Predicted multiple charge density wave phases in monolayer 1T-NbO.

Layered transition-metal dichalcogenides, such as NbSe, have been extensively studied for almost half a century due to their intriguing properties, such as charge density wave (CDW) and superconductivity. Can the layered transition-metal dioxide, such as NbO, be stable and exhibit CDW, given that it has the same crystal structure and electronic configuration as NbSe? Here, we use first-principles calculations to predict that 1T-NbOis possibly stable at high temperatures, but it would undergo two CDW transitions with12×12and13×13periodicities at low temperatures. Both CDW transitions are accompanied by a metal-semiconductor transition. Notably, the13×13CDW phase of NbOpossesses localized magnetic moments and hosts a Mott insulating state. This work offers a fresh outlook on studying CDW and Mott transition in low-dimensional oxide materials.