Superconductivity on the edge

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Science  20 May 2016:
Vol. 352, Issue 6288, pp. 891-892
DOI: 10.1126/science.aaf6604

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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 20 years ago (2), but clear observation of the effect was frustrated by the difficulty of creating coexisting superconducting and quantum Hall states. On page 966 of this issue, Amet et al. (3) report on the interplay between these two states, finding evidence for the unconventional Andreev process. Their results confirm new physics that appears when two correlated states are connected, and opens the door to a range of novel excitations and exotic devices.