Melting of Polarization Vortices Crystals and Chiral Phase Transitions in Oxide Superlattices

TL;DR

This paper investigates the phase transitions of polarization vortex arrangements in oxide superlattices, revealing a chiral crystal melting into a liquid and subsequent loss of handedness at distinct critical temperatures.

Contribution

It introduces a detailed simulation study of polarization vortex phases, identifying two novel phase transitions and their characteristics in oxide superlattices.

Findings

Polarization vortices form a chiral crystal at low temperatures.

A melting transition leads to a chiral liquid with preserved handedness.

Further heating causes loss of handedness, indicating a second phase transition.

Abstract

We study the equilibrium arrangements of polarization vortices in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counter-clockwise vortices alternate positions, leading to a crystal-like structure with well defined handedness. This chiral crystal melts at a critical temperature $T_\mathrm{M}$ into a chiral liquid, where long-range order is lost but handedness is preserved. At even higher temperatures, $T_\mathrm{C}$, a second phase 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.