La@C 60 : A metallic endohedral fullerene

We have produced an endohedrally doped fullerene that shows a metal-like density of states at the Fermi level. Individual La@C60 clusters deposited onto graphite exhibit a zero band gap as observed by scanning tunneling spectroscopy on single clusters at room temperature. Moreover, we find that an isolated La@C60 cluster on graphite shows a reversible opening of a band gap at a transition temperature of ∼28 K. The transition is associated with a freezing of the vibrational motion of the La atom inside the fullerene cage. The metallic behavior of La@C60 is attributed to the presence of a dynamical dipole in the single cluster.