arXiv subjects

K. Watanabe

Publications and source records attributed to K. Watanabe.

At least 19 recordsLinked to original sources

arXiv2026

Spin Relaxation and Electrical Control of Spin Transport in Few-Layer $T_{\rm d}$-MoTe$_2$

We report electrical tuning of spin relaxation in atomically thin $T_{\rm d}$-MoTe$_2$, a representative low-symmetry transition metal dichalcogenide. Weak antilocalization measurements reveal strong spin--orbit interaction with spin--orbit lengths of tens of nanometers, consistent with recent theoretical predictions. Despite the semimetallic nature of $T_{\rm d}$-MoTe$_2$, in the ultrathin limit the spin--orbit scattering time can be tuned electrostatically by up to threefold. We further investigate vertical electric field effects on the spin--orbit length and time, and demonstrate the modulation of the Rashba parameters. Relation between spin--orbit scattering time and the diffusion coefficient indicates that spin relaxation is governed by the D'yakonov--Perel' mechanism in both monolayer and bilayer $T_{\rm d}$-MoTe$_2$. These results establish few-layer $T_{\rm d}$-MoTe$_2$ as a platform for electrically tunable spin transport, thereby providing insight into spin relaxation in low-symmetry transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2026

Microwave spectroscopic evidence of a moiré-commensurate electron crystal in monolayer graphene

We probe the conductivity of the lowest Landau level of a moiré-aligned monolayer graphene device by broadband microwave transmission through a coplanar waveguide ($\sim$50 mK, 15.4--18 T), isolating the longitudinal channel through a low-minus-high frequency band difference $D(ν)$. We resolve a series of minima in $D(ν)$ that drift with field along lines of fixed carrier density rather than fixed Landau filling, the hallmark of moiré-commensurate electron crystals. The strongest minimum is present at every field and matches no fractional quantum Hall or integer Hofstadter state. It lies at band filling $s=1/3$, one electron for every three moiré cells---the generalized Wigner crystal that DaSilva, Jung, and MacDonald predict as the most favorable fixed-density ($t=0$) commensuration. The weaker minima fall near neighboring commensurate fillings.

cond-mat.mes-hall↗

arXiv2026

Beyond the Flat-Band Paradigm: Open Orbits as a Route to Correlated States in Dispersive Moiré Minibands

The realization of fractional Chern insulators has largely relied on isolated flat bands in complex bilayer or multilayer moiré systems with ideal quantum geometry. Whether such phases can emerge in genuinely dispersive bands, and by what mechanism, remains an open question. Here, we propose a mechanism for magnetic-field-induced strong correlations in the highly dispersive bands of monolayer graphene on hexagonal boron nitride. In the low-field classical-to-quantum crossover regime of the second moiré miniband, we show that the interplay of the moiré potential and weak magnetic fields (B < 1.5 T) profoundly alters electron dynamics. Saddle-point van Hove singularities produce extended open orbits that strongly suppress carrier propagation, potentially favoring a regime in which Coulomb interactions become comparatively more important, while weak magnetic fields lift valley degeneracy and trigonal warping redistributes Berry curvature. High-resolution magnetotransport and temperature-dependent measurements reveal fractional-slope transport features and signatures of interaction-driven incompressibility. Because the Hall conductivity is not fully quantized and the longitudinal resistance remains finite, we describe these observations as incipient FCI behavior: transport signatures consistent with a developing fractional Chern insulating state, without claiming a fully developed FCI phase established by complete Hall quantization and vanishing longitudinal resistance. Our results suggest a possible fermiology-driven route toward correlated topological states in dispersive bands in the classical-to-quantum crossover, distinct from the conventional flat-band scenario.

cond-mat.mes-hall↗

arXiv2026

Magnetic-field response of generalized Wigner crystals in twisted MoTe$_2$

We report a magnetotransport study of fractional hole-filling states in the layer-polarized regime of a twisted MoTe$_2$ homobilayer. A perpendicular magnetic field suppresses the resistance peak at the two-thirds filling while enhancing the one-third peak by several orders of magnitude. Both remain near their commensurate densities. We argue that these commensurate states are generalized Wigner crystals whose distinct charge configurations define different magnetic lattices, leading to contrasting responses to spin polarization. Near half filling, we observe a qualitatively different state whose resistance maximum shifts approximately quadratically with field and exhibits pronounced transport anisotropy consistent with a stripe phase. These results reveal a strong dependence of magnetotransport on fractional filling and open a route to magnetic control of transport in moiré charge-ordered states.

