arXiv subjects

Lingjie Du

Publications and source records attributed to Lingjie Du.

12 recordsLinked to original sources

arXiv2023

Excitonic topological order in imbalanced electron-hole bilayers

Correlation and frustration play essential roles in physics, giving rise to novel quantum phases [1-6]. A typical frustrated system is correlated bosons on moat bands, which could host topological orders with long-range quantum entanglement [4]. However, the realization of moat-band physics is still challenging. Here, we explore moat-band phenomena in shallowly-inverted InAs/GaSb quantum wells, where we observe an unconventional time-reversal-symmetry breaking excitonic ground state under imbalanced electron and hole densities. We find a large bulk gap exists encompassing a broad range of density imbalance at zero magnetic field (B), accompanied by edge channels that resemble helical transport. Under an increasing perpendicular B, the bulk gap persists, and an anomalous plateau of Hall signals appears, which demonstrates an evolution from helical-like to chiral-like edge transport with a Hall conductance ~e2/h at 35 Tesla. Theoretically, we show that strong frustration from density imbalance leads to a moat band for excitons, resulting in a time-reversal-symmetry breaking excitonic topological order, which explains all our experimental observations. Our work opens up a new direction for research on topological and correlated bosonic systems in solid states beyond the framework of symmetry-protected topological phases, including but not limited to the bosonic fractional quantum Hall effect.

cond-mat.str-el↗

arXiv2022

Domain Textures in the Fractional Quantum Hall Effect

Impacts of domain textures on low-lying neutral excitations in the bulk of fractional quantum Hall effect (FQHE) systems are probed by resonant inelastic light scattering. We demonstrate that large domains of quantum fluids support long-wavelength neutral collective excitations with well-defined wave vector (momentum) dispersion that could be interpreted by theories for uniform phases. Access to dispersive low-lying neutral collective modes in large domains of FQHE fluids such as long wavelength magnetorotons at filling factor v=1/3 offer significant experimental access to strong electron correlation physics in the FQHE.

cond-mat.mes-hall↗

arXiv2021

Observation of flat bands in gated semiconductor artificial graphene

Flat bands near M points in the Brillouin zone are key features of honeycomb symmetry in artificial graphene (AG) where electrons may condense into novel correlated phases. Here we report the observation of van Hove singularity doublet of AG in GaAs quantum well transistors, which presents the evidence of flat bands in semiconductor AG. Two emerging peaks in photoluminescence spectra tuned by backgate voltages probe the singularity doublet of AG flat bands, and demonstrate their accessibility to the Fermi level. As the Fermi level crosses the doublet, the spectra display dramatic stability against electron density, indicating interplays between electron-electron interactions and honeycomb symmetry. Our results provide a new flexible platform to explore intriguing flat band physics.

cond-mat.mes-hall↗

arXiv2019

Observation of new plasmons in the fractional quantum Hall effect: interplay of topological and nematic orders

Collective modes of exotic quantum fluids reveal underlying physical mechanisms responsible for emergent complex quantum ground states. We observe unexpected new collective modes in the fractional quantum Hall (FQH) regime: intra-Landau-level plasmons in the second Landau level measured by resonant inelastic light scattering. The plasmons herald rotational-symmetry-breaking phases in tilted magnetic fields and reveal long-range translational invariance in these phases. The fascinating dependence of plasmon features on filling factor provide new insights on interplays between topological quantum Hall order and nematic electronic liquid crystal phases. A marked intensity minimum in the plasmon spectrum at Landau level filling factor v = 5/2 strongly suggests that this paired state, which could support non-Abelian excitations, overwhelms competing nematic phases, unveiling the robustness of the 5/2 superfluid state for small tilt angles. At v = 7/3, a sharp and strong plasmon peak that links to emerging macroscopic coherence supports the proposed model of a FQH nematic state at this filling factor.

cond-mat.mes-hall↗

arXiv2017

Tuning Edge States in Strained-Layer InAs/GaInSb Quantum Spin Hall Insulators

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 by up to five folds 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 (TRS) -protected properties consistent with Z2 topological insulator. The InAs/GaInSb bilayers offer a much sought-after platform for future studies and applications of the QSHI.

cond-mat.mes-hall↗

arXiv2015

Observation of a Helical Luttinger-Liquid in InAs/GaSb Quantum Spin Hall Edges

We report on the observation of a helical Luttinger-liquid in the edge of 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.

cond-mat.mes-hall↗

arXiv2015

Gate-Tuned Spontaneous Exciton Insulator in Double-Quantum Wells

It was proposed that a dilute semimetal is unstable against the formation of an exciton insulator, however experimental confirmations have remained elusive. We investigate the origin of bulk energy gap in inverted InAs/GaSb quantum wells (QWs) which naturally host spatially-separated electrons and holes, using charge-neutral point density (no~po) in gated-device as a tuning parameter. We find two distinct regimes of gap formation, that for I), no >> 5x1010/cm2, a soft gap opens predominately by electron-hole hybridization; and for II), approaching the dilute limit no~ 5x1010/cm2, a hard gap opens leading to a true bulk insulator with quantized edge states. Moreover, the gap is dramatically reduced as the QWs are tuned to less dilute. We further examine the response of gaps to in-plane magnetic fields, and find that for I) the gap closes at B// > ~ 10T, consistent with hybridization while for II) the gap opens continuously for B// as high as 35T. Our analyses show that the hard gap in II) cannot be explained by single-particle hybridization. The data are remarkably consistent with the formation of a nontrivial exciton insulator in very dilute InAs/GaSb QWs.

