Kittel law and domain formation mechanism in PbTiO$_3$/SrTiO$_3$ superlattices

TL;DR

This study uses second-principles simulations to explore domain structures in PbTiO$_{3}$/SrTiO$_{3}$ superlattices, revealing the Kittel law's applicability and the dynamic formation of domains via vortex nucleation and annihilation.

Contribution

It demonstrates the relationship between domain period and layer thickness following the Kittel law and uncovers the vortex-driven mechanism of domain formation at finite temperatures.

Findings

Domain period scales with the square root of PbTiO$_{3}$ layer thickness.

Vortex and antivortex nucleation initiates domain formation.

Finite temperature induces spontaneous domain periodicity changes.

Abstract

We report second-principles simulations on the structural and energetic properties of domains in (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. For the explored layer thickness ($n$ ranging between 8 and 16 unit cells) and lateral sizes of the domains, the most stable configuration corresponds to polar domains separated by a sequence of counter-rotating vortices (clockwise/counterclockwise) perpendicular to the stacking direction and acting as domain walls. The balance between the domain wall energy and the electrostatic energy yields to an optimal domain period $\omega$ that is proportional to the square-root of the thickness of the PbTiO$_{3}$ layer, following the Kittel law. For a given lateral size of the simulation box, suboptimal domain structures (with a width larger than the one predicted by the Kittel law) can be obtained in a metastable form. However, at finite temperature, molecular dynamics simulations show the spontaneous change of periodicity, which implies the formation of new domains whose generation is initiated by the nucleation of vortices and antivortices at the interface between the SrTiO$_{3}$ and the PbTiO$_{3}$ layers. The vortices progressively elongate and eventually annihilate with the antivortices yielding the formation of new domains to comply the Kittel law via a topological phase transition.