Abstract
A multi-orbital model for the CoO layer in Na x CoO 2 is discussed for Na-content x close to 1. In this model, the kinetic energy for the degenerate t 2 g orbitals is given by indirect hopping through oxygen, leading naturally to four independent inter-penetrating Kagomé 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 are characterized by a specific spin, charge, orbital or angular momentum order.
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