Survival of the metallic state in a single-hole multiband p-orbital molecular system

Strong correlations and ferromagnetic Hund's coupling lead to diverse electronic phenomena in transition-metal oxides that sensitively depend on the d-orbital electron filling. Fullerides, their p-electron counterparts, exhibit effective antiferromagnetic Hund's coupling in a different energy range. At half-filling (n = 3, three electrons in triply degenerate orbitals), both d- and p-electron systems are Mott insulators due to strong correlations and Hund's coupling. Away from half-filling, in single-electron/hole (n = 1,5) d-orbital systems, Hund's coupling opposes the correlations, reducing the Mott gap and allowing survival of metallicity. Here we report a single-hole multiorbital correlated p-electron system, orthorhombic-structured Yb₂CsC₆₀ comprising pentavalent C₆₀^5- anions, which also exhibits a robust metallic state with no Mott transition, just like in the metastable single-electron cubic-structured CsC₆₀. We assert that particle-hole symmetry holds well in (n = 1,5) fullerides and that their p-electron-derived states are analogous to those in d-orbital solids, providing impetus for further study of these correlated systems.