While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a graphone, however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. The critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in graphone, which offers yet another test by experiment.
@article{arxiv.1911.05244,
title = {Spin-Triplet Excitonic Insulator: The Case of Graphone},
author = {Zeyu Jiang and Wenkai Lou and Yu Liu and Yuanchang Li and Haifeng Song and Kai Chang and Wenhui Duan and Shengbai Zhang},
journal= {arXiv preprint arXiv:1911.05244},
year = {2020}
}