arXiv subjects

Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 433 records · Page 24Linked to original sources

arXiv2025

A Hierarchy of Superconductivity and Topological Charge Density Wave States in Rhombohedral Graphene

Superconductivity and the quantum Hall effect are conventionally regarded as mutually exclusive: superconductivity is suppressed by magnetic fields, whereas the quantum Hall effect relies on them. Here we report a striking exception, where an unconventional superconducting phase is stabilized by an out-of-plane magnetic field and coexists with a re-entrant integer quantum Hall (RIQH) effect in moiré-less rhombohedral hexalayer graphene. The re-entrant quantum Hall state, arising from a bubble-like charge density wave (CDW), provides a natural backdrop for the emergence of superconductivity. Angle-resolved transport reveals that the field-stabilized superconducting phase occupies the same density--displacement-field regime as a stripe-ordered phase at zero field, yet only develops once the stripe is replaced by a bubble-like CDW at finite field. These findings demonstrate a decisive role of CDW order in stabilizing superconductivity in rhombohedral graphene, establishing a new paradigm for the interplay between superconductivity and quantum Hall physics.

cond-mat.mes-hall↗

arXiv2025

Slow quasiparticle dynamics and anyonic statistics in a fractional quantum Hall Fabry-Pérot interferometer

Anyons are particles with fractional exchange statistics that emerge as elementary excitations of fractional quantum Hall phases. Experimentally, their exchange statistics can be measured in the edge-state Fabry-Pérot interferometer. In these devices, the presence of $N_{qp}$ localized anyons in the bulk contributes a phase $N_{qp}θ_a$ to the interference signal. Here we report the observation of large, hysteretic phase jumps in a monolayer graphene Fabry-Pérot interferometer at $ν=1/3$. When the filling factor is increased from $ν<1/3$ towards the center of the plateau, we observe phase slips with magnitude $Δθ\approx 2π/3$, consistent with the addition of individual quasiparticles to the interferometer bulk. In contrast to prior work, however, the phase slips occur as instantaneous jumps in the interference signal, indicative of quasiparticle equilibration times exceeding 20 minutes. We use this long timescale to investigate the effect of changes in interferometer area $A_I$ and $N_{QP}$ independently at fixed magnetic field, revealing a striking memory effect in the phase slip magnitude. In particular, as the $ν=1/3$ plateau is approached from higher filling, we observed phase slips with $Δθ$ significantly larger than $2π/3$ over the same range of gate voltage where quantized jumps are seen for increasing $ν$. We discuss this asymmetry in terms of bulk-edge coupling of quasiparticles localized near the edge or in the bulk, and argue that this effect can be qualitatively reconciled with theoretical expectations for strongly interacting quasiparticles in the presence of weak disorder and strongly nonequilibrium charge dynamics. Our results highlight the key role played by charge dynamics on signatures of the anyon phase, and demonstrate that fractional quasiparticles can be indefinitely localized in nonequilibrium configurations.

cond-mat.mes-hall↗

arXiv2025

Visualizing Electronic Structure of Twisted Bilayer MoTe2 in Devices

The pursuit of emergent quantum phenomena lies at the forefront of modern condensed matter physics. A recent breakthrough in this arena is the discovery of the fractional quantum anomalous Hall effect (FQAHE) in twisted bilayer MoTe2 (tbMoTe2), marking a paradigm shift and establishing a versatile platform for exploring the intricate interplay among topology, magnetism, and electron correlations. While significant progress has been made through both optical and electrical transport measurements, direct experimental insights into the electronic structure - crucial for understanding and modeling this system - have remained elusive. Here, using spatially and angle-resolved photoemission spectroscopy (μ-ARPES), we directly map the electronic band structure of tbMoTe2. We identify the valence band maximum, whose partial filling underlies the FQAHE, at the K points, situated approximately 150 meV above the Γ valley. By fine-tuning the doping level via in-situ alkali metal deposition, we also resolve the conduction band minimum at the K point, providing direct evidence that tbMoTe2 exhibits a direct band gap - distinct from all previously known moire bilayer transition metal dichalcogenide systems. These results offer critical insights for theoretical modeling and advance our understanding of fractionalized excitations and correlated topological phases in this emergent quantum material.

