Induced Superconductivity in the Quantum Spin Hall Edge

August 5, 2014
 General behaviour observed in the topological Josephson junction

General behaviour observed in the topological Josephson junction: a, A map of the differential resistance across the junction, measured with the top gate at VG = 1.05 V, shows the single-slit interference characteristic of a uniform supercurrent density. b, The supercurrent density, extracted for VG = 1.05 V, is consistent with trivial charge transport throughout the bulk of the junction. c, When the top-gate voltage is lowered to VG = −0.425 V, the differential resistance shows a more sinusoidal interference pattern. d, Using the interference envelope measured at VG = −0.425 V, the supercurrent density is clearly dominated by the contribution from the edges. In this regime almost no supercurrent passes through the bulk. [Figure reprinted by permission from Macmillan Publishers Ltd: see S. Hart, H. Ren, T. Wagner, Ph. Leubner... et al., "Induced superconductivity in the quantum spin Hall edge," Nature Physics (2014) doi:10.1038/nphys3036.]

Topological insulators are a newly discovered phase of matter characterized by gapped bulk states surrounded by conducting boundary states. Since their theoretical discovery, these materials have encouraged intense efforts to study their properties and capabilities. Among the most striking results of this activity are proposals to engineer a new variety of superconductor at the surfaces of topological insulators. These topological superconductors would be capable of supporting localized Majorana fermions, particles whose braiding properties have been proposed as the basis of a fault-tolerant quantum computer. Despite the clear theoretical motivation, a conclusive realization of topological superconductivity remains an outstanding experimental goal.

In a Letter in Nature Physics, Prof. Amir Yacoby and colleagues from Harvard and Universität Würzburg, Germany, presented measurements of superconductivity induced in two-dimensional HgTe/HgCdTe quantum wells, a material that becomes a quantum spin Hall insulator when the well width exceeds dC = 6.3 nm. In wells that are 7.5 nm wide, the researchers found that supercurrents are confined to the one-dimensional sample edges as the bulk density is depleted. However, when the well width is decreased to 4.5 nm the edge supercurrents cannot be distinguished from those in the bulk. Their results provide evidence for supercurrents induced in the helical edges of the quantum spin Hall effect, establishing this system as a promising avenue towards topological superconductivity. In addition to directly confirming the existence of the topological edge channels, these results also provide a measurement of their widths, which range from 180 nm to 408 nm.