Naoki Koshizuka

Flux Pinning in Melt-Grown NdBa2Cu3Oy and SmBa2Cu3Oy Superconductors

REBa 2 Cu 3 O y superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T, due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa 2 Cu 3 O y with fine Y 2 BaCuO 5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba 1-x , RE x ) 2 Cu 3 O y phase in a good superconductive matrix

High critical‐current density of Nd(Ba,Nd)2Cu3O7−δ single crystals

We have studied the superconducting properties of Nd(Ba1−xNdx)2Cu3O7−δ (Nd123, x≊0.1) single crystals grown by the traveling‐solvent floating‐zone method under 0.1% O2 in Ar atmosphere. An anomalous peak effect in the magnetization hysteresis (M–B) loop is observed in the Nd123 single crystals as well as in the Nd123 bulk crystals prepared by the oxygen‐controlled melt growth (OCMG) method. The critical current density (Jc) of the Nd123 single crystals is 70 600 A/cm2 in 1.0 T at 77 K for the applied field perpendicular to the a‐b plane. Uniform flux density distribution with the same shape as the sample is observed in the field perpendicular to the a‐b plane by the magneto‐optical flux‐density observation. Finely dispersed white regions in the dark‐field image due to Nd substitutions for Ba are observed in the Nd123 matrix by the transmission electron microscopy. The high Jc value of the Nd123 single crystals in the applied field is explained by the field‐induced pinning centers caused by the Nd–Ba subst...

Large Levitation Force due to Flux Pinning in YBaCuO Superconductors Fabricated by Melt-Powder-Melt-Growth Process

An extremely large levitation force of as high as 30 N at a height of 1 mm was achieved in Ag-doped YBaCuO fabricated by the Melt-Powder-Melt-Growth process using a repulsive force against a 0.4 T rare-earth magnet at 77 K. The combination of a large Jc value and large shielding current loop is the source of such a large levitation force.

Waveguide magneto‐optic isolator fabricated by laser annealing

A compact waveguide optical isolator whose input and output light are TE or TM mode has been realized by making adjacent nonreciprocal and reciprocal TE‐TM mode converters in a (BiGdLu)3(FeGa)5O12 film. Designed configurations of the magnetization directions were produced by making use of local laser annealing of the film grown on a Gd3Ga5O12 substrate of which the face normal is tilted from the [111] axis. The laser‐annealed and nonannealed regions worked as the nonreciprocal and reciprocal mode converters, respectively. The observed isolation agreed with theoretical calculation. An isolation ratio of 12.5 dB was achieved at a wavelength λ=1.15 μm.

Large single crystal Bi-2212 along the c-axis prepared by floating zone method

Abstract A floating zone method was used to study the effects of the growth velocityand the starting compositions of the feed rod on the crystal growth behavior of Bi-2212. It shows that a necessary condition for obtaining large single crystals along the c -axis is that the solid-liquid interface of a rod maintains a smooth interface during crystal growth. A planar growth interface tends to break down into a cellular interface, while increasing the velocity from 0.2 mm/h, or while deviating from the optimum composition of Bi 2.1 Sr 1.9 Ca 1.0 Cu 2.0 O x in a feed rod. Single crystals of up to 20×5.5×1.5 mm 3 along the a -, b - and c -axes have been grown in a 6.4 mm diameter rod with optimum growth conditions. X-ray Laue measurements show that the quality of the single crystals is good. T c onset is 91 K for as-grown crystals.

63Cu(2) nuclear spin-spin relaxation in YBa2Cu3O6.98, YBa2(63Cu)3O7-δ, and YBa2Cu4O8

We have measured the temperature dependence of the transverse relaxation time of 63 Cu(2) nuclear spin in YBa 2 Cu 3 O 6.98 , YBa 2 ( 63 Cu) 3 O 7 , and YBa 2 Cu 4 O 8 . After subtraction of the T 1 process, the decay curve of the spin-echo envelope for YBa 2 Cu 4 O 8 in a strong H 1 followed a single Gaussian form from 280 K to 4.2 K, while for YBa 2 Cu 3 O 6.98 it follows only above T c . The absolute value and the temperature dependence of the decay rate 1/ 63 T G could be understood by the combined contributions of the direct dipole-dipole and the temperature-dependent indirect nuclear spin-spin coupling via χ( q ). From the temperature dependence of 1/ 63 T G for YBa 2 Cu 4 O 8 below T c , we considered that χ( Q ) even below T c remains enhanced. In a viewpoint of the 63 Cu(2) nuclear spin-spin relaxation experiments, so far as YBa 2 Cu 4 O 8 is concerned, the high- T c superconductivity seems to exhibit the d -wave pairing.

High-temperature superconducting edge-type Josephson junctions with modified interface barriers

This paper describes recent results on the fabrication, electrical characteristics, and microstructure of high-temperature superconducting edge-type Josephson junctions with modified interface barriers. The barriers are formed by surface modification of the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// base layer. This process involves structural and chemical modification by ion irradiation and crystallization by annealing. The junctions showed resistively and capacitively shunted junction-like current-voltage characteristics and excellent uniformity. The spread in the critical current for one hundred junctions was smaller than 1/spl sigma/=10% at 4.2 K. The uniformity is now approaching 1/spl sigma/=5%. The junction characteristics have remained the same after two-year room-temperature storage. They also showed no change after high-temperature processing at about 700/spl deg/C. High-resolution transmission electron microscopy revealed that both the crystal structure and chemical composition in relatively thick barriers are different from those of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//.

Nuclear spin-lattice relaxation and Knight shift in YBa2Cu4O8

Abstract The temperature dependences of the 63Cu nuclear spin-lattice relaxation rate 1/T1 and Knight shift K in YBa2Cu4O8 have been measured by nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) techniques. In the normal state, both the 1 T 1 for the Cu(2) sites and the in-plane component of the Knight shift K2⊥ increase rapidly with temperature from Tc up to about 250 K. Approximate T-linear temperature dependence of 1 T 1 was observed in a wide temperature range between 250 and 680 K, that is in general accord with previous analyses.

Application of superconducting magnetic bearings to a 10 kWh-class flywheel energy storage system

Radial type superconducting magnetic bearings have been developed for a 10 kWh-class flywheel energy storage system. The bearings consist of an inner-cylindrical stator of YBCO bulk superconductors and an outer-rotor of permanent magnets. The rotor is suspended without contact via the pinning forces of the bulk superconductors that are arranged such that the c-axis of each superconductor is aligned parallel to the radial direction of the cylinder. In this paper, we describe the implementation of the stator part and report the SMB test results, namely that the rotor was successfully rotated at up to 7,500 rpm. Thus we succeeded in energy storage of 2.24 kWh.

Effect of Y2BaCuO5 on fracture toughness of YBCO prepared by a MPMG process

Abstract The fracture toughness of melt-powder-melt-growth (MPMG) processed YBaCuO samples was measured and the effect on the fracture toughness of the dispersed Y2BaCuO5 (211) particles in the 123 superconducting phase was found. Preferred fracture planes were the (100), (010) and (001) planes, and the fracture toughness of the (001) plane was lower than those of the (100) and (010) planes. The indentation fracture method indicated that K c = 0.88-1.2 MPa m 1 2 in an inert environment for YBaCuO crystals without the 211 particles, when the crack plane is the (100) plane or the (010) plane. The presence of the 211 particles in the 123 matrix increased the fracture toughness in this direction up to 1.6-2.1 MPa m 1 2 .