Temperature dependence of the electronic properties of K3C60 and K4C60 single-phase films investigated by means of electron spectroscopies

The normal state temperature dependence of a metallic K3C60 phase pure film has been investigated by means of core levels and valence band photoemission, carbon K edge x-ray absorption, and electron energy loss spectroscopies and compared with that of a Mott-Hubbard insulating K4C60 film. The anomalous temperature behavior of K3C60, in the range 30 K-600 K, can be consistently interpreted considering the presence of orientational disorder of the C-60 molecules together with the inhomogeneity of the conduction electron distribution at the molecular level. In particular, the changes observed in the photoemission spectra near the Fermi level, are consistent with the behavior predicted for strongly correlated metallic systems in presence of disorder. Also in the case of K4C60 we observe anomalies in the temperature dependence of core levels photoemission data. These anomalies can be explained with the freezing of the molecular motions at low temperature and point to large thermal fluctuations of the alkali atoms around their equilibrium position at high temperature. (C) 2000 American Institute of Physics. [S0021-9606(00)71242-6].