Prediction of dual quantum spin Hall insulator in NbIrTe₄ monolayer

Dual quantum spin Hall insulator (QSHI) is a newly discovered topological state in the 2D material TaIrTe₄, exhibiting both a traditional Z₂ band gap at the charge neutrality point and a Van Hove singularity (VHS) induced correlated Z₂ band gap with weak doping. Inspired by the recent progress in theoretical understanding and experimental measurements, we predicted a promising dual QSHI 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 were found after a 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 around 10 meV, much larger than that from TaIrTe₄. Furthermore, since the new generated band gap is mainly dominated by the 4d orbitals of Nb, electronic correlation effects should be relatively stronger in NbIrTe₄ as compared to TaIrTe₄. Therefore, the dual QSHI state in the NbIrTe₄ monolayer is expected to be a good platform for investigating the interplay between topology and correlation effects.