Transition between large and small electron polaron at neutral ferroelectric domain walls in BiFeO$_3$

TL;DR

This study uses first principles calculations to predict a transition from large to small electron polarons at ferroelectric domain walls in BiFeO3, revealing distinct optical signatures relevant for electronic applications.

Contribution

It introduces the prediction of a density-dependent transition between large and small electron polarons at ferroelectric domain walls using first principles methods.

Findings

Large and small polarons produce different optical absorption peaks.

A critical electron density triggers the transition between polaron types.

Distinct optical signatures can identify polaron states at domain walls.

Abstract

Ferroelectric domain walls are planes within an insulating material that can accumulate and conduct charge carriers, hence the interaction of the domain walls with the charge carriers can be important for photovoltaic and other electronic applications. By means of first principles calculations we predict a transition from a large two-dimensional electron polaron to a small polaron at the domain walls at a critical electron density, with polaron signatures in optical absorption and photoluminescence. We find that large and small polarons at the domain walls create different absorption peaks within the band gap that are not present in the case of pristine domain walls. These are an extended Drude peak in the case of large electron or hole polarons and a narrow mid-gap peak in the case of the small electron polaron.