Dynamic self-polarization of the nuclear spins in a GaAs quantum well.

We demonstrate the self-polarization regime of nuclear spins, originally predicted in 1972 by Dyakonov and Perel and achieved here by lattice temperature reduction into the millikelvin range. We first identify a gallium arsenide-based quantum well structure as highly suited by demonstrating a high nuclear spin polarization, corresponding to an Overhauser field of 3.1 tesla, using optical spin pumping at 1.6-kelvin lattice temperature. Here, adiabatic demagnetization leads to a nuclear spin temperature of 6.4 microkelvins, detected by time-resolved Kerr rotation. By measuring polarized photoluminescence and entering the lattice temperature regime below 500 millikelvins, a sharp zero-field feature in the Hanle electron spin depolarization curve evidences the dynamic nuclear self-polarization under unpolarized optical excitation. The self-polarization results in ultralow nuclear spin temperatures, which we estimate to become as low as 200 nanokelvins.