(a) Time evolutions of the expected energies with the solid curve being associated with the same parameters as those of Fig. 2(a) and the dashed curve with the same parameters except for α = 1. (b) Time evolutions of the expected energies after increasing driving frequency from Ω = 2 to Ω = 5. In the insets we show that the expected energy E₀₀(t) for α = 0.1 approximately equals the stationary state energy E₀₁ indicated by the horizontal line, and the larger Ω value corresponds to smaller deviation from E₀₁. The dashed curves in both figures imply the similar conclusion for α = 1.