Inversion-Symmetry Engineering in Layered Oxide Thin Films

Abstract

Inversion-symmetry breaking is a ubiquitous concept in condensed-matter science: It is a prerequisite for technologically relevant effects such as piezoelectricity, nonlinear optical properties, and spin-transport phenomena. It also determines abstract properties, like the electronic topology in quantum materials. Therefore, the creation of materials where inversion symmetry can be turned on or off by design may be a versatile approach for controlling parity-related functionalities. Here, we engineer inversion symmetry on a sub-unit-cell level in ultrathin hexagonal manganite films. Although an odd number of half-unit-cell layers breaks inversion symmetry, an even number of such layers remains centrosymmetric. Optical second harmonic generation as an inversion-symmetry-sensitive functionality is thus activated and deactivated on demand and at the same time used for in situ tracking of the symmetry state of our films. Symmetry engineering on the sub-unit-cell level thus suggests a new platform for controlled activation and deactivation of symmetry-governed functionalities in oxide-electronic epitaxial thin films.

Nonlinear Light Amplification Governed by Structural Asymmetry

“…[ 8,36,37 ] In particular, the subscript i represents the polarization of emitted second‐harmonic lights, and subscripts j and k represent the polarization of incident FF beams. [ 38,39 ] Accordingly, E j , k (ω) denotes the electric field component of FF beams, while denotes the polarization‐resolved second‐order nonlinear polarization. For simplification, the subscript s can be neglected in the following section.…”

“…[8,36,37] In particular, the subscript i represents the polarization of emitted second-harmonic lights, and subscripts j and k represent the polarization of incident FF beams. [38,39] Accordingly, E j,k (ω) denotes the electric field component of FF beams, while…”

Piezoelectricity across 2D Phase Boundaries

“…Such inversion symmetry breaking on the monolayer 2D materials enables nonlinear optical properties and valley-selectivity. [5] Therefore, there has been a lot of interest in investigating in-plane piezoelectricity in 2D layered materials. [6,7] However, such theoretical and experimental investigations have thus far been limited to homogeneous single-phase 2D materials like transition metal dichalcogenides (MoS 2 , [8] WSe 2 , [9] MoTe 2 [6] ), monochalcogenides (SnS [10,11] ), C/Nbased materials (hBN, [12] g-C3N [13] ), 2D elements (Se [14] ), and ultrathin group III-VI binaries (GaAs, InP, [15] In 2 Se 3 [16] ), etc.…”

“…Therefore, previous studies have been limited to attempts to engineer this symmetry‐breaking behavior by precise growth of an odd number of layers, which requires sophisticated synthesis methods like MBE. [ 5 ] In addition, single‐phase in‐plane piezoelectricity is also limited by edge effects. [ 17 ] An alternate way to engineer centrosymmetry in bulk 3D materials is in the form of heterointerfaces, which break inversion symmetry either by the presence of interfacial strain, in‐built electric field, or charge transfer.…”

Epitaxy of hexagonal ABO$_3$ quantum materials

“…1(b)). Given the half-unit-cell layer-by-layer growth mode achieved by PLD 80 , the symmetry of the ultrathin thin-film system can be alternatingly controlled between preserved and broken inversion symmetry, purely based on either an even or odd total number of half-unit-cell layers 99 . This effect was demonstrated using in-situ SHG 100,101 during hexagonal RMnO 3 epitaxial growth, where the emission of frequency-doubled light was turned on and off in the films by the deposition of each monolayer (Fig.…”