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Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 199 records · Page 11Linked to original sources
arXiv2026

Anomalous magneto-optical response at $\mathrm{RuO_2 / WSe_2}$ van der Waals interface

Ruthenium dioxide ($\mathrm{RuO_2}$) has been proposed as an altermagnetic candidate, although its magnetic ground state remains controversial. Here, we probe weak interfacial magnetic states at the surface of (001)-oriented $\mathrm{RuO_2}$ films using the magnetic proximity effect (MPE) in a van der Waals heterostructure consisting of monolayer tungsten diselenide ($\mathrm{WSe_2}$) atop $\mathrm{RuO_2}$. Temperature-dependent magneto-optical spectroscopy reveals an anomalous excitonic energy shift and a deviation from conventional Varshni behavior below 55 K that are absent in an encapsulated $\mathrm{WSe_2}$ control sample. The anomalous shift reverses sign upon field cooling with opposite magnetic field polarity, indicating a magnetic origin. Polarization-resolved measurements further show a nearly field-independent and fluctuating valley splitting in $\mathrm{WSe_2 / RuO_2}$ in strong contrast to the conventional linear Zeeman splitting observed in the control bare $\mathrm{WSe_2}$ sample. These results suggest that the valley states are governed predominantly by interfacial exchange fields associated with weak surface magnetic states in $\mathrm{RuO_2}$, which do not produce a conventional linear Zeeman response within the applied magnetic field range. Importantly, this approach enables direct optical probing of emergent surface magnetism without introducing an additional ferromagnetic layer, positioning MPE-based optical probing as a tool for investigating weak surface magnetism and offering new possibilities for studying magnetic materials with controversial magnetic states.

cond-mat.mtrl-sci↗
arXiv2026

Propagating Collective Spin-valley Modes in Twisted WSe2

The emergence of neutral collective modes is a hallmark of correlated quantum phases but is often challenging to probe experimentally. In two-dimensional flatband systems, charge responses have been intensively investigated yet neutral excitations remain largely unexplored. In particular, intervalley coherent state (IVC) features a neutral Goldstone mode due to spontaneously broken valley U(1) symmetry. While IVC state has been proposed as a unifying theme across graphene and semiconductor based systems, its defining feature, the neutral Goldstone mode, remains elusive in experiment. Here we investigate space and time resolved transport of neutral modes in twisted WSe2 moire superlattices through a novel ultrafast imaging technique. We uncover two new propagating collective modes with very different velocities, which emerge near the van Hove singularity (VHS) in both intermediate (3.5 to 4 degree) and large (around 5 degree) angle twisted WSe2. The fast-propagating mode has a large speed of about 3 km/s and is consistent with a Goldstone mode for an IVC state, while the slow-moving mode is likely a gapped amplitude mode. They can be understood as the spin-valley analogues of collective modes of a superfluid, whose propagation is imaged for the first time in a condensed matter system. Our study demonstrates a powerful new approach for probing charge-neutral modes in quantum materials and offers key insights into the interplay between charge and spin-valley physics in moire superlattices.

cond-mat.mes-hall↗
arXiv2026

Trion Hall effect in electron-hole double layers

The realization of Coulomb coupled electron-hole double layers in 2D semiconductor heterostructures has enabled the thermodynamic and transport studies of equilibrium exciton fluids without a magnetic field. By doping the exciton fluid with additional electrons/holes, an equilibrium fluid of trions - three particle bound states of electrons and holes - further emerge, providing the platform to explore new transport phenomena associated with such composite particles. Here we report the observation of a Hall effect for trions in MoSe2/WSe2 heterobilayers, which support Coulomb-coupled electron and hole fluids with tunable densities. The Hall effect arises from a Lorentz force on trions under a perpendicular magnetic field. It is manifested in both Hall drag measurements and standard Hall effect measurements on just one of the semiconductor layers. For negatively charged trions, an electron Hall effect is observed even in a hole doped WSe2 monolayer due to the presence of trion drags. The trion Hall effect also disappears when the trions are ionized at elevated temperatures and/or high trion densities. Our work opens the door for realizing quantum oscillations and the quantum Hall effect for trions.

