Excitonic Instability in Quantum Spin Hall Insulators TlX (X = As, Sb, Bi).

Excitonic insulator (EI), a long-sought yet exceptionally scarce macroscopic quantum phase of matter, has remained elusive. We propose that quantum spin Hall insulators (QSHIs) can be promising candidates for realizing excitonic instability. This is enabled by the atypical inverse quasiparticle (QP) correction to the inverted bands of QSHIs, which decouples the synergy of the QP band gap (Eg) and exciton binding energy (Eb). Combining first-principles calculations with many-body perturbation theory, we verify our proposal in the QSHIs TlX (X = As, Sb, Bi) with Eb exceeding the QP Eg. In stark contrast to the previously proposed EI dominated by dark excitons, the EI state in this study is constituted by bright excitons with negative formation energy and large oscillator strength, providing an optically accessible signature for detecting the EI. In particular, the emergence of correlated EI accompanied by nontrivial band topology unveils a novel paradigm of (bright) topological EI.