Fermi-surface-dependent superconducting gap in C6Ca

The discovery of superconductivity in C6Yb and C6Ca (ref. 1) has activated fierce debates on whether it is described within the conventional Bardeen-Cooper-Schrieffer scheme or some other exotic mechanisms are involved, because the superconducting transition temperature (T-c) is significantly higher than that of the alkali-metal graphite intercalation compounds intensively studied in the 1980s (refs 2-4). The key to understand the mechanism of superconductivity lies in the superconducting energy gap associated with the formation of superconducting pairs. Here, we report the first direct observation of a superconducting gap in C6Ca by high-resolution angle-resolved photoemission spectroscopy. We found that the superconducting gap of 1.8-2.0meV opens on the intercalant Fermi surface, whereas the gap is very small or absent on the Fermi surface derived from the pi* band of graphene layers. These experimental results unambiguously establish that the interlayer band has an essential role for the high-T-c superconductivity in C6Ca.