(a) Energy spectrum obtained by numerical diagonalization of the tight-binding Hamiltonian for the spin-up part with λ = 0.6t and the number of unit cells n = 30 along the y-axis direction of the strip lattice with bearded edges. The blue and red dashed lines correspond to the energy given by Eq. (27), respectively. (b) The variations χ_± with k_x. The blue and red solid lines correspond to χ₊ and χ₋, respectively. Here, the green dashed lines locate at .