arXiv subjects

Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 541 records · Page 30Linked to original sources

arXiv2025

Layered semiconductors integrated with polyimide thin films for high-quality valleytronic and quantum-photonic systems

Dielectric integration of layered semiconductors is a prerequisite for fabricating high-quality optoelectronic, valleytronic, and quantum-photonic devices. While hexagonal boron nitride (hBN) is the current benchmark dielectric, exploration of the most suitable dielectric materials covering the complete substrates continues to expand. This work demonstrates the formation of high optical-quality excitons in two widely explored layered semiconductors, WSe$_2$ and WS$_2$, integrated into polyimide (PI) thin films of thicknesses $\approx$500 nm. The photoluminescence (PL) studies at $T$ = 296 K show the formation of neutral excitons $\left(X^0\right)$ and trions in fully-PI-encapsulated 1L-WSe$_2$ with 2-sigma ($2σ$) spatial-inhomogeneity of 4.5 (3.4) meV in $X^0$ emission energy (linewidth), which is $\approx$1/3rd (1/5th), respectively, that of inhomogeneity measured in fully-hBN-encapsulated 1L-WSe$_2$. A smaller $2σ$ of 2.1 (2.3) meV in $X^0$ emission energy (linewidth) has been shown for fully-PI-encapsulated 1L-WS$_2$. Polarization-resolved and excitation power-dependent PL measurements of PI-isolated 1L-TMDs at $T$ = 4 K further reveal formations of high-quality neutral-biexcitons and negatively-charged biexcitons, with degrees of valley-polarization up to 21$\%$ under non-resonant excitation. Furthermore, the fully-PI-encapsulated 1L-WSe$_2$ also hosts single quantum emitters with narrow linewidths and high-spectral stability. This work indicates that PI thin films may serve the purpose of high-quality dielectric material for integrating the layered materials on a wafer scale.

cond-mat.mes-hall↗

arXiv2025

Spin-filter tunneling detection of antiferromagnetic resonance with electrically-tunable damping

Antiferromagnetic spintronics offers the potential for higher-frequency operations and improved insensitivity to magnetic fields compared to ferromagnetic spintronics. However, previous electrical techniques to detect antiferromagnetic dynamics have utilized large, millimeter-scale bulk crystals. Here we demonstrate direct electrical detection of antiferromagnetic resonance in structures on the few-micrometer scale using spin-filter tunneling in PtTe$_2$/bilayer CrSBr/graphite junctions in which the tunnel barrier is the van der Waals antiferromagnet CrSBr. This sample geometry allows not only efficient detection, but also electrical control of the antiferromagnetic resonance through spin-orbit torque from the PtTe$_2$ electrode. The ability to efficiently detect and control antiferromagnetic resonance enables detailed studies of the physics governing these high frequency dynamics.

cond-mat.mtrl-sci↗

arXiv2025

Observation of the Axion quasiparticle in 2D MnBi$_2$Te$_4$

In 1978, Wilczek and Weinberg theoretically discovered a new boson-the Axion-which is the coherent oscillation of the $θ$ field in QCD. Its existence can solve multiple fundamental questions including the strong CP problem of QCD and the dark matter. However, its detection is challenging because it has almost no interaction with existing particles. Similar $θ$ has been introduced to condensed matter and so far studied as a static, quantized value to characterize topology of materials. But the coherent oscillation of $θ$ in condensed matter is proposed to lead to new physics directly analogous to the high-energy Axion particle, the dynamical Axion quasiparticle (DAQ). In this paper, we present the direct observation of the DAQ. By combining 2D electronic device with ultrafast pump-probe optics, we manage to measure the magnetoelectric coupling $θ$ ($θ\proptoα$) of 2D MnBi$_2$Te$_4$ with sub-picosecond time-resolution. This allows us to directly observe the DAQ by seeing a coherent oscillation of $θ$ at ~44 GHz in real time, which is uniquely induced by the out-of-phase antiferromagnetic magnon. Interestingly, in 2D MnBi$_2$Te$_4$, the DAQ arises from the magnon-induced coherent modulation of Berry curvature. Such ultrafast control of quantum wavefunction can be generalized to manipulate Berry curvature and quantum metric of other materials in ultrafast time-scale. Moreover, the DAQ enables novel quantum physics such as Axion polariton and electric control of ultrafast spin polarization, implying applications in unconventional light-matter interaction and coherent antiferromagnetic spintronics. Beyond condensed matter, the DAQ can serve as a detector of the dark matter Axion particle. We estimate the detection frequency range and sensitivity in the critically-lacking meV regime, contributing to one of the most challenging questions in fundamental physics.

