[Perspective] Superconductivity on the edge
Standard superconductors consist of a condensate of paired electrons, called Cooper pairs. The transport behavior of these pairs at junctions can produce exotic effects that are of fundamental and practical interest. When two superconductors are in contact via a normal metal, the pairs must convert to single-particle states to traverse between superconductors. This occurs by the Andreev process, whereby a low-energy electron in the normal metal injects a Cooper pair into the superconductor and generates a hole that reflects back into the metal; coherent, opposite-momentum electron-hole pairs then carry the supercurrent across the metallic junction (1) (see the figure, panel A). In the case of superconductors connected to a quantum Hall state, there are only one-way paths along the junction edges. Here, a new type of Andreev process is predicted to occur, whereby electron and hole states on opposite sample edges carry the supercurrent. This prediction was made more than...