arXiv subjects

Luojun Du

Publications and source records attributed to Luojun Du.

At least 19 recordsLinked to original sources

arXiv2026

Multidimensional Light Detection with Symmetry-engineered Heterojunctions

Miniaturised multidimensional light detection, encompassing full-Stokes polarimetry and spectroscopy in ultracompact footprints, is attracting growing interest for its potential to capture a comprehensive set of light properties in portable platforms. Although significant progress has been made in miniaturised schemes for independent polarisation and spectral detection, achieving simultaneous high-dimensional light detection remains an outstanding challenge that limits their development toward full integration. We overcome this limitation by breaking the rotational and inversion symmetries in a symmetry-engineered van der Waals heterojunction to realise an ultracompact multidimensional photodetector. The dual symmetry breaking gives rise to non-trivial quantum geometric and topological features, enabling simultaneous broadband polarisation- and spectrum-resolved light detection, in contrast to previous van der Waals material-based devices, which could detect only one of these modalities. Our device, with an effective area of only 10 micron x 10 micron, reconstructs full-Stokes polarisation with overall root-mean-square errors below 0.05 and resolves spectral peaks separated by 0.4 nm, capabilities not previously achieved in single-pixel detectors. By unifying high-fidelity polarimetry and sub-nanometre spectroscopy in a single electrically tunable junction, our work eliminates the need for cascaded detection architectures and establishes a foundation for multidimensional detector arrays for integrated photonics, quantum information processing, and precision imaging.

physics.optics↗

arXiv2026

Ferroelectric-tunable quantum nonlinearity of chiral Bloch electrons in a moir\'e system

Sliding ferroelectricity in van der Waals materials shows great potential for designing robust memory devices. However, its thermodynamic behaviors and the coupling with certain quantum effects remain largely unexplored. Here, we demonstrate ferroelectric control over quantum nonlinear transport in a hexagonal boron nitride (hBN) encapsulated twisted double-bilayer graphene moir\'e heterostructure. The ferroelectricity is attributed to the presence of rhombohedral stacking in the top hBN, confirmed by both electrical transport and optical second harmonic generation (SHG) measurements. Remarkably, the polarization magnitude remains temperature-independent across 1.7-200 K, while nucleation time exhibits thermally activated behavior, decreasing with increasing temperature. Furthermore, we demonstrate a ferroelectric-switchable nonlinear Hall effect, attributed to the chiral scattering induced by Berry curvature, with outstanding fatigue-resistant and nonvolatility, demonstrating direct coupling between sliding ferroelectricity and quantum geometric properties. Our results establish sliding ferroelectrics as a platform for exploring electrically programmable Berry curvature physics.

cond-mat.mes-hall↗

arXiv2026

Electrically tunable circular photocurrent via local-field induced symmetry breaking at a metal-MoTe2 interface

Transition metal dichalcogenides (TMDCs) constitute a promising platform for symmetry-engineered responses to circularly polarized light. The high crystal symmetry of centrosymmetric 2H-phase TMDCs inherently forbids the circular photogalvanic effect, thereby necessitating external stimuli such as electric fields or strain to lower the symmetry for its activation. While Schottky junctions provide a ubiquitous built-in field for potentially inducing circular photocurrents, the mechanism for the generation and control of circular photocurrents in TMDCs is not understood. In this study, we fabricated a localized gold-MoTe2 heterostructure and demonstrate a pronounced circular photocurrent at the interface under normal incidence. The photocurrent is attributed to circular photogalvanic effect governed by the strength and direction of the built-in electric field, enabling continuous modulation via an external bias. First-principles calculations show that the gold interface induces a spin splitting in the valence bands of MoTe2, establishing a valley-dependent spin ordering. The observed circular photocurrent from multilayer 2H-MoTe2 under normal incidence indicates the breaking of C3 rotational symmetry by the local in-plane field. These results establish an effective strategy for developing voltage-tunable circularly polarized photodetectors and valleytronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Observation of Integer and Fractional Chern insulators in high Chern number flatbands

