The physical picture behind the oscillating sign of drag in high Landau   levels
  We consider the oscillating sign of the drag resistivity and its anomalous temperature dependence discovered experimentally in a bi-layer system in the regime of the integer quantum Hall effect. We attribute the oscillating sign to the effect of disorder on the relation between an adiabatic momentum transfer to an electron and the displacement of its position. While in the absence of any Landau level mixing a momentum transfer $\hbar \bf q$ implies a displacement of $ql_H^2$ (with $l_H$ being the magnetic length), Landau level mixing induced by short range disorder adds a potentially large displacement that depends on the electron's energy, with the sign being odd with respect to the distance of that energy from the center of the Landau level. We show how the oscillating sign of drag disappears when the disorder is smooth and when the electronic states are localized. 
Rafi Bistritzer and Ady Stern