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

Abstract

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, $\nu$, around 1. For $\nu >0.8$ or $\nu <1.2$, the samples exhibit a microwave resonance whose frequency decreases as $\nu$ 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 $\nu <0.8$ down to as low as $\nu =0.54$, a resonance in the spectra is still present in the Al alloy-disordered samples, though it is partially or completely suppressed at $\nu =3/5$ and $1/2$, and is strongly damped over much of this $\nu$ range. The resonance also shows a striking enhancement in peak frequency for $\nu$ just below 3/4. We discuss possible explanations of the resonance behavior for $\nu <0.8$ in terms of the composite fermion picture.