cond-mat.mes-hall↗
arXiv2026

Graphene Josephson diodes from inherent asymmetric disorder

Josephson diodes are non-reciprocal superconducting devices characterized by different switching currents depending on the current flow direction. They recently attracted considerable theoretical and experimental attention, in view of their possible application as rectifying elements in the field of superconducting electronics, and as probes to investigate symmetry breaking mechanisms in mesoscopic systems. In this work, we show that graphene Josephson junctions provide rectification of supercurrent with an efficiency exceeding 20%. The effect appears applying a mT out-of-plane magnetic field and is enhanced close to the nodes of the Fraunhofer interference pattern. Our theoretical model identifies long-range scattering potentials in the junction as the symmetry-breaking mechanism, which yields supercurrent rectification in highly transparent junctions. While graphene stands as an ultra-clean transmission medium, our work shows that unavoidable residual disorder in a clean two-dimensional system is sufficient to promote this effect. Tailoring of the inversion (mirror) symmetry breaking could be obtained via proper design of external gates.

cond-mat.supr-con↗
arXiv2026

Superconductivity near quarter and half filling of a strongly correlated triangular Hubbard band in twisted trilayer WSe2

The triangular-lattice Hubbard model is one of the paradigmatic settings for studying correlated electronic systems with geometric frustration that can give rise to a variety of phases. Recently, twisted bilayer TMD systems have emerged as attractive experimental platforms for realising such a model in the moderate-correlation regime. Alternating twisted trilayer TMD systems are expected to realise stronger correlations than bilayer systems owing to their mirror symmetry. Here we report the discovery of superconductivity near quarter- and half-filling, together with various correlated phases, in alternating twisted trilayer WSe2 (TTWSe2). At half filling, a robust correlated insulating state persists over a broad range of displacement fields, signalling the strongly correlated regime. Upon doping, superconductivity emerges over an extended region of the phase diagram. Near quarter filling, another superconducting state appears flanked by correlated metals. Our results establish the TTWSe2 system as a compelling platform for studying strongly correlated triangular-lattice Hubbard physics.

cond-mat.supr-con↗
arXiv2026

On-chip superconducting GHz RF reflectometry of the capacitance response in bilayer graphene

In dual-gated bilayer graphene, a perpendicular displacement field opens a band gap that modifies both the channel conductance and the electronic compressibility, motivating measurements that resolve resistive and capacitive responses on the same device. We integrate an hBN-encapsulated bilayer graphene heterostructure with an on-chip superconducting Nb lumped-element LC resonator and carry out RF reflectometry near 4.25 GHz. DC transport and finite-bias spectroscopy on the same device provide a transport reference. Top and bottom gates independently set the carrier density and displacement field. The DC and RF gate maps share the same gate-dependent features, with finite-bias measurements revealing a region of suppressed conductance whose bias extent grows with the displacement field, consistent with a field-induced gap. The gate-dependent resonance-frequency shift is converted to the effective capacitance seen by the resonator using an equivalent-circuit model. The capacitance shows a minimum near the conductance-suppressed region, consistent with reduced electronic compressibility in the gapped bilayer graphene, and exhibits an electron-hole asymmetry. The on-chip configuration probes the gate-dependent admittance of a dual-gated van der Waals heterostructure, providing capacitance-sensitive information that complements DC transport within a single device.

