Andreev Reflection of Helical Edge Modes inInAs/GaSbQuantum Spin Hall Insulator

We present an experimental study of $S\mathrm{\text{−}}N\mathrm{\text{−}}S$ junctions, with $N$ being a quantum spin Hall insulator made of $\mathrm{InAs}/\mathrm{GaSb}$. A front gate is used to vary the Fermi level into the minigap, where helical edge modes exist [ Phys. Rev. Lett. 107, 136603 (2011)]. In this regime we observe a $\sim 2e^2/h$ Andreev conductance peak, consistent with a perfect Andreev reflection on the helical edge modes predicted by theories. The peak diminishes under a small applied magnetic field due to the breaking of time-reversal symmetry. This work thus demonstrates the helical property of the edge modes in a quantum spin Hall insulator.