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Takashi Taniguchi

Publications and source records attributed to Takashi Taniguchi.

At least 37 records · Page 2Linked to original sources
arXiv2026

Remote magnon sensing detects the universal phase stiffness jump in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Phase stiffness governs macroscopic quantum coherence in superfluids and superconductors, providing a sensitive probe of condensate depletion, fluctuations, and phase transitions. However quantitative and spatially resolved measurements remain challenging. Here we report a contactless, local, all-optical measurement of superconducting phase stiffness using propagating antiferromagnetic magnons as remote sensors. In CrSBr, strong exciton-magnon coupling enables optical excitation and detection of magnon wavepackets and precise determination of their group velocity. When CrSBr is interfaced with underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we find that the magnon group velocity is substantially enhanced below the superconducting transition and quantitatively encodes the phase stiffness through Meissner screening currents. Using this capability, we identify a parallel-field-induced Berezinskii-Kosterlitz-Thouless transition through direct observation of the universal phase-stiffness jump, revealing a field-controlled route to expose intrinsic two-dimensional phase-ordering physics in a bulk cuprate. Our work establishes propagating magnons as local probes of superconducting electrodynamics and phase-coherent phenomena in low-dimensional quantum materials.

cond-mat.supr-con↗
arXiv2026

Ambipolar metal-insulator transition in bilayer MoSe2

When Coulomb interactions dominate kinetic energy, dilute two-dimensional carriers form a Wigner solid. Both its melting and disorder-driven localization can drive density-tuned metal-insulator transitions, but their relative roles and interplay have been debated for decades. Atomically thin semiconductors offer a new test because transport and scanning tunneling microscopy can probe the same material, yet poor contacts impede transport where correlations are strongest. Here we introduce contact-free radio-frequency reflectometry that senses sheet resistance through capacitive gate coupling. It resolves the electron and previously inaccessible hole transitions in the same bilayer MoSe2 device, whose carriers share disorder but not mass. Their critical densities differ fourfold yet correspond to comparable Coulomb-to-kinetic-energy ratios, where tunneling microscopy finds solid-liquid coexistence. Magnetoresistance peaks sharply at each transition, reflecting a Pomeranchuk-like, field-driven expansion of the Wigner solid against the correlated liquid. Our results demonstrate interaction-driven transitions in high-quality two-dimensional semiconductors, opening quantitative transport in contact-limited systems.

cond-mat.str-el↗
arXiv2026

Hidden valley dynamics behind vanishing circular polarization in moiré excitons

Optically addressable valley degrees of freedom in transition-metal dichalcogenide heterostructures provide a powerful platform for valleytronic and quantum-optical functionalities. In moiré superlattices, interlayer excitons inherit valley-contrasting optical selection rules while acquiring long lifetimes, electric dipoles, and site-dependent optical responses. However, because conventional measurements typically probe time-integrated valley polarization, the dynamical origin of vanishing polarization has remained elusive. Here, we show that a nearly zero steady-state valley polarization in electrically tunable moiré excitons does not necessarily indicate fast valley relaxation. Helicity-resolved time-resolved photoluminescence reveals a temporal crossing between co- and cross-circularly polarized emission, indicating that helicity-opposite dynamical components coexist and compensate after time integration. A minimal two-channel model, representing A-like and B-like moiré emission channels with opposite optical selection rules and distinct effective decay/depolarization rates, reproduces the observed helicity crossing without invoking a single rapid valley relaxation process. Furthermore, two-dimensional gate-field maps show that the crossing time evolves systematically with electrostatic tuning, demonstrating that the hidden valley dynamics are electrically controllable. These results show that time-integrated circular polarization can give a false-negative indication of valley polarization in multichannel valley emitters.

cond-mat.mes-hall↗
arXiv2026

Robust Diamond Quantum Sensing in Double-Quantum 4-Ramsey Interferometry by Dual-Feedback Scheme