cond-mat.mes-hall↗

arXiv2025

Non-monotonic band flattening near the magic angle of twisted bilayer MoTe$_2$

Twisted bilayer MoTe$_2$ (tMoTe$_2$) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy (ARPES) measurements of tMoTe$_2$, revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory (DFT) captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the K point exhibits a non-monotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe$_2$ as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.

cond-mat.mes-hall↗

arXiv2025

Tunable spin-orbit splitting in bilayer graphene/WSe$_2$ quantum devices

Bilayer graphene (BLG)-based quantum devices represent a promising platform for emerging technologies, such as quantum computing and spintronics. However, their intrinsically weak spin-orbit coupling (SOC) complicates spin and valley manipulation. Integrating BLG with transition metal dichalcogenides (TMDs) enhances the SOC via proximity effects. While this enhancement has been demonstrated in 2D-layered structures, 1D and 0D nanostructures in BLG/TMD remain unrealized, with open questions regarding SOC strength and tunability. Here, we investigate quantum point contacts and quantum dots in two BLG/WSe$_2$ heterostructures with different stacking orders. Across multiple devices, we reproducibly demonstrate spin-orbit splitting up to 1.5 meV - more than 1 order of magnitude higher than in pristine BLG. Furthermore, we show that the induced SOC can be tuned in situ from its maximum value to near-complete suppression via the perpendicular electric field. This enhancement and in situ tunability establish the SOC as a control mechanism for dynamic spin and valley manipulation.

cond-mat.mes-hall↗

arXiv2025

Room Temperature Single Photon Detection at 1550 nm using van der Waals Heterojunction

Single-photon detectors (SPDs) are crucial in applications ranging from space, biological imaging to quantum communication and information processing. The SPDs that operate at room temperature are of particular interest to broader application space as the energy overhead introduced by the cryogenic cooling can be avoided. Although silicon-based single photon avalanche diodes (SPADs) are well matured and operate at room temperature, the bandgap limitation restricts their operation at telecommunication wavelength (1550 nm) and beyond. InGaAs-based SPADs, on the other hand, are sensitive to 1550 nm photons, but suffer from relatively lower efficiency, high dark count rate, afterpulsing probability, and pose hazards to the environment from the fabrication process. In this work, we demonstrate how we can leverage the properties of nanomaterials to address these challenges and realise a room temperature single-photon detector capable of operating at 1550 nm. We achieve this by coupling a low bandgap ($\sim 350~meV$) absorber (black phosphorus) to a sensitive van der Waals probe that is capable of detecting discrete electron fluctuation. We optimize the device for operation at $1550~nm$ and demonstrate an overall quantum efficiency of $21.4\%$ (estimated as $42.8\%$ for polarized light), and a minimum dark count of $\sim 720~Hz$ at room temperature.

quant-ph↗

arXiv2025

Magic continuum in multi-moiré twisted trilayer graphene

Moiré lattices provide a highly tunable platform for exploring the interplay between electronic correlations and band topology. Introducing a second moiré pattern extends this paradigm: interference between the two moiré patterns produces a supermoiré modulation, opening a route to further tailor electronic properties. Twisted trilayer graphene generally exemplifies such a system: two distinct moiré patterns arise from the relative twists between adjacent graphene layers. Here, we report the observation of correlated phenomena across a wide range of twisted trilayer graphene devices whose twist angles lie along two continuous lines in the twist-angle parameter space. Depending on the degree of lattice relaxation, twisted trilayer graphene falls into two classes: moiré polycrystals, composed of periodic domains with locally commensurate moiré order, and moiré quasicrystals, characterized by smoothly varying local moiré configurations. In helically twisted moiré polycrystals, we observe an anomalous Hall effect, consistent with topological bands arising from domains with broken $xy$-inversion symmetry. In contrast, superconductivity appears generically in our moiré quasicrystals. A subset of these systems exhibits signatures of spatially modulated superconductivity, which we attribute to the supermoiré structure. Our findings uncover the organizing principles of the observed correlated phases in twisted trilayer graphene, highlight the critical roles of the supermoiré modulation and lattice relaxation, and suggest a broader framework in which magic conditions arise not as isolated points but as extended manifolds within the multi-dimensional twist-angle space of complex moiré materials.

