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, Al$_x$Ga$_{1-x}$As, for a range of Landau level fillings, $ν$, around 1. For $ν>0.8$ or $ν<1.2$, the samples exhibit a microwave resonance whose frequency decreases as $ν$ moves away from 1. A resonance with this behavior is the signature of solids of quasiparticles or -holes in the partially occupied Landau level, which was previously seen in ultralow disorder samples. For $ν<0.8$ down to as low as $ν=0.54$, a resonance in the spectra is still present in the Al alloy-disordered samples, though it is partially or completely suppressed at $ν=3/5$ and $1/2$, and is strongly damped over much of this $ν$ range. The resonance also shows a striking enhancement in peak frequency for $ν$ just below 3/4. We discuss possible explanations of the resonance behavior for $ν<0.8$ in terms of the composite fermion picture.

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