🔬 mesoscale physics

High-mobility spin transport mediated by indirect excitons in a van der Waals heterostructure

This study demonstrates that van der Waals heterostructures composed of transition-metal dichalcogenides can achieve high-mobility spin transport with a mobility of 10 $^5$ cm $^2$ /(eV $\cdot$ s) by utilizing indirect excitons to suppress particle scattering, thereby overcoming the limitations of standard diffusive spin transport.

Original authors: Zhiwen Zhou, W. J. Brunner, Luc M. Nguyen, C. A. Lozano, G. Mercado, L. H. Fowler-Gerace, L. V. Butov

Published 2026-10-14

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CC BY 4.0

Original authors: Zhiwen Zhou, W. J. Brunner, Luc M. Nguyen, C. A. Lozano, G. Mercado, 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

Spin is a fundamental property of electrons, much like a tiny internal compass needle that points in a specific direction. In the emerging field of spintronics, scientists aim to use this property to carry information, offering a path to faster and more energy-efficient computers than those based on electric charge alone. The challenge, however, is that as these spinning particles move through a material, they frequently bump into other atoms and impurities. These collisions not only slow the particles down but also scramble their spin direction, causing the information to fade away before it can travel far. To build useful devices, researchers need a way to move spin over long distances without losing it to these disruptive collisions.

A team of physicists at the University of California, San Diego, has found a way to overcome this limitation by using a special type of particle called an indirect exciton. These particles are formed in a sandwich of two atomically thin layers of different materials, specifically molybdenum diselenide and tungsten diselenide. In this setup, the electron part of the particle lives in one layer while the hole, which acts like a positive charge, lives in the other. Because they are separated by a tiny gap, they do not crash into each other as quickly as they would if they were in the same layer. This separation allows them to live much longer and move more freely. The researchers discovered that when these particles move together in a coordinated quantum state, they can transport spin with a mobility that is orders of magnitude higher than what is seen in standard materials.

To observe this phenomenon, the researchers built a device using the two-layer material sandwich and shone a focused laser beam onto it. The laser was tuned to create a burst of these indirect excitons with a specific spin direction. By using a camera capable of detecting the light emitted by these particles with extreme precision, they tracked how the spin traveled away from the laser spot over time. They measured how far the spin polarization spread before it faded, comparing this distance to the movement of the particles themselves. The results showed that the spin traveled incredibly efficiently, reaching a mobility of 100,000 square centimeters per electron-volt-second. This value is vastly superior to the spin mobility found in other similar materials, where the movement is typically limited by frequent scattering.

The study revealed that this high-speed transport only happens under specific conditions. When the material is cooled to very low temperatures, below about 10 Kelvin, and the density of particles is just right, the scattering that usually slows things down is suppressed. In this quiet state, the particles move in a way that preserves their spin direction much better than usual. The researchers also found that the spin moves even faster than the particles themselves in some cases. This happens because the particles with the most common spin direction move freely, while the less common ones are slowed down by collisions with the common ones. This difference in speed actually helps maintain the overall spin signal, allowing it to travel further.

This work demonstrates that indirect excitons in these atomically thin layers provide a powerful platform for moving spin without the usual losses. By suppressing the collisions that typically ruin spin transport, the researchers achieved a level of efficiency that was previously out of reach. The findings suggest that these materials could be the key to developing new types of electronic components that rely on spin rather than charge, potentially leading to technologies that are both faster and more energy-efficient. The ability to move spin so effectively at temperatures that are achievable in a laboratory setting marks a significant step forward in understanding how to control quantum properties for practical use.

Technical Summary: High-mobility spin transport mediated by indirect excitons in a van der Waals heterostructure

Problem Statement
Efficient spin transport with high spin mobility is critical for both fundamental physics and the development of spintronic devices, which promise superior energy efficiency and operation speeds compared to conventional charge-based electronics. However, a primary limitation in existing systems is particle scattering. Scattering not only suppresses particle transport but also induces spin relaxation, thereby limiting the distance over which spin can be transferred and preventing the realization of high spin mobilities. While spatially indirect excitons (IXs) in GaAs heterostructures have been explored as a bosonic quantum system capable of suppressing scattering via superfluidity, their utility is restricted to temperatures near a few Kelvin due to low exciton binding energies.

Methodology
To overcome temperature limitations, this work investigates spin transport kinetics mediated by a quantum system of IXs in a van der Waals heterostructure composed of atomically thin transition-metal dichalcogenides (TMDs), specifically a MoSe2/WSe2 bilayer. The study utilizes the following experimental and analytical approach:

Excitation and Detection: Circularly polarized laser pulses (89 ns width) resonant with the direct exciton (DX) in the WSe2 layer generate spin-polarized IXs. The transport is measured using time-, polarization-, and spatially-resolved photoluminescence (PL) imaging.
Quantification: The degree of circular polarization ( $P$ ) and the spin density ( $I_{spin} = I_{\sigma+} - I_{\sigma-}$ ) are tracked over time and space. The transport kinetics are characterized by the $1/e$ decay distance ( $d_{s}^{1/e}$ ) of the spin density profile.
Modeling: A drift-diffusion approach is employed to simulate the kinetics, incorporating diffusion, drift due to IX interactions and in-plane potential landscapes, generation, and recombination. The spin mobility ( $\mu_s$ ) is extracted by fitting the simulated expansion kinetics to the experimental $d_{s}^{1/e}(t)$ data. The model accounts for the mean-field approximation of IX interactions and the Einstein relation for diffusivity.

Key Results
The study demonstrates efficient spin transport characterized by a spin mobility ( $\mu_s$ ) reaching approximately $10^5 \text{ cm}^2/(\text{eV}\cdot\text{s})$ . Key findings include:

High Mobility Regime: This mobility value is orders of magnitude higher than those achieved via standard diffusive spin transport in TMD materials (which are typically in the range of $50$ to $3 \cdot 10^3 \text{ cm}^2/(\text{V}\cdot\text{s})$ for IXs, holes, and electrons, respectively).
Temperature and Density Dependence: The high spin mobility is observed within a specific range of temperatures (below $\sim 10$ K) and excitation densities. The mobility initially increases with density before declining at the highest excitation powers.
Superiority over IX Mobility: In the high-mobility regime, the spin mobility ( $\mu_s$ ) exceeds the total IX mobility ( $\mu$ ). Furthermore, the mobility of majority-spin IXs ( $\mu_+$ ) is found to be higher than that of minority-spin IXs ( $\mu_-$ ).
Mechanism of Enhancement: The observed enhancement is attributed to the suppression of IX scattering. The system exhibits a spin asymmetry where the scattering of minority-spin IXs by the dense majority-spin population suppresses minority transport, while majority-spin transport is largely conserved. This asymmetry increases the net polarization and extends the spin decay length, thereby enhancing the effective spin mobility.

Significance and Claims
The paper claims that the suppression of IX scattering in TMD-based van der Waals heterostructures enables high-mobility spin transport at temperatures significantly higher than those possible in GaAs systems. By leveraging the high exciton binding energies in TMDs, the authors realize a platform where spin transfer is not limited by the rapid spin relaxation typical of diffusive transport. The work establishes that in this quantum system, spin transport can be more efficient than the transport of the constituent particles themselves, offering a pathway for efficient spin-valley transport in solid-state devices. The authors note that these results are consistent with theoretical predictions of IX superfluidity and provide a robust experimental demonstration of high-mobility spin kinetics in 2D materials.

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