Studies of Melting and Sliding Charge-Density - Conduction of Two-Dimensional Electrons on Helium Films

In this thesis, we present a new and unique two -dimensional electron system--surface electrons on a thin helium film supported by a dielectric substrate. An order of magnitude higher electronic density was achieved than attainable on bulk helium. In addition, the random substrate impurity potentials were variable. An experimental cell was designed to measure the transport properties of the surface electrons. Techniques for determining the thickness of a helium film, for measuring the electronic density, for developing an electron source, and for selecting the underlying substrate are discussed. The phase diagram of a screened Coulomb system was studied. The strength of the electron-electron interaction was varied by changing the electronic density or altering the screening from the image charges in the substrate via the film thickness. The melting temperature was determined by a sharp metal-insulator transition due to the pinning of the Wigner lattice. The melting curve is in quantitative agreement with the Kosterlitz-Thouless theory, within the uncertainty in the screening of the electron-electron interaction. The crystalization of the electrons forms a natural charge-density-wave with the phase of the lattice pinned by the random potential provided by the spatial variation of the underlying substrate. A series of experiments were performed to study the conduction properties. The dominant features include a field dependence of the conductivity, a frequency dependence of the conductivity, the effects of a superimposed ac field and conduction noise. These phenomena were found to be similar to those observed in Peierls instability induced charge-density-waves in one -dimensional metals. The Coulomb interaction was found to play a large role in the conduction in this two-dimensional electron system.