Predicted Multiple Charge Density Wave Phases in Monolayer 1T-NbO 2 .

Layered transition-metal dichalcogenides, such as NbSe 2 , 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 2 , be stable and exhibit CDW, given that it has the same crystal structure and electronic configuration as NbSe 2 ? Here, we use first-principles calculations to predict that 1T-NbO 2 is possibly stable at high temperatures, but it would undergo two CDW transitions with √12 ×√12 and √13 ×√13 periodicities at low temperatures. Both CDW transitions are accompanied by a metal-semiconductor transition. Notably, the √13 × √13 CDW phase of NbO 2 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.&#xD.