Coexistence of intrinsic piezoelectricity, ferromagnetism, and nontrivial band topology in Li-decorated Janus monolayer Fe2SSe with a high Curie temperature

Recently, predictions have been made about quantum anomalous Hall (QAH) insulators in Lithium-decorated iron-based superconductor monolayer materials (LiFeX (X = S, Se and Te)) with very high Curie temperatures (0.00,0.00,1.00PRL 125, 086401 (2020)), which combine topological and ferromagnetic orders. It is interesting and useful to achieve the coexistence of intrinsic piezoelectricity, ferromagnetism, and nontrivial band topology in single two-dimensional (2D) materials, namely 2D piezoelectric quantum anomalous Hall insulators (PQAHIs). In this work, 2D Janus monolayer Li2Fe2SSe is predicted to be a room-temperature PQAHI, which possesses dynamic, mechanical, and thermal stabilities. It is predicted to be a half Dirac semimetal without spin-orbit coupling (SOC). It is found that the inclusion of SOC opens up a large nontrivial gap, which means nontrivial bulk topology (QAH insulator), confirmed by the calculation of Berry curvature and the presence of two chiral edge states (Chern number C = 2). Calculated results show that the monolayer Li2Fe2SSe possesses robust QAH states against biaxial strain and electronic correlations. Compared to LiFeX, the glide mirror G ( z ) of Li2Fe2SSe disappears, which will only induce an out-of-plane piezoelectric response. The calculated out-of-plane d (31) of the monolayer Li2Fe2SSe is -0.238 pm V-1, comparable with those of other known 2D materials. Moreover, a very high Curie temperature (about 1000 K) is predicted by using Monte Carlo simulations, which means that the QAH effect can be achieved at room temperature in the Janus monolayer Li2Fe2SSe. Similar to the monolayer Li2Fe2SSe, the PQAHI can also be realized in the Janus monolayer Li2Fe2SeTe. Our work opens a new avenue in searching for PQAHIs with high temperatures and high Chern numbers, which provide a potential platform for multi-functional spintronic applications.