cond-mat.mes-hall↗

arXiv2025

Superconductivity and quantized anomalous Hall in rhombohedral graphene

Inducing superconducting correlations in chiral edge states is predicted to generate topologically protected zero energy modes with exotic quantum statistics. Experimental efforts to date have focused on engineering interfaces between superconducting materials typically amorphous metals and semiconducting quantum Hall or quantum anomalous Hall (QAH) systems. However, the interfacial disorder inherent in this approach can prevent the formation of isolated topological modes. An appealing alternative is to use low-density flat band materials where the ground state can be tuned between intrinsic superconducting and quantum anomalous Hall states using only the electric field effect. However, quantized transport and superconductivity have not been simultaneously achieved. Here, we show that rhombohedral tetralayer graphene aligned to a hexagonal boron nitride substrate hosts a quantized anomalous Hall state at superlattice filling $ν=-1$ as well as a superconducting state at $ν-3.5$ at zero magnetic field. Remarkably, gate voltage can also be used to actuate nonvolatile switching of the chirality in the quantum anomalous Hall state, allowing, in principle, arbitrarily reconfigurable networks of topological edge modes in locally gated devices. Thermodynamic compressibility measurements further reveal a topologically ordered fractional Chern insulator at $ν=2/3$-also stable at zero magnetic field-enabling proximity coupling between superconductivity and fractionally charged edge modes. Finally, we show that, as in rhombohedral bi- and trilayers, integrating a transition metal dichalcogenide layer to the heterostructure nucleates a new superconducting pocket, while leaving the topology of the $ν=-1$ quantum anomalous Hall state intact. Our results pave the way for a new generation of hybrid interfaces between superconductors and topological edge states in the low-disorder limit.

cond-mat.mes-hall↗

arXiv2025

Stability diagram of layer-polarized quantum Hall states in twisted trilayer graphene

In the twisted trilayer graphene (tTLG) platform, the rich beating patterns between the three graphene layers give rise to a plethora of new length scales and reconstructed electronic bands arising from the emergent moiré and moiré-of-moiré superlattices. The co-existing lattices and superlattices interact and compete with each other to determine the overall transport properties of tTLG, the hierarchy of which can be electrostatically controlled by tuning the out-of-plane charge distribution or layer polarization. In this work, we measure the stability diagram of layer-polarized quantum Hall states in tTLG by systematically mapping out layer-specific Chern numbers in each layer, and intra- and interlayer Chern transitions as a function of displacement field D and total carrier density n. In contrast to twisted bilayer systems, the rich interplay between the three atomic layers gives rise to a complex layer-polarized stability diagram with unconventional transport features that evolve rapidly with electric and magnetic fields. The stability diagram quantitatively characterizes the interlayer screening and charge distribution in tTLG with implication of strong inter-atomic-layer Coulomb coupling. Our work provides comprehensive guidance and insights into predicting and controlling layer-polarization and interlayer transitions in tTLG, and for tuning the individual role and interactions of each participating constituent towards novel material properties.

