Electron injection into superconducting trivalent fullerides close to the Mott transition boundary

To date, superconductivity in fullerides has been almost exclusively tuned by (chemical or physical) pressure control of the conduction bandwidth, W at half filling. This contrasts sharply with the extensive control of the superconducting transition temperature, [Formula: see text] in atom-based superconductors such as the cuprates and iron pnictides and chalcogenides via changes in valence (bandfilling). Here, we investigate the effect of doping away from the exactly half-filled [Formula: see text] level in quaternary face-centered-cubic (fcc) — structured fulleride solids with nominal composition ([Formula: see text]Csx)[Formula: see text][Formula: see text] ([Formula: see text]), in which divalent [Formula: see text] ions partially replace monovalent alkali [Formula: see text]/[Formula: see text] ions. The resulting charged-modified fullerides in which the [Formula: see text] bandwidth is also varied with changing x show a dome-shaped dependence of [Formula: see text] on interfullerene separation in analogy with their half-filled antecedents. However, following electron injection beyond half-filling, the superconductivity dome is found to shift towards shorter interfullerene separations, i.e. towards increased conduction bandwidths.