Topographical map of the minimum energy gaps Δ_n (n = 1–6) versus V₀ and V_SAW. The map is divided into several regimes by valleys where band gaps are closed. The rightmost region corresponds to the lowest plateaus of a pumping current of I = 0ef. The plateaus rise up from 0ef to ne f (with an integer n) while we assign parameters from lower right areas to upper left ones.