Rikuo Ogawa

Optimization of single crystal preparation of Bi2Sr2CaCu2Ox superconductor by the travelling solvent floating zone method

Optimization of single crystal preparation of the Bi2.2(Sr, Ca)2.8Cu2.0Oy superconductor by the travelling solvent floating zone method has been performed with the intention of obtaining large single crystals and improving the superconductive transition temperature, Tc. The composition ratio, Sr/Ca, and the oxygen partial pressure of growth atmosphere, PO2 , were varied in the crystal growth. A single crystal typically obtained was a thin plate of the dimensions of 5×50×2 mm3. The thickness of the crystal was the largest in the crystal growth at Sr/Ca=1.4, while no distinct change was observed in the resulting crystals in the range of 22~100 kPa. Electric and magnetic properties, the formal charge of copper ions and existence of ion defects are discussed.

Investigation of Heat-Treating Conditions for Silver-Sheathed Bi2212 Superconducting Coils

Fabrication of silver-sheathed Bi-2212 superconducting coil has been attempted by means of the partial melt growth process. Silver-sheathed Bi-2212 tapes have been cooled down slowly from the temperature range of 875-890°C to 850°C at the rate of 6°C/h. The characteristics of Jc-H, AC magnetic susceptibility and X-ray diffraction pattern have been measured, and the SEM image of the cross section has been observed. The pancake-type test coil has been fabricated using an optimized heat-treating condition. This coil generates 0.22 T in the external magnetic field of 0 T and 0.07 T in the external magnetic field of 6.64 T at 4.2 K.

Large normal resistance and THz electromagnetic‐wave response in YBaCuO step‐edge microbridge Josephson junction

A YBaCuO Josephson junction with a large normal resistance (R) was fabricated and characterized. The junction exhibited an extremely sharp transition on the current‐voltage curve with R=10.8 Ω and IcR=1.8 mV at 15 K. The temperature dependence of critical current (Ic) of the junction was Ic∝(1−T/Tc), while another junction with a smaller R (0.57 Ω) showed a dependence of Ic∝(1−T/Tc)2. The difference can be explained by a picture of a grain boundary junction as a parallel array of superconductor‐normal‐superconductor and superconductor‐insulator‐superconductor elements. From the results of 110 GHz mixing experiments, it was shown that the present junction could be operative for electromagnetic waves over 2 THz.

Properties of Step-Edge Microbridge Josephson Junction Mixer Fabricated by YBaCuO Thin Film

The conversion efficiency and minimum detection power of a step-edge microbridge Josephson junction mixer (SEJJ mixer) were evaluated for both fundamental mixing and harmonic mixing. The SEJJ was fabricated by a YBaCuO thin film and all of the circuits inside the mixer were made from microstrip line circuits in order to reduce the microwave propagation losses. The conversion efficiencies (η) were -31 dB, -46 dB and -53 dB for fundamental mixing, second harmonic mixing and third harmonic mixing, respectively. The dependence of η on the harmonic number (N) was expressed as η\proptN-4.6 for N≦3. The minimum detection power (MDP) of -60 dBm (1.0 nW) was obtained in fundamental mixing, with a linear dynamic range of more than 40 dB.

Millimeter‐wave detection by YBaCuO step‐edge microbridge Josephson junction

Millimeter‐wave detection was investigated using a step‐edge microbridge Josephson junction fabricated by a YBaCuO thin film for both the video detection and harmonic mixing detection. Current–voltage (I–V) characteristics, which could be described by the resistively shunted junction (RSJ) model, were obtained up to 70 K with the IcR product of 150 μV at 70 K. The responsivity and noise equivalent input power were calculated as 2400 W/V and 4.4×10−14 W/√Hz, respectively, in the video detection assuming the RSJ model approximation. Clear dependence of the intermediate frequency output on the applied dc bias voltage was observed in the harmonic mixing detection with the harmonic numbers of 5–16, reflecting the change of dynamic resistance at the constant voltage steps induced by the local oscillation signal on the I–V curve. A minimum detection power of −59.4 dB m and a conversion efficiency (η) of −49.6 dB were obtained for the harmonic number (N) of 5 at 60 K. The relationship between the η and N was expr...

