Temperature dependence of the magnetic field penetration depth inRb3C60measured by ac susceptibility

The temperature dependence of the magnetic field penetration depth λ(T) is determined down to 0.2${\mathit{T}}_{\mathit{c}}$ by the ac-susceptibility measurements on the ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ 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 ${\mathrm{\Delta }}_0$=1.7${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$. Deviations of the experimental dependence λ(T) from BCS are small and can be explained by strong-coupling theory.