cond-mat.mes-hall↗

arXiv2025

Magnon hydrodynamics in an atomically-thin ferromagnet

Strong interactions between particles can lead to emergent collective excitations. These phenomena have been extensively established in electronic systems, but are also expected to occur for gases of neutral particles like magnons, i.e. spin waves, in magnets. In a hydrodynamic regime where magnons are strongly interacting, they can form a slow collective density mode -- in analogy to sound waves in water -- with characteristic low-frequency signatures. While such a mode has been predicted in theory, its signatures have yet to be observed experimentally. In this work, we isolate exfoliated sheets of CrCl$_3$ where magnon interactions are strong, and develop a technique to measure its collective magnon dynamics via the quantum coherence of nearby Nitrogen-Vacancy (NV) centers in diamond. We find that the thermal magnetic fluctuations generated by monolayer CrCl$_3$ exhibit an anomalous temperature dependence, whereby fluctuations increase upon decreasing temperature. Our analysis suggests that this anomalous trend is a consequence of the damping rate of a low-energy magnon sound mode which sharpens as magnon interactions increase with increasing temperature. By measuring the magnetic fluctuations emitted by thin multilayer CrCl$_{3}$ in the presence of a variable-frequency drive field, we observe spectroscopic evidence for this two-dimensional magnon sound mode.

cond-mat.mes-hall↗

arXiv2025

Gate-tunable electroresistance in a sliding ferroelectric tunnel junction

We fabricate and measure electrically-gated tunnel junctions in which the insulating barrier is a sliding van der Waals ferroelectric made from parallel-stacked bilayer hexagonal boron nitride and the electrodes are single-layer graphene. Despite the nominally-symmetric tunnel-junction structure, these devices can exhibit substantial electroresistance upon reversing the ferroelectric polarization. The magnitude and sign of tunneling electroresistance are tunable by bias and gate voltage. We show that this behavior can be understood within a simple tunneling model that takes into account the quantum capacitance of the graphene electrodes, so that the tunneling densities of states in the electrodes are separately modified as a function of bias and gate voltage.

cond-mat.mes-hall↗

arXiv2025

Gating monolayer and bilayer graphene with a two-dimensional semiconductor

Metals are commonly used as electrostatic gates in devices due to their abundant charge carrier densities that are necessary for efficient charging and discharging. A semiconducting gate can be beneficial for certain fabrication processes, in low light conditions, and for specific gating properties. We determine the effectiveness and limitations of a semiconducting gate in graphene and bilayer graphene devices. Using the semiconducting transition metal dichalcogenides molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2), we show that two-dimensional semiconductors can be used to suitably gate the graphene devices under appropriate operating conditions. For single-gated devices, semiconducting gates are comparable to metallic gates below liquid helium temperatures but include resistivity features resulting from gate voltage clamping of the semiconductor. In dual-gated devices, we pin down the parameter range of effective operation and find that the semiconducting depletion regime results in clamping and hysteresis from defect-state charge trapping.

cond-mat.mes-hall↗

arXiv2025

Control of atomic reconstruction and quasi-1D excitons in strain-engineered moiré heterostructures

In two-dimensional nearly commensurate heterostructures, strain plays a critical role in shaping electronic behavior. While previous studies have focused on random strain introduced during fabrication, achieving controlled structural design has remained challenging. Here, we demonstrate the deterministic creation of one-dimensional arrays from initially zero-dimensional triangular moiré patterns in MoSe$_2$-WSe$_2$ heterobilayers. This transformation, driven by the interplay of uniaxial strain and atomic reconstruction, results in one-dimensional confinement of interlayer excitons within domain walls, exhibiting near-unity linearly polarized emission related to the confinement-induced symmetry breaking. The width of the domain walls--and consequently the degree of exciton confinement--can be precisely tuned by the interlayer twist angle. By applying out-of-plane electric field, the confined excitons exhibit energy shifts exceeding 100~meV and changes in the fine-structure splitting by up to a factor of two. Our work demonstrates the potential of strain engineering for constructing designer moiré systems with programmable quantum properties, paving the way for future optoelectronic applications.