cond-mat.mes-hall↗

arXiv2014

De-Pinning Transition of Bubble Phases in a High Landau Level

While in the lowest Landau level the electron-electron interaction leads to the formation of the Wigner crystal, in higher Landau levels a solid phase with multiple electrons in a lattice site of crystal was predicted, which was called the bubble phase. Reentrant integer quantum Hall states are believed to be the insulating bubble phase pinned by disorder. We carry out nonlinear transport measurements on the reentrant states and study the de-pinning of the bubble phase, which is complementary to previous microwave measurements and provides unique information. In this study, conductivity is directly measured with Corbino geometry. Based on the threshold electric field of de-pinning, a phase diagram of the reentrant state is mapped. We discuss an interaction-driven topological phase transition between the integer quantum Hall state and the reentrant integer quantum Hall state.

cond-mat.mes-hall↗

arXiv2014

Images of edge current in InAs/GaSb quantum wells

Quantum spin Hall devices with edges much longer than several microns do not display ballistic transport: that is, their measured conductances are much less than $e^2/h$ per edge. We imaged edge currents in InAs/GaSb quantum wells with long edges and determined an effective edge resistance. Surprisingly, although the effective edge resistance is much greater than $h/e^2$, it is independent of temperature up to 30 K within experimental resolution. Known candidate scattering mechanisms do not explain our observation of an effective edge resistance that is large yet temperature-independent.

cond-mat.mes-hall↗

arXiv2013

Observation of Quantum Spin Hall States in InAs/GaSb Bilayers under Broken Time-Reversal Symmetry

Topological insulators (TIs) are a novel class of materials with nontrivial surface or edge states. Time-reversal symmetry (TRS) protected TIs are characterized by the Z2 topological invariant and their helical property becomes lost in an applied magnetic field. Currently there exist extensive efforts searching for TIs that are protected by symmetries other than TRS. Here we show, a topological phase characterized by a spin Chern topological invariant is realized in an inverted electron-hole bilayer engineered from indium arsenide-gallium antimonide (InAs/GaSb) semiconductors which retains robust helical edges under a strong magnetic field. Wide conductance plateaus of 2e2/h value are observed; they persist to 12T applied in-plane magnetic field without evidence for transition to a trivial insulator. In a perpendicular magnetic field up to 8T, there exists no signature to the bulk gap closing. While the Fermi energy remains inside the bulk gap, the longitudinal conductance increases from 2e2/h in strong magnetic fields suggesting a trend towards chiral edge transport. Our findings are first evidences for a quantum spin Hall (QSH) insulator protected by a spin Chern invariant. These results demonstrate that InAs/GaSb bilayers are a novel system for engineering the robust helical edge channels much needed for spintronics and for creating and manipulating Majorana particles in solid state.

cond-mat.mes-hall↗

arXiv2013

Optimal cooling of a driven artificial atom in dissipative environment

We study microwave-driven cooling in a superconducting flux qubit subjected to environment noises. For the weak decoherence, our analytical results agree well with the experimental observations near the degeneracy point and show that the microwave amplitude for optimal cooling should depend linearly on the dc flux detuning. With the decoherence increasing, more vibrational degrees of freedom couple in, making the ordinary cooling method less effective or even fail. We propose an improved cooling method, which can eliminate the perturbation of additional vibrational degrees of freedom hence keep high efficiency even under the strong decoherence. Furthermore, we point out that the decoherence will modulate the frequency where microwave-driven Landau-Zener transition reaches maximum in both methods, displaying the feature of incoherent dynamics which is important for the optimal cooling of qubits and other quantum systems.

quant-ph↗

arXiv2013

Electromagnetically induced interference at superconducting qubits

We study electromagnetically induced interference at superconducting qubits. The interaction between qubits and electromagnetic fields can provide additional coupling channels to qubit states, leading to quantum interference in a microwave driven qubit. In particular, the interwell relaxation or Rabi oscillation, resulting respectively from the multi- or single-mode interaction, can induce effective crossovers. The environment is modeled by a multi-mode thermal bath, generating the interwell relaxation. Relaxation induced interference, independent of the tunnel coupling, provides deeper understanding to the interaction between the qubits and their environment. It also supplies a useful tool to characterize the relaxation strength as well as the characteristic frequency of the bath. In addition, we demonstrate the relaxation can generate population inversion in a strongly driving two-level system. On the other hand, different from Rabi oscillation, Rabi oscillation induced interference involves more complicated and modulated photon exchange thus offers an alternative means to manipulate the qubit, with more controllable parameters including the strength and position of the tunnel coupling. It also provides a testing ground for exploring nonlinear quantum phenomena and quantum state manipulation, in not only the flux qubit but also the systems with no crossover structure, e.g. phase qubits.

quant-ph↗