Volume 36, Issue 11, 01 November 1965
Index of content:
36(1965); http://dx.doi.org/10.1063/1.1702997View Description Hide Description
Application of voltage along the c axis in rutile single crystals produces an increase in conductivity after a ``delay'' period. During this period, which may amount to several days at room temperature, the current decays as a result of trapping. The subsequent rise in current leads to thermal breakdown unless the current is limited externally. The destruction of the insulating properties reflects a change in the bulk, brought about by an injection of positive space charge from the anode. This charge, believed to be holes, is injected by field emission due to a steepening anode fall and allows the electron current to rise exponentially. The anode fall is approximately 10−3 mm wide and sustains fields of 106 V/cm. Trapping of electrons and holes produces two types of color centers, which are distinguishable by their color and migration in the applied field.
36(1965); http://dx.doi.org/10.1063/1.1702998View Description Hide Description
The derivation is outlined of a general relation connecting the macroscopic plastic strain rate with the configuration and velocity distribution of dislocations in a monocrystal with a single active glide system. This can be simplified for particular cases, the most simple being the case of parallel straight dislocation lines.
A new form of the dynamical theory of plasticity is applied to detailed calculations and compared with previous work. The major modification is the use of the equationv=v* exp[−D/τ] to express the quasi‐viscous behavior of dislocation velocities. Here, v=dislocation velocity, v*= terminal velocity, D=characteristic drag stress, and τ=resolved shear stress. Also, a physical relation between dislocation density and plastic strain is derived.
The nonlinear differential equation that results from combining the strain‐rate equation with an equation that describes the loading machine is integrated. Approximate analytic solutions, and numerical calculations, are given for the upper yield stress and the initial portion of the stress‐strain curve. The effects of initial dislocation density, characteristic drag stress, crosshead speed, machine stiffness, and dislocation multiplication rate on the upper yield stress are calculated. It is shown that the upper yield stress exceeds the cohesive stress at a critical crosshead speed. Delay times are also calculated for rapid load application followed by constancy of the load and compared with experiments for LiF. The effects of strain hardening are considered in a companion paper.
36(1965); http://dx.doi.org/10.1063/1.1702999View Description Hide Description
Mathematical descriptions of the stress‐strain‐time behavior of plastic crystals are developed using a statistical approach to dislocation dynamics. First, the ``easy‐glide'' portions of stress‐strain curves are described in terms of glide band propagation. Then, three models of strain hardening are developed and used in numerical calculations of stress‐strain curves. In one model, the mean density of mobile dislocations first increases and then decreases with increasing plastic strain. In another, strain introduces internal stress fluctuations which decrease the mean velocities of mobile dislocations. In the third (and preferred) model, strain increases the mean viscous drag acting on moving dislocations, thereby decreasing the mean velocity at a given stress. The numerically calculated curves show that the dynamical models provide realistic descriptions.
36(1965); http://dx.doi.org/10.1063/1.1703000View Description Hide Description
The method of analysis developed by the authors for the study of Faraday rotation in terms of the conduction‐current Hall field has been extended to include the effect of the Hall field due to dielectric current. The expression for the angle of rotation derived in this paper is different from that given by Barlow. The contribution of the dielectric‐current Hall field to rotation is found to be smaller than that obtained from Barlow's expressions.
36(1965); http://dx.doi.org/10.1063/1.1703001View Description Hide Description
An experimental investigation of the Faraday rotation at 3 cm in artificial dielectrics composed of paraffin wax and powdered indium antimonide is described. It is found that a rotation which can be ascribed to the dielectric properties of the medium does indeed occur. The experimental results are found to be in reasonable agreement with theoretical predictions made earlier.
36(1965); http://dx.doi.org/10.1063/1.1703002View Description Hide Description
Suppressed positive streamers are produced in a short (3 cm) positive‐point‐to‐plane gap in air by the discharge of the small anodecapacitance. A study is made of the growth of current at the cathode plane, and the growth of luminosity at various positions in the gap with one or two photomultipliers and with a camera. The results are compared with some photographs obtained with an image converter. From a comparison of current and photomultiplier output it is shown that sharp peaks in the current are the result of the arrival of streamer tips at the cathode. The method has allowed measurement of streamer tip velocity at various positions in the gap. It has not proved possible to detect directly any return strokes produced when the streamers reach the cathode, but the subsequent growth of the discharge indicates that they are active in much the same way as previously shown for long (25 cm) gaps.
