Predicted multiple charge density wave phases in monolayer 1T-NbO2

Layered transition-metal dichalcogenides, such as NbSe2, 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 NbO2, be stable and exhibit CDW, given that it has the same crystal structure and electronic configuration as NbSe2? Here, we use first-principles calculations to predict that 1T-NbO2 is possibly stable at high temperatures, but it would undergo two CDW transitions with root 12x root 12 and root 13x root 13 periodicities at low temperatures. Both CDW transitions are accompanied by a metal-semiconductor transition. Notably, the root 13x root 13 CDW phase of NbO2 possesses 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.