arXiv subjects

Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 397 records · Page 22Linked to original sources

arXiv2025

Giant field-tunable nonlinear Hall effect by Lorentz skew scattering in a graphene moire superlattice

The nonlinear Hall effect (NHE) can enable rectification and energy harvesting, and its control by external fields, including gate, strain and magnetic field, has been pursued intensively. However, existing tuning pathways rely predominantly on fully quantum mechanical effects and are typically inefficient, resulting in weak NHE signals that limit further progress. In this work, we report the discovery of a distinct type of NHE in a graphene-hBN moire superlattice, which arises from a classical-quantum cooperative effect called Lorentz skew scattering (LSK), induced by a perpendicular magnetic field. This field-driven NHE exhibits a linear dependence on magnetic field and a pronounced unidirectional angular dependence. Remarkably, its magnitude reaches up to 32% of the linear Hall signal. We show that this giant, field-tunable NHE originating from LSK follows a unique quartic scaling law and produces a record-high nonlinear Hall conductivity (36000 μmV-1Ω-1) near van Hove singularities of moire minibands, which is over an order of magnitude larger than all previously reported NHEs. Our findings establish an efficient, magnetic-field-driven route to giant Hall rectification in high-mobility materials, offering a broadly applicable paradigm for modulating the NHE beyond electrostatic gating.

cond-mat.mes-hall↗

arXiv2025

Phason-driven temperature-dependent transport in moiré graphene

The electronic and vibrational properties of 2D materials are dramatically altered by the formation of a moiré superlattice. The lowest-energy phonon modes of the superlattice are two acoustic branches (called phasons) that describe the sliding motion of one layer with respect to the other. Considering their low-energy dispersion and damping, these modes may act as a significant source of scattering for electrons in moiré materials. Here, we investigate temperature-dependent electrical transport in minimally twisted bilayer graphene, a moiré system developing multiple weakly-dispersive electronic bands and a reconstructed lattice structure. We measure a linear-in-temperature resistivity across the band manyfold above $T\sim{10}$ K, preceded by a quadratic temperature dependence. While the linear-in-temperature resistivity is up to two orders of magnitude larger than in monolayer graphene, it is reduced (approximately by a factor of three) with respect to magic-angle twisted bilayer graphene. Moreover, it is modulated by the recursive band filling, with minima located close to the full filling of each band. Comparing our results with a semiclassical transport calculation, we show that the experimental trends are compatible with scattering processes mediated by longitudinal phasons, which dominate the resistivity over the contribution from conventional acoustic phonons of the monolayer. Our findings highlight the close relation between vibrational modes unique to moiré materials and carrier transport therein.

cond-mat.mes-hall↗

arXiv2025

Single-Ion Sensing in Liquid Using Fluorescent h-BN Point Defects

Understanding the chemical state of individual ions in solutions is crucial for advancing knowledge of complex systems. However, sensing systems at the single-ion level in liquid environments remains a significant challenge. A strategy is introduced that leverages the optical emission properties of point defects in hexagonal boron nitride (h-BN) as single ion sensors. The interaction of optically active h-BN defects with ions in solution leads to distinct spectral shifts, enabling precise visualization and analyzing of individual ions. Using Li+ ions in organic electrolytes as a model, spectral shifts exceeding 10 nm were observed upon ion addition. Application of an external electric field further enhanced these shifts to over 40 nm, enabling real-time monitoring of electrical field induced local perturbations of Li+ ions. Through this approach, individual point defects were shown to spectroscopically distinguish ions of varying charges (e.g., Na+, Mg2+, and Al3+) based on their local electrical field, each producing a distinct spectral shift. This platform allows direct sensing of ions and their chemical states in liquid environments, providing insights into subtle interfacial changes at the single-ion level, with measurable spectral shifts detectable at millisecond temporal resolution and at concentrations down to 10 micromolar range. This capability presents potential applications in various fields involving ions in liquids that include battery technology and environmental science.

