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Electron spin relaxation in graphene with random Rashba field: Comparison of D'yakonov-Perel' and Elliott-Yafet--like mechanisms

Aiming to understand the main spin relaxation mechanism in graphene, we investigate the spin relaxation with random Rashba field induced by both adatoms and substrate, by means of the kinetic spin Bloch equation approach. The charged adatoms on one hand enhance the Rashba spin-orbit coupling locally and on the other hand serve as Coulomb potential scatterers. Both effects contribute to spin relaxation limited by the D'yakonov-Perel' mechanism. In addition, the random Rashba field also causes spin relaxation by spin-flip scattering, manifesting itself as an Elliott-Yafet--like mechanism. Both mechanisms are sensitive to the correlation length of the random Rashba field, which may be affected by the environmental parameters such as electron density and temperature. By fitting and comparing the experiments from the Groningen group [Józsa {\it et al.}, Phys. Rev. B {\bf 80}, 241403(R) (2009)] and Riverside group [Pi {\it et al.}, Phys. Rev. Lett. {\bf 104}, 187201 (2010); Han and Kawakami, {\it ibid.} {\bf 107}, 047207 (2011)] which show either D'yakonov-Perel'-- (with the spin relaxation rate being inversely proportional to the momentum scattering rate) or Elliott-Yafet--like (with the spin relaxation rate being proportional to the momentum scattering rate) properties, we suggest that the D'yakonov-Perel' mechanism dominates the spin relaxation in graphene. The latest experimental finding of a nonmonotonic dependence of spin relaxation time on diffusion coefficient by Jo {\it et al.} [Phys. Rev. B {\bf 84}, 075453 (2011)] is also well reproduced by our model.