Electronic stripe-nematic phases are fascinating, strongly correlated states characterized by spontaneous rotational symmetry breaking. In the quantum Hall regime, such phases typically emerge at half-filled, high-orbital-index ($N\ge 2$) Landau levels (LLs) where the short-range Coulomb interaction is softened by the nodes of electron wave functions. In the lowest ($N=0$) LLs, these phases are not expected. Instead, composite fermion (CF) liquids and fractional quantum Hall states, which are well explained in the picture of weakly interacting CF quasiparticles, are favored. Here, we report the observation of an unexpected stripe-nematic phase in the lowest LL at filling factor $\nu =5/8$ in ultra-high-quality GaAs two-dimensional hole systems, evinced by a pronounced in-plane transport anisotropy. Remarkably, $\nu =5/8$ can be mapped to a half-filled, high-index CF LL ($N_{\mathrm{CF}}=2$), analogous to the $N=2$ hole LL. Our finding signals a novel stripe-nematic phase of CFs driven by the residual long-range interaction among these emergent quasiparticles. This phase is surprisingly robust, surviving up to $\sim 100\mathrm{mK}$. Its absence in electron-type systems suggests that severe LL mixing stemming from the large hole effective mass and nonlinear LL fan diagram plays a crucial role in modifying the CF-CF interaction.

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• Charge density waves
• Composite fermions
• Fractional quantum Hall effect
• Landau levels
• Topological phases of matter
• Two-dimensional electron system