Mobility of a 2D Wigner crystal over liquid 4He and 3He - 4He solution

Spectrum of the coupled electron-ripplon oscillations in the Wigner crystal over liquid helium with surface electron density 1.3 x 109 cm-2 in the temperature range 70-600 mK is studied. The experimental data of resonance width are shown not to be explained by capillary waves attenuation. The consideration is performed using both volume and surface hydrodynamics. A temperature dependence of mobility of the crystal is obtained. Influence of 3He impurities with concentration 0.25% on frequencies and width of the coupled electron-ripplon resonances is studied. It is found that the presence of the 3He impurity causes a noticeable widening of the resonances and decreases the resonant frequencies. The frequency decrease is explained to be due to the change in the surface tension of liquid. (orig.)