Subharmonic phonon-ripplon coupling in the 2D Wigner solid on superfluid helium

The nonlinear response of the liquid-helium surface to the oscillating motion of a two-dimensional electron solid is analyzed. In the nonlinear regime, we found that the effective mass of surface dimples formed under electrons behaves as a singular odd function near subharmonics of the frequency of a ripplon whose wave vector coincides with a reciprocal-lattice vector. This unexpected behavior of the dimple mass is shown to lead to the appearance of new longitudinal phonon-ripplon coupled modes. It also affects in a nontrivial way positions of conventional phonon-ripplon coupled modes. Both these theoretical results explain experimental observations reported previously. Copyright (C) EPLA, 2017