Charge-to-Spin Conversion by the Rashba-Edelstein Effect in 2D van der Waals Heterostructures up to Room Temperature (1905.01371v1)

Abstract: The proximity of a transition metal dichalcogenide (TMD) to graphene imprints a rich spin texture in graphene and complements its high quality charge/spin transport by inducing spin-orbit coupling (SOC). Rashba and valley-Zeeman SOCs are the origin of charge-to-spin conversion mechanisms such as Rashba-Edelstein effect (REE) and spin Hall effect (SHE). In this work, we experimentally demonstrate for the first time charge-to-spin conversion due to the REE in a monolayer WS2-graphene van der Waals heterostructure. We measure the current-induced spin polarization up to room temperature and control it by a gate electric field. Our observation of the REE and inverse of the effect (IREE) is accompanied by the SHE which we discriminate by symmetry-resolved spin precession under oblique magnetic fields. These measurements also allow for quantification of the efficiencies of charge-to-spin conversion by each of the two effects. These findings are a clear indication of induced Rashba and valley-Zeeman SOC in graphene that lead to generation of spin accumulation and spin current without using ferromagnetic electrodes. These realizations have considerable significance for spintronic applications, providing accessible routes towards all-electrical spin generation and manipulation in two-dimensional materials.

Charge-to-Spin Conversion by the Rashba-Edelstein Effect in 2D van der Waals Heterostructures up to Room Temperature

The paper investigates the charge-to-spin conversion within two-dimensional (2D) van der Waals heterostructures composed of transition metal dichalcogenides (TMDs) and graphene. These materials have garnered significant interest due to their promising applications in spintronics, which focuses on utilizing the electron's spin degree of freedom for advanced device functionalities. Specifically, the authors experimentally demonstrate the Rashba-Edelstein Effect (REE) in a monolayer WS2-graphene heterostructure, marking a notable advancement for spin-orbitronics.

Graphene, despite its excellent spin transport properties, has intrinsically weak spin-orbit coupling (SOC). By coupling it with TMDs, the spin-orbit coupling can be enhanced through proximity effects, potentially inducing Rashba and valley-Zeeman SOC in graphene. This leads to efficient charge-to-spin conversion mechanisms such as the REE and spin Hall effect (SHE), enabling spin accumulation and spin-polarized currents without the need for ferromagnetic electrodes.

Key Findings

1. Charge-to-Spin Conversion: The paper demonstrates the charge-to-spin conversion due to REE in WS2-graphene heterostructures and measures the spin polarization induced by charge current up to room temperature. The REE and its inverse (IREE) are discernible due to spin Hall effect (SHE) contributions, segregated by symmetry-resolved spin precession under oblique magnetic fields.
2. Efficiency and Tunability: The strength and efficiency of charge-to-spin conversion mechanisms depend on the position of the Fermi-energy within the TMD-graphene band structure. The authors showcase that a gate electric field can modulate the spin polarization, aligning with theoretical predictions of tunability.
3. Implications for Spintronics: The findings suggest accessible routes for all-electrical spin generation and manipulation in two-dimensional materials, critical for spintronic applications. This offers substantial scope for developing spin-based devices that do not rely on bulk ferromagnetic electrodes.

Numerical Results

The paper provides quantitative measures of spin relaxation times and angles, indicating significant lifetime anisotropy and conversion efficiencies. Spin Hall angles and Rashba conversion metrics are numerically evaluated, with spin Hall angle approximately 0.13 reflecting strong conversion efficiencies.

Implications and Future Directions

The practical implications of this research assert the promise of monolayer TMD-graphene heterostructures in creating efficient spintronic devices. The paper highlights the theoretical and experimental potential for two-dimensional material systems to revolutionize the concept of spin transistors. The observations of tunable charge-to-spin conversion and operation at room temperature indicate potential for device integration into existing technologies.

Future developments could explore the scalability of these heterostructures for commercial applications, further evaluate their behavior under diverse external conditions, and integrate additional materials to expand functionality. Advanced spin-based computation and storage devices could benefit significantly, enhancing the landscape of information technology.

In summation, the paper presents a detailed experimental analysis of charge-to-spin conversion in 2D van der Waals heterostructures, offering insights that pave the way for effective spintronic device architectures in both theoretical exploration and practical application domains.