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_xGa_1-xAs 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 quasiholes in the partially occupied Landau level, and was previously seen only 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, although 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.