Magnetic field effects on the exchange instability of the 2D electron gas

We have studied the effect of an external magnetic field on the photoluminescence of a two-dimensional electron gas (2DEG) formed in a GaAs quantum well for the case of an incipient occupation of the first excited electron subband. As this subband becomes populated the 2DEG undergoes a first-order phase transition driven by exchange terms of the Coulomb interaction, for which a spontaneous spin polarization is expected to occur. For a magnetic field applied perpendicular to the 2DEG we obtain a strong g-factor enhancement due to many-body effects, as determined from Landau level splittings of the densely populated second subband. In contrast, the intensity of the emission peak associated with the spin-polarized phase decreases linearly with field, indicating an in plane spin polarization. From the field dependence of the degree of linear polarization of the emission line F in the parallel field configuration, we were able to estimate the average size of the spin-polarized domains to be about 200 nm.