Experimental evidence of s-wave superconductivity in bulk CaC6

The temperature dependence of the in-plane magnetic penetration depth, lambda(ab)(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 lambda(ab)(T) has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields lambda(ab)(0)=(720 +/- 80) A and Delta(0)=(1.79 +/- 0.08) meV. The ratio 2 Delta(0)/k(B)T(c)=(3.6 +/- 0.2) gives indication for a weakly coupled superconductor.