  Short and long-range impurities have been examined for fractional quantum
Hall systems. There appears to be a consistent computational picture for short
range impurities. In the case of long range impurities, calculations agree
qualitatively with experiment, in that the critical mobility is very sensitive
to long range impurities and the critical mobility for long range impurities is
larger than the critical mobility for short range impurities. The physical
mechanism of this sensitivity and a quantitative understanding remain a
challenging computational issue.
