Microwave pinning modes near Landau filling ν = 1 in two-dimensional electron systems with alloy disorder

We report measurements of microwave spectra of two-dimensional electron systems hosted in dilute Al alloy ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ for a range of Landau level fillings $\ensuremath{\nu}$ around 1. For $\ensuremath{\nu}g0.8$ or $\ensuremath{\nu}l1.2$, the samples exhibit a microwave resonance whose frequency decreases as $\ensuremath{\nu}$ moves away from 1. A resonance with this behavior is the signature of solids of quasiparticles or quasiholes in the partially occupied Landau level, and was previously seen only in ultralow-disorder samples. For $\ensuremath{\nu}l0.8$ down to as low as $\ensuremath{\nu}=0.54$, a resonance in the spectra is still present in the Al alloy-disordered samples, although it is partially or completely suppressed at $\ensuremath{\nu}=\frac{3}{5}$ and $\frac{1}{2}$, and is strongly damped over much of this $\ensuremath{\nu}$ range. The resonance also shows a striking enhancement in peak frequency for $\ensuremath{\nu}$ just below $\frac{3}{4}$. We discuss possible explanations of the resonance behavior for $\ensuremath{\nu}l0.8$ in terms of the composite fermion picture.