Moir\'e flatbands with high Chern numbers (C>1) offer opportunities to study the fractional quantum anomalous Hall effects that go beyond the Landau level paradigm with C=1, which remain unexplored yet. Here, we target the novel topological phases in high Chern number flatbands by designing a new moir\'e system, i.e., twisted rhombohedral trilayer-bilayer graphene. We observe quantized anomalous Hall effects (QAH) with C = 3 at v = 1 and v = 3, demonstrating the high Chern number nature of the flat band from continuum calculations. By fractionally filling the flat band, we observe QAH with C = 2 at even-denominator fractional filling v = 3/2, as well as QAH with C = 3 continuously from v = 1 to the even-denominator v = 3/2 at zero magnetic fields. Most importantly, we observe, for the first time, evidence of an FCI with C = -6/5 at v = 12/5, corresponding to 2/5 filling of a high Chern number flat band with C = -3, verified by both Streda formula analysis and a fractionally QAH. It is also worth noting that Streda formula analysis reveals a signature of another FCI with C = -3/2 at v = 5/2 under finite magnetic fields. Our results demonstrate the tRTBG, which can be naturally extended to other twisted graphene moir\'e superlattices based on rhombohedral graphene multilayers, as a novel platform for hosting unconventional high Chern number correlated topology in the ultra-strong correlated regime that is beyond the paradigm of fractional phases with C < 1.

cond-mat.str-el↗

arXiv2025

Comprehensive Study of Phonon Chirality under Symmetry Constraints

Phonons are quanta of lattice vibrations, and their modes (linear, circular, or stationary) are symmetry-determined. Circularly polarized phonons, possessing nonzero angular momentum (AM), have drawn widespread attention recently. Despite widespread use of pseudo-angular momentum (PAM) and circularly polarized light polarization flips to identify chiral phonons in Raman scattering, their reliability is debated due to symmetry dependence, and experimental verification standards remain lacking. Here, we systematically study phonon chirality and associated phenomena across magnetic point groups. We establish that the AM-PAM correlation is governed by both crystalline symmetry and Wyckoff positions, dictating conditions where nonzero AM manifests in PAM signatures. Crucially, phonons belonging to distinct irreducible representations exhibit distinct experimental benchmarks, enabling direct determination of crystalline chirality and symmetry classification. Furthermore, we report the discovery of a signature for symmetry-induced phenomena, notably a half-wave plate-analogous effect induced by mirror-odd phonons. Meanwhile, we conducted five experiments to validate our theory.

cond-mat.mtrl-sci↗

arXiv2025

Ultrathin Ga$_2$O$_3$ Tunneling Contact for 2D Transition-metal Dichalcogenides Transistor

The development of two-dimensional (2D) transition metal dichalcogenides (TMDs) based transistors has been constrained by high contact resistance and inadequate current delivery, primarily stemming from metal-induced gap states and Fermi level pinning. Research into addressing these challenges is essential for the advancing 2D transistors from laboratory experiments to industrial-grade production. In this work, we present amorphous Ga$_2$O$_3$ as a novel tunneling contact layer for multilayer WS2-based field-effect transistors (FETs) to enhance electrical performance. The addition of this innovative tunneling layer avoid Schottky barrier forming while finally change into a tunneling barrier with the barrier height to just 3.7 meV, near-ideal ohmic contacts. This approach effectively reduces contact resistance to only 2.38 k$Ω\,μ$m and specific contact resistivity as low as $3 \times 10^{-5}$ $Ω$cm$^2$. A record-high electron mobility of 296 cm$^2$ V$^{-1}$ s$^{-1}$ and ON-OFF ratio over 106 are realized for WS$_2$ transistor at room temperature. Compared to other tunneling materials, ultrathin Ga$_2$O$_3$ layer offers scalability, cost-efficient production and broad substrate compatibility, making it well-suited for seamless integration with industrial wafer-scale electronics. A robust device performance remains highly consistent in a large-scale transistor array fabricated on $1.5\times 1.5$ cm$^2$ chips, with the average mobility closing to 200 cm$^2$ V$^{-1}$ s$^{-1}$. These findings establish a new benchmark for contact performance in 2D transistors and prove the potential of tunneling contact engineering in advancing high-performance, scalable 29 pelectronics with promising applications in quantum computing and communication.

cond-mat.mes-hall↗

arXiv2025

Electric field tunable coupling strength and quantum metric hot spots in a moir\'e flatband superconductor

Superconductivity in flatband systems has attracted tremendous attention in condensed matter physics. Alternating twisted multilayer graphene presents a compelling multiband system, with a coexistence of Dirac bands and flat bands, for exploring superconductivity. However, the roles of flat bands and dispersive bands played in determining the superconductivity remain elusive. Here, we focus on the alternating twisted quadralayer graphene to reveal unconventional superconducting behaviors by systematically quantifying individual contributions for both the dispersive bands and the flat bands. The superconductivity is robust, with a strong electrical field tunability, a maximum BKT transition temperature of 1.6 K, and high critical magnetic fields beyond the Pauli limit. By analyzing the Landau fan diagram at zero electric displacement fields, we disentangle Dirac bands and flat bands, revealing a Coulomb interaction-induced band broadening effect. We further quantify the electric-field-dependent evolution of the critical temperature and coherence length, and estimate the flat-band Fermi velocity and superfluid stiffness via critical current measurements. Our results demonstrate an electric field tunable coupling strength within the superconducting phase, revealing unconventional properties with vanishing Fermi velocity and large superfluid stiffness. These phenomena, attributed to substantial quantum metric contributions mediated by Dirac band hybridization, offer new insights into the mechanisms underlying unconventional flatband superconductivity in moir\'e systems.

