In semiconductors, exciton-exciton annihilation limits how densely you can pack these quasiparticles before they destroy each other. Higher mobility normally means higher collision rates and worse annihilation losses. Dense excitonic phases seem forbidden.
Park, Li, Kang, and colleagues show that rhombohedral (3R) stacked MoS₂ bilayers break this trade-off. Spontaneous polarization creates interlayer excitons with built-in electric dipoles. These dipoles repel each other, reducing the probability that two excitons approach within a Bohr radius—the distance needed for annihilation. The rate drops 18-fold compared to monolayers and 2.9-fold compared to nonpolar bilayers.
The key: exciton diffusivity is enhanced in these structures. The excitons move faster, but annihilate less. The repulsive dipole field creates an exclusion zone at the ~1.3 nm scale, so high-speed excitons bounce off each other rather than recombine. The process is governed by encounter probability within the Bohr radius, not by diffusion dynamics.
This decouples two properties that seemed inherently linked. Mobility measures how freely a particle moves. Annihilation rate measures how often moving particles destroy each other. In nonpolar materials, one tracks the other. Here, the built-in dipole inserts a gap between them—an energetic barrier at the collision scale that diffusion dynamics cannot override.
The constraint (spontaneous polarization) enables access to high-density excitonic regimes that annihilation would destroy in any nonpolar material. Faster-moving particles that interact less: a repulsive interaction converts velocity from a destructive to a benign property.