Volume 11, Issue 5, 01 September 1967
Index of content:
11(1967); http://dx.doi.org/10.1063/1.1755071View Description Hide Description
A transient increase of the temperature of the surface of a type II superconductor cooled in a static magnetic field causes flux expulsion and enhances the final diamagnetic moment. It is argued that a net paramagnetic current is induced at the surface of the specimen during the initial cooling in a static magnetic field.Quenching of this surface current by a heat pulse permits flux expulsion.
11(1967); http://dx.doi.org/10.1063/1.1755072View Description Hide Description
Room‐temperature values for all six independent elastic coefficients (at constant electric field) and all four independent piezoelectric stress coefficients have been determined from ultrasonic velocitymeasurements on a single‐crystal specimen of lithiumtantalate grown by the Czochralski technique. The two independent dielectric constants at constant strain were derived from low‐frequency measurements on thin plates of lithiumtantalate, in conjunction with the relevant elastic and piezoelectric data. The numerical results have been used to map the deviation of the elastic energy flux vector from the wave propagation direction for plane elasticwave propagation in the YZ crystallographic plane.
RADIAL DENSITY PROFILE AND CRITICAL MAGNETIC FIELD MEASUREMENT FOR THE HELICAL INSTABILITY IN A HOLLOW PLASMA COLUMN11(1967); http://dx.doi.org/10.1063/1.1755073View Description Hide Description
The characteristics of the current convective instability in a hollow plasma column are described. Langmuir probemeasurements of the radial form of the perturbed electron density reveal that the rotating helix is concentrated towards the outer wall of the discharge vessel. The instability has been shown to occur at a lower critical magnetic field for a hollow plasma cell than for a regular positive column cell of the same dimensions. Neither of these features are considered in previous theoretical work. For both the regular and the hollow positive column, the use of measured electron density profiles in numerically evaluating a stability criterion is shown to give improved agreement between theory and experiment.
11(1967); http://dx.doi.org/10.1063/1.1755074View Description Hide Description
Zeeman effect studies of the 118.65 μm line of a CW water‐vapor laser have shown that the levels involved in the transition have lifetimes between 0.04 and 0.4 μsec and the same total angular momentum, and that the transition has an effective magnetic moment of about 3/4 of a nuclear magneton. These results were obtained by a new spectroscopic technique making explicit use of the saturation behavior of the laser oscillator. This technique enables lifetimes, angular momenta, and magnetic moments to be determined with magnetic field splittings of the order of or less than a tenth of the natural width of the transition.
11(1967); http://dx.doi.org/10.1063/1.1755075View Description Hide Description
Light‐emitting p‐n junctions are scanned by an acoustic domain traveling at soundvelocity in CdS at room temperature. The light elements are heterojunctions formed by depositing p‐Cu2S on n‐CdS. The acoustic domain is generated in CdS by applying a bias electric field which gives the electrons a drift velocity greater than the shear soundvelocity. When the acoustic domain is in the vicinity of a p‐n junction, part of the acoustic energy is transformed into light. As the domain travels between consecutive light elements it replenishes its lost acoustic energy from the bias electric field.
11(1967); http://dx.doi.org/10.1063/1.1755076View Description Hide Description
Experimental results of Raman‐Nath scattering of light by ultrasonic surface and volume waves in quartz are presented. The experiments indicate that the presented method will be a valuable tool for studying the effect of discontinuities upon surface wave propagation. It also appears that the phase velocity of the sound waves may be determined with good accuracy.
11(1967); http://dx.doi.org/10.1063/1.1755077View Description Hide Description
The effect of amplitude of the ultrasonic pulse on the attenuation of pure and impure single crystals of superconductingtin has been studied for the strained as well as annealed specimens. It has been found that small amplitude favors oscillations and the temperature‐dependent attenuation is strongly dependent on the strain content of the specimen and slightly on amplitude.
11(1967); http://dx.doi.org/10.1063/1.1755078View Description Hide Description
The temperature dependence and the absolute sign of the electro‐optic coefficients in single‐crystal LiNbO3 were measured in the temperature range of 20°‐250°C using a vibrating‐mirror Michelson interferometer. The linear electro‐optic effect in LiNbO3 is related through a simple model to the quadratic electro‐optic effect in paraelectric perovskites. The measured magnitude, sign, and the temperature dependence of the electro‐optic effect in this crystal are consistent with this model.
11(1967); http://dx.doi.org/10.1063/1.1755079View Description Hide Description
The nonlinear coefficient d 31 of K.6Li.4NbO3 is equal to d 31 of LiNbO3. Also, the serious problem of optically induced refractive‐index inhomogeneities which has been observed in many nonlinear materials, notably LiNbO3, is not observed in this new material. The electro‐optic half‐wave voltage of this new material is 930 V.
11(1967); http://dx.doi.org/10.1063/1.1755080View Description Hide Description
A novel point tunneling technique for superconductors is reported. This technique employs the intrinsic barrier layer formed between a degenerate GaAs point and the superconducting metalsurface. Excellent agreement between theoretical and experimental conductance curves is obtained for polycrystallinelead.
11(1967); http://dx.doi.org/10.1063/1.1755082View Description Hide Description
The reflectivityspectra of yttriumirongarnet (YIG, Y3Fe5O12) and yttriumgalliumgarnet (YGG, Y3Ga5O12) have been measured in the energy range 2–7.5 eV. Structure is observed in YIG which can be attributed to charge‐transfer‐enhanced crystal field transitions and in YGG to critical point transitions.
STUDIES OF SUPERCONDUCTING Nb‐25% Zr WIRES CARRYING ALTERNATING CURRENT IN AN AXIALLY APPLIED MAGNETIC FIELD11(1967); http://dx.doi.org/10.1063/1.1755083View Description Hide Description
The application of an axially directed magnetic field has been observed to reduce ac losses considerably in superconducting Nb‐25% Zr wires. It has been found that magnetization curves measured with constant‐amplitude ac flowing in a wire sample are paramagnetic‐shifted with respect to curves measured with no externally supplied current. The presence of an axially applied field has been observed to increase the accritical current with a maximum at about 2½ kG applied field.
11(1967); http://dx.doi.org/10.1063/1.1755084View Description Hide Description
GaAs single crystals were grown epitaxially from Ga solution with carrier concentrations in the 1014 cm‐3 range and electron mobilities between 7500 and 9300 cm2/V‐sec at 300°K, and 50,000 and 95,000 cm2/V‐sec at 77°K. A comparison of the theoretical and experimental curves for the mobility vs temperature indicates that the significant scattering mechanisms are ionized impurities and phonons in the temperature range of 77°K to 430°K. This indicates that the epitaxial layers do not contain other mobility‐limiting imperfections to a significant degree. Photoluminescencespectra of the epitaxial layers did not show any emission due to keep lying imperfection levels.