Fig. 2

a Band structures with −6,−8,and−10 % in-plane isotropic strain. b Band structures with different values of in-plane anisotropic strain. Positive strain indicates expansion and negative strain indicates compression. All energies are referenced to Fermi level. The dashed lines are merely a guide for viewing the energy shifts of each band. As band 2 and band 3 are coupled at k _x direction while decoupled at k _y direction, band structure of these two bands become saddle-shaped after band inversion in a way that there is a band crossing point between VBM and CBM in k _y direction. Band 1 and band 3 are decoupled; band inversion does not change the curvature of these two bands; thus, there is a band crossing between VBM and CBM in each k direction. Such band crossing phenomena can be predicted by k·P and group theory