To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Most position sensitive proportional counters (PSPCs) currently used in X-ray diffraction experiments have a dead time longer than 5 μs. Though such PSPCs are useful in measuring weak diffraction diagrams, a faster counter is needed to detect strong X-ray diagrams produced with synchrotron radiation sources. The long dead time of PSPCs using a charge division position read-out is due to the slow analog division circuit plus analog-to-digital converter employed in the present system. A fast processor can be built utilising two high-speed ADCs to digitize voltage signals from the detector, followed by a digital divider to compute position of detected photons. The present paper describes the design of such a processor and some preliminary testings of its performances.
In this paper, a colloidal solution of copper nanoparticles was prepared from a Cu ion aqueous solution with the protein casein surfactant by a liquid phase reduction method at low temperature below 373K. For the casein concentration ranging from 6g/L to 75g/L, the formation of copper nanoparticle colloid were observed. As a result, the peak was observed at the ranging of 450 to 650 nm corresponding to the copper nanoparticle colloid plasmon absorption. As the surfactant concentration increases, the absorption spectrum tends to blue-shift and the particle diameter decreases. Thus, it indicated that the optical property and particle diameter of copper nanoparticle colloidal solution will be controlled by the protein casein surfactant concentration.
Type I storms are the most frequently observed solar phenomena at metre and decimetre wavelengths. Since the first identification of the emission of this type with a large sunspot group a great many type I storms have been recorded with radio-spectrographs, polarimeters and interferometers (see, for example, Wild, Smerd and Weiss, Kundu, Wild). Nevertheless, we can offer no satisfactory answers to the most fundamental questions about type I storms: ‘What kind of disturbances supply energy to a localized coronal region to maintain the storm activity for up to several days?’; ‘What is the emission mechanism responsible for the peculiar features observed in type I storms?’.
The work of Commission 25 covers a wide range of topics concerning the measurement of magnitude, colour and polarisation of astronomical objects. As such, the area of interest covers virtually every field of astrophysical research in the visual and infrared spectral domain. Our reports cover some aspects of photometry and polarimetry as a technique rather than being an account of research highlights over the last three years.
The red variables whose amplitude is larger than 1.3 mag in the MOA database are studied for the LMC. Among 3 196 such stars, 532 stars are likely to be Miras or red semiregular variables. The period–colour relation of these stars is shown.
More than 4000 stars observed in both MOA and DENIS projects showing periodic or quasi-periodic light curves are studied. Almost all Mira stars are located on the classical period-luminosity relation, and the multiplicity of the period-luminosity relation is confirmed for small-amplitude stars. The colour-magnitude diagrams based on the MOA red band, Rm, and Ks constructed for the sequences, form a single strip with small successive shifts.
A high time- and spatial-resolution radio interferometer for solar observations has been constructed at Nobeyama (Figure I.; Nakajima et al. 1994). The Nobeyama Radioheliograph consists of 84 antennas, 0.8m in diameter, arranged on a T-shape lines of 500m in the EW and 220m in the NS directions. The time resolution is 50 ms and the spatial resolution is 10”. The field of view is 40’ at the observing frequency 17GHz, which enables us to watch the whole sun. The radioheliograph has observed hundreds of flares during the few months since the beginning of regular observations in July ‘92, and such powerful performance has never before been demonstrated in the history of solar radio observations.
A large database of CCD photometry for 1.4 million stars towards both the LMC and the SMC, which has been established by the MOA project, is a useful resource to study variable stars. In our preliminary study, variables identified as β Lyrae type stars and Herbig Ae/Be stars have been found amongst blue stars.
A review of the MOA (Microlensing Observations in Astrophysics) project is presented. MOA is a collaboration of approximately 30 astronomers from New Zealand and Japan established with the aim of finding and detecting microlensing events towards the Magellanic Clouds and the Galactic bulge, which may be indicative of either dark matter or of planetary companions. The observing program commenced in 1995, using very wide band blue and red filters and a nine-chip mosaic CCD camera.
As a by-product of these observations a large database of CCD photometry for 1.4 million stars towards both LMC and SMC has been established. In one preliminary analysis 576 bright variable stars were confirmed, nearly half of them being Cepheids. Another analysis has identified large numbers of blue variables, and 205 eclipsing binaries are included in this sample. In addition 351 red variables (AGB stars) have been found. Light curves have been obtained for all these stars. The observations are carried out on a 61-cm f/6.25 telescope at Mt John University Observatory where a new larger CCD camera was installed in 1998 July. From this latitude (44° S) the Magellanic Clouds can be monitored throughout the year.
The challenges associated with meeting 20nm technology requirements for better Cu CMP process uniformity and lower defectivity have been studied. Required improvements in uniformity were obtained through platen process optimization along with evaluation & selection of specific Cu slurries and pads and their performance reported. The principal factors influencing defect formation, including Cu barrier metallurgy, interconnect pattern density and process queue times were studied. Specific new post CMP clean chemistries were evaluated to assess their capability to suppress defect formation and their performance reported. The trade off between uniformity and defect suppression as a function slurry, pad and post Cu CMP clean chemistry is described.
