Abstract :
Synchrotron radiation angle resolved photoemission spectra (PES) were measured on the (111) surface of K3C60 single crystal film at three different temperatures, namely, 190, 220 and 300 K. Dispersions for the subbands of the conduction band along the [111] direction could be observed clearly at 190 K, and the band structure was nearly the same as that at 150 K corresponding to the one-dimensional-disordered orientational structure (1DDS) of K3C60. Most subpeaks smeared and the dispersions could no longer be observed when the sample temperature was raised to 220 K. The PES at room temperature differed remarkably from that at lower temperatures. The change of spectra with temperature is consistent with an orientational phase transition at 200 K, and can be explained by the theory of the fcc antiferromagnetic Ising model, assuming that the 1DD structure at lower temperatures converts to be a mixture of bi-directional structure domains and disordered molecules at higher temperatures.
