Jin NAKAMURA (中村 仁)

Offer Organization: Japan Society for the Promotion of Science, System Name: Grants-in-Aid for Scientific Research, Category: Grant-in-Aid for Scientific Research (C), Fund Type: -, Overall Grant Amount: - (direct: 3900000, indirect: 1170000)
The present study reveals the mechanism of the temperature-induced spin-state transition of LaCoO3 by investigating the high-field magnetization process of the doped LaCoO3 specimens. The Co3+ ions, which are in the low-spin (LS) state in the lowest temperature, are thermally excited to the high-spin (HS) state above about 30 K. However, the number of the Co3+ ions in the HS state is limited to about 15 % due to a large repulsive interaction among HS Co3+ ions. With further increase of the temperature, the remaining LS Co3+ ions are thermally excited to the intermediate-spin (IS) state around 100 K. It is suggested that other physical properties, such as magnetostriction, of LaCoO3 can be understood in accordance with the present model where HS and IS states coexist above 100 K.

Offer Organization: Japan Society for the Promotion of Science, System Name: Grants-in-Aid for Scientific Research, Category: Grant-in-Aid for Scientific Research (C), Fund Type: -, Overall Grant Amount: - (direct: 3600000, indirect: -)
X-ray emission (XES) and obsorption (XAS) spectra near the B K-edge were measured on single-crystalline AlB2 which is an isostructural diboride of superconducting MgB2, and on single-crystalline MgB2. The partial density of states (PDOS) of B-2p-σ and p-π orbitals were derived from the polarization dependence of XES and XAS spectra.
The overall features of PDOS were found to be in good agreement with the result of band calculation of AlB2 and MgB2. There are considerable amounts of PDOS near the Fermi energy in AlB2 similarly to that in MgB2, but there are almost no PDOS in p-σ orbitals of AlB2 near the Fermi energy, i.e., a pseudo-gap in p-σ state and a broad metallic state in p-π state are observed, On the contrary, there are considerable amount of PDOS in p-σ orbitals of MgB2 near the Fermi energy, i.e., broad metallic states in p-π state and p-σ state are observed
The present result indirectly supports scenarios that the p-σ holes play an important role in the occurrence of superconductivity in MgB2.
The effect of electron correlation (EC) on the partial density of state (PDOS) were studied by the polarization dependence of x-ray emission and absorption spectra of MgB2, AlB2 single crystals.
There are some discrepancies between the observed PDOSs and theoretical ones, although these are contrary to the electron-correlation effects. The present results suggest that the EC effect is less than the experimental error (0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.

Offer Organization: Japan Society for the Promotion of Science, System Name: Grants-in-Aid for Scientific Research, Category: Grant-in-Aid for Scientific Research (C), Fund Type: -, Overall Grant Amount: - (direct: 3500000, indirect: -)
The site specific electronic structures of oxygen in PrBa_2Cu_3O_7 and YBa_2Cu_3O_7 compounds were recorded from the measurements of O 1 s total x-ray-fluorescence yield and O Kα x-ray fluorescence emission spectra of PrBa_2Cu_3O_7 and YBa_2Cu_3O_7 single crystals. The polarization dependence of the x-ray absorption spectra(XAS) near oxygen absorption edge in the specimens is attributed to the excitation of four in equivalent oxygen sites : O(1) chain, O (2) and O(3) in-plane, and O(4) apical oxygens and the hole states of the oxygens were specified. Thereafter, we obtained the site specific electronic structure by the O Ka x-ray emission spectra (XES), exciting the target oxygen by the use of polarization and energy dependence of hole state. The evidence of hybridization between the Pr 4f and O p-π orbital was observed in the spectrum of O(2,3) in plane of PrBa_2Cu_3O_7) which was predicted by Fehrenbacher : and Rice. The spectrum is consistent with the band theoretical calculation by Liechtenstein and Mazin, based on the Fehrenbacher and Rice model.
The site specific electronic structures of oxygen in PrBa_2Cu_4O_8 and YBa_2Cu_4O_8 compounds were recorded from the measurements of O 1 s total x-ray-fluorescence yield and O Ka x-ray fluorescence emission spectra of the single crystals. Consistent results with the theory by Fehrenbacher and Rice were obtained also in these compounds