinguishing between first- and second-order peaks is also discussed.
The Low-Temperature Transport Properties of NbSe2
Authors: Lee H. N. S. , McKinzie H. , Tannhauser D. S. , Wold A.
Published: February 2004

The resistivity and Hall effect of single crystals of the two-layer structure of NbSe2 were measured from about 5° to 300°K. A sign change was observed in the Hall coefficient at about 26°K; above this temperature the samples were p type, and below, n type.
Rigorous Dynamic Analysis of Exchange-Coupled Films
Authors: Lin Y. S. , Chang H.
Published: February 2004

The Landau-Lifshitz equation for distributed film systems, which has been formulated by Chang, Lin, and Priver, is applied to analyze both the quasistatic and dynamic magnetization reversal in exchange-coupled films. Each elementary layer of infinitesimal thickness is assumed to reverse magnetization by coherent rotation, subject to both magnetostatic and exchange forces of the other layers. As an example, detailed analysis is made on ``dual uniaxial films,'' which consist of two uniaxial films of different anisotropy fields in intimate contact, and with their easy axes in parallel. Numerical solutions are obtained for magnetization distribution and switching modes, as functions of film parameters and driving fields. Unique in dual uniaxial films, simultaneous switching, sequence-field switching, and separate switching occur in films with strong, moderate, and weak exchange couplings respectively. A convenient method of constructing the critical curves and a comprehensive classification of switching characteristics are given. Finally, a critique is made of Goto's quasistatic analysis based on lumped-constant approximation.
Two-Photon Counting Statistics for Laser and Chaotic Radiation
Authors: Teich M. C. , Diament Paul
Published: February 2004

The relation between the two-quantum photocounting distribution p(n, T) and the intensity fluctuations of the radiation incident on the detector is obtained and compared with the one-quantum results. The actual statistics are evaluated for several cases of interest, including a chaotic source, an amplitude-stabilized wave, and the output of a single-mode laser (Van der Pol oscillator). The two-quantum count rate 〈n〉/T is found to depend on the mean-square intensity of the radiation, in contrast to the one-quantum count rate which is proportional to the mean intensity. The effects of photon correlations in the radiation beam become apparent since the two-quantum distributions manifest a distinctly more positive second derivative than the corresponding one-quantum distributions. Both the low-and the high-count probabilities are therefore increased at the expense of counts near the mean. The quantum-theoretical treatment is found to be equivalent to the semiclassical treatment for density operators possessing a positive-definite weight function in the P-representation. Some possible experiments to verify the theory are discussed and shown to be feasible.