INVESTIGATIONS OF COMPOSITE FERMIONS IN SEMICONDUCTOR NANOSTRUCTURES

We have measured the magnetoresistance of a variety of structures to s earch for effects associated with composite fermions (CF) near the Lan dau filling factor v = 1/2. We find evidence for effects due to random ization of semiclassical ballistic CF trajectories. These produce magn etoresistance features similar to those observed near zero magnetic fi eld. However, we were not able to reproduce the recent CF magnetic foc using experiment despite using devices of very similar quality to thos e used in the original experiment. We also searched, without success, for phenomena due to phase coherence of CF. The relative ease with whi ch the various magnetoresistance effects are seen in CF is discussed, in part with the aid of semiclassical simulations. It is discovered th at inhomogeneities of carrier density cause the magnetoresistance anom alies to be smeared out, largely as a result of spatial variations in the effective magnetic field experienced by the CF. We find also that experiments which are based on randomization of trajectories are more resilient to this spatial variation.