Browsing by Author "Chang, Kai"

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    Evidence for a topological excitonic insulator in InAs/GaSb bilayers
    (Springer Nature, 2017) Du, Lingjie; Li, Xinwei; Lou, Wenkai; Sullivan, Gerard; Chang, Kai; Kono, Junichiro; Du, Rui-Rui
    Electron-hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen-Cooper-Schrieffer (BCS) superconductor. Here, we report optical spectroscopic and electronic transport evidence for the formation of an excitonic insulator gap in an inverted InAs/GaSb quantum-well system at low temperatures and low electron-hole densities. Terahertz transmission spectra exhibit two absorption lines that are quantitatively consistent with predictions from the pair-breaking excitation dispersion calculated based on the BCS gap equation. Low-temperature electronic transport measurements reveal a gap of ~2 meV (or ~25 K) with a critical temperature of ~10 K in the bulk, together with quantized edge conductance, suggesting the occurrence of a topological excitonic insulator phase.
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    Tuning Edge States in Strained-Layer InAs/GaInSb Quantum Spin Hall Insulators
    (American Physical Society, 2017) Du, Lingjie; Li, Tingxin; Lou, Wenkai; Wu, Xingjun; Liu, Xiaoxue; Han, Zhongdong; Zhang, Chi; Sullivan, Gerard; Ikhlassi, Amal; Chang, Kai; Du, Rui-Rui
    We report on a class of quantum spin Hall insulators (QSHIs) in strained-layer InAs / GaInSb quantum wells, in which the bulk gaps are enhanced up to fivefold as compared to the binary InAs / GaSb QSHI. Remarkably, with consequently increasing edge velocity, the edge conductance at zero and applied magnetic fields manifests time reversal symmetry-protected properties consistent with the Z 2 topological insulator. The InAs / GaInSb bilayers offer a much sought-after platform for future studies and applications of the QSHI.