Liquid-crystal polarization ordering and two-step melting in ferroelectric/dielectric superlattices

Static domains are commonly observed in ultra-thin ferroelectric materials at low temperatures. However, upon increasing temperature these domains might become mobile with a concomitant high dielectric response. In this article we report the two-step melting of the polar order in PbTiO₃/SrTiO₃ superlattices upon increasing temperature, resembling the phase sequence of liquid-crystals when the polarization is mainly directed along the c-direction. By means of atomistic second-principles simulations we follow the dynamics of the system and infer the mechanism underneath the melting of the crystalline structure until reaching an isotropic behaviour. Moreover, we quantify the inverse lifetimes of the polarization domains showing a strong dependence on temperature ranging from tens of gigahertz to terahertz in a narrow temperature window.