Anomalous Valley Zeeman Splitting Induced by In‐plane Magnetic Fields in a Mo _0.7 W _0.3 Se ₂ Monolayer

ABSTRACT We report an anomalous valley Zeeman effect in a monolayer with induced by in‐plane magnetic fields. Polarization‐resolved photoluminescence at K reveals linear valley splittings with effective ‐factors of (trion), (intravalley A exciton), and (intervalley exciton) in Voigt geometry. These values exceed both the out‐of‐plane response () and the expected zero splitting for in‐plane fields in binary transition‐metal dichalcogenides. Crucially, resonant inelastic laser‐light scattering yields much smaller ‐factors of 1.16–1.67, consistent with single‐particle Zeeman splitting. The factor‐of‐four ‐factor discrepancy, with near‐constant enhancement across excitonic complexes, indicates that the anomalous valley splitting arises from many‐body interactions. We attribute this effect to anisotropic tensor exchange coupling between excitons and the spin‐polarized Fermi sea. The bright exciton carries valley‐antisymmetric total spin, while the in‐plane field induces Pauli paramagnetic polarization in the Fermi sea. Isotropic exchange cannot couple these orthogonal spin components, but spin‐orbit‐induced anisotropic exchange allows for the coupling and produces valley‐dependent energy shifts. The effect is enhanced because the conduction‐band spin‐orbit splitting nearly vanishes at , unlocking the Fermi sea's in‐plane susceptibility. Our findings establish TMDC alloys near the spin‐orbit crossover as a platform for many‐body‐enhanced valley manipulation using in‐plane magnetic fields.