Giant Spin Lifetime Anisotropy in Graphene Induced by Proximity Effects

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.

Figure 1

Schematic of spin relaxation in graphene-TMDC heterostructures. The tall arrows depict the effective spin-orbit field within the Dirac cones at $K$ and $K^{\prime }$ valleys. Intervalley scattering dominates the in-plane spin dynamics, while overall momentum scattering controls the out-of-plane behavior.

Figure 2

Spin dynamics in the graphene/${\mathrm{WSe}}_2$ system for (a) strong and (b) weak intervalley scattering. The insets show the corresponding momentum relaxation times.

Figure 3

Spin lifetime with strong intervalley scattering for graphene on (a) ${\mathrm{WSe}}_2$ and (b) ${\mathrm{WS}}_2$. The red (blue) lines are for out-of-plane (in-plane) spin lifetime. Solid (dashed) lines are for an impurity density of 0.1% (1%). The open circles are from Eq. (5).

Figure 4

Spin lifetime without intervalley scattering for graphene on (a) ${\mathrm{WSe}}_2$ and (b) ${\mathrm{WS}}_2$. The red (blue) lines are for out-of-plane (in-plane) spin lifetime. The open circles are from Eq. (7), and the insets show the anisotropy for strong and weak intervalley scattering.