A superconductivity dome is created by the electronic structure of the molecular building block of an unconventional superconductor.

Controlled access to the border of the Mott insulating state by variation of control parameters offers exotic electronic states such as anomalous and possibly high-transition-temperature (Tc) superconductivity. The alkali-doped fullerides show a transition from a Mott insulator to a superconductor for the first time in three-dimensional materials, but the impact of dimensionality and electron correlation on superconducting properties has remained unclear. Here we show that, near the Mott insulating phase, the upper critical field Hc2 of the fulleride superconductors reaches values as high as ∼90 T—the highest among cubic crystals. This is accompanied by a crossover from weak- to strong-coupling superconductivity and appears upon entering the metallic state with the dynamical Jahn–Teller effect as the Mott transition is approached. These results suggest that the cooperative interplay between molecular electronic structure and strong electron correlations plays a key role in realizing robust superconductivity with high-Tc and high-Hc2.

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The ternary fulleride, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text], is the most expanded member of the family of face-centered cubic (fcc) structured superconducting fullerides ever accessed with superconductivity surviving at ambient pressure closest to the Mott insulator boundary. Here, we study the evolution of its structural and electronic properties with temperature. At ambient temperature, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text] lies in the Mott–Jahn–Teller (MJT) insulating part of the phase diagram. High-resolution synchrotron X-ray diffraction shows that its structure remains strictly cubic at all temperatures, but the transition to the metallic state at [Formula: see text] 50 K — evident in the evolution of the magnetic susceptibility with temperature — is accompanied by a lattice collapse, [Formula: see text]V/V0 of [Formula: see text]. Bulk superconductivity then emerges on further cooling with a T[Formula: see text] of 25.9 K. The results permit the extension of the electronic phase diagram of A3C[Formula: see text] fullerides as close as possible to the metal–insulator (M–I) crossover.

A solution chemistry synthetic route yields Cs3C60 with a face-centred cubic structure. The described method uses well-established Schlenk techniques and THF as a solvent. The controlled addition of an organo-metallic salt reducing agent prevents the formation of C604− salts. The final product can be precipitated from the solution using hexane as an anti-solvent.