cond-mat.supr-con↗

arXiv2024

The twisting dynamics of large lattice mismatch van der Waals heterostructures

Van der Waals (vdW) homo-/hetero-structures are ideal systems for studying interfacial tribological properties such as structural superlubricity. Previous studies concentrated on the mechanism of translational motion in vdW interfaces. However, detailed mechanisms and general properties of the rotational motion are barely explored. Here, we combine experiments and simulations to reveal the twisting dynamics of the MoS$_2$/graphite heterostructure. Unlike the translational friction falling into the superlubricity regime with no twist angle dependence, the dynamic rotational resistances highly depend on twist angles. Our results show that the periodic rotational resistance force originates from structural potential energy changes during the twisting. The structural potential energy of MoS$_2$/graphite heterostructure increases monotonically from0 to 30 degrees twist angles, and the estimated relative energy barrier is (1.43 +/- 0.36) x 10 J/m. The formation of Moiré superstructures in the graphene layer is the key to controlling the structural potential energy of the MoS$_2$/graphene heterostructure. Our results suggest that in twisting 2D heterostructures, even if the interface sliding friction is negligible, the evolving potential energy change results in a non-vanishing rotational resistance force. The structural change of the heterostructure can be an additional pathway for energy dissipation in the rotational motion, further enhancing the rotational friction force.

cond-mat.mes-hall↗

arXiv2024

Probing band topology in ABAB and ABBA stacked twisted double bilayer graphene

Twisted graphene moire superlattice has been demonstrated as an exotic platform for investigating correlated states and nontrivial topology. Among the moire family, twisted double bilayer graphene (TDBG) is a tunable flat band system expected to show stacking-dependent topological properties. However, electron correlations and the band topology are usually intertwined in the flat band limit, rendering the unique topological property due to stacking still elusive. Focusing on a large-angle TDBG with weak electron correlations, here we probe the Landau level (LL) spectra in two differently stacked TDBG, i.e. ABBA- and ABAB-TDBG, to unveil their distinct topological properties. For ABBA-TDBG, we observe non-trivial topology at zero electric displacement filed, evident from both the emergence of Chern bands from half fillings and the closure of gap at CNP above a critical magnetic field. For ABAB-TDBG, by contrast, we find that the moire band is topologically trivial, supported by the absence of LLs from half fillings and the persistence of the gap at CNP above the critical magnetic fields. In addition, we also observe an evolution of the trivial-to-nontrivial topological transition at finite D fields, confirmed by the emerged Landau fans originating from quarter filling v = 1. Our result demonstrates, for the first time, the unique stacking-dependent topology in TDBG, offering a promising avenue for future investigations on topological states in correlated systems.

cond-mat.mes-hall↗

arXiv2024

Observation of electrical high-harmonic generation

High-harmonic generation (HHG), an extreme nonlinear effect, introduces an unprecedented paradigm to detect emergent quantum phases and electron dynamics inconceivable in the framework of linear and low-order nonlinear processes. As an important manifestation, the optical HHG (o-HHG) enables extraordinary opportunities to underpin attosecond physics. In addition to nonlinear optics, emerging nonlinear electric transport has been demonstrated recently and opens new paradigms to probe quantum phase transition, symmetry breaking, band geometrical and topological properties. Thus far, only electrical second-/third-harmonic generation in perturbative regime has been elucidated, while the electrical HHG (e-HHG) that can advance to extreme non-perturbative physics remains elusive. Here we report the observation of e-HHG up to 300th-order. Remarkably, the e-HHG shows a clear non-perturbative character and exhibits periodic oscillations with the reciprocal of driving current. Further, theoretical simulations corroborate the experiments, suggesting the contribution of singular distribution of Berry curvature near band edges. Our results demonstrate e-HHG in extreme nonlinear regime and may shed light on a plethora of exotic physics and applications, such as extreme non-equilibrium quantum phenomena, ultra-fast and coherent electrical signal generations and detections.

cond-mat.mes-hall↗

arXiv2024

Circular Photocurrents in Centrosymmetric Semiconductors with Hidden Spin Polarization

