arXiv subjects

Takashi Taniguchi

Publications and source records attributed to Takashi Taniguchi.

At least 145 records · Page 8Linked to original sources

arXiv2026

Layer-Hybridized Wigner Crystals in MoSe2/WS2 Moiré Superlattice

Transition metal dichalcogenide moiré heterobilayers with type-II band alignment provide a versatile platform for layer-polarized generalized Wigner crystals, in which strong Coulomb interactions drive charge ordering at fractional lattice fillings. With a finite interlayer band offset, an out-of-plane electric field can tune layer-resolved moiré bands through resonance and enable controllable interlayer hybridization. Although hybridized Mott insulators have been previously demonstrated, whether fractional charge-ordered states can survive such hybridization remains elusive. Here we drive an H-stacked MoSe2/WS2 moiré heterobilayer through a type-I-to-type-II band-alignment transition and realize layer-hybridized Mott insulator and generalized Wigner crystals. For fillings below one electron per moiré cell, tunneling delocalizes electrons and modifies Wigner crystallization. However, above one electron per cell, Coulomb repulsion overcomes tunneling and favors layer-separated occupation, stabilizing stronger charge-ordered states. These results establish electrically tunable hybridized moiré heterobilayers as a powerful platform for engineering correlated charge order and exploring fractional Chern phases and emergent magnetism.

cond-mat.mes-hall↗

arXiv2026

Dual-Mode Exciton Coupling in Epitaxially Registered Organic-Inorganic 2D Heterocrystals

Two-dimensional (2D) heterocrystals comprising molecules and semiconductors can serve as an ideal platform for studying interfacial excitons and for future optoelectronic applications, yet the energy and charge flow across these atomically sharp interfaces remain unclear. In this work, we investigated PTCDA-MoS2 as a prototypical 2D organic-inorganic heterostructure and revealed dual-mode exciton coupling between the constituent crystals. Monolayer-resolved PTCDA molecular crystals were grown on monolayer MoS2 via physical vapor assembly, and their crystallographic details, including the stacking angle, were determined by electron diffraction. Upon the formation of the heterostructures, PTCDA's photoluminescence was completely quenched because of organic-to-inorganic hole transfer, whereas that of MoS2 increased markedly with PTCDA thickness. Using differential reflectance and photoluminescence excitation spectroscopy, we found that the enhancement arises from two distinct mechanisms. Ground-state charge transfer injects holes into MoS2, which suppresses negative trion formation and enhances the radiative recombination of neutral excitons. In addition, resonant energy transfer, enabled by spectral overlap between PTCDA and MoS2, diverts excitation energy from PTCDA to MoS2. Our findings reconcile previously proposed mechanisms and establish a unified framework in which charge and energy transfer cooperate to govern exciton coupling at organic-inorganic interfaces.

cond-mat.mtrl-sci↗

arXiv2026

Charge Tunable Optical Nonlinearity of Moiré Exciton-Polaritons

Transition metal dichalcogenides represent a versatile platform to study strong light-matter interactions based on excitons and electrons in ordered lattices. Twist-engineering of moiré structures further enables the manipulation of the polaritonic nonlinearities via engineering the exciton landscape on the nanoscale. In this work, we demonstrate in-situ control of the optical saturation-based nonlinearity of moiré exciton-polaritons by phase space restriction via charge doping. Strong exciton-photon coupling is established in a gate-controllable MoTe$_2$-MoSe$_2$ heterobilayer, embedded in a spectrally-tunable open cavity. A small gate voltage can effectively lower the necessary polariton density by one order of magnitude to achieve a similar nonlinear saturation effect as in the charge-neutral case. Our microscopic description successfully explains the observed phenomena in the framework of Pauli blocking for the moiré superlattices with charge preoccupation.

cond-mat.mes-hall↗

arXiv2026

Optical switching of magnetic order in few-layer CrSBr

Manipulating magnetism with light is crucial for both fundamental understanding and technological advancements of information storage devices. The layered antiferromagnet CrSBr, part of the recently emerging class of two-dimensional van der Waals magnets, offers a unique path towards optical control of magnetism via magneto-excitons, which allow optical readout of the spin alignment and also provide a strong absorption channel. Here, we use exciton absorption and photoluminescence to demonstrate optical switching of the magnetic order in bi- and trilayer CrSBr. Using a continuous-wave laser with a power as low as a few microwatts at near-critical external magnetic fields, we demonstrate both local and remote switching of the magnetic configuration in extended lateral domains and further employ this mechanism to deterministically prepare the zero-field magnetic configuration. Our results establish optical switching as a useful means of controlling magnetism in CrSBr, with potential applications in magneto-optoelectronic devices.

