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Strong and weak second-order topological insulators with hexagonal symmetry and ℤ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow /><mml:mn>3</mml:mn></mml:msub></mml:math> index
We propose second-order topological insulators (SOTIs) whose lattice structure has a hexagonal symmetry ${C}_{6}$. We start with a three-dimensional weak topological insulator constructed on a stacked triangular lattice, which has only side topological surface states. We then introduce an additional mass term which gaps out the side surface states but …