System dependence of the magnetic-field penetration depth in C60 superconductors

We have performed muon spin relaxation (μSR) measurements of the magnetic field penetration depth λ in Rb3C60, K3C60, and Na2CsC60. In a plot of Tc versus the muon spin relaxation rate σ(T → 0) ∝ λ−2, we observe a monotonic variation, following a roughly linear relation Tc ∝ σ. Assuming a clean-limit situation, where λ−2 ∝ ns/m∗ (carrier density / effective mass), the μSR results imply a variation Tc ∝ ns/m∗, opposite to the variation expected in a simple band picture where a decreasing band width (increasing m∗) with increasing lattice constant (increasing Tc) would have reduced ns/m∗.