We present the scanning tunneling microscope-induced luminescence (STL) imaging of defects in optoelectronic materials. Resolution is first discussed using cross-sectional images of InGaAs/GaAs quantum dots. Proof of concept is then provided through the nanometer-scale imaging of GaN layers and quantum wells. The expected λ=356±25 nm range dominates the low temperature STL of GaN. Mapping of luminescence shows circular non-emitting areas around threading dislocations. Extent of dark areas suggests a hole diffusion length of Ld=30–55 nm, in agreement with reported values. The expected λ=450±35 nm range dominates the STL from a buried InGaN/GaN multiple quantum well. Imaging reveals 30–100 nm wide smooth fluctuations of luminescence.
A series of 100 nm thick InGaN films with In content up to 14% has been grown by MOVPE on SiC substrates. Optical characterization was carried out by means of reflectance spectrometry, photoluminescence and cathodoluminescence. Optical properties of the samples have been correlated with the microstructural properties measured by atomic force microscopy, transmission electron microscopy and X-ray diffraction data. Results indicate a dependence of the optical properties on the In content in the sample, as well as on the residual stress in the films induced by Indium incorporation. Part of the strain is reduced elastically by formation of pinholes which reach the InGaN/GaN interface, where first misfit dislocations are observed to form. Our results show that luminescence is directly correlated with the strain distribution in the layers. Pinholes are observed to act as nonradiative recombination sites for carriers, while strain relaxation around pinholes may enhance luminescence emission. We discuss the influence of strain with respect to In incorporation, the appearance of piezoelectric fields and effects due to strain induced band bending.
We present an optical investigation of thin Zn-doped GaAs layers embedded in bulk GaAs, by means of stationary optical spectroscopy. The samples were grown by metalorganic vapor phase epitaxy (MOVPE) The concentration of doped Zn acceptors were aimed at 2×1020/cm3 in 4 nm wide doping regions. We observed a novel optical radiative transition (denoted as F-emission) appearing in photoluminescence (PL) spectra below the energy position of the transition between the free electrons and holes bound to acceptors in bulk GaAs. The F emission shows a strong dependence on excitation intensity and temperature. The energy position varies from 1.46 eV to 1.49 eV when the excitation density increases from about 40 mW/cm2 to 23 W/cm2. Our results indicate that this emission is related to the transition between spatially separated electrons and holes. The holes are located in the p-type Zn δ-doped region, while the electrons are located in the undoped GaAs region.
Porous layers and free-standing membranes were fabricated by anodic etching of n-GaP substrates in a sulphuric acid solution. Micro-Raman analysis of the interaction between the longitudinal optical phonons and plasmons in porous membranes allowed us to obtain specific information about the electro-optical properties of microstructured GaP. In particular, apart from the carrier exhausted areas surrounding the pores, the existence of conductive regions was demonstrated. A comparative analysis of the secondary electron and panchromatic cathodoluminescence (CL) images evidenced an increase in the emission efficiency caused by porosity. Data concerning the spectral distribution of CL in bulk and porous samples are presented.
We present optical and microstructural characterization of nanocrystalline silicon superlattices (nc-Si SLs). Our samples have better than 5 % Si nanocrystal size distribution and a long range order along the direction of growth provided by periodically alternating layers of Si nanocrystals and SiO2. Flat and chemically abrupt nc-Si/SiO2 interfaces with a roughness of < 4Å are confirmed by transmission electron microscopy (TEM), Auger elemental microanalysis, X-ray small angle reflection, and low-frequency Raman scattering. Photoluminescence (PL) in our structures has been studied in details including time-resolved and steady-state PL spectroscopy in a wide range of temperature, excitation wavelength and power. Resonantly excited PL spectra show phonon steps proving that the PL originates in Si nanocrystals. Electrical measurements show signature of phonon-assisted tunneling proving low defect density nc-Si/SiO2 interface.
Light detection system combined with a UHV-scanning tunneling microscope (STM) was applied to the study of silver films deposited on Si(111) surfaces. Photon maps clearly show single atom height steps and terraces on an Ag(111) surface with high spatial resolution of nanometer scale. Chemical reaction on the Ag surface with residual gas was clearly revealed in the photon map. In the photon map of the thin Ag film of 2˜3 ML in thick, no contrast appears between the terraces, and a characteristic bright contrast appears at the single atom height steps. The local plasmon model does not readily explain those contrasts.
In this paper, we showed the significant reduction of the energy spacing between ground state and excited state emissions from InAs/GaAs quantum dots due to interface interdiffusion induced by thermal treatment. In addition, the strong narrowing of the luminescence linewidth of the ground state and excited state emissions from the InAs dot layers for the annealed samples indicates an improvement of the size-distribution of the QDs. Large blue-shift of the energy positions of both emissions was also observed. High resolution X-Ray diffraction experiments give strong evidence of the interface atom interdiffusion in the annealed samples. This work shows ability to tune the wavelength for applications like infrared detectors and lasers based on intrasubband transitions of self-assembled QDs.
We present the results of the magneto-photoluminescence measurements performed on modulation doped GaAs/AlGaAs heterostructures in high magnetic fields (up to 60T) and low temperatures (0.37−1.5K). With increasing magnetic field we observe the formation of the triplet and singlet states of negatively charged magneto-excitons (X−) in addition to the neutral exciton (X0). Their behavior with field strongly depends on the sample geometry. In the case of a modulation doped quantum well (QW) with a well-width of 200A, the and states cross at a magnetic field of about 40T, whereas for a modulation-doped single heterojunction (SHJ) these states show no crossing over the whole range of available fields.