cond-mat.mes-hall↗

arXiv2025

Thermoelectricity of moiré heavy fermions in MoTe2/WSe2 bilayers

Tunable Kondo lattice and heavy fermion physics have been recently reported in moiré materials, but most of the studies have focused on the electrical and magnetic properties. Quantitative thermoelectric measurements, which can reveal entropic information of the heavy fermions, have yet to be achieved. Here, we report a comprehensive thermoelectric study on the moiré heavy fermion phase realized in hole-doped angle-aligned MoTe2/WSe2 bilayers. By electrically gating the material to the Kondo lattice region of the phase diagram, we observe a sign change in the Seebeck coefficient near the Kondo coherence temperature, where the heavy fermion phase with an electron-like Fermi surface evolves into an itinerant Fermi liquid with a hole-like Fermi surface. We compare the results with the semiclassical Mott relation and discuss the observed discrepancies. In addition to the thermal dissociation of Kondo singlets in the heavy Fermi liquid, a sign change accompanied by a strong peak in the Seebeck coefficient is also observed near a Zeeman breakdown of the Kondo singlets, signaling an entropy accumulation. Our results provide entropic information on both the formation and breakdown of heavy fermions in moiré semiconductors.

cond-mat.str-el↗

arXiv2025

Defect Engineered Hexagonal-Boron Nitride Enables Ionic Conduction for Lithium Metal Batteries

The practical implementation of lithium-metal anodes has been hindered by uncontrollable dendrite formation and interfacial instability. This study presents a defect-engineering approach of a chemically stable and electrically insulating interfacial layer of hexagonal boron nitride (h-BN) that markedly enhances ionic conductivity through argon ion irradiation. Initially, the electrochemical performance from commercially available, large-area chemical vapor deposition (CVD)-grown h-BN films with industrial-scale argon ion implantation motivated our subsequent detailed investigations using lab-scale exfoliated single-crystal h-BN flakes. Integration of these exfoliated flakes into a hybrid microfluidic-microelectronic chip provided direct evidence that controlled vacancy defects transform h-BN into an efficient lithium-ion conductor while preserving its intrinsic electrical insulation. Experimental validation confirmed improved lithium-metal anode stability, achieving dendrite-free cycling with Li plating/stripping Coulombic efficiencies exceeding 99.5% about 1000 cycles. Further assemble of irradiated h-BN in lithium-sulfur batteries effectively mitigates the polysulfide shuttle effect, sustaining over 97% specific capacity around 300 cycles. These results establish a robust, scalable interface-engineering route for next-generation lithium-metal batteries that combine high ionic transport with excellent electrical insulation.

cond-mat.mtrl-sci↗

arXiv2025

Super-Moiré Spin Textures in Twisted Antiferromagnets

Stacking two-dimensional (2D) layered materials offers a powerful platform to engineer electronic and magnetic states. In general, the resulting states, such as Moiré magnetism, have a periodicity at the length scale of the Moiré unit cell. Here, we report a new type of magnetism -- dubbed a super-Moiré magnetic state -- which is characterized by long-range magnetic textures extending beyond the single Moiré unit cell -- in twisted double bilayer chromium triiodide (tDB CrI$_3$). We found that at small twist angles, the size of the spontaneous magnetic texture increases with twist angle, opposite to the underlying Moiré periodicity. The spin-texture size reaches a maximum of about 300 nm in 1.1$°$ twisted devices, an order of magnitude larger than the underlying Moiré wavelength, and vanishes at twist angles above 2$°$. Employing scanning quantum spin magnetometry, the obtained vector field maps suggest the formation of antiferromagnetic Néel-type skyrmions spanning multiple Moiré cells. The twist-angle-dependent study combined with large-scale atomistic simulations suggests that complex magnetic competition between the Dzyaloshinskii--Moriya interaction, magnetic anisotropy, and exchange interactions controlled by the relative rotation of the layers produces the topological textures which arise in the super-Moiré spin orders.

quant-ph↗

arXiv2025

Mapping the twist angle dependence of quasi-Brillouin zones in doubly aligned graphene/BN heterostructures

