Strongly anisotropic spin relaxation in graphene/transition metal dichalcogenide heterostructures at room temperature

Abstract: Graphene has emerged as the foremost material for future two-dimensional spintronics due to its tuneable electronic properties. In graphene, spin information can be transported over long distances and, in principle, be manipulated by using magnetic correlations or large spin-orbit coupling (SOC) induced by proximity effects. In particular, a dramatic SOC enhancement has been predicted when interfacing graphene with a semiconducting transition metal dechalcogenide, such as tungsten disulphide (WS$_2$). Signatures of such an enhancement have recently been reported but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and WS$_2$. By using out-of-plane spin precession, we show that the spin lifetime is largest when the spins point out of the graphene plane. Moreover, we observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, indicating that the strong spin-valley coupling in WS$_2$ is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials.

• The paper demonstrates a pronounced anisotropy in spin relaxation, with out-of-plane lifetimes nearly 10 times longer than in-plane ones.
• It employs both in-plane and out-of-plane spin precession measurements to reveal the impact of proximity-induced spin-orbit coupling on graphene.
• The findings highlight the potential for tuning spin transport in graphene, paving the way for innovative spintronic and quantum devices.

Anisotropic Spin Relaxation in Graphene/TMDC Heterostructures

The paper presents a detailed investigation into the anisotropic spin relaxation dynamics in graphene interfaced with transition metal dichalcogenides (TMDCs), specifically tungsten disulfide (WS$_2$) and molybdenum disulfide (MoS$_2$), at room temperature. This research is significant because it reveals unique insights into the interaction of graphene with TMDCs and their influence on spin dynamics, an area of intense interest in the development of future spintronic devices.

Key Findings

The study demonstrates an unambiguous anisotropy in spin relaxation within graphene/TMDC heterostructures. The spin lifetime, a critical parameter for spintronic applications, is shown to vary by an order of magnitude depending on the spin orientation with respect to the graphene plane. Spins oriented out of the plane exhibit significantly longer relaxation times compared to those aligned parallel to the plane. This anisotropy is quantified by the ratio $\zeta \equiv \tau^\perp_s / \tau^\parallel_s$, which reaches values approximately around 10 in these heterostructures as measured at room temperature.

The researchers employed both in-plane and out-of-plane spin precession measurements to elucidate the spin dynamics. These techniques revealed that a spin's relaxation length is notably longer when they are oriented perpendicular (out-of-plane) compared to when they are parallel (in-plane), hence affirming the presence of strong anisotropic relaxation.

Implications and Theoretical Considerations

The findings highlight the imprinting of TMDC's spin-orbit coupling (SOC) characteristics onto graphene, suggesting strong proximity-induced effects. The study implies that strong SOC in TMDCs, such as WS$_2$ and MoS$_2$, introduces significant anisotropic spin relaxation in graphene, thus imprinting spin-valley coupling phenomena. The realized anisotropy in the spin system provides an additional degree of freedom in designing graphene-based spintronic devices, allowing for innovative approaches in spin manipulation and control.

This anisotropy can potentially be leveraged in developing spin filters or spin-valleytronic devices that selectively allow the passage of spins based on their orientation. The ability to modulate spin transport through gate voltages further underlines the versatility of these heterostructures for use in advanced electronic and spintronic applications.

Future Prospects

The results suggest further exploration into the thickness dependence of TMDCs on proximity effects and spin relaxation in graphene could provide more robust control over spintronic properties. Additionally, investigating the impact of various TMDC materials and the influence of factors such as substrate-induced strain or interlayer twist on the anisotropy in graphene could expand the applicability of such heterostructures.

In conclusion, the research extends the understanding of spin dynamics in hybrid 2D materials, particularly focusing on anisotropic effects due to proximity-induced SOC in graphene/TMDC systems. This understanding is pivotal to advancing graphene's role in next-generation spintronic devices, offering pathways to capitalize on spin and valley degrees of freedom in quantum computing and information processing technologies.