Interface-Controlled Ferroelectricity at the Nanoscale

TL;DR

This study uses first-principles calculations to explore how interface bonding influences ferroelectricity in ultrathin KNbO3 films, revealing that local polarization can persist even when net polarization is suppressed by interface effects.

Contribution

It demonstrates how interface bonding constrains atomic displacements and induces a ferroelectric double-domain structure in nanoscale KNbO3 films, advancing understanding of ferroelectricity at the nanoscale.

Findings

Net polarization vanishes below critical thickness (~1 nm for Pt, 1.8 nm for SrRuO3)

Local polarization persists down to a single unit cell thickness

Interface bonding induces a double-domain ferroelectric state

Abstract

Recent experimental results demonstrate that in thin films ferroelectricity persists down to film thickness of a few unit cells. This finding opens an avenue for novel electronic devices based on ultathin ferroelectrics, but also raises questions about factors controlling ferroelectricity and the nature of the ferroelectric state at the nanoscale. Here we report a first-principles study of KNbO3 ferroelectric thin films placed between two metal electrodes, either SrRuO3 or Pt. We show that the bonding at the ferroelectric-metal interface imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode in thin ferroelectric films. This does not, however, quench local polarization. If the interface bonding is sufficiently strong the ground state represents a ferroelectric double-domain structure, driven by the intrinsic oppositely-oriented dipole moments at the two interfaces. Although the critical thickness for the net polarization of KNbO3 film is finite - about 1 nm for Pt and 1.8 nm for SrRuO3 electrodes - local polarization persists down to thickness of a unit cell.