When monolayer graphene is crystallographically aligned to hexagonal boron nitride (BN), a moiré superlattice is formed, producing characteristic satellite Dirac peaks in the electronic band structure. Aligning a second BN layer to graphene creates two coexisting moiré patterns, which can interfere to produce periodic, quasi-periodic or non-periodic superlattices, depending on their relative alignment. Here, we investigate one of the simplest realizations of such a double-moiré structure, graphene encapsulated between two BN layers, using dynamically rotatable van der Waals heterostructures. Our setup allows \textit{in situ} control of the top BN alignment while keeping the bottom BN fixed. By systematically mapping the charge transport as a function of BN angular alignment, we identify the simultaneous signatures of the original moirés, super-moirés, and a third set of features corresponding to quasi-Brillouin zones (qBZ) formed when the system's periodicity becomes ill-defined. Comparing our measurements with theoretical models, we provide the first experimental mapping of the qBZs as a function of angular alignment. Our results establish a direct experimental link between moiré interference and qBZ formation, opening new avenues for engineering electronic structures in multi-aligned 2D heterostructures.

cond-mat.mes-hall↗

arXiv2025

Spin injection and emission helicity switching in a 2D perovskite/WSe2 heterostructure

The initialization and control of a long-lived spin population in lead halide perovskites are prerequisites for their use in spintronic applications. Here, we demonstrate circular polarization of the interlayer exciton emission in a (BA)2PbI4/WSe2 monolayer heterostructure. The helicity of this emission is controlled by tuning the energy of the excitation laser through the manifold of exciton resonances of the WSe2 monolayer, together with an emerging interlayer absorption feature of the heterostructure. Theoretical calculations show that this resonance arises from hybridized (BA)2PbI4/WSe2 states in the valence band. This hybrid character enables its observation in both linear absorption and ultrafast pump-probe spectroscopies, and plays a key role in controlling the sign of the helicity of the interlayer exciton emission. The tunable spin polarization demonstrated here, with the WSe2 monolayer effectively acting as a tunable spin filter, represents an important step toward the use of 2D perovskites in opto-spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Sliding Disassembly of van der Waals Heterostructures

Many recent advances in our understanding of two-dimensional (2D) electron systems stem from van der Waals (vdW) heterostructures. The assembly process relies on the weak bonding across interfaces between layered vdW compounds, making it possible to construct exceptionally clean heterostructures from chemically and structurally distinct materials - a challenging task for traditional thin-film growth techniques. Here we demonstrate an additional, dynamic degree of freedom afforded by vdW interfaces, wherein we use microstructured polymer stamps to disassemble and reconfigure vdW heterostructures by sliding. We apply this technique to alter the dielectric environment of monolayer graphene, perform scanning tunneling microscopy on semiconducting and air-sensitive monolayers, and manipulate strain-sensitive moiré materials. Together these demonstrations suggest a new paradigm for assembling and dynamically modifying van der Waals heterostructures, with the potential to reveal new insights into 2D electron systems.

cond-mat.mes-hall↗

arXiv2025

Does Moire Matter? Critical Moire Dependence with Quantum Fluctuations in Graphene Based Integer and Fractional Chern Insulators

Rhombohedral multilayer graphene has emerged as a powerful platform for investigating flat-band-driven correlated phenomena, yet most aspects remain not understood. In this work, we systematically study the moire-dependent band topology in rhombohedral hexalayer graphene. For the first time we demonstrate that the moire twist angle plays a crucial role in the formation of the moire Chern insulators in rhombohedral hexalayer graphene/hexagonal boron nitride (RHG/hBN) moire superlattices. In the moire-distant regime at filling factor v = 1, only systems with a twist angle θ < 1.1° exhibit an integer moire Chern insulator, while the fractional Chern insulator at v = 2/3 requires smaller twist angle to be stabilized. Our theoretical modelling, which includes quantum fluctuations and exact diagonalization results, suggests that mean-field theory, which has been widely adopted, does not explain the twist-angle dependence of the v = 1 phase diagram, and that correlation effects are crucial. Moreover, we realize two distinct stacking configurations ( /Xi=0 and /Xi=1) between graphene and hBN, and find that both cases can yield a Chern insulator at v = 1. Our experimental work upends the current mean-field paradigm, illuminates how quantum fluctuations and moiré effects shape the RHG/hBN phase diagram, and paves the way for future understanding and engineering of topological correlated states in rhombohedral graphene moire systems.