cond-mat.mes-hall↗

arXiv2026

Interferometry reveals spin-singlet fractional quantum Hall edges in graphene

The edge modes of spin-singlet fractional quantum Hall (FQH) phases are manifestations of multicomponent topological order and SU(2) spin symmetry. An archetype is the spin-unpolarized state at $ν$=2/3, whose edge is expected to host spatially coexisting yet counter-propagating charge and neutral spin modes. Despite efforts, its edge properties, including spin coherence and spin-charge separation, have remained elusive owing to the difficulty of resolving spins in FQH edge transport. Here, we develop a spin-sensitive probe of graphene FQH edges by using a p-n junction to interface a target FQH state with a probe integer quantum Hall (QH) state of opposite polarity. An Aharonov-Bohm (AB) interferometer forms along the interface, when the target and probe edge channels carry the same spin. We identify the spin-unpolarized and polarized edges of the $ν$=2/3 states at lower and higher magnetic fields, respectively, through spin-dependent interference. A novel multiparticle AB interference between two electrons of opposite spin emerges from spin-charge separation and recombination on the unpolarized edge, featuring a spin swap. Our results establish edge transport as a probe of spin-singlet topological orders, with implications for parafermion platforms in graphene-superconductor hybrids.

cond-mat.mes-hall↗

arXiv2026

Extraordinary high room-temperature carrier mobility in graphene-WSe$_2$ heterostructures

High charge carrier mobilities play a fundamental role for high-frequency electronics, integrated optoelectronics as well as for sensor and spintronic applications, where device performance is directly linked to the magnitude of the carrier mobility. Van der Waals heterostructures formed by graphene and hexagonal boron nitride (hBN) already outperform all known materials in terms of room temperature mobility. Here, we show that the room temperature carrier mobility of today's best graphene/hBN devices can be surpassed by more than a factor of three by heterostructures formed by tungsten diselenide (WSe$_2$), graphene and hBN, which can have mobilities as high as 350,000 cm$^2$/(Vs) and resistivities as low as $15$ Ohm. The resistivity of these devices shows a significantly weaker temperature dependence than the one of graphene on any other known substrate. Notably, the reduced temperature dependence and the resulting mobility enhancement in graphene show an unexpected relation to the thickness of the WSe$_2$ layer and to the minimum conductivity at the charge neutrality point, suggesting a role of increased charge disorder and/or enhanced screening and questioning the current understanding of electron-phonon scattering in graphene-based van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2026

Electrically Tunable Two-Component Exciton Condensate in a Coulomb-Coupled Graphene Trilayer

Multicomponent condensates possess internal phase degrees of freedom unavailable to a single-component condensate, yet their components are rarely controllable in solids. Here we realize a graphene trilayer with negligible interlayer tunnelling in which the layer-specific carrier densities are continuously tuned by electrostatic gating. Quantum-capacitance measurements demonstrate that charge-incompressible quantum Hall states at total filling factors 1 and 2 persist across the full range of layer-filling configurations and continuously connect the three bilayer exciton-condensate limits. This persistence provides evidence for a trilayer excitonic state. Static Hartree-Fock and time-dependent Hartree-Fock calculations yield two independent finite phase-stiffness eigenmodes and two linearly dispersing Goldstone modes, respectively, when all three layers are partially filled, whereas only one phase-stiffness eigenmode and one linear Goldstone mode remain when one layer is unfilled. The stiffness eigenmodes rotate continuously between the two adjacent-layer exciton bases as charge is transferred among the layers, revealing electrical control of the condensate-mode composition. Together, the experimental and theoretical results support the identification of a two-component exciton condensate with a continuously tunable internal structure.