cond-mat.mes-hall↗

arXiv2026

Lifshitz transitions and isospin polarization in twist-decoupled monolayer-bilayer graphene

Bernal-stacked bilayer graphene (BLG) hosts correlated electronic phases tied to low-energy Lifshitz transitions at saddle points in its valence band. To access this regime, ultralow charge disorder and control over a vertical electric field are simultaneously required. Here, we employ a twist-decoupled monolayer (MLG) to bias a proximal BLG in the absence of an external displacement field (D). We thereby reveal three-fold degenerate quantum Hall states at D = 0, with multiple transitions driven by doping, magnetic and electric field. Spontaneous broken symmetry in the vicinity of the valence band edge is signaled by the emergence of quantum oscillations with anomalous frequencies and large quasiparticle mass. These results indicate that electronic interactions in BLG are preserved in presence of an atomically close MLG, while showcasing the potential of CVD-grown graphene multilayers for the exploration of correlated phases of matter.

cond-mat.mes-hall↗

arXiv2026

Emergence of Propagating Exciton-Polaritons in Hybrid Waveguide-van der Waals Heterostructures

Integrating few-layer materials into photonic circuits is a promising concept for novel on-chip photonic applications. We incorporate transition metal dichalcogenides (TMDs) with femtosecond-laser-written surface waveguides, which are embedded in fused silica chips. Our novel low-temperature optical spectroscopy setup enables coupling to the waveguide and simultaneous focus from the top to the TMD layer for a distinct excitation and collection of micro-photoluminescence ($μ$PL) signals in several measurement geometries. Along these lines, we observe spectral changes of the A exciton for encapsulated TMD monolayers when capturing the $μ$PL signal propagating through the waveguide. Depending on the thickness of the encapsulation with hexagonal boron nitride (hBN), these changes manifest as energetic redshifts of the A exciton, or even a splitting of the A exciton into two components. We attribute this behavior to strong coupling of the waveguide mode and the exciton in the sample, giving rise to the formation of propagating exciton-polaritons. Our interpretation is supported by calculations for a simplified model of a slab waveguide in the vicinity of an exciton by using the transfer matrix method. Having proven to be a highly adaptable framework for the study of propagating polaritons, our experimental platform likewise holds great promise for harnessing the unique properties of exciton-polaristons in integrated photonic circuits.

cond-mat.mes-hall↗

arXiv2026

Sub-nm range momentum-dependent exciton transfer from a 2D semiconductor to graphene

Heterostructures made from atomically thin semiconductors (here MoSe$_2$) and graphene are uniquely poised to investigate photoinduced charge and energy transfer in the 2D limit. Here, using picosecond time-resolved photoluminescence spectroscopy at cryogenic temperatures on two types of MoSe$_2$/graphene heterostrutures, we unveil key features of the underlying mechanisms. First, the shortening of the MoSe$_2$ bright exciton lifetime is marginally affected by the number of graphene layers to which MoSe$_2$ is coupled. Second, exciton transfer vanishes when a sub-nm thick spacer of hexagonal boron nitride decouples MoSe$_2$ from graphene. These results indicate that charge tunneling govern bright exciton relaxation in MoSe$_2$/graphene and that longer-range, Förster-type energy transfer (FRET) does not affect bright excitons. However, sub-ps FRET to graphene accelerates the relaxation of ``hot'' excitons formed upon optical excitation, leading to photoluminescence quenching factors that exceed expectations based on the shortening of the bright exciton lifetime. Our work has direct implications for energy harvesting and funneling using van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2026

Competing Chern states revealed by quasiparticle charging in moiré rhombohedral graphene

Moiré materials realize a versatile platform for exploring the physics of fractional Chern insulators (FCIs). The recently observed evolution from FCIs to an extended quantum anomalous Hall background upon lowering the electronic temperature in moiré rhombohedral graphene (mRG)8 raises a fundamental question: Is it caused by a failure to equilibrate the edge states of an FCI or by a genuine phase transition in the bulk from an FCI to a generalized anomalous Hall crystal? Here we address this question by probing quasiparticle charging in a mesoscopic mRG antidot device and by bulk resistance measurements, both of which are bulk-sensitive and free from complications from edge states. Tunneling to the mRG antidot reveals quasiparticles carrying one electron charge for both Chern states at filling factors ν=1 and 2/3 at low temperatures. Temperature dependence measurements of the bulk resistance near ν=2/3 further suggest a thermodynamic phase transition from an FCI to a generalized anomalous Hall crystal at temperatures below about 150mK. The results clearly exclude the edge state equilibration scenario and favor the phase transition scenario. Our work establishes mesoscopic probes as a powerful approach to uncover competing ground states in moiré materials and provides a basis for probing fractionalized excitations in FCIs.

