Correlation Effects on the Doped Triangular Lattice in View of the Physics of Sodium-Rich NaxCoO2

The peculiar correlation effects on the triangular lattice are studied by means of the rotationally invariant slave boson method in a cellular cluster approach. Hence nonlocal correlations are included in a short-range regime. Their impact for the single-band Hubbard model is studied at half filling, i.e., on the Mott transition, and with doping. Using the realistic band structure of NaxCoO2, we may also shed light on the cobaltate physics for x >= 1/3, with the in-plane transition from antiferromagnetic tendencies towards the onset of ferromagnetism for a finite Hubbard U.