Volume 43, Issue 10, 01 October 1972

Anomalous resonance of strontium titanate
View Description Hide DescriptionAn anomalous resonance exhibited by perovskitestrontiumtitanate crystals are obtained by cooling the samples with an applied electric field. A plausible mechanism involves a mechanical oscillation with a frequency determined by the domain boundary spacing, which is coupled to the electric field through the piezoelectric strain constant.

Acoustical holography with a scanned linear array
View Description Hide DescriptionA mechanically scanned linear detector array is analyzed for acoustical holography applications in which the sound transmitter is stationary. The theory of hologram formation is extended to include linear and nonlinear scanning as well as temporal reference phase. Both linear translatory scan and oscillatory mirror (cylindrical) scan are treated theoretically and experimentally. A rotating linear array is noted as yet another mechanical scanning technique. Emphasis is placed on an electronically simulated reference wave for the linear array under translatory and oscillating mirror scan. Attention centers on simulation of a plane reference wave for cylindrical scan by means of a temporal reference phase. The electronic reference technique is broadened to include curvilinear arrays. A rectilinear array for hydroacoustic holography receives particular mention; typical linear and cylindrical holograms obtained with this system are presented. Also shown are holograms occurring with a simulated plane reference wave for cylindrical scan. Applications of a scanned linear array in aeroacoustic and hydroacoustic holography, medical ultrasonics, and nondestructive testing are specifically mentioned.

Electric and optic behavior of aligned samples of nematic liquid crystals
View Description Hide DescriptionThe orientation behavior of nematic liquid crystals was studied by measurements of I vs V and I vs T combined with observation of their optical behavior. The measurements were performed at low dc voltages in the region around the Helfrich threshold for the formation of domains. A voltage region in which the electric conductivity decreases by increasing the electric field is found. It is also observed that in certain cases the whole crystal orientates itself through a cooperative motion. The change in conductivity with increasing electric field can be partially accounted for by the anisotropyproperties of the material, although other phenomena, such as the flow of the material and charge injection at the electrodes, may play a role. The temperature behavior of I gives an indication that charge carrier velocity is governed by the same processes as viscosity.

Correlation of electronic surface properties and surface structure on cleaved silicon surfaces
View Description Hide DescriptionThe clean, cleaved Si (111) surface exhibits a 2×1 LEED structure which converts to the Si (111)‐7 structure upon anneal at elevated temperatures. The conversion temperature was determined to be 370°C. Measurements taken at room temperature indicate that, after passing through a maximum, the surface conductivity decreases with increasing annealing temperature up to the conversion temperature. For higher annealing temperatures the surface conductivity increases again as the Si (111)‐7 structure develops. The correlation of LEEDpattern with the change of surface conductivity demonstrates that the surface states at the clean Si (111) surface are strongly dependent on surface atomic arrangement with respect to superstructure and domain boundaries.

The 5×20 surface structure of gold
View Description Hide DescriptionA precision RHEED investigation of the so‐called Au (100) (5×20) surface structure has shown that its basic unit is in reality a primitive hexagonal cell, or a similar cell rotated 90° with respect to the first. The cell is repeated by translation in two dimensions, and coincidence occurs in one basic. Au repeat direction only. The designation 5×20, which implies coincidence in two directions, is therefore inappropriate and misleading.

A new representation of the strain field associated with the cube‐edge dislocation in a model of a α‐iron
View Description Hide DescriptionThe displacement field around an edge dislocation core is examined in an attempt to quantitatively modify the linear elastic Volterra solution. The approach derives from a computer simulation to obtain the equilibrium atomic positions in a dislocated bcc crystallite using Johnson's potential to model α‐iron. Unlike many previous calculations of this nature which employ rigid boundary conditions throughout the relaxation, the elastic media bounding the crystallite is here allowed to adjust in response to atomic readjustments within the crystallite. The procedure used to achieve flexible boundaries and its application to this problem is discussed in detail. This scheme permits linear elastic displacements at distances relatively far from the dislocation center. The difference between the computed strain field and the Volterra prescription results from nonlinear effects in the core, and is found to correspond to a net expansion of 0.25b ^{2} per unit dislocation length. This is in accord with findings of experimental studies relating lattice parameters to dislocation densities. The details of this residual field are fairly well described in terms of an elliptical line source expansion center located approximately two Burgers vectors below the tip of the extra half‐plane. Thus, these results, while obtained for model material, suggest that an edge dislocation may be characterized by the sum of two linear elastic fields; the Volterra field and an elliptical expansion field. It is found that this result is not explainable by third‐order perturbation theories. Finally, we note that the addition of an elliptical dilatation field should have a number of important applications to dislocationtheory and specific examples are outlined.

