Degenerate conduction-band minima, or `valleys, in materials such as Si, AlAs, graphene, and MoS$_2$ 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 $2.4times10^6$ cm$^{2}$V$^{-1}$s$^{-1}$ at a density of $2.2times10^{11}$ 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 $ u=8/17$, and even the elusive $ u=1/5$ quantum Hall state.
Download