arXiv subjects

T. Taniguchi

Publications and source records attributed to T. Taniguchi.

At least 37 records · Page 2Linked to original sources

arXiv2026

Chern junctions in Moiré-Patterned Graphene/PbI2

Expanding the moire material library continues to unlock novel quantum phases and emergent electronic behaviors. Here, we introduce PbI2 into the moire family and investigate the magnetotransport properties of moire superlattice in a hexagonal boron nitride/graphene/PbI2 heterostructures. In the high-field quantum Hall regime, we observe robust dissipationless transport at the charge neutrality point, indicative of incompressible states at filling factor vh = 0. Additionally, a fractional conductance plateau at 2/3 e2/h emerges, which we attribute to a Chern junction between domains with distinct Chern numbers originating from moire-modulated and conventional integer quantum Hall states. The moire Hofstadter spectrum displays an unconventional flavor sequence, likely influenced by proximity-induced spin-orbit coupling from the PbI2 layer. We also see coherent electronic interference along lines with Chern number vm = -2. These observations provide compelling evidence for the formation of moire domains that nontrivially interrupt incompressible quantum Hall states, reflecting the strong moire potential in the BN/graphene/PbI2 superlattice. We suggest that the moire Hofstadter spectrum coupled with the proximity-induced spin-orbit interaction from PbI2 gives rise to a high magnetic field topological insulator phase explaining ballistic transport at the charge neutrality point in the graphene monolayer.

cond-mat.mes-hall↗

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↗

arXiv2025

Photoexcitation of moiré-trapped interlayer excitons via chiral phonons

Moiré 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 moiré-trapped interlayer excitons in 2H-stacked MoSe$_2$/WSe$_2$ heterobilayers via chiral $E^{\prime\prime}$ in-plane optical phonons at the Γ-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 moiré systems, opening new opportunities for valleytronic and quantum photonic applications.

quant-ph↗

arXiv2025

Interaction-limited conductivity of twisted bilayer graphene revealed by giant terahertz photoresistance

Identifying the microscopic processes that limit conductivity is essential for understanding correlated and quantum-critical states in quantum materials. In twisted bilayer graphene (TBG) and other twist-controlled materials, the temperature dependence of metallic resistivity follows power-law scaling, with the exponent spanning a broad range, rendering standard transport measurements insufficient to unambiguously identify the dominant scattering processes and giving rise to competing interpretations ranging from phonon-limited transport and umklapp scattering to strange metallicity and heavy fermion renormalization. Here, we use terahertz (THz) excitation to selectively raise the electron temperature in TBG while keeping the lattice cold, enabling a direct separation of electron-electron and electron-phonon contributions to resistivity. We observe a giant THz photoresistance, reaching up to 70% in magic-angle devices, demonstrating that electronic interactions dominate transport even in regimes previously attributed to phonons, including the linear-in-temperature resistivity near the magic angle. Away from the magic angle, we observe coexisting photoresistance and robust quadratic-in-temperature resistivity at extremely low carrier densities where standard electron-electron scattering mechanisms (umklapp and Baber inter-band scattering) are kinematically forbidden. Our analysis identifies the breakdown of Galilean invariance in the Dirac-type dispersion as a possible origin of the interaction-limited conductivity, arising from inter-valley electron-electron collisions. Beyond twisted bilayer graphene, our approach establishes THz-driven hot-electron transport as a general framework for disentangling scattering mechanisms in low-density quantum materials.

cond-mat.mes-hall↗

arXiv2025

Spectroscopy of Wigner crystal polarons in an atomically thin semiconductor

Strongly interacting electrons in two-dimensional systems can spontaneously break translational symmetry, forming a periodic Wigner crystal. Although these crystals have been realized in several platforms, experimental studies of their collective many-body excitations in the absence of a magnetic field remain an outstanding challenge. Here, we access this regime optically by uncovering Wigner crystal polarons: novel light-matter excitations arising from the dressing of excitons by collective excitations of the Wigner crystal. These hybrid quasiparticles manifest as new optical resonances in cryogenic reflectance spectra of a charge-tunable WSe$_2$ monolayer, appearing concurrently with previously identified exciton umklapp transitions. In contrast to the latter, the energies of Wigner crystal polarons are governed not only by the electronic lattice constant but also by their hybridization with attractive exciton-polarons, whose strength is controlled by electronic interactions. These novel many-body excitations provide an optical interface to the spin state of the Wigner crystal, which as we demonstrate, can be controlled both magnetically and optically. Our work establishes layered materials as a unique platform for exploring dynamical impurity dressing by strongly correlated electronic orders.