cond-mat.mes-hall↗
arXiv2026

Ising Dirac fermions across a topological phase transition

Dirac fermions have attracted significant interest due to their relativistic dispersions and close connections to topological physics, yet they are generally expected to be gapped in two-dimensional systems with strong Ising spin orbit coupling, making their realization in such materials an outstanding challenge. Here we report the emergence of six fold degenerate Dirac fermions in an Ising moire system across a quantum spin Hall transition in twisted WSe2. In a 3.65 degree device, we observe a quantum spin Hall phase at high electric fields with nearly quantized resistance h/(2e2), and a Dirac semimetal phase over a broad range of electric fields near zero field. Magnetotransport measurements of the Dirac phase exhibit a half-integer Landau fan sequence, characteristic of Dirac fermions, with six-fold degeneracy on the hole-doped side and two fold degeneracy on the electron-doped side. Temperature dependence shows weakly metallic behavior consistent with a semimetallic state. Our twist-angle-dependent transport measurements map out a complete phase diagram and identify a critical twist angle of 3.3 degree, establishing the phase boundary between the quantum spin Hall and Dirac semimetal regimes. Our work establishes a new route to realizing Dirac fermions in strongly spin orbit coupled moire systems through a topological phase transition, providing a promising platform for high mobility spintronics.

cond-mat.mes-hall↗
arXiv2026

Engineering electrically-switchable quantum anomalous Hall states by spin-orbit coupling

Nonvolatile gate-driven switching of quantum anomalous Hall (QAH) states in graphene moiré systems provides a promising route toward topological electronics based on chiral edge states. However, deliberate use of this switching mechanism requires control over both the magnetic properties and metastability of QAH states. While previous demonstrations mostly relied on the intrinsic magnetic energy landscape of moiré devices, here we show that this landscape can be engineered through proximity coupling to WSe2. We find that proximitizing twisted monolayer-bilayer graphene by WSe2 reshapes the magnetization reversals responsible for nonvolatile electrical switching of QAH states. We attribute this effect to the proximity-induced spin-orbit coupling (SOC), which can lock spin and valley and modify the magnetization of the competing states involved in switching compared with non-proximitized graphene systems. Our findings establish proximity-induced SOC as a new way to engineer magnetic properties and switchable magnetic states in graphene-based systems. We further demonstrate that strong magnetic metastability in tMBG allows the magnetic states to be gate-tuned between QAH and metallic regimes, and between QAH states with Chern numbers |C| = 2 and 1 without resetting the magnetic state. This functionality points toward new device architectures based on QAH chiral edge states.

cond-mat.mes-hall↗
arXiv2026

Tuning proximity-induced spin-orbit coupling in graphene/WSe$_{2}$ heterostructures

Recently, proximity-induced spin-orbit coupling (SOC) has been observed in heterostructures consisting of monolayer graphene (ML-G) and transition metal dichalcogenides (TMDCs) such as WSe$_{2}$. Successful tuning of SOC in graphene/WSe$_{2}$ heterostructures by applying mechanical pressure and electric fields was also demonstrated in previous studies. In addition, theoretical calculations predicted a strong dependence of the proximity-induced SOC on the twist angle between graphene and TMDC. Here, we put these predictions to experimental test in ML-G/ML-WSe$_{2}$/hBN-heterostructures, where the twist angle is determined by aligning fractured edges, and by crystallographic etching of graphene. By performing weak anti-localization measurements, we determine the strength of the Rasbha-type SOC ($λ_\mathrm{R}$) and the valley-Zeeman-type SOC ($λ_\mathrm{VZ}$). Our experiments confirm a strong twist angle dependence of the proximity-induced SOC in agreement with theoretical predictions. Finally, we demonstrate the tunability of the SOC strength via mechanical pressure, which is in agreement with earlier findings.

