Visualizing molecular orientational ordering and electronic structure in Csn C60 fulleride films

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature. Here we report a cryogenic scanning tunneling microscopy study of the submolecular structural and electronic properties of expanded fullerene C60n-films with various cesium dopings. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of C60n- A nions, yet the tunneling conductance spectra are all robustly characteristic of energy gaps, hallmarks of Jahn-Teller instability and electronic correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of n=1, 2, 3, and 4, apart from which it moves toward band edges with concomitant electronic states within the energy gap. Our findings establish the universality of Jahn-Teller instability and clarify the relationship between the doping and structural and electronic properties in CsnC60 fullerides.