Evidence for Helical Edge Modes in Inverted InAs/GaSb Quantum Wells

We present an experimental study of low temperature electronic transport in the hybridization gap of inverted InAs/GaSb composite quantum wells. Electrostatic gate is used to push the Fermi level into the gap regime, where the conductance as a function of sample length and width is measured. Our analysis shows strong evidence for the existence of helical edge modes proposed by Liu et al [Phys. Rev. Lett., 100, 236601 (2008)]. Edge modes persist inspite of sizable bulk conduction and show only a weak magnetic field dependence - a direct consequence of gap opening away from zone center.Comment: 4 pages, 4 figure

Finite Conductivity in Mesoscopic Hall Bars of Inverted InAs/GaSb Quantum Wells

We have studied experimentally the low temperature conductivity of mesoscopic size InAs/GaSb quantum well Hall bar devices in the inverted regime. Using a pair of electrostatic gates we were able to move the Fermi level into the electron-hole hybridization state, and observe a mini gap. Temperature dependence of the conductivity in the gap shows residual conductivity, which can be consistently explained by the contributions from the free as well as the hybridized carriers in the presence of impurity scattering, as proposed by Naveh and Laikhtman [Euro. Phys. Lett., 55, 545-551 (2001)]. Experimental implications for the stability of proposed helical edge states will be discussed.Comment: 5 pages, 4 figure

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

Contrastive pretraining for semantic segmentation is robust to noisy positive pairs

Domain-specific variants of contrastive learning can construct positive pairs from two distinct images, as opposed to augmenting the same image twice. Unlike in traditional contrastive methods, this can result in positive pairs not matching perfectly. Similar to false negative pairs, this could impede model performance. Surprisingly, we find that downstream semantic segmentation is either robust to the noisy pairs or even benefits from them. The experiments are conducted on the remote sensing dataset xBD, and a synthetic segmentation dataset, on which we have full control over the noise parameters. As a result, practitioners should be able to use such domain-specific contrastive methods without having to filter their positive pairs beforehand.Comment: 8 pages, 8 figure

Andreev Reflection of Helical Edge Modes in InAs=GaSb Quantum Spin Hall Insulator

We present an experimental study of S−N−S junctions, with N being a quantum spin Hall insulator made of InAs/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 ∼2e2/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