Quantum Many - Body Effect of Two - Dimensional Electron Systems on Helium Films

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We have investigated a two - dimensional (2-D) electron system formed on a superfluid helium film. A variety of novel quantum phenomena such as quantum melting of the Wigner crystal, a low - density quantum fluid state and its quantum Hall effects have been expected for the electrons on high - dielectric constant materials. However, experiments have failed to realize stable 2-D electrons. In this study we have attempted to form 2-D electrons and to make conductivity measurement. The results are summarized as follows:1. We have made and constructed a dilution refrigerator, a ^4He cryostat, and a AC conductivity measurement setup. We have developed a novel conductivity cell, in which the height of helium surface is controlled by a piezoelectric motor. This makes a fine thickness. We have employed an atomically flat sapphire substrate (dielectric constant 〜9). We have succeeded to form the very low density electrons, and taken conductivity data in a wide range of temperature.2. In order to control the coupling of the electrons to helium surface excitations (ripplons), we have conducted the conductivity measurement for electrons on ^3He - ^4He liquid mixtures. We have quite unexpectedly observed resonance phenomena under perpendicular magnetic field. We have revealed that the resonances result from an inhomogeneity of electron density, and the addition of ^3He atoms to liquid ^4He enhances and stabilizes the density inhomogeneity.We have observed an anomalous nonlinear transport phenomenon in the 2-D Wigner crystal on liquid ^3He revealed theoretically that the nonlinearity is attributed to a strong damping effect inherent in liquid ^3He. This finding gives an important clue for future experiments on electrons on ^3He films at ultraslow temperatures.