Koichi Nakao

Band Structures and Optical Properties of Semiconducting Layer Compounds GaS and GaSe

The band structures of GaS and GaSe near the Fermi levels are derived in a semiempirical way. In view of the anisotropic structures, the π band structures are mainly studied. The valence band consists of a π band with the heavy effective masses. The conduction band at the center of the Brillouin zone has the light effective masses for the both directions parallel and perpendicular to the layers, while its minimum at the zone edge has a two-dimensional character. The characteristics of the optical properties of these compounds are the existence of the sharp peaks associated with the nearly two-dimensional pair bands at the saddle points and further the limited spatial extension of an exciton along the c -axis. Most of the structures in reflectivities in GaS and GaSe and the binding energy of an exciton in GaSe are explained fairly well.

A laboratory instrument for generating magnetic fields over 200 T with single turn coils

An instrument has been developed that reliably generates pulsed magnetic fields in the range 150-250 T for experimental applications. A 100 kJ, 40 kV capacitor bank with 18 nH internal inductance and 2.9 m Omega internal resistance is discharged into single turn coils made of copper sheet of 3 mm thickness. 150 T are obtained in 10 mm diameter with a rise time (quarter period) of 2.5 mu s, 200 T are obtained in 6 mm ID within 2 mu s and 250 T in 4 mm ID within 1.8 mu s. Although the coils explode violently and generate a strong axial blast, there is no movement towards the magnetic field volume and even delicate samples can easily be protected to survive an experiment unharmed. This is demonstrated with two experiments: infra-red cyclotron resonance in a GaAs-AlGaAs superlattice and Faraday rotation in GaP.

Doping effect of Zr and Ti on the critical current density of MgB2 bulk superconductors prepared under ambient pressure

Abstract The doping effect of Ti and Zr on the superconducting properties and the microstructure of sintered MgB 2 bulk materials has been studied. The transition metal is found to work as a sintering assistant, which significantly enhances the grain connection of the MgB 2 bulk materials during sintering processing. With this technique, we have fabricated Ti-doped MgB 2 bulk materials consisting of fine 10 nm scale particles and achieved a high J c over 1 MA/cm 2 in 0 T at 20 K. The large enhancement of J c is explained by the excellent connection between grains and a high density of pinning centers served by grain boundaries and MgO nanoparticles.

Reliable fabrication process for long-length multi-filamentary coated conductors by a laser scribing method for reduction of AC?loss

We have developed a reliable fabrication process based on a laser scribing method for multi-filamentary coated conductors with a low alternating current loss. This process consists of shallow laser irradiation and two-step chemical etching. The shallow laser irradiation, which penetrates into only a masking polymer tape and a silver stabilizing layer, suppresses generation of dross that is formed due to melting and hardening of a metal substrate. The two-step chemical etching individually for the stabilizing and the superconducting layers prevents over-etching of the superconducting layer and thus suppresses degradation of the critical current (Ic). By using the developed laser scribing process, we realized an improvement of processing speed and a reduction of Ic degradation at the same time. Subsequently, we developed reel-to-reel equipment to apply this process to coated conductors several hundred meters long. We succeeded in fabricating 280?m and 70?m long coated conductors with 5?mm width divided into three and five filaments, respectively. It was confirmed that they had one-third and one-fifth hysteresis loss, respectively, in comparison with that before processing, and high inter-filament resistance, over 0.1?M??cm. The typical degradation rates of Ic in the developed laser scribing process were less than 20%.

Magnetization measurements in very high pulsed fields produced by a single-turn coil system

Abstract Magnetization measurements are tried in megagauss fields produced by a single-turn coil system at temperatures down to 4.2 K. The measuring apparatus and techniques are described as well as the observed metamagnetic transition of the 1-D Ising antiferromagnet CsCoCl3. Application to other systems is also discussed.

Production of ultra-high magnetic fields and their application to solid state physics

Abstract A review is given on the recent progress of techniques for generating ultra-high magnetic fields in the megagauss range. Discussion is focused on the techniques using condenser bank discharge which have been developed at ISSP (Institute for Solid State Physics) with a view to applications in solid state physics. By the electromagnetic flux compression technique, pulsed high fields up to 350 T have been generated. By the single-turn coil technique, fields well above 150 T have been generated without destroying samples. The megagauss fields are succesfully applied in a variety of experiments.

Pulsed laser deposition of BaFe2(As,P)2 superconducting thin films with high critical current density

Superconducting BaFe2(As0.6P0.4)2 (Ba122:P) thin films were fabricated on MgO(100) substrates by a pulsed laser deposition (PLD) method using a second-harmonic Nd:YAG (yttrium aluminum garnet) laser. Structural investigation by means of x-ray diffraction confirmed both c-axis orientation and in-plane alignment, or epitaxial growth on the substrate. The film exhibited Tc(onset)?=?26.5?K and Tc(zero)?=?24.0?K. High Jc values of 3.5?MA?cm?2 at 4.2?K in the self-field and over 1?MA?cm?2 at 10?K under 1?T were also obtained. Fabrication of Ba122:P film by Nd:YAG PLD seems to be a promising approach for preparing superconducting tapes, since a high-Tc and high-Jc film can be relatively easily obtained.

Magnetisation measurements in ultra-high magnetic fields produced by a single-turn coil system

A technique has been developed for measuring magnetisation in very high magnetic fields (up to 90 T) produced by single-turn coils. The system is applied for measurements of high-field magnetisation of magnetic substances in a wide temperature range from room temperature down to 4.2 K. In a simple magnetic system, the one-dimensional Ising antiferromagnet CsCoCl3, the magnetisation curve was successfully obtained up to 90 T including a stepwise change around 40T at 4.2 K.

Computer Analysis of Megagauss Field Generation by Condenser Bank Discharge

Experiments on the generation of very high magnetic fields in the megagauss range (>100 T) by use of a large current from condenser banks are analysed by a computer simulation. The analysis is performed for the electromagnetic flux compression with both the θ-pinch and z-pinch configuration as well as the single-turn coil technique. The distribution of the current and temperature of the liner or the coil is appropriately treated by a 2-dimensional analysis dividing the cross-section into many mesh sections. The deformation of the liner during the implosion is calculated by a finite-element method. A good agreement between the calculation and recent experiments was obtained.

Observation of Current Distribution in High-

We have carried out observations of current flow in processed RE123 coated conductors using a scanning Hall-probe microscope. Using this system, the position of defects and the resulting disturbed current flow could be observed. The two-dimensional current distribution in a 10-filament formed coated conductor (400 mum width and 100 mum space) was also investigated. Current sharing and the current distribution in each filament were quantitatively estimated. Furthermore, current penetration in quasi-static AC current mode was also measured.