TETRAHIS(DIMETHYLAMINO)ETHYLENE-C-60 - MULTICOMPONENT SUPEREXCHANGE AND MOTT FERROMAGNETISM

An interacting, quasi-one-dimensional, orbitally degenerate hopping model is introduced, motivated by the structure of tetrahis(dimethylamino)ethylene (TDAE)-C-60. The ground state is a Mott insulator for average ionization C-60(-), With several possible magnetically ordered phases. The effective Hamiltonian is derived via a multicomponent superexchange transformation. Orbital degeneracy is represented as an additional ''isospin'' degree of freedom. We explore the phase diagram of the one-dimensional model in interaction parameter space, and identify various special points: (i) an SU(4) antiferromagnet (solved by Sutherland and approximated by a large-N mean-field theory), (ii) a spin-ferromagnet/isospin-antiferromagnet, and (iii) an isospin-ferromagnet/spin-antiferromagnet The exact ground states for (ii) and (iii) are proven using Marshall's theorem. Away from these points we determine the phase diagram in the classical limit. Three-dimensional ordering for case (ii) is found using interchain mean-field theory. We find two transition temperatures: T-C proportional to root J(perpendicular to)J(parallel to) for the ferromagnetic spin order and T-N proportional to J(perpendicular to) for the antiferromagnetic isospin ordering, where J(parallel to), J(perpendicular to) are the intrachain and interchain couplings, respectively. We discuss the relevance of our theory to recent experiments on TDAE-C-60.