Polar surface engineering in ultrathinMgO(111)∕Ag(111): Possibility of a metal-insulator transition and magnetism

Abstract

A recent report [Kiguchi et al., Phys. Rev. B 68, 115402 (2003)] that the (111) surface of $5\mathrm{MgO}$ layers grown epitaxially on $\mathrm{Ag}\left(111\right)$ becomes metallic raises a question of what will happen when we have fewer $\mathrm{MgO}$ layers. Here we have revealed, first experimentally with electron energy-loss spectroscopy, that $\mathrm{MgO}\left(111\right)$ remains metallic even when one-layer thick. We have confirmed this theoretically with the density functional theory, where metallization turns out to depend on the nature of the substrate. We finally predict, with a spin-density functional calculation, that a magnetic polar surface can be possible.