| We demonstrate dynamical nuclear spin polarization (DNSP) in the absence of an external magnetic field, using resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. The optically pumped electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our observations suggest a new mechanism for spin-polarization where spin exchange with an electron reservoir plays a crucial role. |
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