Theoretical proposal and material realization of ferromagnetic negative charge-transfer energy insulator

Here we propose a type of ferromagnetic semiconductors: ferromagnetic negative charge-transfer energy insulator (FNCTEI). In FNCTEI, the negative charge-transfer states strongly enhance the ferromagnetic (FM) exchange interactions and the orbital hybridization gap permits the magnetic molecular orbitals as the underlying magnetic units rather than local atomic orbitals. Thus, the FM exchange interactions are rather strong and decay slowly due to the large spreading of magnetic molecular orbitals. This is distinct from the superexchange mechanism where FM exchange interactions are quite weak as summarized in the well-known Goodenough-Kanamori-Anderson semiempirical rules. Through first-principle calculations with the hybrid functional, PbO-type CrAs monolayer is mapped out to be a FNCTEI, which possesses a band gap $\ensuremath{\sim}0.35$ eV, FM nearest-/next-nearest-neighbor exchange coupling strength $\ensuremath{\sim}57/40$ meV, and a high ${T}_c\ensuremath{\sim}1500$ K respectively. It is believed that the theoretical prediction of FNCTEI supports the hypothesis by Khomskii and Sawatzky in 1997 [Solid State Commun. 102, 87 (1997)].