cond-mat.mes-hall↗

arXiv2025

Family of Unconventional Superconductivities in Crystalline Graphene

Unconventional superconductors exhibit multiple broken symmetries and exceed the range of the Bardeen-Cooper-Schrieffer (BCS) theory. For instance, time-reversal symmetry can be broken in addition to the gauge symmetry, resulting in superconductors that can be enhanced or induced by a magnetic field. However, such unconventional superconductivities are more vulnerable to impurities than their BCS counterparts, requiring highly ordered and clean material systems to observe them. Crystalline rhombohedral multilayer graphene is a promising platform to explore unconventional superconductivity due to its superior material quality and gate-tunable strong correlation effects. Here we report transport measurements of rhombohedral tetralayer and pentalayer graphene, where a spectrum of superconductivities in a clean limit are observed. Three of them (SC2-4) show highly unusual enhancements by magnetic fields: 1. SC2 is strengthened by an in-plane field; 2. SC3 is boosted by a small out-of-plane field; 3. SC4 is induced by an in-plane field. All these superconductors are robust against an in-plane field up to 8.5 Tesla, exceeding the Pauli limit of conventional superconductors by tens of times and suggesting their unconventional nature. Moreover, we observed that proximitized spin-orbit coupling generates a plethora of new superconductors in the phase diagram, while maintaining the high quality of bare rhombohedral graphene. Our work establishes a family of new superconductors in rhombohedral multilayer graphene, which also provides an ideal platform to engineer non-Abelian quasiparticles by proximitizing with quantum anomalous Hall states existing in the same material system.

cond-mat.mes-hall↗

arXiv2025

Highly tunable band structure in ferroelectric R-stacked bilayer WSe$_2$

Transition metal dichalcogenide homobilayers unite two frontiers of quantum materials research: sliding ferroelectricity, arising from rhombohedral (R) stacking, and moiré quantum matter, emerging from small-angle twisting. The spontaneous polarization of ferroelectric R-stacked homobilayers produces a highly tunable band structure, which, together with strain-induced piezoelectricity, governs the topology and correlated electronic phases of twisted bilayers. Here we present a systematic low-temperature optical spectroscopy study of R-stacked bilayer WSe$_2$ to quantitatively establish its fundamental electronic and ferroelectric properties. Exciton and exciton-polaron spectroscopy under doping reveals a pronounced electron-hole asymmetry that confirms type-II band alignment, with the conduction and valence band edges located at the $Λ$ and K valleys, respectively. Through distinct excitonic responses and tunable interlayer-intralayer exciton hybridization under displacement fields, we uncover the coexistence of AB and BA ferroelectric domains. Using exciton-polarons as a probe, we directly measure the intrinsic polarization field and extract the interlayer potential. Finally, we demonstrate electric-field-driven symmetric switching of the valence band maximum, attributed to ferroelectric domain switching. These results provide a complete experimental picture of the band alignment, spontaneous polarization field, and domain dynamics of R-stacked WSe$_2$, establishing key parameters to understand twisted bilayers and enabling new ferroelectric and excitonic device opportunities.

cond-mat.mtrl-sci↗

arXiv2025

Probing the Nanoscale Excitonic Landscape and Quantum Confinement of Excitons in Gated Monolayer Semiconductors

Engineering and probing excitonic properties at the nanoscale remains a central challenge in quantum photonics and optoelectronics. While exciton confinement via electrical control and strain engineering has been demonstrated in 2D semiconductors, substantial nanoscale heterogeneity limits the scalability of 2D quantum photonic device architectures. In this work, we use cathodoluminescence spectroscopy to probe the excitonic landscape of monolayer $WS_2$ under electrostatic gating. Exploiting the high spatial resolution of the converged electron beam, we resolve a homojunction arising between gated and ungated regions. Moreover, we reveal an exciton confinement channel arising from an unconventional doping mechanism driven by the interplay between the electron beam and the applied gate fields. These findings offer new insights into the optoelectronic behavior of monolayer semiconductors under the combined influence of electron-beam excitation and electrostatic gating. Our approach provides a pathway for exciton manipulation at the nanoscale and opens opportunities for controlling quantum-confined exciton transport in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2025

Optical Switching of Moiré Chern Ferromagnet

Optical manipulation of quantum matter offers a non-contact, high-precision and fast control. Fractional Chern ferromagnet states in moiré superlattices are promising for topological quantum computing, but an effective optical control protocol has remained elusive. Here, we demonstrate robust optical switching of integer and fractional Chern ferromagnets in twisted MoTe2 bilayers using circularly polarized light. Highly efficient optical manipulation of spin orientations in the topological ferromagnet regime is realized at zero field using a pump light power as low as 28 nanowatts per square micrometer. Utilizing this optically induced transition, we also demonstrate magnetic bistate cycling and spatially resolved writing of ferromagnetic domain walls. This work establishes a reliable and efficient optical control scheme for moiré Chern ferromagnets, paving the way for dissipationless spintronics and quantized Chern junction devices.

