arXiv subjects

Rui-Rui Du

Publications and source records attributed to Rui-Rui Du.

At least 19 recordsLinked to original sources

arXiv2026

Successive Phase Transitions from a Composite Fermion Liquid to a Fractional Quantum Hall State at {\nu}=3/2 Driven by In-Plane Magnetic Field

We report an even-denominator fractional quantum Hall state at {\nu} = 3/2 induced entirely by in-plane magnetic field B_|| in an ultra-high-mobility GaAs quantum well. As B_|| increases, the system undergoes two successive transitions: from a composite fermion liquid to a soft-gap FQH state (B_||~12.2 T), then via a topological phase transition to a hard-gap robust FQH state (B_||~14.7 T), accompanied by a daughter state at {\nu} = 19/13. We present systematic data, and discuss a possible scenario in interpreting these findings. Our work demonstrates that topological order may be engineered through k-space Fermi contour splitting under an in-plane magnetic field.

cond-mat.str-el↗

arXiv2026

Symmetry Breaking and Transition to Robust Excitonic Topological Order in InAs/GaSb Bilayers

Symmetry and topology are fundamental concepts deeply intertwined in various fields of physics, especially in the studies of quantum phases of matter. The critical role that Coulomb interactions play in symmetry breaking during topological transitions is a fundamental problem that has not been fully understood. Utilizing gated indium arsenide-gallium antimonide bilayers, we demonstrate that Coulomb interactions play a critical role in symmetry breaking and topological transitions. Whereas the quantum spin Hall insulator (QSHI) dominates the high-density regime, gating the system into the dilute regime enhances interlayer Coulomb interactions and leads to an emergent excitonic topological order (ETO) with spontaneous time-reversal-symmetry breaking. Moreover, applying a magnetic field drives a transition from the QSHI to the ETO accompanied by Coulomb-induced spin-rotation-symmetry breaking, which selects triplet electron-hole pairing in the lowest Landau levels. These results underscore an intricate interplay between symmetry and topology under Coulomb interactions in electron-hole bilayers.

cond-mat.mes-hall↗

arXiv2026

Observation of Superfluidity and Meissner Effect of Composite Bosons in GaAs Quantum Hall System

The quantum Hall effect (QHE) is theoretically understood as a superfluid condensate of composite bosons (CBs) -- bound states of electrons and magnetic flux quanta. While dissipationless transport is consistent with this picture, other signatures of superfluidity, such as the Meissner effect, remain elusive. Here, we present direct experimental evidence for CB superfluidity by probing the system's response to a controlled, time-varying magnetic field in Corbino disk geometries. We simultaneously observe the quantized Laughlin charge pumping and a new, quantized charge accumulation phenomenon, governed by the relation $\Delta Q_{\rm a}/e = \nu\,(\Delta \Phi/\Phi_0)$. This relation signifies that the system actively maintains the fixed electron-to-flux ratio that defines the CBs, neutralizing excess flux by drawing in a precise number of electrons. Crucially, devices with multiple concentric top gates reveal that this charge accumulation is uniformly distributed across the bulk of the QHE fluid, demonstrating that it is a collective, bulk property rather than an edge effect -- a key signature of a superfluid condensate. Furthermore, the presence of a top gate determines the screening mechanism: in a "grand canonical" setting with a gate, low Coulomb energy favors a charge-mediated screening (generalized Meissner effect); without a gate, the system enters a "canonical" regime, exhibiting fixed electron density like type-II superconductors. These observations confirm the CB superfluid nature of the QHE ground state and establish a versatile platform for studying macroscopic quantum coherence and its screening transitions in two dimensions.

cond-mat.mes-hall↗

arXiv2026

Laughlin pumping assisted by surface acoustic waves

The quantum Hall effect is a fascinating electrical transport phenomenon signified by precise quantization of Hall conductivity $\sigma_\mathrm{xy}$ and vanishing longitudinal conductivity $\sigma_\mathrm{xx}$. Laughlin proposed an elegant explanation in which adiabatic insertion of a flux tube pumps charge through the system. This analysis unveils the fundamental role of gauge invariance and provides a compelling argument about the fractional charge of fractional quantum Hall states. While it has been used extensively as a theoretical tool, a quantitative experimental investigation is lacking despite multiple attempts. Here we report successful realizations of Laughlin pumping in several integer and fractional quantum Hall states. One essential technical innovation is using surface acoustic waves to periodically clear the charges accumulated during the pumping process. Magnetic fluxes are inserted at a constant rate so there is no need to perform complicated data fitting. Furthermore, our setting can reliably extract $\sigma_\mathrm{xx}$ that is several orders of magnitude lower than the limit of conventional techniques. Effective energy gaps can be deduced from the temperature dependence of $\sigma_\mathrm{xx}$, which are drastically different from those provided by conventional transport data. This work not only brings a famous gedanken experiment to reality but also serves as a portal for many future investigations.

