Rosseinsky et al. reported 'Superconductivity at 28 K in RbxC60' in Physical Review Letters in 1991.
The potassium analogue K₃C₆₀, reported the same year by Hebard, Rosseinsky, Haddon et al. in Nature, superconducts at 18 K and was then the highest transition temperature for a molecular superconductor.
In M₃C₆₀ the alkali ions sit in the interstitial holes of the C₆₀ lattice and Tc grows with unit-cell volume; Cs₃C₆₀ reaches about 38–40 K but only under applied pressure.