The mechanism of the yellow luminescence in GaN has been studied. The yellow band is observed in microcrystals synthesized from Ga and NH3 by direct reaction, but is not observed in needle-like crystals grown by sublimation-recrystallization. Doping with carbon emphasizes the yellow band. The characteristic excitation band is observed in C-doped GaN. The yellow band is due to a radiative transition from a shallow donor with a depth of 25 meV to a deep acceptor with a depth of 860 meV. The relation between the characteristic excitation band and the emission band is interpreted by the simple configuration coordinate model. The deep acceptor is thought to be a complex consisting of a gallium vacancy and a carbon atom substituted for a nearest neighbour of the gallium sites.

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The thermal decomposition pathway of an ultrathin oxide layer on Ge(100) and Si(100) surfaces is examined by synchrotron radiation photoelectron spectroscopy and ultraviolet photoelectron spectroscopy with helium I radiation. The as-prepared oxide layer consists of a mixture of oxides, namely, suboxides and dioxides, on both the surfaces. Upon annealing, the oxide layers decompose and desorb as monoxides. However, we find that the decomposition pathways are different from each other. On annealing Ge oxides, GeO2 species transform to GeO and remain on the surface and desorb at >420 °C. In contrast, annealing of Si oxides results in the transformation of SiO to SiO2 up to temperatures (∼780 °C) close to the desorption. At higher temperatures, SiO2 decomposes and desorbs, implying a reverse transformation to volatile SiO species.

We characterize the iron and cobalt catalysts for carbon nanotube growth in chemical vapor deposition (CVD) by using electron microscopy. Nanoparticles of iron and cobalt exhibit a melting point drop in the methane ambient. Nanoparticles after nanotube growth are identified as Fe 3 C and Co 3 C for iron and cobalt, respectively. Those results indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation into graphite (nanotubes) and metal carbide as the carbon uptake in the catalyst melt increases. This supports the vapor-liquid-solid mechanism for nanotube growth by CVD. Iron or cobalt silicide formation causes the poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO 2 base, which can prevent silicidation of iron particles.

During the chemical vapor deposition of multiwalled carbon nanotubes using the vapor phase delivery of a metal-organic (ferrocene) catalyst precursor, a strong selectivity for growth on patterned SiO 2 /Si substrates has been observed. A mechanism for this selective growth is described here. Delivered metal particles (Fe) on Si and SiO 2 regions were investigated using several high-resolution characterization techniques. Active iron catalyst (γ iron) particles were formed on the silicon oxide surface resulting in the formation of highly aligned nanotubes on this substrate. However, in the Si regions, stable FeSi 2 and Fe 2 SiO 4 particles were formed due to chemical reactions between silicon surface and Fe particles at high temperature leading to an inhibition of nanotube growth in the Si regions.

Summary:Purpose: Very fast activity was investigated on the ictal EEGs of epileptic spasms to elucidate the pathophysiology of West syndrome (WS) and related disorders from a novel point of view.Methods: The traces of scalp ictal EEG of spasms temporally were expanded in 11 patients whose clinical diagnosis was symptomatic WS in six, cryptogenic WS in two, Aicardi syndrome in one, and symptomatic generalized epilepsy after WS in the remaining two. Time evolution of averaged power spectra of the ictal fast activity also was analyzed in each patient.Results: Rhythmic gamma activity with frequency ranging from 50 to 100 Hz was detected in a total of 345 of 537 spasms.Fast activity was seen bilaterally in nine patients, was lateralized to one hemisphere in another, and appeared independently on each hemisphere in the remaining infant with Aicardi syndrome. Power spectra showed a clear peak corresponding to spasmassociated gamma rhythm, with frequency centering ∼65 Hz and ranging from 51 to 98 Hz. The morphology and spectral characteristics of ictal gamma rhythm were completely different from those of muscle activity or alternating current (AC) artifacts.Conclusions: Spasm-associated gamma activity was clearly detected on the scalp. This observation may provide a clue to the pathophysiology of spasms.

A global computer simulation of the interaction of the solar wind with the earth's magnetosphere was executed by using a three‐dimensional magnetohydrodynamic model. As a result, we were able to reproduce quasi‐steady‐state magnetospheric configurations and a Birkeland field‐aligned current system which depend on the polarity of the z component of the interplanetary magnetic field (IMF). Twin convection cells and a dawn to dusk electric potential of 30–100 kV appeared at the equator in the magnetosphere. Four types of field‐aligned currents were observed. Region 1 and 2 field‐aligned currents generated for all IMF conditions were 0.6–1.0×106 A and 0.15–0.61×106 A, respectively, in the total current. Region 1 currents at high latitudes are generated from the field‐aligned vorticity at the flanks through a viscous interaction and are strengthened by a twisting of open magnetic field lines in the tail region for southward IMF. On the other hand, the low‐latitude region 2 currents probably are generated mainly from the inner pressure gradient of the plasma sheet. The region 1 current obtained from the simulation was in good agreement with an estimate from our theoretical analysis of the localized Alfvén mode. The other two types of field‐aligned currents are the dayside magnetopause currents in the dayside cusp region, which increase for northward IMF, and the dayside cusp currents for southward IMF. The cusp currents are associated with a twisting of open magnetic field lines in the magnetopause region.