Pinning mode of 2D electron system with short-range alloy disorder

At the low Landau filling ($\nu )$ termination of the fractional quantum Hall effect (FQHE) series, a two-dimensional electron system (2DES) becomes an insulator, which is identified in sufficiently low-disorder samples as a form of pinned Wigner solid. The microwave conductivity spectrum of such a solid shows a striking resonance, which is understood as a pinning mode, in which pieces of solid oscillate within the disorder potential. We report on the observation of the pinning mode of a 2DES that resides within Al$_{x}$Ga$_{1-x}$As with x=0.85{\%}. For a carrier density of n= 8.7 x10$^{10}$ /cm$^{2}$, a resonance with a peak frequency (f$_{pk})$ of about 5 GHz appears as $\nu $ goes below the 2/3 FQHE. A local minimum in resonance amplitude vs. $\nu $ occurs around $\nu $ =1/2. We will discuss the contribution of the alloy disorder to f$_{pk}$.