cond-mat.mes-hall↗

arXiv2026

Probing the Potential Profile of Twisted Bilayer Graphene via Fabry-Pérot Interference

We use Fabry-Pérot interference to probe the internal potential profile of large-angle twisted bilayer graphene. We trace anomalous resistance oscillations in the nominally unipolar regime to a hidden cavity formed by unintentional local doping in our device. By analyzing zero-field interference patterns, we determine the location and size of this inhomogeneity. Magnetotransport measurements support the model, distinguishing local cavity modes from global resonances through their different magnetic dependence. Simulations using our extracted profile reproduce the experimental features. Our results highlight interference spectroscopy as a simple, non-invasive probe for identifying local defects and internal potential barriers in ballistic devices.

cond-mat.mes-hall↗

arXiv2026

Quasiparticle phono-conversion: filming carriers coalescing into excitons

Condensed matter physics is replete with phenomena involving high-energy free particles coalescing into low-energy bound few-particle states. While the cooling of the individual particles is well understood, the crucial step by which cold free carriers form a bound state remains elusive, involving complex energy and momentum relaxation pathways. Here, by combining ultrafast time- and momentum-resolved photoemission spectroscopy on a monolayer WSe$_2$ with the first-principles excitonic-Bloch equations, we resolve the conversion of initially free electrons and holes at the bandedges into bound excitons. With unprecedented energy resolution, we observe the transient \textit{coexistence} of free-carrier and excitonic bands, accompanied by a transfer of spectral weight between the two. We establish the phononic origin of exciton formation and ascribe this coexistence to a sequential relaxation cascade toward the lowest-energy excitonic states, wherein intermediate states remain weakly populated. The efficiency of this process is controlled by valley multiplicity, large-momentum phonon emission and spin-flip processes. By elucidating how bound states emerge from their elementary constituents, our results point to strategies for engineering exciton formation, with direct implications for optical materials and devices that operate with excitons or free carriers.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous metal and superconducting phases in rhombohedral graphene

Two-dimensional superconductivity is now well established in graphene-based systems, with many such realizations showing evidence for unconventional pairing. Yet in several of the gate-tuned phases that otherwise exhibit clear signatures of superconductivity, the resistance does not vanish as temperature is lowered, instead saturating at a finite value. Here we report a systematic study of this behavior in rhombohedral graphene on a WSe$_2$ substrate, finding regions of gate space with zero-resistance superconductivity alongside others with finite saturation resistance. At zero magnetic field, these regions appear as isolated pockets in gate space that otherwise exhibit strikingly similar phenomenology, including abrupt transitions to the normal state as temperature, perpendicular magnetic field, and current are raised above critical values. A small in-plane field expands and merges these pockets without qualitatively altering their behavior, producing a sharp boundary at millikelvin base temperature between states of zero or finite resistance. The finite-resistance state reproduces key phenomenology associated with the anomalous metal, a state that has been observed in thin-film superconductors for decades but lacks an accepted theoretical explanation. The tunability and reproducibility of ultra-clean rhombohedral graphene place strong constraints on extrinsic explanations and provide a new platform for understanding this behavior.

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arXiv2026

Third-order nonlinear transport in a percolative two-dimensional superconductor

Percolative superconductivity frequently arises in two-dimensional van der Waals materials due to reduced dimensionality, enhanced quantum fluctuations, and complex electron-phonon interactions, providing a unique platform where normal electrons coexist with Cooper pairs. We report the observation of substantial third-order nonlinear transport in a trilayer $1T^\prime$-MoTe$_2$ superconductor within its percolative transition regime. The third-harmonic longitudinal voltage ($V_{\|}^{3ω}$) exhibits a clear cubic dependence on excitation current below a threshold, with both its magnitude and nonlinear coefficient strongly correlated with the superconducting state. This nonlinear response is semiquantitatively captured by the superconducting fluctuation within the time-dependent Ginzburg-Landau theory, where nonlinear transport arises due to fluctuating Cooper pairs. Our results demonstrate that third-order nonlinear transport serves as a sensitive probe of superconducting transitions in percolative systems and establish a foundation for exploring higher-order transport phenomena in strongly correlated systems.

