Stability of type-II Weyl points in MnBi_2–_xSb_xTe₄ based on orbital interactions

One strategy of realizing ideal Weyl semimetals – with only one pair of Weyl points in the Brillouin zone – is to break only time-reversal symmetry while preserving inversion symmetry, as has been experimentally investigated in the ferromagnetic phase of the Weyl semimetal MnBi₂Te₄ [1]. Whether these Weyl points are of type-II character is yet debated, since this character is sensitive to MnBi₂Te₄ lattice parameter changes on the order of 1%. Such stringent lattice parameter requirements can in fact be relaxed by tuning MnBi_2–xSb_xTe₄ alloy composition x from 0 to 0.5. Employing density functional theory, we demonstrate the robustness of this procedure by articulating the conditions of realizing type-II Weyl points based on general principles of orbital interactions as applied to MnBi₂Te₄, including zone folding of p_z orbital dispersions and modifications of these band dispersions due to spin-orbit interaction [2]. The stability of type-II Weyl points in MnBi_1.5Sb_0.5Te₄ is thus intimately associated with orbital interactions, providing a generalizable strategy for future efforts in the rational design and engineering of topological electronic structures.