Isotope effect in superconducting fullerides: nonadiabatic contribution to electronic pairing and partial substitution anomaly

Summary form only given. The isotope effect is calculated in a simple BCS-type model in which purely electronic attraction on C/sub 60/ molecules is modulated by zero point intramolecular motion, as first proposed by Chakravarty et.al. We have found new terms increasing the magnitude of the isotope exponent, particularly the nonadiabatic contribution is found which is proportional to /spl omega//W (/spl omega/ being vibron intermolecular frequency and W the electronic intermolecular bandwidth) and is thus more sensitive to intermolecular electronic motion and through it to disorder. The case of isotopically disordered fullerides (in which the enhanced isotope effect has been found experimentally known as partial substitution anomaly) is considered qualitatively and the case of isotope effect in alkali earth metal doped fulleride superconductors is analyzed.