Surface electron states on the quasi-two-dimensional excess-electron compoundsCa2NandY2C

Compounds having excess electrons from the formal valence viewpoint (electrides) are a new class of materials, which often take low-dimensional structures. We studied the (001) surface electronic structures of quasi-two-dimensional electrides ${\mathrm{Ca}}_{2}\mathrm{N}$ and ${\mathrm{Y}}_{2}\mathrm{C}$ by density functional theory using a slab model. Both materials were found to have a clean surface state well separated in energy from the bulk states. Furthermore, this state virtually floats above the surface and may be considered to be a hallmark of two-dimensional electrides. For ${\mathrm{Ca}}_{2}\mathrm{N}$, a tight-binding model in the Wannier representation was derived and analyzed, from which we concluded that the surface state, described by extra-surface $s$-like orbitals, is a Tamm state originating from an abrupt increase in potential energy at the surface.