Volume 37, Issue 8, 01 July 1966

Charge Growth Equation for Belts of Insulating Materials Passing over Grounded Rollers
View Description Hide DescriptionThe magnitude of the electrostaticcharge density on an insulating belt of photographic film increases as the belt passes repeatedly over a set of grounded metal rollers. The charge densityq asymptotically approaches an equilibrium value q_{e} according to the relation q = q_{e} (1 − e^{−n/τ} ), where n is the number of roller passages and τ is an arbitrary constant. An equivalent circuit which describes this behavior is presented. The circuit elements are an electromotive force, a contact resistance, a leakage resistance, and a capacitance between the belt and the roller. The physical equivalents of these elements and the possibility of their measurement are discussed.

High‐Frequency Behavior of the Surface‐Wave Helicon Instability
View Description Hide DescriptionThe surface‐wave instability in helicon wave propagationanalyzed by Baraff and Buchsbaum is reinvestigated here for the purpose of extending the analysis to values of ωτ greater than unity. We find that although collisions are essential for the mechanism of instability, the formulas and analysis of Baraff and Buchsbaum, derived for ωτ«1, are unchanged until ωτ exceeds some large value ω_{0}τ. This value ω_{0}τ is typically much greater than unity. At higher frequencies the surface wave penetrates from one boundary to the next and some of the Baraff‐Buchsbaum approximations start to lose validity.

Surface‐Wave Instability in Helicon Propagation. II: Effect of Collisional Losses
View Description Hide DescriptionThe surface‐wave instabilityanalyzed by Baraff and Buchsbaum in the limit of infinite ω_{ c }τ is investigated here for ω_{ c }τ large but finite. In the previous analysis, the carrier drift velocity required to establish the instability was found to be proportional to (K _{1}−K _{2}), the effective dielectric mismatch across the interface at which the surface wave was excited. Here we find that with ω_{ c }τ large but finite, this threshold velocity is increased by an amount proportional to (ω_{ c }τ)^{−1} and inversely proportional to (K _{1}−K _{2}). For a given value of ω_{ c }τ there thus exists an optimum choice of (K _{1}−K _{2}) which minimizes the required threshold velocity. These new results can be obtained from heuristic arguments which do not require the calculation of field components beyond lowest nonvanishing order in (ω_{ c }τ)^{−1}. The heuristic arguments are here justified rigorously for the physically interesting case of small (K _{1}−K _{2}).

Saturation and Spectral Characteristics of the Stokes Emission in the Stimulated Brillouin Process
View Description Hide DescriptionAn analysis is given of the saturationeffect in the stimulated Brillouin scattering of coherent light waves. It is shown that the three coupled nonlinear waveequations describing the complex amplitudes of the forward‐traveling primary and acoustic waves and the backward‐traveling Stokes wave can be replaced for room‐temperature situations by two coupled first‐order nonlinear equations for the intensities of the primary and Stokes waves. These resulting equations are solved exactly and the solutions describe completely the process of photoelastic amplification of the coherent Stokes wave via stimulated Brillouin scattering in both the linear and the nonlinear saturated regimes. The results also give a more realistic estimate of the intensity of the hypersonic wave generated in the process than that made on the basis of either the Manley‐Rowe relationship, which does not take into account the losses, or the usual linear theories, which do not take into account the saturationeffect. It is believed that the Stokes emission observed in the usual experiments on stimulated Brillouin scattering of ruby laser light is due to photoelastic amplification via the stimulated process of Stokes noise generated by the scattering of the laser light via the normal Brillouin process by the thermal phonons present throughout the electrostrictive medium. A detailed analysis of this situation is also given. Explicit formulas for the spectralcharacteristics and the total integrated intensities of the Stokes and acoustic waves are obtained. The results also describe the noise characteristics of the photoelastic backward‐wave amplifier. As an illustration, all these results are applied to the case of stimulated Brillouin scattering of ruby laser light in quartz.

