NMR Studies of Insulating, Metallic, and Superconducting Fullerides: Importance of Correlations and Jahn–Teller Distortions

Abstract

Many unique properties of C₆₀-based materials stem from the fact that purely molecular properties survive in the solid state, adding a level of complexity to their description. As the molecules are only weakly bonded by Van der Waals interactions, the overlap between their electronic wave functions is small and narrow bands are formed. Because of the narrowness of these bands (typically 0.5 eV), these materials are expected to be strongly correlated systems. The Coulomb repulsion between electrons is indeed estimated to be of the order of 1 eV, much larger than the bandwidth, so that the equilibrium between localizing electrons to avoid the energy cost of a double occupancy of one site and delocalizing electrons to gain kinetic energy is uncertain. These strong correlations probably play a role in the various electronic properties observed as a function of the filling of the lowest unoccupied molecular orbital by electrons from alkali ions, such as superconductivity in A₃C₆₀ (and only A₃C₆₀), the half-filled case, but also insulating phases for A₄C₆₀, which cannot be explained by a simple band picture. Equivalently, the strong Coulomb repulsion will tend to increase the average time spent by one electron on a given molecule, because hopping to the next ball might not be favorable, and during that time, “molecular physics” is relevant and will therefore be intimately mixed into the electronic properties of the solid. This suggests that molecular features that would be typically washed out in simple metals may survive here.

One interesting example is the Jahn–Teller effect, which can lead to a spontaneous distortion of the C₆₀ molecule. It turns out to be particularly important in fullerides because of the strong electron–phonon coupling and the icosahedral symmetry of the C₆₀ molecule. This high symmetry leads to large degeneracies in electronic and phonon levels which are at the origin of the Jahn–Teller effect. Although it is a purely molecular property, we will show that it has important consequences in the solid state, as it is very sensitive to the number of electrons occupying a C₆₀ ball. This number will change with time if hopping takes place, leading to an interesting competition between delocalization of electrons and stabilization of Jahn–Teller distortion. Unfortunately, the distortions themselves are very small and possibly dynamic, so that they are hardly directly detectable, leading to a lack of experimental basis in the analysis of this effect.

The purpose of this chapter is to introduce the basic notions related to the Jahn–Teller effect in fullerides and to examine how it affects the properties of these solids, mainly in the case of alkali-doped fullerides. A brief survey of the theoretical notions important to understand this effect will be given in the first part. As illustration, we will present examples of a few cases where clear manifestations of Jahn–Teller-related effects have been observed, mostly chosen in diluted salts where the molecules can be considered as isolated. We will then focus our attention on alkali-doped fullerides, and particularly the relationship between metallic and insulating phases. We will argue that the different stability of the Jahn–Teller distortion for an odd or even charge of a C₆₀ ball considerably affects the behavior of these systems. This will be developed through the study of three different situations encountered in these systems. The second part deals with compounds where static distortions are created as defects within a metallic phase. We will show that for these distortions the electrons are localized in pairs, even though the stoichiometry corresponds to an odd number of electrons per C₆₀. In the third part, we will argue that the insulating nature of most even stoichiometries can be understood by strong electronic correlations enhanced by the Jahn–Teller distortion. In the fourth part, we will generalize the formation of electronic pairs via Jahn–Teller distortions in metals to the dynamic case. Finally, we will conclude that altogether, this set of data strongly supports the idea that Jahn–Teller-related interactions are a major source of electronic correlations for these solids.