In a superconductor, electrons travel in pairs and can flow past obstacles without resistance. On page 481 of this issue, Nandi et al.1 demonstrate the existence of a strange analogue of this electronic behaviour in a system of two adjacent conducting layers in which the pairing is instead between an electron in one layer and the absence of an electron in the other layer. This strange form of pairing causes the current in the system to flow without dissipation, as long as the separate currents in each layer are exactly equal in magnitude and opposite in direction. As the physicist Isidor Isaac Rabi once remarked when confronted with an unexpected item from the menu of the delicatessen that is our Universe, “Who ordered that?”.
The 'fluid' model of electricity, developed in the eighteenth century by Benjamin Franklin and others, postulates that electrical charge is a continuous quantity associated with an excess or deficit in the density of some mysterious fluid that smoothly pervades the Universe. This model is perfectly adequate for many practical purposes, but in 1897, J. J. Thomson discovered the electron and concluded that electrical charge is quantized — it occurs in tiny but discrete units carried by elementary particles.
The discreteness of charge is responsible for a phenomenon known as Coulomb drag. To understand what this is, consider two thin metal layers that are separated by an electrically insulating barrier but that are so close to each other that the distance between them (about 10nanometres) is comparable to the distance between individual electrons within each layer (Fig. 1). The electrons in each layer therefore 'see' the graininess of the charge density of the electrons in the other layer. Because of their mutual Coulomb force, electrons in each layer can collide with each other. These collisions cause electrons flowing in one layer (the drive current) to 'drag' the electrons in the other layer, producing a drag current that flows in the same direction as the drive current. An analogous phenomenon occurs when a piece of sandpaper is pushed past another piece: the discrete grains of sand in each sheet produce a mutual friction that drags the second sheet along in the same direction as the first.
