We investigate the temperature dependence of the pseudospin phase transition at filling factor $\nu =2$ in the quantum Hall regime. Experiments were performed on a high-quality, wide AlAs quantum well, where the pseudospin degree of freedom arises naturally from the near degeneracy and equivalence of the two conduction-band valleys. The anisotropic $g$-factor inherent to AlAs enabled independent measurement of the spin resonance response for each valley, thereby allowing quantification of the pseudospin polarization. The observed critical behavior of the pseudospin polarization yielded a critical temperature ${\text{T}}_c=0.9\mathrm{K}$ for the phase transition. The value of ${\text{T}}_c$ was further verified by the temperature dependence of the resistance spike that emerges at the phase transition.

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• Ferromagnetism
• Phase transitions
• Quantum Hall effect
• Transport phenomena
• Valley degrees of freedom
• Quantum wells
• Semiconductor compounds
• Strongly correlated systems
• Two-dimensional electron system
• Electron spin resonance