cond-mat.mes-hall↗

arXiv2025

Stacking-induced ferroelectricity in tetralayer graphene

Recent studies have reported emergent ferroelectric behavior in twisted or moiré-engineered graphene-based van der Waals heterostructures, yet the microscopic origin of this effect remains under debate. Pristine mono- or few-layer graphene lacks a permanent dipole due to its centrosymmetric lattice, making the emergence of ferroelectricity unlikely. However, mixed-stacked graphene, such as the ABCB tetralayer configuration, breaks both inversion and mirror symmetry and has been theoretically predicted to support electrically switchable dipoles. ABCB graphene represents the simplest natural graphene polytype exhibiting intrinsic out-of-plane polarization, arising from asymmetric charge carrier distribution across its layers. Here, we report robust ferroelectric behavior in dual-gated, non-aligned ABCB tetralayer graphene encapsulated in hexagonal boron nitride. The device exhibits pronounced hysteresis in resistance under both top and bottom gate modulation, with the effect persisting up to room temperature. This hysteresis originates from reversible layer-polarized charge reordering, driven by gate-induced transitions between ABCB and BCBA stacking configurations -- without requiring moiré superlattices. Our findings establish stacking-order-induced symmetry breaking as a fundamental route to electronic ferroelectricity in graphene and open pathways for non-volatile memory applications based on naturally occurring mixed-stacked multilayer graphene.

cond-mat.mes-hall↗

arXiv2025

Superballistic conduction in hydrodynamic antidot graphene superlattices

Viscous electron flow exhibits exotic signatures such as superballistic conduction. In order to observe hydrodynamics effects, a 2D device where the current flow is as inhomogeneous as possible is desirable. To this end, we build three antidot graphene superlattices with different hole diameters. We measure their electrical properties at various temperatures and under the effect of a perpendicular magnetic field. We find an enhanced superballistic effect, suggesting the effectiveness of the geometry at bending the electron flow. In addition, superballistic conduction, which is related to a transition from a non-collective to a collective regime of transport, behaves non-monotonically with the magnetic field. We also analyze the device resistance as a function of the size of the antidot superlattice to find characteristic scaling laws describing the different transport regimes. We prove that the antidot superlattice is a convenient geometry for realizing hydrodynamic flow and provide valuable explanations for the technologically relevant effects of superballistic conduction and scaling laws.

cond-mat.mes-hall↗

arXiv2025

Electrical switching of Chern insulators in moire rhombohedral heptalayer graphene

In orbital Chern insulators, the chemical potential acts as a tuning knob to reverse chirality in dissipationless edge currents, enabling electric-field control of magnetic order-key for future quantum electronics. Despite the rise of orbital Chern insulators, electrically switchable quantum anomalous Hall effect (QAHE) remains rare, necessitating further investigation. Here, we demonstrate electric-field-induced reversal of orbital Chern insulators in a moire superlattice composed of rhombohedral heptalayer graphene (r-7LG) aligned with hexagonal boron nitride. At one electron per moire unit cell, two emerging Chern insulating phases - one pointing away from and the other toward graphene's charge neutrality point in the phase diagram of carrier density (n) versus magnetic field (B) - exhibit energetic competition modulated by both n and B. This switchable QAHE chirality in r-7LG demonstrates a layer-number dependent response: similar phenomena in moire r-6LG require much higher magnetic fields and are absent in thinner rhombohedral graphene. Our findings establish moire-engineered rhombohedral graphene as a promising platform for exploring topological quantum materials with electrically controllable chiral edge modes and magnetic order.

cond-mat.mes-hall↗

arXiv2025

Gate-tunable hot electron extraction in a two-dimensional semiconductor heterojunction

