Superexchange and electron correlations in alkali fullerides AC60, A=K, Rb, Cs

Superexchange interactions in alkali fullerides AC(60) are derived for C(60) molecular ions separated by interstitial alkali-metal ions. We use a multiconfiguration approach which comprises the lowest molecular orbital states of the C(60) molecule and the excited s and d states of the alkali-metal atom A. Interactions are described by the valence bond (Heitler-London) method for a complex (C(60)-A-C(60))(-) with two valence electrons. The electronic charge transfer between the alkali-metal atom and a neighboring C(60) molecule is not complete. The occupation probability of excited d and s states of the alkali atom is not negligible. In correspondence with the relative positions of the C(60) molecules and A atoms in the polymer crystal, we consider 180 degrees and 90 degrees (angle) superexchange pathways. For the former case the ground state is found to be a spin singlet separated from a triplet at approximately 20 K. For T<20 K there appear strong spin correlations for the 180 degrees superexchange pathway. The results are related to spin lattice relaxation experiments on CsC(60) in the polymerized and in the quenched cubic phase.