Dip problem of the electron mobility on a thin helium film - art. no. 073401

Electrons floating above liquid helium form an ideal two-dimensional system with an extremely high mobility. However, the mobility can change substant ially when decreasing the thickness of the helium film from bulk to a thin film of a few hundred Angstrom. Furthermore it is observed that for certain film thicknesses there is a pronounced dip in the mobility. We present the oretical investigations and measurements concerning this problem. Taking in to account the roughness of the substrate, which supports the helium film, we find theoretically a discontinuity in the chemical potential of the elec trons which results in a diplike behavior in the electron current and hence in the electron mobility. This scenario is supported by direct measurement s of the electron current on substrates with different roughness and at dif ferent electron densities.