Observation of Anisotropic Stripe Phases at Even Denominator Fillings $\nu $ $=$ 3/2, 5/2 and 7/2 in a Narrow AlAs Quantum Well

Anisotropic stripe phases are observed at high-index, half-filled Landau levels in high quality 2D carrier systems with high mobility. Here, we report the observation of such phases at filling factors $\nu =$ 3/2, 5/2 and 7/2 under tilted magnetic field in a narrow (56.6 {\AA}-wide) AlAs quantum well where electrons occupy an out of plane (Z) valley with a density of 3x10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$ and a modest mobility of about 7 m$^{\mathrm{2}}$/Vs. At a critical tilting angle, when $\nu =$ 3/2 undergoes a transition from the N$=$ 0 (Z0$\uparrow )$ to the N$=$ 1 (Z1$\downarrow )$ Landau level, a resistance spike emerges, indicating a ferromagnetic transition. For higher tilting angles, the ground state at $\nu =$ 3/2 develops into a stripe phase with the resistance along B$_{\mathrm{\vert \vert }}$ exhibiting a minimum whose value is about 6 times smaller (in a Hall-bar geometry) than the resistance perpendicular to B$_{\mathrm{\vert \vert }}$ which shows a maximum. Similar phenomenon happens for $\nu =$ 5/2 and 7/2, when they undergo transitions from Z0$\uparrow $ to Z2$\downarrow $ and Z0$\uparrow $ to Z3$\downarrow $, respectively, at yet higher tilt angles. Furthermore, the anisotropic phases appear to be quite robust as they persist even at 1.8 K.