Fulleride superconductivity tuned by elastic strain due to cation compositional disorder

Dynamical fluctuations of the elastic strain in strongly correlated systems are known to affect the onset of metal-to-insulator or superconducting transitions. Here we report their effect on the properties of a family of bandwidth-controlled alkali-intercalated fullerene superconductors. We introduce elastic strain through static local structural disorder in a systematic and controllable way in the fcc-structured K$_x$Cs$_{3-x}$C$_{60}$ (with potassium content, 0.22 $leq$ x$_K$ $leq$ 2) series of compositions by utilizing the difference in size between the K$^+$ and Cs$^+$ co-dopants. The occurrence of the crossover from the Mott–Jahn–Teller insulating (MJTI) state into the strongly correlated Jahn–Teller metal (JTM) on cooling is evidenced for the compositions with x$_K$ < 1.28 by both synchrotron X-ray powder diffraction (SXRPD) – anomalous reduction of the unit cell volume – and $^{133}$Cs NMR spectroscopy – sudden suppression in the $^{133}$Cs spin-lattice relaxation rates. The emerging superconducting state with a maximum critical temperature, T$_c$ = 30.9 K shows a characteristic dome-like dependence on the unit-cell volume or equivalently, on the ratio between the on-site Coulomb repulsion, U, and the bandwidth, W. However, compared to the parent Cs$_3$C$_{60}$ composition in which cation disorder effects are completely absent, the maximum T$_c$ is lower by ∼12%. The reduction in T$_c$ displays a linear dependence on the variance of the tetrahedral-site cation size, $sigma_T^2$, thus establishing a clear link between structural-disorder–induced attenuation of critical elastic strain fluctuations and the electronic ground state.