Electrode-tunable nonlocal Rashba-Edelstein effect and layer-selective chirality switch in WSe$_2$-intercalated bilayer graphene

In the quest to build faster, smaller, and more efficient electronic devices, scientists have long looked toward a field called spintronics. Unlike traditional electronics, which rely on the movement of electric charge to carry information, spintronics uses a fundamental property of electrons known as "spin." You can think of spin as a tiny, intrinsic magnetism that makes each electron act like a microscopic bar magnet pointing either up or down. By manipulating these magnetic directions, researchers hope to create devices that process data with less energy and greater speed. For years, a major challenge has been how to control this spin without using large, bulky magnets, which are difficult to manage at the microscopic scale. One promising solution involves placing ordinary materials next to special ones that naturally twist the electron's spin, a phenomenon known as the proximity effect. However, most existing designs rely on a single layer of material touching another, which limits how much control scientists can exert over the electron's behavior.

A team of researchers has now proposed a new way to overcome these limits by creating a sandwich-like structure that behaves like a single, unified material with unique properties. In a study published recently, the scientists explored a device made by stacking two layers of graphene, a material consisting of a single sheet of carbon atoms, with a single layer of tungsten diselenide placed right in the middle. Instead of treating the middle layer as a passive spacer, they found that it actively connects the two graphene sheets, merging their electronic states into a new, synthetic form of bilayer graphene. This connection is strong enough that the electrons no longer belong to just the top or bottom sheet; they exist in a shared state across the entire structure. This setup allows the researchers to switch the behavior of the electrons simply by changing which electrical contacts they use, revealing a hidden ability to generate spin currents that was previously inaccessible.

The researchers began by constructing this three-layer stack and analyzing how the electrons move through it. They discovered that the middle layer of tungsten diselenide does more than just sit between the graphene sheets; it acts as a bridge that allows the wavefunctions of the electrons in the top and bottom layers to overlap significantly. This overlap creates a hybrid state where the electrons are delocalized, meaning they are spread out across both graphene layers simultaneously. This is a crucial departure from standard designs where the layers remain distinct. Because the energy scale of this connection is comparable to the strength of the spin-twisting effect induced by the middle layer, the system operates in a regime where the two effects must be treated together rather than as separate, minor influences. This unique balance gives rise to a set of behaviors that cannot be found in simpler, single-interface devices.

One of the most striking findings is the ability to generate spin in one layer by sending an electric current through the other. In a standard setup, if you push a current through a material, you might expect to create a buildup of spin in that same material. Here, the researchers found that by injecting a current into the top graphene layer, they could generate a measurable accumulation of spin in the bottom layer, and vice versa. This nonlocal effect means that the charge and the resulting spin are separated in space, connected only by the hybridized electronic structure. The study shows that this effect is robust, meaning it persists even if the layers are slightly twisted relative to one another or if an external electric field is applied to the device. This stability suggests that the phenomenon is a fundamental property of the hybrid structure rather than a fragile coincidence.

The team also demonstrated that they could selectively switch the direction of the spin generation simply by choosing which electrodes to activate. When they sent current through the top layer and measured the spin in the top layer, they observed a specific signal. When they did the same for the bottom layer, they observed a signal of the exact opposite sign. This layer-selective behavior reveals a hidden form of spin-orbit coupling that exists even when the overall structure appears to have no net spin effect. In the symmetric state, where the top and bottom layers are identical, the local spin effects cancel each other out globally, making the device appear neutral. However, by probing the layers individually, the researchers could uncover these opposing chiralities, effectively acting as a switch that selects which layer's spin signature is active.

To confirm these findings, the researchers used advanced computer simulations to model the electronic structure of the stack. They calculated how the electrons would behave under various conditions, including different levels of doping, which is the process of adding extra electrons to the material, and different angles of twist between the layers. The simulations showed that the synthetic bilayer graphene maintains its unique properties across a range of conditions. Even when an electric field was applied to break the symmetry between the top and bottom layers, the nonlocal charge-to-spin conversion remained active. The results indicate that the device can function as a versatile platform for spintronics, where the configuration of the electrical contacts alone is sufficient to select between different spintronic functionalities without the need for external magnetic fields.

The implications of this work extend beyond a single device. By showing that interlayer hybridization can be used to create a synthetic material with tunable spin properties, the study opens a new path for designing van der Waals heterostructures. These are materials built by stacking different two-dimensional layers like Lego bricks. The ability to create a state where interlayer coupling and spin-orbit effects are of comparable strength suggests that future devices could be engineered to perform complex logic operations or signal processing tasks that are currently impossible. The researchers emphasize that their findings are based on theoretical calculations and simulations, which provide a strong foundation for experimental verification. If realized in the laboratory, such devices could lead to a new generation of electronic components that are more efficient and capable of handling information in ways that current technology cannot.

The study also highlights the importance of symmetry in these systems. In the ideal, symmetric configuration, the device exhibits a "hidden" Rashba effect, where the spin textures in the two layers are opposite and cancel out globally. This cancellation is what makes the nonlocal effect possible, as it allows the system to support a state where spin can be generated in one layer while the other remains neutral, or where the spin in one layer is the mirror image of the other. When the symmetry is broken, for instance by applying an electric field, the cancellation is lifted, and the device can exhibit a net spin effect. This tunability offers a way to control the spin behavior of the material dynamically, simply by adjusting the voltage applied to the device.

Ultimately, the work presents a clear picture of how a simple change in architecture—inserting a single layer of tungsten diselenide between two sheets of graphene—can fundamentally alter the electronic landscape. It transforms a standard stack of materials into a synthetic system with emergent properties. The ability to switch between local and nonlocal spin generation, and to select the chirality of the spin based on the electrode configuration, provides a new toolkit for manipulating electron spin. As the field of spintronics continues to evolve, such discoveries of robust, tunable, and nonlocal effects in van der Waals heterostructures will likely play a central role in the development of next-generation electronic technologies. The findings suggest that by carefully engineering the interface between layers, scientists can unlock physical regimes that were previously thought to be out of reach, paving the way for devices that are both more powerful and more energy-efficient.