Inversion-asymmetry-induced spin splitting observed in the quantum oscillatory magnetization of a two-dimensional electron system

We have investigated the de Haas-van Alphen (dHvA) effect of a two-dimensional electron system in an AlGaAs/GaAs heterojunction which exhibited structure inversion asymmetry. Using torque magnetometry at 30 mK the magnetic quantum oscillations are found to display beating patterns under large tilt angles. We attribute these features to spin splitting of the Fermi surface due to spin-orbit interaction. Such beating patterns in the dHvA effect have been predicted by Bychkov and Rashba more than two decades ago but have not been reported before. From the beat node positions in the magnetization M we estimate the zero-field spin splitting to be about 200 mu eV. Interestingly we find characteristic phase changes in the dHvA signal accompanied by a shift of the discontinuous jump in M from the high-field to the low-field side of the dHvA oscillations. This observation is unexpected and needs further theoretical explanation.