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

Connectivity-Enforcing Hough Transform for the Robust Extraction of Line Segments

Global voting schemes based on the Hough transform (HT) have been widely used to robustly detect lines in images. However, since the votes do not take line connectivity into account, these methods do not deal well with cluttered images. In opposition, the so-called local methods enforce connectivity but lack robustness to deal with challenging situations that occur in many realistic scenarios, e.g., when line segments cross or when long segments are corrupted. In this paper, we address the critical limitations of the HT as a line segment extractor by incorporating connectivity in the voting process. This is done by only accounting for the contributions of edge points lying in increasingly larger neighborhoods and whose position and directional content agree with potential line segments. As a result, our method, which we call STRAIGHT (Segment exTRAction by connectivity-enforcInG HT), extracts the longest connected segments in each location of the image, thus also integrating into the HT voting process the usually separate step of individual segment extraction. The usage of the Hough space mapping and a corresponding hierarchical implementation make our approach computationally feasible. We present experiments that illustrate, with synthetic and real images, how STRAIGHT succeeds in extracting complete segments in several situations where current methods fail.Comment: Submitted for publicatio