Observation of Wannier-Stark localization at the surface of BaTiO\(_3\) films by photoemission

Observation of Bloch oscillations and Wannier-Stark localization of charge carriers is typically impossible in single-crystals, because an electric field higher than the breakdown voltage is required. In BaTiO\(_3\) however, high intrinsic electric fields are present due to its ferroelectric properties. With angle-resolved photoemission we directly probe the Wannier-Stark localized surface states of the BaTiO\(_3\) film-vacuum interface and show that this effect extends to thin SrTiO\(_3\) overlayers. The electrons are found to be localized along the in-plane polarization direction of the BaTiO\(_3\) film.