Effective Interaction between the inter-penetrating Kagome lattices in   Na_xCoO_2
  A multi-orbital model for a CoO_2-layer in Na_xCoO_2 is derived. In this model the kinetic energy for the degenerate t_2g-orbitals is given by indirect hopping over oxygen, leading naturally to the concept of four inter-penetrating Kagome lattices. Local Coulomb interaction couples the four lattices and an effective Hamiltonian for the interaction in the top band can be written in terms of fermionic operators with four different flavors. Focusing on charge and spin density instabilities, a big variety of possible metallic states with spontaneously broken symmetry are found. These states lead to different charge, orbital, spin and angular momentum ordering patterns. The strong superstructure formation at x=0.5 is also discussed within this model. 
Martin Indergand, Yasufumi Yamashita, Hiroaki Kusunose and Manfred   Sigrist