Electron-drag effects in coupled electron systems

The advances in the techniques used for fabrication and lithography of semiconductors have made it possible to study bi-layer systems made of two electronic layers separated by distances of several hundred ångströms. In this situation the electrons in layer 1 are distinguishable from those in layer 2, and can communicate through the direct inter-layer Coulomb interaction. In particular, if a current is applied to one of the layers, the electrons in the second will be dragged, giving rise to a transresistance ρ_D. In this article we review recent theoretical and experimental developments in the understanding of this effect. At very low temperatures it turns out that phonons dominate the transresistance. The direct Coulomb interaction and plasmon excitations are important at temperatures T >0.1T_F, with T_F the Fermi temperature. If a magnetic field is applied, the transresistance is increased, in a very interesting interplay between ρ_D and Landau quantization. The non-dissipative drag is also reviewed.