Strongly Correlated van der Waals Oxide: 2H-NbO₂

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 2H-type transition metal dioxide, 2H-NbO₂, via a soft-chemical approach from the only 2H-type layered oxide LiNbO₂. 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 2H-NbO₂ is a correlated insulator with a half-filled single band, which endows exotic superconductivity to Li_1-xNbO₂, 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.