We developed a highly sensitive and long-term stable diamond quantum magnetometer by integrating a dual-feedback system that compensates for common- and differential-mode frequency shifts in double-quantum (DQ) 4-Ramsey interferometry. Detecting extremely weak magnetic-field signals requires extensive signal accumulation to enhance the signal-to-noise ratio. However, deviations from the optimal operating frequencies caused by thermal drift or environmental magnetic noise severely hinder the long-term stability of such a sensing system. To overcome the limitation, we first derived analytical solutions for the signal response to common- and differential-mode detunings based on a detuning-inclusive Hamiltonian to theoretically clarify the robustness limit in DQ 4-Ramsey sensing. Guided by this theoretical model, we introduced synchronized frequency modulation and a dual-feedback scheme to independently track and compensate for these frequency shifts while maintaining an optimal magnetic-field response. The proposed method enabled stable continuous operation against temperature variations of 2.8 K. By maintaining a magnetic field sensitivity of 7.0 pT/Hz^1/2 in the frequency range of 5-100 Hz for a prolonged duration of 4 hours, we achieved an extremely low noise floor of 64 fT/Hz^1/2. During prolonged continuous sensing, the system exhibited the expected 1/T^1/2 scaling with integration time T, and prevented both the underestimation of amplitude and waveform distortion. These results establish a robust approach for long-term DQ 4-Ramsey diamond magnetometry and demonstrate its potential for detecting weak biomagnetic signals.

quant-ph↗
arXiv2026

Direct synthesis of moiré graphene with twist-tuned hybridization

Twisted van der Waals (vdW) systems provide a powerful platform for exploring emergent correlated phenomena, including superconductivity, magnetism, and topological phases. The twist angle offers a continuous tuning parameter for modifying the underlying Hamiltonian, however, precise and reproducible control remains challenging. Existing approaches are limited by single-use device architectures, restricted angular precision, or vertical probing geometries. Here we introduce a reconfigurable graphene-based moiré platform in which two independently fabricated layers are mechanically assembled and rotated in situ using a micro-electromechanical system (MEMS). Raman spectroscopy confirms the formation and robustness of the twist-induced moiré potential and enables continuous, deterministic control of the twist angle. The moiré potential folds the electronic band structure and activates the longitudinal optical (LO) phonon mode, allowing us to map its dispersion and directly visualize the moiré momentum. Angle-dependent electronic band hybridization is resolved through the evolution of the double-resonant 2D mode. Near \(0^\circ\), an emergent asymmetry of the \(Γ\)-point G-mode phonon reveals phonon band folding and hybridization driven by the moiré superlattice potential and interlayer relaxation. Our platform provides a reproducible Raman reference for twisted bilayer graphene while eliminating device-to-device variability. More broadly, it establishes the twist angle as a deterministically controllable parameter for systematic studies of correlated and topological phenomena in moiré quantum materials.

cond-mat.mes-hall↗
arXiv2026

Terahertz nanoscopy of quantum-geometric photovoltaics in bilayer graphene

The bulk photovoltaic effect (BPVE) provides a direct link between quantum geometry and nonlinear light-matter interactions, yet conventional far-field measurements cannot disentangle BPVE from contact- and interface-induced photothermoelectric signals. Here we develop cryogenic, polarization-sensitive terahertz microscopy with deep-subwavelength resolution and apply it to dual-gated AB bilayer graphene. Near resonance between the band gap and terahertz photon energy, we observe enhanced optical conductivity and spatially resolved BPVE. Polarization-dependent measurements reveal a characteristic twofold angular symmetry and a distinct shift-vector direction, both governed by crystalline symmetries and quantum geometry. The evolution of BPVE with carrier density and temperature provides a sensitive probe of the fine electronic structure of bilayer graphene, revealing signatures of trigonal warping and electron-hole asymmetry. Agreement with theory demonstrates that the terahertz nanoscopy of BPVE established here offers an optical lens into the interplay between band and quantum geometry in quantum materials.

cond-mat.mes-hall↗
arXiv2026

Orientational origin of nonlinear shear rheology in unentangled polymer melts

Universal rheological properties of unentangled polymer melts are investigated through the stretch-orientation decomposition of polymer dynamics. By externalizing stretch from the explicit state of each Rouse mode, we construct a constitutive model with orientational kinetics that preserves the Rouse relaxation spectrum. The resulting model yields the observed steady-shear scalings of the shear viscosity and first normal-stress coefficient, predicts a finite second normal-stress coefficient, and approximately reproduces the Cox-Merz and Gleissle-Osaki rules. The derived steady-oscillatory shear correspondence further identifies an elastic contribution, associated with the first normal stress difference, that is absent from the conventional Cox-Merz rule.

cond-mat.soft↗
arXiv2026

Symmetry Engineering Enables Deterministic Ensemble Polarization Control in Hexagonal Boron Nitride

