Wigner crystal in a two-dimensional electron system in the vicinity of filling factor 1/5: Acoustic studies

By simultaneous measurements of the attenuation and velocity of surface acoustic waves propagating in proximity to a high-quality GaAs quantum well we study the complex AC conductance of the two-dimensional electron system. Focusing on the vicinity of the filling factor $ u=1/5$ we confirm that the insulating states formed closely to this value of $ u$ are pinned Wigner crystals.