🔬 mesoscale physics

Efficient transport kinetics of indirect excitons in van der Waals heterostructure

This paper reports the observation of efficient transport kinetics with anomalously high mobility in spatially indirect excitons within van der Waals heterostructures, a phenomenon that persists despite in-plane disorder and aligns with predictions of exciton superfluidity.

Original authors: Zhiwen Zhou, W. J. Brunner, E. A. Szwed, H. Henstridge, L. H. Fowler-Gerace, L. V. Butov

Published 2026-06-02

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CC BY 4.0

Original authors: Zhiwen Zhou, W. J. Brunner, E. A. Szwed, H. Henstridge, L. H. Fowler-Gerace, L. V. Butov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a crowded dance floor where everyone is trying to move from one side to the other. Usually, if the floor is messy, uneven, or full of obstacles (like chairs or people standing still), the dancers get stuck, bump into things, and move very slowly. This is how most "excitons" (tiny particles of light and matter) behave in new, high-tech materials called van der Waals heterostructures. Scientists have long known that these messy "floors" usually trap the particles, stopping them from traveling far.

However, in this study, researchers at UC San Diego discovered something surprising: under specific conditions, these particles suddenly start moving like a super-fast, perfectly synchronized swarm, gliding over the messy floor as if the obstacles weren't even there.

Here is a breakdown of what they found, using simple analogies:

1. The Characters: "Indirect Excitons" (The Long-Lived Travelers)

Think of an exciton as a pair of dancers: one is an electron (a negative charge) and the other is a hole (a positive charge). Usually, they hold hands tightly and stay in the same spot. But in this experiment, the researchers put them in a special sandwich made of two ultra-thin layers of material (MoSe2 and WSe2).

Because the layers are separated, the electron and the hole are forced to stay in different "rooms" but are still connected by an invisible string. This is called an Indirect Exciton (IX).

The Superpower: Because they are in different rooms, they can't easily "kiss" and disappear (recombine). This gives them a much longer life than normal particles. It's like giving a traveler a map that lasts for hours instead of minutes, allowing them to travel much further.

2. The Problem: The "Messy Floor"

The material they used isn't perfectly smooth. It has a bumpy, disordered landscape (like a floor covered in random pebbles or a crumpled rug).

Normal Expectation: In physics, when particles try to move across a bumpy floor, they get stuck in the valleys (localization) or bounce off the bumps (scattering). They move slowly and randomly, like a drunk person stumbling home. Scientists expected these excitons to behave this way, traveling only a tiny distance before getting stuck.

3. The Discovery: The "Super-Slide"

The researchers shined a laser on the material to create a cloud of these excitons and watched how fast the cloud spread out over time.

What they saw: Instead of stumbling and spreading out slowly (diffusion), the cloud expanded in a straight, rapid line. It grew so fast that the distance it traveled doubled every second, rather than just creeping forward.
The Analogy: Imagine dropping a drop of ink in water. Usually, it spreads out slowly and gets fuzzy at the edges. In this experiment, the ink didn't just spread; it shot forward like a bullet, maintaining a sharp, fast-moving front.

4. The "Magic" Conditions

This super-fast movement didn't happen all the time. It only worked when the "dancers" were:

Cold enough: If the room was too hot (above about 10 Kelvin, which is very cold, near absolute zero), the particles started jittering too much and the magic stopped.
Just the right crowd size: If there were too few particles or too many, the fast movement stopped. It only worked at a "Goldilocks" density.

5. Why is this happening? (The "Superfluid" Theory)

The paper suggests that the reason these particles can glide over the bumpy floor is that they have entered a state called superfluidity.

The Analogy: Think of a crowd of people trying to walk through a narrow, crowded hallway. Usually, everyone bumps into each other and gets stuck. But if everyone suddenly starts holding hands and moving in perfect unison (like a synchronized swim team), they can flow through the crowd without bumping into anything. The "bumps" on the floor no longer matter because the group moves as one single, smooth entity.
The researchers found that the particles were moving with "anomalously high mobility," meaning they faced almost no friction or resistance, even though the material was messy. This behavior matches theories predicting that excitons can become superfluids in these materials.

Summary

The paper reports that by cooling down a specific type of layered material and hitting it with a laser at just the right intensity, the researchers made tiny light-particles (excitons) move incredibly fast and far. They didn't get stuck on the material's natural bumps. Instead, they seemed to flow like a frictionless liquid, a behavior that scientists believe is a sign of superfluidity. This is a big deal because it proves these particles can travel long distances efficiently, which is a key step for understanding how energy moves in quantum systems.

