2025/10/14 by Rahul Verma, Yash Vardhan, Verma, Rahul +5 · 1 citation
Physics and Astronomy · #Topological Materials and Phenomena #Quantum and electron transport phenomena #Atomic and Subatomic Physics Research
paper · pdf · doi:10.48550/arxiv.2510.12935
Quantum spin Hall (QSH) insulators are versatile platforms for exploring exotic quantum phases, especially when combined with high-order van Hove singularities (VHSs) that enhance electron correlations. However, perfect spin Hall quantization is often hindered by spin mixing from strong spin-orbit coupling, and the emergence of such VHSs is highly sensitive to material-specific electronic structures. Here, we predict a class of seven-layered square-octagonal MA2Z4 (M = Mo/W, A = Si/Ge, Z = Pnictogen) isomers that host a robust, large-gap QSH phase with nearly quantized spin Hall conductivity and intrinsic high-order VHSs. Topological and symmetry analyses reveal that compounds with Z = P, As, and Sb are ℤ2 nontrivial with spin Chern number Cs = 1 and support Sz-polarized edge states, while those with Z = N are trivial insulators. The QSH phase features an Sz-conserving spin Hamiltonian consistent with an emergent spin U(1) quasi-symmetry, yielding spin Hall conductivity ∼ 2e2/h. Notably, MA2(As, Sb)4 compounds exhibit quasi-flat bands near the Fermi level in the inverted regime, with WSi2Sb4 additionally hosting four high-order VHSs at generic momentum points. These results position square-octagonal MA2Z4 materials as robust QSH insulators for realizing quantized spin Hall conductivity and correlated topological phases, including fractionalized states and possibly non-Abelian anyons.