Two-dimensional quantum materials can host original electronic phases that arise from the interplay of electronic correlations, symmetry, and topology. In particular, breaking internal symmetry that acts simultaneously on the pseudospin and the spatial degree of freedom realizes a nematic phase ordering. Here, we report evidence of a quantum Hall nematic phase with an underlying SU(3) internal symmetry carried by the valley flavor of $\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{S}{\mathrm{n}}_x\mathrm{Se}$ quantum wells. Under SU(3) symmetry, this two-dimensional Dirac system exhibits a spin-polarized ground Landau level that is threefold valley degenerated, giving a robust $h/3e^2$ resistance plateau. A control over the valley pseudospin is further obtained by means of an in-plane magnetic field that acts as a valley Zeeman field, in analogy with the spin Zeeman field. By revealing the emergence of a $h/2e^2$ resistance plateau under tilted magnetic field, we demonstrate a quantum Hall valley nematic phase and the concomitant SU(3) symmetry breaking. An explicit SU(3) flavor symmetry breaking is achieved. Such evidence is of fundamental interest to shape the rich quantum Hall physics of a unique SU(3) system.

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• Dirac fermions
• Electrical properties
• Nematic order
• Quantum Hall effect
• Valley degrees of freedom