Effects of short-ranged disorder on pinning modes of 2D electron system near $\nu =$1

We performed microwave measurements on 2-D electron systems (2DES) in heterostructures of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As/ Al$_{0.3}$Ga$_{0.7}$As in which the 2DES resides in dilute Al alloy, with x$=$0.21, 0.33 and 0.85{\%}. The dilute Al atoms are randomly distributed [1]. Around Landau filling $\nu =$1,the samples exhibit microwave resonances which differ from pinning mode resonances observed [2] near $\nu =$1 in unalloyed samples. The alloyed samples have larger resonance frequencies, and resonances that exist over wider ranges of $\nu $, extending to \textbar $\nu $-1\textbar $\sim$ 0.2. Also in the disordered samples, the resonances are not quasiparticle-quasihole symmetric around $\nu =$1, and there is strong frequency and $\nu $ dependence in the spectra away from rational fractional $\nu $, down to 0.6. \\[4pt] [1] W. Li \textit{et al.}, \textit{Appl.Phys. Lett. }\textbf{83}, 2832 (2003).\\[0pt] [2] Y. P. Chen et al., Phys. Rev. Lett. \textbf{91},016801 (2003).