Self similar unsteady boundary layers

Summary This paper deals with unsteady laminar boundary layers contiguous to self-similar flows. The following general result is obtained: when the self-similar structure of the external flow is apparent in the boundary layer system of coordinates, and when there is no heat transfer through the wall, then the boundary layer is self-similar too. This property can be applied to a lot of known self-similar motions. Some examples are considered for which the final form of the boundary layer equations is formulated.     

Characteristics of bistable CW GaAs junction lasers operating above the delay-transition temperature

The conditions for bistable operation of CW GaAs junction lasers are developed in terms of the previously published double-acceptor trap theory. The experimental CW operation of such devices is shown to agree well with the theoretical results. In addition the fabrication of these bistable lasers is described and several pulsed experiments are reported that indicate a significant increase in the number of trapping centers in the vicinity of the junction.     

Proton bombarded double heterostructure LED’s

In this paper we report the first experimental results for double heterostructure (DH) Burrus LED’s in which current confinement is achieved by proton bombardment techniques. Both p-type and n-type active layers with carrier concentrations ranging from 10¹⁶ cm⁻³ to 2 × 10¹⁸ cm⁻³ were investigated. Devices with shallow surface bombardments (at 100 keV) were compared to devices with deep bombardments (at 390 keV), which penetrate past the p-n junction. Measurements were made of the shape of the light emitting area, device capacitance, 3 dB bandwidth, external efficiency, and operating reliability. These results have been compared with those obtained from LED’s fabricated from the same crystals with conventional oxide-masking techniques. It has been found that deep proton bombardment provides precise control of the. light emitting area as well as the largest 3 dB bandwidths through reductions in the device capacitance. Comparisons of the output power for the various devices established that the overall device efficiency improved in going from deep to shallow bombardment conditions. However, in all cases, the highest efficiencies were achieved with conventional oxide confinement as a result of enhanced reflection at the SiO₂-metal interface. These observations have led to the fabrication of an innovative LED structure which incorporates both oxide-masking for improved efficiency and proton bombardment for improved 3 dB bandwidth. Under operation at 100°C and junction current densities of approximately 3 kA/cm², the reliability of LED’s with proton bombardment confinement was found to be equal to or better than LED’s with conventional oxide confinement.     

Anomalous self-diffusion in a single crystal of α-zirconium

Self-diffusion in an α-zirconium single crystal was investigated over the temperature range 779 to 1128 K. The measurements were made by simultaneous diffusion of ⁹⁵Zr and its radioactive daughter ⁹⁵Nb and by subsequent ion-beam-sputter sectioning (IBS). The activity of ⁹⁵Zr in each section was determined from the 724 keV γ peak using a Ge-Li spectrometer. Gaussian penetration curves were observed. The diffusivities vary in the range between $\text{3.28 × 10}{}^{\mathrm{?23}}\text{m}{}^2\text{s}{}^{\mathrm{?1}}$ and $\text{1.04 × 10}{}^{\mathrm{?18}}\text{m}{}^2\text{s}{}^{\mathrm{?1}}$. The present results are compared with previous studies of self-diffusion in α- and β-Zr. The Arrhenius plot of self-diffusion coefficients in α-Zr is strongly curved downward. This means that an anomalous behaviour of self-diffusion is observed not only in the β- but also in the α-phase of Zr. Some results on the diffusion of ⁹⁵Nb in α-Zr are reported in an Appendix.     

Spiking in light pulses from GaAs Q-switched junction lasers

Using a detection system with 150-ps resolution, a spiking behavior has been detected in theQ-switched light output from GaAs junction lasers. At the lower currents in theQ-switching region, a single light spike, whose width is about 300 ps, is observed. At higher currents, additional light spikes appear whose widths and spacings decrease as the current is increased. At the highest pumping levels, only the initial spike is clearly resolved and its width has decreased to less than 200 ps. Qualitative agreement is obtained from a simple theory based on the standard rate equations. The necessary modifications to the theory are discussed and results of computer calculations are presented which predict that the width of the initial spike can be much less than 100 ps at sufficiently high pumping levels.     

Resonant modes of GaAs junction lasers--III: Propagation characteristics of laser beams with rectangular symmetry

In Parts I and II of this work [1], [2], the resonant modes of GaAs lasers, as predicted from a proposed model, are found to be in very good agreement with experimental results that emphasize the frequency domain. This paper, Part III, supplements the previous work by investigating the spatial field distributions of junction-laser beams. For the first time, the propagation characteristics of the beams are described mathematically in a manner that is consistent with detailed beamwidth measurements in the Fresnel and Fraunhofer regions of the diffraction field. This correlation between theory and experiment is made possible by the use of a lens-slit combination that ensures the generation of reproducible beams with rectangular symmetry. The astigmatic beam solutions describing the propagation characteristics beyond the slit include beam-broadening effects produced both by the lens and by phase variations at the laser mirror, along the junction plane. These latter variations have not been previously reported. Their existence is deduced from our measurements beyond the slit, which show a greater beam expansion along the junction plane than can be theoretically accounted for by the lens system alone. Near threshold, at a distance of one beamwidth from the center of the mirror illumination, the phase lag is found to have a current-dependent magnitude of a few degrees.     

