Kazuyuki Shibutani

Coherent Kondo State in a Dense Kondo Substance: CexLa1-xCu6

The electrical resistivity, magnetoresistance and magnetic susceptibility of single crystalline Ce x La 1- x Cu 6 ( x =0-1) have been measured down to mK region. As the Ce concentration is increased, the consecutive change from the dilute (incoherent) Kondo regime to the Kondo lattice (coherent) regime is observed. The characteristic features in the coherent Kondo regime are clearly observed in the resistivity and the magnetoresistance above x =0.7. The low temperature resistivity in Ce x La 1- x Cu 6 shows a T 2 dependence and the residual resistivity follows the Nordheim law. As for the coherence effect in CeCu 6 , the resistivity follows a T 2 dependence below 0.1 K, the magnetoresistance changes from negative to positive below about 0.15 K within applied magnetic fields and the susceptibility shows an enhanced Pauli paramagnetism below 1 K.

Characterization of Kondo Lattice Substance: CeCu6

Impurity effects of the starting material Ce and the crucible on the Kondo lattice formation of CeCu 6 have been studied through the measurements of the electrical resistivity, thermoelectric power, negative magnetoresistance and magnetic susceptibility.

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.

Fabrication of superconducting joints for Bi-2212 pancake coils

To evaluate the possibility of persistent current operation for NMR/MRI using a Bi/sub 2/Sr/sub 2/CaCuO/sub 8/ (Bi-2212) superconductor at 4.2 K, the temperature dependence of magnetic relaxation of the single crystal has been measured. It was found that the operating current must be one-fourth of J/sub c/ at 4.2 K to satisfy the stability condition for NMR/MRI operation ( Delta J/J less than 0.005 ppm/h). A superconducting coil made of Bi-2212 silver-sheathed wire was fabricated by the partial melt growth process. The authors were successful in generating 1.60 T in a zero external magnetic field at 4.2 K with a coil made of silver-sheathed Bi-2212 tape which was 100- mu m thick, 10-mm wide and 150-m long. Superconducting jointed pancake coils with critical current reaching 80 A in an external magnetic field of 1.0 T at 4.2 K were also fabricated.<<ETX>>

Structural and superconducting properties of oxygenated La2−χMχCuO4+y (M=Sr, Ba; O≦χ<0.1)

Abstract Oxygen stoichiometry, superconductivity and structural properties of La 2−χ M χ CuO 4+ y (M=Sr, Ba; 0≦χ P O 2 =360 atm at 600°C were studied by TG-DTA, electrical resistivity, magnetic susceptibility and X-ray diffraction measurements. All oxygenated samples showed superconducivity. The phase separation into two phases with different orthorhombic strain was observed in the samples with 0≦χ≦0.03 under ambient atmosphere, and the phase with the smaller orthorhombic strain was the superconducting one. The superconductivity and the phase separation in this system appear to be related to the average valence of Cu ions and the structural properties, respectively, which are affected by both oxygen nonstoichiometry and substitution.

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.

Improvement of J/sub c/ in Bi-2212-Ag composite superconductor by controlling Po/sub 2/ on the partial melt process

In a partial melt process for Bi-2212-Ag composite tapes, round and rectangular shape superconductors, the effects of oxygen partial pressure (Po/sub 2/) on the critical current density (J/sub c/) have been studied. It was found that the J/sub c/ of Bi-2212-Ag composite tapes increased from 4.5/spl times/10/sup 4/ A/cm/sup 2/ to 2.2/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K in zero magnetic field with increasing Po/sub 2/ from 0.1 bar to 1.0 bar. SEM observation for the cross sectional image and J/sub c/ measurement revealed that the partial melt process under higher Po/sub 2/ was effective in realizing well-aligned structure along the silver wall. These results were explained by the idea that crystallization of the Bi-2212 system starts at the interface between the liquid phase and the oxygen rich gas phase. In order to prove the potentiality of this process for the rectangular shape conductors, fabrication of a solenoid magnet has been attempted using rectangular shape conductors under high oxygen partial pressure. The typical results of this solenoid magnet, 2.7/spl times/10/sup 4/ A/cm/sup 2/ in J/sub c/ with generating magnetic field 0.70 T, and 4.3/spl times/10/sup 4/ A/cm/sup 2/ in quenching current density with generating magnetic field 1.13 T have been observed at 4.2 K in the self magnetic field.<<ETX>>

Fabrication of silver-sheathed Bi(2:2:1:2) superconducting magnet by means of partial melt and slow cooling process

Abstract Fabrication of silver-sheathed Bi(2:2:1:2) superconducting coil has been attempted by means of the partial melt and slow cooling process. In order to establish the fabrication process of long length Bi (2:2:1:2) Ag tape with high Jc, the nominal compositions and heat-treating conditions have been optimized. d.c. and a.c. current operation around 20 K has been carried out using GM-cryo-cooler, a.c. loss characteristics have been discussed. The large scale magnet (81/185 mm in inner/outer diameter and 218 mm long) succeeded in generating magnetic field of 0.68 T at the center part of room temperature bore of 45 mm in diameter at 20 K. In order to obtain the guide for material selection between these materials for the cryogen-free application, a comparative study of Jc-B characteristics for Bi(2:2:1:2) and Bi(2:2:3) tapes has been done at 4.2 and 27 K. Detailed measurements of V-I characteristics have been performed at 27 and 64 K as a function of applied magnetic field for Bi (2:2:1:2) Ag and Bi (2:2:2:3) Ag . The E−J curves are well described by an expression, E ∼ exp [ − ( J 0 J ) μ ] below their transition magnetic fields Hg, rather than the commonly used power law, i.e. E ∼ Jn, where μ, J0 and n are constants. For realizing the cryogen-free medium field magnet (e.g. H ∼ 10 T), Bi (2:2:1:2) Ag will be useful only below 20 K and Bi (2:2:1:2) Ag will be feasible at temperature range from 20 to 30 K.

Operation of superconductively jointed Bi-2212 solenoidal coil in persistent current mode

Abstract A Bi 2 Sr 2 Ca 1 Cu 2 O x superconducting solenoidal coil with a persistent current switch (PCS) has been newly designed and fabricated, for the first time as a superconductively jointed oxide superconducting coil. The operation behaviour of the solenoidal coil at 4.2 K has been evaluated when the PCS is off or on. When the PCS was off, the solenoidal coil worked well, generating magnetic fields of over 95% of the designed value. After the PCS was turned on, a magnetic field of about 100 gauss was continuously generated for 100 h and then the PCS was turned off. The residual resistance of the entire circuit was estimated to be below 1.6 × 10 −11 Ω from the last 1900 min (= 31.7 h) of operation. This resistance value reaches the practical level of commercial metallic superconducting magnets used for magnetic resonance spectroscopy, being operated in the persistent current mode. This work has proved that the Bi-2212 solenoidal coil has great potential for practical use with persistent current mode operation.

Operation of Bi-2212 superconducting magnet with room temperature bore of 45 mm diameter at 20 K

Abstract A Bi-2212 superconducting magnet with a practical room temperature (RT) bore of 45 mm diameter has been fabricated. The magnet was cooled by a Gifford-McMahon cryocooler and generated magnetic fields up to 0.68 T at 20 K. Oxide superconducting magnets with such a large RT bore require high homogeneity of the critical current density in the silver-sheathed Bi-2212 tape over long ranges such as 1500 m. This problem has been solved by optimizing the time for calcination of the Bi-2212 powder and employing a new heat process.