Room-temperature field-free valley exciton splitting in monolayer semiconductors achieved by perpendicular magnetic proximity effect

The magnetic proximity effect at ferromagnet/semiconductor interface has been demonstrated as an effective method to generate and control valley splitting states in the semiconductor, which has potential for valley-based information processing devices. However, currently, this method typically requires cryogenic temperature or continuous application of external fields, due to limitation of the ferromagnet used. Here, we report room-temperature valley exciton splitting in semiconducting monolayer WS2 without the need of an external magnetic field. This is achieved by interfacing the monolayer WS2 with a room-temperature ferromagnet L10-phase FePt possessing strong perpendicular magnetic anisotropy, which provides a field-free perpendicular magnetic proximity effect and permanently lifts the valley degeneracy of WS2 via Zeeman interaction. Circularly polarized photoluminescence measurements reveal that the intensity and energy of exciton emissions show strong dependence on the excitation and detection helicity and the magnetization direction of FePt, and the valley exciton splitting can reach very large values of 4.9, 11.7, and 19.2 meV for neutral exciton, trion, and defect-bound exciton, respectively.