Gate-tunable chiral spin mode in WSe2/WS2 moiré superlattices.

The interplay between many-body and spin-orbit coupling (SOC) effects can lead to previously unobserved phenomena in solids. Chiral spin modes (CSMs) are collective spin excitations that arise from such interplay and connect states with opposite chirality. Here, we report a gate-tunable CSM that occurs between conduction SOC-split minibands in near-0°-twisted WSe2/WS2 bilayers. This mode manifests as a sharp resonance with giant Raman efficiency in the pseudovector-symmetry channel of the Raman spectra. By varying the filling, the CSM transitions from a chiral spin exciton to an excitonic polaron. The spin-flip nature is directly confirmed by the Zeeman effect, and the filling dependence indicates a charge-transfer insulator at unity filling. Our results demonstrate that transition metal-dichalcogenide moiré superlattices are a fertile platform for exploring exotic low-lying collective excitations.