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Notes: Annihilation characteristics of positrons in SiO2/Si structure were studied by using a monoenergetic positron beam in the temperature range between 50 K and room temperature. In the SiO2 film, positrons formed positronium (Ps) and they annihilated from localized states in open spaces. Below 100 K, the Ps formation was found to be suppressed. This fact was attributed to the trapping of positrons by point defects in the SiO2 film at low temperature. The depth distribution of such traps was not homogeneous; its concentration in the central region of the SiO2 film was higher than that in the regions near the surface or the interface. The potential of monoenergetic positrons for the detection of the point defects provides unique information for depth distributions of hole traps in SiO2 films. © 1995 American Institute of Physics.

Notes: The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open-space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho-Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open-space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.

Notes: Frequency dependence of power loss of MnZn ferrite was investigated in relation with the absolute complex permeability, μa(=μa−jμa″), in the frequency range 10 kHz–5 MHz and the temperature range 23–120 °C. Residual loss, Pr contributes more than half of the total power loss, Pc, at a frequency above 1 MHz and exhibits a close relationship with μa″. The specimen with grain size of 4 μm and ZnO content of 0 mol % indicates μa′ of 1280 and Pc of 140 kW/m3 at 1 MHz and 50 mT. In the second measurement at the remanent state after the dc magnetization of 400 A/m, these values, however, increased and then gradually recovered with time. This behavior is considered to be related to the diffusion of Fe2+ through cation vacancy. © 2000 American Institute of Physics.

Notes: Heteroepitaxial layers of InP with thickness D ranging from 0.1 to 6.0 μm were grown by low-pressure metalorganic chemical vapor deposition on (001) surfaces of GaAs substrates. Their dislocation structure, induced strains, and nature of the radiative recombinations were investigated as a function of D with transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. For D〈2 μm, the films are highly dislocated with a tangle of interfacial and threading dislocations above the heterointerface. The spatial extent of the interfacial dislocations and the density of threading dislocations increase with increasing D. For D(approximately-greater-than)2 μm the portion of the layers away from the heterointerface by more than 1.5 μm shows a decrease in the density of threading dislocations and a dramatic improvement in the crystalline quality with increasing D.Typical dislocation densities in the neighborhood of the top surface are in the mid 107 cm−2 range when D surpasses 4.0 μm. Concomitant with the improved crystalline quality, the following observations are made. Firstly, the full width at half maximum of the x-ray rocking curves diminishes from values larger than 500 arcsec for D〈1.0 μm to about 200 arcsec for D(approximately-greater-than)4.0 μm. Secondly, the near-band-edge photoluminescence transitions, which for D〈2.0 μm are predominantly determined by defect-induced band tailing, display excitonic character. Thirdly, below-band-gap transitions due to interfacial defects decrease in intensity. Biaxial compressive strain is present in the layers because of lattice mismatch and differences in linear thermal expansion with the substrate.The strain removes the degeneracy between the light- and heavy-hole states at the top of the valence band, and consequently with increasing temperature above 12 K recombinations from the conduction to the split valence bands are observed in the photoluminescence spectra for all D. The identification of such transitions follows from their temperature dependence and the activation energy yield for the thermalization of the holes. The measured valence-band splitting decreases from 12.5 meV for D=0.3 μm to saturation values of 5.6 meV for D(approximately-greater-than)3.0 μm, indicating strain relaxation with D in qualitative agreement with x-ray determinations. Quantitative differences between both methods are realized and are attributed to a temperature dependence of the differential linear thermal expansion. The contribution to the strain from the lattice mismatch is much larger than expected from equilibrium models. The dislocation generation at different stages during the growth is inferred from the strain relaxation against D and the observed location of the dislocations throughout the layers.

Notes: GaAs layers grown on (001) InP surfaces by low-pressure metalorganic chemical vapor deposition were investigated with photoluminescence spectroscopy, x-ray diffraction, Raman scattering, and transmission electron microscopy. Correlations between the optoelectronic properties, the strain relaxation, and the structural defects were established for layer thickness D ranging between 0.1 and 3.0 μm. A comparison with the case of InP layers grown on GaAs substrates is presented. Radiative recombinations to split light- and heavy-hole valence bands near the zone center are seen at 12 K in the photoluminescence spectra. The splitting is due to a biaxial tensile strain. With increasing temperature, the heavy-hole transitions gain intensity and at around 140 K they are the only features in the spectra. In the 12–50 K temperature range the intensity ratio between the heavy- and light-hole transitions also depends on laser power. The hole activation energy determined from the temperature dependence of the intensity ratio above 50 K agrees with the valence-band splitting. The strain for D(approximately-greater-than)0.3 μm arises from differences in linear thermal expansion and has contributions from the lattice mismatch in thinner layers. The strain values yielded by x-ray diffraction are smaller than those obtained from the valence-band splitting measured with photoluminescence. The difference is attributed to a temperature dependence of the linear thermal expansion, which was corroborated by the shifts of the longitudinal optical phonon frequencies measured with Raman spectroscopy at 300 and 12 K. A comparison is made of the absolute magnitude of the strain and the x-ray diffraction linewidths for heteroepitaxial GaAs and InP layers on InP and GaAs substrates, respectively. The contribution to the strain from the lattice mismatch relaxes in GaAs faster than in InP and the GaAs x-ray linewidths are narrower for D〈1.0 μm. These results are understood in terms of the growth habit and the behavior of threading and interfacial dislocations.

