Narrow Results

A theoretical model based upon a one-dimensional anharmonic oscillator is proposed in order to describe strong interactions in molecular solids. Vibrational energy levels are studied in terms of the associated vibrational quantum number; in particular, classical limit is discussed. Kinetic energy corresponding to a typical collision process is calculated. In addition, Morse-type potential interaction is found to be an approximation to our model.

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, $T_c$ 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 ${\text{C}}_{60}^{3-}$ level in quaternary face-centered-cubic (fcc) — structured fulleride solids with nominal composition (${\text{Rb}}_{2.5-x}$Cs_x)${\text{Ba}}_{0.5}$${\text{C}}_{60}$ ($0\le x\le 2.5$), in which divalent ${\text{Ba}}^{2+}$ ions partially replace monovalent alkali ${\text{Rb}}^{+}$/${\text{Cs}}^{+}$ ions. The resulting charged-modified fullerides in which the $t_{1u}$ bandwidth is also varied with changing x show a dome-shaped dependence of $T_c$ 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.