Centrosymmetric materials with site inversion asymmetries possess hidden spin polarization, which remains challenging to be converted into spin currents because the global inversion symmetry is still conserved. This study demonstrates the spin-polarized DC circular photocurrents (CPC) in centrosymmetric transition metal dichalcogenides (TMDCs) at normal incidence without applying electric bias. The global inversion symmetry is broken by using a spatially-varying circularly polarized light beam, which could generate spin gradient owing to the hidden spin polarization. The dependences of the CPC on electrode configuration, illumination position, and beam spot size indicate an emergence of circulating electric current under spatially inhomogeneous light, which is associated with the deflection of spin-polarized current through the inverse spin Hall effect (ISHE). The CPC is subsequently utilized to probe the spin polarization and ISHE under different excitation wavelengths and temperatures. The results of this study demonstrate the feasibility of using centrosymmetric materials with hidden spin polarization and non-vanishing Berry curvature for spintronic device applications.

cond-mat.mtrl-sci↗

arXiv2024

Sliding ferroelectric memories and synapses

Ferroelectric materials with switchable electric polarization hold great promise for a plethora of emergent applications, such as post-Moore's law nanoelectronics, beyond-Boltzmann transistors, non-volatile memories, and above-bandgap photovoltaic devices. Recent advances have uncovered an exotic sliding ferroelectric mechanism, which endows to design atomically thin ferroelectrics from non-ferroelectric parent monolayers. Although notable progress has been witnessed in understanding its fundamental properties, functional devices based on sliding ferroelectrics, the key touchstone toward applications, remain elusive. Here, we demonstrate the rewritable, non-volatile memory devices at room-temperature utilizing a two-dimensional (2D) sliding ferroelectric semiconductor of rhombohedral-stacked bilayer molybdenum disulfide. The 2D sliding ferroelectric memories (SFeMs) show superior performances with a large memory window of >8V, a high conductance ratio of above 106, a long retention time of >10 years, and a programming endurance greater than 104 cycles. Remarkably, flexible SFeMs are achieved with state-of-the-art performances competitive to their rigid counterparts and maintain their performances post bending over 103 cycles. Furthermore, synapse-specific Hebbian forms of plasticity and image recognition with a high accuracy of 97.81% are demonstrated based on flexible SFeMs. Our work demonstrates the sliding ferroelectric memories and synaptic plasticity on both rigid and flexible substrates, highlighting the great potential of sliding ferroelectrics for emerging technological applications in brain-inspired in-memory computing, edge intelligence and energy-efficient wearable electronics.

cond-mat.mes-hall↗

arXiv2024

Interplay of Landau quantization and interminivalley scatterings in a weakly coupled moir\'e superlattice

Double layer quantum systems are promising platforms for realizing novel quantum phases. Here, we report a study of quantum oscillations (QOs) in a weakly coupled double layer system, composed of a large angle twisted double bilayer graphene (TDBG). We observe two different QOs at low temperature, one with a periodicity in carrier density (n), i.e. Shubnikov de Haas oscillation (SdHO) due to Landau quantization, and the other one in displacement field (D), resulting a grid pattern. We quantify the interlayer coupling strength by measuring the interlayer capacitance from the grid pattern with a capacitance model, revealing an electron hole asymmetry. At high temperature when SdHO are thermal smeared, we observe resistance peaks when LLs from two minivalleys in the moir\'e Brillion zone are aligned, regardless of carrier density; eventually, it results in a two fold increase of oscillating frequency in D, serving as a smoking gun evidence of the magneto intersubband oscillations (MISO) in a double layer system. The temperature dependence of MISO suggests electron-electron interaction between two minivalleys play a crucial rule in the scattering, and the scattering times obtained from MISO thermal damping are found to be correlated with the interlayer coupling strength. Our study reveals an intriguing interplay among Landau quantization, moir\'e band structure, and scatterings.

cond-mat.mes-hall↗

arXiv2023

Room-temperature correlated states in twisted bilayer MoS$_2$

Moiré superlattices have emerged as an exciting condensed-matter quantum simulator for exploring the exotic physics of strong electronic correlations. Notable progress has been witnessed, but such correlated states are achievable usually at low temperatures. Here, we report the transport evidences of room-temperature correlated electronic states and layer-hybridized SU(4) Hubbard model simulator in AB-stacked MoS$_2$ homo-bilayer moiré superlattices. Correlated insulating states at moiré band filling factors v = 1, 2, 3 are unambiguously established in twisted bilayer MoS$_2$. Remarkably, the correlated electronic states can persist up to a record-high critical temperature of over 285 K. The realization of room-temperature correlated states in twisted bilayer MoS$_2$ can be understood as the cooperation effects of the stacking-specific atomic reconstruction and the resonantly enhanced interlayer hybridization, which largely amplify the moiré superlattice effects on electronic correlations. Furthermore, extreme large non-linear Hall responses up to room-temperature are uncovered near correlated insulating states, demonstrating the quantum geometry of moiré flat conduction band.

