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

TL;DR

This study investigates microwave resonances in two-dimensional electron systems with alloy disorder near Landau level filling factor 1, revealing how disorder affects the pinning modes of electron solids and their dependence on filling factors.

Contribution

It provides new insights into microwave resonance behavior in alloy-disordered 2D electron systems near $ u=1$, highlighting the impact of alloy disorder on pinning modes and composite fermion states.

Findings

Resonance frequency decreases as $ u$ moves away from 1.

Resonance persists down to $ u=0.54$ despite alloy disorder.

Resonance is suppressed or damped at certain fractional fillings.

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.