小泉 昭久

Temperature dependence of extended X-ray absorption fine structure (EXAFS) is studied in the high-Tc BiPbSrCa Cu oxide superconductor. Measurements were made at the Cu K-edge in the temperature range from 26.9 to 300 K. Interatomic distances and mean square relative displacement (MSRD: σ(2) of CuO bonds are determined by the least-squares refinement. An anomalous increase in the axial component of the MSRD is found near Tc while its equatorial component decreases normally with decreasing temperature. © 1992.

The polarization-dependent EXFAS of the Cu K-edge has been measured on a uniaxially oriented YBa2Cu3O7-° superconductor. Information on the axial and equatorial CuO bonds is separated by EXAFS analysis taking into account anharmonicity. An anomalous increase in the 2nd- and 4th-order cumulants has been found around Tc both the components. The anharmonic lattice vibrations relating with the apical oxygen should correlate with the superconductivity. © 1991.

Polarized Cu K X-ray absorption fine structure (XAFS) has been measured on a single crystal of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. From the temperature dependence of extended X-ray absorption fine structure (EXAFS) signal, anomalies of the local structure around 100 K and 60 K have been confirmed. This is the first report on the local structure consistent with the anomalies reported in other physical properties. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

Magnetic effects in the X-ray absorption process have been measured using circularly polarized X-rays emitted from an elliptical multipole wiggler. Magnetic XANES spectra on the K-edge of Fe and Ni taken by the transmission method show positive and negative spin polarizations, respectively, near the Fermi level which are consistent with the results in literature. A large positive magnetic XANES spectrum on the Nd L2-edge of Nd2Fe14B compound measured by the fluorescent method indicates that the unoccupied 5d-bands are polarized due to a significant hybridization between 4f- and 5d-states. The magnetic XANES spectrum on the Nd L3-edge has a structure below the Fermi level, suggesting a quadrupole transition contribution of the partially vacant majority band of 4/-states. © 1991, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

X-ray Absorption Fine Structure (XAFS) measurements have been carried out for Fe-Ni alloys which undergo fee → bcc martensitic transformation. From EXAFS analysis, the local structural change occurs at a higher temperature than Ms and the bcc structure is locally more stable around Fe atom than around Ni atom. © 1990, All rights reserved.

The temperature dependence of the extended X-ray absorption fine structure (EXAFS) is studied in the high Tc superconductors, YBa2Cu3O7-δ. The measurements were done at the Cu K-edge for samples of two orthorhombic phases (Tc≈90 K and ≈58 K, respectively) and a nonsuperconducting tetragonal phase. Interatomic distances and mean square relative displacements σ2 for Cu-O bonds are determined by the least squares refinement. The results indicate that values of σ2 increase near Tc for both the orthorhombic samples. It is concluded that this anomalous behavior related to Tc is caused by an anomalous vibration of oxygen atoms in the Ba-O layer. Changes in the Cu-O distances from 300 to 20 K are not found. © 1989.

The local and average structures of YBa2Cu3O7-δ doped with Zn were studied by the combined use of extended X-ray absorption fine structure (EXAFS) analysis and Rietveld refinement of neutron powder diffraction data. X-ray absorption measurements at the Zn K-edge indicate that the Zn atom has four short (2.02 Å) and one long (2.25 Å) Zn-O bonds. The chemical environments of the Cu and Y atoms agree well with the structure determined by neutron diffraction. The mechanism of Tc of depression accompanying Zn doping is discussed. © 1989.

An extended X-ray absorption fine structure (EXAFS) method was applied to YBa2Cu3-xFexOy with x=0.05, 0.1, 0.2, 0.3, 0.4 and 0.5. Fe atoms are surrounded by six oxygen atoms with a distance of about 1.85 Å, and they mainly substitute for the Cu(1) site over the whole concentration range. The local structures around Cu atoms and Y atoms are also reported. © 1989 The Japan Society of Applied Physics.

X-ray absorption spectroscopy has been used to study a new superconducting phase in the Bi-Ca-Sr-Cu-O system. X-ray absorption measurements at the Cu K-edge indicate that a copper ion is surrounded by five coordinated oxygen atoms, and that the Cu-O distance is 1.91 Å. Measurements at the Bi LIII-edge give the interatomic distance between bismuth and oxygen as 2.10 Å. © 1988 The Japan Society of Applied Physics.

The temperature dependence of the EXAFS (Extended X-ray Absorption Fine Structure) method has been carried out to study high-Tc superconductors of composition Y1-xBaxCu03-δ with x=0.6. The anomalous behavior of the Debye-Waller factor in the superconducting transition region is observed, which may result from the breathing-mode vibration of oxygen atoms around Cu atoms, as reported in the oxide superconductor La-Sr-Cu-O system. © 1987, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.