-0.2 0.0 0.2 -0.2 0.0 0.2 -0.2 0.0 0.2 -0.2 0.0 0.2 k y ( Å ) -1 Tu n i n gGLYPH<.notdef>t h eGLYPH<.notdef>t o p o l o g i c a lGLYPH<.notdef>h e l i c a lGLYPH<.notdef>D i r a cGLYPH<.notdef>e l e c t r o nGLYPH<.notdef>d e n s i t y 2L k ( Å ) -1 0 -0.4 -0.2 -0.1 0.0 0.1 -0.1 0.0 0.1 -0.1 0.0 0.1 (b) (d) (e) (f) (c) Low High 0GLYPH<.notdef>L 0.1L (a) 0 -0.4 -0.2 (g) (h) (i) 0GLYPH<.notdef>L 0.1L .01L 0.5L 1L 2L EGLYPH<.notdef>GLYPH<.notdef>(eV) B EGLYPH<.notdef>GLYPH<.notdef>(eV) B x Kramers’ point GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s50>GLYPH<s53> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s50>GLYPH<s48> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s49>GLYPH<s53> GLYPH<s45>GLYPH<s48>GLYPH<s46>GLYPH<s49>GLYPH<s48> E$_{F}$ IntensityGLYPH<.notdef>(arb.GLYPH<.notdef>units) -10 200K -20 300K 10K 0 BCB BVB Kramers’ point NO$_{2}$ 0 -1 1 0 -1 1 2 kGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>(ÅGLYPH<.notdef>) x -1 M M K K GLYPH<c71> k x k y -0.2 0 -0.4 0 0.1 -0.1 T =GLYPH<.notdef>GLYPH<.notdef>10k T =GLYPH<.notdef>GLYPH<.notdef>300K kGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>(ÅGLYPH<.notdef>) y -1 EGLYPH<.notdef>GLYPH<.notdef>(eV) B EGLYPH<.notdef>GLYPH<.notdef>(eV) B (a) (c) (c) BiGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>CaGLYPH<.notdef>GLYPH<.notdef>SeGLYPH<.notdef>GLYPH<.notdef>.GLYPH<.notdef>NO 2-x x 3 2 (b) (e) GLYPH<c71> BiGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>CaGLYPH<.notdef>GLYPH<.notdef>SeGLYPH<.notdef>GLYPH<.notdef>.NO$_{2}$ 3 2-$_{GLYPH<c100>}$ GLYPH<c100> k x k y kGLYPH<.notdef>GLYPH<.notdef>GLYPH<.notdef>(ÅGLYPH<.notdef>) x -1 0 0.1 -0.1 ChiralGLYPH<.notdef>DiracGLYPH<.notdef>groundGLYPH<.notdef>state Kramers’ nodal helicalGLYPH<.notdef>groundGLYPH<.notdef>state Kramers’ point -0.2 0 -0.4 (d) (f) -0.15 -0.1 EGLYPH<.notdef>-GLYPH<.notdef>EGLYPH<.notdef>GLYPH<.notdef>(eV) D -0.05 (g) (h) E$_{Dirac}$ DensityGLYPH<.notdef>ofGLYPH<.notdef>statesGLYPH<.notdef>GLYPH<.notdef>(arb.GLYPH<.notdef>units) 0 1 2 DOS 0. 0 background 1 2 TopologicalGLYPH<.notdef>orderGLYPH<.notdef>atGLYPH<.notdef>roomGLYPH<.notdef>temperature 300K 300K FIG. 7: Hsieh, Xia, Qian et al. , NATURE 460, 1101 (2009). FIG. 8: Hsieh, Xia, Qian et al. , NATURE 460, 1101 (2009). 17