cond-mat.mes-hall↗
arXiv2026

Observation of a Mott quantum spin Hall insulator in twisted WSe2

Quantum spin Hall (QSH) insulators and Mott insulators are conventionally regarded as distinct insulating phases, arising from band topology and strong Coulomb interactions, respectively. Here, we report the observation of QSH edge transport in a magnetic-field-stabilized Mott insulating state at half filling of the second moire band in a 2.29 degree twisted WSe2 device. This state exhibits a resistance plateau identical to that of the single-particle QSH state at full filling of the first moire valence band, indicating the same number of helical edge channels. Electrical transport measurements reveal nearly quantized resistance that is insensitive to vertical electric field, out-of-plane magnetic field, and temperature below 5 K. Pronounced nonlocal transport and strong negative in-plane magnetoconductance further support helical edge conduction, establishing robust edge transport in the strongly correlated regime. Temperature-dependent Hall measurements reveal a characteristic temperature scale of approximately 10 K, corresponding to an energy scale of about 1 meV. Our results demonstrate that spin-conserved QSH edge states can persist in a half filled, strongly correlated insulating phase and under external magnetic field, opening a route toward interaction-resilient topological transport in moire quantum materials.

cond-mat.mes-hall↗
arXiv2026

Supermoiré Chern mosaic in helical trilayer WSe2

Helically twisted multilayers offer access to moiré physics beyond the single-superlattice paradigm, yet their correlated and topological transport properties remain largely unexplored in semiconductor moiré materials. Here we report magnetotransport measurements of helical trilayer WSe2, in which two coupled moiré patterns relax into a supermoiré landscape composed of inequivalent local topological domains with distinct electronic structures and unequal spatial areas. By electrostatic tuning, we identify a trilayer-hybridized regime where interactions and real-space reconstruction combine to generate a plethora of magnetic and topological states absent in the twisted bilayers. At moiré filling factor $ν$ = -1, we observe a ferromagnetic insulating state that is robust against magnetic field and accompanied by a non-quantized anomalous Hall response ~-4 kOhms. This behaviour is consistent with a time-reversal-symmetry-breaking supermoiré Chern mosaic, in which the Hall response arises from the non-cancelling contributions of local domains with opposite Chern character arranged by the relaxed structure. Under strong magnetic fields, a symmetry-broken Chern insulating state (C = 1) emerges near $ν$ = -2/3, displaying a much larger positive Hall response together with strongly enhanced longitudinal resistance, suggestive of field-reconstructed topological minibands and domain-boundary scattering. These results establish relaxed supermoiré semiconductor trilayers as a platform for spatially organized magnetism and topology beyond the bilayer limit.

cond-mat.str-el↗
arXiv2026

Strain-programmable exciton diffusion in moiré heterostructures

Moiré superlattices in van der Waals heterostructures have recently gained significant attention as an intriguing platform for studying correlated electronic systems and exotic excitonic properties. Previous reports, however, focused on creating and modulating moiré heterostructures through interlayer twisting or lattice constant mismatches, limiting controls on symmetry of heterostructures. In this work, we show that strain significantly alters the geometry of moiré superlattices by breaking the C3 rotational symmetry. We realize strain-induced moiré superlattices by intentionally regulating interlayer strain in WSe2-MoSe2 heterostructures, which is manifested by linearly polarized interlayer exciton emission coupled to the strain direction. Furthermore, interlayer exciton diffusion was preferentially guided along the stretched moiré superlattice orientations over a wide spatial range, reflecting the strain-modified moiré potentials. Our work highlights strain tuning as a versatile tool for designing moiré superlattices and programming excitonic transport, which opens pathways for van der Waals logic and information processing devices.

cond-mat.mes-hall↗
arXiv2026

Layer-parity-dependent interfacial coupling in Nb$_3$Cl$_8$/graphene van der Waals heterostructures

