🔬 mesoscale physics

Transport of indirect excitons and exciton mediated spin transport in a van der Waals heterostructure in magnetic fields

This study demonstrates that indirect excitons and their mediated spin transport in a MoSe $_2$ /WSe $_2$ van der Waals heterostructure exhibit long-range propagation of up to ~100 micrometers under magnetic fields up to 8 T, with decay distances that correlate between the two phenomena and vary non-monotonically with exciton density.

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

Published 2026-09-30

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CC BY 4.0

Original authors: Zhiwen Zhou, W. J. Brunner, E. A. Szwed, 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

In the microscopic world of modern electronics, scientists are constantly searching for ways to move information without generating heat or wasting energy. One promising avenue involves tiny packets of light and matter called excitons. Imagine an exciton as a pair of particles, an electron and a hole, that are attracted to each other like opposite ends of a magnet. Usually, they stay close together, but in a special type of layered material, researchers can separate them into different layers. This separation creates a long-lived particle that can travel much farther than its ordinary counterparts before disappearing. Because these particles carry a property called spin, which relates to their magnetic orientation, they offer a potential way to transport information over long distances without the signal fading away. Understanding how these particles move, especially when influenced by powerful magnetic forces, is crucial for developing future technologies that could process data more efficiently.

A team of physicists at the University of California San Diego recently investigated how these separated particles behave when subjected to strong magnetic fields. They worked with a sandwich-like structure made of two different atom-thin materials, molybdenum diselenide and tungsten diselenide, stacked on top of each other. By shining a laser onto this structure, they created a stream of these long-lived particles and watched how they spread out. The researchers applied magnetic fields as strong as eight tesla to see if the invisible force would alter the particles' journey. What they found was that these particles could travel remarkably far, covering distances of up to one hundred micrometers, which is roughly the width of a human hair. Even more surprisingly, the particles maintained their spin orientation over these same long distances, meaning they could carry magnetic information without losing it to the environment.

The behavior of these particles was not a simple straight line. As the researchers increased the number of particles created by the laser, the distance they traveled first grew longer and then became shorter again. This pattern held true whether the magnetic field was present or absent. The scientists observed that the distance the spin information traveled was directly linked to how far the particles themselves moved. When the particles traveled further, the spin information traveled just as far. This tight connection suggests that the ability to move the information depends entirely on the stability and movement of the particles carrying it. The team noted that this long-range movement was consistent with theories about how particles behave in a repeating, grid-like landscape created by the mismatch between the two atomic layers, a phenomenon known as a moiré pattern.

The study also highlighted a key difference between this new material and older semiconductor materials used in previous experiments. In those older materials, strong magnetic fields typically made the particles heavier and slowed them down, drastically reducing how far they could travel. However, in this new stack of atom-thin layers, the magnetic fields did not suppress the movement. The particles continued to travel long distances even under the influence of the strong magnetic force. This resilience is likely due to the fact that the particles in this material are held together much more tightly than in older materials, making them less sensitive to the magnetic forces that usually disrupt their motion. The researchers concluded that these findings confirm that long-distance transport of both particles and their spin information is possible in these materials, even when subjected to significant magnetic fields, opening a path for further exploration into how these systems might be used in future devices.

Technical Summary: Transport of Indirect Excitons and Exciton-Mediated Spin Transport in a van der Waals Heterostructure in Magnetic Fields

Problem Statement
Spatially indirect excitons (IXs), or interlayer excitons, formed in van der Waals heterostructures (HS) composed of transition metal dichalcogenides (TMDs) offer a unique platform for studying long-range transport and spin dynamics. Unlike direct excitons, IXs possess long lifetimes due to the spatial separation of electrons and holes, enabling potential Bose-Einstein condensation and long-range spin transport. While long-range IX transport and IX-mediated spin transport have been observed in TMD HS at zero magnetic field, the behavior of these phenomena under high magnetic fields remains an open question.

Previous studies in GaAs/AlGaAs HS demonstrated that high magnetic fields strongly suppress IX transport distances. This suppression is attributed to the coupling between the exciton's internal structure and its center-of-mass motion, which significantly enhances the exciton mass when the cyclotron energy becomes comparable to the binding energy. However, TMD HS possess significantly higher IX binding energies (hundreds of meV) compared to GaAs/AlGaAs HS (a few meV), suggesting that the high-magnetic-field regime might not be reached in TMDs at similar field strengths. This paper investigates whether the long-range IX transport and spin transport observed in TMDs at zero field persist in magnetic fields up to 8 T, and how these transport properties depend on exciton density and magnetic field strength.

