  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 $1/N$, 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.
