Moiré-patterned twisted SrTiO₃ bilayer membranes

Moiré superlattices formed from two-dimensional materials exhibit diverse emergent physical phenomena such as unconventional superconductivity, a flat band structure, ferroelectricity, and ferromagnetism. Recent advances in fabricating freestanding membranes in three-dimensional materials have now made it possible to fabricate twisted bilayers of three-dimensional materials, in particular membranes of complex oxide materials.

We fabricated high-quality ultrathin SrTiO₃ single crystal membranes using a Sr₂CaAl₂O₆ sacrificial layer. The epitaxial SrTiO₃ ultrathin film and Sr₂CaAl₂O₆ sacrificial layer were grown using pulsed laser deposition with atomic layer control facilitated by high-pressure RHEED. To transfer the membranes to create a bilayer, and to control the twisted angle of the SrTiO₃ membrane, we coated a supporting layer (Polypropylene carbonate and Poly (methyl methacrylate)) then floated the sample on water to etch the sacrificial layer. The transferred twisted bilayer exhibited an atomically flat surface with high crystallinity. Through scanning transmission electron microscopy and electron energy loss spectroscopy, we observed ordered Moiré patterns with locally reconstructed electronic structures at coincidence site lattice conditions. We will discuss the interfacial structural analysis via synchrotron X-ray, local electrical properties through scanning probe microscopy, and macroscopic electrical properties.