Fractionally quantized Hall effect in two-dimensional systems of extreme electron concentration

We report magnetotransport measurements, down to 0.050 K and up to 20 T, in two-dimensional systems with dilute and dense electron densities. The emphasis is on the latter regime, where the Hall resistance shows quantization for level filling factors at =43 and 53, and the magnetoresistance presents well-defined structures at =73 and 83. These results indicate that the fractional quantum Hall effect is a general phenomenon not restricted to the lowest-orbital Landau level.