In condensed matter physics, the excitonic insulator (EI) state represents an exotic macroscopic quantum coherent phase, which, however, remains elusive. In this work, we reveal that bright EI is likely to be realized in $\mathrm{Tl}\mathit{X}{\left(\mathrm{C}{\mathrm{H}}_3\right)}_2$ ($\mathit{X}=\mathrm{As}$, Sb) as a quantum spin Hall insulator (QSHI), based on the first-principles many-body perturbation theory. In particular, we highlight that the weakened dielectric screening due to the strong spin-orbit coupling (SOC) enhances the exciton binding energy ($E_{\mathrm{b}}$), and the anomalous inverse quasiparticle (QP) correction suppresses the band gap ($E_{\mathrm{g}}$) broadening. In the light of these two factors, the band dispersion can be dramatically altered, and the linear scaling relation between $E_{\mathrm{b}}$ and $E_{\mathrm{g}}$ can be successfully broken, paving the way for the excitonic instability and the possible formation of EI. It is of importance that ground-state bright excitons can provide direct spectroscopic evidence for excitonic condensation. Our results present a paradigm for EI formation that directly couples to light and enables ways for the experimental detection of EI.

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• Excited-state quantum phase transitions
• Excitons
• Topological insulators
• 2-dimensional systems
• Bose-Einstein condensates
• Density functional theory
• First-principles calculations
• GW method
• Green's function methods