Electron spin dynamics of the superconductor CaC 6 probed by ESR

Conduction electron spin resonance (CESR) was measured on a thick slab of ${\mathrm{CaC}}_{6}$ in the normal and superconducting state. Nonlinear microwave absorption measurements in the superconducting state describe ${\mathrm{CaC}}_{6}$ as an anisotropic BCS superconductor. CESR data in the normal state characterize ${\mathrm{CaC}}_{6}$ as a three-dimensional metal. The analysis suggests that the scattering of conduction electrons is dominated by impurities and supports the description of superconductivity in the dirty limit. A surprising increase of the CESR intensity below ${T}_{c}$ cannot be explained by the theoretically predicted change in spin susceptibility. It is interpreted as a vortex enhanced increase of the effective skin depth. 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.