Domain wall-localized phonons in BiFeO3: spectrum and selection rules

Peng Chen,Sergey Artyukhin,Roberto Cingolani,Louis Ponet,Keji Lai

doi:10.1038/s41524-020-0304-y

Abstract

Ferroelectric domain walls (DWs) are nanoscale topological defects that can be easily tailored to create nanoscale devices. Their excitations, recently discovered to be responsible for GHz DW conductivity, hold promise for faster signal transmission and processing compared to the existing technology. Here we find that DW phonons have unprecedented dispersion going from GHz all the way to THz frequencies, and resulting in a surprisingly broad GHz signature in DW conductivity. Puzzling activation of nominally forbidden DW sliding modes in BiFeO3 is traced back to DW tilting and resulting asymmetry in wall-localized phonons. The obtained phonon spectra and selection rules are used to simulate scanning impedance microscopy, emerging as a powerful probe in nanophononics. The results will guide the experimental discovery of the predicted phonon branches and design of DW-based nanodevices operating in the technologically important frequency range.

Highlights

1234567890():,; INTRODUCTION The seminal work of Seidel et al.[1] demonstrated that DC conductivity is higher at domain walls (DWs) in BiFeO3 (BFO) than in the bulk, enabling signal transmission along the walls
The conductivity results from the excitation of soft DW-localized phonon modes, that are at the heart of switching, microwave dielectric loss, and dielectric constant enhancement in ferroelectrics[15,18,19]
We use the Ginzburg–Landau–Devonshire (GLD) model for DWs in BFO to capture the energetics of interacting ferroelectric polarization and strain

Summary

Introduction

INTRODUCTION The seminal work ofSeidel et al.[1] demonstrated that DC conductivity is higher at domain walls (DWs) in BiFeO3 (BFO) than in the bulk, enabling signal transmission along the walls. The conductivity results from the excitation of soft DW-localized phonon modes, that are at the heart of switching, microwave dielectric loss, and dielectric constant enhancement in ferroelectrics[15,18,19]. They correspond to oscillations of the DW plane and can be excited by an AC electric field that favors one of the domains during its half period (Fig. 1a). We show that the frequency of these phonons can go from GHz all the way up to THz, going beyond the frequency range of modern surface acoustic wave-based cell phone transducers

Methods

Results

Conclusion