cond-mat.mes-hall↗

arXiv2025

Lessons from $\alpha$-RuCl3 for pursuing quantum spin liquid physics in atomically thin materials

Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from the field of atomically thin materials. We primarily focus on the antiferromagnetic Mott insulator $\alpha$-RuCl3, which exhibits Kitaev couplings and is readily exfoliated to single- or few-layer sheets, and thus serves as a test bed for developing probes of Kitaev phenomena in atomically thin materials and devices. We introduce the Kitaev model and how it is realized in $\alpha$-RuCl3 and other material candidates; and cover $\alpha$-RuCl3 synthesis and fabrication into van der Waals heterostructure devices. A key discovery is a work-function-mediated charge transfer that heavily dopes both the $\alpha$-RuCl3 and proximate materials, and can enhance Kitaev interactions by up to 50%. We further discuss a wide range of recent results in electronic transport and optical and tunneling spectroscopies of $\alpha$-RuCl3 devices. The experimental techniques and theoretical insights developed for $\alpha$-RuCl3 establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases.

cond-mat.str-el↗

arXiv2025

Hybridization Gap and Edge States in Strained-layer InAs/In0.5Ga0.5Sb Quantum Spin Hall Insulator

The hybridization gap in strained-layer InAs/InxGa1-xSb quantum spin Hall insulators (QSHIs) is significantly enhanced compared to binary InAs/GaSb QSHI structures, where the typical indium composition, x, ranges between 0.2 and 0.4. This enhancement prompts a critical question: to what extent can quantum wells (QWs) be strained while still preserving the fundamental QSHI phase? In this study, we demonstrate the controlled molecular beam epitaxial (MBE) growth of highly strained-layer QWs with an indium composition of x = 0.5. These structures possess a substantial compressive strain within the In0.5Ga0.5Sb QW. Detailed crystal structure analyses confirm the exceptional quality of the resulting epitaxial films, indicating coherent lattice structures and the absence of visible dislocations. Transport measurements further reveal that the QSHI phase in InAs/In0.5Ga0.5Sb QWs is robust and protected by time-reversal symmetry. Notably, the edge states in these systems exhibit giant magnetoresistance when subjected to a modest perpendicular magnetic field. This behavior is in agreement with the Z2 topological property predicted by the Bernevig-Hughes-Zhang (BHZ) model, confirming the preservation of topologically protected edge transport in the presence of enhanced bulk strain.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear transport of Wigner solid phase surrounding the two-flux composite fermion liquid

We have investigated the low temperature (T) transport properties of fractional quantum Hall (FQH) states in a high-mobility two-dimensional hole gas. According to the composite fermion (CF) model, FQH states stemming from a half-filled Landau level, specifically at filling factors ${\nu}=p/(2p+1) (p=\pm 1,\pm 2,\pm 3,...)$, can be associated with two-flux-attached CFs at the corresponding Lambda filling factor p. The zero-resistance minima and Hall plateaus of these states exhibit unusual temperature dependencies, characterized by rapid increases in width below a threshold temperature around 100 mK. Differential conductivity measurements from Corbino samples reveal that the regimes surrounding the CF liquid display clear nonlinear transport characteristics. This nonlinearity implies that each CF liquid is surrounded by CF solid phase composed of dilute CF excitations. Quantitatively, the applied electric field E influences the motion of CF solid in a way analogous to T, which is dubbed the "E-T duality". Our analysis indicates that this E-T duality is consistent with the Berezinskii-Kosterlitz-Thouless theory in two-dimensional phase transitions.

cond-mat.mes-hall↗

arXiv2024

Mott Gap Softening Coinciding with Spin Correlations Collapse in a-RuCl3

The Mott insulator a-RuCl3, featuring the intertwined interplay of spin-orbit coupling (SOC) and Kitaev spin correlations, provides an unparalleled platform for probing quantum many-body physics. Using scanning tunneling microscopy/spectroscopy (STM/STS), we compare temperature-dependent dI/dV spectra between in-situ grown monolayers and exfoliated bulk samples. Both systems exhibit pronounced Mott gap softening near 110 K, manifested by spectral weight transfer from Hubbard bands toward the Fermi level, resulting in low-energy correlated charge delocalization. Although this gap softening coincides with Kitaev paramagnetic and structural phase transitions in bulk crystals, monolayer studies provide compelling insights. By eliminating structural phase transition in monolayer sample, we suggest that spin correlations, rather than Coulomb interactions alone, may govern charge dynamics within the Mott-Hubbard framework, challenging conventional Mott-Hubbard paradigms. These results resolve a long-standing controversy regarding the Mott gap magnitude in a-RuCl3 and experimentally confirm the critical role of spin correlations in Mott physics.

