  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.
