Fig. 3: Electronic properties of free zigzag edge states vs. DB states.

a STS dI/dV spectra on/off a free bismuthene zigzag edge, measured at positions marked in b. The bulk spectrum (gray) shows an insulating gap. Spectra taken on the edge (red, green) verify the existence of a metallic edge channel smoothly filling the gap. b Constant-current STM topography of a free zigzag edge (V_set = −1.2 V, I_set = 20 pA). The edge is divided into straight edge segments of variable length separated by kinks (indicated by white-dashed line). Colored squares: positions, where the spectra in a are taken. c Energy-dependent dI/dV signal versus position along the white-dashed path in b. The periodic modulation corresponds to the structural edge periodicity 2a_Bi. d dI/dV spectra on/off a bismuthene DB, at positions marked in e. The bulk spectrum (gray) agrees with the one in a. The spectra taken on the DB (green, blue, red) (nearly) fill the gap and are characterized by the presence of intense peaks at discrete energies, in remarkable contrast to the zigzag edge spectra in a. e STM topography of a DB (V_set = −0.85 V, I_set = 80 pA). Similar to the free zigzag edge, the DB consists of piecewise straight sections connected by kinks. Colored squares: positions, where the spectra in d are taken. f Energy-dependent (w-integrated) dI/dV signal versus position along the DB in b. Energy-dependent standing wave excitations are observed at the marked energies E₁ to E₄, additional to the structural 2a_Bi modulation. g Two-dimensional dI/dV maps of the DB in e, taken at the discrete energies E₁ to E₄ in f. The corresponding (w-integrated) dI/dV profile along the DB is shown above each map. The modulation wavelength λ is extracted from the dI/dV profiles. Gray-shaded regions: guide to the eye to mark the extension of the effective resonator. The scale bar is 1nm. h Power spectrum of the dI/dV map in f. Yellow line: guide to the eye with a slope of 3.6 eVÅ.