Incoherent dark excitons extend Floquet-like physics across time and momentum

In the world of semiconductors, the behavior of electrons is usually governed by the material's natural structure, but scientists have long sought a way to reshape these electronic landscapes on demand. One powerful method involves using light to create a rhythmic, repeating force that mixes different energy states, a phenomenon known as Floquet physics. Imagine a drumbeat that is so steady and strong it forces the drumhead to vibrate in a completely new pattern; in a similar way, intense, coherent light waves can hybridize, or blend, the energy bands of a material, creating new states that do not exist in the dark. However, this effect has historically been fleeting. It relies on the light wave maintaining a perfect, synchronized rhythm, known as coherence. As soon as the light pulse ends or the material's internal vibrations scramble this rhythm, the effect vanishes, typically lasting only a fraction of a trillionth of a second. This extreme brevity has limited the practical use of such engineered states, confining them to the realm of ultrafast experiments rather than stable devices.

A team of researchers at the Okinawa Institute of Science and Technology, along with collaborators from Italy, the United States, and Japan, has now discovered a way to achieve a similar reshaping of electronic states that lasts nearly a thousand times longer. By using a specific type of material stack made from two layers of atomically thin crystals, they found that a dense crowd of invisible, "dark" particles can mimic the effects of a rhythmic light drive without needing the light to stay coherent. These particles, called interlayer excitons, are formed when an electron and a hole (a missing electron) get trapped in different layers of the material. Because they are separated, they cannot easily recombine and disappear, allowing them to persist for nearly a nanosecond. The researchers demonstrated that a dense population of these long-lived particles creates a static, unchanging force that hybridizes the material's energy bands just as a coherent light wave would, but with the added ability to be tuned by twisting the layers of the material. This discovery opens a path to engineering electronic properties that are robust and long-lasting, moving beyond the fragile, split-second world of coherent light.

To explore this, the scientists constructed a sandwich of two different materials: a single layer of tungsten diselenide placed on top of a single layer of molybdenum disulfide. They carefully rotated one layer relative to the other at specific angles, creating a twisted structure. Using a sophisticated camera capable of capturing the energy and momentum of electrons flying off the surface, they shined a pulse of laser light tuned to create a massive number of excitons within the tungsten diselenide layer. Almost immediately, these excitons scattered and transformed into the interlayer variety, with electrons settling in the molybdenum disulfide layer and holes remaining in the tungsten diselenide. The researchers then waited, observing the material at different time delays after the initial laser pulse.

At very short times, the material behaves as expected, but the team focused their attention on what happened much later, specifically at a delay of 100 picoseconds. By this point, any initial rhythmic coherence from the laser pulse had long since vanished. Yet, the electronic structure of the material had not returned to its original state. Instead, the researchers observed a dramatic change in the way electrons moved through the top layer of the material. The natural, smooth curve of the energy band had transformed into a shape resembling a sombrero, with a dip in the center and a rise around the edges. This specific shape, known as a Mexican-hat dispersion, is a signature of strong mixing between different energy bands. Crucially, this transformation persisted for nearly a nanosecond, a duration that is orders of magnitude longer than the fleeting existence of coherent light-driven states.

The study revealed that the strength of this band mixing was directly tied to the number of these dark excitons present. As the density of the excitons increased, the hybridization effect grew stronger in a linear fashion, meaning that doubling the number of particles doubled the strength of the effect. This finding aligns with recent theoretical predictions that suggested a dense, incoherent crowd of particles could generate a static coupling between energy bands, acting like a permanent switch rather than a flickering light. The researchers confirmed that this effect was not a temporary artifact of the laser pulse but a stable property of the material driven by the sheer presence of the excitons.

Furthermore, the team discovered a new way to control this phenomenon by adjusting the twist angle between the two crystal layers. By changing the angle, they could shift the momentum of the dark excitons. This allowed them to mix energy bands that were not just separated in energy, but also separated in momentum space. In simpler terms, they could engineer the electronic landscape to connect points that were previously far apart in the material's structure. This capability adds a powerful new degree of freedom to the field of Floquet engineering, allowing scientists to design electronic states with specific momentum properties that were previously unattainable.

The implications of this work extend beyond the immediate observation. The ability to create long-lived, engineered electronic states using incoherent excitons suggests a route to stable, controllable materials that do not rely on the difficult maintenance of ultrafast laser coherence. The twisted bilayer structure used in the experiment acts as a flexible platform where the energy landscape, momentum offsets, and confinement can be tuned with precision. Because these dark excitons are inherently long-lived and can exist at high densities, they satisfy the conditions needed for more complex collective behaviors, such as exciton condensation, which could lead to new phases of matter. The researchers have established a robust method to manipulate the fundamental properties of semiconductors, offering a pathway to explore novel electronic states that are both persistent and tunable, bridging the gap between transient quantum effects and practical material engineering.