arXiv subjects

Kenji Watanabe

Publications and source records attributed to Kenji Watanabe.

At least 487 records · Page 27Linked to original sources

arXiv2025

Observing Laughlin's pump using quantized edge states in graphene

Laughlin's thought experiment of quantized charge pumping is central to understanding the integer quantum Hall effect (IQHE) and the topological origin of its conductance quantization. Its direct experimental observation, however, has been hindered by the difficulty of realizing clean electronic edges. We address this by fabricating ultra-small, lithographically defined contacts on graphene. This creates a Corbino-equivalent system, with well-confined inner edge states. Crucially, the small contact size induces strong energy quantization of the edge states. This quantization allows us to directly resolve the spectral flow associated with Laughlin's pump. By tracing the finite-size resonances of the inner edge, we observe clear oscillations in conductance as a function of magnetic field and carrier density. The oscillation period scales with contact size, consistent with quantized charge transfer. Thus, our results provide a direct observation of the spectral flow underlying Laughlin's pump. The simplicity of the graphene platform makes this approach scalable and robust for exploring fundamental topological effects.

cond-mat.mes-hall↗

arXiv2025

Brightening interlayer excitons by electric-field-driven hole transfer in bilayer WSe2

We observe the interlayer A1s^I, A2s^I, and B1s^I excitons in bilayer WSe2 under applied electric fields using reflectance contrast spectroscopy. Remarkably, these interlayer excitons remain optically bright despite being well separated from symmetry-matched intralayer excitons-a regime where conventional two-level coupling models fail unless unphysically large coupling strengths are assumed. To uncover the origin of this brightening, we perform density functional theory (DFT) calculations and find that the applied electric field distorts the valence-band Bloch states, driving the hole wavefunction from one layer to the other. This field-driven interlayer hole transfer imparts intralayer character to the interlayer excitons, thereby enhancing their oscillator strength without requiring hybridization with bright intralayer states. Simulations confirm that this mechanism accounts for the major contribution to the observed brightness, with excitonic hybridization playing only a minor role. Our results identify interlayer hole transfer as a robust and general mechanism for brightening interlayer excitons in bilayer transition metal dichalcogenides (TMDs), especially when inter- and intralayer excitons are energetically well separated.

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arXiv2025

Observation of Cavity-Mediated Nonlinear Landau Fan and Modified Landau Level Degeneracy in Graphene Quantum Transport

Recent studies on cavity-coupled two-dimensional electron gas demonstrate that vacuum-field engineering can tailor electronic transport properties of materials. By achieving ultra-strong coupling between a terahertz resonator and mesoscopic graphene, we demonstrate that cavity vacuum fields can alter the effective degeneracies of Landau levels, resulting in a nonlinear Landau fan diagram for massless Dirac fermions while preserving quantum-Hall quantization. Specifically, by leveraging graphene's gate-tunability, we observe that quantum-Hall features, minimum longitudinal and quantized Hall conductance for a given filling factor, occur at carrier densities reduced by more than 20 percent compared to systems without cavity. Theoretical analysis attributes this effect to the virtual cavity photon mediated transitions between the non-equidistant Landau levels in graphene, significantly reducing their effective degeneracy. This study paves the way for investigating cavity quantum electrodynamics in highly tunable, atomically thin two-dimensional crystals.

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arXiv2025

Revealing electron-lattice decoupling by Peltier thermometry and nanoscale thermal imaging in graphene

Electrical currents in low-dimensional quantum materials can drive electrons far from equilibrium, creating stark imbalance between electron and lattice temperatures. Yet, no existing methods enable simultaneous nanoscale mapping of both temperatures at cryogenic conditions. Here, we introduce a scanning probe technique that images the local lattice temperature and extracts electron temperature at gate-defined p-n junctions in graphene. By applying an alternating electrical current and analyzing first- and second-harmonic responses, we disentangle Joule heating from the Peltier effect-the latter encoding the local electron temperature. This enables the first spatially resolved cryogenic imaging of both phenomena in graphene. Even under modest current bias, the electron temperature increases by nearly three orders of magnitude more than the lattice temperature, revealing strong electron-phonon decoupling and indicating a previously unrecognized electron cooling pathway. Our minimally invasive method is broadly applicable to van der Waals heterostructures and opens new avenues for probing energy dissipation and non-equilibrium transport in correlated and hydrodynamic electron systems.

