Visualizing molecular orientational ordering and electronic structure in CsnC60 fulleride films

TL;DR

This study uses cryogenic STM to explore how cesium doping affects the molecular orientation and electronic properties of C60 fulleride films, revealing universal Jahn-Teller effects and doping-dependent electronic structures.

Contribution

It provides detailed insights into the structural and electronic tunability of Cs-doped C60 films, clarifying the interplay between doping, structure, and electronic states.

Findings

Large tunability of C60 orientational ordering with doping

Robust energy gaps indicating Jahn-Teller instability

Fermi level shifts with doping, revealing electronic state evolution

Abstract

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 sub-molecular structural and electronic properties of expanded fullerene C60 films with various cesium (Cs) doping. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of C60 anions, 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 among the doping, structural and electronic structures in CsnC60 fullerides.