Through ${}^{13}\mathrm{C}$ NMR spin lattice relaxation ( $T_1$) measurements in cubic ${\mathrm{Na}}_2{\mathrm{C}}_{60}$, we detect a gap in its electronic excitations, similar to that observed in tetragonal $A_4{\mathrm{C}}_{60}$. This establishes that Jahn-Teller distortions (JTD) and strong electronic correlations must be considered to understand the behavior of even electron systems, regardless of the structure. Furthermore, in metallic ${\mathrm{Na}}_2{\mathrm{CsC}}_{60}$, a similar contribution to $T_1$ is also detected for ${}^{13}\mathrm{C}$ and ${}^{133}\mathrm{Cs}$ NMR, implying the occurrence of excitations typical of JT distorted ${\mathrm{C}}_{60}^{2-}$ (or equivalently ${\mathrm{C}}_{60}^{4-}$). This supports the idea that dynamic JTD can induce attractive electronic interactions in odd electron systems.

• Received 7 December 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.4680