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arXiv2025

Transport Evidence for Wigner Crystals in Monolayer MoTe2

The crystallization of charge carriers, dubbed the Wigner crystal, is anticipated at low densities in clean two-dimensional electronic systems (2DES). While there has been extensive investigation across diverse platforms, probing spontaneous charge and spin ordering is hindered by disorder effects and limited interaction energies. Here, we report transport evidence for Wigner crystals with antiferromagnetic exchange interactions in high-quality, hexagonal boron nitride encapsulated monolayer MoTe2, a system that achieves a large interaction parameter (r_s) at proper hole densities. A density-tuned metal-insulator transition (MIT) occurring at 3.1E10^11 cm-2 (corresponding to r_s~32) and pronounced nonlinear charge transport in the insulating regime at low temperatures signify the formation of Wigner crystals. Thermal melting of the crystalline phase is observed below approximately 2 K via temperature-dependent nonlinear transport. Magnetoresistance measurements further reveal a substantial enhancement of spin susceptibility as approaching the MIT. The temperature dependence of spin susceptibility in the Wigner crystal phase closely follows the Curie-Weiss law, with the extracted negative Weiss constant illustrating antiferromagnetic exchange interactions. Furthermore, we have found the system exhibits metallic-like differential resistivity under finite DC bias, possibly indicating the existence of a non-equilibrium coherent state in the depinning of Wigner crystals. Our observations establish monolayer MoTe2 as a promising platform for exploring magnetic and dynamic properties of Wigner crystals.

cond-mat.str-el↗

arXiv2025

Highly anisotropic spin transport in ultrathin black phosphorus

In anisotropic crystals, the direction-dependent effective mass of carriers can have a profound impact on spin transport dynamics. The puckered crystal structure of black phosphorus (BP) leads to direction-dependent charge transport and optical response, suggesting that it is an ideal system for studying anisotropic spin transport. To this end, we fabricate and characterize high mobility encapsulated ultrathin BP-based spin-valves in four-terminal geometry. Our measurements show that in-plane spin-lifetimes are strongly gate tunable and exceed one nanosecond. Through high out-of-plane magnetic fields, we observe a five-fold enhancement in the out-of-plane spin signal case compared to in-plane and estimate a colossal spin lifetime anisotropy of ~ 6. This finding is further confirmed by Oblique Hanle measurements. Additionally, we estimate an in-plane spin-lifetime anisotropy ratio of up to 1.8. Our observation of strongly anisotropic spin transport along three orthogonal axes in this pristine material could be exploited to realize directionally tunable spin transport.

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arXiv2025

Probing Phonon Modes in Reconstructed twisted Homo and Hetero Bilayer System

Twist angle engineering in van der Waals homo and hetero-bilayers introduces profound modifications in their electronic, optical and mechanical properties due to lattice reconstruction. In these systems, the interlayer coupling and atomic rearrangement strongly depend on the twist angle, leading to the formation of periodic Moire superlattices. At small twist angles, significant lattice relaxation results in the emergence of domain structures separated by one dimensional soliton networks, influencing electronic band structures and phonon modes. Here we systematically investigate the impact of lattice reconstruction on phonon renormalization in twisted bilayer graphene (TBLG,homo) and graphene-hBN Moire superlattices(hetero). Using Raman spectroscopy, we identify distinct phonon behaviours across different twist angle regimes. In TBLG, we observe the evolution of the G peak, including broadening, splitting, and the emergence of additional peaks in the small angle range 0.3 to 1 degree, attributed to Moire modified phonon interactions. At large twist angles, the peaks gradually merge back into a single feature, reflecting the reduced impact of lattice reconstruction. Similarly, in hBN graphene Moire superlattices, we detect Moire induced Raman peaks above and below the G peak, while the central G peak remains largely invariant to twist angle variation. The theoretical calculations uncover Moire phonon modes originating from different stacking regions providing insights into phonon renormalization. Our results establish a direct link between twist angle, lattice reconstruction, Moire phonons, and interlayer coupling, offering a fundamental framework for understanding phonon engineering in twisted bilayer systems. These findings pave the way for controlling phononic, optoelectronic and heat flow properties in next generation van der Waals heterostructures.