Strongly correlated two-dimensional systems provide compelling platforms for investigating exotic quantum phenomena. Niobium chloride (Nb$_3$Cl$_8$), a single-band Mott insulator, exhibits a remarkable out-of-plane polarization in its topmost layer that oscillates with layer parity, manifesting as an odd-even effect. Using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), this layer-parity-dependent polarization can be effectively characterized through surface morphology and potential mapping, enabling the unambiguous identification of different surface phases. We then fabricated dual-gate Hall devices by coupling different surface phases of Nb$_3$Cl$_8$ with monolayer graphene to investigate how the topmost-layer out-of-plane polarization influences interfacial coupling and the resulting transport behavior. Our results reveal significant phase-dependent variations in charge transfer, carrier densities, and hybridization gaps (25.2 meV for Phase 1 and 30.0 meV for Phase 2). Density functional theory calculations corroborate these experimental findings, showing that distinct out-of-plane polarizations in the topmost layer lead to different orbital overlaps and interfacial coupling strengths. These findings highlight the critical importance of surface polarization and orbital orientation in engineering the properties of strongly correlated van der Waals heterostructures.

cond-mat.mtrl-sci↗
arXiv2026

Layer-parity-defined surface polarization in Nb$_3$Cl$_8$ for excitonic modulation at van der Waals interfaces

The intrinsic symmetry breaking in the breathing kagome lattice of layered Nb$_3$Cl$_8$ provides a unique mechanism for realizing electrically polar surfaces. In each monolayer, the trimerization of Nb atoms breaks inversion and mirror symmetries, generating an out-of-plane electric dipole. The AB-stacked $α$ phase arranges adjacent layer dipoles antiferroelectrically, leaving the uncompensated surface polarization strictly governed by layer parity. Here, using atomic force microscopy operated in Kelvin probe force microscopy mode, we directly visualize layer-dependent polarization states in exfoliated Nb$_3$Cl$_8$ flakes and resolve a pronounced odd-even oscillation of the surface electrostatic potential. Beyond this parity-locked antiferroelectric order, we further identify intralayer polar domains in which local atomic reconstructions of the breathing kagome network reverse the out-of-plane dipole of the surface layer, producing ferroelectric-like stacking configurations. By interfacing monolayer MoSe$_2$ with Nb$_3$Cl$_8$, we demonstrate that these surface-polarization textures effectively modulate adjacent excitonic emission through domain-dependent interfacial band alignment and charge transfer. Our findings establish Nb$_3$Cl$_8$ as an intrinsic layer-polarized van der Waals platform and show that layer parity provides powerful structural degree of freedom for programming excitonic and optoelectronic responses at van der Waals interfaces.

cond-mat.mtrl-sci↗
arXiv2026

Layer-Polarization-Driven Metal-Insulator Transition in multi-band Graphene Moire' Superlattices

Graphene/hBN moiré superlattices provide a highly tunable platform for exploring emergent quantum phases in low-dimensional systems. Here, we investigate the moiré superlattice formed between hBN and ABA-stacked trilayer graphene (TLG), an inherently multi-band system. We demonstrate that the moiré potential is not merely a perturbation but a tool to hybridize the distinct massless and massive electronic sectors of TLG. By applying a perpendicular displacement field to tune layer polarization, we drive a fundamental reconstruction of the electronic band structure. Specifically, increasing the displacement field evolves the system from a multi-band regime to an effectively single-band regime at low energies, accompanied by a metal--insulator transition at the hole-doped secondary Dirac point. This transition originates from a redistribution of carriers across graphene layers that selectively enhances their coupling to the extrinsic moiré potential. Quantum capacitance measurements provide direct evidence for the suppression of the density of states at the hole-side secondary Dirac point, consistent with gap opening and the emergence of a displacement-field-tuned band gap. Theoretical calculations reproduce these observations and identify layer-selective coupling to the moiré potential as the underlying mechanism. These results demonstrate electrical control of an emergent insulating phase in a low-dimensional moiré system, and highlight that layer polarization and layer-selective coupling in multi-band moiré heterostructures provide a powerful route for engineering topological and correlated phases through band structure reconstruction and electron interactions.

cond-mat.mes-hall↗
arXiv2026

1/3 Fractional and Gapless Integer Quantum Anomalous Hall States in Rhombohedral Graphene

