  Conduction Electron Spin Resonance (CESR) was measured on a thick slab of
CaC6 in the normal and superconducting state. A surprising increase of the CESR
intensity below Tc can not be explained by the theoretically predicted change
in spin susceptibility. It is interpreted as a vortex enhanced increase of the
effective skin depth. Non-linear microwave absorption measurements in the
superconducting state describe CaC6 as an anisotropic BCS superconductor. The
study of the spin dynamics in the superconducting state and the discovery of
the vortex enhanced increase of the skin depth poses a challenge to theory to
provide a comprehensive description of the observed phenomena. CESR data in the
normal state characterize CaC6 as a three-dimensional (3D) metal. The analysis
suggests that the scattering of conduction electrons is dominated by impurities
and supports the description of superconductivity in the dirty limit.
