Experimental evidence of s-wave superconductivity in bulk CaC${}_6$

Abstract

The temperature dependence of the in-plane magnetic penetration depth, ${\lambda }_{ab}(T)$, has been measured in a c-axis oriented polycrystalline CaC${}_6$ 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 ${\lambda }_{ab}(0)=(720\pm 80)$ Angstroem and $\Delta (0)=(1.79\pm 0.08)$ meV. The ratio $2\Delta (0)/k_{{}_B}{T}_c=(3.6\pm 0.2)$ gives indication for a conventional weakly coupled superconductor.

To appear in Phys. Rev. Lett