🔬 mesoscale physics

Layer-selective chirality switch in bilayer graphene intercalated by Janus monolayers

This paper predicts that intercalating bilayer graphene with nonmagnetic WSSe or magnetic MnSSe Janus monolayers induces a layer-selective switch in Rashba spin textures, enabling independent gate-tunable control of opposite spin current directions in the top and bottom graphene layers for advanced spintronic applications.

Original authors: Marko Milivojević

Published 2026-07-08

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CC BY 4.0

Original authors: Marko Milivojević

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 you have a sandwich made of two slices of bread (graphene) with a special filling in the middle. In this scientific paper, the researchers are building a very specific kind of electronic "sandwich" to control how tiny magnetic particles called spins move.

Here is the breakdown of their discovery in simple terms:

1. The Problem: Graphene is Too "Clean"

Think of graphene (a single layer of carbon atoms) as a super-fast highway for electricity. It's great, but it has a flaw for a specific type of technology called spintronics (which uses the "spin" of electrons instead of just their charge to store data). Graphene is too "clean"—it doesn't naturally interact with spin very well. It's like a highway where cars (electrons) zoom by, but they don't spin their wheels or turn left/right on their own.

2. The Solution: The "Janus" Filling

To fix this, the researchers put a special "filling" between the two graphene slices. This filling is a Janus monolayer.

The Analogy: Think of a Janus coin or a two-faced god. One side of this filling is made of Sulfur, and the other side is made of Selenium. They are different, just like the two faces of a coin.
The Setup: They made two versions of this sandwich:
1. The Non-Magnetic Version: Using a Janus layer made of Tungsten, Sulfur, and Selenium (WSSe).
2. The Magnetic Version: Using a Janus layer made of Manganese, Sulfur, and Selenium (MnSSe).

3. The Magic Trick: Opposite Directions

When they put this Janus filling between the two graphene slices, something amazing happened. The filling acted like a divider that separated the top and bottom slices, but it also gave them a "push" in opposite directions.

The Top Slice: The Janus filling pushed the spins in the top graphene layer to spin in a clockwise direction.
The Bottom Slice: Because the Janus filling is "two-faced" (different atoms on top and bottom), it pushed the spins in the bottom graphene layer to spin in a counter-clockwise direction.

It's as if you had two lanes of traffic on a bridge. The filling made the cars in the top lane turn right, while the cars in the bottom lane turned left, all at the same time.

4. The Control Switch: A Light Switch for Spin

The most exciting part is how you control this. Usually, to change the direction of a spin current, you have to physically flip a magnet or change the material.

In this new design, you can act like a light switch.

If you turn on the "Top Switch" (apply a voltage to the top layer), the spin current flows one way.
If you turn on the "Bottom Switch" (apply a voltage to the bottom layer), the spin current flows the opposite way.

Because the two layers are separated by the Janus filling, you can choose which "lane" to use without messing up the other. This creates a layer-selective switch.

5. Why This Matters

The researchers found that this setup works for both the non-magnetic and magnetic versions of the Janus filling.

The non-magnetic version (WSSe) is something that has already been made in real labs, so this is a practical, real-world possibility right now.
The magnetic version (MnSSe) is a theoretical model that shows the idea works even if the filling is magnetic, proving the concept is very robust.

Summary

The paper predicts that by sandwiching a "two-faced" (Janus) layer between two sheets of graphene, you can force the top sheet to spin electrons one way and the bottom sheet to spin them the other way. By simply turning a voltage knob on the top or bottom, you can instantly switch the direction of the spin current. This offers a simple, direct way to build new types of electronic switches that use spin instead of just charge.

Technical Summary: Layer-selective chirality switch in bilayer graphene intercalated by Janus monolayers

Problem Statement
Spintronic applications require materials with significant spin-orbit coupling (SOC) or magnetic ordering, properties often absent in high-mobility materials like graphene. While van der Waals (vdW) heterostructures allow for the engineering of desired properties via the proximity effect, practical spin logic devices require not only the generation of spin currents but also the ability to control their direction. Previous proposals for controlling spin current direction, such as reversing ferroelectric polarization in graphene/In $_2$ Se $_3$ heterostructures, rely on active manipulation of the ferroelectric material. This work addresses the need for a more direct mechanism to control the chirality of the Rashba-induced spin texture, which determines the direction of spin current flow, by engineering opposite spin textures in bilayer graphene layers simultaneously.

