Mott-Hubbard-like Behavior of the Energy Gap ofA4C60(A=Na,K,Rb,Cs)andNa10C60

We have studied the low energy electronic excitations of $A_4{\mathrm{C}}_{60}$ $(A=\mathrm{Na},\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ and ${\mathrm{Na}}_{10}{\mathrm{C}}_{60}$ using electron energy-loss spectroscopy in transmission. Our results provide evidence for a Mott-Hubbard ground state in these compounds together with a Jahn-Teller distortion of the fullerene molecules. The behavior of the energy gap as a function of lattice parameter and crystal symmetry could be successfully modeled within this framework. We conclude that a comprehensive understanding of the $A_4{\mathrm{C}}_{60}$ and $A_3{\mathrm{C}}_{60}$ materials requires the consideration of both a Jahn-Teller distortion owing to strong electron-phonon coupling and electron correlation effects.