cond-mat.str-el↗

arXiv2024

Magneto-Induced Topological Phase Transition in Inverted InAs/GaSb Bilayers

We report a magneto-induced topological phase transition in inverted InAs/GaSb bilayers from a quantum spin Hall insulator to a normal insulator. We utilize a dual-gated Corbino device in which the degree of band inversion, or equivalently the electron and hole densities, can be continuously tuned. We observe a topological phase transition around the magnetic field where a band crossing occurs, that is accompanied by a bulk-gap closure characterized by a bulk conductance peak (BCP). In another set of experiments, we study the transition under a tilted magnetic field (tilt angle $\theta$). We observe the characteristic magneto-conductance around BCP as a function of $\theta$, which dramatically depends on the density of the bilayers. In a relatively deep-inversion (hence a higher density) regime, where the electron-hole hybridization dominates the excitonic interaction, the BCP grows with $\theta$. On the contrary, in a shallowly-inverted (a lower density) regime, where the excitonic interaction dominates the hybridization, the BCP is suppressed indicating a smooth crossover without a gap closure. This suggests the existence of a low-density, correlated insulator with spontaneous symmetry breaking near the critical point. Our highly controllable electron-hole system offers an ideal platform to study interacting topological states as proposed by recent theories.

cond-mat.mes-hall↗

arXiv2024

Incommensurate Charge Super-modulation and Hidden Dipole Order in Layered Kitaev Material $\alpha$-RuCl$_3$

The magnetism of Kitaev materials has been widely studied, but their charge properties and the coupling to other degrees of freedom are less known. Here we investigate the charge states of $\alpha$-RuCl$_3$, a promising Kitaev quantum spin liquid candidate, in proximity to graphite. We discover that few-layered $\alpha$-RuCl$_3$ experiences a clear modulation of charge states, where a Mott-insulator to weak charge-transfer-insulator transition in the 2D limit occurs by means of heterointerfacial polarization. More notably, distinct signals of incommensurate charge and lattice super-modulations, regarded as an unconventional charge order, accompanied in the insulator. Our theoretical calculations have reproduced the incommensurate charge order by taking into account the antiferroelectricity of $\alpha$-RuCl$_3$ that is driven by dipole order in the internal electric fields. The findings imply that there is strong coupling between the charge, spin, and lattice degrees of freedom in layered $\alpha$-RuCl$_3$ in the heterostructure, which offers an opportunity to electrically access and tune its magnetic interactions inside the Kitaev compounds.

cond-mat.str-el↗

arXiv2024

Superballistic flow of viscous electron fluid induced by microwave irradiation in quantum point contact

We measure the resistance oscillation of quantum point contact (QPC) under microwave (MW) radiation. What is different from the common resistance oscillation induced by edge magnetoplasmon (EMP) is that at lower magnetic field ({\omega}>{\omega}_c), photoconductance is positive (negative) with weak (strong) MW radiation. This transport phenomenon is proved to be related to superballistic flow of electrons through QPC. The distinction between the regions {\omega}>{\omega}_c and {\omega}<{\omega}_c is attributed to different absorption rate of MW radiation. Violent absorption occurs when cyclotron orbits or current domains are destroyed in QPC region.

cond-mat.mes-hall↗

arXiv2024

Interaction between Surface Acoustic Wave and Quantum Hall Effects

Surface Acoustic Wave (SAW) is a powerful technique for investigating quantum phases appearing in two-dimensional electron systems. The electrons respond to the piezoelectric field of SAWthrough screening, attenuating its amplitude and shifting its velocity, which is described by the relaxation model. In this work, we systematically study this interaction using orders of magnitude lower SAW amplitude than that in previous studies. At high magnetic fields when electrons form highly correlated states such as the quantum Hall effect, we observe an anomalously large attenuation of SAW while the acoustic speed remains considerably high, inconsistent with the conventional relaxation model. This anomaly exists only when the SAW power is sufficiently low.

cond-mat.mes-hall↗

arXiv2023

Insulator-to-metal Mott transition facilitated by lattice deformation in monolayer $\alpha$-RuCl$_3$ on graphite

