Visualizing molecular orientational ordering and electronic structure in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cs</mml:mi><mml:mi>n</mml:mi></mml:msub><mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:mn>60</mml:mn></mml:msub></mml:mrow></mml:math> fulleride films
Sha Han, Mengxue Guan, Can‐Li Song, Yilin Wang, Mingqiang Ren, Sheng Meng, Xu-Cun Ma, Qi‐Kun Xue
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 submolecular structural and electronic properties of expanded fullerene ${\mathrm{C}}_{60}^{n\ensuremath{-}}$ films with various cesium dopings. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of ${\mathrm{C}}_{60}^{n\ensuremath{-}}$ 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 $n\phantom{\rule{4pt}{0ex}}=1$, 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 ${\mathrm{Cs}}_{n}{\mathrm{C}}_{60}$ fullerides.