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(1998)Journal ArticleThe first results obtained using a SOI device for microdosimetry applications are presented. Microbeam and broadbeam spectroscopy methods are used for determining minority carrier lifetime and radiation damage constants. A spectroscopy model is presented which includes the majority of effects that impact spectral resolution. Charge collection statistics were found to substantially affect spectral resolution. Lateral diffusion effects significantly complicate charge collection
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(1999)Journal ArticleFlux pinning force and irreversibility lines have been investigated for melt-textured YBa2Cu3Oy crystals containing Y2BaCuO5 (211) particles with an average size ranging from 20 to 0.02 microns. We found that the irreversibility lines are increasingly improved and that the pinning force as well as the Jc is greatly enhanced as the average size of the 211 particles decreases. The Fp-H curve exhibits two peaks in the samples containing very fine 211 particles (nanometer size particles), which suggests that an addition pinning mechanism is involved in this type of samples. The experimental results can be well explained with combining the interface-pinning mechanism and the match effect.
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(1998)Journal ArticleYBa2Cu3O7-y with high grain alignment has been successfully fabricated by a modified powder melting process at a temperature of ~930oC which is near the eutectic point of the starting materials Y2BaCuO5, BaCuO2, and CuO. In terms of the levitation force and YBa2Cu3O7-y grain alignment, the best result was shown in a sample having a Y2BaCuO5:BaCuO2:CuO molecular ratio of 1:3:5. In the present work, the Cu-surplus eutectic liquid was completely absorbed by use of a Y2BaCuO5 substrate, by which the final composition was driven to stoichiometric YBa2Cu3O7-y. The results were comparable to those obtained by the conventional melt-texture-growth process. A new invariant point apparently occurred at ~930-940oC owing to an interfacial reaction between YBa2Cu3O7-y matrix and Y2BaCuO5 substrate.
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(1998)Journal ArticleDuring magnetoresistance measurement of Bi-2212 single crystals, we found the resistance in the c-axis direction exhibited a huge magnetic enhancement (H perpendicular to the a-b-plane) while the onset transition temperature along the c-axis showed a large depression when the magnetic fields were over a certain value (H > 180 Oe). No such phenomena were observed in small applied magnetic fields (H < 180 Oe) and in a-b-plane magnetoresistance measurements. By changing different probes (eight probes being attached to the sample, each side having four probes) for the measurement of the magnetoresistance along the c-axis with H = 2000 Oe, we found that the resistance along the c-axis in the temperature region T/Tc > 0.90 was only related to the measuring dimensions and Tc onset remained unchanged. Our results provided evidence that both Lorentz-force-independent fluctuation-induced dissipation and Lorentz-force-dependent vortex motion dissipation exist in the mixed state.
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(1999)Journal ArticleA survey towards a selection of 35 methanol maser and/or ultracompact (UC) H ii regions, reported in Papers I and II and by Norris et al., has been conducted in the near-infrared (NIR). Out of 25 methanol maser sites surveyed, 12 are associated with a NIR counterpart. Out of 18 UC H ii regions (8 of which overlap with maser emission), 12 are associated with a NIR counterpart. Counterparts can be confidently identified not only by the positional agreements, but also by their unusually red colours. Spectral types for the embedded stars can be unambiguously determined for six sources, all of which imply massive, ionizing stars. One of these infrared sources has methanol maser emission, but no UC H ii region. It is possible that the maser emission associated with this source arises from a pre-UC H ii phase of massive stellar evolution or it could be that nearly all the ultraviolet photons are absorbed by dust within the UC H ii region. We have modelled the spectral energy distributions (SEDs) for some sources and find that a single blackbody can be used to estimate the stellar luminosity, but cannot represent the whole infrared SED. A two-component blackbody model and a radiative transfer model were also used to derive essential parameters of the infrared sources. The radiative transfer model also indicates which infrared sources are relatively young and which are older. Both models show that silicate absorption at 9.7 μm must be a dominant feature of these SEDs.
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(1999)Journal ArticleUsing near-infrared imaging and spectroscopy, we discuss the discovery of six emission-line sources within 20 pc of Sgr A West but outside of the central parsec. The objects are coincident with stars seen in the J, H, and K images; all have Brγ (2.166 μm) in emission, several also have He i (2.058 and 2.112/3 μm), and some have He ii (2.189 μm) emission lines. Comparison of the H- and K-band spectra of the newly discovered stars with recently published infrared spectral atlases of optically classified stars suggests that most of these stars are similar to massive stars in transition (e.g., Ofpe/WN9, B[e], and LBV), although one is definitely a WN6 star. Two of the potential stars are associated with the radio emission regions A–D near Sgr A East, three are near or within the radio emission regions H1–H8 (nomenclature from Yusef-Zadeh & Morris), the last is near G0.10+0.02 and is likely to be associated with the G0.12+0.02 (Arches) cluster of stars, discussed in a previous paper. The stars are shown to be significant sources of ionization for their associated radio emission regions, with several capable of completely ionizing the regions. These results provide additional support for the idea that these thermal radio regions, and similar such regions in the Galactic center, are indeed photoionized by hot young stars.
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