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Perfect Andreev Reflection of Helical Edge Modes in InAs/GaSb Quantum Wells

We present an experimental study of inverted InAs/GaSb composite quantum wells in the hybridization regime and contacted by superconducting electrodes. A front gate is used to vary the Fermi level into the mini-gap, where recent experiments indicate existence of helical edge modes [arXiv:1105.0137]. Zero bias dips in differential resistance are observed across the mini-gap, suggesting transport dominated by Andreev reflection processes. Evolution of the mini-gap differential resistance with applied bias as well as measured mini-gap excess current of 150 nA are in good agreement with the prediction of perfect Andreev reflection of the helical edge modes, which is necessitated by the absence of back-scattering channels. The perfect Andreev reflection occurs in spite of a finite barrier at the interface and shows strong sensitivity to time-reversal breaking - hallmarks of the helical nature of quantum spin Hall edges.