|Budget Amount *help
¥3,770,000 (Direct Cost: ¥3,500,000、Indirect Cost: ¥270,000)
Fiscal Year 2007: ¥1,170,000 (Direct Cost: ¥900,000、Indirect Cost: ¥270,000)
Fiscal Year 2006: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2005: ¥1,600,000 (Direct Cost: ¥1,600,000)
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.