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 = 6×1011cm−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.
@article{arxiv.1805.12420,
title = {Observation of spin-valley coupling induced large spin lifetime anisotropy in bilayer graphene},
author = {Johannes Christian Leutenantsmeyer and Josep Ingla-Aynés and Jaroslav Fabian and Bart J. van Wees},
journal= {arXiv preprint arXiv:1805.12420},
year = {2018}
}