Effective time-reversal symmetry breaking in the spin relaxation in a graphene quantum dot

TL;DR

This paper investigates spin relaxation in a graphene quantum dot, revealing that intrinsic and Rashba spin-orbit couplings enable B-field independent relaxation at low fields and distinct behaviors from GaAs due to broken time-reversal symmetry.

Contribution

It demonstrates how spin relaxation mechanisms in graphene quantum dots differ from traditional semiconductors, highlighting the role of time-reversal symmetry breaking and phonon interactions.

Findings

Spin relaxation rate is B-field independent at low fields.

Absence of van Vleck cancellation in graphene quantum dots.

Distinct B-field dependence of relaxation rate compared to GaAs.

Abstract

We study the relaxation of a single electron spin in a circular gate-tunbable quantum dot in gapped graphene. Direct coupling of the electron spin to out-of-plane phonons via the intrinsic spin-orbit coupling leads to a relaxation time T_1 which is independent of the B-field at low fields. We also find that Rashba spin-orbit induced admixture of opposite spin states in combination with the emission of in-plane phonons provides various further relaxation channels via deformation potential and bond-length change. In the absence of valley mixing, spin relaxation takes place within each valley separately and thus time-reversal symmetry is effectively broken, thus inhibiting the van Vleck cancellation at B=0 known from GaAs quantum dots. Both the absence of the van Vleck cancellation as well as the out-of-plane phonons lead to a behavior of the spin relaxation rate at low magnetic fields which is markedly different from the known results for GaAs. For low B-fields, we find that the rate is constant in B and then crosses over to ~B^2 or ~B^4 at higher fields.