(color online) The influence of exchange field on edge states. (a) Energy gap as a function of the exchange field h ₁. Energy spectrums corresponding to points A (b), B (c), and C (d) in panel (a) are plotted versus k _x (with ). The first three sub-bands are labeled with red, green, and blue colors, respectively. The parameters used in the calculation are and the thickness of Na₃Bi thin films l = 5.