Negative electron drag and holelike 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 to that for zero field. This negative drag requires that the electrons have a holelike dispersion. Density dependence measurements in the integer quantum Hall regime show that the 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 holelike dispersion is present in one-half of each Landau level. Negative drag is argued to be a consequence of disorder. [S0031-9007(98)07296-2].