Low temperature electronic transports in the presence of a density gradient

In this paper, we review low temperature electronic transport results in high quality two-dimensional electron systems. We discuss the quantization of the diagonal resistance, R-xx, at the edges of several quantum Hall states. Each quantized R-xx value turns out to be close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, R-xy. Moreover, peaks in R-xx occur at different positions in positive and negative magnetic fields. All three R-xx features can be explained quantitatively by a similar to 1% cm electron density gradient. Furthermore, based on this observation, the well known but still enigmatic resistivity rule, relating R-xx to dR(xv)/dB, finds a simple interpretation in terms of this gradient. In another sample, at 1.2 K, R-xx we observe a strongly linear magnetic field dependence. Surprisingly, this linear magnetoresistance also originates from the density gradient. Our findings throw an unexpected light on the relationship between the experimentally measured R-xx and the diagonal resistivity rho(xx). (c) 2006 Elsevier Ltd. All rights reserved.