Electrons on Helium in a Quantizing Magnetic Field

Abstract

Electrons above the surface of superfluid helium are held in a vertical potential well formed by the repulsion of the helium atoms and the attraction of the image or polarization charges induced in the liquid, which may be augmented by a vertical electric holding field F _z Below 1K all the electrons can be in the quantum ground state as far as vertical motion is concerned. However, the electrons are free to move parallel to the helium surface (the x–y plane) and form a non-degenerate two-dimensional electron gas (2DEG), density n, with a Maxwell-Boltzmann distribution of thermal velocities. Their mobility µ in a horizonatal electric field is limited by collisions with He atoms from the vapour and with the surface wave excitations, or ripplons, on the liquid helium. Electron-electron correlations are also very important and lead to a phase transition to a classical electron solid at low temperatures. This solid phase prevents the Fermi degenerate gas regime from being reached for electrons on bulk liquid. This coupled 2-D system of electrons and helium is of great interest both for the properties of the electrons and for the information which can be obtained about the helium itself.