Magnetic-field penetration depth in K₃C₆₀ measured by muon spin relaxation

The discovery^1-3 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 (T_c = 19.3K), a very high upper critical field (H_c2 (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*/m_e) = 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 T_F = 470 K. In the relationship between T_F and T_c, K₃C₆₀ conforms to the trend exhibited by 'exotic' superconductors^6,7 such as the Chevrel phase compounds, the copper oxides and the organic BEDT systems.