arXiv subjects

Rui-Rui Du

Publications and source records attributed to Rui-Rui Du.

At least 37 records · Page 2Linked to original sources

arXiv2019

Anomalous Conductance Oscillations in the Hybridization Gap of InAs/GaSb Quantum Wells

We observe the magnetic oscillation of electric conductance in the two-dimensional InAs/GaSb quantum spin Hall insulator. Its insulating bulk origin is unambiguously demonstrated by the antiphase oscillations of the conductance and the resistance. Characteristically, the in-gap oscillation frequency is higher than the Shubnikov-de Haas oscillation close to the conduction band edge in the metallic regime. The temperature dependence shows both thermal activation and smearing effects, which cannot be described by the Lifshitz-Kosevich theory. A two-band Bernevig-Hughes-Zhang model with a large quasiparticle self-energy in the insulating regime is proposed to capture the main properties of the in-gap oscillations.

cond-mat.mes-hall↗

arXiv2019

Resistive signature of excitonic coupling in an electron-hole double layer with a middle barrier

We study the interlayer scattering mediated by long-range Coulomb interaction between electrons (density n) and holes (p) in a double-layer system. The gated device is made of InAs (e) and InGaSb (h) quantum wells separated by a AlSb middle barrier such that the interlayer tunneling is negligibly small. By using independent-layer contacts we measure the transport tensor \r{ho}_xx and \r{ho}_xy that are solely from the InAs layer, while sweeping p in the InGaSb layer. We found a strongly enhanced resistive scattering signal as the carrier densities approach a total charge neutrality, n = p, which cannot be described by the Fermi-liquid theory. Results of data analysis for density, temperature, and magnetic field dependences are consistent with the emergence of excitonic coupling between the two layers, stressing the dominance of Coulomb interaction even in the presence of disorder.

cond-mat.str-el↗

arXiv2018

Thermopower and Nernst measurements in a half-filled lowest Landau level

Motivated by recent proposal by Potter et al. [Phys. Rev. X 6, 031026 (2016)] concerning possible thermoelectric signatures of Dirac composite fermions, we perform a systematic experimental study of thermoelectric transport of an ultrahigh-mobility GaAs/AlxGa1-xAs two dimensional electron system at filling factor v = 1/2. We demonstrate that the thermopower Sxx and Nernst Sxy are symmetric and anti-symmetric with respect to B = 0 T, respectively. The measured properties of thermopower Sxx at v = 1/2 are consistent with previous experimental results. The Nernst signals Sxy of v = 1/2, which have not been reported previously, are non-zero and show a power law relation with temperature in the phonon-drag dominant region. In the electron-diffusion dominant region, the Nernst signals Sxy of v = 1/2 are found to be significantly smaller than the linear temperature dependent values predicted by Potter et al., and decreasing with temperature faster than linear dependence.

cond-mat.mes-hall↗

arXiv2018

Microwave Photocurrent from the Helical Edge state of InAs/GaInSb Bilayers

We measure microwave photocurrent in devices made from InAs/GaInSb bilayers where both insulating bulk state and conducting edge state were observed in the inverted-band regime, consistent with the theoretical prediction for a quantum spin Hall (QSH) insulator. It has been theoretically proposed that microwave photocurrent could be a unique probe in studying the properties of QSH edge states. To distinguish possible photoresponse between bulk state and helical edge state, we prepare Hall bar and Corbino disk from the same wafer. Results show that the Corbino disk samples have a negligible photocurrent in the bulk gap while clear photocurrent signals from the Hall bar samples are observed. This finding suggests that the photocurrent may carry information concerning the electronic properties of the edge states.

cond-mat.mes-hall↗

arXiv2018

Stability of the 5/2 Fractional Quantum Hall State in a Corbino Disc Sample with In-plane Electric Fields

Among the various geometries to study the fractional quantum Hall effect in two dimensional electron gas, the Corbino disc owns the advantage to probe the bulk properties directly. In this work we explore the influence of in-plane electric fields on the stability of the 5/2 fractional quantum Hall state realized in Corbino geometry. The effect of weak electric fields is investigated at ultra-low temperatures in order to compare with a theoretical proposal of enhanced Pfaffian state under weak electric fields.

cond-mat.mes-hall↗

arXiv2017

Low-Temperature Conductivity of Weakly Interacting Quantum Spin Hall Edges in Strained-Layer InAs/GaInSb

We report low-temperature transport measurements in strained InAs/Ga0.68In0.32Sb quantum 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.

cond-mat.mes-hall↗

arXiv2017

Simultaneous Measurements of Microwave Photoresistance and Cyclotron Reflection in the Multi-Photon Regime