Hot carrier solar cells (HCSCs), harvesting excess energy of the hot carriers generated by above-band-gap photoexcitation, is crucial for pushing the solar cell efficiency beyond the Shockley Queisser limit, which is challenging to realize mainly due to fast hot-carrier cooling. By performing transient reflectance spectroscopy in a MoSe2/hBN/WS2 junction, we demonstrate the gate-tunable harvest of hot electrons from MoSe2 to WS2. By spectrally distinguishing hot-electron extraction from lattice temperature increase, we find that electrostatically doped electrons in MoSe2 can boost hot-electron extraction density (n_ET) by factor up to several tens. Such enhancement arises from interaction between hot excitons and doped electrons, which converts the excess energy of hot excitons to excitations of the Fermi sea and hence generates hot electrons. Moreover, n_ET can be further enhanced by reducing the conduction band offset with external electric field. Our results provide in-depth insights into design of HCSCs with electrostatic strategies.

cond-mat.mes-hall↗

arXiv2025

Moiré enhanced flat band in rhombohedral graphene

The fractional quantum anomalous Hall effect (FQAHE) is a fascinating emergent quantum state characterized by fractionally charged excitations in the absence of magnetic field,which could arise from the intricate interplay between electron correlation, nontrivial topology and spontaneous time-reversal symmetry breaking. Recently, FQAHE has been realized in aligned rhombohedral pentalayer graphene on BN superlattice (aligned R5G/BN), where the topological flat band is modulated by the moiré potential. However, intriguingly, the FQAHE is observed only when electrons are pushed away from the moiré interface. The apparently opposite implications from these experimental observations, along with different theoretical models, have sparked intense debates regarding the role of the moiré potential. Unambiguous experimental observation of the topological flat band as well as moiré bands with energy and momentum resolved information is therefore critical to elucidate the underlying mechanism. Here by performing nanospot angle-resolved photoemission spectroscopy (NanoARPES) measurements, we directly reveal the topological flat band electronic structures of R5G, from which key hopping parameters essential for determining the fundamental electronic structure of rhombohedral graphene are extracted. Moreover, a comparison of electronic structures between aligned and non-aligned samples reveals that the moiré potential plays a pivotal role in enhancing the topological flat band in the aligned sample. Our study provides experimental guiding lines to narrow down the phase space of rhombohedral graphene, laying an important foundation for understanding exotic quantum phenomena in this emerging platform.

cond-mat.mes-hall↗

arXiv2025

Resistance hysteresis in twisted bilayer graphene: Intrinsic versus extrinsic effects

Hysteresis in resistance under magnetic field sweeps is a key signature for identifying magnetism in twisted bilayer graphene and similar systems. However, such sweeps can induce extrinsic thermal effects, complicating interpretations. Distinguishing intrinsic magnetic ordering from extrinsic thermal influences is crucial. In this study, we report hysteresis in the longitudinal resistance ($(R_{xx}$)) of a near magic-angle twisted bilayer graphene (TBG) sample under an in-plane magnetic field ($(B_{||}$)). The hysteresis phase appears at the edge of the superconducting dome, diminishes deep within the superconducting regime, and reemerges near the superconducting critical temperature ($(T \sim T_c$)). The hysteresis magnitude and coercive fields strongly depend on the magnetic field sweep rate ($(dB/dt)$) and exhibit transient relaxation in time-series measurements. Notably, similar hysteresis behavior was observed in the temperature profile of the sample stage, measured using a calibrated temperature sensor under analogous magnetic field cycles, suggesting extrinsic thermal origins rather than intrinsic magnetic ordering. These findings underscore the importance of carefully distinguishing intrinsic and extrinsic effects in resistance hysteresis observed in mesoscopic van der Waals systems.

cond-mat.mes-hall↗

arXiv2025

Optical detection of charge defects near a graphene transistor using the Stark shift of fluorescent molecules

