a strong spin splitting of the localized defect state

Due to the semiconductor nature of atomically thin transition metal dichalcogenides (TMDs), it has soon been recognized that defects can play important roles in the electron transport and the optical transition in TMDs. In particular, it was demonstrated recently that the point defect (e.g. W vacancy) in WSe₂ can behave as single quantum emitters, which are at the heart of the quantum optics and photonic quantum-information technologies. One of the essential steps towards the realistic application of the point defects in monolayer semiconductor is to tune the localized electronic structure trough the defect engineering. Here, we report a single potassium atom decoration of the W vacancy in monolayer WSe₂. Combing the scanning microscope/spectroscopy and the first principle calculation, we unveiled the atomic and electronic structures of the K/W-vac defect. We show that the K atom behaves as a surprising p-type dopant that shifts the acceptor states upwards, and narrows the local gap by about 0.2 eV compared with the intrinsic W-vac. Most interestingly, the symmetry breaking by the K atom leads to significant spin-orbit coupling splits of the localized defect states (> 100 meV). A local magnetic moment is predicted to form when the charging effect from the substrate was excluded.