Electronic structure and superconducting gap of silicon clathrateBa8Si46studied with ultrahigh-resolution photoemission spectroscopy

We study the electronic structure and superconducting transition of silicon clathrate ${\mathrm{Ba}}_8{\mathrm{Si}}_{46}{(T}_c=8\mathrm{}\mathrm{K})$ using photoemission spectroscopy. We observe a narrow band at the Fermi level ${(E}_F),$ whose width (∼0.3 eV) is substantially smaller than that of band structure calculations (∼1.5 eV). Ultrahigh-resolution measurements show a superconducting gap at 5.4 K $[2\Delta {\mathrm{}\left(0\right)/k}_B{T}_c=3.51\mathrm{}].$ Fine structures associated with phonons are observed within 70 meV of $E_F.$ These results characterize ${\mathrm{Ba}}_8{\mathrm{Si}}_{46}$ as a weak-coupling superconductor most probably driven by phonon.