Edge-mode superconductivity in a two-dimensional topological insulator

Abstract

Topological superconductivity is an exotic state of matter that supports Majorana zero-modes, which have been predicted to occur in the surface states of three-dimensional systems, in the edge states of two-dimensional systems, and in one-dimensional wires. Localized Majorana zero-modes obey non-Abelian exchange statistics, making them interesting building blocks for topological quantum computing. Here, we report superconductivity induced in the edge modes of semiconducting InAs/GaSb quantum wells, a two-dimensional topological insulator. Using superconducting quantum interference we demonstrate gate-tuning between edge-dominated and bulk-dominated regimes of superconducting transport. The edge-dominated regime arises only under conditions of high-bulk resistivity, which we associate with the two-dimensional topological phase. These experiments establish InAs/GaSb as a promising platform for the confinement of Majoranas into localized states, enabling future investigations of non-Abelian statistics.

Random-matrix theory of Majorana fermions and topological superconductors

“…The topological Altland-Zirnbauer ensembles still lack an experimental platform of similar versatility, but there is a great variety of candidate systems. We are particularly excited by the recent progress in quantum spin-Hall insulators (HgTe or InAs quantum wells) with superconducting electrodes (Hart et al, 2014;Knez, Du, and Sullivan, 2012;Pribiag et al, 2014). The quantum dot geometry of Fig.…”

Odd-frequency triplet superconductivity at the helical edge of a topological insulator

“…However, it is fair to say that it is very difficult to unambiguously probe the emergence of unconventional SC because -often times -conventional and unconventional signatures of SC look alike. Several recent experiments have demonstrated (as a first step towards the detection of unconventional SC) that helical liquids as boundary states of quantum spin Hall systems can indeed be brought in proximity to s-wave superconductors [10] and serve, for instance, as conducting channels of a Josephson junction [11,12]. Importantly, helicity guarantees perfect local Andreev reflection [13] -the conversion of an electron into a hole with opposite spin -at the interface between the normal and the proximity-induced superconducting region called NS junction.…”

Even-odd flux quanta effect in the Fraunhofer oscillations of an edge-channel Josephson junction

“…The broad conducting pathway that appears along the NS interface will be gapped by the superconducting proximity effect, but a narrow gapless channel may remain because superconductivity only becomes effective at some penetration length ξ 0 from the NS interface. (Reference [4] estimates ξ 0 240 nm, comparable to the estimated width of the edge states.) This channel provides a connection between the helical edge states that is nonhelical, meaning that either spin can propagate in both directions.…”

“…Turning now to the experiment that motivated this analysis [4], we first of all notice that the observed even-odd effect appears already for the first few peaks around zero field. An explanation in terms of a Lorentz-force induced asymmetry in the current distribution is therefore unlikely [19][20][21][22][23].…”

“…We are motivated by recent work on proximity induced superconductivity in quantum spin-Hall (QSH) insulators [2][3][4][5][6][7][8], which in one series of experiments [4] showed Fraunhofer oscillations with an even-odd effect: Large peaks in the critical current at even multiples of h/2e alternate with smaller peaks at odd multiples.…”