Dr. Wanli Li, Princeton University

Controlling disorder in 2DEG and the physics of Quantum Hall plateau-to-plateau transition

Disorder plays a critical role in the solid state physics. The usual way people treat disorder is to accept its existence and model its effect, while we are trying to manipulate disorder in experimental systems. We have investigated two-dimensional electrons embedded in Al_xGa_1-xAs-Al0_.33Ga_0.67As heterostructures with various Al concentration x. The Al alloy potential fluctuation has a range of atomic size, and an amplitude of 1.13eV, which is three orders of magnitude larger than the micrometer-ranged Coulomb potential fluctuation from remote ionized impurities. The relative weight of the short-ranged disorder to the long-ranged disorder increases linearly with x. We have studied the quantum Hall plateau-to-plateau transition, a localization-delocalization transition, in these systems. In a sample with x=0.85%, where short-ranged alloy disorder dominates, we found (dRxy/dB)|Bc~T^-k established with the universal exponent k=0.42 from 1.2K down to 10mK. In samples of the under-dosed alloy regime, we have observed a crossover between a high temperature region with scaling exponent k=0.58 and a low temperature region with k=0.42. The crossover temperature T_c increases with the weight of the short-ranged disorder. In a sample of the over-dosed alloy regime, k=0.58 is observed to persists down to the lowest temperature of the measurement limit. Exponents 0.42 and 0.58 are suggested to represent different classes of localization-delocalization transitions related to short-ranged and long-ranged disorder, respectively.