Fractional Quantum Hall Effect in AlAs 2D Electrons

We study two-dimensional (2D) electrons confined to an AlAs quantum well, with density n = 1.5 x 10¹¹ cm^-2 and unprecedented high mobility, 10⁶ cm²V^-1s^-1. The 2D electrons in this system occupy conduction-band minima (valleys) with anisotropic dispersion and effective mass. The magneto-resistance traces exhibit a rich sequence of fractional quantum Hall states near filling factor ν = ½, up to high-order fractions 8/15 and 8/17. We study the temperature dependence of these states, and extract energy gaps and composite fermion effective masses. We compare these with the relevant parameters in other 2D systems such as 2D electrons confined to GaAs quantum wells.