S_F as a function of J_z for various system size. The lines represent the cases for L = 100, 200, 400, 800, and 1600, respectively, when going upward. The inset shows a schematic phase diagram on the J_x + J_y + J_z = 1 plane of the Kitaev model. The A phase is gapped, while the B phase is gapless. A topological QPT takes place at J_z = 0.5 along the J_x = J_y line (as indicated by the dashed line).