Conductivity of two-dimensional crystals over liquid helium for different holding fields

The results are presented of the experimental study of electron crystals over liquid helium with surface electron density of 3.2×108 and 6.4×108 cm−2 (melting temperatures 0.4 and 0.58 K) at temperature 83 mK and for holding electric fields 300-1200 V/cm. The measurements are performed in the frequency range 1-14 MHz where the coupled phonon-ripplon resonances are observed in the experimental cell. The real and imaginary parts of the complex conductivity of the crystal have been obtained as a result of analysis of an electron layer response on exciting ac voltage with frequency corresponding to the mode (0,1) of the coupled phonon-ripplon oscillations. An analysis of the results allows us to suppose that structural defects of the electron crystal play an essential role in dissipation processes.