Large Ca isotope effect in the CaC6 superconductor

We have measured the Ca isotope effect coefficient, alpha(Ca), in the newly discovered superconductor CaC6 and find a value of 0.53(2). This result shows that the superconductivity is dominated by coupling of the electrons by Ca phonon modes. The C phonons contribute very little, assuming that this material is a conventional electron-phonon coupled superconductor. Thus, in contrast to another layered material MgB2, where high-energy phonons in the B layers are responsible for the superconductivity, in layered CaC6 the phonons responsible for superconductivity are primarily low-energy modes of the intercalated Ca.