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 TaIrTe_4, which exhibits both a traditional Z_2 band gap at the charge neutrality point and a Van Hove singularity (VHS) that induces a correlated Z_2 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 NbIrTe_4 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 Z_2 band gap. The VHS-induced non-trivial band gap is approximately 10 meV, significantly larger than that of TaIrTe_4. Furthermore, as the newly generated band gap is mainly dominated by the 4d orbitals of Nb, the electronic correlation effects should be stronger for NbIrTe_4 than for TaIrTe_4. Therefore, the dual QSHI state in the NbIrTe_4 monolayer is expected to provide a strong platform for investigating the interplay between topologies and correlation effects.