Hidden Antipolar Order Parameter and Entangled Néel-Type Charged Domain Walls in Hybrid Improper Ferroelectrics
M. H. Lee1*, C.-P. Chang1, F.-T. Huang2, G. Y. Guo3,4, B. Gao2, C. H. Chen1,2, S.-W. Cheong2, M.-W. Chu1
1Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
2Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey, USA
3Department of Physics, National Taiwan University, Taipei, Taiwan
4Physics Division, National Center for Theoretical Sciences, Hsinchu, Taiwan
* Presenter:M. H. Lee
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 Néel-type twin-like antipolar domain walls (few nm) between the head-to-head and tail-to-tail domains. The finite-width Néel 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.

Keywords: TEM, Ferroelectric