Merohedral disorder and impurity impacts on superconductivity of fullerenes

Abstract

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb₃C₆₀ films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.