Prediction of the Dual Quantum Spin Hall Insulator in the NbIrTe4 Monolayer

The dual quantum spin Hall insulator (QSHI) is a newly discovered topological state in the two-dimensional (2D) material TaIrTe4, which exhibits both a traditional Z2 band gap at the charge neutrality point and a Van Hove singularity (VHS) that induces a correlated Z2 band gap with weak doping. Inspired by the recent progress in theoretical understanding and experimental measurements, a promising dual QSHI is predicted in the counterpart material of the NbIrTe4 monolayer by first-principles calculations. In addition to the well-known band inversion at the charge neutrality point, two new band inversions are found after a charge density wave (CDW) phase transition when the chemical potential is near the VHS: one direct and one indirect Z2 band gap. The VHS-induced non-trivial band gap is approximately 10 meV, significantly larger than that of TaIrTe4. Furthermore, as the newly generated band gap is mainly dominated by the 4d orbitals of Nb, the electronic correlation effects should be stronger for NbIrTe4 than for TaIrTe4. Therefore, the dual QSHI state in the NbIrTe4 monolayer is expected to provide a strong platform for investigating the interplay between topologies and correlation effects.