Quantum Hall effect in Coulomb drag: Interlayer friction in strong magnetic fields

We study the Coulomb drag between two spatially separated electron systems in the quantum Hall regime. At a fixed temperature, the drag is peaked near the transitions between plateaus. The temperature dependence of the drag is a probe of the dynamics of the Hall system in a frequency and wave-vector regime inaccessible to transport measurements in a single layer. Most strikingly, in the transition regions it directly measures an exponent η, discussed by Chalker, that is characteristic of the fractal structure of the critical eigenstates.