Fig. 3: Surface topological phase diagram and partial Fermi surface.

Surface topological phase diagrams as a function of (M_y, M_z) and (M_z, μ) are shown in (a) and (b). The darker the blue color is, the larger the surface energy gap is. The region colored in white has no energy gap and could indicate the existence of either a surface topological phase transition or a partial Fermi surface (PFS). Note that embedded second-order topological phase (ET₂) and PFS can coexist. c The surface spectrum of a PFS along k_x with (M_y, M_z) = (0.5, 0.05). An example of the surface spectrum with coexisting ET₂ and PFS is shown in (d) with (M_y, M_z) = (0.9, 0.05).