Dynamical coupling of surface acoustic waves to electrons on helium

We report on the dynamical coupling between high-frequency piezoelectric surface acoustic waves (SAWs) and a two-dimensional (2D) system of electrons floating on liquid helium. This coupling leads to a reduction in the velocity of the SAW and attenuation of SAW energy, which reflects the high-frequency, wavevector-dependent conductivity of 2D electron sheet. In our device, a piezoelectric substrate incorporates a micro-channel array filled with superfluid helium above which a 2D ensemble of electrons are trapped. Underlying electrodes capacitively couple to the electron system and are used to control the areal density of electrons. Employing pulsed time-of-flight SAW techniques, we measure the attenuation and the velocity shift of SAWs as function of electron density at T = 1.5 K where the electron system is in liquid state. We also present SAW response near the phase transition regime from the electronic liquid to the Wigner crystal.