Role of dynamic Jahn-Teller distortions in Na2C60 and Na2CsC60 studied by NMR.

Through 13C NMR spin lattice relaxation ( T1) measurements in cubic Na2C60, we detect a gap in its electronic excitations, similar to that observed in tetragonal A4C60. 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 Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR, implying the occurrence of excitations typical of JT distorted C( 2-)60 (or equivalently C( 4-)60). This supports the idea that dynamic JTD can induce attractive electronic interactions in odd electron systems.