Cyclotron resonance at fractional Landau-level fillings

Distinct shifts of the cyclotron resonance position correlating with various fractional Landau-level fillings were observed for high-mobility electron space-charge layers in GaAs. In the magnetic quantum limit the shifts are reversed for incompressible liquids in the filling factor regimes ν<0.5 and 0.5<ν<1. We propose an explanation based on a many-body induced renormalization of the electron effective mass.