R. Klingeler, G. Kann, I. Wirth, S. Eisebitt, P.S. Bechthold, M. Neeb, and W. Eberhardt

The density of electronic states in the vicinity of the Fermi level has been investigated on individual La@C6o, Ce@C6o, and C6o clusters deposited on highly oriented pyrolytic graphite (HOPG) using scanning tunneling spectroscopy (STS). The question weather a metallic or semiconducting behavior is observed depends on the type of encapsulated atom. Ce@C60 exhibits a non-metal1ic behavior with a band gap of about 0.3 eV, whereas La@C60 is found metallic at room temperature (RT). However, for La@C6o low temperature STS reveals the reversible opening of a 40 meV gap below 28 K, which is most likely due to the freezing of the "rattling" mode. The spectra are interpreted in terms of free C6o, including charge transfer and Jahn- Tel1er-distortion.