Weak anisotropy and disorder dependence of the in-plane magnetoresistance in high-mobility (100) Si-inversion layers

We report studies of the magnetoresistance (MR) in a two-dimensional electron system in (100) Si-inversion layers, for perpendicular and parallel orientations of the current with respect to the magnetic field in the 2D plane. The magnetoresistance is almost isotropic; this result does not support the suggestion of its orbital origin. In the hopping regime, however, the MR contains a weak anisotropic component that is nonmonotonic in the magnetic field. We found that the field, at which the MR saturates, varies for different samples by a factor of 2 at a given carrier density. Therefore, the saturation of the MR cannot be identified with the complete spin polarization of free carriers.