arXiv subjects

T. Taniguchi

Publications and source records attributed to T. Taniguchi.

At least 19 recordsLinked to original sources

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 $\nu$=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 $\nu$=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

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

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 Moir\'e 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 moir\'e 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 moir\'e 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 moir\'e superlattice with a moir\`e 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 = t\kappa_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 moir\'e 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 $\pi$-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 moir\'e Hofstadter spectrum

Magnetic fields generally suppress phase-coherent Josephson transport, limiting superconducting interferometry to relatively low fields. Here we show that moir\'e-engineered graphene Josephson junctions can overcome this constraint. Using ballistic graphene/hBN junctions, we establish phase-coherent Andreev transport through Fabry-P\'erot oscillations and Fraunhofer interference that persist across both the primary Dirac cone and reconstructed moir\'e 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 moir\'e 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

Dipolar interlayer excitons in transition metal dichalcogenide alloy heterobilayers

Interlayer excitons in transition metal dichalcogenide (TMD) heterobilayers possess a permanent electric dipole moment and long recombination lifetimes, making them a promising platform for exploring excitonic many-body physics. Here, we report dipolar interlayer excitons in a MoS$_{1.4}$Se$_{0.6}$/MoSe$_2$ heterobilayer encapsulated in hexagonal boron nitride. Low-temperature photoluminescence measurements reveal a distinct emission peak at $\sim1.4$ eV, attributed to radiative recombination of interlayer excitons. The emission exhibits a blueshift with increasing excitation power, indicating repulsive dipole-dipole interactions. Time-resolved photoluminescence measurements uncover nanosecond-scale lifetimes, consistent with the spatial separation of electrons and holes across the two layers. These findings establish chalcogen-alloyed TMD heterobilayers as a versatile platform for engineering dipolar excitons and tuning excitonic interactions in van der Waals materials.

cond-mat.mes-hall↗

arXiv2026

Tuning the low-energy band structure in twisted bilayer WSe2

Tuning the electronic structures of two-dimensional (2D) material-based heterostructures is of crucial importance for their use in functional next-generation electronics. Here, through angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nano-ARPES), we systematically track the evolution of the near-Fermi-level electronic structure of bilayer WSe2 over a large range of twist angle. While the momentum positioning of the valence band maxima is independent of twist angle, we find that the energetic separation between the hole bands at the K point of the Brillouin zone and the higher binding-energy hole band at {\Gamma} can be varied in excess of 100 meV. We explore the mechanisms underpinning this evolution and discuss the implications for tuning both the size of the band gaps, and the efficiency of the spin-dependent electron-phonon coupling channels in homobilayer transition metal dichalcogenide devices.

cond-mat.mtrl-sci↗

arXiv2026

Incommensurate Antiferromagnetic Order in the Fe-substituted Bi-2201 Cuprate in the Heavily Overdoped Regime

Elastic neutron scattering experiments showed incommensurate antiferromagnetic peaks in 5% Fe-substituted Bi-2201 cuprate in the non-superconducting heavily overdoped regime. The incommensurability delta~0.21 is comparable to that observed in Fe-substituted Bi-2201 in the overdoped regime. [Hiraka et al., Phys. Rev. B 81, 144501 (2010).] The magnetic correlation length is comparable between the overdoped and non-superconducting heavily overdoped regimes. It is plausible that incommensurate antiferromagnetic order is induced and stabilized by Fe in the heavily overdoped regime, which suggests a robust antiferromagnetic correlation beyond the superconducting dome in the phase diagram.

cond-mat.supr-con↗

arXiv2026

Electron-phonon coupling across the TMD/hBN van der Waals interface

Many-body interactions can couple electronic states in one layer with collective excitations in the adjacent layer, providing a route to tailor properties of heterostructures. However, detecting and quantifying interlayer many-body interactions proved a major challenge. Here, we demonstrate that quasiparticles in monolayer transition metal dichalcogenides (TMDs) are dressed by a remote cloud of phonons in the adjacent hexagonal boron nitride slab. Using angle resolved photoemission, we identify replica bands in the TMDs which are a clear fingerprint of long-range electron-phonon interaction. We develop a modified Fr\"ohlich model that shows semi-quantitative agreement with the experimental spectral functions. Our analysis shows that remote electron-phonon coupling is a generic property of interfaces with hBN. This has implications for electron mobilities in 2D materials, for superconductivity and possibly for moir\'e correlated phases.

