Atomic scale crystal field mapping of polar vortices in oxide superlattices

Sandhya Susarla,Lane W Martin,Sujit Das,Javier Junquera,Colin Ophus,Peter Ercius,Pablo García-Fernández,Pablo Aguado-Puente,Margaret Mccarter,Ramamoorthy Ramesh

doi:10.1038/s41467-021-26476-5

Abstract

Polar vortices in oxide superlattices exhibit complex polarization topologies. Using a combination of electron energy loss near-edge structure analysis, crystal field multiplet theory, and first-principles calculations, we probe the electronic structure within such polar vortices in [(PbTiO3)16/(SrTiO3)16] superlattices at the atomic scale. The peaks in Ti L-edge spectra shift systematically depending on the position of the Ti4+ cations within the vortices i.e., the direction and magnitude of the local dipole. First-principles computation of the local projected density of states on the Ti 3d orbitals, together with the simulated crystal field multiplet spectra derived from first principles are in good agreement with the experiments.

Highlights

IntroductionUsing a combination of electron energy loss near-edge structure analysis, crystal field multiplet theory, and first-principles calculations, we probe the electronic structure within such polar vortices in [(PbTiO3)16/(SrTiO3)16] superlattices at the atomic scale
Polar vortices in oxide superlattices exhibit complex polarization topologies
We studied the crystal field of the Ti L-edge in polar vortices formed in [(PbTiO3)16/(SrTiO3)16]8 superlattices with a combination of high-resolution STEM-energy loss spectroscopy (EELS) mapping using a state-ofthe-art direct electron detector and spectrometer (Gatan Continuum with a K3 detector), first-principles calculations, and crystal field multiplet theory

Summary

Introduction

Using a combination of electron energy loss near-edge structure analysis, crystal field multiplet theory, and first-principles calculations, we probe the electronic structure within such polar vortices in [(PbTiO3)16/(SrTiO3)16] superlattices at the atomic scale. Electron energy loss spectroscopy (EELS) in the STEM mode uses inelastically scattered electrons to probe the core-shell excitations (empty density of states) of transition metals at atomic resolution[20,21,22,23,24,25]. We studied the crystal field of the Ti L-edge in polar vortices formed in [(PbTiO3)16/(SrTiO3)16]8 superlattices with a combination of high-resolution STEM-EELS mapping using a state-ofthe-art direct electron detector and spectrometer (Gatan Continuum with a K3 detector), first-principles calculations, and crystal field multiplet theory. We answer three important questions: (i) How are the t2g and eg orbitals affected by the local polarization and tetragonality? (ii) How does the rotation of the Ti 3d-orbitals affect the local crystal field? and, (iii) What is the crystal field at the vortex core, a special region where exotic effects such as local negative capacitance[13] have been reported?

Methods

Results

Conclusion