Abstract
Charged polar interfaces such as charged ferroelectric walls or heterostructured interfaces of ZnO/(Zn,Mg)O and LaAlO3/SrTiO3, across which the normal component of electric polarization changes suddenly, can host large two-dimensional conduction. Charged ferroelectric walls, which are energetically unfavourable in general, were found to be mysteriously abundant in hybrid improper ferroelectric (Ca,Sr)3Ti2O7 crystals. From the exploration of antiphase boundaries in bilayer-perovskites, here we discover that each of four polarization-direction states is degenerate with two antiphase domains, and these eight structural variants form a Z4 × Z2 domain structure with Z3 vortices and five distinct types of domain walls, whose topology is directly relevant to the presence of abundant charged walls. We also discover a zipper-like nature of antiphase boundaries, which are the reversible creation/annihilation centres of pairs of two types of ferroelectric walls (and also Z3-vortex pairs) in 90° and 180° polarization switching. Our results demonstrate the unexpectedly rich nature of hybrid improper ferroelectricity.
Highlights
Charged polar interfaces such as charged ferroelectric walls or heterostructured interfaces of ZnO/(Zn,Mg)O and LaAlO3/SrTiO3, across which the normal component of electric polarization changes suddenly, can host large two-dimensional conduction
To unveil the origin of these abundant charged ferroelectric domain walls (FE domain walls (DWs)), we have explored the complete connectivity of DWs in Ca2.55Sr0.45Ti2O7 (CSTO; FE TcE790 K) and Ca3Mn1.9Ti0.1O7 (CMTO; FE TcE360 K) single crystals with in-plane polarization along the pseudo-tetragonal [110] directions[3,30,33], with mapping of antiphase boundaries (APBs) using transmission electron microscopy (TEM)
Our results reveal that the formation of a unique Z4 Â Z2 domain topology with Z3 vortices is responsible for the presence of abundant charged FE DWs in CSTO
Summary
Charged polar interfaces such as charged ferroelectric walls or heterostructured interfaces of ZnO/(Zn,Mg)O and LaAlO3/SrTiO3, across which the normal component of electric polarization changes suddenly, can host large two-dimensional conduction. W Present address: Chongqing University of Science and Technology, Over the last decade, two-dimensional (2D) conduction in heterostructured interfaces with polar discontinuity[1,2] or compositionally homogeneous charged interfaces such as charged ferroelectric domain walls (FE DWs)[3,4,5,6,7,8,9,10] has attracted enormous attention for emergent phenomena and new material functionalities. Charged domain walls (DWs), some of which are highly conducting, were found to be abundant in the hybrid improper FE (Ca,Sr)3Ti2O7 Charged FE DWs, some of which are highly conducting, were found to be mysteriously abundant in the recently discovered Ruddlesden-Popper-type HIF (Ca,Sr)3Ti2O7 crystals[3]
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