0_3160 Magnetic/Topological excitonic insulators combine magnetism/nontrivial-topology and spontaneous exciton condensation, with dual functionality of magnetic/topological insulators and excitonic insulators. Yet, they are very rare and little is known about their formation. In this presentation I will describe our recent progress in finding and designing two-dimensional magnetic excitonic insulators and topological excitonic insulators using first-principles GW-BSE calculations in accordance with the selection rule. We reveal an unusual electronic state (dubbed as half excitonic insulator) in monolayer 1T-MX₂ (M = Co, Ni and X = Cl, Br). Its one spin channel has a many-body ground state due to excitonic instability, while the other is characterized by a conventional band insulator gap. We predict the semi-hydrogenated graphene (known as graphone) as a spin-triplet excitonic insulator with a critical temperature of 11.5 K. We find that a mechanism dubbed as parity frustration prevents excitonic instability in usual topological insulators, and those whose band inversion is independent of spin-orbit coupling are possible candidates. We verify this on four monolayer double-transition-metal carbides (MXenes), which show a robust thermal-equilibrium exciton condensation, being sufficient for topological applications at room temperature.References

1. Zeyu Jiang, Yuanchang Li, Wenhui Duan, and Shengbai Zhang, PRL 122, 236402 (2019).
2. Zeyu Jiang, Wenkai Lou, Yu Liu, Yuanchang Li, Haifeng Song, Kai Chang, Wenhui Duan, and Shengbai Zhang, PRL 124, 166401 (2020).
3. Dong Shan and Yuanchang Li, https://arxiv.org/abs/2303.16627v1 (2023).