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Role of dynamic Jahn-Teller distortions in Na_(2)C_(60) and Na_(2)CsC_(60) studied by NMR

Through C-13 NMR spin lattice relaxation (T_1) measurements in cubic Na_(2)C_(60), we detect a gap in its electronic excitations, similar to that observed in tetragonal A_(4)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 Na_(2)CsC_(60), a similar contribution to T_1 is also detected for C-13 and Cs-133 NMR, implying the occurrence of excitations typical of JT distorted C_(60)^(2-) (or equivalently C_(60)^(4-)). This supports the idea that dynamic JTD can induce attractive electronic interactions in odd electron systems.

Na_(2)C_(60); NMR; Gap in electronic spectrum; Jahn-Teller distortions; Electronic correlations

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