cond-mat.mes-hall↗

arXiv2026

Electron spin resonance driven photogalvanic effect in graphene-based structures

We report an electron-spin-resonance-driven linear photogalvanic effect (LPGE) in unbiased monolayer graphene and WSe_2/graphene heterostructures. Under linearly polarized 45--75 GHz radiation, the photovoltage exhibits pronounced resonant features in both Faraday and Voigt geometries. Multiple resonances associated with the electron spin resonance in graphene are observed for both out-of-plane and in-plane magnetic-field orientations. Their magnetic-field positions vary linearly with frequency, their amplitudes reverse sign across the charge-neutrality point, and the resonant contribution has the opposite sign to the nonresonant Drude photogalvanic background. We develop a microscopic theory in which radiation-induced momentum alignment followed by skew scattering generates both contributions. Their opposite signs originate from the orthogonal momentum alignments produced by indirect Drude absorption and direct spin-resonant transitions. The theory describes well the main features of the observed resonant photocurrent and provides a microscopic description of ESR-induced LPGE in two-dimensional systems. These results establish the photogalvanic response as a probe of ESR in unbiased micron-scale graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2026

Photo- and thermally-induced huge layer decoupling in twisted bilayer WSe$_2$

Twisted bilayer systems host a wealth of emergent phenomena, such as flat-band superconductivity, ferromagnetism, and ferroelectricity, arising from moiré superlattices and unconventional interlayer coupling. Despite their central role, direct and quantitative access to the three-dimensional atomic arrangement in these systems has remained elusive due to their nanoscale dimensions. Here, we introduce an automated dark-field electron tomography technique that enables quantitative three-dimensional structural analysis of atomically thin materials with sub-Å precision. By applying this method to twisted bilayer WSe$_2$, we precisely visualize the twist-angle-dependent structural relaxation appearing as the AB/BA stacking domains separated by 10-20 nm domain walls.In the marginally twisted region ($θ\leq 0.1^\circ$), we uncover a significant expansion of the interlayer spacing compared to the bulk configuration, exceeding 0.1 Å, along with a remarkable temperature-driven interlayer decoupling. Ultrafast measurements further reveal optically induced interlayer separation of ~0.2 Å on the picosecond timescale, attributed to transient exciton formation. These findings not only establish a powerful approach for visualizing hidden out-of-plane structures in atomically thin micro-flake materials, but also uncover the intrinsic fragility and dynamical tunability of interlayer coupling in moiré-engineered 2-dimensional materials.

cond-mat.mes-hall↗

arXiv2026

Electronic bistability, discontinuous switching and stochasticity in a two-dimensional semiconductor

Bistability - two stable electronic states under the same bias - underlies switching and memory, but is usually absent in transistors and must be engineered through material means: doped tunnel junctions, filaments in memristors, or phase transitions. Here we demonstrate a transistor with intrinsic electronic bistability in a single chemically homogeneous crystal. In dual-gated black phosphorus, whose band gap narrows under a perpendicular electric field due to a giant Stark effect, the gates not only modulate carrier density but also reshape the band profile, forming interband tunnel junctions in the channel. Transport across the two-gate parameter space reveals competing conduction regimes - diffusive, two tunnelling channels and Zener breakdown - whose interplay produces negative differential conductance and transconductance, discontinuous switching, and hysteresis with the state set by gate history. Moreover, the switching remains intrinsically stochastic, yet statistically stable within a narrow range of gate voltages, providing an electrically programmable source of randomness. Devices based on this principle should be realisable in other two-dimensional semiconductors, opening a route to next-generation computing architectures in which nonlinearity, switching, memory and stochasticity are integrated within a single electrostatically programmable element.

cond-mat.mes-hall↗

arXiv2026

Out-of-equilibrium spin-valley dynamics of ferromagnets in topological Chern bands

