Inversion-symmetry engineering in sub-unit-cell-layered oxide thin films

TL;DR

This paper demonstrates a method to reversibly control inversion symmetry in ultrathin layered oxide films by precise sub-unit-cell growth, enabling tailored symmetry-related properties for advanced electronic applications.

Contribution

It introduces a novel approach to engineering inversion symmetry in oxide thin films through sub-unit-cell growth control, applicable across different materials and heterostructures.

Findings

Inversion symmetry can be toggled by controlling the number of half-unit-cell layers.

Symmetry control is achieved independently of the rare-earth element R.

The method is effective in heterostructures with mixed constituents.

Abstract

Inversion symmetry breaking is a ubiquitous concept in condensed-matter science. On the one hand, it is a prerequisite for many technologically relevant effects such as piezoelectricity, photovoltaic and nonlinear optical properties and spin-transport phenomena. On the other hand, it may determine abstract properties such as the electronic topology in quantum materials. Therefore, the creation of materials where inversion symmetry can be turned on or off by design may be the ultimate route towards controlling parity-related phenomena and functionalities. Here, we engineer the symmetry of ultrathin epitaxial oxide films by sub-unit-cell growth control. We reversibly activate and deactivate inversion symmetry in the layered hexagonal manganites, h-RMnO$_3$ with R = Y, Er, Tb. While an odd number of half-unit-cell layers exhibits a breaking of inversion symmetry through its arrangement of MnO$_5$ bipyramids, an even number of such half-unit-cell layers takes on a centrosymmetric structure. Here we control the resulting symmetry by tracking the growth in situ via optical second-harmonic generation. We furthermore demonstrate that our symmetry engineering works independent of the choice of R and even in heterostructures mixing constituents with different R in a two-dimensional growth mode. Symmetry engineering on the atomic level thus suggests a new platform for the controlled activation and deactivation of symmetry-governed functionalities in oxide-electronic epitaxial thin films.