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

TL;DR

This paper demonstrates that orbital disproportionation driven by Jahn-Teller instability is a universal feature across alkali-doped fullerides, regardless of their electronic correlation strength, revealing a common underlying mechanism.

Contribution

The study provides the first comprehensive analysis showing that Jahn-Teller instability and orbital disproportionation occur in A$_4$C$_{60}$ fullerides, contrasting with their behavior in A$_3$C$_{60}$.

Findings

Jahn-Teller instability is present in A$_4$C$_{60}$.

A$_4$C$_{60}$ exhibits an uncorrelated band-insulating state.

Orbital disproportionation is a universal feature of fullerides.

Abstract

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$_{60}$. 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. 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.