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 C₆₀ (with potassium content, 0.22 ≤ x _K ≤ 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 ¹³³Cs NMR spectroscopy - sudden suppression in the ¹³³Cs 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 Cs₃C₆₀ 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, σ _T ², thus establishing a clear link between structural-disorder-induced attenuation of critical elastic strain fluctuations and the electronic ground state.