Multivalley two-dimensional electron system in an AlAs quantum well with mobility exceeding 2 x 106 cm2 V-1 s-1

Degenerate conduction-band minima, or valleys, in materials such as Si, AlAs, graphene, and MoS2 allow them to host two-dimensional electron systems (2DESs) that can access a valley degree of freedom. These multivalley 2DESs present exciting opportunities for both pragmatic and fundamental research alike because not only are they a platform for valleytronic devices, but they also provide a tool to tune and investigate the properties of complex many-body ground states. Here, we report ultra-high-quality, modulation-doped AlAs quantum wells containing 2DESs that occupy two anisotropic valleys and have electron mobilities peaking at 2.4 x 10(6) cm(2) V-1 s(-1) at a density of 2.2 x 10(11) cm(-2). This is more than an order of magnitude improvement in mobility over previous results. The unprecedented quality of our samples is demonstrated by magneto-transport data that show high-order fractional quantum Hall minima up to the Landau level filling nu = 8/17, and even the elusive nu = 1/5 quantum Hall state.