2004 Fiscal Year Final Research Report Summary
Study on the metal-insulator transition of Ti oxide single crystals caused from the band filling by means of photoemission and inverse-photoemission spectroscopies
| Project/Area Number |
15540313
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Condensed matter physics I
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| Research Institution | HIROSHIMA UNIVERSITY |
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Principal Investigator |
SATO Hitoshi Hiroshima University, Hiroshima Synchrotron Radiation Center, Associate Professor, 放射光科学研究センター, 助教授 (90243550)
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| Co-Investigator(Kenkyū-buntansha) |
TANIGUCHI Masaki Hiroshima University, Graduate School of Science, Professor, 大学院・理学研究科, 教授 (10126120)
NAMATAME Hirofumi Hiroshima University, Hiroshima Synchrotron Radiation Center, Professor, 放射光科学研究センター, 教授 (10218050)
SHIMADA Kenya Hiroshima University, Hiroshima Synchrotron Radiation Center, Associate Professor, 放射光科学研究センター, 助教授 (10284225)
IGA Fumitoshi Hiroshima University, Graduate School of Advanced Sciences of Matter, Associate Professor, 大学院・先端物質科学研究科, 助教授 (60192473)
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| Project Period (FY) |
2003 – 2004
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| Keywords | Y_<1-x>Ca_xTiO_3 / Metal-Insulator Transition / Electron Correlation / Hubbard Band / Photoemission Spectroscopy / Inverse-Photoemission Spectroscopy |
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| Research Abstract |
Distorted perovskite-type Y_<1-x>Ca_xTiO_3 is a system which moves from the Mott insulator YTiO_3(x=0) to the band insulator CaTiO_3(x=1) across the metallic phase, by controlling the Ti 3d band filling with x. In particular, Y_<1-x>Ca_xTiO_3 with x=0.39 exhibits a sharp metal-insulator transition (MIT) around 150 K (high-temperature phase : insulator, low-temperature phase : metal).
We have carried out for x=0.39 the Ti 3p-3d resonant photoemission and inverse-photoemission spectroscopies (PES abd IPES) and observed the occupied and unoccupied Ti 3d densities of states, that is, the remnant of the lower and upper Hubbard bands split by the electron correlation. The experimental results revealed that the U_<eff>-value, determined from their energy separation, increases with decreasing temperature. Since the Coulomb interaction energy of the bare Ti 3d electrons is considered to be almost intraatomic value, the U_<eff> reduction reflects that the Ti 3d bands become wider at low temperature.
In order to analyze the IPES spectra of x=0.39, we have measured the O 1s soft x-ray absorption spectra in comparison with the IPES spectra. The subtle structure, possibly corresponding to the coherent part in the PES spectra, seems to appear in the lower energy side of the unoccupied Ti 3d structure. This component is considered to be closely related to MIT. We have to reconfirm the existence by the IPES with higher energy resolution.
We have also PES for x=0.39 using hard x-ray (several keV), which is more bulk-sensitive. The lower Hubbard band shifts to the shallower binding-energy side compared with the PES spectra with several ten eV. This suggests that the correlation effect is strong near the surface. In addition, for the metallic phase, a new component has been found in the Ti 2p PES, probably being attributed to the well screened state as observed in the Mn 2p PES of La_<1-x>Sr_xMnO_3 in the metallic phase [1].
[1]K.Horiba et al., Phys.Rev.Lett. 93,236401 (2004).
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