Polar instability under electrostatic doping in tetragonal SnTiO3

Abstract

How to make the metallicity compatible with a polar structure for forming a ``polar metal'' has been an interesting and important topic since the polar structure was discovered in metallic $\mathrm{LiOs}{\mathrm{O}}_{3}$. Here, we present robust polar instability under electrostatic doping in tetragonal $\mathrm{SnTi}{\mathrm{O}}_{3}$ based on our first-principles calculations. The mechanism for polar distortion surviving free carriers is investigated from the ``weak-coupling'' perspective. The contributions of different interactions between two polar modes in a ``ferroelectric'' phase transition are also studied. We found that the short-range interaction contributes the most for lowering the total energy during the ferroelectricphase transition. Moreover, the dipole-dipole interaction between two polar modes of oxygen atoms provides the largest energy gain among local interactions, which cannot be screened out by doping. We propose that the promising candidates for noncentrosymmetric metals obtained by doping are ferroelectrics with bonding states responsible for polar distortions away from the Fermi level. We believe that this mechanism sheds light on a method to obtain noncentrosymmetric metals from a large amount of ferroelectric perovskite oxides by doping.