cond-mat.mes-hall↗

arXiv2025

Evidence of Momentum Space Condensation in Rhombohedral Hexalayer Graphene

Spontaneous symmetry breaking provides a powerful window into the nature of underlying electronic orders. In strongly correlated systems, multiple symmetry-breaking orders can arise simultaneously. and their interplay generates an intricate landscape of quantum phases that has remained a central focus of condensed-matter research. In this work, we report a previously unidentified electronic phase in rhombohedral hexalayer graphene, distinguished by the simultaneous breaking of rotational, time-reversal, and inversion symmetries. Broken rotational symmetry is evidenced through anisotropic transport in angle-resolved measurements, while the onset of both the anomalous Hall effect and the nonlinear Hall effect signals the breaking of time-reversal and inversion symmetries. These combined signatures reveal an emergent order consistent with momentum-space condensation, a theoretically anticipated phenomenon realized here experimentally for the first time. This mechanism establishes a natural framework for understanding a broader class of correlated phases known to emerge from the flat bands of two-dimensional materials.

cond-mat.mes-hall↗

arXiv2025

Probing moire excitons in MoSe2/WSe2 heterobilayers by combined micro-photoluminescence and lateral force microscopy

We study interlayer excitons in MoSe2/WSe2 heterobilayers, by combining lateral force microscopy and micro-photoluminescence spectroscopy. This allows us to correlate the spatial profile of the moiré superlattice with the distribution of optically active states accessible to interlayer excitons. In heterostructures where a few degrees twist angle is imposed between the MoSe and WSe crystallographic axes, we show that a continuous moiré lattice is realized across areas close to the optical diffraction limit. In such regions, the photoluminescence reduces to a few narrow-band lines only, energetically distributed consistently with the geometry of the moiré lattice. This correlation reveals that interlayer excitons explore a controlled periodic confinement, paving the way towards implementations of Bose-Hubbard models.

cond-mat.mes-hall↗

arXiv2025

Direct imaging of magnetotransport at graphene-metal interfaces with a single-spin quantum sensor

Magnetotransport underlines many important phenomena in condensed matter physics, such as the Hall effect and magnetoresistance (MR) effect. Thus far, most magnetotransport studies are based on bulk resistance measurements without direct access to microscopic details of the spatial transport pattern. Here, we report nanoscale imaging of magnetotransport using a scanning single-spin quantum magnetometer, which is demonstrated in a graphene-metal hybrid device at room temperature. By visualizing the current flow at elevated magnetic fields (~0.5 T), we directly observe the Lorentz deflection of current near the graphene-metal interface, which is a hallmark of magnetotransport. Combining the local current distribution with global resistance measurements, we reveal that transport properties of the hybrid are governed by a complex interplay of intrinsic MR around the Dirac cone, carrier hydrodynamics, interface resistance, and the nanoscale device geometry. Furthermore, accessing the local transport pattern across the interface enables quantitative mapping of spatial variations in contact resistance, which is commonly present in electronic devices made from two-dimensional materials yet non-trivial to characterize. Our work demonstrates the potential of nanoscale current imaging techniques for studying complex electronic transport phenomena that are difficult to probe by resistance-based measurements.

cond-mat.mes-hall↗

arXiv2025

Interaction of moire-induced quantum Hall channels in a locally gated graphene junction

Manipulating electron quantum 1D channels is an important element in the field of quantum information due to their ballistic and phase coherence properties. In GaAs and graphene based two dimensional gas systems, these edge channels have been investigated with both integer and fractional quantum Hall effects, contributing to the realization of electron interferometer and anyon braiding. Often, at the p-n junction in the quantum Hall (QH) regime, the presence of a depletion region due to a band gap or the formation of gaps between the zeroth Landau levels (zLL) suppresses interaction between the co-propagating edge channels of opposing doping regimes and helps to preserve the phase coherence of the channels. Here, we observe a new type of p-n junction in hexagonal boron nitride aligned graphene that lacks both the zLL and band gap. In this system, a van Hove singularity (vHS) emerges at the p-n junctions under magnetic fields of several Tesla, owing to the doping inversion near the secondary Dirac point. By fabricating devices with independently tunable global bottom and local top gates, we enable the study of interactions between p-type and n-type QH edge channels through magnetic breakdown associated with the vHS. These findings provide valuable insights into the interactions of superlattice-induced QH edge channels in hBN-aligned graphene.

