Hiroshi Sawa

Superconductivity in the Bi−Sr−Cu−O system

A superconducting transition with an onset temperature near 8 K was observed for the Bi-Sr-Cu-O system through electrical resistivity and magnetic susceptibility measurements. The superconductivity is of a bulk nature and reproducible.

Superconductivity in the Nd-Sr-Ce-Cu-O System

A new high-Tc superconductor Nd-Sr-Ce-Cu-O system has been found by the resistivity and magnetization measurements. The highest onset transition temperature is about 28 K. The superconducting phase may be described as (Nd1-xSrx-yCey)2CuO4-δ.

A New Family of Superconducting Copper Oxides: (Ln1-xCex)2(Ba1-yLny)2Cu3O10-δ (Ln: Nd, Sm, EU)

A new family of high- T C superconductors (Ln 1- x Ce x ) 2 (Ba 1- y Ln y ) 2 Cu 3 O 10-δ (Ln: Nd, Sm, EU) has been found by resistivity and magnetization measurements. The highest onset transition temperature is about 43 K. The structure is proposed on the basis of electron diffractions and Rietveld analysis of powder X-ray diffraction data.

Metal ordering and oxygen displacements in (Nd, Sr, Ce)2CuO4 − y

Abstract The structural parameters of a fully oxidized, superconducting sample and a deoxidized, non-superconducting sample of Nd 1.32 Sr 0.41 Ce 0.27 CuO 4 − y were refined by Rietveld analysis of TOF neutron powder diffraction data. No appreciable structural differences were found between the two samples. Its structure consists of K 2 NiF 4 - and Nd 2 CuO 4 -type units stacked alternately along the c -axis. Ce 4+ ions, whose oxidation state was confirmed by XPS, occupy an eight-coordinate site together with Nd 3+ ions in the Nd 2 CuO 4 units. On the other hand, a nine-coordinate site in the K 2 NiF 4 units is filled with Nd 3+ and Sr 2+ ions. Each Cu atom is coordinated to five oxide ions in a distorted square-pyramidal configuration. O(2) atoms at the apical positions of the CuO 5 pyramids are displaced from an ideal 2c site to an 8j site with lower symmetry, which is mainly caused by the occupation of the same nine-coordinate site by Nd 3+ and Sr 2+ ions. The occupation factors of site O(2) were refined to be ca. 0.23, giving the 4 − y value of ≈3.92. The formal charge of Cu, +1.98, suggests that this superconductor has an unhomogeneous domain structure in which oxygen-rich superconducting regions and oxygen-poor non-superconducting regions coexist within each particle.

Successive magnetic phase transitions in Nd2CuO4

A neutron scattering study on Nd 2 CuO 4 , a mother material of the electron-doped superconductor (Nd 1- x Ce x )CuO 4- y , was carried out. Reentrant magnetic phase transitions associated with spin reorientation of antiferromagnetic ordering were observed. La 2 NiO 4 -type order was stabilized in the range 80 K\({\lesssim}T{\lesssim}\)250 K and at \(T{\lesssim}40\) K. Between 40 K\({\lesssim}T{\lesssim}\)80 K, La 2 CuO 4 -type order was realized. Diffuse scattering due to the two-dimensional spin fluctuation was also observed at around T ≃250 K.

Crystal Substructure of the Bi-Sr-Cu-O System

A substructure with a pseudotetragonal unit cell is proposed for a low-Tc superconducting oxide Bi-Sr-Cu-O system on the basis of Rietveld analysis of powder X-ray diffraction data. It is a layered structure having the composition Bi2Sr2CuOy. The relationship between the crystal structure of the Bi-Sr-Ca-Cu-O system and that of Bi-Sr-Cu-O system was discussed.

Oxygen deficiency and atomic displacements in superconducting (Ba1−xNdx)2(Nd1−yCey)2Cu3O8+z

Abstract The structural parameters of the superconductor (Ba 0.633 Nd 0.367 ) 2 (Nd 0.675 Ce 0.325 ) 2 Cu 3 O 8+z (z≈0.91) were refined by the Rietveld analysis of TOF neutron powder diffraction data. This oxide belongs to a new family of solid solutions (Ba 1−x Ln x ) 2 (Ln 1−y Ce y ) 2 Cu 3 O 8+z ( Ln = Nd , Sm , Eu and Gd ). It has a body-centered tetragonal structure with space group I4/mmm and lattice constants a =0.38747(4) nm and c =2.8599(3) nm. Its structure can be derived by stacking tetragonal Ba 2 YCu 3 O 6+z -and Nd 2 CuO 4 -type slabs alternately along the c -axis; they are jointed through two-dimensional hole-doped CuO 2 sheets. Each Cu cation on the CuO 2 sheet is coordinated to five oxide ions in a distorted square-pyramidal configuration. The Ba 2 YCu 3 O 6+z -type slab accommodates Ba(Nd) atoms, whereas each Nd(Ce) atom in the Nd 2 CuO 4 -type slab has four short and four long Nd(Ce)-O bonds. Cu and O atoms within the Ba 2 YCu 3 O 6+z -type slab are considerably displaced from ideal positions. An O site between two Ba(Nd)O sheets is highly deficient (occupancy: 0.114), as is observed in corresponding oxygen sites in the tetragonal structures of Ba 2 YCu 3 O 6+z (z 2−x Ln 1+x Cu 3 O 7±z ( Ln = La , Pr , Nd , Sm and Eu ). Bulk superconductivity is induced by intercalating a sufficient amount of O at this position to increase the concentration of (Cu-O) + holes on the CuO 2 sheet.

Planar defects in the new superconducting oxide (Eu1−xCex)2(Ba1−yEuy)2Cu3Oz observed by high-resolution transmission electron microscopy

Planar defects in the 2-2-3 type of new superconducting oxide, R2B2Cu3Oz [R is (Eu1-xCex), (Ba1-yEuy); x and y are about 0.33 and z is about 8.54], recently discovered by Sawa et al. (to be published in J. Phys. Soc. Jpn.) are examined by high-resolution transmission electron microscopy. The planar defects are derived by the local replacements of R2O2-layers by R-layers to form defect stabs of 1.2 nm thickness, having local structure similar to that of RBa2Cu3Oy.

Long Period Structures in the Bi-Sr-Cu-O System

The structure of the superconducting Bi-Sr-Cu oxide system was studied by means of electron diffraction. Three kinds of long period structures were found in this system. The relation between these long period structures and the nonstoichiometry in this system is discussed.

Copper Spin Correlations in a Two-Dimensional Antiferromagnet Pr2CuO4-δ

Neutron scattering experiments were performed to study the antiferromagnetic structure and the two-dimensional short-range correlation of copper spins in nondoped Pr 2 CuO 4-δ , the parent material of the electron-doped oxide superconductor Pr 2- x Ce x CuO 4-δ , in the temperature range from 325 K to 10 K. A single-crystal sample used in the present study orders at T N =249±1 K with the antiferromagnetic La 2 NiO 4 structure. A highly inelastic two-dimensional spin correlation was observed above and below T N , whose qualitative behavior is similar to the spin-fluid state observed in La 2 CuO 4-δ .