We simultaneously measure photoresistance with electrical transport and plasmon-cyclotron resonance (PCR) using microwave reflection spectroscopy in high mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field. Multi-photon transitions are revealed as sharp peaks in the resistance and the cyclotron reflection on samples with various carrier densities. Our main finding is that plasmon coupling is relevant in the cyclotron reflection spectrum but has not been observed in the electrical conductivity signal. We discuss possible mechanisms relevant to reflection or dc conductivity signal to explain this discrepancy. We further confirm a trend that higher order multi-photon features can be observed using higher carrier density samples.

cond-mat.mes-hall↗

arXiv2017

Stability of the $N = 2$ bubble states in Corbino geometry under a tilted field

We measured the activation energy gap of the N = 2 bubble states using tilted-field technique in two GaAs/AlGaAs samples with Corbino geometry, and observed monotonically decreasing energy gaps with the increasing total magnetic field. The observed tilt angle dependence is discussed based on magneto-orbital coupling and finite-size effect. We also found a bubble state at an uncommon filling factor of 4.67, with an unanticipated energy gap of up to 5.35 K and opposite in-plane field dependence.

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↗

arXiv2016

Multi-Photon Transitions in Coupled Plasmon-Cyclotron Resonance Measured by Millimeter-Wave Reflection

We construct a low-temperature microwave waveguide interferometer for measuring high-frequency properties of two-dimensional electron gases (2DEGs). Coupled plasmon-cyclotron resonance (PCR) spectra are used to extract effective mass, bulk plasmon frequency, and carrier relaxation times. In contrast to traditional transmission spectroscopy, this method does not require sample preparation and is nondestructive. PCR signals can be resolved with a microwave power source as low as 10 nW. We observe PCR in the multi-photon transition regime, which has been proposed to be relevant to the microwave-induced resistance oscillations.

cond-mat.mes-hall↗

arXiv2016

Composite Fermion States around 2D Hole Landau Level Filling Factor 3/2 in Tilted Magnetic Fields

Transport measurements under a tilted magnetic field were performed on a series of C-doped (001) AlGaAs/GaAs/AlGaAs two-dimensional hole samples. Due to a large g-factor, Zeeman energy is large and comparable to the cyclotron energy in these samples. On the other hand, it was found that the in-plane component g// is small, and the effect of tilted magnetic field is mainly to increase the effective mass of holes. We investigate the spin transition of composite fermion states around Landau level (LL) filling factor 3/2. We found that the ν = 4/3 state encounters a partial to full spin polarization transition, conforming to the same pattern as that of electron samples. In addition, high-resistance phase emerges at ν = 3/2 under very high tilt angles. We interpret both of these phenomena as a consequence of LL crossings that are mainly driven by the orbital effects. The roles that the spin degrees of freedom play in FQH states around ν = 3/2 in these systems will be discussed.

cond-mat.mes-hall↗

arXiv2016

Effective g-factors of carriers in inverted InAs/GaSb bilayers

We perform tilt-field transport experiment on inverted InAs/GaSb which hosts quantum spin Hall insulator. By means of coincidence method, Landau level (LL) spectra of electron and hole carriers are systematically studied at different carrier densities tuned by gate voltages. When Fermi level stays in the conduction band, we observe LL crossing and anti-crossing behaviors at odd and even filling factors respectively, with a corresponding g-factor of 11.5. It remains nearly constant for varying filling factors and electron densities. On the contrary, for GaSb holes only a small Zeeman splitting is observed even at large tilt angles, indicating a g-factor of less than 3.

cond-mat.mtrl-sci↗

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

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↗

arXiv2011

Perfect Andreev Reflection of Helical Edge Modes in InAs/GaSb Quantum Wells

We present an experimental study of inverted InAs/GaSb composite quantum wells in the hybridization regime and contacted by superconducting electrodes. A front gate is used to vary the Fermi level into the mini-gap, where recent experiments indicate existence of helical edge modes [arXiv:1105.0137]. Zero bias dips in differential resistance are observed across the mini-gap, suggesting transport dominated by Andreev reflection processes. Evolution of the mini-gap differential resistance with applied bias as well as measured mini-gap excess current of 150 nA are in good agreement with the prediction of perfect Andreev reflection of the helical edge modes, which is necessitated by the absence of back-scattering channels. The perfect Andreev reflection occurs in spite of a finite barrier at the interface and shows strong sensitivity to time-reversal breaking - hallmarks of the helical nature of quantum spin Hall edges.

cond-mat.mes-hall↗

arXiv2011

Anisotropy of zero-resistance states in InN films under an in-plane magnetic filed

We report low temperature current-voltage measurements on n-type InN films grown by molecular beam epitaxy. The zero-resistance state with a large critical current around 1 mA has been observed at 0.3 K. Under in-plane field configuration, the zero-resistance state shows a large anisotropy in critical current for B parallel and perpendicular to applied current. The ratio of critical current between B parallel and perpendicular to the applied current can be up to 2.5, when B = 0.15T. The anisotropy is explained by the vortex flow in the context of type II superconductivity. We have thus established an important aspect of the phenomenology of superconductivity in an otherwise typical narrow gap semiconductor.

cond-mat.supr-con↗