Creating heterostructures with graphene/graphite is a practical method for charge-doping $\alpha$-RuCl$_3$, but not sufficient to cause the insulator-to-metal transition. In this study, detailed scanning tunneling microscopy/spectroscopy measurements on $\alpha$-RuCl$_3$ with various lattice deformations reveal that both in-plane and out-of-plane lattice distortions may collapse the Mott-gap in the case of monolayer $\alpha$-RuCl$_3$ in proximity to graphite, but have little impact on its bulk form alone. In the Mott-Hubbard framework, the transition is attributed to the lattice distortion-facilitated substantial modulation of the electron correlation parameter. Observation of the orbital textures on a highly compressed monolayer $\alpha$-RuCl$_3$ flake on graphite provides valuable evidence that electrons are efficiently transferred from the heterointerface into Cl3$p$ orbitals under the lattice distortion. It is believed that the splitting of Ru $t_{2g}$ bands within the trigonal distortion of Ru-Cl-Ru octahedra bonds generated the electrons transfer pathways. The increase of the Cl3$p$ states enhance the hopping integral in the Mott-Hubbard bands, resulting in the Mott-transition. These findings suggest a new route for implementing the insulator-to-metal transition upon doping in $\alpha$-RuCl$_3$ by deforming the lattice in addition to the formation of heterostructure.

cond-mat.str-el↗

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↗

arXiv2023

Realization of independent contacts in barrier-separated InAs/GaSb quantum wells

InAs/GaSb double quantum wells (QWs) separated by a 100 \AA\ AlSb middle barrier are grown by molecular beam epitaxy. We report a nanofabrication technique that utilizes the surface Fermi level pinning position in InAs $[E_f^s(\rm InAs)]$ for realizing independent electric contacts to each well. In particular, separate ohmic contacts to the upper InAs quantum well are achieved by selectively etching down to the InAs, while contacts to the lower GaSb quantum well are obtained by the depletion method. For the latter, the upper InAs quantum well is locally pinched off by top etched trenches capped with a remaining 2-3 nm InAs layer. As a result of a relatively low $E_f^s(\rm InAs)$, applying a negative bias gate potential will create a conducting hole channel in GaSb, and hence a separate ohmic contact to the lower quantum well. This method is demonstrated with experiment and the support of a self-consistent band bending calculation. A number of experiments on separately probing Coulomb and tunnel-coupled InAs/GaSb systems now become accessible.

cond-mat.mes-hall↗

arXiv2022

Hydrodynamic charge transport in GaAs/AlGaAs ultrahigh-mobility two-dimensional electron gas

Viscous fluid in an ultrahigh-mobility two-dimensional electron gas (2DEG) in GaAs/AlGaAs quantum wells is systematically studied through measurements of negative magnetoresistance (NMR) and photoresistance under microwave radiation, and the data are analyzed according to recent theoretical work by e.g., Alekseev, Physical Review Letters 117,166601 (2016). Size-dependent and temperature dependent NMR are found to conform to the theoretical predictions. In particular, transport of 2DEG with relatively weak Coulomb interaction (interparticle interaction parameter r_s<1) manifests a crossover between viscous liquid and viscous gas. The size dependence of microwave induced resistance oscillations and that of the '2nd harmonic' peak indicate that 2DEG in a moderate magnetic field should be regarded as viscous fluid as well. Our results suggest that the hydrodynamic effects must be considered in order to understand semiclassical electronic transport in a clean 2DEG.

cond-mat.mes-hall↗

arXiv2022

Tunneling spectroscopic signatures of charge doping and associated Mott transition in ${\alpha}$-RuCl${_3}$ in proximity to graphite

The layered Mott insulator ${\alpha}$-RuCl${_3}$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer ${\alpha}$-RuCl${_3}$ using a scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on ${\alpha}$-RuCl${_3}$ layers in proximity to graphite. In the single-layer ${\alpha}$-RuCl${_3}$ in direct contact with graphite, distinct states in the Mott-gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in ${\alpha}$-RuCl${_3}$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the ${\alpha}$-RuCl${_3}$ upper-layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott-gap within the upper-layers, suggesting that an unconventional Mott-transition is occurring in these layers. The results show that the heterostructure comprised of ${\alpha}$-RuCl${_3}$ and graphite is a good platform for investigating the doping physics in ${\alpha}$-RuCl${_3}$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin-liquid candidates.

cond-mat.str-el↗

arXiv2020

Cryogen-free one hundred microKelvin refrigerator

Temperature below 100 microKelvin is achieved in a customized cryogen-free dilution refrigerator with a copper-nuclear demagnetization stage. The lowest temperature of conduction electrons of the demagnetization stage is below 100 microKelvin as measured by a pulsed platinum NMR thermometer and the temperature can remain below 100 microKelvin for over 10 hours. An up to 9 T demagnetization magnetic field and an up to 12 T research magnetic field can be controlled independently, provided by a coaxial room-temperature-bore cryogen-free magnet.

physics.ins-det↗