Technical Summary: Efficient Transport Kinetics of Indirect Excitons in van der Waals Heterostructures

Problem Statement
Spatially indirect excitons (IXs), formed by electrons and holes in separated layers of a heterostructure, possess long lifetimes that enable the formation of quantum bosonic states and long-range transport. While van der Waals (vdW) heterostructures composed of transition metal dichalcogenides (TMDs) offer a promising platform for studying IXs due to their high binding energies and potential for room-temperature stability, transport in these systems is typically hindered by disordered in-plane potentials. Previous studies in vdW TMD heterostructures have characterized IX transport as diffusive or sub-diffusive, with short decay distances (typically a few micrometers) caused by localization and scattering within disordered or moiré potentials. Furthermore, existing continuous-wave (cw) studies have been limited in their ability to distinguish between regular diffusion and non-diffusive transport regimes (such as ballistic transport or drift) because they do not resolve the time-dependent kinetics of the exciton cloud expansion.

Methodology
The authors investigated IX transport in a MoSe₂/WSe₂ vdW heterostructure using time-resolved optical imaging. The experimental setup involved optically generating IXs via focused laser excitation resonant with the direct exciton (DX) transition in the WSe₂ layer. The excitation utilized laser pulses with a step-like rectangular shape (84 ns width, 300 ns period) to initiate transport.

The study employed time- and spatially-resolved imaging to track the expansion of the IX photoluminescence (PL) cloud. The spatial extent of the cloud was quantified by the $1/e$ decay distance, $d_{1/e}(t)$ , determined by fitting the PL profiles to exponential decays. To characterize the transport kinetics, the authors compared the experimental $d_{1/e}(t)$ data against three theoretical models:

Diffusion: Modeled by the diffusion equation with a single parameter, the diffusivity $D$ .
Ballistic Transport: Modeled by an equation describing transport with a constant velocity $v$ .
Drift-Diffusion: A more complex mean-field model incorporating both interaction-induced drift (due to repulsive IX-IX interactions) and drift due to the in-plane potential energy landscape ( $U$ ). This model uses the IX mobility $\mu$ as the primary fitting parameter, relating diffusivity and mobility via the Einstein relationship ( $D' = \mu k_B T$ ).

Key Results
The experiments revealed a regime of "efficient IX transport kinetics" characterized by an anomalously high mobility and a specific growth pattern of the cloud expansion:

Kinetic Behavior: In a specific range of low temperatures ( $T \lesssim 10$ K) and intermediate excitation densities, the square of the decay distance ( $d_{1/e}^2$ ) exhibited a nearly quadratic growth with time. This corresponds to a nearly linear growth of $d_{1/e}$ with time, indicative of transport with a nearly constant velocity (ballistic or drift-dominated) rather than the linear growth of $d_{1/e}^2$ characteristic of diffusion.
Parameter Dependence: This efficient transport regime was observed only within a narrow window of parameters. It vanished at high temperatures (where thermal energy exceeds the disorder potential) and at both lower and higher excitation densities.
Mobility and Scattering: The derived IX mobility ( $\mu$ ) was anomalously high, implying a significantly extended mean free time and mean free path compared to previous studies in vdW heterostructures. This high mobility persists despite the presence of substantial in-plane disorder (estimated at $\sim 10$ meV), which is much larger than the thermal energy ( $k_B T$ ) and the interaction energy shift in the observed regime.
Model Comparison: While diffusion and drift-diffusion models could fit the data, the observed behavior (quadratic growth of $d_{1/e}^2$ ) and the specific parameter dependence (suppression at high $T$ and high density) were inconsistent with standard diffusive transport mechanisms, which typically predict enhanced transport with increasing temperature or density due to delocalization and screening.

Significance and Claims
The paper claims that the observed efficient transport kinetics, characterized by anomalously high mobility in the presence of strong disorder, is qualitatively consistent with theoretically predicted indirect exciton superfluidity in vdW heterostructures.

Superfluidity Connection: The suppression of transport at high temperatures and the non-monotonic dependence on density (enhancement followed by suppression) align with predictions from the Bose-Hubbard model for a superfluid phase occurring at specific boson densities per lattice site.
Scattering Suppression: The ability of IXs to maintain long-range ballistic-like transport despite significant disorder potentials suggests a suppression of scattering, a hallmark of superfluidity.
Platform Validation: The work establishes vdW TMD heterostructures as a viable platform for exploring quantum bosonic transport phenomena and high-temperature superfluidity, leveraging the high binding energies of TMD excitons.

The authors conclude that their findings provide direct evidence of efficient IX transport kinetics that cannot be explained by standard diffusive models and are consistent with the emergence of a superfluid state in these materials.