Hidden Quantum Hall Stripes in Al${}_x$Ga${}_{1-x}$As/Al${}_{0.24}$Ga${}_{0.76}$As Quantum Wells

We report on transport signatures of hidden quantum Hall stripe (hQHS) phases in high ($N>2$) half-filled Landau levels of Al${}_x$Ga${}_{1-x}$As/Al${}_{0.24}$Ga${}_{0.76}$As quantum wells with varying Al mole fraction $x<{10}^{-3}$. Residing between the conventional stripe phases (lower $N$) and the isotropic liquid phases (higher $N$), where resistivity decreases as $1\mathrm{/}N$, these hQHS phases exhibit isotropic and $N$-independent resistivity. Using the experimental phase diagram we establish that the stripe phases are more robust than theoretically predicted, calling for improved theoretical treatment. We also show that, unlike conventional stripe phases, the hQHS phases do not occur in ultrahigh mobility GaAs quantum wells, but are likely to be found in other systems.