cond-mat.mes-hall↗

arXiv2025

Optical Control of Integer and Fractional Chern Insulators

Optical control of topology, particularly in the presence of electron correlations, is a fascinating topic with broad scientific and technological impact. Twisted MoTe$_2$ bilayer (tMoTe$_2$) is a newly discovered zero-field fractional Chern insulator (FCI), exhibiting the fractionally quantized anomalous Hall (FQAH) effect. Since the chirality of the edge states and sign of the Chern number are determined by the underlying ferromagnetic polarization, manipulation of ferromagnetism would realize control of the CI/FCI states. Here, we demonstrate control and switching of ferromagnetic polarization, and thus the CI and FCI states by circularly polarized optical pumping in tMoTe$_2$. At low optical excitation power, we achieve on-demand preparation of ferromagnetic polarization by optical training, i.e., electrically tuning the system from non-ferromagnetic to desirable ferromagnetic states accompanied with helicity-selective optical pumping. With increased excitation power, we further realize direct optical switching of ferromagnetic polarization at a temperature far below the Curie temperature. Both optical training and direct switching of ferromagnetism are most effective near CI/FCI states, which we attribute to a gap enhanced valley polarization of photo-injected holes. We show that the magnetization can be dynamically switched by modulating the helicity of optical excitation. Spatially resolved measurements further demonstrate optical writing of a ferromagnetic, and thus a CI (or FCI) domain. Our work realizes precise optical control of a topological quantum many-body system with potential applications in topological spintronics, quantum memories, and creation of exotic edge states by programmable patterning of integer and fractional QAH domains.

cond-mat.mes-hall↗

arXiv2025

Optical control over topological Chern number in moiré materials

Controlling quantum matter with light offers a promising route to dynamically tune its many-body properties, ranging from band topology to superconductivity. However, achieving such optical control for strongly correlated electron systems in the steady-state has remained elusive. Here, we demonstrate all-optical switching of the spin-valley degree of freedom of itinerant ferromagnets in twisted MoTe2 homobilayers. This system uniquely features flat valley-contrasting Chern bands and exhibits a range of strongly correlated phases at various moiré lattice fillings, including Chern insulators and ferromagnetic metals. We show that the spin-valley orientation of all of these phases can be dynamically reversed by resonantly exciting the attractive polaron transition with circularly-polarized light. These findings not only constitute the first direct evidence for non-thermal switching of a ferromagnetic spin state at zero magnetic field, but also demonstrate the possibility of dynamical control over topological order parameter, paving the way for all-optical generation of chiral edge modes and topological quantum circuits.

cond-mat.mes-hall↗

arXiv2025

3D microwave imaging of a van der Waals heterostructure

Van der Waals (vdW) heterostructures offer a tunable platform for the realization of emergent phenomena in layered electron systems. While scanning probe microscopy techniques have proven useful for the characterization of surface states and 2D crystals, the subsurface imaging of quantum phenomena in multi-layer systems presents a significant challenge. In 3D heterostructures, states that occupy different planes can simultaneously contribute to the signal detected by the microscope probe, which complicates image analysis and interpretation. Here we present a quantum imaging technique that offers a glimpse into the third dimension by resolving states out of plane: it extracts the charge density landscape of individual atomic planes inside a vdW heterostructure, layer by layer. As a proof-of-concept, we perform layer-resolved imaging of quantum Hall states and charge disorder in double-layer graphene using milliKelvin microwave impedance microscopy. Here the discrete energy spectrum of the top layer enables transmission of microwaves through gapped states, thus opening direct access to quantum phases in the subsurface layer. Resolving how charge is distributed out-of-plane offers a direct probe of interlayer screening, revealing signatures of negative quantum capacitance driven by many-body correlations. At the same time, we extract key features of the band structure and thermodynamics, including gap sizes. Notably, by imaging the charge distribution on different atomic planes beneath the surface, we shed light on the roles of surface impurities and screening on the stability of fractional quantum Hall states. We also show that the uppermost graphene layer can serve as a top gate: This unlocks access to a wide range of phenomena that require displacement field control, from fractional Chern insulators in Moiré superlattices to correlated states in multilayer graphene.

