Unveiling the Spin-Valley Structure of Dipolar Exciton Ladders in R-stacked WSe$_2$/WS$_2$ Moiré Heterobilayers

Abstract

Localized interlayer excitons in moiré heterobilayers can form dipolar exciton ladders, yet their internal spin-valley structure remains unresolved. Here, we use helicity-resolved magneto-photoluminescence to identify the microscopic origin of the ladder in R-stacked WSe$_2$/WS$_2$ at charge neutrality and one-electron filling of the moiré lattice. At charge neutrality, the first two emission peaks correspond to a spin-triplet interlayer exciton and a triplet-triplet two-exciton state separated by 38 meV, reflecting the on-site dipolar interaction. The opposite Zeeman response of the apparent third rung of the ladder rules out its assignment as a spin-conserving three-exciton state and instead identifies it as a triplet-singlet two-exciton configuration with a 22 meV offset set by the WS$_2$ conduction-band spin splitting. At one-electron filling, the correlated electronic background gives rise to charged one- and two-exciton states and intervalley/intravalley two-exciton configurations, while reducing the effective exciton-exciton interaction. Our results establish a spin-valley-resolved picture of dipolar exciton ladders beyond simple occupation-number physics in moiré heterobilayers.