Diffusivity of sodium in lead
View Description Hide DescriptionThe rate of diffusion of ^{22}Na in lead has been measured at temperatures between 249 and 313°C by sectioning diffusion couples prepared by pressure welding a sodium‐lead alloy with pure lead. Over the observed temperature range the ^{22}Na diffusivity is three to four times that of lead self‐diffusivity, and the activation energy for sodiumdiffusion is approximately equal to that for lead self‐diffusion. These results indicate that Na diffuses in lead predominantly via a vacancy mechanism. The contrast between this behavior and that of the monovalent noble metal solutes suggests that a necessary condition for appreciable interstitial solution in hosts such as lead is that the size of the interstices delineated by the host ions should be equal to or larger than the metallic volume of the solute atom.

Circular disclinations and interface effects
View Description Hide DescriptionThe elastic fields of a circular wedge disclination and a twist disclination in the two‐phase materials and a twist disclination in a plate are given. The elastic strain energies and forces on the loops are then obtained to arrive at the studies of the interface effects on the loops.

Structure of electroplated and vapor‐deposited copper films. II. Effects of annealing
View Description Hide DescriptionThe effects of annealing on some structural parameters of electroplated and vapor‐deposited copperfilms have been investigated by x‐ray diffraction and electrical resistivitymeasurements. The results can be described in terms of two annealing stages. Stage I, occuring only in electroplated films in the temperature range 25–200°C, is characterized by annealing out of twins due to the diffraction and electrical resistivitymeasurements. The results can be described in terms of two molecules. Stage II, occuring above 200°C, is characterized by annealing out of dislocations and growth of particle sizes. The kinetics of the annealing stages are discussed.

Measurement of conductivity by induction
View Description Hide DescriptionThe exact solution to the problem of determining the conductivity of a metallic cylinder from the measurement of the decay of induced currents is compared to the approximate solution which considers the cylinder to be a single time‐constant R‐L network. We consider the general case where the sample partially fills the primary coil and where the primary coil has an arbitrary time constant. Over a wide range of values for the coupling coefficient and over a wide range of values for the ratio of the dominant time constant τ_{1} to the primary time constant τ_{ P′ }, we calculate the error involved in the approximate approach.

Theory of the measurement of thickness and conductivity of cylindrical shells by an inductive method
View Description Hide DescriptionWe consider the case of a solid cylinder of arbitrary conductivity, covered by a thin shell of conductivity σ_{ s }. The coated cylinder is assumed to be located inside a primary exciting coil, and also inside a secondary detection coil. A time‐varying current in the primary induces currents in the sample, and these currents are reflected at the secondary. We derive expressions for the time constants of the decay of the secondary voltage. We relate the dominant time constant to the conductivity and the thickness of the shell. If the conductivity of the shell is known, then the simple measurement of a time constant allows the determination of the thickness of the shell. The special case where the conductivity of the inner cylinder is zero is analyzed in detail. The method of measuring the thickness of the shell by induction is shown to be of practical importance in the case of thin metallic films deposited on a cylindrical substrate. We show that the dominant time constant for this case depends linearly on the thickness of the film. Thus measurements on a cylindrical shell are shown to be more convenient than measurements on flat films, where the dominant time constant is proportional to the square of the thickness of the film.

Cadmium selenide films deposited at high substrate temperature as tunneling barriers between tin electrodes
View Description Hide DescriptionVery thin films of CdSe have been prepared as tunneling barriers in Sn–CdSe–Sn tunnel junctions. The CdSe barriers were prepared at a substrate temperature of 115°C. High substrate temperatures were used since under such conditions dielectric films of CdSe can be produced. Conductance measurements on these junctions at 300°K as well as 77°K showed that the conductance depended exponentially on the thickness of the CdSe barrier within the thickness range studied, 50–100 Å. This result is consistent with tunnelingtheory. In addition, at 1.4°K a sudden jump in current occurred at a voltage corresponding to the Sn superconducting energy gap. The magnitude of the current jump agrees with the theory of quasi‐particle tunneling. The shape of the I‐V curve as well as its weak dependence on temperature also indicate that tunneling is the conduction mechanism in these junctions. Since thin CdSefilms are usually not continuous, it is necessary to oxidize those areas of the Sn electrode where no CdSe is present. A method is given for determining the pinhole area. From a knowledge of the pinhole area, one can estimate the fraction of the total current which flows through the CdSe. Our results indicate that essentially all the current flows through the CdSe. This conclusion is supported by the pronounced longitudinal optical (LO) phonon structure in the tunneling data. The LO phonon structure is characteristic of the CdSe barrier. Typical changes of 20% in conductance are observed as a result of excitation of LO phonons in the CdSe layer by the tunneling electrons. Measurement of the conductance as a function of the CdSe thickness show that the conductance of the tunnel junctions can be controlled in a fairly reproducible manner by varying the thickness of the CdSe layer.

Radiation from an electric dipole in anisotropic media
View Description Hide DescriptionBased on the single‐fluid theory of magnetohydrodynamics(MHD), the investigation deals with a comparative study on the radiation characteristics of an electric dipole in homogeneous anistropic media of infinite extent with a uniform magnetostatic field. Attention is specifically focussed on the excitation of waves in a frequency range below the ion cyclotron frequency. In the asymptotic evaluation of the far fields, critical cases of the dispersion curves having points of inflexion or unbounded branches are briefly considered. The normalized power density as a function of the polar angle θ (being measured from the axis of the magnetic field) is calculated, and it is shown that both compressibility and Hall effects enhance the angular zone of radiation for increasing values of the parameters χ and λ.

