Abstract
The understanding of spin dynamics and relaxation mechanisms in clean graphene, and the upper time and length scales on which spin devices can operate, are prerequisites to realizing graphene-based spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with Rashba spin-orbit fields as low as a few tens of μeV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from a Dyakonov-Perel type for SiO2 substrates to a broadening-induced dephasing for hBN substrates is described. The energy dependence of spin lifetimes, their ratio for spins pointing out-of-plane and in-plane, and the scaling with disorder provide a global picture about spin dynamics and relaxation in ultraclean graphene in the presence of electron-hole puddles.
Highlights
The tantalizing prospect of graphene spintronics was initiated by Tombros and coworkers[1], who first reported long spin diffusion length in large area graphene
A comprehensive picture of the spin dynamics of massless Dirac fermions in the presence of weak spin-orbit coupling fields is of paramount importance for further exploitation and manipulation of the spin, pseudospin and valley degrees of freedom[7,31,32,33]
We show numerically that a weak uniform Rashba spin-orbit coupling (SOC), induced by an electric field or the substrate, yields spin lifetimes from 50 ps up to several nanoseconds
Summary
Electron-hole puddles are real-space fluctuations of the chemical potential, induced by the underlying substrate, which locally shift the Dirac point[37,38,39]. The onsite energy profiles were found to obey a Gaussian distribution, with standard deviations of σ = 5.5 and 56 meV for hBN and SiO2 substrates, respectively. From such information, we can tune Δ to obtain suitable disorder profiles for the onsite energy of the π-orbital. To fully characterize the role of electron-hole puddles, we evaluate the transport time τp using a real-space order-N approach, which computes the diffusion coefficient D(E, t).
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