A potassium doped μm thick C60 film: direct observations of threshold doping and phases evolution

Abstract

The evolution of the valence band spectra of K doped μm thick C60 film is presented. The data shows directly that there exists a threshold concentration before the first stable phase, K3C60, starts to appear. Below the threshold concentration, no Fermi emission is seen and the K atoms distributed randomly in the bulk retain their 4s electron whose binding energy is at 0.9eV below the Fermi level. This finding differs from all the previous experiments, which reported the immediate appearance of the K3C60 phase with a distinct Fermi cutoff upon K doping. A second stable phase, K6C60, evolves upon further doping with two characteristic states at 0.5 and 2.0eV and coexists with the K3C60 phase. Our work also demonstrates for the first time that the superfulleride phase which arises from the filling of the t1g band can be obtained at high doping. This phase is found on the surface.