Observation of Spin-Valley-Coupling-Induced Large Spin-Lifetime Anisotropy in Bilayer Graphene

Abstract

We report the first observation of a large spin lifetime anisotropy in bilayer graphene (BLG) fully encapsulated between hexagonal boron nitride. We characterize the out-of-plane (τ ⊥ ) and in-plane (τ ) spin lifetimes by oblique Hanle spin precession. At 75 K and the charge neutrality point (CNP) we observe a strong anisotropy of τ ⊥ /τ = 8 ± 2. This value is comparable to graphene/TMD heterostructures, whereas our high quality BLG provides with τ ⊥ up to 9 ns, a more than two orders of magnitude larger spin lifetime. The anisotropy decreases to 3.5 ± 1 at a carrier density of n = 6 × 10 11 cm −2 . Temperature dependent measurements show above 75 K a decrease of τ ⊥ /τ with increasing temperature, reaching the isotropic case close to room temperature. We explain our findings with electric field induced spin-valley coupling arising from the small intrinsic spin orbit fields in BLG of 12 µeV at the CNP.

Heterostructures of graphene and hBN: Electronic, spin-orbit, and spin relaxation properties from first principles

“…So far, anisotropies of ξ ≈ 1 have been measured for graphene on hBN and SiO 2 26,27,38,92,93 , in agreement with our averaged parameter results. A first indication of large anisotropies was found in hBN encapsulated bilayer graphene heterostructures 69,70 . There it was shown, that the anisotropy ξ decreases with increasing carrier density, in line with our results for monolayer graphene.…”

“…Typically the SR anisotropy ratio ξ = τ s,z /τ s,x is 0.5, reflecting the fact that two spin-orbit field components can flip an out-ofplane spin, but only one component can flip the in-plane spin. In contrast, as recently predicted 71 and soon experimentally realized 69,70,72,73 , 2D materials offer so far unrivaled control over ξ. It was found that graphene on a transition metal dichalcogenide (TMDC) has ξ ≈ 10, due to the strong valley Zeeman spin-orbit fields, being induced from the TMDC into graphene.…”

“…There is now experimental evidence that in hBN encapsulated bilayer graphene, SR is due to SOC 69,70 . Finally there is a graphene-based structure in which spins live long (10 ns) and SOC is strong enough, relative to other spin-dependent interactions, to play a dominant role and be used for spin manipulation.…”

“…On the other hand, the SR anisotropy in encapsulated bilayer graphene is also giant (ξ ≈ 10), but the spin lifetime is three orders of magnitude larger, up to 10 ns 69,70 . Remarkably, the SR anisotropy ξ sharply increases as a transverse electric field is applied 70 at a fixed doping.…”

Induced spin-orbit coupling in twisted graphene–transition metal dichalcogenide heterobilayers: Twistronics meets spintronics

“…These heterostructures can posses functionalities that the individual constituent layers may not have. In order to increase the SOC in graphene, one of the most actively pursued directions is to interface it with materials that have strong intrinsic SOC, such as transition metal dichalcogenides (TMDCs) [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . TMDCs are expected to be good candidates for graphene spintronics for two reasons: i) it was shown that TMDC substrates do not degrade the mobility of graphene 23,26 , and ii) they host a strong intrinsic SOC of the order of 100 meV (10 meV) in their valence (conduction) band 27 and hence can potentially be suitable materials for proximity induced SOC.…”