Impurity diffusion of Fe in stoichiometric UC

Measurements have been made between 1800 and 2260°C of the diffusion coefficient D of ⁵⁹Fe tracer in coarse polycrystalline near-stoichiometric UC. The results give very large values for D, around 5 × 10⁻⁸ cm²/s, comparable with the C diffusion rate and 10⁵ to 10⁶ times greater than the self-diffusion of the U in the same temperature range. The enhancement of the U-self-diffusion rate by the presence of Fe, previously observed by Matzke, is much too small to be compatible, on the basis of a vacancy mechanism, with the very large Fe diffusion rates we report here. Accordingly it is proposed that Fe may dissolve, at least partially, in interstitial solution and diffuse by a Miller-type interstitial-vacancy pair mechanism. The likely similar behaviour of other solutes is also briefly discussed.     

Internal Q-switching in GaAs junction lasers

InternallyQ-switched light pulses have been obtained from junction lasers. It is believed that this is a completely new observation for semiconductor lasers. Using specially fabricated diodes, narrow bursts of light were detected immediately after the termination of the injection current pulse. The effect persists for a wide variation in the length of the current pulse, from less than 2 ns to several μs. The width of theQ-switched light pulse itself is less than 0.4 ns, this value being the resolution of our detection system. Its energy increases rapidly with the amplitude of the injection current. The occurrence of stimulated emission after the end of the injection pulse indicatesQ-switching due to a reduction of the internal absorption. This reduction allows those injected carriers that have not yet spontaneously recombined to produce the narrow burst of stimulated light. TheQ-switching is observed over a current amplitude range that is a very strong function of temperature. This range can be relatively large. One diode at 150°K showedQ-switching after current pulses from 1.2 to 5.0 amperes; for amplitudes greater than 5.0 amperes, normal stimulated emission occurred during the current pulse. As the temperature is increased, theQ-switched pulse is first observed near Tt, the so-called "transition temperature" where trapping effects first start to cause long delays between the application of the current pulse and the onset of stimulated emission. Therefore, it is believed that the same traps are involved in both the long delays and theQ-switching. The latter is observed only in diodes with low Ttvalues where the absorption due to traps accounts for a large proportion of the total losses of the laser. A model explaining these effects will be presented.     

Self-diffusion of Ti,Zr, and Hf in their hcp phases,and diffusion of Nb95 in hcp Zr

Preliminary values for the self-diffusion constants of -(hexagonal cp) Hf and new values for the self-diffusion constants of -(hcp) Zr are given. A new determination of the latter was considered necessary since existing values disagree widely and the experimental methods employed in their determination are not considered to be sound. Values obtained by the authors for the self-diffusion constants of -Ti are reviewed.The activation energies obtained are much smaller than those predicted by relationships based on physical properties of the elements and, within the experimental error, they have similar values to those corresponding to the -(body-centred cubic) phases. Frequency factors are too small to satisfy Zener's theory; when interpreted according to a vacancy model they give negative activation entropies, and the relationship D ₀()/D ₀() is approximately the same for the three elements.It is suggested that the same diffusion mechanism operates in both the - and -phases. If two mechanisms operate in the -phase, the low temperature one is the same as operates in the -phase.     

Genetic epidemiology of multiple sclerosis

Random Amplified Polymorphic DNA (RAPD) analysis is a new technology of molecular marking which has proved very powerful in detecting genetic diversity at the level of population. The genomic DNAs used in our experiment were extracted from fresh leaves taken from 59 individuals sampled from three natural populations in Yan An, Shanxi Province. Through more than 2,000 PCRs, deep-going RAPD analysis was carried out on DNA samples from 49 individuals. The percentage of polymorphic RAPD loci found in these three populations were respectively 27.2%, 18.6% and 5.4%; the average genetic distances within population, 0.055, 0.036 and 0.008; the average genetic distances between populations (I-II), (I-III) and (II-III), 0.105, 0.096 and 0.060. The genetic diversity of A. brachypus within and between populations was found, for the first time, to be rather poor, thus revealing innate factors as the cause contributing to its endangered status. In addition, our work also provides basic materials for elucidating the underlying cause of its endangerment and for its protection biology.