Kiyoshi Kuroda

Phase Diagrams in the Solid Solution Nd–Ba–Cu–O System and NdBa2Cu3O7-δ Single Crystals Growth by the Traveling-Solvent Floating-Zone Method

We have investigated the pseudo-binary phase diagrams between NdBa2Cu3O7-δ (Nd123) and several Ba–Cu–O compositions in air and reduced oxygen atmosphere by DTA, quenching, and X-ray powder diffraction measurements. We found that in air the wider liquidus line exists in the 60–90% concentration of (1/13) Ba3Cu10O13 [40–10% (1/6) Nd123] at 940–1090° C. Under 0.1% O2 in Ar, the liquidus line on the same pseudo-binary line shifts to the 65–92.5% concentration of (1/13) Ba3Cu10O13 [35–7.5% (1/6) Nd123] at 850–1000° C. Furthermore, we tried to grow single crystals of Nd123 based on these phase diagrams by traveling-solvent floating-zone method under 0.1% O2 in Ar. We could get a single crystal rod having the composition of Nd1.02Ba1.98Cu3O7-δ and the T C~93 K in all through the length of the crystal rod.

NdBa2Cu3O7 − δ single crystal growth by the traveling-solvent floating-zone method

Large single crystals are very important to not only for applications but also for the basic study of high-Tc oxide superconductors. We have succeeded in growing large size ((A) 4 mm o × 20 mm l and (B) 3 mm o × 32 mm l) NdBa2Cu3O7-δ (Nd123) crystals using an improved Traveling-Solvent Floating-Zone (TSFZ) method. The crystal (A) consists of four grains, and the largest grain with the size of 2mm × 4mm has been proved to be of single crystal from the Laue diffraction pattern and Tc=94K (onset) by the SQUID magnetometer. The other crystal (B) consists of single grain.

Improvement of Irreversibility Field by Ca Substitution in Nd123 Single Crystals

The field dependence of J c is studied for Nd1-x Ca x Ba2Cu3O7-δ [(x =0, 0.01, 0.02, 0.05, 0.10): N(C)BCO] single crystals grown by the traveling solvent floating zone (TSFZ) method. Values of J C, peak field (H peak) and irreversibility field (H irr) are evaluated from magnetization curves of these crystals. H peak and H irr values as a function of T / T C increase compared with those of Ca free single crystals. The enhancement of H irr is ascribed to the increase of vortex coupling along the C-axis due to decrease of anisotropy with increase of carrier concentration by Ca substitution. The increase of H peak is explained by the increase of T C and H C2 of weak superconducting Nd/Ba substituted regions with Ca doping.

Crystallinity of Large YBa2Cu3Oy Single Crystals Grown by the Solute Rich Liquid-Crystal Pulling Method

The solute rich liquid-crystal pulling method, a modified Czochralski-pulling technique, can be used to grow very large single crystals, larger than 1 cm along each axis. In order to examine the crystallinity of the YBa2Cu3Oy single crystals grown by this technique, neutron diffraction was applied for six crystals subjected to various annealing conditions. The typical mosaicity was smaller than 1° along the c-axis and 2° in the ab-plane. Some crystals were of high quality, with a mosaicity of less than 0.2° for every direction. The oxygen inhomogeneity present in the as-grown samples was eliminated by prolonged-duration oxygen annealing. Furthermore, we succeeded in observing the orthorhombic II and III (ortho-II and III) phases, and determined the correlation lengths along the a-axis as ξa=16 A and ξa=19 A for y≈6.60 (ortho-II) and y≈6.70 (ortho-III), respectively.

FLUX PINNING BY SUBSTITUTION IN ND1BA2CU3OY SINGLE CRYSTALS

Magnetization and magneto-transport studies have been performed on high quality Nd1Ba2Cu3Oy (Nd123) single crystals grown by traveling solvent floating zone method in a reduced oxygen partial pressure. The angle dependent magnetization scales well. The magnetization loop splits into two peaks for H∥c. The first peak disappears when the c-axis is tilted away from the field displaying the influence of twin boundary for H∥c. The magnetotransport measurement by a six-terminal configuration shows correlation of the vortices along the c-direction of the Nd123 crystal. The pinning force increases markedly in the twinned crystal in comparison to the detwinned Nd123 crystal. The above measurements along with the studies of volume pinning forces in twinned and detwinned crystals and their microstructure reveal that the interplay between the twin planes and the substitutional Nd–Ba sites are responsible for high pinning in this material, especially in the high field region.

Observation of Mesoscopic Structures in NdBa2Cu3O7-δ by Small-Angle Neutron Scattering

In order to clarify the origin of the high critical current density (Jc) of NdBa2Cu3O7-δ (Nd123), we investigated the inhomogeneities in this material by small-angle neutron scattering (SANS) experiments. As a result of numerical calculations, it was found that the anisotropic scattering patterns and radial intensity profiles can be explained by the existence of elliptic cylindrical objects of which the cylindrical axis is parallel to the crystallographic c-axis. Long and short elliptic axis lengths and the height of the elliptic cylinders in the high-Jc sample are about 400, 200 and 100 nm, and those of the low-Jc sample are about 200, 100 and 50 nm. Since these two samples have been heat-treated in different manners, it is suggested that sizes of the elliptic cylinders are affected by the heat treatments and this change in scale, as a result, increases or decreases the Jc characteristics of the samples.

Neutron Scattering and Microstructural Studies of the Crystalline Perfection of SRL-CP Single Crystals of Y1Ba2Cu3O7−δ

The SRL-CP growth method, a modified Czochralski-pulling technique, can produce very large single crystals, longer than 1cm along each axis. In order to verify the Y1Ba2Cu3O7−δ crystal purity grown by this technique, a neutron scattering experiments have been performed on an as-grown (Tc=60K) and reduced sample. The as-grown sample in particular, is oriented very well along the c-axis, ≤1° (FWHM). However, in both samples, low angle, ≤4°, twist boundaries about the c-axis are present. The as-grown crystal shows distinct regions with differing oxygen contents; the bulk of the crystal is tetragonal, whereas, close to the surface it is orthorhombic.