Giant exchange-mediated exciton splitting and linear optical dichroism in monolayer MoS₂ in a proximity-induced moiré system

Moiré patterns of transition metal dichalcogenides (TMDs), formed by twisting or lattice mismatch, have been widely explored to discover emergent electronic and excitonic phenomena. We investigate a new design principle to induce a similar moiré effect in a TMD monolayer by stacking it on top of a material with alternating ferroelectric domains. Using ab initio GW plus Bethe-Salpeter equation approach, we find that the low energy spectrum is dominated by quasi-one dimensional Wannier excitons, with a confinement length of about 2 nm. Unlike degenerate valley excitons with circular dichroism in pristine TMDs, these excitons are energetically split and show linear dichroism as a combined effect of confinement and exchange interaction. Extracting a model Hamiltonian focused on the low-energy spectrum, we explore the interplay of confinement and exchange interaction in determining the excitonic properties and phases. We show how the extreme exciton confinement generated by a proximity moiré system yields larger exciton splitting compared to what is currently possible with lithographic approaches, offering new possibilities for valley engineering in TMDs.