Researchers report that the way two atomically thin semiconductor layers are stacked can make a large difference to how bound electron–hole pairs, called excitons, interact and survive. By comparing two types of WSe2/WS2 moiré heterobilayers—R-stacked (0° twist) and H-stacked (60° twist)—they show that the H-stacked geometry boosts repulsion between excitons on neighboring lattice sites and produces a long-lived, valley-polarized excitonic Mott state that resists relaxation for unusually long times.

The team made high-quality moiré superlattices by stacking monolayers of WSe2 and WS2 with controlled twist angles, encapsulating them in hexagonal boron nitride, and placing them on a mirror-coated substrate. They used helicity-resolved transient photoluminescence (PL) — a time-resolved way to measure light emitted with a given circular polarization — together with first-principles–informed modelling. Measurements were taken at short delays (1 ns) after excitation so the excitons had relaxed into the moiré potential but before tunneling and nonradiative losses dominated.

The key finding is that H-stacked excitons have a different charge shape than R-stacked ones. In R-stacks the electron and hole sit above one another and form a vertical dipole. In H-stacks the electron and hole are laterally displaced, producing an out-of-plane dipole plus a pronounced in-plane quadrupole. This quadrupolar geometry raises the effective intersite repulsion Vxx by at least a factor of two compared with the dipolar R-stacked case. Experimentally this shows up as a larger density-driven blueshift of the single-occupancy emission line (about 11 meV in H-stacks versus about 5 meV in R-stacks at unit filling), and a smaller onsite doublon energy (the second emission line sits ~27 meV above the first in H-stacks versus ~36 meV in R-stacks).