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 T_c versus the muon spin relaxation rate σ(T → 0) ∝ λ^-2, we observe a monotonic variation, following a roughly linear relation T_c ∝ σ. Assuming a clean-limit situation, where λ^-2 ∝ n_s/m^* (carrier density / effective mass), the μSR results imply a variation T_c ∝ n_s/m^*, opposite to the variation expected in a simple band picture where a decreasing band width (increasing m^*) with increasing lattice constant (increasing T_c) would have reduced n_s/m^*.