cond-mat.mtrl-sci↗

arXiv2026

Spin-Valley Relaxation of Rydberg Excitons

Rydberg excitons, characterized by large spatial extension and reduced electron-hole overlap, must have a spin-valley dynamics different from that of ground state excitons. Here we report a direct measurement of spin relaxation of Rydberg excitons in high-quality WSe2 monolayer using continuous-wave and time-resolved optical orientation experiments. Excited excitonic states exhibit exceptionally large photoluminescence circular polarization, approaching 90% for the 3s state. Time-resolved measurements reveal a strong increase of the spin relaxation time with the principal quantum number, from ~2 ps for the 1s exciton to ~75 ps for the 3s exciton. A microscopic model based on electron-hole exchange-driven spin relaxation quantitatively reproduces the observed trend, demonstrating that Rydberg excitons enable tunable spin-valley dynamics in two-dimensional semiconductors.

cond-mat.other↗

arXiv2026

Gate-Tunable Photoresponse of Graphene Josephson Junctions at Terahertz Frequencies

Graphene Josephson junctions (JJ) provide a promising platform for ultra-broadband quantum sensing of light owing to graphene's frequency-independent absorption, vanishing electronic heat capacity, and weak electron-phonon coupling, which enable rapid suppression of the critical current through radiation-induced electron heating. Existing investigations have been confined to the microwave and infrared regimes, where competing detector technologies are already established; by contrast, the terahertz (THz) band - where sensitivity is most urgently lacking and no mature quantum sensor exists - has remained largerly unexplored. Here we demonstrate a strong photoresponse of graphene JJs at THz frequencies, establishing a first experimental step towards graphene-based THz quantum sensors. Under low-intensity illumination, we observe a pronounced suppression of the critical current that generates a strong photovoltage (Vph) under current bias. By tracking this Vph and independently measuring the electron temperature as a function of absorbed power, we extract a responsivity of 88 kV W^-1 and a noise-equivalent power of 45 aW Hz^-1/2 at 1.7 K. Furthermore, gate tunability of our JJ enables access to a regime where hysteretic current-voltage characteristics persist up to 0.9 K, offering a potential route toward single-photon THz detection beyond millikelvin (mK) temperatures. These findings establish graphene JJ as a versatile platform for broadband cryogenic radiation sensing and point towards their use as quantum sensors at THz frequencies.

cond-mat.mes-hall↗

arXiv2026

Trion transfer in mixed-dimensional heterostructures

Charged excitons, or trions, offering unique spin and charge degrees of freedom, have primarily been investigated in doped systems where charges are long considered indispensable. Here, we present an alternative route to ultra-efficient trion emission from an intrinsic, defect-free semiconductor via a transfer mechanism. By exciting trions in two-dimensional tungsten-diselenide donors and transferring them into one-dimensional carbon-nanotube acceptors in mixed-dimensional heterostructures, we circumvent the usual carrier requirement, overcoming intrinsic Auger-quenching limitations. Benefitting from a reservoir effect induced by dimensional heterogeneity, this process achieves trion emission efficiencies increased by over 100-fold compared to conventional doping-based approaches, and remains robust across diverse doping conditions. Our findings extend the exciton transfer paradigm to the three-body quasiparticles, offering a new platform for advancing excitonic physics and trion-based optoelectronic/spintronic applications.

cond-mat.mes-hall↗

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 moir\'e 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-\r{A} 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 ($\theta \leq 0.1^\circ$), we uncover a significant expansion of the interlayer spacing compared to the bulk configuration, exceeding 0.1 \r{A}, along with a remarkable temperature-driven interlayer decoupling. Ultrafast measurements further reveal optically induced interlayer separation of ~0.2 \r{A} 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 moir\'e-engineered 2-dimensional materials.

cond-mat.mes-hall↗

arXiv2025

Photoexcitation of moir\'e-trapped interlayer excitons via chiral phonons

Moir\'e superlattices in transition-metal dichalcogenide semiconductor heterobilayers enable the quantum confinement of interlayer excitons with large out-of-plane permanent electric dipoles and spin-valley control. Here, we report a novel phonon-assisted excitation mechanism of individual moir\'e-trapped interlayer excitons in 2H-stacked MoSe$_2$/WSe$_2$ heterobilayers via chiral $E^{\prime\prime}$ in-plane optical phonons at the {\Gamma}-point. This excitation pathway preserves valley-selective optical selection rules and enables deterministic generation of individual interlayer excitons with defined helicity, emitting within a spectrally narrow energy spread. Through photoluminescence excitation spectroscopy in both the ensemble and quantum emitter regimes, we identify a fixed phonon energy of $\sim$23 meV mediating the process. First-principles calculations corroborate the symmetry and energy of the relevant phonon mode and its coupling to interlayer excitons, providing microscopic support for the observed valley-selective phonon-assisted excitation mechanism. Our results highlight the utility of chiral phonons as a tool for controlled excitation of quantum emitters in TMD moir\'e systems, opening new opportunities for valleytronic and quantum photonic applications.

quant-ph↗