cond-mat.mtrl-sci↗

arXiv2023

Weyl phonons in chiral crystals

Chirality is an indispensable concept that pervades fundamental science and nature, manifesting itself in diverse forms such as chiral quasiparticles and chiral structures. Of particular interest are Weyl phonons carrying specific Chern numbers and chiral phonons doing circular motions in crystals. Up to now, Weyl and chiral phonons have been studied independently and the interpretations of chirality seem to be different in these two concepts, impeding our understanding. Here, we demonstrate that Weyl and chiral phonons are entangled in chiral crystals. Employing a typical chiral crystal of elementary tellurium (Te) as a case study, we expound on the intrinsic relationship between Chern number of Weyl phonons and pseudo-angular momentum (PAM) of chiral phonons. In light of the mutual coupling, we propose Raman scattering as a new technique to demonstrate the existence of Weyl phonons in Te, by detecting the chirality-induced energy splitting between the two constituent chiral phonon branches for Weyl phonons. By using the same experimental approach, we also observe the obstructed phonon surface states for the first time.

cond-mat.mtrl-sci↗

arXiv2023

Observation of first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene

Twisted graphene multilayers are highly tunable flatband systems for developing new phases of matter. Thus far, while orbital ferromagnetism has been observed in valley polarized phases, the long-range orders of other correlated phases as well as the quantum phase transitions between different orders mostly remain unknown. Here, we report an observation of Coulomb interaction driven first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene (TDBG). At zero magnetic field, the transitions are revealed in a series of step-like abrupt resistance jumps with prominent hysteresis loop when either the displacement field (D) or the carrier density (n) is tuned across symmetry-breaking boundary near half filling, indicating a formation of ordered domains. It is worth noting that the good turnability and switching of these states gives a rise to a memory performance with a large on/off ratio. Moreover, when both spin and valley play the roles at finite magnetic field, we observe abundant first-order quantum phase transitions among normal metallic states from charge neutral point, orbital ferromagnetic states from quarter filling, and spin-polarized states from half filling. We interpret these first-order phase transitions in the picture of phase separations and spin domain percolations driven by multi-field tunable Coulomb interactions, in agreement with Lifshitz transition from Hartree-Fock calculations. The observed multi-filed tunable domain structure and its hysteresis resembles the characteristics of multiferroics, revealing intriguing magnetoelectric properties. Our result enriches the correlated phase diagram in TDBG for discovering novel exotic phases and quantum phase transitions, and it would benefit other twisted moiré systems as well.

cond-mat.mes-hall↗

arXiv2023

Electron-infrared phonon coupling in ABC trilayer graphene

Stacking order plays a crucial role in determining the crystal symmetry and has significant impacts on electronic, optical, magnetic, and topological properties. Electron-phonon coupling, which is central to a wide range of intriguing quantum phenomena, is expected to be intricately connected with stacking order. Understanding the stacking order-dependent electron-phonon coupling is essential for understanding peculiar physical phenomena associated with electron-phonon coupling, such as superconductivity and charge density waves. In this study, we investigate the effect of stacking order on electron-infrared phonon coupling in graphene trilayers. By using gate-tunable Raman spectroscopy and excitation frequency-dependent near-field infrared nanoscopy, we show that rhombohedral ABC-stacked trilayer graphene has a significantly stronger electron-infrared phonon coupling strength than the Bernal ABA-stacked trilayer graphene. Our findings provide novel insights into the superconductivity and other fundamental physical properties of rhombohedral ABC-stacked trilayer graphene, and can enable nondestructive and high-throughput imaging of trilayer graphene stacking order using Raman scattering.

physics.optics↗

arXiv2022

Probing Electronic States in Monolayer Semiconductors through Static and Transient Third-Harmonic Spectroscopy

Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, we probe various relevant electronic states in monolayer MoS2, such as multiple excitonic Rydberg states and free-particle energy bands, with a high relative contrast of up to >200 via broadband (from ~1.79 to 3.10 eV) static third-harmonic spectroscopy, which is further supported by theoretical calculations. Moreover, we introduce transient third-harmonic spectroscopy to demonstrate that third-harmonic generation can be all-optically modulated with a modulation depth exceeding ~94% at ~2.18 eV, providing direct evidence of dominant carrier relaxation processes, associated with carrier-exciton and carrier-phonon interactions. Our results indicate that static and transient third-harmonic spectroscopies are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.

physics.optics↗