Giant Spin Lifetime Anisotropy in Graphene Induced by Proximity Effects

Abstract

We report on fundamental aspects of spin dynamics in heterostructures of graphene and transition metal dichalcogenides (TMDCs). By using realistic models derived from first principles we compute the spin lifetime anisotropy, defined as the ratio of lifetimes for spins pointing out of the graphene plane to those pointing in the plane. We find that the anisotropy can reach values of tens to hundreds, which is unprecedented for typical 2D systems with spin-orbit coupling and indicates a qualitatively new regime of spin relaxation. This behavior is mediated by spin-valley locking, which is strongly imprinted onto graphene by TMDCs. Our results indicate that this giant spin lifetime anisotropy can serve as an experimental signature of materials with strong spin-valley locking, including graphene/TMDC heterostructures and TMDCs themselves. Additionally, materials with giant spin lifetime anisotropy can provide an exciting platform for manipulating the valley and spin degrees of freedom, and for designing novel spintronic devices. [3,4], where the combined system might be engineered for specific applications [5] or might enable the exploration of new phenomena [6,7]. In the field of spintronics, graphene has exceptional charge transport properties but weak spin-orbit coupling (SOC) on the order of 10 µeV [8], which makes it ideal for long-distance spin transport [9-11] but ineffective for generating or manipulating spin currents. To advance towards spin manipulation, recent work has focused on heterostructures of graphene and magnetic insulators [12][13][14][15][16] or strong SOC materials such as transition metal dichalcogenides (TMDCs) and topological insulators [17][18][19]. The SOC induced in graphene by a TMDC could enable phenomena such as topological edge states [20] or the spin Hall effect [21][22][23].To this end, a variety of recent experiments have explored spin transport in graphene/TMDC heterostructures [21,[24][25][26][27][28][29]. Magnetotransport measurements revealed that graphene in contact with WS 2 exhibits a large weak antilocalization (WAL) peak, revealing a strong SOC induced by proximity effects [24][25][26]30]. Fits to the magnetoconductance yielded spin lifetimes τ s ≈ 5 ps, which is two to three orders of magnitude lower than graphene on traditional substrates [10,31]. It was later asserted that after the removal of a temperatureindependent background, τ s becomes at most only a few hundred femtoseconds [26]. Nonlocal Hanle measurements, meanwhile, have revealed spin lifetimes up to a few tens of picoseconds [27][28][29] that can be tuned by a back gate [28,29]. Finally, charge transport measure-

Spin transport in high-mobility graphene on WS2 substrate with electric-field tunable proximity spin-orbit interaction

“…For both electron and hole regimes (stronger for the hole regime), we observe the fingerprints of the D'yakonov-Perel' type mechanism for spin-relaxation, similar to WAL measurements [3,4]. For Gr-on-WS 2 , the ratio of the out-of-plane to the in-plane ∆R NL ( therefore τ s ) in the electron-doped regime is less than one, an indicative of an in-plane Rashba-type system [11,20]. For the hole doped regime, we observe an enhanced out-of-plane spin-signal [15] which suggests a higher τ ⊥ s for the out-of-plane spins.…”

“…A higher τ h s in the encapsulated region is possibly due to a combined effect of an intrinsically reduced spin-orbit coupling in the hole regime [11,19] and modification of the electric-field induced Rashba SOC [3,4,20]. This can be seen in two features evident from Fig.…”

“…On the other hand, spin-transport measurements in graphene fully supported on a TMD substrate do not have this drawback and can distinguish the possible effects of spin-absorption via the TMD or a proximity-induced SOC, due to a uniform carrier density and identical effect of the substrate present everywhere in graphene, and iii) in contrast with the TMD-on-graphene geometry [12][13][14][15] where graphene partially shields the back-gate induced electric field to the TMD, and one cannot clearly comment on the TMD's conducting state and correlate its effect on spin-transport in graphene, the inverted Gr-on-TMD geometry does not have this drawback. Lastly, it is worth exploring the possibility of recently observed spin-relaxation anisotropy for in-plane and out-of-plane spins in Gr-TMD heterostructures [11,14,15] in our system.…”

“…For Gr-transition metal dichalcogenide (TMD) heterostructures, an enhanced intrinsic spinorbit coupling (SOC) in the order of 5-15 meV can be induced in graphene, along with a meV order valley-Zeeman splitting due to inequivalent K and K' valleys in graphene [6,9], a Rashba SOC due to breaking of the inversion symmetry at the graphene-TMD interface [3,4] with a possibility of spin-valley coupling [10,11]. This unique ability of the graphene-TMD interface makes it an attractive platform for studying the spin-related proximity induced effects in graphene.…”

Microscopic Linear Response Theory of Spin Relaxation and Relativistic Transport Phenomena in Graphene

“…Our theory sets the stage to study the spin dynamics in that regime. This topic has become of renewed great interest due to recent progresses in graphene-based heterostructures, where the spin relaxation anisotropy has been recognized as a viable tool to estimate the induced large spin-orbital effects [72][73][74][75]. …”