Magnetic-field-induced spin polarization of AlAs two-dimensional electrons

Two-dimensional (2D) electrons in an in-plane magnetic field become fully spin polarized above a field $B_P,$ which we can determine from the in-plane magnetoresistance. We perform such measurements in modulation-doped AlAs electron systems, and find that the field $B_P$ increases approximately linearly with 2D electron density. These results imply that the product $|g^{*}|m^{*},$ where $g^{*}$ is the effective g factor and $m^{*}$ the effective mass, is a constant essentially independent of density. While the deduced $|g^{*}|m^{*}$ is enhanced relative to its band value by a factor of $\sim 4,$ we see no indication of its divergence as 2D density approaches zero. These observations are at odds with results obtained in Si, but qualitatively confirm spin-polarization studies of 2D GaAs carriers.