Nonlinear Features of Phonon–Ripplon Modes in the Electron Crystal Over Liquid Helium

The resonance spectra of coupled phonon–ripplon oscillations in two-dimensional electron crystals over liquid helium are measured at different driving electric potentials. The crystals with surface electron densities n _s = (3.2 − 12) × 10⁸ cm⁻² are studied at holding electric fields E _⊥ = 590−1180 V/cm at temperature T≈ 80 mK. It is found that an increase of the driving potential leads to a change of resonance curves: the curves become more flat, asymmetric, and splintered depending on electron density and holding potential. The data obtained are used to estimate the electron effective mass in the crystal. It is shown for the first time that the effective electron mass in the crystal increases as the driving field increases.