The fractional quantum anomalous Hall (FQAH) effect occurs in moiré superlattices in both twisted bilayer MoTe$_2$ and rhombohedral $n$-layer graphene aligned to hexagonal boron nitride (R$n$G/hBN) as a novel quantum phase driven by intertwined electron correlation and topology. Although several fractional states in the Jain sequence have been identified, the $1/3$ state, the most robust and fundamental state in conventional fractional quantum Hall (FQH) systems, was missing in either FQAH system. Determining whether it exists would have a major impact on understanding the mechanism of FQAH, especially in the theoretically still-debated R$n$G/hBN system. Here we report the FQAH effect at moiré filling factor $ν= 1/3$ in R$5$G/hBN moiré superlattice devices, through a combination of quantum capacitance and transport measurements. By tuning the displacement field, we observed a topological phase transition from a $1/3$ fractional Chern insulator (FCI) to a trivial charge density wave state. With the inclusion of the $1/3$ state, the FQAH states in R$5$G/hBN now exhibit a surprising level of particle-hole symmetry about half-filling, closely resembling the behavior of FQH states in the lowest Landau level. Additionally, we perform compressibility and transport measurements at a filling of one electron per moiré unit cell, $ν=1$, and also for $ν\lesssim 1$, where previous transport measurements displayed the extended quantum anomalous Hall (EQAH) effect. While our transport measurements show no change between the integer quantum anomalous Hall state (IQAH) and the EQAH region, compressibility measurements reveal a distinct transition from a gapped IQAH state to a gapless and highly compressible EQAH state. Our direct thermodynamic characterization of the rich phase diagram paves the way to engineering of anyon braiding and non-Abelian quasiparticles at zero magnetic field.

cond-mat.mes-hall↗
arXiv2026

Fermiology and the Candidate Chiral Superconductor in Rhombohedral Tetralayer Graphene

Chiral superconductivity, in which the phase of the superconducting order parameter winds in momentum space, has long been sought for its close link to topological superconductivity. Recent work reported a superconductor in rhombohedral multilayer graphene emerging from a time-reversal symmetry broken normal state, suggesting that it could be a chiral superconductor. However, the possibility of chirality depends on the symmetry and structure of the normal-state Fermi surface, which have not been directly measured. Here we measure quantum oscillations in rhombohedral tetralayer graphene over a broad range of the phase diagram, including the superconducting region. At densities well above the onset of superconductivity, we reproduce previously-reported oscillations consistent with a spin- and valley-polarized quarter metal with a single simply-connected Fermi pocket. As the carrier density is reduced, we find a transition to a complex "multitone" state that persists through the superconducting region. This state's spectrum of quantum oscillations is incompatible with a simply-connected quarter metal. The next-simplest candidate normal states suggested by our microscopic modeling (fully-polarized annular, nematic, and three-pocket states) are inconsistent with our measurements, albeit difficult to rule out entirely. The normal state is thus seen to be richer than previously envisaged, reshaping the search for the superconducting mechanism and the possible chirality of the pairing channel.

cond-mat.supr-con↗
arXiv2026

Fractional High-Chern Insulator in Twisted Rhombohedral Graphene

The realization of fractional Chern insulators opens up the possibility of exploring fractionally charged excitations and anyonic statistics in the absence of a magnetic field. A central question is whether lattice-based systems can give rise to radically new states, distinct from those observed in traditional fractional quantum Hall systems. In this work, we investigate a new type of moiré flat band system composed of Bernal bilayer graphene and rhombohedral tetralayer graphene. We discover an unprecedented richness of quantum anomalous Hall insulators with Chern numbers from C = 1 to C = 7 at v = 1 and around v = 3. Remarkably, we observe an exotic fractional Chern insulator with C = 7/3 around v = 2/3 which is beyond all known fractional Chern insulators described by either the Jain sequence or current high Chern theory. Our work expands the understanding of fractionally charged excitations beyond the Landau level basis and offers a new moire platform for exploring anyons.

cond-mat.mes-hall↗