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 (${\tau }_{\perp }$) and in-plane (${\tau }_{\parallel }$) spin lifetimes by oblique Hanle spin precession. At 75~K and the charge neutrality point (CNP) we observe a strong anisotropy of ${\tau }_{\perp }/{\tau }_{\parallel }$ = 8 $\pm$ 2. This value is comparable to graphene/TMD heterostructures, whereas our high quality BLG provides with ${\tau }_{\perp }$ up to 9~ns, a more than two orders of magnitude larger spin lifetime. The anisotropy decreases to 3.5 $\pm$ 1 at a carrier density of n = $6\times 10^{11}$cm${}^{-2}$. Temperature dependent measurements show above 75~K a decrease of ${\tau }_{\perp }/{\tau }_{\parallel }$ 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~$\mu$eV at the CNP.

5 pages, 4 figures + supplementary information (6 pages, 11 figures)