Unconventional isotope effects in superconducting fullerides

Although widely accepted, the phonon-mediated, BCS-like theory of superconducting A₃C₆₀ fullerides (A=K, Rb, Cs) cannot reproduce correctly all their parameters, even in its strong-coupling, Migdal-Eliashberg limit. The fundamental difficulty, ascribed to intrinsically close phonon and electron energy scales (respectively at 0.2 and 0.25 eV), has been overcome by dynamical mean-field theories (DMFT), which, unlike ME, consider electron-phonon and electron-electron interactions on an equal footing. The unconventional phenomena predicted in the new framework include, among others, isotope effects on spin susceptibility, totally absent from standard theories. We have tested these predictions, finding a significant dependence on the isotopic mass in both T_c and, more importantly, in the normal-state Pauli susceptibility χ_P. The comparative measurement of χ_P in two different K₃C₆₀ samples (85%¹³C-enriched vs. natural abundance), both by SQUID magnetometry as well as by ¹³C NMR Knight shift, definitely confirms the presence of an isotope effect on susceptibility, although a quantitative agreement with theory is still missing.