Giant Spontaneous Valley Polarization and Adsorption-Induced Topological Phase Transition in Single-Layer 2H-NbTe₂

Quantum spin Hall state usually emerges in the non-magnetic system, which is typically incompatible with ferromagnetism. Here, we predict that two-dimensional (2D) ferrovalley semiconductor single-layer (SL) 2H-NbTe₂ can be transformed into a 2D room-temperature quantum spin Hall insulator through hydrogen (H) atom adsorption. The SL 2H-NbTe₂ is found to possess a giant spontaneous valley polarization of 274 meV, which is much larger than those of most available ferrovalley materials. Upon H atom adsorption, a transition from ferromagnetism to non-magnetism emerges. More interestingly, H-adsorbed NbTe₂ is predicted as a quantum spin Hall insulator with a direct band gap of 110 meV (equal to a working temperature of 1267 K). The predicted rich quantum effects render the 2H-NbTe₂ a promising candidate for the practical valleytronic and topological electronics.