Optical polarization is a key degree of freedom in quantum photonic and sensing technologies, enabling efficient light--matter coupling and directional emission. However, in defect ensemble-based quantum systems, orientational averaging across many defects suppresses optical anisotropy and eliminates a deterministic polarization axis. Here, symmetry engineering is introduced as a strategy to restore collective optical anisotropy in negatively charged boron vacancy (V_B^-) ensembles hosted in hexagonal boron nitride (h-BN). By imposing anisotropic in-plane tensile strain through lithographically defined nano-ridge arrays, in-plane symmetry is broken and the ensemble emission dipole is aligned with the ridge-defined strain symmetry axis. Polarization-resolved photoluminescence reveals that the polarization visibility scales with the magnitude of strain anisotropy, while the emission orientation rigidly follows the engineered symmetry axis across devices with varying strain direction. This deterministic alignment is independent of crystal orientation, establishing the optical foundation for polarization-defined ensemble spin readout and programmable photonic integration in two-dimensional quantum materials

cond-mat.mtrl-sci↗
arXiv2026

Visualization of Tunable Electronic Structure of Monolayer TaIrTe$_4$

Monolayer TaIrTe$_4$ is an ideal platform to explore the tunability of band topology where charge doping converts the quantum spin Hall insulator (QSHI) state into a dual QSHI state. However, experimental visualization of its band structure has been lacking despite its importance in interpreting this topological phase transition. Here we report the band structure of monolayer TaIrTe$_4$ using micro-focused angle-resolved photoemission spectroscopy ($μ$ARPES). Observed dispersions agree with the Heyd-Scuseria-Ernzerhof (HSE) calculations, establishing the insulating ground state without strong electronic correlations. Furthermore, we show an electron-hole asymmetry in the doping response, where hole doping is readily induced by electrostatic gating, whereas electron doping via gating and alkali-metal deposition yields band renormalization instead of a rigid shift. Fractional-charge calculations demonstrate that added electrons shrink the band gap and drive band renormalization. Thus, our work identifies the microscopic mechanism by which induced charges reshape the band topology of monolayer TaIrTe$_4$, beyond the rigid shift behavior.

cond-mat.str-el↗
arXiv2026

Polymer viscoelasticity from an orientational closure with implicit stretch

We propose a deviatoric-stress constitutive equation derived from orientational kinetics, while implicitly retaining contributions from conformational stretch. Starting from the dumbbell model, we show that conformational stretch affects orientational dynamics through an effective stress scale and rotational diffusivity, which determine an effective relaxation time. The orientation-based equation is derived by assuming an approximate relation between orientational anisotropy and the stretch state. Within the proposed framework, constant effective coefficients yield steady-oscillatory shear correspondences closely related to the empirical Cox-Merz and Gleissle-Osaki relations. For model-specific effective coefficients, the upper-convected Maxwell model is characterized by a constant relaxation time and an increasing modulus under steady shear, whereas finite extensibility with Peterlin preaveraging shortens the relaxation time and suppresses the modulus increase. With coefficients determined from steady-shear states, the proposed equation successfully reproduces stress responses under transient shear, large-amplitude oscillatory shear, and uniaxial extension. These results show that the effects of conformational stretch on orientational kinetics can be retained implicitly, providing a constitutive description based on a deviatoric-stress state reconstructable from standard rheometry and a physical interpretation of how stretch enters polymer viscoelasticity.

cond-mat.soft↗
arXiv2026

Capacitance sensing in bilayer graphene with gate reflectometry

In the search for topological states in bilayer graphene, capacitance measurement methods are widely utilized in bridge type readouts or source-drain reflectometry setups. In this paper we demonstrate an alternative readout method based on gate reflectometry. We probe the capacitance of a BLG flake encapsulated between hBN and WSe$_2$ through a lumped-element LC resonator coupled to the top gate electrode of the heterostructure. The microwave readout reliably tracks the opening of a displacement field-induced band gap in the device. Compared to simultaneous DC transport measurements, the RF signal proves to be less affected by the percolating transport channels caused by sample disorder. Furthermore, we use the gate based readout to identify a small-angle alignment between the hBN and BLG flake. From the fixed frequency microwave readout, we extract the device capacitance change between the insulating and metallic state of the bilayer and utilize a comprehensive capacitance network model to explain its origin. Finally, we propose an advanced, multiplexed double-resonator readout scheme capable of simultaneously measuring top and bottom gate capacitances to extract the layer polarizability of the system. This work establishes gate reflectometry as a highly scalable and powerful methodology for exploring topological phases and internal charge dynamics in van der Waals heterostructures.

