Dimensional evolution of the electronic and structural properties of K$_{x}$C$_{60}$ multilayers studied by Scanning Tunneling Microscopy

We investigate the effect of dimensionality on the properties of potassium doped C$_{60}$ (K$_{x}$C$_{60})$ by studying thin films with precisely controlled doping levels and layer structures using scanning tunneling microscopy and spectroscopy. We observe systematic variation in spatial and electronic structure as the films change from the 2D to the quasi-3D regime. In metallic K$_{3}$C$_{60}$, the large electronic density of states at the Fermi level (E$_{F})$ is seen to split, with a small gap opening at E$_{F}$. In the Jahn-Teller-induced K$_{4}$C$_{60}$ insulator, the energy gap around E$_{F}$ increases monotonically with increased film thickness. In K$_{5}$C$_{60}$, the spectra change from a re-entrant metal in the first layer to an insulator in the third layer. These trends can be explained by considering the increase of Coulomb repulsion in multilayers as screening from the metal substrate is reduced. These results highlight the role of strong electron correlation and dimensionality in determining the properties of doped fullerides.