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Abstract:
We introduce and investigate an effective five-band model for t(2g) and e(g) electrons to describe doped cobalt oxides with Co3+ and Co4+ ions in two-dimensional CoO2 triangular lattice layers, as in Na1-xCoO2. The effective Hamiltonian includes anisotropic kinetic energy (due to both direct Co-Co and indirect Co-O-Co hoppings), on-site Coulomb interactions parameterized by intraorbital Hubbard repulsion U and full Hund's exchange tensor, crystal field terms and Jahn-Teller static distortions. We study it using correlated wave functions on 6 x 6 clusters with periodic boundary conditions. The computations indicate a low S = 0 spin to high S = 2 spin abrupt transition in the undoped systems when increasing strength of the crystal field, while intermediate S = 1 spins are not found. Surprisingly, for the investigated realistic Hamiltonian parameters describing low-spin states in CoO2 planes, doping generates high S = 5/2 spins at Co4+ ions that are pairwise bound into singlets, seen here as pairs of up and down spins. It is found that such singlet pairs self-organize at higher doping into lines of magnetic ions with coexisting antiferromagnetic and ferromagnetic bonds between them, forming stripe-like structures. The ground states are insulating within the investigated range of doping because computed HOMO-LUMO gaps are never small enough.
