Localization in a two-dimensional electron system above a helium film

The rf conductivity of surface electrons (SE) above a helium film is measured for different film thicknesses. As the helium film thickness d is varied from 250 to 800 Å, the rf electron mobility μ (ω) increases from 250 to 1.5 · 104 cm2/V · sec, the latter value being typical of SE above bulk helium. It has been established that for very large values of d the dependence of μ (ω) on d is exponential. The results obtained are attributed to both the localization of SE at inhomogeneities of the polarization potential of the substrate, caused by the substrate roughness, and the transition of electrons from the localized to the delocalized state as the film thickness increased.

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