Browsing by Author "Du, Rui-Rui"
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Item 5/2 state in high election density gallium arsenide/aluminum gallium arsenide quantum well(2010) Zhang, Chi; Du, Rui-RuiThis Master of Science Thesis is concerned with electronic transport in the higher Landau levels (LL) in a two-dimensional electron system, where novel many-body electronic phases have been observed. Particular attention is paid to the even-denominator fractional quantum Hall states at LL filling factors 5/2 and 7/2, and the anisotropic states at 9/2 and 11/2. In a high electron density (n = 6.3 x 10 11cm-2), high mobility (mu = 1 x 107cm2/Vs) modulation-doped GaAs/Al0.24Ga 0.76As quantum well, we observed the nu = 5/2 quantum Hall plateau at a high magnetic field B = 10 T. In contrast to previous findings in a lower density system, electronic transport at nu = 9/2 and nu = 11/2 is essentially isotropic. Anisotropic transport at 9/2 and 11/2 can be induced by an in-plane magnetic field, B//. Depending on the B// direction, the nu = 5/2 diagonal resistances in a high B// either remain isotropic or become strongly anisotropic. Our data suggest a new regime for electronic transport in higher LLs.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 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 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 Experimental studies of the fractional quantum Hall effect and the Wigner solids(2010) Zhang, Chi; Du, Rui-RuiThis Ph.D. thesis is composed of two parts: the first part is concerned with electron transport in the higher Landau levels (LL) in a two-dimensional electron system (2DES); the second part is focused on the Wigner Crystal in two-dimensional hole system (2DHS). We report on the high Landau-level (LL) magnetotransport (including tilt-fields) in a high purity modulation-doped GaAs/AlGaAs quantum Well (QW) with twice the electron density of standard samples. A quantized nu = 5/2 Hall plateau is observed at B ∼ 10 T, with an activation gap Delta 5/2 ∼ 125 mK; the plateau can persist up to a ∼ 25° tilt-field. This finding is discussed in the context of proposed Moore-Read Pfaffian (Pf) wave function (or Anti-Pfaffian (APf)) being possible ground states at 5/2. The tilted-fields induce background resistance at 5/2 that could be either isotropic or anisotropic, depending simply on in-plane magnetic field orientation with respect to the GaAs crystalline axis. Such data indicate a substantial coupling between the 5/2 collective phases and the GaAs crystal. In a high hole density (p = 2.0x1011 cm-2), high mobility (micro = lx106 cm2/V s) 20 nm wide GaAs/AlGaAs quantum well, we observed reentrant insulating phases around Landau-level filling factor nu = 1/5 and nu = 2/9 at very high magnetic fields. Previous experiments reported the reentrant insulating phases around the nu = 1/3 FQHE state in dilute 2DHS and around the nu = 1/5 FQH liquid in 2DES, respectively. It is rather interesting that our experimental results in the hole system look like the former electron results at low fields. Our T-dependent conductance measurements exhibit rather intriguing behaviors: the Arrhenius plot of the conductance (vs. 1/T) suggests a certain energy scale in the melting procedure of the solid phase. We also observed a large threshold for electric field in the differential conductance measurements, which is an indication of the sliding motion of Wigner solid driven by an external field. These phenomena are related to the depinning of Wigner solids under various magnetic fields. A possible explanation is that the effective pinning potential increases with the magnetic fields.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 Landau level crossing in a spin-orbit coupled two-dimensional electron gas(AIP Publishing, 2015) Wu, Xing-Jun; Li, Ting-Xin; Zhang, Chi; Du, Rui-RuiWe have studied experimentally theﾠLandau levelﾠ(LL) spectrum of a two-dimensionalﾠelectron gasﾠ(2DEG)ﾠin an In0.53Ga0.47As/InPﾠquantum wellﾠstructure by means of low-temperature magneto-transport coincidence measurement in vectorﾠmagnetic fields.ﾠIt is well known that LL crossing occurs in tiltedﾠmagnetic fieldsﾠdue to a competition between cyclotron energy andﾠZeeman effect.ﾠRemarkably, here we observe an additional type of level-crossing resulting from a competition between Rashba andﾠZeeman splittingﾠin a smallﾠmagnetic field,ﾠconsistent with theﾠtheoreticalﾠprediction for strongly spin-orbit coupledﾠ2DEG.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 Measurement of Magnetoplasmon Resonance in a High-Mobility 2-Dimensional Dot Array using a Micro-calorimeter(2013-07-16) Ball, Jason; Du, Rui-Rui; Kiang, Ching-Hwa; Foster, MatthewWe used a calorimetric method to measure the microwave absorption of a high-mobility 2-dimensional electron gas in small magnetic fields. The calorimeter responded well to the signal, even when reduced to a single dot. Curiously, in the smaller dot lattices we found the zero-field plasmon resonances to be much less the than expected values; however a second, larger sample yielded better values. As a result of the high mobility of the sample we also saw evidence of the second harmonic of the plasmon frequency coupling with the cyclotron resonance, although a corresponding edge mode was not found. The calorimetric method appears to be a valid means of measuring signal absorption in a 2DEG and has potential for probing other types of systems.