Signatures of Gapless Andreev Bound States in HgTe Based Topological Insulators

Jonas Wiedenmann, Wuerzburg University

The search for new exotic states in superconductors has recently experienced a boost due to the experimental discovery of topological insulators (TIs). Such a TI in proximity with a conventional superconductor is predicted to lead to the emergence of induced p-wave superconductivity. Due to their inverted bandstructure, HgTe quantum wells of appropriate thickness are a 2D TI and exhibit the Quantum Spin Hall effect. Zero energy states (sometimes called Majorana bound states) occur at vortices or at edges of such triplet pairing superconductors.

In this talk I will focus on detecting two experimental signatures of such zero energy states. We show from the ac Josephson effect that the supercurrent across such a Josephson junction made of a HgTe based topological insulator, varies 4Pi periodically with the superconducting phase difference. Namely we identify a doubling of the Shapiro steps compared to conventional superconductors ([1],[2]) and are able to measure the Josephson emission at half the Josephson frequency [3]. Furthermore, by applying a magnetic flux to the junction, we are able to show that the supercurrent is predominantly carried by edge states.

[1] Bocquillon et al. Arxiv: 1601.08055
[2] Wiedenmann et al. Nature Communications, 7:10303, 2016
[3] Deacon et al. Arxiv: 1603.09611