Orbital disproportionation of electronic density is a universal feature of alkali-doped fullerides

Alkali-doped fullerides A$_n$C$_{60}$ show a remarkably wide range of electronic phases in function of A = Li, Na, K, Rb, Cs and the degree of doping, $n=$ 1-5. While the presence of strong electron correlations is well established, recent investigations give also evidence for dynamical Jahn-Teller instability in the insulating and the metallic phase of A$_3$C$_{60}$ [1, 2]. To reveal the interplay of these interactions in fullerides with even $n$, we address the electronic phase of A$_4$C$_{60}$ with accurate many-body calculations within a realistic electronic model including all basic interactions extracted from first principles [3]. We find that the Jahn-Teller instability is always realized in these materials too. More remarkably, in sharp contrast to strongly correlated A$_3$C$_{60}$, A$_4$C$_{60}$ displays uncorrelated band-insulating state despite pretty similar interactions present in both fullerides. Our results show that the Jahn-Teller instability and the accompanying orbital disproportionation of electronic density in the degenerate LUMO band is a universal feature of fullerides. [1] N. I. and L. F. C., Phys. Rev. Lett. 111, 056401 (2013). [2] N. I. and L. F. C., Phys. Rev. B 91, 035109 (2015). [3] N. I. and L. F. C., Nat. Commun. 7, 13093 (2016).