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 Microwave Spectroscopy on Two Dimensional Electron/Hole Gases(2018-06-04) Zhang, Jie; Du, Rui-RuiWe develop new techniques to explore how electrons and holes in 2D semiconductors behave under microwave radiation in a low temperature regime. To observe cyclotron resonance (CR) in GaAs/AlGaAs quantum wells, thermal methods with the sample and thermometer sealed in a vacuum can in a 3He environment and reflection spectroscopy via power sensing are developed. This configuration has high sensitivity and can detect CR with only 10nW of microwave power. It obviates the need for sampling thinning/wedging which is required in conventional transmission spectroscopy and hence preserves sample quality. With these advantages, we are able to detect narrow CR peaks with a small full width at half maximum (FWHM). Carrier effective masses are measured in various two dimensional electron gases (2DEGs) and two dimensional hole gases (2DHGs). Transport scattering time and single particle relaxation time are two important time scales that can be extracted from this measurement. The ratio of these times can be used as an indication of scattering angle scale limited by carrier mobility. The most exciting feature of this measurement is that a multi-photon phenomenon is observed for the first time in this system other than when using photoresistance measurements. This is made possible due to the use of millimeter waves, as opposed to the more traditional terahertz and infrared regimes used in earlier configurations. We further investigate the density dependence of the observable order of the multi-photon transitions and find that it is more prominent in higher density samples. By simultaneously measuring microwave reflectance and electrical resistance, we find that the plasmon-coupled CR mode is only present in the optical signal, not in the electrical. We attribute this discrepancy to the difference in the ability to pick up the corresponding signal in the scattering process. Though experimental data is very convincing, theoretical explanation is still needed to account for these phenomena. We also analyzed the photoresistance of the InAs/GaSb inverted bilayer. This system is known for its marvelous helical edge state in the quantum spin Hall effect (QSHE). By tuning the back gate, the Fermi level can be positioned in the bulk gap where the conductivity only comes from the edge state. This is verified by taking measurements on a Corbino disk where the edge state is shunted. We find that the photocurrent is prohibited in the charge neutral point (CNP) due to the lack of spin flipping mechanisms. A finite magnetic field provides a source for this process from which we see an enhancement of the photocurrent. Further effort in observing gap opening and resonance with the Zeeman energy needs to be made to understand this interesting system. Therefore, better wafer quality is strongly desired.Item Millimeter wave transmission spectroscopy of two-dimensional electron and hole systems(2010) Stone, Kristjan Jakob; Du, Rui-RuiIn order to explore how electrons and holes in 2D semiconductors behave at 3He temperatures under millimeter wave irradiation, we developed a new probe and measurement technique. Our samples are specially grown high-mobility GaAs/AlGaAs 2D electron or hole systems that have been modulation doped with Si or C respectively and etched into Hall bars. We also use microwave irradiation waveguide techniques to probe edge magnetoplasmons in 2D electron systems and find that the periodic resistance oscillations in the magnetic field are independent of the length between the leads measured. This demonstrates that the propagation of edge states is a non-local effect, contrary to previously established research. We confirm microwave induced resistance oscillations using a newly developed probe that delivers microwaves from a frequency generator down to the sample via a coax line and coplanar waveguides. Due to the low frequency range (2 -- 40 GHz) and high irradiation powers available, we are able to observe microwave induced resistance oscillations and newly revealed fractional microwave induced resistance oscillations. The probe that we develop for this new measurement makes previously unattainable non-Faraday as well as Faraday irradiation geometries accessible. In addition to measuring quantum transport, it also allows us to measure the transmission of microwaves across the sample. We establish a differential measurement technique that instantaneously removes the background signal leaving only the transmission from the 2D system, also reducing the preparation time required. This is accomplished with a gated high-mobility sample prepared to allow for microwaves to be irradiated from the back. The advantage of this new technique is that it accommodates any gated/polished sample which can be mounted on the specially designed sample holder. From this arrangement we are able to measure the cyclotron resonance transmission minima of both the 2D electron and hole systems. We can then use the known values for the effective mass and cyclotron time constant as a confirmation that our new probe can successfully make the expected measurements.Item Nanostructure Investigations of Nonlinear Differential Conductance in NdNiO3 Thin Films(2014-12-03) Hardy, Will J; Natelson, Douglas; Du, Rui-Rui; Biswal, Sibani LisaTransport measurements on thin films of NdNiO3 reveal a crossover to a regime of pronounced nonlinear conduction below the well-known metal-insulator transition temperature. Surprising evolution of the transport properties is observed at temperatures well below this transition, which appears consistent with a gradual formation of a gap in the hole-like Fermi surface of this strongly correlated system. A model of Landau-Zener breakdown fails to describe the I(V) characteristics at high temperatures, where Ohmic transport dominates, but becomes increasingly more appropriate as the temperature approaches 2 K.