Optical control of magnetization and spin blockade in graphene quantum dots

We show that the magnetization of triangular graphene quantum dots with zigzag edges can be manipulated optically. When the system is charge neutral, the magnetic moment can be first erased by addition of a single electron spin with a gate, then restored by absorption of a photon. The conversion of a single photon to a magnetic moment results in a many-body effect, optical spin blockade. The effect demonstrated here can potentially lead to efficient spin to photon conversion, quantum memories, and single-photon detectors.

Figure 1

Schematic illustration of optical control of magnetization and origin of optical spin blockade: Creation of magnetic moment S; erasure of S with addition of a single electron, which through e-e interactions destroys S; restoration of S by absorption of a single photon that creates an exciton, which restores magnetic moment S through e-e and e-h interactions.

Figure 2

Noninteracting (left panels) and many-body (right panels) energy spectra showing the ground-state total spin of (a) charge neutral, (b) charged, and (c) charged and photoexcited quantum dot with seven zero-energy states.

Figure 3

Ground-state total spin as a function of filling of the zero-energy band of the system described in Fig. 2, with and without optical activation. Magnetization of the system is stabilized by the presence of an exciton. Optically allowed and blockaded transitions are shown with blue and red arrows, respectively.

Figure 4

Allowed and blockaded optical transitions due to spin conservation rule in (a) the absorption and (b) emission many-body spectra. Corresponding absorption and emission lines are shown in (c).