Unprotected edge modes in quantum spin Hall insulator candidate materials

The experiments in quantum spin Hall insulator candidate materials, such as HgTe/CdTe and InAs/GaSb heterostructures, indicate that addition to the topologically protected helical edge modes, these multilayer heterostructures may also support additional states, which can contribute scattering transport. We use first-principles calculations derive an effective tight-binding model for HgTe/CdTe, HgS/CdTe, we show all materials states are sensitive termination. trace microscopic origin of back a minimal supporting flat bands with nontrivial geometry gives rise polarization charges at edges. transform into when coupled each other form Hamiltonian describing full heterostructure. Interestingly, wells far away from Fermi level so they do not transport, but HgS/CdTe appear within bulk energy gap, giving possibility multimode Finally, demonstrate because modes nontopological it is possible remove them gap by modifying potential, example, help side gate or chemical doping.