Field expressions for a circular loop antenna in terms of a new set of functions
View Description Hide DescriptionAn exact expression for the fields produced by an electrically small loop antenna is obtained in terms of a series involving a new set of functions, namely and , where − ∞<σ<∞ and 0≤γ≤1/2. Series expansion, approximate expressions for γ^{2}σ≪1 and recursion relations for the functions are derived. These functions permit the exact evaluation of the fields in the Fresnel and near‐field regions of the loop antenna. In the case that the distance to the point of observation is of the order of a wavelength or greater, but greater than the loop radius, the field expression is given in terms of the function q _{1}(σ,γ) = f _{1}(σ,γ) + ie _{1}(σ,γ). Numerical results describing the behavior of the functions f _{1}(σ,γ) and e _{1}(σ,γ) are given. Field expressions for the electrically large loop antenna are also derived.

Bow‐shock effects on probe measurements
View Description Hide DescriptionProbe measurements of pressure and magnetic field are of importance in electromagnetic‐shock‐tube diagnostics. The effect of the bow shock on such measurements, in transverse shock waves in the ionizing‐hydromagnetic regimes, is considered. The bow‐shock jump equations are first solved, then the total stagnation pressure is obtained assuming an isentropic compression towards the stagnation point. The pressure measured at the stagnation point of the probe is seen to be less than the free‐stream total gas pressure. The magnetic field is considered in terms of a simple model taking into account the field and flow distributions outside of the stagnation streamline. This analysis shows that the magnetic fieldmeasured at a coil is also less than the free‐stream magnetic field.

Transient behavior of a cavity
View Description Hide DescriptionAn expression for the transient electric field setup in a lossless cylindrical cavity by a short pulse of electrons has been derived by means of the Laplace transformation. It is found that longitudinal and radial fields are set up in the cavity. Expressions for the electric field have been evaluated as a function of time for several different points within a representative cavity.

Electromagnetic surface fields produced by a pulse‐excited loop buried in the earth
View Description Hide DescriptionA transient excited magnetic dipole immersed in a conducting half‐space is considered. Explicit results for the field waveforms observed on the surface are obtained for the case where the exciting current is an exponential time function. It is shown that the shape of the waveform is diagnostic of the relative location of the buried source. The results have possible application in mine rescue operations.

Nanosecond and picosecond laser‐produced CD_{2} plasmas
View Description Hide DescriptionA preliminary comparison is made of plasmas created by irradiation of CD_{2} slab targets by single high‐energy Nd^{3+}glass laser pulses of 3.5‐nsec (35 J max) and 3–10‐psec (10 J max) duration. For the nanosecond irradiations we find electron temperatures of 350–400 eV and appreciable C^{5+} ions at 20‐keV expansion energy. Ion expansion energies for a picosecond pulse are about one‐third those for a nanosecond irradiation of comparable energy. Up to 50% of the incident pulse energy may be accounted for in plasma expansion in either case. In each case, the specularly reflected energy back into the input f/5.0 lens is small: 0.4% and 0.8% for the nanosecond and picosecond times, respectively. Detailed ion analysis shows the target composition to be CD_{2−x }H_{ x }, where 0.2<x<0.9 with traces of oxygen also observable.

Scattering of obliquely incident waves by inhomogeneous fibers
View Description Hide DescriptionThe problem of the interaction of obliquely incident electromagnetic waves with a radially inhomogeneous fiber cylinder is treated analytically. By formulating this problem in terms of the two‐point boundary‐value problem, we eliminate the need to evaluate untabulated functions or to invert large matrices. Numerical results for the differential scattering cross sections are obtained for the case of an inhomogeneous fiber whose radial dielectric variation may be described by the Luneberg profile. It is found that the induced cross‐polarized scattered field as well as the characteristics of the total scattered fields are influenced significantly by the inhomogeneity of the dielectric medium.

Magnetic focusing of a relativistic electron beam: experiment
View Description Hide DescriptionA magnetic mirror is used to control and focus a 2.0‐MeV 70‐kA electron beam.Mirror ratios of 3.7 and 5.9 and magnetic fields of 2.0, 2.5, and 5.0 kG are used. A 500‐μ air pressure is used in the drift tube to minimize the beam's electrostatic and magnetic self‐forces. The surface‐energy deposition in a sample placed at the peak of the 3.7 to 1 bottle is approximately twice as large as that obtained with beam self‐focusing without an external field. The fluence and energy as a function of axial position in the region of low‐grad B_{z} agree with the results of adiabatic theory and are found to be independent of magnetic field strength and nearly independent of mirror ratio for the selected ranges. Reproducibility in delivered energy, fluence, and surface‐energy deposition is significantly greater than that obtained without an external magnetic field. The induced B_{z} field due to beam rotation or plasma diamagnetism is approximately 1% of the applied field.