Researchers report numerical simulations for spin relaxation in graphene/TMDC heterostructures. Their calculations indicate a spin lifetime anisotropy that is orders of magnitude larger than anything observed in graphene until now.
Published in Physical Review Letters, spintronics researchers of the ICN2 Theoretical and Computational Nanoscience Group led by ICREA Professor Stephan Roche have gleaned potentially game-changing insight into the mechanisms governing spin dynamics and relaxation in graphene/TMDC heterostructures. Not only do their models give a spin lifetime anisotropy that is orders of magnitude larger than the 1:1 ratio typically observed in 2D systems, but they point to a qualitatively new regime of spin relaxation.
Spin relaxation is the process whereby the spins in a spin current lose their orientation, reverting to a natural disordered state. This causes spin signal to be lost, since spins are only useful for transporting information when they are oriented in a certain direction. This study reveals that the rate at which spins relax in graphene/TMDC systems depends strongly on whether they are pointing in or out of the graphene plane, with out-of-plane spins lasting tens or hundreds of times longer than in-plane spins. Such a high ratio has not previously been observed in graphene or any other 2D material.
In the paper, aptly titled “Giant Spin Lifetime Anisotropy in Graphene Induced by Proximity Effects,” lead author Aron Cummings reports that this behavior is mediated by the spin-valley locking induced in graphene by the TMDC, which ties the lifetime of in-plane spin to the intervalley scattering time. This causes in-plane spin to relax much faster than out-of-plane spin. Furthermore, the numerical simulations suggest that this mechanism should come into play in any substrate with strong spin-valley locking, including the TMDCs themselves.
Effectively inducing a spin filter effect — the ability to sort or tweak spin orientations — these findings give reason to believe that it might one day be possible to manipulate, and not just transport, spin in graphene.
These simulations have since been borne out experimentally by colleagues in the ICN2 Physics and Engineering of Nanodevices Group, led by ICREA Professor Sergio Valenzuela.