Nuclear effects in ultrafast quantum-well spin-dynamics

Ultrafast optical methods are used to study low temperature nuclear spin polarisation and electron spin precession and relaxation in an oblique magnetic field in undoped GaAs/AlGaAs quantum wells. We find that electron localisation dramatically extends electron spin relaxation and accelerates nuclear spin polarisation which is manifest as a large time-dependent Overhauser shift in electron Larmor precession frequency. Nuclear polarisation is a two-stage process, which we suggest involves contact hyperfine interaction at electron localisation centres and nuclear spin diffusion to the remaining nuclei of the sample. Special samples are used to investigate nuclear spin diffusion through the barriers, which is found to be an order of magnitude slower than within the wells, consistent with extra dipolar and quadrupolar disorder in the alloy.