2006 Fiscal Year Final Research Report Summary
Metal-insulator transition near the superconducting phase of two-dimensional cobalt oxides
| Project/Area Number |
17540323
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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 II
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| Research Institution | Nagoya University |
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Principal Investigator |
KOBAYASHI Yoshiaki Nagoya University, Division of Material Science, Associated Professor, 大学院理学研究科, 助教授 (60262846)
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| Co-Investigator(Kenkyū-buntansha) |
SATO Masatoshi Nagoya University Division of Material Science, Professor, 大学院理学研究科, 教授 (40092225)
YASUI Yukio Nagoya University, Division of Material Science, Assistant Professor, 大学院理学研究科, 助手 (80345850)
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| Project Period (FY) |
2005 – 2006
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| Keywords | Na_xC0O_2・yH_2O / NMR Knight shift / nuclear electric quadrupole resonance / Cooper pairs / singlet / Na_<0.5>CoO_2 / neutron scattering / charge ordering |
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| Research Abstract |
We have produced principally three results, which are of considering importance to the mechanism of the formation of the Cooper pairs in Na_xCoO_2・yH_2O superconductors.
(1) We have revealed that the superconducting pairs in the superconducting state of Na_xCoO_2・yH_2O are in the spin-singlet state, by the measurements of the ^<59>Co-NMR Knight shift for both magnetic field (H) directions parallel and perpendicular to the CoO_2planes in the superconducting state using single phase single crystals.
(2) The superconducting transition temperatures T_c of Na_xCoO_2・yH_2O have been found to exhibit the systematic dependence on the ^<59>Co nuclear electric quadrupole resonance frequency ν_Q. In the T-3ν_Q phase diagram, the superconducting region of 3ν_Q, spreading over the region between 〜12.0 and 〜12.9 MHz, is divided into two regions by the non-superconducting phase between 〜12.5 and 〜12.6 MHz.
We have investigated the origin of the non-superconducting phase by detailed experimental measurem
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ents of ^<59>Co-NQR and ^2D-NMR and found that the non-superconducting phase is in the charge-density-wave (CDW) or the charge disproportionate state, in which the magnetic moments induced inhomogeneously by this transition become frozen at low temperatures.
(3) We have revealed that the x dependence of the electronic nature of Na_x,CoO_2 which is the parent compound of Na_<0.3>CoO_2・1.3H_2O superconductors. In particular, the successive phase transitions observed in Na_<0.5>CoO_2 at 87 K (T_<c1>) and 53 K (T_<c2>) have been studies. Na_<0.5>CoO_2 indicates the antiferromagnetic ordering below T_<c1>, where we have determined the unusual magnetic structure by ^<59>Co-NMR and neutron diffraction studies.
It has been found from the X dependence of χ-T and 1/T_1-T curves that the two-dimensional nature of the electrons of Na_xCoO_2 is enhanced and the antiferromagnetic correlation increases in this system as the Na concentration x decreases. Therefore, the superconducting pairs are expected to be in the singlet state. This is consistent with the results of the Knight shift studies in Na_<0.3>CoO_2-1.3H_2O. Less
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Research Products
(22 results)
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[Journal Article] Magnetic and Metal-Insulator Transitions in β-Na_<0.5>CoO_2 and γ-Na_<0.5>CoO_2 -NMR and Neutron Diffraction Studies-2006
Author(s)
H.Watanabe, Y.Mori, M.Yokoi, T.Moyoshi, M.Soda, Y.Yasui, Y.Kobayashi, M.Sato, N.Igawa, K.Kakurai
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Journal Title
Journal of the Physical Society of Japan Vol.75, No.3
Pages: 034716(1-8)
Description
「研究成果報告書概要(欧文)」より
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