Electromagnetic Behavior of Thin‐Film Structures
View Description Hide DescriptionA general method is presented for studying the response of thin‐film structures to electromagnetic excitations. It is shown that for thin films which can be characterized by a complex, nonlocal conductivity tensor σ, a simple closed‐form solution can be obtained relating the secondary sources at the films to the external excitations. From this relation the dispersions and interactions of resonant surface waves, as well as the scattering properties of thin‐film geometries, can be studied. Several geometries consisting of plasma layers, dielectric films, and resistive and superconductive films serve to demonstrate the power and simplicity of this method.

Liquid‐Nitrogen Cooling of a Ruby Rod
View Description Hide DescriptionNumerical solutions of the one‐dimensional heat‐flow equation have been obtained for a liquid‐nitrogen end‐cooled ruby laser rod. Temperature distributions, cooling times, and heat fluxes have been obtained for four initial rod temperatures from 150° to 350°K.

Role of MnO Substrates in Enhanced Photoemission from Cs_{3}Sb
View Description Hide DescriptionManganese oxide films are frequently used as substrates for Cs_{3}Sb photocathodes prepared on glass windows. Such photocathodes have significantly enhanced sensitivity at wavelengths in the range 5500–6500 Å. Photovoltaic and photoemission measurements indicate that the enhanced photosensitivity results from band bending in the Cs_{3}Sb at the MnO substrate interface. Band bending provides an electrostatic field, which reduces the barrier height for photoemission from near the substrate.

Vacuum Electrical Breakdown between PlaneParallel Copper Electrodes
View Description Hide DescriptionMeasurements have been made of prebreakdown currents between thoroughly outgassed planeparallel copperelectrodes in ultrahigh vacuum, for electrode separations in the range 0.03 cm to 0.2 cm. These currents are found to be in good agreement with the FowlerNordheim theory of field emission from the pure metal surface. From combined measurements of prebreakdown current and breakdown voltage at different electrode separations it is deduced that the cathode microscopic field at breakdown is constant and of magnitude (6±1)×10^{7} V/cm.
Spectroscopicmeasurements of both resonance line absorption and line fluorescence have revealed that the density of neutral copper vapor present in the interelectrode space during the application of electric fields only fractionally (<1%) less than the breakdown field is considerably less than that necessary for volume ionization to occur. For the same experimental conditions, infrared radiation observations have revealed no hot spots on the anode surface.
From the measurements it is concluded that mechanisms involving the amplification of fieldemissioncurrent in electrode vapor produced by steady evaporation from either the anode or the cathode do not explain the electrical breakdown of vacuum between extended copper surfaces. Further, the production of electrode vapor by the detachment of heated anode material under the influence of the force of the applied electric field is unlikely.

Interaction between a Dislocation and an Assembly of Dislocation Arrays
View Description Hide DescriptionThe force exerted upon a lone dislocation by an assembly of dislocation arrays is discussed in the continuum approximation. The arrays are parallel and in equilibrium with a constant applied stress, but no further restrictions are imposed.
For both edge and screw dislocation arrays, the force parallel to the glide planes can exhibit absolute maxima and minima only at the glide planes, and the sign of the force throughout the medium is governed by the distribution functions evaluated at the glide plane ends. Some numerical results in agreement with the analysis are given for screw dislocations upon two parallel glide planes; a ``shadow effect'' is also demonstrated for this case. The analysis is also valid for the interaction between a dislocation and an assembly of parallel cracks.

Snoek Atmosphere Dislocation Pinning in Tantalum
View Description Hide DescriptionSnoek pinning of dislocations due to oxygen in tantalum was investigated using internal friction and simultaneous deformation techniques. The magnitudes of the pinning in the two oxygen concentrations used (5 ppm and 100 ppm) were found to be approximately 20 kg/cm^{2} and 150 kg/cm^{2}, respectively. This shows that Snoek pinning can contribute a significant fraction of the yield stress in material with high concentrations of dissolved interstitials.

Molybdenum‐Silicon Schottky Barrier
View Description Hide DescriptionAn ideal metal‐semiconductor Schottky barrier contact was made by chemically depositingthin films of molybdenum on n‐type silicon by the hydrogen reduction of molybdenum pentachloride at temperatures between 390°C and 500°C. Current‐voltage, capacity‐voltage, and photoelectric measurements were used to investigate the characteristics of molybdenum‐silicon diodes thus produced. The junction is shown to be very close to the ideal Schottky barrier with the barrier height measured with respect to the Fermi energy of 0.57±0.02 eV.

