(a) The ratio ℛ versus ℏ_eff with m = 0.001 (triangles), 0.01 (circles), and 1 (squares). Dashed lines (in red) denote the exponential fitting, i.e., ℛ ∝ e^−αℏ_eff. (b) Phase diagram of quantum diffusion as a function of ℏ_eff and m, where circles and triangles indicate the threshold values of ℏeffc and ℏeffd which correspond to the appearance of classically chaotic diffusion and dynamical localization, respectively. The coupling strength is ε = 2. Other parameters are the same as those in Fig. 2.