Mott-Hubbard-like behavior of the energy gap of A(4)C(60) (A = Na, K, Rb, Cs) and Na10C60

We have studied the low energy electronic excitations of A(4)C(60) (A = Na, K, Rb, Cs) and Na10C60 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)C(60) and A(3)C(60) materials requires the consideration of both a Jahn-Teller distortion owing to strong electron-phonon coupling and electron correlation effects.