Laser Amplifier Noise at 3.5 Microns in Helium‐Xenon
View Description Hide DescriptionThe noise added to the amplified signal in laser amplifiers results from spontaneous emission in the laser medium. A measurement of the intensity of the spontaneous emission noise has been performed at the high‐gain 3.508‐μ line in a dc‐excited He‐Xe discharge. Measurements were made with the laser amplifier operating under normal low‐level signal conditions as well as under high‐level, saturated conditions. It is found that the measured values of the noise power agree closely with the expressions derived for a four‐level excitation system using a plane wave propagation model. The low‐level measurement yields a population inversion of 2.6±0.3. For an amplifier with an unsaturated gain of 17 dB and a saturated gain of 1 dB, the increase in the noise output power is only 7 dB over that of an equivalent unsaturated 1‐dB amplifier. The measurement of the noise power under saturation gave a ratio of the saturated to the unsaturated upper level population of 0.4. The measurements indicate that the 3.508‐μ line has good noise properties. The experiments and the theoretical analysis apply to a one‐dimensional geometry in which variations in the transverse direction are negligible.

Vacancy and Interstitial Cluster Production in Neutron‐Irradiated α Iron
View Description Hide DescriptionVacancy cluster and interstitial cluster production in α iron was computed by simulating atomic collision cascades on a computer. Each collision was determined by the Erginsoy‐Vineyard interatomic potential for α iron. The computed densities of displacement spikes produced by primary knock‐on atoms with energies above 3 keV correlated qualitatively with the degrees of irradiation hardening observed by Harries et al. in ferritic steel specimens for five different neutron energy spectra. The computed total displaced atom densities did not correlate with the degrees of irradiation hardening observed by Harries et al.Annealing simulations indicated that displacement spikes produced by primary knock‐on atoms with energies below 2.5 keV should not contribute importantly to irradiation hardening in specimens irradiated at the temperature (∼60°C) adopted by Harries et al. The volume of collided atoms involved in a collision cascade usually exhibited a marked orientation along 〈110〉 directions as did the associated displacement spike. Spikes are therefore oriented in the primary slip planes of α iron and each could serve in toto as a barrier to dislocation motion.

Variation of the Gain Factor of GaAs Lasers with Photon and Current Densities
View Description Hide DescriptionThe variation of the gain factor with the threshold current was studied in two cases, viz., (1) a reflective film was applied on one end of a GaAs laser, and (2) antireflective films were applied on both ends of the laser. In (1) the threshold current is reduced to one third as compared with the case in which no film is applied. The gain factor increases 30%. In (2) the threshold current becomes eleven times greater and the gain factor is reduced to one fourth.
These phenomena were analyzed in the light of spontaneous and stimulated lifetime of electrons in the p region. A formula was obtained giving the gain factor as a function of the density of photons and of current. When the current is constant, the gain factor is inversely proportional to (P+1), where P is the density of photons. At the threshold current the gain factor is inversely proportional to the sum of quasi‐Fermi levels, F_{n} and F_{p} .
The saturation effect of a light amplifier at a fixed current observed by Crowe and Craig and also the variation of the gain factor with the threshold current can be calculated by this formula.

Chemisorbed Coincidence Lattices on Rhodium
View Description Hide DescriptionThe previous low‐energy electron diffraction(LEED) work of the author on coincidence lattices formed by chemisorbed monolayers is extended to include two new structures, one formed by oxygen and the other by CO, on the (100) face of Rh. These structures are analyzed and found to be coincidence lattices, that is, high‐coverage structures which are out of step with the substrate lattice in particular crystallographic directions but come into coincidence with it every several substrate lattice positions. The oxygen structure forms a slightly distorted close‐packed arrangement which is out of step in the Rh [11] direction by 1/7 of the Rh‐Rh separation. The CO structure is out of step in the Rh [10] direction by ⅙ of the Rh face‐centered unit cell. It is concluded that the coincidence lattice is a general type of chemisorbed structure and that contrary to the general presumption, and particularly for high‐coverage structures, the chemisorbed species do not always reside in positions of simple coordination with respect to the substrate.

