Among the superconducting graphite intercalation compounds,  exhibits the highest critical temperature T = 11.5 K, which further increases under applied pressure (15.1 K at 8 GPa). Superconductivity in these compounds is thought to be related to the role of an interlayer state, a free electron like band lying roughly  above the Fermi level; superconductivity only occurs if the interlayer state is occupied. Analysis of pure  using a high quality ultraviolet light revealed to conduct angle-resolved photoemission spectroscopy measurements. The opening of a superconducting gap in the π* band revealed a substantial contribution to the total electron–phonon-coupling strength from the π*-interlayer interband interaction.