Takehiko Tsukamoto

Numerical Matrix Analysis for Performances of Wideband 100GHz Branch-Line Couplers

The wideband waveguide 3-dB branch-line coupler suitable for use in SSB heterodyne receivers in radio astronomy is designed and its performances are reported. The coupler is designed by a numerical analytical method by using matrices based on the circuit theory and has good performances in the range of frequencies from 84GHz to 116GHz.The numerical analytical method presented in this manuscript enables quick calculations for performances of RF components and possesses advantages over electromagnetic simulation software in common use.

Emission Wavelength Dependence of Internal Quantum Efficiency in InGaN Nanowires

The internal quantum efficiency (IQE) of InGaN nanowires with different emission wavelength of 485, 515, 555, and 580 nm has been studied by means of photoluminescence (PL) spectroscopy. It was found from the analysis of IQE as a function of excitation power density that the IQE was unchanged at about 100% under weak excitation conditions at low temperature. This indicated that the effects of nonradiative recombination processes were negligibly small at low temperature. Moreover, the IQE increased from 5 to 12% with increasing emission wavelength from 485 to 580 nm. Since the clear correlation between the IQE and the PL blue shift due to band filling effects of localized states was observed, the increase in the IQE reflected the increase in the effect of exciton localization with increasing indium composition.

Alternating-current transport losses of melt-cast processed Bi-2212 bulk superconductor bars

Using a melt-casting method, we have fabricated two pieces of Bi-2212 bulk superconductor bar with square and rectangular cross-sections, and we have investigated the alternating-current (ac) transport self-field losses at 77 K. Despite the main contribution of hysteresis loss of the superconductor, there is some difference in the loss behaviour between these two samples. To elucidate the origin, we make numerical calculations on the ac transport self-field losses as a function of current amplitude I0 below the critical current Ic. At a fixed I0, the calculated values using the uniform Jc distribution and the actual cross-sectional geometry are much higher than the experimental data for the sample with a square cross-section 7.5 × 7.5 mm2, while there is good agreement between the calculation and the experiment for the sample with a rectangular cross-section 4.5 × 13.6 mm2. The discrepancy appearing in the sample with a square cross-section is ascribed to the actual Jc distribution, which is confirmed by critical current measurements when scraping off the sample. The local Jc value decreases significantly in going from the surface to the interior of the sample. This suppresses the extension of the flux-penetration region to the interior under ac current transmission and lowers the loss generation compared with the calculated results obtained by the uniform Jc distribution.

Superconducting Properties of Melt-Cast Processed Bi2Sr2CaCu2Oy Compounds

A significant enhancement of the superconducting properties in Bi2Sr2CaCu2Oy compounds has been achieved by using melt-casting process (MCP). The weak links have been largely eliminated in the MCP samples. The samples show sharp drops associated with the onset of intragranular superconductivity. The value of Tc increases for SrSO4 addition up to approximately 10wt%. The zero resistance is observed at 90.4K for the 10wt% SrSO4 added sample annealed at 800°C in air. The SrSO4 admixture improves the superconducting properties of the intergranular regions.

First-year Engineering Education with the Creative Electrical Engineering Laboratory

The Department of Electrical and Electronic Engineering in Toyota National College of Technology has put great emphasis on fundamental subjects. We introduced the creative electrical engineering laboratory into the first-year engineering education since 1998. The laboratory concentrates on the practice exercise. The final questionnaire of students showed that our first-year education is very effective to promote students motivation and their scholastic ability in engineering.

AC Transport Properties of Bi-2212 Oxide Superconductors

The AC transport properties of Bi-2212 single crystals have been investigated through measurement of electrical resistance. The quenching current was measured by AC transport method measuring a resistive voltage with a sample immersed in liquid nitrogen. The AC quenching current Iq is nearly independent of the frequency f below 2kHz. For f≧2kHz, Iq decreases with f, and is less than 50% of Iq(0Hz) at f=20kHz. This result suggests that the temperature of sample rises for f≧2kHz, because of the AC losses and the Joule’s heat from contact resistance.