System dependence of the magnetic-field penetration depth in C 60 superconductors

Abstract

We have performed muon spin relaxation (μSR) measurements of the magnetic field penetration depth λ in Rb 3C 60, K 3C 60, and Na 2CsC 60. 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 ∗ .