cond-mat.mes-hall↗

arXiv2025

Strong Correlation Driven Quadrupolar to Dipolar Exciton Transitions in a Trilayer Moiré Superlattice

The additional layer degree of freedom in trilayer moiré superlattices of transition metal dichalcogenides enables the emergence of novel excitonic species, such as quadrupolar excitons, which exhibit unique excitonic interactions and hold promise for realizing intriguing excitonic phases and their quantum phase transitions. Concurrently, the presence of strong electronic correlations in moiré superlattices, as exemplified by the observations of Mott insulators and generalized Wigner crystals, offers a direct route to manipulate these new excitonic states and resulting collective excitonic phases. Here, we demonstrate that strong exciton-exciton and electron-exciton interactions, both stemming from robust electron correlations, can be harnessed to controllably drive transitions between quadrupolar and dipolar excitons. This is achieved by tuning either the exciton density or electrostatic doping in a trilayer semiconducting moiré superlattice. Our findings not only advance the fundamental understanding of quadrupolar excitons but also usher in new avenues for exploring and engineering many-body quantum phenomena through novel correlated excitons in semiconducting moiré systems.

cond-mat.mes-hall↗

arXiv2025

Photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers featuring sharp excitonic lines and diverse grain structures

Recent studies have demonstrated that using h-BN as a substrate for the growth of transition metal dichalcogenides can significantly reduce excitonic linewidths. However, many other optical parameters still require optimization. In this work, we present a detailed study of the low-temperature photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers on h-BN substrates, comparing them to state-of-the-art exfoliated monolayers encapsulated in h-BN. We demonstrate that a quantitative comparison between samples requires accounting for interference effects and Purcell enhancement or suppression of the emission. By accounting for these effects in both photoluminescence and Raman signals, we show that the overall intrinsic luminescence efficiency is proportional to the sample coverage. Consequently, we find that exciton diffusion and edge effects are negligible in spectroscopy of MBE-grown samples, even for nanometer-sized crystals.

cond-mat.mes-hall↗

arXiv2025

Thermal transport mapping in twisted double bilayer graphene

Two-dimensional (2D) materials have attracted significant interest due to their tunable physical properties when stacked into homo- and hetero-structures. Twisting adjacent layers introduces moiré patterns that strongly influence the material electronic and thermal behavior. In twisted graphene systems, the twist angle critically alters phonon transport, leading to reduced thermal conductivity compared to Bernal-stacked configurations. However, experimental investigations into thermal transport in twisted structures remain limited. Here, we study the local thermal properties of twisted double bilayer graphene (TDBG) using Scanning Thermal Microscopy (SThM). We find an increase in thermal resistance of $0.3 \pm 0.1 \times 10^6 KW^{-1}$ compared to untwisted bilayers, attributed to changes in both intrinsic thermal conductivity and the tip-sample interface. These results, supported by analytical modeling, provide new insight into thermal transport mechanisms in twisted 2D systems and offer a pathway toward thermal engineering in twistronic devices.

cond-mat.mes-hall↗

arXiv2025

Dynamically tunable hydrodynamic transport in boron nitride-encapsulated graphene

Over the past decade, graphene has emerged as a promising candidate for exploring the viscous nature of electronic flow facilitated by the availability of extremely high-quality devices employing a graphene channel encapsulated within dielectric layers of hexagonal boron nitride (hBN). However, the level of disorder in such systems is mainly determined by the device fabrication protocols, making it impossible to obtain a tunability between the impurity-dominated and the viscous transport within the same device. In this work, using a combination of ultraviolet (UV) radiation and gate electric field, we have demonstrated a dynamic modulation of charge hydrodynamics, quantified in the thermal and electrical transport by the extent of departure from the Wiedemann-Franz (WF) Law in monolayer graphene devices at room temperature. We achieved this by tuning the disorder level continuously and reversibly using UV light to create transient trap states in the encapsulating hBN dielectric. With progressive UV radiation, we observed a dramatic increase in the momentum-relaxing scattering relative to that between the electrons and also the Lorentz number, by nearly a factor of ten, with increasing disorder, thereby approaching the restoration of the WF law in highly disordered graphene. Our experiments outline a potent strategy to tune the fundamental mechanism of charge flow in state-of-the-art graphene devices.

cond-mat.mes-hall↗