IMF2017

Topologically nontrivial dipolar patterns like vortices, hedgehogs (monopoles) and skyrmions are not commonly expected to appear in bulk ferroelectric materials. Indeed, unlike ferroelectric nanostructures or relaxors, bulk ferroelectrics exhibit neither depolarizing nor random local fields that can render topological defects energetically favorable. Furthermore, polar bulks also appear to fall short of alternative, "topological", defect stabilization mechanisms as a result of inherent discrete symmetries of these systems. In this study, we combine homotopy theory and first-principles-based effective Hamiltonian approach to explore stability of topological defects in bulk BaTiO₃. Our results show that, against all odds and theoretical expectations, this proper ferroelectric material can exhibit stable topological point defects in its tetragonal polar phase and stable topological line defects in its orthorhombic polar phase. The stability of such defects originates from a novel mechanism of topological protection related to finite-temperature fluctuations of local dipoles. Large-scale effective Hamiltonian Monte Carlo simulations are then conducted to confirm these theoretical predictions. The results of our work, hence, reveal a novel mechanism of topological protection that can be realized in proper ferroelectrics and provide a theoretical framework for investigations of topological defects in systems with finite underlying symmetries.