FIG. 2.

The winding number for the SSH model. The curves plot the locus of (h_x, h_y) as q runs over the BZ. For $J^{\prime }/J>1$ the vector (h_x, h_y) does not encircle the origin, but for $J^{\prime }/J<1$ it encircles the origin once, indicating that these two cases have winding numbers (23) of N = 0 and 1, respectively.