We present a recipe for an electronic two-dimensional (2D) higher order topological insulator (HOTI) on a triangular lattice that can be realized in a large family of materials. The essential ingredient is mirror symmetry breaking, which allows for a finite quadrupole moment and trivial ${\mathbb{Z}}_2$ index. The competition between spin-orbit coupling and the symmetry-breaking terms gives rise to four topologically distinct phases; the HOTI phase appears when symmetry breaking dominates, including in the absence of spin-orbit coupling. We identify triangular monolayer adsorbate systems on the (111) surface of zincblende/diamond type substrates as ideal material platforms and predict the HOTI phase for $X=(\mathrm{Al},\mathrm{B},\mathrm{Ga})$ on SiC.

• Received 11 July 2022
• Revised 8 November 2022
• Accepted 15 February 2023

DOI:https://doi.org/10.1103/PhysRevB.107.115130