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 $\nu =\frac{4}{3} \mathrm{and} \frac{5}{3}$, and the magnetoresistance presents well-defined structures at $\nu =\frac{7}{3} \mathrm{and} \frac{8}{3}$. These results indicate that the fractional quantum Hall effect is a general phenomenon not restricted to the lowest-orbital Landau level.