Exchange interaction-driven dynamic nuclear polarization in Mn-doped InGaAs/GaAs quantum dots

Abstract

We investigated optical spin orientation and dynamic nuclear polarization (DNP) in individual self-assembled InGaAs/GaAs quantum dots (QDs) doped by a single Mn atom, a magnetic impurity providing a neutral acceptor A0 with an effective spin J=1. We find that the spin of an electron photocreated in such a quantum dot can be efficiently oriented by a quasiresonant circularly polarized excitation. For the electron spin levels which are made quasidegenerate by a magnetic field compensating the exchange interaction Δe with A0, there is however a full depolarization due the anisotropic part of the exchange. Still, in most studied QDs, the spin polarized photoelectrons give rise to a pronounced DNP which grows with a longitudinal magnetic field until a critical field where it abruptly vanishes. For some QDs, several replica of such DNP sequence are observed at different magnetic fields. This striking behavior is qualitatively discussed as a consequence of different exchange interactions experienced by the electron, driving the DNP rate via the energy cost of electron-nucleus spin flip-flops.