Browsing by Author "Du, Lingjie"
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Item Depinning transition of bubble phases in a high Landau level(American Physical Society, 2015) Wang, Xuebin; Fu, Hailong; Du, Lingjie; Liu, Xiaoxue; Wang, Pengjie; Pfeiffer, L.N.; West, K.W.; Du, Rui-Rui; Lin, XiIn the higher Landau levels (N>0) a reentrant integer quantum Hall effect (RIQHE) state, which resides at fractional filling factors but exhibits integer Hall plateaus, has been previously observed and studied extensively. The nonlinear dynamics of the RIQHE were measured by microwave resonance, with the results consistent with an electronic bubble phase pinned by impurities. We have carried out depinning experiments on the N=2 bubble phases by using Corbino geometry, where depinning threshold values have been systematically measured as a function of magnetic fields and temperatures. Domain sizes and pinning potential of the bubble phases have been estimated from the nonlinear transport data.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 Experiments on quantum phases in InAs/GaSb bilayers: Topological insulator and exciton condensation(2016-10-12) Du, Lingjie; Du, Rui-RuiRecent developments in Quantum Spin Hall (QSH) effect have triggered much attention in inverted InAs/GaSb Quantum wells (QWs), which are the leading material in QSH systems. Inverted InAs/GaSb QWs are a type II heterostructure with the broken gap, where two dimensional (2D) electrons and holes are confined in spatially separated QWs. From the 1970s until now, the ground state of this structure has been discussed between two candidates: exciton insulator (BCS type exciton condensation) and hybridization gap. The QSH effect was theoretically proposed in the bulk hybridization gap. Although pioneer works about QSH effect have been performed, the conductive hybridization gap limits further exploration. For example, the existence of the QSH effect in this system is still not conclusive. In this thesis, through double-gate modulation, we investigated the whole phase in the inverted band of this structure. We observed two distinct quantum phases: time reversal symmetry (TRS) QSH insulator in the deeply inverted regime and exciton insulator in the shallowly inverted regime. In the deeply inverted regime, with the strain effect in InGaSb QW, we realized the insulating hybridization gap for the first time, which gave us the opportunity to observe TRS QSH effect in this system for the first time. With the largest bulk gap in known QSH systems, we observed the helical edges had the longest coherence length (nearly 13μm) and were more stable against temperature, compared with previous results, which paved the way to construct the room temperature topological circuit. In the shallowly inverted regime, the quantized plateau of QSH effect was observed in mesoscopic devices for the first time. Surprisingly, this helical edge mode was robust under the high magnetic field, demonstrating the first TRS broken QSH insulator. This novel quantum phase could not be understood in the single particle topological theory. Further studies showed that the bulk gap was dominated by exciton gap instead of hybridization gap. We performed the low temperature transport and Terahertz transmission measurement on the bulk exciton gap, and observed the solid evidence for the existence of BCS-like exciton condensation, which was under search for more than fifty years. Furthermore, we performed one dimensional Coulomb drag experiments in the topological circuit. We observed positive and negative drag results dependent on the temperature, indicating the charge symmetry and many-body correlation.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 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 Scaling properties of the plateau transitions in the two-dimensional hole gas system(American Physical Society, 2016) Wang, Xuebin; Liu, Haiwen; Zhu, Junbo; Shan, Pujia; Wang, Pengjie; Fu, Hailong; Du, Lingjie; Pfeiffer, L.N.; West, K.W.; Xie, X.C.; Du, Rui-Rui; Lin, XiThe behavior of phase coherence is studied in two-dimensional hole gas through the integer quantum Hall plateau-to-plateau transition. From the plateau transition as a function of temperature, scaling properties of multiple transitions are analyzed. Our results are in good agreement with the assumption of the zero-point fluctuations of the coherent holes, and support the intrinsic saturation of the coherence time at low temperature limit. The critical exponent p can also be determined under the scheme of the zero-point fluctuations. The similarity and difference in experimental observations between quantum Griffiths singularity and plateau transition is discussed. The spin-orbit coupling effect's influence on the plateau transition is explored by comparing the results from different transitions.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.