SYSTEM DEPENDENCE OF THE MAGNETIC-FIELD PENETRATION DEPTH IN C-60 SUPERCONDUCTORS

We have performed muon spin relaxation (mu SR) measurements of the mag netic field penetration depth lambda in Rb3C60, K3C60, and Na2CsC60. I n a plot of T-c versus the muon spin relaxation rate sigma(T-->0) prop ortional to lambda(-2), we observe a monotonic variation, following a roughly linear relation T-c proportional to sigma. Assuming a clean-li mit situation, where lambda(-2) proportional to n(s)/m (carrier densi ty / effective mass), the mu SR results imply a variation T-c proporti onal to n(s)/m, opposite to the variation expected in a simple band p icture where a decreasing band width(increasing m) with increasing la ttice constant (increasing T-c) would have reduced n(s)/m.