cond-mat.mes-hall↗

arXiv2025

2D Excitonics with Atomically Thin Lateral Heterostructures

Semiconducting transition metal dichalcogenides (TMDs), such as MoSe$_2$ and WSe$_2$, exhibit unique optical and electronic properties. Vertical stacking of layers of one or more TMDs, to create heterostructures, has expanded the fields of moiré physics and twistronics. Bottom-up fabrication techniques, such as chemical vapor deposition, have advanced the creation of heterostructures beyond what was possible with mechanical exfoliation and stacking. These techniques now enable the fabrication of lateral heterostructures, where two or more monolayers are covalently bonded in the plane of their atoms. At their atomically sharp interfaces, lateral heterostructures exhibit additional phenomena, such as the formation of charge-transfer excitons, in which the electron and hole reside on opposite sides of the interface. Due to the energy landscape created by differences in the band structures of the constituent materials, unique effects such as unidirectional exciton transport and excitonic lensing can be observed in lateral heterostructures. This review outlines recent progress in exciton dynamics and spectroscopy of TMD-based lateral heterostructures and offers an outlook on future developments in excitonics in this promising system.

cond-mat.mtrl-sci↗

arXiv2025

Gate-tunable Josephson diodes in magic-angle twisted bilayer graphene

We report low-temperature measurements of two adjacent, gate-defined Josephson junctions (JJs) in magic-angle twisted bilayer graphene (MATBG) at a moiré filling factor near $ν= -2$. We show that both junctions exhibit a prominent, gate-tunable Josephson diode effect, which we explain by a combination of large kinetic inductance and non-uniform supercurrent distribution. Despite their proximity, the JJs display differences in their interference patterns and different diode behavior, underscoring that microscopic inhomogeneities such as twist angle variations shape the non-uniform supercurrent and drive the diode behavior. As a result, the nonreciprocal supercurrent can be tuned by gate voltage, enabling tuning of the diode efficiency and even reversing the polarity at fixed magnetic fields. Our findings offer potential routes for tailoring Josephson diode performance in superconducting quantum circuits.

cond-mat.mes-hall↗

arXiv2025

Impact of irradiation conditions on the magnetic field sensitivity of spin defects in hBN nano flakes

We study $V_{\mathrm{B}}^-$ centres generated by helium focused ion beam (FIB) irradiation in thin ($\sim$70 nm) hBN nanoflakes, in order to investigate the effect of implantation conditions on the key parameters that influence the magnetic field sensitivity of $V_{\mathrm{B}}^-$ quantum sensors. Using a combination of photoluminescence, optically detected magnetic resonance, and Raman spectroscopy, we examine the competing factors of maximising signal intensity through larger $V_{\mathrm{B}}^-$ concentration against the degradation in spin coherence and lattice quality observed at high ion fluences. Our results indicate that both the $V_{\mathrm{B}}^-$ spin properties and hBN lattice parameters are largely preserved up to an ion fluence of $10^{14}$ ions/cm$^2$, and beyond this significant degradation occurs in both. At the optimal implantation dose, an AC magnetic sensitivity of $\sim 1\,μ\mathrm{T}/\sqrt{\mathrm{Hz}}$ is achieved. Using the patterned implantation enabled by the FIB, we find that $V_{\mathrm{B}}^-$ centres and the associated lattice damage are well localised to the implanted regions. This work demonstrates how careful selection of fabrication parameters can be used to optimise the properties of $V_{\mathrm{B}}^-$ centres in hBN, supporting their application as quantum sensors based on 2D materials.

physics.app-ph↗

arXiv2025

TbPt6Al3: A rare-earth-based g-wave altermagnet with a honeycomb structure

The magnetic properties of the Tb-honeycomb lattice compound TbPt6Al3, which crystallizes in the NdPt6Al3-type trigonal structure, have been studied by the measurements of electrical resistivity, magnetization M(T, B), and specific heat on single-crystalline samples. The magnetic susceptibility, M(T)/B, for B || c = 0.1 T shows a cusp at TN = 3.5 K, which temperature decreases with increasing the magnitude of B || c, while M(T)/B for B || a = 0.1 T remains constant with decreasing temperature below TN. This anisotropic behavior suggests a collinear antiferromagnetic (AFM) order of the Tb3+ moments pointing along the c axis. The data of M(T)/B for T > 10 K on the single crystal and that of inelastic neutron scattering from powdered samples have been simultaneously analyzed using the crystal field model. The analysis reveals the non-Kramers doublet ground state for the Tb3+ ion under the trigonal crystal field. The neutron powder diffraction measurement shows that the collinear AFM structure with a magnetic propagation vector k = [0, 0, 0] is associated with moments of 5.1 μB/Tb pointing along the c axis. Comparison of the magnetic point group with the nontrivial spin Laue group indicates that TbPt6Al3 is classified into bulk g-wave altermagnets.

