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

TL;DR

This paper studies how spin relaxation and decoherence behave in graphene quantum dots with spin-orbit interactions, revealing non-monotonic magnetic field dependence and the dominance of relaxation processes in decoherence.

Contribution

It derives an effective Hamiltonian for spin-phonon interactions in graphene quantum dots and analyzes the non-monotonic behavior of spin relaxation times due to Rashba spin-orbit coupling.

Findings

Non-monotonic dependence of T_1 on magnetic field due to Rashba coupling.

Rapid decrease of T_1 near spin-level anticrossing.

Decoherence time T_2 is approximately twice T_1, dominated by relaxation processes.

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 T_1 and decoherence time T_2 within the framework of the Bloch-Redfield theory. In this model, the emergence of a non-monotonic dependence of T_1 on the external magnetic field is attributed to the Rashba spin-orbit coupling-induced anticrossing of opposite spin states. A rapid decrease of T_1 occurs when the spin and orbital relaxation rates become comparable in the vicinity of the spin-mixing energy-level anticrossing. By contrast, the intrinsic spin-orbit interaction leads to a monotonic magnetic field dependence of the spin relaxation rate which is caused solely by the direct spin-phonon coupling mechanism. Within our model, we demonstrate that the decoherence time T_2 ~ 2 T_1 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.