Spin-Orbital Model For Fullerides

The multiorbital Hubbard model in the strong-coupling limit is analyzed for the effectively antiferromagnetic Hund's coupling relevant to fulleride superconductors with three orbitals per molecule. The localized spin-orbital model describes the thermodynamics of the half-filled (three electron) state with total spin 1/2, composed of a singlon and a doublon placed on two of the three orbitals. The model is solved using the mean-field approximation, and magnetic and electric ordered states are clarified through the temperature dependences of the order parameters. Combining the model with the band structure from ab initio calculation, we also semiquantitatively analyze the realistic model and the corresponding physical quantities. In the A15-structure fulleride model, there is an antiferromagnetic ordered state, and subsequently the two-orbital ordered state appears at lower temperatures. It is argued that the origin of these orbital orders is related to the T-h point-group symmetry. As for the fcc fulleride model, a time-reversal-broken orbital ordered state is identified. Whereas the spin degeneracy remains in our treatment for a geometrically frustrated lattice, it is expected to be lifted by some magnetic ordering or quantum fluctuations, but not by the spin-orbital coupling, which is effectively zero for fullerides in the strong-coupling regime.