The temperature dependence of the in-plane magnetic penetration depth,
λab(T), has been measured in a c-axis oriented polycrystalline
CaC6 bulk sample using a high-resolution mutual inductance technique. A
clear exponential behavior of λab(T) has been observed at low
temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the
standard BCS theory yields λab(0)=(720±80) Angstroem and
Δ(0)=(1.79±0.08) meV. The ratio 2Δ(0)/kBTc=(3.6±0.2)
gives indication for a conventional weakly coupled superconductor.Comment: To appear in Phys. Rev. Let