Transport kinetics in excitonic Mott insulator

In the quantum world, particles do not always behave like solitary travelers; sometimes, they move as a single, coordinated crowd. When certain particles called bosons are cooled to extremely low temperatures and confined in a repeating pattern, they can enter a state where they flow without any resistance, a phenomenon known as superfluidity. However, if the density of these particles becomes too high, they can suddenly stop moving entirely, locking themselves into place to form a Mott insulator. This transition from a frictionless flow to a rigid stop is a fundamental prediction of physics, but observing it requires a system so pure that the particles are not stopped by random bumps or impurities in the material, but only by their own interactions with one another. For decades, scientists have sought a clean enough platform to watch this specific switch happen, distinguishing the natural locking of particles from the accidental jamming caused by a messy environment.

Researchers at the University of California, San Diego, have now captured this transition in action using a unique sandwich of two atom-thin materials. They created a device by stacking a single layer of molybdenum diselenide on top of a single layer of tungsten diselenide. When light hits this structure, it creates special particles called indirect excitons. These are pairs consisting of an electron in one layer and a hole (a missing electron) in the other, held together by electrical attraction but separated by the gap between the layers. This separation gives them a long life, allowing them to travel far and settle into a calm state before they disappear. Because the atomic layers are slightly twisted relative to each other, they form a repeating, honeycomb-like landscape of energy hills and valleys, known as a moiré potential. This landscape acts like a grid of tiny traps where the excitons can sit.

The team focused on how these excitons move across this grid as they increased the number of particles. At lower densities, the excitons moved freely, spreading out from the spot where the laser hit them. As the researchers increased the density to a specific point where the grid was about half full, the particles moved with extraordinary speed and efficiency, a behavior consistent with a superfluid state. In this regime, the particles seemed to glide over the landscape without getting stuck, proving that the material was clean enough to allow collective quantum states to form. This high-speed movement was crucial because it ruled out the possibility that the particles were being slowed down by random defects in the material.

However, the story changed dramatically when the researchers pushed the density even higher, filling the grid so that nearly every trap contained a particle. At this point, the smooth, fast flow suddenly collapsed. Instead of continuing to spread out, the cloud of particles began to shrink back toward the center. The researchers observed that the distance the particles traveled increased at first, but then reversed, contracting as the density reached the point where the grid was completely full. This contraction was the smoking gun for the Mott insulator state. It showed that the particles had become so crowded that they could no longer hop from one trap to the next, effectively freezing in place due to their own mutual repulsion. This was not a result of the material being dirty; the initial fast movement proved the material was clean, meaning the stop was a deliberate, collective decision by the particles themselves.

Along with this shrinking cloud, a distinct peak in the density of particles appeared right at the center where the laser hit. In the earlier, faster-moving stages, the particles spread out smoothly, fading away gradually from the center. But in the high-density state, the particles piled up at the origin, creating a sharp spike in their numbers before dropping off. This accumulation happened because the particles could not move away from the center once the grid was full. The researchers confirmed these observations by running computer simulations that modeled the particles moving through the grid. These simulations, which did not rely on complex mathematics but rather on the physical rules of how the particles interact, successfully reproduced both the initial expansion and the subsequent contraction of the cloud, as well as the central peak.

The study also revealed that this delicate quantum behavior is fragile. When the temperature was raised above 10 Kelvin, the effect vanished, and the particles returned to a slower, more ordinary spreading motion. This temperature limit aligns with theoretical expectations for when such quantum states can survive. By watching the excitons expand and then contract, the team provided direct evidence of the transition from a superfluid to a Mott insulator. They demonstrated that in a sufficiently clean material, particles can be forced to stop moving not because they are stuck in a mess, but because they have filled every available spot and are forced to stay put by the rules of quantum mechanics. This work offers a clear window into how matter can change its fundamental nature simply by becoming more crowded, turning a flowing fluid into a rigid solid through the power of interaction alone.