Cyclotron resonance for 2D electrons on helium films above rough substrates

An investigation of the microwave absorption for two-dimensional electron systems (2DES) on helium films and in the presence of a cyclotron resonance (CR) magnetic field are presented. Measured data are explained by a recently proposed two-fraction model of the 2DES, which makes the general structure of the microwave absorption understandable. The fraction of localized and free electrons can be precisely determined and its dependence on the thickness of the helium film above the roughness of the underlying solid substrate is understood.