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arXiv2025

Interaction-Driven Topological Transitions in Monolayer TaIrTe$_4$

Discovering materials that combine topological phenomena with correlated electron behavior is a central pursuit in quantum materials research. Monolayer TaIrTe$_4$ has recently emerged as a promising platform in this context, hosting robust quantum spin Hall insulator (QSHI) phases both within a single-particle gap and within a correlation-induced gap arising from van Hove singularities (vHSs), accessed via electrostatic doping. Its intrinsic monolayer nature offers exceptional tunability and the potential to realize a versatile array of interaction-driven topological phases. In this work, we combine theory and experiment to map the phase landscape of monolayer TaIrTe$_4$. Using Hartree-Fock calculations, we investigate the interaction-driven phase diagram near the vHSs under commensurate filling conditions. By systematically tuning the dielectric screening and strain, we uncover a rich set of ground states--including QSHI, trivial insulator, higher-order topological insulator, and metallic phase--among which are interaction-driven topological phase transitions. Experimentally, we perform both local and nonlocal transport measurements across a broad set of devices, which--due to unavoidable strain variations during fabrication-realize several phases consistent with theoretical predictions. Together, our results lay the groundwork for understanding correlation-driven topological phenomena in TaIrTe$_4$ and open new directions for engineering exotic quantum phases in low-dimensional materials beyond the limitations of moiré superlattices.

cond-mat.str-el↗

arXiv2025

Half-integer thermal conductance in the absence of Majorana mode

Considering a range of candidate quantum phases of matter, half-integer thermal conductance ($κ_{\text{th}}$) is believed to be an unambiguous evidence of non-Abelian states. It has been long known that such half-integer values arise due to the presence of Majorana edge modes, representing a significant step towards topological quantum computing platforms. Here, we challenge this prevailing notion by presenting a comprehensive theoretical and experimental study where half-integer two-terminal thermal conductance plateau is realized employing Abelian phases. Our proposed setup features a confined geometry of bilayer graphene, interfacing distinct particle-like and hole-like integer quantum Hall states. Each segment of the device exhibits full charge and thermal equilibration. Our approach is amenable to generalization to other quantum Hall platforms, and may give rise to other values of fractional (electrical and thermal) quantized transport. Our study demonstrates that the observation of robust non-integer values of thermal conductance can arise as a manifestation of mundane equilibration dynamics as opposed to underlying non-trivial topology.

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arXiv2025

Anomalies in G and 2D Raman Modes of Twisted Bilayer Graphene Near the Magic Angle

The role of twist angle ($θ_t$) in tailoring the physical properties of heterostructures is emerging as a new paradigm in two-dimensional materials. The influence of flat electronic bands near the magic angle ($\sim$1.1$^{\circ}$) on the phononic properties of twisted bilayer graphene (t-BLG) is not well understood. In this work, we systematically investigate the G and 2D Raman modes of t-BLG samples with twist angles ranging from $\sim$0.3$^{\circ}$ to $\sim$3$^{\circ}$ using micro-Raman spectroscopy. A key finding of our work is the splitting of the G mode near the magic angle due to moiré potential induced phonon hybridization. The linewidth of the low-frequency component of the G mode (G$^-$), as well as the main component of the 2D mode, exhibits enhanced broadening near the magic angle due to increased electron-phonon coupling, driven by the emergence of flat electronic bands. Additionally, temperature-dependent Raman measurements (6-300 K) of magic-angle twisted bilayer graphene sample ($θ_t \sim$ 1$^{\circ}$) reveal an almost tenfold increase in phonon anharmonicity-induced temperature variation in both components of the split G mode, as compared to Bernal-stacked bilayer graphene sample, further emphasizing the role of phonon hybridization in this system. These studies could be important for understanding the thermal properties of the twisted bilayer graphene systems.