cond-mat.str-el↗

arXiv2025

Hard and soft phase slips in a Fabry-Pérot quantum Hall interferometer

Quantum Hall Fabry-Pérot interferometers are sensitive to the properties of bulk quasiparticles enclosed by the interferometer loop, with the interference phase containing information about both the quasiparticle statistics and the Coulomb-mediated bulk-edge coupling. Previous studies have explored the role of the bulk-edge coupling in an equilibrium picture where quasiparticles enter and exit the interferometer rapidly compared to the timescale over which the interferometer phase is measured. Here, we present data from a monolayer graphene quantum Hall interferometer in the integer quantum Hall regime at $ν= -1$ and $ν= -2$. Quantum interference shows phase slips associated with the entrance of quasiparticles to the interferometer bulk. Tracing the dependence of these phase slips on the magnetic field, we show that the equilibration time can become as long as several minutes. We further use our multi-gated geometry to identify two classes of phase slips. The first is associated with the addition of a quasiparticle to a bulk `puddle' of quasiparticles uniformly coupled to the entire chiral edge state, while the second is associated with the addition of a quasiparticle trapped by a defect site that couples predominantly to the closest portion of the edge.

cond-mat.mes-hall↗

arXiv2025

Exciton Formation in Two-Dimensional Semiconductors

The optical properties of atomically thin semiconductors are dominated by excitons, tightly bound electron-hole pairs, which give rise to particularly rich and remarkable physics. Despite their importance, the microscopic formation mechanisms of excitons remain very poorly understood due to the complex interplay of concurrent phenomena occurring on an ultrafast timescale. Here, we investigate the exciton formation processes in 2D materials based on transition metal dichalcogenide (TMD) monolayers using a technique based on the control of excitation light polarization. It allows us to distinguish between the two competing models of exciton formation: geminate and bimolecular formation. The geminate process is the direct formation of the exciton from the initially photogenerated electron hole pair before the loss of correlation between them, whereas the bimolecular process corresponds to the random binding of free electron hole-pairs from the initially photogenerated plasma. These processes control the exciton formation time. Our findings reveal that the luminescence intensity is higher by up to 40% for circularly polarized excitation compared to linearly polarized excitation for laser energy above the free carrier gap. We show that this spin-dependent exciton emission is a fingerprint of the bimolecular formation process. Importantly, we observe that exciton linear polarization (valley coherence) persists even for laser excitation energies exceeding the gap. We demonstrate that it is the result of a fraction of excitons formed by a geminate process. This shows that two formation processes coexist for excitation energies above the gap, where both mechanisms operate concurrently.

cond-mat.mtrl-sci↗

arXiv2025

Dielectric environment engineering via 2D material heterostructure formation on hybrid photonic crystal nanocavity

Hybrid integration of two-dimensional (2D) materials with nanophotonic platforms has enabled compact optoelectronic devices by leveraging the unique optical and electronic properties of atomically thin layers. While most efforts have focused on coupling 2D materials to pre-defined photonic structures, the broader potential of 2D heterostructures for actively engineering the photonic environment remains largely unexplored. In our previous work, we employed single types of 2D material and showed that even monolayer flakes can locally induce high-$Q$ nanocavities in photonic crystal (PhC) waveguides through effective refractive index modulation. Here, we extend this concept by demonstrating that further transferring of 2D material flakes onto the induced hybrid nanocavity to form heterostructures enable more flexibility for post-fabrication dielectric environment engineering of the cavity. We show that the high-$Q$ hybrid nanocavities remain robust under sequential flake stacking. Coupling optically active MoTe$_{2}$ flake to these cavities yields enhanced photoluminescence and reduced emission lifetimes, consistent with Purcell-enhanced light-matter interactions. Additionally, encapsulation with a top hBN layer leads to a significant increase in the cavity $Q$ factor, in agreement with numerical simulations. Our results show that these heterostructure stacks not only preserve the cavity quality but also introduce an additional degrees of control -- via flake thickness, refractive indices, size and interface design -- offering a richer dielectric environment modulation landscape than what is achievable with monolayers alone, providing a versatile method toward scalable and reconfigurable hybrid nanophotonic systems.

physics.optics↗