Observation of square-like moire lattice and quasicrystalline order in twisted rock-salt nitrides

Twistronics, which exploits moire modulation of lattice and electronic structures in twisted bilayers, has emerged as a powerful approach to engineer novel quantum states. Recent efforts have expanded beyond two dimensional van der Waals (vdWs) crystals to more complex, strongly correlated materials, where interfacial moire effects can dominate physical properties. Here we demonstrate a generalizable route to fabricate twisted bilayers of transition metal nitrides with vdWs like interfaces, using freestanding CrN membranes as a model system. Twisted bilayer CrN (tCrN) is realized by employing cubic alkaline earth metal monoxides as sacrificial layers, enabling the assembly of clean, controllable interfaces. Electron ptychography reveals well defined, periodic square moire superlattices in tCrN. For a twist angle of 16.3 degree, we identify a nearly commensurate moire lattice with coincident Cr columns, whereas at 45 degree we uncover localized octagonal quasicrystalline order with clear self-similarity. These results establish a practical platform for twisted TMNs and open avenues to explore moire-induced atomic configurations and emergent correlated phenomena in nitride based heterostructures.

💡 Research Summary

In this work the authors extend the concept of twistronics, previously confined to van‑der‑Waals (vdW) two‑dimensional crystals, to three‑dimensional transition‑metal nitrides (TMNs). They develop a generalizable fabrication route that yields clean, vdW‑like interfaces between two CrN membranes. The key enabling step is the use of a water‑soluble alkaline‑earth metal monoxide (BaO) as a sacrificial layer. Epitaxial BaO (∼5 nm) and CrN (∼16 nm) are grown by pulsed‑laser deposition on SrTiO₃ (001) under ultra‑high vacuum (≤2 × 10⁻⁸ Torr) to avoid oxidation of CrN. After selective dissolution of BaO in de‑ionized water, large‑area freestanding CrN (FS‑CrN) membranes are obtained. X‑ray diffraction, rocking curves, Laue oscillations and X‑ray photoelectron spectroscopy confirm the high crystallinity, correct stoichiometry (Cr²⁺, N³⁻) and the release of compressive strain upon membrane release.

The FS‑CrN sheets are then stacked with a controlled twist angle using a NaCl‑assisted transfer method. Four‑dimensional scanning transmission electron microscopy (4D‑STEM) combined with electron ptychography provides atomic‑resolution imaging of both individual Cr sublattices and the interfacial moiré pattern. For a twist angle of 16.3°, a nearly commensurate Σ = 25 coincidence‑site lattice (CSL) emerges: Cr columns from the two layers coincide periodically along