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arXiv2025

Brightening dark trions in WS2 monolayers via introducing atomic sulfur vacancies

Understanding the effects of atomic defects on the optical functionality of two-dimensional (2D) layered materials is critical to develop novel optical and optoelectronic applications of these ultimate materials. Herein, we correlate sulfur vacancies (VS) and luminescence properties of dark trions in monolayer WS2 through introducing VS defects and conducting a systematic optical spectroscopic characterization at cryogenic and room temperatures. It is unraveled that the VS defects can brighten the dark trions via introducing a stronger spin-orbit coupling due to the space inversion symmetry broken by the defects. Furthermore, the wavefunction localization of the dark trions bound at VS defects results in significant enhancement of the phonon scattering from the K2 valley phonons and hence makes the K2 phonon replica dominant in the emission spectrum. Theoretical calculations of the temperature-dependent photoluminescence spectra with quantum mechanics-based multimode Brownian oscillator model show strong support for the above arguments. Brightening the dark excitons not only sheds light on the understanding of the intriguing excitonic properties of 2D semiconductors, but also may open a way for regulating the optoelectronic performance of two-dimensional semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Observing the Birth of Rydberg Exciton Fermi Polarons on a Moire Fermi Sea

The optical spectra of two-dimensional (2D) semiconductors are dominated by tightly bound excitons and trions. In the low doping limit, trions are often described as three-body quasiparticles consisting of two electrons and one hole or vice versa. However, trions are more rigorously understood as quasiparticles arising from the interaction between an exciton and excitation of the Fermi sea - referred to as exciton Fermi polaron. Here we employ pump-probe spectroscopy to directly observe the formation of exciton Fermi polarons in a model system composed of a WSe2 monolayer adjacent to twisted bilayer graphene (tBLG). Following the pump-injection of Rydberg excitons in WSe2, a time-delayed probe pulse tracks the development of Rydberg exciton Fermi polarons as interactions with localized carriers in the tBLG moire superlattice evolve. Both the exciton Fermi polaron relaxation rate and binding energy are found to increase with electron or hole density. Our findings provide insight into the optical response of fundamental excitations in 2D Van der Waals systems and reveal how many-body interactions give rise to emergent quasiparticles.

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arXiv2025

Emergence of Chern metal in a moiré Kondo lattice

A Chern metal is a two-dimensional metallic state of matter carrying chiral edge states. It can emerge as a doped Chern insulator, but theoretical studies have also predicted its emergence near a Kondo breakdown separating a metallic chiral spin liquid and a heavy Fermi liquid in a frustrated lattice. To date, the latter exotic scenario has not been realized. Here, we report the observation of a Chern metal at the onset of the magnetic Kondo breakdown in a frustrated moiré Kondo lattice--angle-aligned MoTe2/WSe2 bilayers. The state is compressible and is manifested by a nearly quantized Hall resistance but a finite longitudinal resistance that arises from a bad metallic bulk. The state also separates an itinerant and a heavy Fermi liquid and appears far away from the band inversion critical point of the material, thus ruling out its origin from simply doping a Chern insulator. We demonstrate the presence of a chiral edge state by nonlocal transport measurements and current-induced quantum anomalous Hall breakdown. Magnetic circular dichroism measurements further reveal a magnetization plateau for the Chern metal before a metamagnetic transition at the Kondo breakdown. Our results open an opportunity for moiré engineering of exotic quantum phases of matter through the close interplay between band topology and Kondo interactions.

cond-mat.str-el↗

arXiv2025

Tunable Band Inversion in Trilayer Graphene

Displacement field control of elecronic bands in low-dimensional systems is a promising route toward engineering emergent quantum phases. Here, we report displacement-field-induced band inversion and modulation of the Berry phase of low-energy quasi particles in high-mobility Bernal-stacked trilayer graphene (TLG). Using quantum oscillations, we track the evolution of the Fermi surface and topological properties of Dirac-like gully bands that emerge under a finite interlayer potential. We observe a striking sequence of transitions: at low displacement field $D$, the gullies are characterized by a Berry phase of $2π$ and large effective mass, indicating massive fermions. As $D$ increases, the Berry phase abruptly shifts to $π$ and the effective mass reaches a minimum, signaling the onset of massless Dirac behavior. At higher $D$, the Berry phase returns to $2π$, and the effective mass increases again, consistent with a band inversion. These findings demonstrate a rare, reversible topological phase transition - massive to massless to massive - driven entirely by an external displacement field. Despite robust theoretical predictions [\textit{Phys. Rev. B} \textbf{87}, 085424 (2013), \textit{Phys. Rev. B} \textbf{87}, 115422 (2013), and \textit{Phys. Rev. B} \textbf{101}, 245411 (2020)], this evolution of the band topology had escaped experimental detection. Our results establish TLG as a tunable platform for nanoscale control of band topology. They establish a means to tune between massive and Dirac-like dispersions dynamically providing a foundation for exploring field-switchable topological phenomena in layered 2D systems.