cond-mat.mes-hall↗
arXiv2026

Topological Fermi-polaron Polaritons with Electrically Invertible Topology

Topological polaritons provide a route to engineer optical Chern bands, but realizing large gaps or efficient electrical tunability remains challenging because time-reversal-symmetry breaking in semiconductor polariton systems is typically limited by weak Zeeman splittings. Here, we demonstrate a topological Bose-Fermi mixture --- Fermi-polaron polaritons ---where large gaps and electrically invertible topology emerge from the interplay of Fermi-polaron many-body interactions, photonic symmetry, and strong light--matter coupling. Using a gate-controlled monolayer transition-metal dichalcogenide integrated with a symmetry-engineered photonic crystal, we create Fermi-polaron polaritons with topological band structures. The direct coupling between band topology and gate-tunable many-body interactions provides a mechanism for amplified time-reversal-symmetry breaking beyond bare Zeeman splitting and enables electrical inversion of topological gaps, Berry curvature, and Chern numbers. These results establish an electrically reconfigurable platform for interaction-driven topological states and programmable topological photonics.

physics.optics↗
arXiv2026

Atomic-Scale Imaging of Lattice Relaxation and Topological Flat Bands in Helical Trilayer Graphene

Helical trilayer graphene (HTG) has emerged as a highly tunable moire quantum material that hosts strong electronic correlations and nontrivial band topology. However, the atomic-scale lattice structure and local electronic properties have remained largely unexplored. Here we present a comprehensive real-space study of HTG using a combination of scanning near-field optical microscopy and low-temperature scanning tunneling microscopy. We directly image supermoire lattice relaxation, revealing large triangular domains separated by sharp domain walls, as well as stripe domains connected by smoothly varying boundaries. Atomic-scale spectroscopy uncovers flat bands with a honeycomb electronic texture and one-dimensional boundary states confined to domain walls. By systematically varying the twist angle, we identify a magic angle of approximately 1.9°, substantially larger than the 1.6 degree predicted by theory. Our results establish a direct microscopic link between lattice relaxation and flat bands in HTG.

cond-mat.mes-hall↗
arXiv2026

Strongly coupled ultracompressed self-cavity plasmon polaritons at millikelvin temperatures

Van der Waals heterostructures exhibit self-cavity resonances from sub-terahertz to terahertz frequencies and thus offer an attractive route toward realizing cavity-modified phases of matter. Spectroscopy of such systems calls for high frequency resolution at millikelvin temperatures, which is extremely challenging with existing methods. Here we demonstrate a new platform for realizing and probing such systems. Superconducting layers define self-cavity structures with electromagnetic volume compression of order $10^7$. With an integrated graphene layer, we observe underdamped cavity plasmon polaritons at frequencies as low as 0.04 THz and which exhibit avoided crossings with cavity modes. To measure such spectra, we deploy a millikelvin-compatible sub-THz spectrometer based on a superconducting circuit. This work paves the way for simultaneous spectroscopy and transport measurements of cavity-matter hybrid systems at millikelvin temperatures.

cond-mat.mes-hall↗
arXiv2026

Incipient superconductivity and tunable Chern insulators in twisted bilayer-trilayer graphene

Moiré superlattices assembled by twisting Bernal and rhombohedral multilayer graphene host a rich set of interaction-driven magnetic and topological states, yet superconductivity has not been observed in these systems except in proximity to a transition-metal dichalcogenide. Here we report incipient superconductivity and tunable Chern insulators in twisted bilayer-trilayer graphene encapsulated by hexagonal boron nitride. Across twist angles from $θ= 1.05^\circ$ to $1.50^\circ$, Chern insulators form at integer and fractional moiré fillings for electron doping, with Chern numbers up to |C| = 3 set by twist angle and tuned by doping. At $θ= 1.18^\circ$, a symmetry-broken metallic region forms for hole doping and hosts a trivial insulator at band filling $ν= -2$. Displacement field alone drives this insulator into a pocket of incipient superconductivity with a sharp transition, a well-defined critical current, and a critical temperature that peaks near the insulating boundary, although the resistance does not fall to zero. In-plane magnetic field expands the pocket, which persists to more than four times the weak-coupling Pauli limit, and stabilizes a second pocket in which Fraunhofer-like modulation of the critical current signals phase-coherent pairing. Twisted bilayer-trilayer graphene thus offers a single gate-tunable system in which pairing can be interfaced with Chern insulators whose topology is itself an adjustable parameter.

