Synchrotron radiation angle-resolved photoemission study of the orientational phase transition for K3C60

Synchrotron radiation photoemission spectra were measured along the [111] direction of K3C60 single crystal film at 190K, 200K and 300K. Conduction band dispersions can be observed at 190K, and the band structure was nearly the same as that at 150K corresponding to the one-dimensional-disordered orientational structure (IDDS) of K3C60. When the sample temperature increased to 220K, most subpeaks of the conduction band vanished and the dispersions could not be observed. This result coincides with the existence of an orientational phase transition occurred at 200K. The fcc nearest-neighbor antiferromagetic Ising model was found to be suitable to analyz the temperature dependence of the band structure. The phase transition is due to the one-dimensional-disordered orientational structure at lower temperatures turned to be the mixture of BDS domains and disordered molecules at higher temperatures. The number of disordered molecules was deduced to be about 40 percent of the total molecular number at 220K. At room temperature, the electronic structure was drastically different from that at low temperatures because of the dynamical disorder Of C-60 molecules.