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arXiv2025

Tunability of Robust Exciton-Trion Polaritons in Atomically Thin WS2 Monolayers

Herein, we present an experimental demonstration of the robust exciton-trion polaritons (ETPs) by measuring and simulating the resonance reflectance spectra of various configurational WS2 monolayers with different dielectric screenings. Moreover, the oscillator strength and decoherent behavior of such hybrid ETPs can be tuned via utilizing dielectric screening effect. The effect is attributed to the regulation of the Coulomb coupling between excitons and trions by changing the surrounding dielectric constant. The demonstration and tunability of the robust ETPs offers a novel pathway for researching novel phases of quantum matter in a quantum many-body physics regime.

cond-mat.mtrl-sci↗

arXiv2025

Polarized electroluminescence with magnetic spectral tuning in van der Waals magnet CrSBr

Polarized wavelength-tunable electroluminescence (EL) represents a critical on-demand functionality for next-generation optoelectronics. While conventional van der Waals (vdW) EL devices offer discrete wavelength switching constrained by fixed emission states, we report a novel platform enabling continuous spectral tuning combined with intrinsically polarized emission. By leveraging exciton-assisted inelastic tunneling in the anisotropic magnet CrSBr, our devices achieve uniform EL with a near unity degree of linear polarization ($\approx$ 94.3$\%$). The strong magneto-electronic coupling in CrSBr facilitates continuous magnetic-field-controlled spectral tuning through spin canting-induced band renormalization. This work establishes vdW magnets as a versatile platform for developing reconfigurable polarized light sources with simultaneous spectral and polarization control.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast intrinsic optical-to-electrical conversion dynamics in graphene photodetector

Optical-to-electrical (O-E) conversion in graphene is a central phenomenon for realizing anticipated ultrafast and low-power-consumption information technologies. However, revealing its mechanism and intrinsic time scale require uncharted terahertz (THz) electronics and device architectures. Here, we succeeded in resolving O-E conversion processes in high-quality graphene by on-chip electrical readout of ultrafast photothermoelectric current. By suppressing the RC time constant using a resistive zinc oxide top gate, we constructed a gate-tunable graphene photodetector with a bandwidth of up to 220 GHz. By measuring nonlocal photocurrent dynamics, we found that the photocurrent extraction from the electrode is instantaneous without a measurable carrier transit time across several-micrometer-long graphene, following the Shockley-Ramo theorem. The time for photocurrent generation is exceptionally tunable from immediate to > 4 ps, and its origin is identified as Fermi-level-dependent intraband carrier-carrier scattering. Our results bridge the gap between ultrafast optical science and device engineering, accelerating ultrafast graphene optoelectronic applications.

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arXiv2025

On-chip transfer of ultrashort graphene plasmon wavepackets using terahertz electronics

Steering transport of ultrashort polariton wavepackets is essential for achieving on-chip integrated nanocircuits with tightly confined electromagnetic fields towards ultrafast information processing. However, conventional optical techniques have struggled to integrate the necessary components for transferring polariton signals. Here, we address this challenge by electrically generating, manipulating, and reading out terahertz graphene plasmon-polariton wavepackets on-chip. By injecting an electrical pulse into graphene via an ohmic contact, we achieve coherent conversion of the pulse into a plasmon wavepacket exhibiting a pulse duration of 1.2 ps and extreme three-dimensional spatial confinement within a volume of $2.1 \times 10^{-18} m^3$. We reveal the transport properties of plasmons along graphene ribbons in different dielectric environments, providing a basis for designing graphene plasmonic circuits. Furthermore, we find that the conversion efficiency between the electrical pulses and plasmon wavepackets reaches ~30% thanks to the absence of a momentum mismatch. With unprecedented controllability, our platform represents a significant advance in on-chip handling of plasmonic signals in various van der Waals heterostructures.

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