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 superconductors 4. The potassium-doped compound 3 K3C60 has a relatively high transition temperature (T(c) = 19.3 K), a very high upper critical field (H(c2)(T --> 0) almost-equal-to 50T) and a short superconducting coherence length 5 (xi = 26 angstrom), in common with the copper oxide superconductors. Here we report muon-spin-relaxation measurements of the magnetic-field penetration depth-lambda in K3C60. The temperature dependence of lambda 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-lambda(T --> 0) is about 4,800 angstrom, which implies a ratio of superconducting carrier density to effective mass to be n(s)/(m*/m(e)) = 1.2 x 10(20) cm-3 if one assumes the 'clean limit'. Combining this result with the value of xi, 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.