Spin-Vibronic Superexchange in Mott-Hubbard Fullerides

In the Mott-Hubbard cubic fulleride ${\mathrm{Li}}_3({\mathrm{NH}}_3{)}_6{\mathrm{C}}_{60}$ the superexchange energy is found to be much smaller than the rotational quantum for Jahn-Teller deformations at fullerene sites. This gives rise to a new type of superexchange interaction involving threefold degenerate vibronic ground states of ${\mathrm{C}}_{60}^{3-}$ ions. In contrast with spin-orbital models, the spin-vibronic superexchange can be only antiferromagnetic and shows a significant vibronic reduction of the superexchange amplitude, in agreement with magnetic susceptibility data. As a function of the transfer parameters, two quadrupolar fully dynamical vibronic orders with quenched vibronic moments on sites develop in the ground state.

Figure 1

Hund’s (a) and Jahn-Teller (b) electronic configurations of ${\mathrm{C}}_{60}^{3-}$. $|{\Phi }_i⟩$ are adiabatic orbitals.

Figure 2

Vibronic quadrupole ordered states: (a) ferro type (I), $T=(-2/3,-2/3,-2/3)$ and (b) antiffero type (II), $T_1=(0,0,1)$, $T_2=(0,0,-1)$.

Figure 3

Ratio of transfer parameters for LUMO orbitals of nearest neighbor fullerenes in a bcc lattice, calculated within Harrison and Wolfsberg-Helmholz approximations.