Negative magnetoresistance of a silicon 2deg under in-plane magnetic field due to spin-splitting of upper subbands

We examine the effect of an in-plane magnetic field on the resistance of a 2-dimensional electron system confined in a silicon quantum well when the Fermi energy is tuned through the upper valley-subband edge while the electrons are otherwise valley-polarized. In contrast to previous experiments on valley-degenerate systems which only showed positive magnetoresistance, when the Fermi energy is at or near the upper valley-subband edge, the magnetoresistance is found to show a distinct negative contribution which is interpreted as being due to spin polarization of the upper valley-subband.