Methodology
The study utilized a MoSe2/WSe2 van der Waals heterostructure, where adjacent monolayers form separated electron and hole layers. The heterostructure was assembled using a dry-transfer peel technique with a twist angle of approximately 1.1°, creating a moiré superlattice with a period of ~17 nm.

Excitation and Detection: IXs were generated via focused laser excitation resonant with the direct exciton (DX) in the WSe2 layer. The propagation of IXs was characterized by spatially resolved imaging of the photoluminescence (PL) intensity.
Spin Transport Characterization: Spin transport was measured using spatially and polarization-resolved imaging. The circularly polarized PL intensity ( $I_{\sigma+}$ and $I_{\sigma-}$ ) was used to calculate the spin density profile ( $I_{spin} = I_{\sigma+} - I_{\sigma-}$ ) and the degree of circular polarization.
Experimental Conditions: Measurements were conducted in magnetic fields ranging from -8 T to +8 T. The laser excitation power ( $P_{ex}$ ) was varied from 10 $\mu$ W to 5000 $\mu$ W to modulate the IX density.
Data Analysis: The transport characteristics were quantified by the $1/e$ decay distance ( $d_{1/e}$ ) for total IX intensity and ( $d^s_{1/e}$ ) for spin density. These values were derived by fitting the PL profiles to exponential decays in the region from the excitation spot ( $x=0$ ) to the heterostructure edge ( $x=9$ $\mu$ m).

Key Results

Long-Range Transport in Magnetic Fields: The study observed long-range IX transport and long-range IX-mediated spin transport in magnetic fields up to 8 T. The $1/e$ decay distances for both IX transport ( $d_{1/e}$ ) and spin transport ( $d^s_{1/e}$ ) reached approximately 100 $\mu$ m.
Non-Monotonic Density Dependence: For all studied magnetic fields, the decay distances exhibited a non-monotonic dependence on IX density (controlled by $P_{ex}$ ). Both $d_{1/e}$ and $d^s_{1/e}$ initially increased with increasing density, reached a maximum, and then decreased at higher densities. This behavior mirrors observations made in zero magnetic field.
Correlation Between Spin and Charge Transport: A strong correlation was found between the decay distance of spin transport ( $d^s_{1/e}$ ) and the decay distance of IX transport ( $d_{1/e}$ ). As the IX transport range increased, the spin transport range increased correspondingly across a broad range of excitation densities and magnetic fields.
Absence of Magnetic Suppression: Unlike GaAs/AlGaAs HS, where magnetic fields up to 10 T strongly suppress transport distances due to mass enhancement, no such suppression was observed in the MoSe2/WSe2 HS. The transport distances remained robust up to 8 T.

Significance and Claims
The authors claim that the observed long-range transport and its non-monotonic density dependence in magnetic fields are consistent with the behavior seen in zero magnetic fields and support the interpretation of the system within the Bose-Hubbard model. Specifically:

The transition from localization to long-range transport and back to localization with increasing density is qualitatively consistent with the transition between insulating (Mott/Bose glass) and superfluid phases in a periodic moiré potential.
The lack of magnetic suppression in TMD HS, in contrast to GaAs/AlGaAs HS, is attributed to the high IX binding energy in TMDs. This high binding energy prevents the system from entering the high-magnetic-field regime (where cyclotron energy $\ge$ binding energy) at 8 T, thereby avoiding the strong mass enhancement that suppresses transport in GaAs-based systems.
The correlation between spin and charge transport decay lengths confirms that the suppression of scattering in the IX condensate effectively suppresses spin relaxation, enabling efficient long-range spin transport even in the presence of magnetic fields.

The paper concludes that the MoSe2/WSe2 heterostructure maintains its capability for long-range IX and spin transport in magnetic fields up to 8 T, distinguishing its physics from that of conventional semiconductor heterostructures and highlighting the potential of TMDs for high-temperature superfluidity and spintronic applications.

Technical Summary: Transport of Indirect Excitons and Exciton-Mediated Spin Transport in a van der Waals Heterostructure in Magnetic Fields

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