Methodology
The study employs a multi-scale theoretical approach combining first-principles calculations with effective modeling:

Density Functional Theory (DFT): Calculations were performed using Quantum ESPRESSO with van der Waals corrections, dipole corrections, and fully-relativistic pseudopotentials to treat SOC. Two heterostructures were modeled:
- Gr/WSSe/Gr: Bilayer graphene intercalated by a nonmagnetic Janus WSSe monolayer.
- Gr/MnSSe/Gr: Bilayer graphene intercalated by a magnetic Janus MnSSe monolayer (specifically a type-A antiferromagnet with out-of-plane spin configuration).
- Projected density of states (pDOS) and layer-resolved band structures were analyzed to confirm the decoupling of graphene layers and the absence of hybridization near the Fermi level.
Tight-Binding Modeling: Effective Hamiltonians were constructed for the top and bottom graphene layers.
- For the nonmagnetic case (WSSe), the model includes orbital terms, intrinsic SOC, and Rashba SOC ( $H_{orb} + H_I + H_R$ ).
- For the magnetic case (MnSSe), an exchange-proximity term ( $H_{ex}$ ) was added to account for the antiferromagnetic ordering of the MnSSe layer.
- Parameters for these models were extracted by fitting to the DFT band structures and spin expectation values near the K and K' points.
Rashba-Edelstein Effect Calculation: The charge-to-spin conversion efficiency ( $\alpha_{REE}$ ) was calculated using linear response theory and the Kubo formula (Smrčka-Středa formulation). This involved computing non-equilibrium spin accumulation ( $\delta S_y$ ) and charge current ( $\delta J_x$ ) in response to an electric field, integrating Fermi surface and Fermi sea contributions.

Key Contributions and Results

Layer Decoupling and Asymmetry: The intercalation of Janus monolayers (WSSe or MnSSe) breaks the horizontal mirror symmetry of AA-stacked bilayer graphene. The intrinsic structural asymmetry of the Janus layers (different S and Se environments for top and bottom graphene) shifts the Dirac cones of the two graphene layers to different chemical potentials, effectively decoupling them.
Opposite Rashba Chirality:
- In Gr/WSSe/Gr, the top and bottom graphene layers experience opposite effective electric fields due to the inverted proximity geometry. This induces Rashba SOC with opposite signs ( $\lambda_R \approx -0.309$ meV for top, $+0.490$ meV for bottom).
- In Gr/MnSSe/Gr, the magnetic ordering induces opposite exchange interactions in the two layers. While the exchange interaction dominates the energy scale, the Rashba SOC also exhibits opposite signs ( $\lambda_R \approx -0.018$ meV for top, $+0.105$ meV for bottom).
Layer-Selective Chirality Switch: The opposite signs of the Rashba SOC lead to opposite chiralities of the in-plane spin texture in the top and bottom layers. Consequently, the Rashba-Edelstein coefficient ( $\alpha_{REE}$ ) has opposite signs for the two layers.
Gate-Tunable Spin Current: The calculations demonstrate that the direction of the spin current can be controlled independently by gating the top or bottom graphene layer. Activating the top layer generates a spin current in one direction, while activating the bottom layer generates a spin current in the opposite direction.
Magnitude Differences: In both heterostructures, the magnitude of $|\alpha_{REE}|$ is larger for the bottom graphene layer, reflecting stronger proximity-induced Rashba coupling in that layer.

Significance and Claims
The paper claims that bilayer graphene intercalated by Janus monolayers represents a promising platform for gate-tunable, layer-selective spintronic devices. The primary significance lies in the mechanism of the chirality switch:

Unlike previous proposals that rely on switching the polarization of a ferroelectric material, this mechanism is inherent to the heterostructure geometry.
The switch is activated simply by selecting which graphene layer carries the current via electrostatic gating, offering a simpler and more direct route for controlling spin current direction.
The Gr/WSSe/Gr system is highlighted as an experimentally accessible platform, given that Janus WSSe monolayers have already been synthesized.
The Gr/MnSSe/Gr system serves as a model case to demonstrate the generality of the mechanism in the presence of both proximity-induced SOC and out-of-plane exchange interactions, noting that the energy difference between in-plane and out-of-plane magnetic configurations is negligible (0.19 meV per Mn atom), suggesting experimental stabilization is feasible with suitable substrates.

The work concludes that this approach paves the way for spin logic devices based on vdW heterostructures where spin current direction is controlled by layer-selective gating.