Understanding quantum matter far from equilibrium is a central goal of modern physics. Twisted MoTe2 bilayers constitute a promising platform for exploring this frontier by combining strong Coulomb interactions, nontrivial band geometry, and optical control. Here, we exploit this setting to investigate the role of topology and many-body correlations in the out-of-equilibrium dynamics of ferromagnets in Chern bands. Using a focused circularly polarized light pulse, we create a local magnetic domain oriented opposite to an external magnetic field and directly image its subsequent spin-valley relaxation in spatially and time-resolved low-temperature experiments. We demonstrate that in the vicinity of both integer and fractional Chern insulating states, the dynamics is governed by qualitatively different mechanisms than in ferromagnetic metals. Whereas metallic domains collapse by shrinking, Chern domains melt via thermal activation, resulting in drastically different temporal spin evolution and orders-of-magnitude longer relaxation times. These findings demonstrate the influence of topology and strong correlations on far-from-equilibrium collective spin phases, opening new opportunities for dynamical control of ferromagnets in the quantum Hall regime.

cond-mat.mes-hall↗

arXiv2026

Correlated Insulator Moiré Bolometer

Light incident on an insulator is generally not expected to turn it into a metal without invoking intense ultrafast excitation that leads to transient structural transitions. Here we show that magic-angle twisted bilayer graphene tuned to half filling of the moiré band provides a notable exception to this expectation. We find that weak beam of long-wavelength photons, with energies comparable to the flat-band width, selectively heat the low-heat-capacity electronic subsystem, thereby suppressing the correlated gap. This produces a giant resistance change governed not by a persistent photocarrier population, but by the extreme sensitivity of a many-body correlated gap to weak electronic heating. The resulting photon-driven insulator-to-metal transition produces a broadband low-noise photoresponse with voltage responsivity exceeding millivolts per nW of absorbed power. The mechanism is dual to superconducting hot-electron response: radiation-heated electrons suppress a many-body order, but in reverse the correlated insulator melts into a metal, providing robustness to magnetic fields of several tesla and a sharp insulator-to-metal resistive contrast. Our results establish correlated flat-band systems as a platform for ultra-sensitive detection of faint long-wavelength radiation.

cond-mat.mes-hall↗

arXiv2026

Quantized heat flow in moiré chern bands of bilayer graphene

When electrons are subjected simultaneously to a magnetic field and a periodic potential, they form the fractal Hofstadter spectrum, whose topological gaps host quantum Hall and Chern insulating states with distinct Chern numbers. While electrical transport has established the topology of these states, whether their heat transport is likewise universal has remained unexplored. Here, we measure the thermal conductance of quantum Hall, Chern insulator, and interaction-driven symmetry-broken Chern insulator states in a bilayer graphene-hexagonal boron nitride moiré superlattice with a moirè wavelength of $\sim$14 nm using Johnson-noise thermometry. We find that the thermal conductance ($G_Q$) is quantized in units of the thermal conductance quantum ($G_Q = tκ_0T$) and is determined solely by the Chern number ($t$), independent of the microscopic origin of the topological state. By directly revealing universal topological heat transport in Hofstadter bands, our work establishes thermal conductance as a stringent probe of moiré topological matter and provides a route to investigating more exotic phases, including fractional Chern insulators.

cond-mat.mes-hall↗

arXiv2026

Fermion parity of an Andreev molecule probed by nonlocal Josephson effect

Fermion parity is a fundamental property of superconducting many-body states. Here, we show that the global fermion parity of a delocalized superconducting state can be detected locally by exploiting the nonlocal Josephson effect. Using a carbon nanotube-based Andreev molecule formed by two coupled quantum-dot Josephson junctions, we observe a pronounced nonlocal Josephson response and demonstrate the formation of delocalized Andreev molecular states extending across both junctions. We further show that changes in the molecular ground-state parity manifest as characteristic $π$-phase shifts in the nonlocal response. Supported by a minimal theoretical model, these results identify global fermion parity as an experimentally accessible degree of freedom in hybrid superconducting circuits that can be readily revealed through the nonlocal Josephson effect.

