The discovery1-3 of superconductivity in C60 doped with the alkali metals potassium and rubidium has introduced a new family of three-dimensional molecular superconductors4. The potassium-doped compound3 K3C60 has a relatively high transition temperature (Tc = 19.3K), a very high upper critical field (Hc2 (T⇒ 0)≈50T) and a short superconducting coherence length5 (£ = 26 Å), in common with the copper oxide superconductors. Here we report muon-spin-relaxation measurements of the magnetic-field penetration depth λ in K3C60. The temperature dependence of λ and of the muon spin relaxation rate indicate that the superconducting energy gap is isotropic, without nodes or zero points. The low-temperature penetration depth λ (T⇒ 0) is about 4,800 Å, which implies a ratio of superconducting carrier density to effective mass to be n2/(m*/me) = 1.2 x 1020 cm-3 if one assumes the 'clean limit'. Combining this result with the value of ε, we estimate the Fermi temperature TF = 470 K. In the relationship between TF and Tc, K3C60 conforms to the trend exhibited by 'exotic' superconductors6,7 such as the Chevrel phase compounds, the copper oxides and the organic BEDT systems.
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