Transport properties and electronic states of the thermoelectric oxide NaCo_2O_4

Physical properties of the thermoelectric oxide NaCo_2O_4 are briefly reviewed. The high thermoelectric properties of this material are attributed to the substantially enhanced effective mass, which comes from the large entropy of Co$^{4+}$ in the low spin state. The large entropy confined in the CoO_2 block causes a spin-density-wave transition at 22 K upon Cu substitution for Co, which can be regarded as ``order from disorder''.Comment: 4 Pages, $ figures, Proceedings of The 2nd Hiroshima Workshop: Transport and Thermal Properties of Advanced Material

Electronic band structure and Fermi surface of Ag5_5Pb2_2O6_6

We present electronic band structure of Ag$_5$Pb$_2$O$_6$ with layered hexagonal structure containing one-dimensional chains and two-dimensional Kagom\'{e} layers of silver. A half-filled conduction band shows extremely simple, single nearly-free-electron-like Fermi surface. The conduction band is composed of an antibonding state of Pb-$6s$ and O-$2p$ mixing with Ag-4d and $5s$. Mass enhancement in the state density at the Fermi energy is expected to be negligibly small by comparing with the specific-heat data. Calculated Fermi velocity is consistent with small anisotropy observed in transport properties. Doping effects on the electronic structure are also discussed.Comment: 6 pages, 9 figures; fig7 replaced, reference 6 adde

Study of the triangular lattice tV model near x=1/3

We study extended Hubbard model on a triangular lattice near doping $x=1/3$, which may be relevant for the recently discovered superconductor Na$_x$CoO$_2 \cdot y$H$_2$O. By generalizing this model to $N$ fermionic species, we formulate a meanfield description in the limit of large $N$. In meanfield, we find two possible phases: a renormalized Fermi liquid and a \rt3rt3 charge density wave state. The transition between the two phases is driven by increasing the nearest neighbor repulsion and is found to be first order for doping $x=1/3$, but occurs close to the point of the local instability of the uniform liquid. We also study fluctuations about the uniform meanfield state in a systematic 1/N expansion, focusing on the residual interaction of quasiparticles and possible superconducting instabilities due to this interaction. Upon moving towards the CDW instability, the increasing charge fluctuations favor a particular $f$-wave triplet state. (This state was recently discussed by Tanakaet al, cond-mat/0311266). We also report a direct Gutzwiller wavefunction study of the spin-1/2 model.Comment: 9 pages, 5 figure