Induced superconductivity in the quantum spin Hall edge

Abstract

Topological insulators are a newly discovered phase of matter characterized by a gapped bulk surrounded by novel conducting boundary states [1,2,3]. 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 [4,5]. 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 [6]. Despite the clear theoretical motivation, a conclusive realization of topological superconductivity remains an outstanding experimental goal.Here we present measurements of superconductivity induced in two-dimensional HgTe/HgCdTe quantum wells, a material which becomes a quantum spin Hall insulator when the well width exceeds dC = 6.3 nm [7]. In wells that are 7.5 nm wide, we find 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. These results provide evidence for superconductivity induced in the helical edges of the quantum spin Hall effect, a promising step toward the demonstration of one-dimensional topological superconductivity.Our results also provide a direct measurement of the widths of these edge channels, which range from 180 nm to 408 nm.Topological superconductors, like topological insulators, possess a bulk energy gap and gapless surface states. In a topological superconductor, the surface states are predicted to manifest as zero-energy Majorana fermions, fractionalized modes which pair to form conventional fermions. Due to their non-Abelian braiding statistics, achieving control of these Majorana modes is desirable both fundamentally and for [9], and on their direct engineering using s-wave superconductors combined with topological insulators or semiconductors [10,11]. Particularly appealing are implementations in one-dimensional (1D) systems, where Majorana modes would be localized to the ends of a wire. In such a 1D system, restriction to a single spin degree of freedom combined with proximity to an s-wave superconductor would provide the basis for topological superconductivity [12]. Effort in this direction has been advanced by studies of nanowire systems [13,14,15,16,17,18] and by excess current measurements on InAs/GaSb devices [19]. Given the wide interest in Majorana fermions in one dimension, it is essential to expand the search to other systems whose properties are suited toward their control.An attractive route toward a 1D topological superconductor uses as its starting point the twodimensional (2D) quantum spin Hall (QSH) insulator. This topological phase of matter was recently predicted [20,21] and observed [22,23] in HgTe/HgCdTe quantum wells thicker than a critical thickness d C = 6...

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.…”

Superconducting quantum spin Hall systems with giant orbitalgfactors

“…In particular, the quantum spin-Hall effect [5, 6, 9, 10] allows one to generate and convert charge and spin currents in protected edge channels [11]. Combining quantum spin-Hall systems (QSHSs) with superconductors (SCs) leads to a broader spectrum of interesting observable phenomena [12][13][14]. These include quantum interference effects reported in [13,14], indicating superconducting transport through the edge states in the QSH regime.…”

“…Combining quantum spin-Hall systems (QSHSs) with superconductors (SCs) leads to a broader spectrum of interesting observable phenomena [12][13][14]. These include quantum interference effects reported in [13,14], indicating superconducting transport through the edge states in the QSH regime. Understanding edge superconductivity in QSHS/SC hybrids is also instrumental to the proposals to realize Majorana zero modes in topological insulators (see, e.g., [15] and reviews [16,17]).…”