cond-mat.mes-hall↗
arXiv2026

Interlayer dark excitons in a van der Waals heterostructure

Interlayer excitons in transition metal dichalcogenide (TMD) heterostructures exhibit long lifetimes and long-range transport, making them promising for excitonic devices and quantum many-body phases, such as Bose-Einstein condensates. Achieving these goals requires a precise understanding of spin-allowed bright and nominally spin-forbidden dark excitons, because the lowest-energy exciton species governs population, transport, and condensation. Despite substantial progress, unambiguously distinguishing singlet and triplet interlayer excitons has been challenging, as moiré excitons in these heterostructures can mimic their optical signatures. Here, we report the direct spectroscopic identification of bright (singlet) and dark (triplet) interlayer excitons in high-quality, dual-gated WSe$_2$/hBN/WSe$_2$ homobilayers. Electric-field-dependent photoluminescence and reflectance reveal two momentum-direct interlayer transitions with distinct spin configurations. The interlayer dark excitons obey selection rules that differ from those of bright excitons. Strikingly, interlayer dark excitons retain strong valley polarization, even with their ultralong lifetime exceeding microseconds. Finally, we demonstrate twist-angle control, wherein twist-induced electron-hole momentum mismatch modulates interlayer exciton emission. These results provide critical insights into the electronic and excitonic structure of TMD heterostructures, opening new avenues for excitonic many-body physics and optoelectronic devices.

cond-mat.mes-hall↗
arXiv2026

Interlayer Fermi Polarons in Bilayer MoTe$_2$

Atomic bilayers of transition metal dichalcogenides (TMDs) host quantum phases governed by the layer degree of freedom, including bilayer Wigner crystals, fractional Chern insulators, and exciton condensates. These phases are probed primarily through exciton spectroscopy, yet it remains poorly understood how excitons and carriers interact to form Fermi polarons in bilayers, where both the impurity and the Fermi sea carry a layer pseudospin. Progress has been limited because most TMD bilayers have momentum-indirect optical bandgaps, in which non-radiative decay and inhomogeneous broadening obscure the intrinsic spectra. Here, we show that bilayer MoTe$_2$, unlike most TMD bilayers, retains a direct optical bandgap, providing a clean platform for studying bilayer Fermi-polaron physics. In a dual-gated device, an out-of-plane electric field continuously tunes the hybridization between intralayer and interlayer excitons, forming layer-coherent excitons. Upon electrostatic doping, the excitonic spectrum evolves into multiple polaron branches, controlled by both carrier doping and the out-of-plane electric field. Among these, we identify a polaron with no analog in monolayers, in which a layer-coherent exciton is dressed by carriers in the opposite layer, and is quantitatively captured by our field-theoretic model. Our results establish that pseudospin structure in both the impurity and the bath reshapes polaron formation, opening new avenues to many-body states such as Bose-Einstein condensates with interlayer coherence.

cond-mat.mes-hall↗
arXiv2026

Topological Hall Effect Induced by Chiral Spin Textures at the Ferroelectric/Ferromagnetic Interface

Chiral spin textures, largely driven by the Dzyaloshinskii-Moriya interaction, offer significant potential for next-generation computing technologies due to their chirality and topological stability. Ferroelectric/ferromagnetic van der Waals heterostructures are particularly appealing because they can combine interfacial inversion-symmetry breaking and spin-orbit coupling to promote interfacial Dzyaloshinskii-Moriya interaction, while switchable ferroelectric polarization provides a nonvolatile tuning knob. This study investigates interfacial chiral spin textures in few-layer Fe$_3$GeTe$_2$/$α$-In$_2$Se$_3$ heterostructures. Two groups of topological Hall signals are identified just below and above the coercive field, and thickness-dependent transport reveals a notable reduction in critical temperature with increasing Fe$_3$GeTe$_2$ layer thickness. Low-temperature magnetic force microscopy images reveal two types of magnetic bubbles with opposite magnetic contrasts near the coercive field, each associated with distinct topological Hall signals. Together with atomistic spin-dynamics simulations and first-principles calculations, these results support the formation of interfacial DMI-stabilized chiral spin textures. Switching the ferroelectric polarization of the $α$-In$_2$Se$_3$ layer further enables nonvolatile modulation of both anomalous and topological Hall effects. The resulting ferroelectric and magnetic bistabilities generate four distinguishable Hall resistance states programmable by electric and magnetic fields. These findings highlight the potential of van der Waals interfaces for advanced device applications.

cond-mat.mtrl-sci↗