ISOTOPE EFFECT ON 82-PERCENT C-13 SUBSTITUTED K3C60

In order to test the isotope effect in the superconducting doped fullerenes, we made the K3C60 compound in which the C60 molecules are enriched in C-13. The C-13 concentration in C60 powder, as determined by mass spectrometry, was found to be 82%. A standard sample (K3C60)-C-12 was prepared in the same conditions. The superconducting transition was measured on both samples by an ac susceptibility technique. The onset transition temperature observed at T(c) = 19.6+/-0.1 K for the standard sample is lowered by 1.8K in the C-13 compound which leads to alpha almost-equal-to 1.45+/-0.3 in the usual BCS formula T(c) approximately M(-alpha). From C-13 NMR measurements of the relaxation time T1 on these two compounds, the isotopic shift leads to alpha - 1.1+/-0.3 in a magnetic field of 9.4T, which remains strongly higher than the theoretical value alpha = 0.5.