Nuclear spin bi-stability in semiconductor quantum dots

The confinement of electrons on the nano-scale and the control of their spin is leading to both new physics and to novel devices with functionality at the quantum level. In III-V semiconductors where manipulation of a single electron spin has been possible in nano-structures, new challenges and opportunities arise due to the hyperfine interaction with the spin reservoir of lattice nuclei. In this work we show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of the tens of thousands of atoms constituting an InGaAs island can be driven into a bistable regime, in which either a thresholdlike enhancement or reduction of the local nuclear field by up to 3 Tesla can be generated by varying the intensity of light. We show that such a nuclear spin "switch" can be controlled by both external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of spin transfer from electrons to the nuclei of the dot. A comprehensive theory describing the nuclear bi-stability phenomenon in optically pumped dots is presented.