Topological materials refer to solid structures with topologically protected electronic states or other degrees of freedom.A prime example is a Ch insulator in which the occupied bands of states are organized with a non-trivial topology as characterized by a nonzero Ch integer, exhibiting gapless chiral excitations on the boundaries, which are robust against scatterings.In the presence of certain discrete symmetries, such as time reversal or mirror reflection, one can have more general topological classes of insulators exhibiting helical or spin-momentum locked boundary excitations, with a lesser degree of robustness against scatterings [1].