Negative Electron Drag and Hole-Like behavior in the Integer Quantum   Hall Regime
  Electron drag between two two-dimensional electron gases in magnetic fields has been observed with a polarity opposite that for zero field. This negative drag requires that the electrons have a hole-like dispersion. Density dependence measurements in the integer quantum Hall regime show that drag is negative only when the upper Landau level of one layer is more than half filled while the other is less than half filled, indicating that hole-like dispersion is present in a half of each Landau level. Negative drag is argued to be a consequence of disorder. 
X. G. Feng (1), S. Zelakiewicz (1), H. Noh (1), T. J. Ragucci(1), T.   J. Gramila (1), L. N. Pfeiffer (2), K. W. West (2) ((...