Spin-Orbit Proximity Effect in Graphene (1412.0920v1)

Abstract: The development of a spintronics device relies on efficient generation of spin polarized currents and their electric field controlled manipulation. While observation of exceptionally long spin relaxation lengths make graphene an intriguing material for spintronics studies, modulation of spin currents by gate field is almost impossible due to negligibly small intrinsic spin orbit coupling (SOC) of graphene. In this work, we create an artificial interface between monolayer graphene and few-layers semiconducting tungsten disulfide (WS2). We show that in such devices graphene acquires a SOC as high as 17meV, three orders of magnitude higher than its intrinsic value, without modifying any of the structural properties of the graphene. Such proximity SOC leads to the spin Hall effect even at room temperature and opens the doors for spin FETs. We show that intrinsic defects in WS2 play an important role in this proximity effect and that graphene can act as a probe to detect defects in semiconducting surfaces.

• The paper demonstrates that WS₂ proximity enhances graphene’s spin-orbit coupling up to 17 meV, nearly 1000 times its intrinsic value.
• The study shows that intrinsic WS₂ defects, such as sulfur vacancies, create localized states that interact with graphene electrons.
• The graphene/WS₂ heterostructure maintains high electron mobility (~50,000 cm²/V·s) while enabling the room-temperature spin Hall effect, promising for spintronic applications.

Spin-Orbit Proximity Effect in Graphene

This paper presents a pioneering paper on inducing a significant enhancement of spin-orbit coupling (SOC) in graphene by leveraging the proximity effect with tungsten disulfide (WS₂). The objective was to address the challenge of graphene's inherently weak intrinsic SOC, which limits its potential in spintronics applications. The enhancement of SOC was achieved without structurally modifying the graphene, a critical accomplishment given that previous methods, such as chemical functionalization or metal adatom adsorption, tend to introduce disorder, thereby impairing graphene's charge and spin transport properties.

Key Findings

• Proximity-Induced SOC: The paper reports a considerable increase in SOC up to 17 meV, roughly three orders of magnitude greater than graphene's intrinsic SOC. This is achieved by forming an interface of monolayer graphene with a few layers of semiconducting WS₂ via the dry transfer method. The SOC acquired is significant enough to observe the spin Hall effect at room temperature, a characteristic that can facilitate the realization of novel spintronic devices, such as spin field-effect transistors (FETs).
• Role of WS₂ Defects: Intrinsic defects within WS₂ play a pivotal role in this proximity SOC enhancement. Specifically, these defects, such as sulfur vacancies determined through X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations, introduce localized states within the bandgap that interact with graphene electrons. This interaction significantly contributes to the observed substantial SOC enhancement.
• High Mobility-Conductivity: The graphene/WS₂ heterostructure demonstrates the retention of high electron mobility, with values as high as 50,000 cm²/V.s, akin to those found in devices fabricated on boron nitride substrates. This remarkable blending of high mobility with strong proximity-induced SOC presents an optimal case for practical spintronic applications.
• Non-Local Transport Measurements: The non-local measurement configuration elucidates the origin of enhanced spin transport properties in these devices. Devices exhibit a non-local signal with a large magnitude and distinct electron-hole asymmetry, reinforcing the non-trivial nature of the spin transport phenomena arising from the presence of WS₂ defects.

Implications and Future Directions

The findings have theoretical implications in elucidating the mechanism of proximity-induced SOC via defect states in TMDCs. These insights pave the way for better understanding and tuning SOC in graphene and other 2D materials, potentially leading to advances in material engineering for spintronic applications.

From a practical perspective, this research showcases the viability of graphene/WS₂ heterostructures in developing next-generation spintronic devices. The room-temperature operability and high SOC make such devices promising candidates for low-power, high-speed electronics that exploit spin-dependent properties.

This paper opens up multiple avenues for further research. One potential direction includes exploring other TMDCs with different defect types and concentrations, thus broadening the scope of controllable SOC enhancement. Additionally, understanding the interplay between charge puddles and defect states could provide further insight into manipulating electronic properties in 2D heterostructures.

Overall, this paper contributes significantly to the knowledge base of graphene-based spintronics, offering practical solutions and new theoretical perspectives to surmount the challenges posed by graphene's weak intrinsic SOC.