Katsuo Kato

High Tc Superconductor YBa2Cu3Oy–Oxygen Content vs Tc Relation–

High Tc superconductor YBa2Cu3Oy, having various oxygen contents have been examined by a.c. susceptibility, d.c. conductivity and Raman spectrum measurements. The Tc is greatly affected by the oxygen content, being lowered by decreasing oxygen content. Particularly, the Tc drops markedly when the compound takes a tetragonal form. To confirm the y vs temperature relation, the compound was reexamined by thermogravimetry. The new results cast doubt on our previous y value (6.5>y>5.8).

Superconductivity of YBa2Cu3-xMxOy (M = Co, Fe, Ni, Zn)

The superconductivity of YBa2Cu3-xMxOy, (M = Co, Fe, Ni, Zn; x = 0.1-0.4) was examined by means of ac susceptibility measurements. The results indicated that Cu in the Cu2 site (between the Y and the BaO layer) plays a more substantial role in superconductivity than that in the Cu1 site (between the BaO layers). Combining this with our earlier results for the La1+xBa2-xCu3Oy system, we suggest that holes doped mainly in the O 2p orbitals of the rock-salt-type BaO layers form Cooper pairs through some medium (spin fluctuations or possibly other collective modes) excited in the Cu2 layer.

Thermogravimetric and High-Temperature X-Ray Studies on the Orthorhombic-to-Tetragonal Transition of YBa2Cu3Oy

Thermogravimetric experiment was performed to determine oxygen content in the high Tc superconductor YBa2Cu3Oy. The y varies from 5.8 to 6.5 depending on temperature and oxygen fugacity, indicating that the average valence of copper ions is between 2 and 1. The oxygen content is strongly affected by quenching process of a sample. High-temperature X-ray diffraction study showed orthorhombic-to-tetragonal transition (second or possibly higher order) at around 900 K. Combining this fact with the thermogravimetric data, we concluded that the compound having y less than ~6.0 has a tetragonal symmetry.

Identification of the Superconducting Phase in the Nd–Ce–Sr–Cu–O System

Phase search experiments were performed to identify the superconducting phase in the Nd-Ce-Sr-Cu-O system. A single phase superconductor was obtained form the nominal composition (Nd0.66Ce0.135Sr0.205)2CuOy. It has a tetragonal cell with a=3.8558(1) and c=12.487(1) A. From powder X-ray diffraction and electron microscopic observations, it was confirmed that the metal positions in this phase are identical to those in the K2NiF4 structure but with cationic order of Nd(Ce) and Sr. Three structure models including oxygen positions are proposed and discussed. Although we could not be absolutely conclusive, the structure which consists of alternative stacking of K2NiF4- and Nd2CuO4-type slabs seems to be most likely.

On the 110 K Superconductor in the Bi-Ca-Sr-Cu-O System

We found a new compound in the Bi-Ca-Sr-Cu-O system which has a different structure from the 80 K superconductor Bi2(Ca, Sr)3Cu2Oy. The c-dimension of this compound is ~37 A and is ~6 A longer than that of Bi2(Ca, Sr)3Cu2Oy. Although we have not obtained a single-phased sample yet, the clear correlation between the diamagnetic susceptibility just below 110 K and the content of this compound included in the sample strongly suggests that the compound with c≈37 A is responsible for the superconductivity of Tc~110 K.

X-Ray Study of Compressibilities of β-(BEDT-TTF)2I3 under Hydrostatic Pressure

Values of the lattice constants of the organic conductor β-(BEDT-TTF) 2 I 3 up to 2 GPa at room temperature are obtained by Buerger precession method and the compressibilities d x / x d p [/GPa] are determined as -0.044, -0.032 and -0.026, where x = a , b and c , respectively. The anisotropy factor of the compressibilities is less than 2, in contrast with the quasi-one-dimensional case of (TMTSF) 2 PF 6 . The difference is caused by the two-dimensional character of the electronic structure originated from the corrugated network of BEDT-TTF molecules.

High-Resolution Electron Microscopy of Planer Defects and Dislocation in Ba2YCu3Oy

Two types of planar defects in Ba2YCu3Oy (y is around 6.9), previously reported [Y. Matsui et al.: Jpn. J. Appl. Phys. 26 (1987) L777] are re-examined by high-resolution transmission electron microscopy. The first is due to additional [CuO] one-dimensional chains inserted in the basal plane to form double chains, and the second to additional [CuO2] layers inserted near the Y layers to form double pyramids. The local formation of the latter type of planar defect may possibly be the origin of the edge-dislocation with Burgers vector [100] or [010].

Superconductivity of La1+xBa2-xCu3Oy System: Which Layers are Doped with Carriers?

The La1+xBa2-xCu3Oy system was examined by means of ac susceptibility measurement and thermogravimetry. The superconducting transition point is affected by the La/Ba ratio: Tc decreases monotonously with increasing x, and the sample having x of at least 0.3 did not show superconductivity above 4.2 K. Moreover, it was found that Tc was dramatically reduced by the introduction of a very small quantity of oxygen defects into the rock-salt-type Ba-O layers (2g site in P4/mmm). From this fact and earlier results on YBa2Cu3Oy and La2-xSrxCuO4, we suggest that charge carriers are holes doped mainly in the O 2p orbitals of the Ba-O layers, being consistent with a large number of experimental results reported thus far.

Structure refinement of the incommensurate composite crystal Sr1.145TiS3 through the Rietveld analysis process

The structure of the incommensurate composite crystal of the trigonal sulfide Sr 1.145 TiS 3 , M r =244.42, has been analyzed on the basis of the four-dimensional superspace group P R3m 1s using powder X-ray diffraction data and the Rietveld analysis process. The unit cell and other crystal data are a 1 =a 2 =11.5108 (2), a 3 =2.99094 (5) A, a=[000.57227 (2)], V=343.19 A 3 , D x =3.548 Mg m -3 and Z=3. R wp was 0.104 on an incommensurate modulated structure model with 47 structural parameters. The crystal is composed of columns of face-shared polyhedra, (TiS 6/2 )∞, and rows of Sr, and both are parallel to the c axis

Magnetic Properties of NaV6O11

Magnetic properties and electrical resistivity of NaV 6 O 11 single crystal have been investigated. Magnetic transition was observed at 64.2 K. This oxide having a hexagonal crystal structure exhibited a uniaxial magnetic anisotropy with the easy axis of magnetization parallel to [001]. The spontaneous magnetization at 5 K was 1.7µ B per formula unit. The resistivity perpendicular to [001] showed an anomaly at the Curie temperature similar to that of ferromagnetic metals, while that parallel to [001] showed no such anomaly. This behavior suggests the itinerant character of d electrons.