摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1657417

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1657418

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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 theql<1 approximation.

ISSN:0021-8979    

DOI:10.1063/1.1657419

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1657420

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Laser oscillations of wavelength 3.39 &mgr; are reported for a low‐voltage discharge in He&sngbnd;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.

ISSN:0021-8979    

DOI:10.1063/1.1657421

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

An anomalous self‐diffusion behavior has been observed in single‐crystalline bismuth. The penetration profiles do not follow Fick's laws of diffusion and disagree with the results reported by Seith. For the semi‐infinite rod configuration, the concentration of210Bi tracer decreased exponentially with depth, but there was not necessarily an excess of tracer on the surface and the profiles did not vary with time. Penetration was correlated with mechanical twins, but was not directly amenable to a Fisher grain‐boundary analysis. Volume diffusion was not observed. The results have been interpreted by a modified grain‐boundary treatment in which the diffusing species is trapped. Integration of the trapped material over time with a Laplace transform results in the profileC=s(k/D′)1/2exp[−y(k/D′)1/2], which is in functional agreement with observation. The quantity (k/D)′1/2appears to be a property of Bi, and may be regarded as a characteristic inverse trapping distance.

ISSN:0021-8979    

DOI:10.1063/1.1657422

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1657423

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1657424

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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 &mgr; in diameter, were seen to grow to unusual lengths, occasionally exceeding 50 &mgr;.

ISSN:0021-8979    

DOI:10.1063/1.1657425

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

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.

ISSN:0021-8979    

DOI:10.1063/1.1657426

出版商:AIP    

年代:1969

数据来源: AIP