36(1965); http://dx.doi.org/10.1063/1.1703003View Description Hide Description
The dynamics of ``web'' growth have been investigated, with particular emphasis on the meniscus. The meniscus deflections have been expressed as a function of surface tension, and pulling force. An effort is being made to describe the parameters which influence surface tension, and its reflection on the process of web growth, and the properties of the material. The degree of supercooling, crucible design, and the control of thermal gradients have profound effects on the quality of the crystal and its physical dimensions.
36(1965); http://dx.doi.org/10.1063/1.1703004View Description Hide Description
When a portion of a liquid dielectric was subjected to an electric dc field of high intensity, the level rose locally. The maximum rise attained by different liquids above the liquid level outside the field increased with their dielectric constant and polarizability.
The following observations indicate that electrostriction rather than electrophoresis was the cause of the observed rise in liquid level. (a) Molten polymers, which have such high viscosities that electrophoretic motion of ions or charged particles is virtually suppressed, rose to the same heights as fluid liquids. (b) Neither inverting the direction of the field nor (c) adding 0.05% finely dispersed aluminum or 0.15% ultramarine powder to molten polyethylene affected the rise in the dc field qualitatively or quantitatively.
Repulsion occurred in very high fields, producing a slight depression in the liquid level.
36(1965); http://dx.doi.org/10.1063/1.1703005View Description Hide Description
A directional dependence of the propagation of nonlinearly excited spin waves in anisotropicferrites has been observed by measuring the radiation from the edge of a (110) YIG disk. The disk was excited by a parallel pump field normal to its plane and rotated through 360 deg. Measurements were made at dc field intervals of 10 Oe from 1800 to 2800 Oe (the experimental minimum of the butterfly curve). Radiation was observed at half the parallel pump frequency over a dc field range of 248 Oe (from 2294 to 2542 Oe). The direction for maximum radiation was along a [1̄10] axis in the plane of the disk, and a strong directional dependence was observed when the pump field was less than 4 dB above threshold. The ratio of maximum to minimum output as the disk was rotated was at least 10 dB. The directional dependence was found to be essentially independent of pump field strength up to 4 dB above threshold. Any further increase in pump field strength reduced the amount of radiation, and for pump field strengths higher than 8 dB above threshold no radiation was observed.
36(1965); http://dx.doi.org/10.1063/1.1703006View Description Hide Description
The internal quantum efficiency and spectral distribution of radiation from GaAs electroluminescent diodes is examined with a careful investigation of the optical absorption of both the n‐type and p‐type layers of the diodes. A group of diodes so fabricated as to allow necessary corrections for internal absorption and reflection has internal quantum efficiencies greater than 50%. These diodes have more than 85% of all photons in the principal peak and the temperature variation of the emission is explainable on the basis of changes of the absorption with temperature. In addition, the shape of the emission can be interpreted in terms of radiation originating in a compensated p‐type layer of relatively low carrier concentration.
36(1965); http://dx.doi.org/10.1063/1.1703007View Description Hide Description
Recent measurements of the thermal (K) and electrical (σ) conductivities of the rare earth metals(gadolinium,terbium, and dysprosium), indicate that the ratio Kσ−1 T −1 is considerably larger than the value predicted by the Wiedemann‐Franz law. At low temperatures, a significant lattice contribution to the thermal conductivity (and possibly some magnon contribution) seems to account for the experimental results. At higher temperatures, the bipolar mechanism of heat transport is probably responsible for the observed behavior. If this interpretation is correct, other transport properties should exhibit anomalous behavior. In particular, the Nernst and Ettingshausen effects should be anomalously large and therefore these rare earth metals may be useful in devices for refrigeration or electrical power generation.
36(1965); http://dx.doi.org/10.1063/1.1703008View Description Hide Description
Two important effects of the addition of Mo to the laser material CaWO4:Nd have been found and are discussed: (1) an increased susceptibility to photolytic coloring, and (2) a drastic change in the vacuum reduction coloring spectra. Therefore, it is necessary to specify the Mo concentration in CaWO4:Nd crystals in order to understand either photolytic or reduction processes. The Mo content in the crystals has been obtained from x‐ray and optical fluorescence analysis. The distribution coefficient of Mo in CaWO4 has been determined to be 0.7 from crystal growth studies. The Nd concentration is obtained in terms of the optical absorption cross sections for the 580 and 740 mμ absorption peaks. For these two wavelengths, the extraordinary ray cross sections are 10 and 2.6×10−20 cm2; the ordinary ray cross sections are 8.0 and 1.6×10−20 cm2.
