Strongly Correlated van der Waals Oxide: 2-NbO.

Two-dimensional (2D) materials are attracting attention as an advanced class of materials. However, despite the strongly correlated electron systems exhibiting emergent properties, transition metal oxides are not recognized as 2D materials due to their infrequent van der Waals (vdW) crystal structures. Herein, we present a synthesis route for a vdW 2-type transition metal dioxide, 2-NbO, via a soft-chemical approach from the only 2-type layered oxide LiNbO. By combining epitaxially stabilized thin films with a nanoscale thickness and solvothermal reaction, we successfully deintercalated Li, exceeding the bulk limit, while maintaining the single-crystal nature. We demonstrate that the synthesized 2-NbO is a correlated insulator with a half-filled single band, which endows exotic superconductivity to LiNbO, whose quantum critical phase diagram has similarities to those of cuprates and Moiré superlattices. Our findings indicate that "strongly correlated vdW oxides" could be an approach for introducing Mott physics to 2D systems and a key that connects each research field.