Observation of spin-valley coupling induced large spin lifetime anisotropy in bilayer graphene

TL;DR

This study reports the first observation of a large spin lifetime anisotropy in bilayer graphene, driven by spin-valley coupling, with significant implications for spintronics applications.

Contribution

It demonstrates the existence of a large spin lifetime anisotropy in bilayer graphene due to spin-valley coupling, with detailed temperature and carrier density dependence.

Findings

Spin lifetime anisotropy of 8 at 75 K and CNP

Maximum out-of-plane spin lifetime of 9 ns in high-quality BLG

Anisotropy decreases with temperature, approaching isotropy near room temperature

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.