Prediction of thickness limits of ideal polar ultrathin films

TL;DR

This study uses density functional theory to predict how ultrathin polar films of ACuO2 transition from electronic to atomic reconstruction as they become thinner, revealing a critical thickness below which structural changes dominate.

Contribution

It introduces an analytical model explaining the preference for atomic over electronic reconstruction in ultrathin polar films, validated by DFT calculations on ACuO2/SrTiO3 systems.

Findings

A transition occurs at fewer than five unit cells in SrCuO2 films.

Atomic reconstruction becomes more favorable than electronic reconstruction as film thickness decreases.

The model applies broadly to other polar film systems.

Abstract

Competition between electronic and atomic reconstruction is a constantly recurring theme in transition-metal oxides. We use density functional theory calculations to study this competition for a model system consisting of a thin film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar structure to a chain-type thin film accompanied by substantial changes to the electronic structure is predicted for a SrCuO2 film fewer than five unit cells thick. An analytical model explains why atomic reconstruction becomes more favorable than electronic reconstruction as the film becomes thinner, and suggests that similar considerations should be valid for other polar films.