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 Quantum Transport and Microwave Response in Modulated High-Mobility Two-Dimensional Electron Systems(2011) Dai, Yanhua; Du, Rui-RuiTwo-dimensional electron gas (2DEG) in microwave irradiation has been an intriguing system to study transport properties of electrons for a decade, during which extensive important phenomena have been discovered, such as microwave induced resistance oscillation (MIRO), zero resistance state(ZRS) and zero conductance state(ZCS). Along this direction, this thesis is devoted to new phenomena recently discovered in such systems, 1) microwave induced cyclotron harmonic peak (2wc spike) in ultraclean 2DEG without antidots inducing electric modulation potential 2) Aharonov- Bohm(AB) oscillations in microwave irradiated 2DEG with modulation potential. Those phenomena originate from quantum interference, from either interplay between two different kinds of scattering or different electrons transition paths. In addition to microwave induced phenomena, other nonlinear effects such as Zener tunneling and geometric resonance(GR) have been investigated further in the past few years and collected in this thesis. Zener tunneling has been found in two-dimensional hole gas(2DHG) with a large damping factor, which inspires more questions on the unique band structures of holes. GR has been studied in ultra-clean modulated 2DEG , in which more oscillation peaks show up. In the GR regime with an inplane magnetic field applied, the antisymmetric hall oscillations lead us to propose the existence of a magnetic lattice along with an artificial antidots lattice and a current lattice in such systems.Item Quantum transport in inverted indium arsenide/gallium antimonide composite quantum wells(2010) Knez, Ivan; Du, Rui-RuiWe present a comprehensive study of low temperature quantum transport in double gated InAs/GaSb composite quantum wells. Recently, it has been proposed that this system in inverted regime should exhibit the topologically insulating (TI) phase, characterized by an energy gap in the bulk and gapless edge modes, protected from backscattering by time reversal symmetry. We sweep the Fermi level through the bulk mini-gap, observing resistance peaks and finding strong evidence for the existence of the mini-gap; however, the mini-gap does not show insulating behavior, with a residual bulk conductivity which is a few times larger then the expected contribution from the edge. Our data indicate, that bulk conductivity is not an issue of "dirt", which can be improved by simply reducing the amount of disorder, but a fundamental property of strongly coupled electron-hole systems in realistic materials, which must be considered in studies of proposed TI edge modes.Item Quantum transport in spatially modulated two-dimensional electron and hole systems(2009) Yuan, Zhuoquan; Du, Rui-RuiTwo-dimensional electron (2DES) and hole (2DHS) systems have attracted intense research attentions in past decades. A 2DES or 2DHS modulated by one-dimensional or two-dimensional spatially periodic potential shows particular importance because the existence of modulation provides a tunable parameter for exploring interaction between electrons and scattering centers presenting on the two-dimensional systems. This thesis documents a systematic experimental study, in collaboration with Bell Labs, of electronic transport in very-high mobility 2DES and 2DHS in GaAs/AlGaAs quantum structures. Fabrication of triangular antidot lattice in 2DES, as well as low-temperature transport and photoconductivity properties in spatially modulated 2DES, has been studied. Strong Geometric resonance (GR), up to seven peaks resolved, is observed in the longitudinal magnetoresistance because of high mobility of 2DES after fabrication of antidot lattice. Photoresistance shows clear millimeterwave-induced resistance oscillations (MIRO) but with heavily damping amplitudes, and magnetoplasmon resonance (MPR) is also observed as well. GR, MIRO and MPR are decoupled from each other in our modulated 2DES. These experimental findings pave the way for studies of nonlinear transport in modulated 2DES. Magnetotransport measurements on a new material, the Carbon delta-doped 2DHG in GaAs/AlGaAs quantum well, indicate that the 2DHG has a transport scattering time compatible with those in very-high mobility 2DES. However, photoresistance measurement shows much weaker MIRO in the 2DHS than that in 2DES with compatible transport scattering time. Low-temperature transport measurements on Landau, Zeeman, and spin-orbital parameters imply that the C-doped 2DHS has small zero field spin splitting and large effective g-factor. As part of the thesis work, the thesis also presents a development of low-temperature/high magnetic field (300mK/12T) scanning Hall probe microscope (SHPM) technique for measuring small local magnetic fields at low temperature and an algorithm for calculating the current density from measured magnetic fields based on Fourier transformation technique. Integration of SHPM and the algorithm provides a practical tool for imaging the current distribution and a powerful method to explore electronic transport properties of 2DES and 2DHS.