Akihisa Koizumi

EXAFS and neutron diffraction studies of local and average structures for YBa2Cu2.8Zn0.2O7−δ

Abstract The local and average structures of YBa 2 Cu 3 O 7−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 A) and one long (2.25 A) Zn-O bonds. The chemical environments of the Cu and Y atoms agree well with the structure determined by neutron diffraction. The mechanism of T c of depression accompanying Zn doping is discussed.

Temperature dependence of the exafs spectrum in YBa2Cu3O7−δ compounds

Abstract The temperature dependence of the extended X-ray absorption fine structure (EXAFS) is studied in the high T c superconductors, YBa 2 Cu 3 O 7− δ . The measurements were done at the Cu K-edge for samples of two orthorhombic phases ( T c ≈90 K and ≈58 K, respectively) and a nonsuperconducting tetragonal phase. Interatomic distances and mean square relative displacements σ 2 for Cu-O bonds are determined by the least squares refinement. The results indicate that values of σ 2 increase near T c for both the orthorhombic samples. It is concluded that this anomalous behavior related to T c 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.

EXAFS Study on the Local Structure of a New High-Tc Superconductor in the Bi-Sr-Ca-Cu-O System

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 A. Measurements at the Bi LIII-edge give the interatomic distance between bismuth and oxygen as 2.10 A.

EXAFS Studies on Y1-xBaxCuO3-δ Superconductors

The temperature dependence of the EXAFS (Extended X-ray Absorption Fine Structure) method has been carried out to study high- T c superconductors of composition Y 1- x Ba x CuO 3-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.

Temperature dependence of EXAFS spectrum in BiPbSrCaCu oxide superconductor

Abstract Temperature dependence of extended X-ray absorption fine structure (EXAFS) is studied in the high- T c BiPbSrCa 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 CuO bonds are determined by the least-squares refinement. An anomalous increase in the axial component of the MSRD is found near T c while its equatorial component decreases normally with decreasing temperature.

Magnetic X-ray absorption measurements at the Pt L2,3- and Fe K-edge in ordered and disordered FePt alloys

Abstract Magnetic X-ray absorption near the edge structure (magnetic XANES) as a new technique is applied to ordered and disordered FePt alloys in order to study the magnetic local environment effect. From the magnetic XANES at the Fe K-edge, it is suggested that the 3d-electronic states of Fe are sensitively influenced by the local environment depending on concentration and/or symmetry. The magnitude of the magnetic XANES at the Pt L2,3-edges is not directly corresponding to the magnetic moment of the Pt atom.

EXAFS Study on Fe-Doped YBa2Cu3Oy

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 A, 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.

Polarization-dependent EXAFS study on anharmonicity in YBa2Cu3O7−δ

Abstract The polarization-dependent EXFAS of the Cu K -edge has been measured on a uniaxially oriented YBa 2 Cu 3 O 7−° superconductor. Information on the axial and equatorial CuO bonds is separated by EXAFS analysis taking into account anharmonicity. An anomalous increase in the 2nd- and 4th-order cumulants has been found around T c both the components. The anharmonic lattice vibrations relating with the apical oxygen should correlate with the superconductivity.

Magnetic Moment and Intensity of Magnetic X-ray Dichroism at the K-edge in Fe73.6Ni26.4 Alloy

The temperature dependence of circularly magnetic x-ray dichroism (CMXD) spectra at the Fe K-edge has been measured in a Fe73.4Ni26.4 alloy. The CMXD signal intensities at the Fe K-edge are proportional to the total magnetization of the alloy.

XAFS studies on martensitic transformation in Fe-Ni alloys

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 M s and the bcc structure is locally more stable around Fe atom than around Ni atom.