2007 Fiscal Year Final Research Report Summary
Theory of anomalous properties and novel superconductivity due to many body effects in cobalt oxides
| Project/Area Number |
17540317
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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 | Niigata University |
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Principal Investigator |
ONO Yoshiaki Niigata University, Institute of Science and Technology, Professor (40221832)
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| Co-Investigator(Kenkyū-buntansha) |
OKUNISHI Kouichi Niigata University, Institute of Science and Technology, Assistant Professor (30332646)
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| Project Period (FY) |
2005 – 2007
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| Keywords | cobalt oxides / electron correlation / d-p model / frustration / superconductivity / charge order / orbital order / metal-insulator transition |
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| Research Abstract |
The specific features of the cobalt oxides Na_xCoO_2 and the H2_O intercalated superconductor Na_xCoO_2・yH_2O are the geometrical fluctuation of the CoO_2 plane which consists of a triangular lattice of Co atoms, and the orbital degeneracy of three t2g bands for Co 3d electrons. We investigate the electronic states of the CoO_2 plane using the 11 band d-p model on a two-dimensional triangular lattice, where the tight-binding parameters are determined so as to fit the LDA band structure. The model includes the Coulomb interaction on a Co site: the intra- and inter-orbital direct terms U and U', the exchange coupling J and the pair-transfer J', and is solved by using the dynamical mean-field theory, the slave-boson method, the random phase approximation and the Hartree-Fock approximation. What we found are:
1. Due to the strong correlation effect of the Co d-electrons, the renormalized bands become narrow and six hole pockets of the e'g band disappear for x>0.4 leaving a large hole Fermi surface of the at band.
2. When H_2O is intercalated, the trigonal distortion becomes large and e'g hole pockets reemergence. Then, the spin singlet superconductivity, where the order parameters for e'_g and at orbital are opposite sign, takes place.
3. When the Na content x approaches 0.75, the T-linear specific heat coefficient and the thermopower S are largely enhanced due to the effects of the ferromagnetic fluctuation which becomes important near the in-plane ferromagnetism observed for x>0.75.
4. The effect of the one-dimensional Na order at x=0.5 enhances the Fermi surface nesting resulting in the metallic antiferromagnetism below T_c1, and, then, a coexistence of the spin, charge and orbital order takes place below T_c2(<T_c1) where the system becomes insulator. The obtained results are consistent with the successive phase transitions observed in Na_0.5CoO_2.
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