36(1965); http://dx.doi.org/10.1063/1.1703009View Description Hide Description
The current‐voltage characteristics of zinc‐diffused gallium arsenide diodes have been measured between 4.2° and 300°K, with carrier concentrations in the n‐type region varying from 6×1015 to 9×1018 cm−3. In the forward direction it is shown that the current is due to tunneling at low temperatures and high carrier concentrations whereas it arises from thermal processes involving recombination in the space‐charge region at higher temperatures and low carrier concentrations. The temperature at which the current changes from predominantly tunneling to predominantly thermal in origin has been determined as a function of the n‐type carrier concentration. In the reverse direction the current in these diodes appears to be dominated by tunneling at all temperatures and doping densities tested.
36(1965); http://dx.doi.org/10.1063/1.1703010View Description Hide Description
Measurements of the photoresponse of germanium bicrystals are reported here with two specific results in mind:
(1)The detailed response at higher optical frequencies. The measured transition steps at larger than band gapenergies can be associated with the specific band structure of the dislocation plane taking the stress field into consideration.
(2) The detailed bias or field dependence yields a linear law for the photovoltage. It is found that the electronic dislocation model, as used earlier, explains the linear field dependence of the photoresponse.
36(1965); http://dx.doi.org/10.1063/1.1703011View Description Hide Description
An analysis is made of the loss of x‐ray intensity for the reflection technique with respect to geometric arrangement of the specimen. Good agreement has been obtained between the analysis and experimental data from various materials of different lattice structures. The relative loss of intensity was found decreasing with higher index planes, i.e., shorter d spacings. This is considered advantageous in comparison with the transmission techniques of which the loss of intensity is greater for higher index planes.
Electrical Properties and Resonance Scattering in Heavily Doped n‐Type GaSb and Related Semiconductors36(1965); http://dx.doi.org/10.1063/1.1703012View Description Hide Description
Data are presented of the resistivity ρ and Hall coefficient RH vs temperature T from 4.2°K to room temperature for seven n‐type moderately heavily Te‐doped samples of GaSb. The ρ‐vs‐T curve shapes are similar at low temperatures to those previously measured on n‐GaAs and n‐InAs, and they are presumably caused by the same near‐resonance scattering mechanism as proposed earlier to explain the n‐GaAs and n‐InAs results. An unusual rise in the low‐temperature mobility with increasing carrier concentration in the n‐GaSb samples can also apparently be understood on the basis of the near‐resonance scattering model.
36(1965); http://dx.doi.org/10.1063/1.1703013View Description Hide Description
36(1965); http://dx.doi.org/10.1063/1.1703014View Description Hide Description
By means of x‐ray diffraction techniques, the presence of twin relationships in silicon epitaxially deposited on sapphire was demonstrated and the relative amounts determined in a semiquantitative manner. A correlation was made with the surface structure observed by means of replica electron microscopy and the twinning relationships. The minor twin relationships appear to be present as long thin microtwins. Besides the four silicon‐sapphire orientation relationships previously reported, a fifth relationship has been found. The density of twins deposited on substrates near the (1̄1̄23) orientation appears to be greatly dependent on the substrate orientation. Possible explanations for this phenomenon are discussed. Discrepancies with the work of other investigators are described.
Determination of the Mean Size of Disordered Regions Induced in Germanium by Fast‐Neutron Bombardment at Low Temperature36(1965); http://dx.doi.org/10.1063/1.1703015View Description Hide Description
Thermal conductivitymeasurements have been performed on germanium at 80°K during fast‐neutron bombardment. By using Callaway's formula it is established that the change in thermal conductivity can be related to the size of disordered regions by a simple expression. From experimental data, it is found that the mean radius of disordered regions is about 70 Å.
36(1965); http://dx.doi.org/10.1063/1.1703018View Description Hide Description
The theoretical transit time is derived for propagation of spin waves across a normally‐magnetized disk. The exchange‐free case is described first. The group velocity is obtained for magnetostatic spin waves in an infinite slab. This is combined with the inhomogeneous internal field of a disk to compute the time delay of a wave packet in the disk. The time delay obtained from a computer calculation is compared with an approximate calculation. The dispersion relation is then modified to include exchange effects and the delay time is computed for this case. Experimental observations of microwave pulse‐echo behavior in disks of yttriumirongarnet are compared with these theoretical results.