Hard antiphase domain boundaries in strontium titanate unravelled using machine-learned force fields

We investigate the properties of hard antiphase boundaries in ${\mathrm{SrTiO}}_{3}$ using machine-learned force fields. In contrast to earlier findings based on standard ab initio methods, for all pressures up to $120\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$ the observed domain wall pattern maintains an almost perfect N\'eel character in quantitative agreement with Landau-Ginzburg-Devonshire theory, and the in-plane polarization ${P}_{3}$ shows no tendency to decay to zero. Together with the switching properties of ${P}_{3}$ under reversal of the N\'eel order parameter component, this provides hard evidence for the presence of rotopolar couplings. The present approach overcomes the severe limitations of ab initio simulations of wide domain walls and opens avenues toward concise atomistic predictions of domain-wall properties even at finite temperatures.