Crossover from the nonuniversal scaling regime to the universal scaling regime in quantum Hall plateau transitions

We present in this Brief Report our experimental results on the quantum Hall plateau-to-plateau transition in long-range Coulombic disordered two-dimensional electron systems embedded in the Al_xGa _1-xAs-Al_0.32Ga_0.68 As heterostructures (with x=0% and 0.21%) in a large temperature range from 1.2 K down to 1 mK. In these samples a crossover behavior is observed from the high-temperature, nonuniversal scaling regime to the low-temperature, universal scaling regime, with the temperature exponent κ changing from κ=0.58 to 0.42, respectively. The crossover temperature increases with increasing x, from 120 mK for x=0% to 250 mK for x=0.21%. When the Al concentration reaches x=0.85% at which the short-range random alloy potential dominates the disorder, the crossover temperature is beyond 1.2 K and the universal scaling is observed over two decades of temperature.