Nonmonotonic spin relaxation and decoherence in graphene quantum dots with spin-orbit interactions

Abstract

We investigate the spin relaxation and decoherence in a single-electron graphene quantum dot with Rashba and intrinsic spin-orbit interactions.We derive an effective spin-phonon Hamiltonian via the Schrieffer-Wolff transformation in order to calculate the spin relaxation time T1 and decoherence time T2 within the framework of theBloch- Redfield theory. In this model, the emergence of a nonmonotonic dependence of T1 on the external magnetic field is attributed to the Rashba spin-orbit coupling-induced anticrossing of opposite spin states. A rapid decrease of T1 occurs when the spin and orbital relaxation rates become comparable in the vicinity of the spin-mixing energylevel anticrossing. By contrast, the intrinsic spin-orbit interaction leads to a monotonicmagnetic field dependence of the spin relaxation ratewhich is caused solely by the direct spin-phonon couplingmechanism.Within our model, we demonstrate that the decoherence time T2 ≈ 2T1 is dominated by relaxation processes for the electron-phonon coupling mechanisms in graphene up to leading order in the spin-orbit interaction. Moreover, we show that the energy anticrossing also leads to a vanishing pure spin dephasing rate for these states for a super-Ohmic bath.