Nonadiabatic Pauli susceptibility in fullerene compounds

The Pauli paramagnetic susceptibility ? is unaffected by the electron-phonon interaction in the Migdal- Eliashberg context. The adiabatic assumption of Migdal's theorem, however, is of questionable validity when applied to very narrow-band systems as the fullerene compounds. In these materials therefore the nonadiabatic effects are in principle relevant and quantities such as ? could be seriously affected by the electron-phonon interaction. In this paper we investigate the Pauli spin susceptibility in the nonadiabatic regime by following a conserving approach based on Ward's identity. We find that a sizable renormalization of ? due to electron- phonon coupling appears when nonadiabatic effects are taken into account. The intrinsic dependence of ? on the electron-phonon interaction gives rise to a finite and negative isotope effect which could be experimentally detected in fullerides. In addition, we find an enhancement of the spin susceptibility with temperature increas- ing, in agreement with the temperature dependence of ? observed in fullerene compounds. The role of elec- tronic correlation is also discussed.