Theory of domain structure in electron-doped ferroelectric thin films*

To reduce the energy cost of strong depolarizing fields, insulating ferroelectric often films break up into nanoscale ``Kittel'' domains of opposite polarization that are separated by uncharged 180^? domain walls. Here, I report on recent self-consistent solutions of coupled Landau-Ginzburg-Devonshire and Schrödinger equations for an electron-doped ferroelectric thin film. The model is based on LaAlO3/SrTiO3 interfaces in which the SrTiO3 substrate is made ferroelectric by cation substitution or strain. Electron doping destabilizes the Kittel domains. As the two-dimensional electron density increases, there is a smooth crossover to a zigzag domain wall configuration. The domain wall is positively charged, but is compensated by the electron gas, which attaches itself to the domain wall and screens depolarizing fields. The domain wall approaches a flat head-to-head configuration in the limit of perfect screening. The polarization profile may be manipulated by an external bias voltage and the electron gas may be switched between surfaces of the ferroelectric film.