cond-mat.mes-hall↗

arXiv2025

Atomically-resolved exciton emission from single defects in MoS$_2$

Understanding how atomic defects shape the nanoscale optical properties of two-dimensional (2D) semiconductors is essential for advancing quantum technologies and optoelectronics. Using scanning tunneling spectroscopy (STS) and luminescence (STML), we correlate the atomic structure and optical fingerprints of individual defects in monolayer MoS$_2$. A bilayer of hexagonal boron nitride (hBN) effectively decouples MoS$_2$ from the graphene substrate, increasing its band gap and extending the defect charge state lifetime. This enables the observation of sharp STML emission lines from MoS$_2$ excitons and trions exhibiting nanoscale sensitivity to local potential fluctuations. We identify the optical signatures of common point defects in MoS$_2$: sulfur vacancies (Vac$_\text{S}^-$), oxygen substitutions (O$_\text{S}$), and negatively charged carbon-hydrogen complexes (CH$_\text{S}^-$). While Vac$_\text{S}^-$ and O$_\text{S}$ only suppress pristine excitonic emission, CH$_\text{S}^-$ generate defect-bound exciton complexes ($A^-X$) about 200\,meV below the MoS$_2$ exciton. Sub-nanometer-resolved STML maps reveal large spectral shifts near charged defects, concurrent with the local band bending expected for band-to-defect optical transitions. These results establish an atomically precise correlation between structure, electronic states, and optical response, enabling deterministic engineering of quantum emitters in 2D materials.

cond-mat.mtrl-sci↗

arXiv2025

Optical Injection and Detection of Long-Lived Interlayer Excitons in van der Waals Heterostructures

Interlayer excitons in semiconducting bilayers separated by insulating hBN layers constitute a promising platform for investigation of strongly correlated bosonic phases. Here, we report an optical method for the generation and characterization of long-lived interlayer excitons. We confirm the presence of tightly bound interlayer excitons by measuring 1s and 2s intralayer excitons in each layer concurrently. Using a pump-probe technique, we find interlayer exciton lifetimes up to 8.8 $μ$s, increasing with the thickness of the hBN. With optical access to long-lived interlayer excitons, our approach provides a new route to explore degenerate Bose--Fermi mixtures of excitons and itinerant electrons with high spatial and temporal resolution.

cond-mat.mes-hall↗

arXiv2025

Mapping the moiré potential in multi-layer rhombohedral graphene

Rhombohedral graphene (rG) aligned with hexagonal boron nitride (hBN) has been shown to host flat bands that stabilize various strongly correlated quantum phases, including Mott insulators, integer, and fractional quantum anomalous Hall phases. In this work, we use scanning tunneling microscopy/spectroscopy (STM/STS) to visualize the dispersion of flat bands with doping and applied displacement fields in a hBN-aligned rhombohedral trilayer graphene (rtG)/hBN moiré superlattice. In addition to the intrinsic flat bands of rtG induced by the displacement field, we observe low-energy features originating from moiré potential-induced band folding. Real-space variations of the spectroscopic features allow us to quantify the spatial structure of the moiré potential at the rtG/hBN interface. Importantly, we find that accurately capturing the moiré site-dependent spectra requires incorporating a moiré potential acting on the top graphene layer with a sign opposite to that of the bottom layer into the continuum model. Our results thus provide key experimental and theoretical insights into understanding the role of the moire superlattice in rG/hBN heterostructures.

cond-mat.mes-hall↗

arXiv2025

Thermoelectric effect at the quantum Hall-superconductor interface

The interfaces of quantum Hall insulators with superconductors have emerged as a promising platform to realise interesting physics that may be relevant for topologically protected quantum computing. However, these interfaces can host other effects which obscure the detection of the desired excitations. Here we present measurements of the thermoelectric effect at the quantum Hall-superconductor interface. We explain the heat transport by considering the formation of a hotspot at the interface, which results in a non-equilibrium distribution of electrons that can propagate across the superconductor through vortex cores. The observed thermoelectric effect results in a voltage which changes sign on quantum Hall plateaus and responds to the rearrangement of vortices in the wire. These observations highlight the complex interplay of thermal and charge phenomena at the quantum Hall -- superconductor interfaces and should be considered when interpreting transport measurements in similar systems.

cond-mat.mes-hall↗

arXiv2025

Investigating the Ferroelectric Potential Landscape of 3R-MoS$_2$ through Optical Measurements

