Temperature dependence of the magnetic field penetration depth in Rb3C60 measured by ac susceptibility

The temperature dependence of the magnetic field penetration depth λ(T) is determined down to 0.2T_c by the ac-susceptibility measurements on the Rb₃C₆₀ fine powder with small, about 1 K, variation in the grains' superconducting transition temperatures. The λ(T) temperature dependence is exponential at low temperatures, which allows us to determine the energy gap Δ₀=1.7k_BT_c. Deviations of the experimental dependence λ(T) from BCS are small and can be explained by strong-coupling theory.