Highly charged 180 degree head-to-head domain walls in lead titanate

Kalani Moore,Ursel Bangert,Alan Harvey,Andrew J. Bell,J. Marty Gregg,Jennifer Mackel,Thomas E. Hooper,Charlotte Cochard,Eoghan N. O’Connell,Michele Conroy

doi:10.1038/s42005-020-00488-x

Abstract

Charged domain walls (DWs) in ferroelectric materials are an area of intense research. Microscale strain has been identified as a method of inducing arrays of twin walls to meet at right angles, forming needlepoint domains which exhibit novel material properties. Atomic scale characterisation of the features exhibiting these exciting behaviours was inaccessible with the piezoresponse force microscopy resolution of previous work. Here we use aberration corrected scanning transmission electron microscopy to observe short, stepped, highly charged DWs at the tip of the needle points in ferroelectric PbTiO3. Reverse Ti4+ shift polarisation mapping confirms the head-to-head polarisation in adjacent domains. Strain mapping reveals large deviations from the bulk and a wider DW with a high Pb2+ vacancy concentration. The extra screening charge is found to stabilise the DW perpendicular to the opposing polarisation vectors and thus constitutes the most highly charged DW possible in PbTiO3. This feature at the needle point junction is a 5 nm × 2 nm channel running through the sample and is likely to have useful conducting properties. We envisage that similar junctions can be formed in other ferroelastic materials and yield exciting phenomena for future research.

Highlights

Charged domain walls (DWs) in ferroelectric materials are an area of intense research
‘The interface is the device’ said Herbert Kroemer in his Nobel laureate speech in 20001. He was referring to semiconductor heterostructures, this idea has held true for ferroic materials where domain walls (DWs) have been found to possess a high degree of functional tunability with extremely low dielectric loss[2,3,4,5,6,7]
The superconducting charged needle points (CNPs) form between arrays of twin walls; two DWs bend towards each other to intersect a third, perpendicular twin DW, which results in CNP junctions

Summary

Introduction

Charged domain walls (DWs) in ferroelectric materials are an area of intense research. Microscale strain has been identified as a method of inducing arrays of twin walls to meet at right angles, forming needlepoint domains which exhibit novel material properties. The extra screening charge is found to stabilise the DW perpendicular to the opposing polarisation vectors and constitutes the most highly charged DW possible in PbTiO3 This feature at the needle point junction is a 5 nm × 2 nm channel running through the sample and is likely to have useful conducting properties. In PbTiO3 (PTO) single crystals, needle-tip domains form due to arrays of 90° ferroelastic twin domains meeting at right angles These regions have been highlighted by Salje et al.[18,19] as potentially exciting, as they could constitute DW-based Josephson junctions in systems where some of the DW components are in the superconducting state. Shifts in the Ti4+ atomic columns show that the charged DW is polarised head-to-head with a high density of ionic vacancies trapped between the directly opposing electric fields

Methods

Results

Conclusion