Nonadiabatic Pauli susceptibility in fullerene compounds

The Pauli paramagnetic susceptibility chi 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 chi 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 chi due to electron-phonon coupling appears when nonadiabatic effects are taken into account. The intrinsic dependence of chi 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 increasing, in agreement with the temperature dependence of chi observed in fullerene compounds. The role of electronic correlation is also discussed.