Melting of crystals of polarization vortices and chiral phase transitions in oxide superlattices

Abstract

We study the equilibrium arrangements of polarization vortices in (PbTiO3 )n/(SrTiO3 )n superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counterclockwise vortices alternate positions, leading to a crystal-like structure with well-defined handedness. This chiral crystal melts at a critical temperature TM into a chiral liquid where long-range order is lost but handedness is preserved. At even higher temperatures TC, a second pase transition occurs at which the chiral liquid of polarization vortices loses its handedness. Both phase transitions can be readily identified by the adequate choices of order parameters.