X-ray absorption spectroscopy investigations on oxidized Ni/Au contacts to p-GaN.

X-ray absorption spectroscopy was used to investigate the electronic structure of as-deposited and oxidized Ni/Au contacts to p-GaN and to elucidate the mechanism responsible for low impedance. X-ray absorption near edge spectra of Ni K- and L3,2-edges clearly indicate formation of NiO on the sample surface after annealing. The reason for low impedance may be attributed to increase in hole concentration and existence of p-NiO layer on the surface.

X-ray absorption spectroscopy studies of Ba(1-x)Ca(x)TiO3.

We report x-ray absorption near edge spectroscopy (XANES) of Ca and O K-edges of Ba(1-x)Ca(x)TiO3 (x = 0, 0.01, 0.08, 1) and understand the spectral features related to the electronic structure of these perovskites. The XANES spectra of Ca K-edge possess a pre-edge peak similar to other 3d transition metals like Ti, Ni when present in perovskite structure and provides information about p-type or hole doping. Presence of considerable amount of 3d states justifies the reason to consider it as a light 3d transition metal. The O K-edge spectra display characteristic spectral features assigned as e(g) and t2g and show strong dependence on concentration.

Advanced field-measurement method with three orthogonal Hall probes for an elliptically polarizing undulator.

A three-orthogonal-Hall-probe assembly with an 'on the fly' mapping method has been developed to characterize an elliptically polarizing undulator (EPU). The underlying design concept is that it can measure the three real field components without any field correction under a reliable and synchronization measurement method. Therefore, the relative central position shift, orthogonal angle and the planar Hall effect error between the three Hall probes should be calibrated and readjusted. Experimental results demonstrate that this method can yield an r.m.s. reproducibility of 10 G cm for the three field components and 2 G for the peak field strength. Under precision conditions this system can completely measure the three on-axis field components within 2 min for a 4 m-long EPU.

The commissioning of a flexible low-cost multipurpose X-ray beamline at SRRC.

A low-cost multipurpose X-ray beamline has been commissioned at the bending magnet B15 of the Synchrotron Radiation Research Center (SRRC). The beamline is constructed in such a way to accommodate the various needs of small research consortia, from universities or research institutes, whose requirements of a beamline facility are quite diverse while under limited funding support. Flexibility is the special feature of this beamline. It is capable of performing quick test measurements without a prolonged reviewing process. Switching between different techniques, such as white-beam irradiation, EXAFS and X-ray scattering, can be achieved within 1 h. Novel experiments, such as energy-dispersive small-angle scattering experiments and energy-dispersive reflectivity measurements, can also be performed.

Detection of a phase transition in red cell membranes using positronium as a probe.

Positron lifetimes in human red cell ghost membranes have been measured as a function of temperature from 3 degrees C to 25 degrees C. A marked sudden change in the ortho-positronium annihilation rate was found at 16-18 degrees C during the heating cycle and at 18-20 degrees C in the cooling cycle. Such sudden change of microenvironment in the membranes sensed by ortho-positronium is attributed to the sudden change of water diffusion rate through the membranes which is a consequence of the sudden change in free volume, or fluidities in the lipid layers.