Optimum Bias Conditions for K-Band Microwave Mixing Using YBaCuO Step-Edge Microbridge Josephson Junction

Characteristics of the K-band microwave mixing using a YBaCuO step-edge microbridge Josephson junction were systematically investigated, and the optimum bias conditions were experimentally determined. A large intermediate frequency (IF) output signal was observed at every midway between the Shapiro steps, and the maximum IF output was obtained between the 0th and 1st steps (0-1) for the local oscillation (LO) power level of I0/Ic=0.2-0.3 (absolute LO power of -49.9 dBm). The maximum conversion efficiency was estimated to be -26.6 dB. Although the IF output (0-1) was observed for a wide range of LO power, it vanished or decreased radically under some peculiar conditions (I0/Ic?0 and ?(I0/Ic)/?ILO?0). These characteristics were qualitatively accounted for by the basic Josephson-mixing theory.

Reversible magnetization of Bi2Sr2CaCu2O8 single crystals at 30 K ≤ T < Tc

Abstract We report magnetization measurements on high-quality Bi 2 Sr 2 CaCu 2 O 8 single crystals in H ∥ the c-axis. Our reversible magnetization data at 30 K ≤ T T c show strong deviations from the linear dependence of M on ln ( H ) in the intermediate-field region. The Ginzburg-Landau parameter κ, obtained by using the Hao-Clem variational model, was found to vary significantly with the field range for the data fitting. This unusual behavior can not be explained by the thermal fluctuations of the vortices in layered superconductors.

139La NQR study of antiferromagnetic La2NiO4+δ

The magnetic properties in the La 2 NiO 4+δ system have been investigated by 139 La nuclear quadrupole resonance (NQR). The structure or the NQR spectrum at 1.4 K was found to depend sensitively on annealing procedures of the compounds, and therefore on δ. A well-characterized spectrum was obtained for samples oxygenated (δ≧0.066) and deoxygenated (δ∼0.022) with different structures, exhibiting the existence of the internal magnetic fields H mt of ∼17.8 kOe nearly parallel to the basal Ni–O plane and ∼19.3 kOe perpendicular to the plane. The strength of H mt at the La site was significantly large when compared with that of ∼1 kOe in La 2 CuO 4 and could not be explained simply by the dipole field caused by Ni spin moments.

139La NQR Study in Superconducting La2-xCuO4-y

Nuclear quadrupole resonance (NQR) measurements were performed on superconducting La 2- x CuO 4- y ( T C ∼30 K) synthesized by high-temperature and high-pressure oxygen enrichment. At liquid 4 He temperature, the existence of a weak and broad line has been found around 6.35 MHz which is the resonance frequency expected in stoichiometric La 2 CuO 4 when an internal magnetic field is absent. The spin-echo intensity of this new signal increases in weak magnetic field (∼100 Oe) which is characteristic of the resonance in the mixed state of the superconductors. On the other hand, the dominant lines corresponding to the magnetic phase do not show any significant changes in contrast with the case of the Sr-doped samples. These results indicate that superconductivity occurs in the regions where antiferromagnetic ordering is suppressed sufficiently by the carrier doping.

Cryogenic Fe-Mn Austenitic Steels

It is taken for granted that structural materials used for superconducting magnets of a fusion reactor must combine strength, toughness and fatigue resistance especially in case of welded structures at cryogenic temperatures. Austenitic Fe-Mn alloys were expected to be promising candidates to meet the above requirements. However, they proved to have some problems to be overcome in terms of cryogenic brittle fractures. To optimize the mechanical properties, chromium, nickel, nitrogen and other elements were added to Fe-Mn alloy. Consequently, 22Mn-13Cr-5Ni steel has been developed as the candidate steel. Optimum manufacturing conditions have been established and the steel has been tested at cryogenic temperatures. The results show that the steel maintains satisfactory strength of more than 1.2 GPa and fracture toughness of more than 200 MPa\(\sqrt m \) at 4 K.