Short-range disorder induced RIQHE in the lowest Landau level

We have studied the magneto-transport of two dimensional electron systems with various amount of short-range alloy disorder. Our samples are Al$_x$Ga$_{1-x}$As-Al$_{0.32}$Ga$_ {0.68}$As heterostructures with the Al concentration $x$ ranging from 0 to 0.85\%, and the electron mobility varies from 1.2$\times $10$^7$cm$^2$/V.s down to 8.9$\times $10$^5$cm$^2 $/V.s within this $x$ range. We have two major observations in the high magnetic field regime. First, we have found that the amplitude of the fractional quantum Hall gaps is independent on $x$. Second, and more surprisingly, we have observed a $\nu $=1 reentrant integer quantum Hall effect (RIQHE) between the Landau level filling factor $\nu $=2/3 and $\nu $=3/5 in the sample with $x$=0.85\%. Between the quantum Hall Plateaus of $\nu $=2/3 and $\nu $=3/5, the Hall resistance is observed to be quantized to $h/e^2$ while the longitudinal resistance reaches a deep minimum.