Optical Signature of Moiré Superlattices Formed by Twisted SrTiO ₃ Membranes

ABSTRACT Moiré superlattices formed at the interfaces of mismatched lattices have attracted significant interest over the past decade due to their large tunability of band parameters and interactions among electrons, spins, and lattices. Superlattices made from twisted perovskite oxides may have strong structure and potential modulation, but evidence of such modulation over macroscopic areas, particularly at large twisting angles, has not been clearly demonstrated so far. Here, millimeter‐scale twisted oxide bilayers are fabricated at angle, close to the simple coincidence site lattice condition , from freestanding SrTiO 3 membranes. We observe new low‐frequency vibrational modes whose Raman activity, according to molecular dynamics simulations, is greatly enhanced by an asymmetric, twisted interface between the SrO and TiO 2 layers. Such an interface is energetically favorable from first‐principles calculations and is corroborated by the observation of strong second‐harmonic generation from the interface, whose amplitude is comparable to that from the bare SrTiO 3 surface throughout the bilayer region. The results are consistent with interlayer coupling, which is enhanced by high‐temperature annealing and confirmed by cross‐sectional scanning transmission electron microscopy. Our work sheds light on the structural behavior of twisted oxides and provides directions for tuning their phononic and nonlinear optical properties in future studies.