(color online) The energy band diagrams with different U₃/U₂ on the basis of U₂ = 6 (0.05) for (a), (b) N = 3 and (c), (d) N = 4. The dotted line represents the corresponding critical condition of Eq. (5), i.e., U₃/U₂ = −2 for N = 3 while U₃/U₂ = −1 for N = 4. Note that E₀, E₁, E₂, E₃, and E₄ are denoted by blue, black, green, red, and purple lines, respectively.