In recent years, sliding ferroelectricity has emerged as a topic of significant interest due to its possible application in non-volatile, reconfigurable storage devices. This phenomenon is unique to two-dimensional van der Waals materials, where out-of-plane ferroelectric polarization switching is induced by relative in-plane sliding of adjacent layers. The intrinsic stacking order influences the resulting polarization, creating distinct polarization regions separated by domain walls. These regions and the domain walls can be manipulated using an applied vertical electric field, enabling a switchable system that retains the environmental robustness of van der Waals materials under ambient conditions. This study investigates 3R-MoS$_2$ using various optical measurement techniques at room temperature. The spatially resolved optical measurements reveal apparent signal changes corresponding to different ferroelectric stacking orders and variations in layer count. Our findings demonstrate that fast optical mapping at room temperature is a reliable method for probing ferroelectric potential steps in 3R-stacked MoS$_2$ samples, thereby facilitating the identification of the ferroelectric configuration. This approach does not require a conductive substrate or an electrical contact to the sample, making it more versatile than traditional atomic force probe techniques.

cond-mat.mtrl-sci↗

arXiv2025

Non-ohmic to ohmic crossover in the breakdown of the quantum Hall states in graphene under broadband excitations

Graphene, through the coexistence of large cyclotron gaps and small spin and valley gaps, offers the possibility to study the breakdown of the quantum Hall effect across a wide range of energy scales. In this work, we investigate the breakdown of the QHE in high-mobility graphene Corbino devices under broadband excitation ranging from DC up to 10 GHz. We find that the conductance is consistently described by variable range hopping (VRH) and extract the hopping energies from both temperature and field-driven measurements. Using VRH thermometry, we are able to distinguish between a cold and hot electron regime, which are dominated by non-ohmic VRH and Joule heating, respectively. Our results demonstrate that breakdown in the quantum Hall regime of graphene is governed by a crossover from non-ohmic, field-driven VRH to ohmic, Joule-heating-dominated transport.

cond-mat.mes-hall↗

arXiv2025

Twist-tuned exchange and hysteresis in a bilayer van der Waals magnet

Moiré superlattices in twisted bilayers enable profound reconstructions of the electronic bandstructure, giving rise to correlated states with remarkable tunability. Extending this paradigm to van der Waals magnets, twisting creates spatially varying interlayer exchange interactions that stabilize emergent spin textures and the coexistence of ferromagnetic and antiferromagnetic domains. Here, we demonstrate the emergence of robust magnetic hysteresis in bilayer CrSBr upon twisting by an angle of ~ 3°. This is observed as the corresponding hysteretic evolution of the exciton energy, that directly correlates with the bilayer magnetic state, in magnetic field dependent photoluminescence measurements. A two-sublattice model captures this behavior, attributing it to the twist-induced reduction of interlayer exchange that stabilizes both parallel and antiparallel spin configurations across a broad field range. Comparison with experiment enables quantitative extraction of the effective exchange strength. Remarkably, the system exhibits coherent averaging across the moiré supercell, yielding an effective monodomain response characterized by switching into the antiferromagnetic state, rather than forming spin textures or fragmented domains. Spatially resolved measurements further uncover local variations in hysteresis loops, consistent with position-dependent modulation of the average exchange interaction. Our results establish twist engineering as a powerful route to programmable magnetic memories in two-dimensional magnets, harnessing the robustness of antiferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2025

Skyrmionic Transport and First Order Phase Transitions in Twisted Bilayer Graphene Quantum Hall Ferromagnet

Large-angle twisted bilayer graphene (TBLG) realizes a multicomponent quantum Hall (QH) platform of spin, valley and layer pseudospins with strong Coulomb interaction-driven symmetry broken phases. Here, we investigate the low energy Landau-level spectrum of layer-decoupled TBLG and identify skyrmion-textured charged excitations and a field-induced insulating transition to an intervalley coherent state at zero-filling factor. Symmetric potential difference perpendicular to TBLG demonstrated layer coherent population of ground states with uniform energy barriers, while the charge imbalance in the layers at finite displacement field led to multidomain nucleation and a pronounced hysteresis in the exchange-dominated transport regime suggesting first order phase transitions between different QH ferromagnetic ground states.

cond-mat.mes-hall↗