Surface Conduction on Insulating BaTiO₃ Crystal Suggesting an Intrinsic Surface Electron Layer

We report conduction measurements on the clean, free surface of transparent insulating BaTiO₃ single crystals in high vacuum. We find that the insulating BaTiO₃ crystals exhibit surface conductance that is dependent on the spontaneous polarization, whereas no conduction perpendicular to the surface is observed. The surface conduction shows semimetallic temperature dependence and persists down to at least 100 K. The observations suggest a two-dimensional electron on a clean, free ferroelectric surface that may be regarded as a ferroelectric metal. The results have important implications for understandings of the fundamental properties of ferroelectrics, the size effect, and ferroelectric devices.