Dynamic nuclear polarization in n-GaAs - free versus localized electrons

In spin dependent information processing using III-V semiconductors, the large electron-nuclear coupling plays an important role, e.g. in affecting the electron spin coherence through the hyperfine interaction. In our experiment, optically injected spin-polarized electrons are used to generate a dynamic nuclear polarization (DNP) in bulk n-GaAs. This results in a tiny variation of the electron Larmor precession frequency, which is probed by time resolved Kerr rotation. A saturated Overhauser field on the order of several 10 mT is obtained, depending on the helicity of the pump beam. Our experimental data indicate a significant difference of the DNP time constant for localized and itinerant electrons. This is explained by considering a model involving Fermi contact hyperfine interaction, spin exchange between donor electrons and itinerant electrons and nuclear spin diffusion.