Temperature‐Modulated Optical Absorption in Semiconductors
View Description Hide DescriptionTemperature‐modulatedoptical absorptionmeasurements on Si, GaAs,CdS, and KTaO_{3} near the absorption edges are described and interpreted. These measurements show that the experiment is a simple and accurate method of measuring the band‐gap energy of semiconductors, and of determining whether the gap is direct or indirect. Information is also obtained on phonon‐assisted absorption mechanisms near the band edge and some phononenergies can be deduced. The measurements on Si and CdS verify previous interpretations of the band edge based on optical absorption, and the results from GaAs are nearly identical to those obtained from the electric field modulated absorption experiment. The measurements on KTaO_{3} indicate that this material has an indirect gap with an energy of 3.45 eV.

Depletion Capacitance and Diffusion Potential of Gallium Phosphide Schottky‐Barrier Diodes
View Description Hide DescriptionThe diffusion potential V_{BO} and the intercept V _{0} of the 1/C ^{2} vs bias plots for n‐type GaP‐metal Schottky‐Barrier diodes have been measured. Photoresponse measurements indicate that V_{BO} is not sensitive to the thickness δ of the interfacial separation between the GaP and the metal; V _{0} is observed to increase rapidly with δ. The 1/C ^{2} plots are linear in all cases, with slopes independent of V _{0}. The values for the donor density N_{D} , calculated in the usual way from slopes of the 1/C ^{2} plots, seem to agree within experimental error with N_{D} calculated from the resistivity of the GaP.
Several models are proposed for the metal‐interfacial‐layer‐semiconductor system in order to explain this behavior. The GaP‐metal diodes are shown to be best characterized by a model which includes a bias‐dependent charge in surface states at the semiconductor surface.

Attenuation Length Measurements of Photoexcited Electrons in CuBr Films
View Description Hide DescriptionThe attenuation length of photoexcited electrons in evaporated layers of CuBr was estimated by measuring the photoemission yield as a function of thickness. The attenuation length was found to be less than the optical absorption depth at photon energies between 8 and 11 eV. At hν=9.7 eV, the attenuation length is approximately 51 Å.

Transient Spin‐Wave Buildup in Ferrites
View Description Hide DescriptionThe equations of motion for spin waves in a ferrite, together with the equations for the uniform mode, have been numerically integrated for the case in which the ferrite is spherical in shape and subjected to an applied magnetic field which rises linearly in time. The equations for the uniform mode are completely general and allow for arbitrary amplitude of the mode, so that parametric couplings which lead to spin‐wave instabilities of various orders are accounted for, including the ordinary first‐order and second‐order spin‐wave instabilities. The spin‐wave equations are solved to first order in the spin‐wave amplitudes.
The equations have been solved under the conditions of a linearly rising magnetic field having arbitrary rate of rise, together with a dc applied magnetic field of arbitrary magnitude, and for arbitrary angle between the orientations of the rising and dc fields. Assuming that spin waves are initially excited to thermal amplitudes, a numerical integration is performed over k space to find the aggregate spin‐wave growth as a function of time, and the energy in the uniform mode as a function of time in the presence of the amplified spin waves. These results define the conditions under which useful transient devices can be operated for the generation of microwave energy in pulsed magnetic fields, to take advantage of the large rf magnetization available in ferrites when operated beyond the usual limiting values of the uniform mode which apply under steady‐state operation.

Mössbauer Study of the Structure and Decomposition of Wustite
View Description Hide DescriptionA series of Mössbauer, x‐ray, and magnetic measurements has been made on wustite (Fe_{1−x }O). The main new results are:
(a) The dependence of the Mössbauer quadrupole splitting on heat treatment is very similar to that of the atomic constant, confirming Hoffman's conclusions on the variation of x.
(b) The Zeeman Mössbauer lines of untreated wustite below the Curie temperature are completely smeared out; for a sample treated so as to give minimum quadrupole splitting (minimum x) these lines appear.
(c) In a heat‐treated sample a Zeeman splitting in the Mössbauer spectrum is observed well above the bulk wustite Curie temperature. This is interpreted as the imposition by the magnetic decomposition products of a magnetic ordering on the otherwise paramagnetic residual wustite with which they are in intimate contact.