Mobility of 2D Electrons on Pure 4He and on 3He-4He Dilute Solution

The mobility of 2D electrons on pure He-4 and on 0.5 % solution of He-3 in He-4 was investigated for different electron densities in the temperature range 0.12 to 1.3 K. The electrons in the same electron density show the same transition temperature from liquid state to Wigner crystal state in both pure He-4 and in the solution. In the high temperature range where the gas-scattering is dominant, the electrons show a smaller mobility in the solution than in the pure He-4 due to the electron collision with He-3 gas atoms which have a higher vapor pressure. In the middle temperature range where the ripplon-scattering is dominant, the mobility in the solution is smaller than in He-4. This is explained by a smaller surface tension caused by He-3 atoms collected at the surface. In the low temperature range where electrons are in the Wigner crystal state, the mobility gradually increases with decreasing temperature in the solution, while it stays almost constant in the pure He-4. The mobility increase is more pronounced in the low electron density. The results are qualitatively in agreement with the existing theory which includes the bulk He-3 quasiparticle reflection from surface dimples and the effect of the surface layer of He-3 atoms.