Figure 2: Expected activation energy as a function of relative density imbalance between the layers, . For voltage drop measured in the drive layer, the results are asymmetric with around , as shown explicitly for (see text). For measurements in the drag layer, the data is reflected around ; the measured result would follow the higher of the two activation energies, yielding a result symmetric in bias. Inset: activation energy at zero bias versus layer separation. Reproduced from .