Jahn–Teller orbital glass state in the expanded fcc Cs3C60 fulleride

Abstract

The most expanded fcc-structured alkali fulleride, Cs₃C₆₀, is a Mott insulator at ambient pressure because of the weak overlap between the frontier t_1u molecular orbitals of the C₆₀³⁻ anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical temperature, T_c of 35 K upon compression. The effect of the localised t_1u³ electronic configuration on the properties of the material is not well-understood. Here we study the relationship between the intrinsic crystallographic C₆₀³⁻ orientational disorder and the molecular Jahn–Teller (JT) effect dynamics in the Mott insulating state. The high-resolution ¹³C magic-angle-spinning (MAS) NMR spectrum at room temperature comprises three peaks in the intensity ratio 1 : 2 : 2 consistent with the presence of three crystallographically-inequivalent carbon sites in the fcc unit cell and revealing that the JT-effect dynamics are fast on the NMR time-scale of 10⁻⁵ s despite the presence of the frozen-in C₆₀³⁻ merohedral disorder disclosed by the ¹³³Cs MAS NMR fine splitting of the tetrahedral and octahedral ¹³³Cs resonances. Cooling to sub-liquid-nitrogen temperatures leads to severe broadening of both the ¹³C and ¹³³Cs MAS NMR multiplets, which provides the signature of an increased number of inequivalent ¹³C and ¹³³Cs sites. This is attributed to the freezing out of the C₆₀³⁻ JT dynamics and the development of a t_1u electronic orbital glass state guided by the merohedral disorder of the fcc structure. The observation of the dynamic and static JT effect in the Mott insulating state of the metrically cubic but merohedrally disordered Cs₃C₆₀ fulleride in different temperature ranges reveals the intimate relation between charge localization, magnetic ground state, lifting of electronic degeneracy, and orientational disorder in these strongly-correlated systems.