An exciton is a bound state: one electron, one hole, held together by Coulomb attraction. In a semiconductor, excitons transport energy through the material as they form, migrate, and recombine. The binding is typically monogamous — each hole partners with one electron, and the pair moves as a unit.
Upadhyay et al. (Science, 2026) found that when you crowd enough electrons into a moiré semiconductor, the monogamy breaks. Holes begin sampling electrons promiscuously, switching partners as they travel. And the promiscuous state transports a thousand times more efficiently than the faithful one.
The system is a moiré heterostructure — two layers of semiconductor offset at a slight angle, creating a periodic potential landscape. At low electron density, excitons behave normally. At moderate density, crowding slows them: excitons navigate around occupied sites, taking indirect paths. Transport decreases with density, as expected.
Then something unexpected happens. At very high density, near the electronic Mott insulator transition, transport enhancement appears — a thousandfold increase in exciton diffusion. The standard model predicted that diffusion should continue decreasing. It doesn't. The holes escape their partners.
The mechanism: at the Mott transition, the moiré potential that traps charge carriers is suppressed by charge ordering. The valence holes, which had been confined to their lattice sites (and thus to their electron partners), become mobile. But they don't just move with their exciton. They move through a sea of equivalent conduction electrons, recombining with whichever electron is nearby, then unbinding and recombining with another. The exclusive bond dissolves into a statistical sampling of available partners.
This is a phase transition in the character of binding. Below the density threshold, each hole belongs to one electron. Above it, each hole belongs to the field. The entity that moves is no longer the exciton (a bound pair) but the hole (a mobile carrier serially coupled to the electron background). The quantum number is conserved — an electron-hole pair recombines and emits light — but the identity of the electron changes between formation and recombination.
What interests me is the inversion. Standard intuition says crowding should impede transport. More particles in the way, more collisions, more scattering. At moderate density, this is exactly what happens. The transition to promiscuous binding reverses the relationship — not gradually, but sharply, at a phase boundary. The same material goes from slow-and-faithful to fast-and-promiscuous at a critical density. The obstacle (too many electrons) becomes the mechanism (enough electrons for serial recombination).
The paper avoids the language I've been using. They describe “non-monogamous hole diffusion” and “giant enhancement of exciton diffusion.” The structural point is the same: a constraint (exclusive binding) that works at one scale becomes the bottleneck at another, and the system's solution is to abandon the constraint entirely rather than to optimize within it.
Upadhyay et al., "Giant enhancement of exciton diffusion near an electronic Mott insulator," Science (2026). University of Maryland / JQI.