Touko Sugiura

Raman scattering of InGaAsP lattice-matched to GaAs in the region of immiscibility

We have studied the Raman spectra of the InxGa1-xAsyP1-y quaternary alloys lattice-matched to GaAs in the immiscible region. The spectra exhibit four modes of behavior due to four binary compositions: GaP-, InP-, GaAs-, and InAs-like phonon modes. It was found that the phonon spectra in the immiscible region are well explained by the isodisplacement model of Inoshita [J. Appl. Phys. 56 (1984) 2056].

Raman Scattering Study of InGaN Grown by Metalorganic Vapor Phase Epitaxy on (0001) Sapphire Substrates

We have studied the asymmetric broadening of the Raman spectra of InxGa1-xN grown on sapphire substrates with the aid of the spatial correlation model. The asymmetric broadening of the E2 phonon mode is enhanced in the region of immiscibility by increasing the indium molar fraction. The correlation length, which corresponds to the decay length of the E2 phonon mode, was estimated for the first time. It was on the order of 6–10 nm, which is on the same order of magnitude as the size of the columnar structure suggested by transmission electron microscope analyses.

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.

Raman Scattering Study of InGaAsP Quaternary Alloys Grown on InP in the Immiscible Region

We have studied the asymmetric broadening of the Raman spectra of InxGa1-xAsyP1-y grown on InP in the immiscible region using the spatial correlation model. The compositional dependence of Raman peaks are well explained by a simple model which does not account for the immiscibility. However, the broadening of the Raman line shape is greatly enhanced in the samples in the region of the immiscibility. In particular, the asymmetry of the spectral peak of the InAs-like longitudinal optical phonon is found to be enhanced in the immiscible region. This shows that the samples which include the immiscibility have been structurally changed to suppress the uniform distribution of the long wavelength phonons.

Raman Intensity of Phonon Modes in InGaAsP Quaternary Alloys Grown on (100) InP in the Region of Immiscibility

The Raman intensities of optical phonon modes in InxGa1-xAsyP1-y quaternary alloys lattice matched to (100) InP are studied in the compositional region of immiscibility. The peak intensity as well as the integral intensity of each phonon modes are investigated in terms of the relative density of corresponding bonds. It was found that the integral intensity is well interpreted in terms of the bond density. The replacement of a gallium atom with an indium atom produces a remarkable variation in the peak intensity.

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.

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.