Negative magneto-drag of double layer 2DEGs

We present friction measurements (drag) between two 2DEGs in a magnetic field in the integer quantum Hall regime. At matched densities, the double peak in the drag that occurs when the longitudinal resistances of each 2DEG do not show any sign of spin splitting is observed. For unequal densities in the 2DEGs, a 1/B periodic oscillation around zero appears. Both effects are well described by a phenomenological model assuming a negative drag between electrons with opposite spin and a positive drag between electrons with the same spin. Experiments in tilted magnetic field and on low-mobility samples show that when the spin gap becomes large, the negative drag disappears.