Two-dimensional crystals and their heterostructures unlock access to a class of photonic devices, bringing nanophotonics from the nanometer scale down to the atomic level where quantum effects are relevant. Single-photon emitters (SPEs) are central in quantum photonics as quantum markers linked to their electrostatic, thermal, magnetic, or dielectric environment. This aspect is exciting in two-dimensional (2D) crystals and their heterostructures, where the environment can be abruptly modified through vertical stacking or lateral structuring, such as moiré or nano-patterned gates. To further develop 2D-based quantum photonic devices, there is a need for quantum markers that are capable of integration into various device geometries, and that can be read out individually, non-destructively, and without additional electrodes. Here, we show how to optically detect charge carrier accumulation using sub-GHz linewidth single-photon emitters coupled to a graphene device. We employ the single molecule Stark effect, sensitive to the electric fields generated by charge puddles, such as those at the graphene edge. The same approach enables dynamic sensing of electronic noise, and we demonstrate the optical read-out of low-frequency white noise in a biased graphene device. The approach described here can be further exploited to explore charge dynamics in 2D heterostructures using quantum emitter markers.

cond-mat.mes-hall↗

arXiv2025

Imaging moiré flat bands and Wigner molecular crystals in twisted bilayer MoTe2

Two-dimensional semiconducting moiré materials have emerged as a highly tunable platform for exploring novel quantum phenomena. Recently, tMoTe2 has attracted significant attentions due to the observation of the long-sought fractional quantum anomalous Hall effect. However, a comprehensive microscopic understanding of the tMoTe2 moiré superlattice remains elusive. Here, we report STM/STS studies in dual-gated tMoTe2 moiré devices with twist angles ranging from 2.3 to 3.8 deg. The device consists of two independent back-gates, one enables an ohmic contact for tMoTe2, while the other fine-tunes the Fermi level of tMoTe2. This dual-gate control enables direct measurement of the electronic structure in tMoTe2 under varied displacement fields and moiré filling factors, by fine tuning the gate voltage and the tip bias. Our STS spectra and spatial imaging reveal that the low-energy moiré flat bands are predominantly localized in the XM and MX regions of the moiré superlattice. At zero E-field, these bands form a honeycomb lattice with non-trivial topology, whereas an applied E-field drives a transition into two distinct triangular lattices with trivial topology. The spatial distributions align with large-scale first-principle calculations, demonstrating that the topological flat bands arise from the K-valley hybridization between the top and bottom MoTe2 layers. Furthermore, we show that the effective moiré potential depth can be controlled via gate and tip biases. At sufficient potential depths, we observe the emergence of Wigner molecular crystals, transitioning MX triangular lattice into a Kagome lattice at MX moiré filling factor 3. These results elucidate the microscopic origin of topological flat bands in tMoTe2 and demonstrate electric-field control of topology and correlated electronic orders, paving the way to engineer exotic quantum phases in moiré simulators.

cond-mat.mes-hall↗

arXiv2025

Symmetry broken states at high displacement fields in ABA trilayer graphene

In this Letter, we present a comprehensive study of magnetotransport in high-mobility trilayer graphene (TLG) devices under a transverse displacement field, focusing on symmetry-broken Landau levels (LLs) from monolayer-like and bilayer-like bands. A striking displacement-field-induced enhancement of the Landé g-factor is observed in the zeroth Landau level of the monolayer-like band, highlighting the role of strong electron-electron interactions. Additionally, we find a rich landscape of LL crossings in the Dirac gully region, accompanied by phase transitions between spin-, gully-, and valley-polarized LLs. These experimental observations are successfully modeled using calculations based on optimized tight-binding parameters. Furthermore, our results reveal significant particle-hole asymmetry in the sequence of LLs in the Dirac gullies, attributed to differing g-factor values for electrons and holes. This asymmetry underscores the limitations of non-interacting models in capturing the complexities of strongly correlated multiband systems. This work provides new insights into the interplay of symmetry-breaking mechanisms and strong correlations in Bernal-stacked trilayer graphene, advancing our understanding of quantum transport phenomena in multiband systems.

cond-mat.mes-hall↗