Spin-Triplet Topological Excitonic Insulators in Two-dimensional Materials

TL;DR

This paper predicts high-temperature spin-triplet topological excitonic insulators in 2D quantum spin-Hall insulators like AsO and Mo2TiC2O2, revealing new topological phases with unique optical properties.

Contribution

It introduces the theoretical prediction of topological excitonic insulators with spin-triplet pairing in 2D QSHIs, combining first-principles calculations and models.

Findings

Existence of high-temperature topological EIs in 2D QSHIs.

Spin-triplet EI phase preserves time-reversal symmetry.

Distinct optical signatures during phase transition.

Abstract

Quantum spin-hall insulator (QSHI) processes nontrivial topology. We notice that the electronic structures of some particular QSHIs are favorable for realization of excitonic insulators (EIs). Using first-principles many-body perturbation theory ($GW$+BSE) and $k \cdot p$ model, we show that high-temperature ($T$) topological EIs with unlike spin can exist in such QSHIs with non-vanishing band gaps, e.g. 2D AsO and $\text{Mo}_2\text{Ti}\text{C}_2\text{O}_2$. Spin-triplet type EI phase induced by strong electron-hole interaction preserves time-reversal symmetry and the topological characteristics. A novel optical selection rule exists, upon going through the phase transition from the normal QSHIs to the topological EIs, absorption spectroscopy shows pronounced $T$-dependent changes, providing guidance for future experimental detections. The demonstrated coupling between EIs and topology also means that rich physics exists in such materials which retain such interdisciplinary features.