Notes: Heteroepitaxial InP layers were grown by metalorganic chemical vapor deposition on novel GaAs buffer layers on Si substrates. The GaAs buffers were prepared by molecular beam epitaxy and show a reduced threading dislocation density achieved by inserting one nanometer thick Si interlayers. The structural and optoelectronic properties of the heteroepitaxial InP layers, which were investigated by transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy, show improvements attributed to the optimized GaAs buffer layers.

Notes: Self-sustained pulsating optical power and kink level in AlGaAs semiconductor lasers are remarkably improved by introducing a multiquantum well (MQW) structure in the active layer. Stable fundamental transverse mode operation at output power up to 50 mW and self-sustained pulsation at output power up to 40 mW are obtained simultaneously for MQW lasers with antireflective and high-reflective coatings on the facets. Low-noise characteristics (relative intensity noise of less than 10−13 Hz−1 under 3–4% optical feedback) are obtained in the output power range from 7 to 17 mW in MQW lasers with high-reflective coating on the rear facet. These results suggest that low-noise high-power characteristics can be achieved in self-sustained pulsating lasers with a MQW active layer.

Notes: Abstract: Norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylalanine (DOPA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured simultaneously by high performance liquid chromatography with electrochemical detection in extracts of olfactory bulbs at various intervals after chemical or surgical deafferentation. Chemical deafferentation of mice by intranasal irrigation with Triton X-100 or of rats by olfactory axotomy resulted in a rapid progressive decline of DA and DOPAC and an associated rise in NE in the olfactory bulb. However, after several weeks, these values returned to pre-lesion levels concomitant with reinnervation of the bulb by the afferent neurons. In contrast, deafferentation by procedures known to prevent reinnervation of the bulb by the afferent chemoreceptor neurons (i.e., a ZnSO4 solution in mice or a surgical procedure in rats) completely blocked the return to pre-lesion values of DA, DOPAC, and NE. The specificity of these effects was demonstrated by the inability of intranasal administration of the neurotoxin 6-hydroxydopamine to alter DA levels, resulting instead in a significant decline in olfactory bulb NE content. These data demonstrate that the DA content of the olfactory bulb can be influenced by either chemical or surgical modulation of the afferent pathway in two different species. This offers additional support for our hypothesis of transsynaptic regulation of intrinsic DA neurons of the bulb by the afferent olfactory chemoreceptor neurons.

Notes: Abstract: Carnosine has now been demonstrated by chemical analysis to be present in rat olfactory mucosa on day 16 of gestation. The tissue content of this dipeptide then increases progressively during fetal and postnatal life. Radioactive carnosine can be isolated from cultured embryonic rat olfactory mucosa incubated with [14C]β-alanine as early as 13–14 days of gestation. The amount of incorporation also increases progressively with the initial age of the explant and with time in culture indicating in vitro maturation of the carnosine synthesis capability of olfactory tissue. To test whether the level of β-alanine was limiting the synthesis of carnosine, we evaluated the effect of elevated β-alanine levels on tissue carnosine content. Exogenous β-alanine caused an increase in the tissue content of carnosine at several ages in vivo and in vitro. In adult animals this increase was observed in olfactory bulb, olfactory muscosa, and skeletal muscle. However, there was no associated alteration in carnosine synthetase activity. In addition, the different half-lives of carnosine in olfactory tissue and muscle seemed unaltered, arguing against any effect on degradative enzymes. Thus, tissue carnosine levels are regulated, at least in part, by substrate availability. The early appearance of carnosine synthetic capacity during prenatal development indicates that this enzyme activity should be a valuable aid in studying early events in olfactory neuron maturation.

Notes: Background : Nitrates decrease the tone of the lower oesophageal sphincter, and may thus induce gastro-oesophageal reflux.Aim : In the present study, we evaluated electrogastrographic changes and heart-rate variability before and after the administration of nitrates.Methods : In 15 patients with chest pain treated with nitrates, electrocardiography and percutaneous electrogastrography were performed before and after administration of nitrates. Autonomic nervous system function was evaluated by spectral analysis of heart-rate variability and serial changes in low frequency and high frequency power, and the low frequency/high frequency ratio were compared. Electrogastrograms were analysed by obtaining peak power amplitudes and their dominant frequencies.Results : After the administration of nitrates (isosorbide dinitrate), high frequency power, an index of parasympathetic nervous activity, was significantly decreased, whereas the low frequency/high frequency ratio, an index of sympathetic nervous activity, was significantly increased. The mean peak amplitude of the electrogastrogram significantly increased postprandially both before and after treatment. After isosorbide dinitrate treatment, however, mean peak amplitudes after a meal were significantly lower than those obtained before treatment. The mean dominant frequency of the electrogastrogram did not vary before and after treatment.Conclusions : The present study suggests that nitrates inhibit gastrointestinal motility by decreasing autonomic nervous activity.