Mitsuru Matsuura

Synthesis and thermoelectric properties of type-I clathrate Ge30P16Te8

The tellurium-containing cationic type-I clathrate Ge30P16Te8 was synthesized and its thermoelectric properties were investigated. This clathrate showed a p-type conductivity, while most of the clathrates are n type. The Ge30P16Te8 clathrate had a band gap energy of 0.62eV and a room temperature thermal conductivity of 9mWcm−1K−1, which is as low as those of germanium clathrates such as anionic Sr8Ga16Ge30 and cationic Ge38Sb8I8. The electronic structure of the Ge30P16Te8 clathrate was also calculated using an ab initio method.

Properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells

The fundamental properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are investigated theoretically. The mode number and the energies of optical phonons, the high frequency and static dielectric constants of Ga1−xInxN crystals, are obtained by using an improved mixed crystal theory. The carrier-longitudinal optical (LO)-phonon interaction in Ga1−xInxN mixed crystals is studied by a perturbational polaron theory. The feature of optical phonons in Ga1−xInxN/GaN quantum wells is studied based on the dielectric continuum approach. The frequencies and the dispersion relation of the interface phonons are obtained. The 1s and 2s exciton binding energies and the exciton–LO-phonon interaction energies in Ga1−xInxN/GaN quantum wells are calculated numerically. Based on the present theoretical results, the general properties of Ga1−xInxN mixed crystals and Ga1−xInxN/GaN quantum wells are discussed.

Fabrication of high-permeability ferrite by spark-plasma-sintering method

Abstract Using spark-plasma-sintering (SPS) method, Mn–Zn ferrite platelet cores with few vacancies were successfully fabricated. Laminated cores composed of Mn–Zn or Ni–Zn–Cu ferrites and permalloy layers were also prepared by the SPS method. The laminated cores had large saturation magnetic flux density and high permeability at high frequencies. These soft magnetic cores may be available as the magnetic cores of a magnetic head used in a card reader or the soft magnetic substrates of the perpendicular recording hard disks.

Two-dimensional shrinkage of the exciton wavefunction in quantum wells probed by optical absorption

Abstract The oscillator strength of the lowest 1s excitons in GaAsAlAs quantum wells is studied as a function of well layer thickness Lz by means of optical absorption. It is enhanced more than the 1 L z dependence with the decrease of Lz. The results present the first full experimental observation of two-dimensional shrinkage of the exciton wavefunction in quantum wells.

Recording characteristics of Co‐γFe2O3 perpendicular magnetic recording media

The recording characteristics of Co‐γFe2O3 perpendicular magnetic recording hard disk media were investigated. Sliding contact recording was performed to evaluate the intrinsic high‐density recording performance of the media with using a metal‐in‐gap‐type ring head. The half‐voltage density D50 of 176 kFRPI and low‐density reproduced voltage Ep of 143 nVp‐p/[turn μm (m/s)] were obtained for the optimal medium whose Co‐γFe2O3 layer thickness was 0.13 μm and perpendicular coercivity was 1800 Oe. The pass wear durability test was performed at 2 m/s, or at a disk rotation of 830 rpm. The reproduced voltage did not decrease with time, and scratches and damage were not observed on the tested track even after 18 000 000 passes. The Co‐γFe2O3 medium is one of the candidates for ultrahigh‐density recording media because of its specific advantages; superior high density recording performance and hardness tolerable for sliding contact use.

Oscillator strength of excitons in quantum wells

Abstract The oscillator strengths of excitons in quantum wells are discussed theoretically. The selection rules for both allowed and forbidden transition cases in a one-dimensional well are derived and discussed. It is also shown that for a two-dimensional well, oscillator strengths are much larger than for a one-dimensional well.

Fabrication of ultrathin Ni-Zn ferrite films using electron cyclotron resonance sputtering method

Well-crystallized Ni–Zn ferrite (Ni0.4Zn0.6Fe2O4) highly oriented ultrathin films were obtained at a substrate temperature of 200 °C by a reactive sputtering method utilizing electron cyclotron resonance microwave plasma, which is very effective to crystallize oxide or nitride materials without heat treatment. Thin films of Ni–Zn ferrite deposited on a MgO (100) underlayer showed an intense X-ray-diffraction peak of (400) from the Ni–Zn ferrite as compared to similar films deposited directly onto thermally oxidized Si substrates. A 1.5-nm-thick Ni–Zn ferrite film, which corresponds to twice the lattice constant for bulk Ni–Zn ferrite, crystallized on a MgO (100) underlayer.

Fabrication of circulator with coplanar wave guide structure

A circulator with a coplanar wave-guide structure, whose external form is rectangular, was designed and analyzed by using a three-dimensional finite-element method. In this analysis, the nonreciprocal transmission characteristics with an insertion loss of 1.0 dB and an isolation of 33 dB appeared at around 7 GHz. The bandwidth defined as the band with isolation of over 20 dB was 50 MHz. A circulator was fabricated based on the design, and the transmission characteristic of the circulator was measured. The fabricated circulator showed an insertion loss of 4.9 dB and an isolation of 28 dB at around 8 GHz. The bandwidth at isolation over 20 dB was 70 MHz.

Characterization of CuCl nanocrystals in SiO2 matrix fabricated by inductively coupled plasma-assisted sputtering deposition

We have investigated crystal characteristics and optical properties of CuCl nanocrystals embedded in a SiO2 glass matrix, which is fabricated by a novel rf-magnetron sputtering technique combined with an inductively coupled plasma (ICP). The diffraction spots caused by the (111), (220), and (311) planes of the zinc-blende structure of CuCl crystals evidently appear on the electron diffraction patterns under inductively coupled plasma-assisted magnetron-sputtering deposition. Nanoscale particles with the mean particle radius of 4.0 nm are observed in the transmission-electron-microscope images. These results indicate that CuCl nanoparticles are crystallized with a very high quality. This yields optical properties in samples such as the sharp exciton-absorption bands named Z3 and Z1,2. These bands appear with a blueshift, as compared to the bulk-exciton energies caused by the quantum-size effect for the exciton. Only the free-exciton luminescence-band appears without other luminescence bands caused by an im...

Perpendicular Co-Cr magnetic recording media prepared by sputtering using ECR microwave plasma

Perpendicular Co-Cr media were deposited on polyimide substrates by sputtering using an electron-cyclotron-resonance microwave plasma in an Ar sputtering gas pressure ranging from 3/spl times/10/sup -2/ to 8/spl times/10/sup -2/ Pa at a target to substrate distance from 170 to 230 mm. The accelerating voltage of the ions which bombard the media surface during deposition drastically affects the crystallographic and magnetic properties of Co-Cr films. Co-Cr media (50 nm in thickness) with good preferred crystal orientation (/spl Delta//spl theta//sub 50/ less than 5 degrees), high perpendicular magnetic anisotropy (H/sub k/ higher than 4 kOe), high perpendicular coercivity (H/sub c/spl perp// over 1400 Oe) and no initial layer were successfully deposited when the ion accelerating voltage was reduced less than about 20 V.