Ferroelectricity in crystals is associated with the displacement of ions or rotations of polar units. Here we consider the dipole created by donor doping ($D^+$) and the corresponding bound polaron ($e^-$).A dipole of 6.15 Debye is predicted, from Berry phase analysis, in the Ruddlesden-Popper phase of ${ \rm Sr_3Ti_2O_7}$. A characteristic double-well potential is formed, which persists for high doping densities. The effective Hubbard $U$ interaction can vary the defect state from metallic, a two-dimensional polaron, through to a zero-dimensional polaron. The ferroelectric-like behavior reported here is localized and distinct from conventional spontaneous lattice polarization.
Categories: cond-mat.mtrl-sci