Tetrahis(dimethylamino)ethylene-C₆₀: 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)-C₆₀. The ground state is a Mott insulator for average ionization C₆₀^-, 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∝ √J_⊥J_∥ for the ferromagnetic spin order and T_N∝J_⊥ for the antiferromagnetic isospin ordering, where J_∥, J_⊥ are the intrachain and interchain couplings, respectively. We discuss the relevance of our theory to recent experiments on TDAE-C₆₀.