cond-mat.mes-hall↗

arXiv2026

High-field Josephson effect enabled by a moiré Hofstadter spectrum

Magnetic fields generally suppress phase-coherent Josephson transport, limiting superconducting interferometry to relatively low fields. Here we show that moiré-engineered graphene Josephson junctions can overcome this constraint. Using ballistic graphene/hBN junctions, we establish phase-coherent Andreev transport through Fabry-Pérot oscillations and Fraunhofer interference that persist across both the primary Dirac cone and reconstructed moiré minibands. We then demonstrate phase-coherent Josephson interference up to 6 T in the fractal Hofstadter-butterfly regime, well beyond the range expected for conventional ballistic graphene junctions. Comparison with Hofstadter-spectrum calculations reveals that superconductivity survives where the moiré potential transforms Landau levels with quenched group velocity into dispersive magnetic Bloch bands with finite quasiparticle group velocity, enabling extended electron-hole Andreev trajectories across the junction. Our results show that Hofstadter minibands can stabilize phase-coherent superconductivity deep into the parameter domain conventionally associated with the quantum Hall regime, establishing a new platform for high-field superconducting interferometry.

cond-mat.mes-hall↗

arXiv2026

Evidence for a $\bar{K}NN$ quasi-bound state in the $γd \to K^0Λp$ reaction

The $γd \to K^{0}Λp$ reaction has been studied to search for a $\bar{K}NN$ quasi-bound state using the LEPS2 solenoid spectrometer at SPring-8. A localized enhancement concentrated at low $q$ is observed in the $(M_{Λp},q)$ distribution below the $K^-pp$ mass threshold, where $M_{Λp}$ is the $Λp$ invariant mass and $q \equiv |\vec{p}_γ-\vec{p}_{K^{0}}| = |\vec{p}_{Λp}|$ is the momentum transfer. A two-dimensional $(M_{Λp},q)$ fit demonstrates that the enhancement near the threshold is statistically significant, yielding a local significance of $7.3,σ$. The enhancement is characterized by effective shape parameters including the Breit--Wigner mass $M$ and width $Γ$, and a Gaussian form-factor momentum scale $Q$: $M = 2.354 \pm 0.011(\mathrm{stat.})^{+0.009}_{-0.005}(\mathrm{syst.})~\mathrm{GeV}/c^{2}$, $Γ= 0.055 \pm 0.023(\mathrm{stat.})^{+0.031}_{-0.009}(\mathrm{syst.})~\mathrm{GeV}/c^{2}$, and $Q = 0.350 \pm 0.041(\mathrm{stat.})^{+0.035}_{-0.010}(\mathrm{syst.})~\mathrm{GeV}/c$. The obtained results support the existence of a $\bar{K}NN$ quasi-bound state in photoproduction.

nucl-ex↗

arXiv2026

Origins of Cosmic Rays in the Galactic-extragalactic Transition Energy Range

Cosmic rays arrive at Earth with energies ranging from $10^9$ to over $10^{20}$ eV. One of the open questions in high-energy cosmic ray science concerns the origin of the highest-energy cosmic rays that can be accelerated by Galactic sources, and the transition energy beyond which only extragalactic sources can provide. Measuring the mass composition gives essential information for comparing measurements to source and propagation models, both from the abundances at the source and from the maximum attainable energy which is proportional to the particle charge (and hence its mass). The highest-energy cosmic rays from the Galaxy are found in a range of $10^{16}$ to $10^{18}$ eV which is well suited for radio detection. Building on a decade of experience in measuring cosmic rays at LOFAR, we show that SKA-Low, augmented with an array of small particle detectors, is well suited to advance the field by measuring the mass composition of cosmic rays across this energy range.

astro-ph.HE↗

arXiv2026

Coulomb screening of superconductivity in magic-angle twisted bilayer graphene

The origin of superconductivity in magic-angle twisted bilayer graphene has been a subject of intense debate. While some experimental evidence indicated an unconventional pairing mechanism, efforts to tune the critical temperature by screening the Coulomb interactions have been unsuccessful, possibly indicating a conventional phonon-mediated pairing. Here we study a double-layer electronic system consisting of two twisted graphene bilayers in immediate proximity of each other but remaining electronically decoupled. By increasing the carrier density in one bilayer, we completely suppressed both the superconductivity and the correlated-insulator state in the adjacent magic-angle graphene. The observation of such a screening effect offers strong support for an unconventional mechanism of Cooper pairing in magic-angle twisted bilayer graphene, shedding new light on the underlying physics governing their properties.

cond-mat.mes-hall↗