The magnetoresistance of a narrow single quantum well is spectacularly
different from the usual behavior. At filling factors 2/3 and 3/5 we
observe large and sharp maxima in the longitudinal resistance instead
of the expected minima. The peak value of the resistance exceeds those
of the surrounding magnetic field regions by a factor of up to 3. The
formation of the maxima takes place on very large time scales which s
uggests a close relation with nuclear spins. We discuss the properties
of the observed maxima due to a formation of domains of different ele
ctronic states. [S0031-9007(98)07239-1].