# Polar discontinuities, emergent conductivity, and critical twist-angle-dependent behaviour at wafer-bonded ferroelectric interfaces

## TL;DR

Researchers used wafer bonding to create twisted ferroelectric interfaces, revealing twist-angle-dependent conductivity and polar discontinuities.

## Contribution

Demonstrated wafer bonding for creating non-van der Waals twisted interfaces with polar discontinuities and twist-dependent conductivity.

## Key findings

- Twisted ferroelectric interfaces show emergent two-dimensional conductivity.
- Conductivity is suppressed at specific twist angles with aperiodic lattices.
- Local dipolar switching and charge screening collapse occur at sparse twist angles.

## Abstract

Probing novel properties, arising from twisted interfaces, has traditionally relied on the stacking of exfoliated two-dimensional materials and the spontaneous formation of van der Waals bonds. So far, investigations involving intimate covalent or ionic bonds have not been a focus. Yet, we show here that an established technique, involving thermocompressional wafer bonding, works well for creating twisted non-van der Waals interfaces. We have successfully bonded z-cut lithium niobate single crystals to create ferroelectric oxide interfaces with strong polar discontinuities and have mapped the associated emergent interfacial conductivity. In some instances, a dramatic change in microstructure occurs, involving local dipolar switching. A twist-induced collapse in the capability of the system to effec8tively screen interfacial bound charge is implied. Importantly, this only occurs around specific moiré twist angles with sparse coincident lattices and associated short-range aperiodicity. In quasicrystals, aperiodicity is known to induce pseudo-bandgaps and we suspect a similar phenomenon here.

Wafer bonding has allowed the synthesis of twisted interfaces which support polar discontinuities in ferroelectric lithium niobate. Two-dimensional sheet conductivity arises but is suppressed when twist angles lead to interfacial lattice aperiodicity.