Dimensionality and bonding characters of Cu _2 Si self-assembled on Si(111)

The electron band structure of single-layer Cu _2 Si self-assembled on Si(111) has been investigated using angle-resolved photoemission spectroscopy. Cu _2 Si is a two-dimensional (2D) Dirac nodal-line semimetal that is protected by the in-plane mirror reflection symmetry. Indeed, the electron band structure of Cu _2 Si shows non-dispersive states as a function of photon energy, whereas the electron band structure from the substrate shows strong photon energy dependence, supporting the 2D nature of Cu _2 Si. Polarization dependence shows that the spectral intensity of the electron band structure of Cu _2 Si is strongly but not completely suppressed when measured using photons with in-plane polarization. These findings indicate that Cu _2 Si is deformed along the out-of-plane direction, so that the out-of-plane mirror symmetry becomes important in the topological properties of Cu _2 Si when it is placed on an Si(111) substrate.