Hidden Antipolar Order Parameter and Entangled Neel-Type Charged Domain Walls in Hybrid Improper Ferroelectrics

TL;DR

This paper uncovers a hidden antipolar order parameter in hybrid improper ferroelectrics, revealing Neel-type antipolar domain walls that help screen charged domain walls without free carriers, advancing understanding of complex domain topologies.

Contribution

It introduces the concept of a hidden antipolar order parameter in hybrid improper ferroelectrics and describes the formation of Neel-type antipolar domain walls that mitigate charge divergence.

Findings

Existence of hidden antipolar order parameter in (Ca,Sr)3Ti2O7.

Formation of Neel-type antipolar domain walls.

Charged domain-wall screening by antipolar ingredients.

Abstract

Hybrid improper ferroelectricity (HIF) denotes a new class of polar instability by the mixture of two octahedral-distortion modes and can feature the coexistence of abundant head-to-head and tail-to-tail polar domains, of which the domain walls tend to be charged due to the respective screening charges with an opposite sign. However, no such coexisting carriers are available in the materials. Using group-theoretical, microscopic, and spectroscopic analyses, we established the existence of hidden antipolar order parameter in model HIF (Ca,Sr)3Ti2O7 by the condensation of a weak, previously unnoticed antipolar lattice instability, turning the order-parameter spaces to be multicomponent with the distinct polar-antipolar intertwining and accompanied formation of Neel-type twin-like antipolar domain walls (few nm) between the head-to-head and tail-to-tail domains. The finite-width Neel walls and correlated domain topology inherently lift the polar divergences between the domains, casting an emergent exemplification of charged domain-wall screening by an antipolar ingredient. Comparisons to topological defects in improper-ferroelectrics hexagonal manganites were discussed.