Browsing by Author "Sullivan, Gerard"
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Item Andreev Reflection of Helical Edge Modes in InAs=GaSb Quantum Spin Hall Insulator(American Physical Society, 2012) Knez, Ivan; Du, Rui-Rui; Sullivan, GerardWe 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.Item Effectiveﾠg-factors of carriers in inverted InAs/GaSb bilayers(AIP Publishing, 2016) Mu, Xiaoyang; Sullivan, Gerard; Du, Rui-RuiWe perform tilt-fieldﾠtransportﾠexperiment on inverted InAs/GaSb, which hosts quantum spin Hallﾠinsulator.ﾠBy means of coincidence method,ﾠLandau levelﾠ(LL) spectra of electron and hole carriers are systematically studied at different carrier densities tuned by gate voltages. When Fermi level stays in theﾠconduction band,ﾠwe observe LL crossing and anti-crossing behaviors at odd and even filling factors, respectively, with a correspondingﾠg-factor of 11.5. It remains nearly constant for varying filling factors and electron densities. On the contrary, forﾠGaSbﾠholes, only a smallﾠZeeman splittingﾠis observed even at large tilt angles, indicating aﾠg-factor of less than 3.Item 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-RuiElectron-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.Item Images of Edge Current inﾠInAs/GaSbﾠQuantum Wells(American Physical Society, 2014) Spanton, Eric M.; Nowack, Katja C.; Du, Lingjie; Sullivan, Gerard; Du, Rui-Rui; Moler, Kathryn A.Quantum spin Hall devices with edges much longer than several microns do not display ballistic transport; that is, their measured conductances are much less thanﾠe2/hﾠper edge. We imaged edge currents inﾠInAs/GaSbﾠquantum wells with long edges and determined an effective edge resistance. Surprisingly, although the effective edge resistance is much greater thanﾠh/e2, it is independent of temperature up to 30ﾠK within experimental resolution. Known candidate scattering mechanisms do not explain our observation of an effective edge resistance that is large yet temperature independent.Item Low-temperature conductivity of weakly interacting quantum spin Hall edges in strained-layer InAs/GaInSb(American Physical Society, 2017) Li, Tingxin; Wang, Pengjie; Sullivan, Gerard; Lin, Xi; Du, Rui-RuiWe report low-temperature transport measurements in strainedﾠInAs/Ga0.68In0.32Sbﾠquantum wells, which supports time-reversal symmetry-protected helical edge states. The temperature and bias voltage dependence of the helical edge conductance for devices of various sizes are consistent with the theoretical expectation of a weakly interacting helical edge state. Moreover, we found that the magnetoresistance of the helical edge states is related to the edge interaction effect and the disorder strength.Item Microwave photocurrent from the edge states of InAs/GaInSb bilayers(American Physical Society, 2018) Zhang, Jie; Li, Tingxin; Du, Rui-Rui; Sullivan, GerardMotivated by the recent low-temperature experiments on bulk FeSe, we study the electron correlation effects in a multiorbital model for this compound in the nematic phase using the U(1) slave-spin theory. We find that a finite nematic order helps to stabilize an orbital selective Mott phase. Moreover, we propose that when the d- and s-wave bond nematic orders are combined with the ferro-orbital order, there exists a surprisingly large orbital selectivity between the xz and yz orbitals even though the associated band splitting is relatively small. Our results explain the seemingly unusual observation of strong orbital selectivity in the nematic phase of FeSe, uncover new clues on the nature of the nematic order, and set the stage to elucidate the interplay between superconductivity and nematicity in iron-based superconductors.Item Observation of a Helical Luttinger Liquid in InAs/GaSb Quantum Spin Hall Edges(American Physical Society, 2015) Li, Tingxin; Wang, Pengjie; Fu, Hailong; Du, Lingjie; Schreiber, Kate A.; Mu, Xiaoyang; Liu, Xiaoxue; Sullivan, Gerard; Csáthy, Gábor A.; Lin, Xi; Du, Rui-RuiWe report on the observation of a helical Luttinger liquid in the edge of an InAs/GaSb quantum spin Hall insulator, which shows characteristic suppression of conductance at low temperature and low bias voltage. Moreover, the conductance shows power-law behavior as a function of temperature and bias voltage. The results underscore the strong electron-electron interaction effect in transport of InAs/GaSb edge states. Because of the fact that the Fermi velocity of the edge modes is controlled by gates, the Luttinger parameter can be fine tuned. Realization of a tunable Luttinger liquid offers a one-dimensional model system for future studies of predicted correlation effects.Item Observation of Edge Transport in the Disordered Regime of Topologically Insulating InAs/GaSb Quantum Wells(American Physical Society, 2014-01) Knez, Ivan; Rettner, Charles T.; Yang, See-Hun; Parkin, Stuart S.P.; Du, Lingjie; Du, Rui-Rui; Sullivan, GerardWe observe edge transport in the topologically insulating InAs=GaSb system in the disordered regime. Using asymmetric current paths we show that conduction occurs exclusively along the device edge, exhibiting a large Hall signal at zero magnetic fields, while for symmetric current paths, the conductance between the two mesoscopicly separated probes is quantized to 2e2=h. Both quantized and self-averaged transport show resilience to magnetic fields, and are temperature independent for temperatures between 20 mK and 1 K.Item Robust Helical Edge Transport in Gated InAs/GaSb Bilayers(American Physical Society, 2015) Du, Lingjie; Knez, Ivan; Sullivan, Gerard; Du, Rui-RuiWe have engineered electron-hole bilayers of inverted InAs/GaSb quantum wells, using dilute silicon impurity doping to suppress residual bulk conductance. We have observed robust helical edge states with wide conductance plateaus precisely quantized to 2e2/h in mesoscopic Hall samples. On the other hand, in larger samples the edge conductance is found to be inversely proportional to the edge length. These characteristics persist in a wide temperature range and show essentially no temperature dependence. The quantized plateaus persist to a 12ﾠT applied in-plane field; the conductance increases from 2e2/h in strong perpendicular fields manifesting chiral edge transport. Our study presents a compelling case for exotic properties of a one-dimensional helical liquid on the edge of InAs/GaSb bilayers.Item 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-RuiWe 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.Item Tuning the charge states in InAs/GaSb or InAs/GaInSb composite quantum wells by persistent photoconductivity(AIP Publishing, 2017) Tong, Bingbing; Han, Zhongdong; Li, Tingxin; Zhang, Chi; Sullivan, Gerard; Du, Rui-RuiWe have experimentally studied the persistent photoconductivity (PPC) in inverted InAs/GaSb and InAs/GaInSb quantum wells, which can be tuned into a bulk-insulating state by electron-hole hybridization. Specifically we tune the bulk band structure and carriers with light-emitting diode (LED) illuminations. The persistent photoconductivity could be negative or positive, depending on the specific doping structure and the illuminating photon energy. Compared to the widely-used electro-statically gating method, our findings provide a more flexible and non-invasive way to control the band structures and charge states in InAs/GaSb and InAs/GaInSb quantum wells (QWs).