THE ISOTOPE EFFECT IN THE DOPED-FULLERENE SUPERCONDUCTORS

Measurements [C.C. Chen and C.M. Lieber, Science 259, 655 (1993)] of t he isotope effect in Rb3C60 superconductors exhibit a larger shift in the T-c of a Rb-3(C-12(60))(0.5)(C-13(60))(0.5) superconductor than ex pected on the basis of existing theories. It is proposed that the tran sfer of inter-molecule H-g vibrations (vibron) from one C-60 cluster t o another in a single cluster type compound Rb-3(C-13(0.5))(60), is di fferent from that in the two cluster type compound, Rb-3(C-12(60))(0.5 )(C-13(60))(0.5). This difference leads to a reduction of the electron -vibron coupling constant for the latter superconductor. The T(c)s of mixed Rb-3(C-12(60))(1-c)(C-13(60))(1-c)(C-13(60))c superconductors ar e calculated for c < 0.5. Copyright (C) 1996 Elsevier Science Ltd