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arXiv2026

Large Thermopower with Sign-Alternating Quantum Oscillations in Insulating Monolayer WTe2

The detection of Landau-level-like energy structures near the chemical potential of an insulator is essential to the search for a class of correlated electronic matter hosting charge-neutral fermions and Fermi surfaces, a long-proposed concept that remains elusive experimentally. Here we introduce and demonstrate that the magneto-thermoelectric response of a quantum insulator can reveal critical information not available via other approaches. We report the observation of large thermopower together with quantum oscillations (QOs) in the hole-doped insulating state of monolayer tungsten ditelluride (WTe2) in magnetic fields. The measured low temperature magneto-thermopower exceeds k_B/e by more than an order of magnitude, where k_B is the Boltzmann constant and e the elementary charge. This large thermopower is a characteristic of an insulating state, consistent with high resistivity. However, as the magnetic field is swept, QOs develop in the thermopower, which remarkably undergoes sign-changes that mimic the quantum characteristic of metals due to Landau quantization. The resistivity of the sample is orders of magnitude larger than the value expected from the QOs. The sign-change in the thermoelectric response directly implies the presence of a field-induced Landau-level-like structure at the chemical potential of the insulator. Neither the large thermopower nor the sign-changes can be induced by the metallic gate nearby. Our results demonstrate a new dilemma for investigating low energy excitations in correlated materials featuring mixed quantum characteristics of metals and insulators.

cond-mat.str-el↗

arXiv2026

Same-spin Andreev reflections in the quantum Hall regime: the role of loss

The interfaces of superconductors and topological materials hold promise for realizing exotic states and excitations. An important example is provided by the chiral Andreev edge states (CAES), which are formed at interfaces between quantum Hall (QH) states and superconductors (SC). CAES combine electron and hole amplitudes which are hybridized via Andreev reflections. This study explores the spin properties of the CAES through selective spin filtering of the QH edge channels. We find robust evidence of spin-flips accompanying the Andreev processes: electrons can be reflected from the superconductor as holes in the same spin channel. We demonstrate that the distribution of the reflection probabilities is exponential and then use random matrix theory to account for this observation. Finally, we observe Andreev reflections in the spin-polarized ν = 1 case, which is enabled by particle loss. Our findings shed light on the mechanism underlying Andreev reflections of spin-polarized chiral states. They also demonstrate the importance of considering non-Hermiticity when constructing topological superconductors in hybrid materials.

cond-mat.mes-hall↗

arXiv2026

Scanning Gate Microscopy Modulation of Supercurrent in Graphene Josephson Junctions

Graphene Josephson junctions represent an excellent platform for quantum technologies, thanks to the combination of high carrier mobility, ballistic transport, and large gate-tunable critical currents, preserved even under quantizing magnetic fields. Investigating the spatial distribution of supercurrent flow could be crucial for elucidating transport mechanisms and advancing the engineering of these devices. In this work, we employ a Scanning Gate Microscope to investigate supercurrent transport in hBN-encapsulated graphene Josephson junctions contacted by Niobium leads. We study the supercurrent modulation as a function of the applied tip voltage bias and tip-to-sample distance, and provide a complete characterization of the tip-induced modulation. Our experimental results are quantitatively consistent with numerical simulations and pave the way towards local mapping and manipulation of gate-tunable superconducting phenomena with unprecedented spatial resolution.

cond-mat.mes-hall↗

arXiv2026

Visualizing the Impact of Quenched Disorder on 2D Electron Wigner Solids

Electron Wigner solids (WSs)1-12 provide an ideal system for understanding the competing effects of electron-electron and electron-disorder interactions, a central unsolved problem in condensed matter physics. Progress in this topic has been limited by a lack of single-defect-resolved experimental measurements as well as accurate theoretical tools to enable realistic experiment/theory comparison. Here we overcome these limitations by combining atomically-resolved scanning tunneling microscopy (STM) with neural-quantum-state quantum Monte Carlo (NQS-QMC) simulation of disordered 2D electron WSs to discover new disorder-induced physical regimes of correlated electron behavior. STM was used to image the electron density ($n_e$) dependent evolution of electron WSs in gate-tunable bilayer MoSe2 devices with varying long-range ($n_\mathrm{LR}$) and short-range ($n_\mathrm{SR}$) disorder densities. These images were compared to NQS-QMC simulations using realistic disorder maps extracted from experiment, thus allowing the roles of different disorder types to be disentangled. We identify two distinct physical regimes for disordered electron WSs that depend on $n_\mathrm{SR}$. For $n_\mathrm{SR} \lesssim n_e$ the WS behavior is dominated by long-range disorder and features extensive mixed solid-liquid phases, a new type of local re-entrant melting/crystallization, and prominent Friedel oscillations. In contrast, when $n_\mathrm{SR} \gg n_e$ these features are suppressed and a more robust amorphous WS phase emerges that persists to higher ne, highlighting the importance of short-range disorder in this regime. Our work establishes a powerful framework for studying disordered quantum solids via a combined experimental-theoretical approach.

cond-mat.str-el↗

arXiv2026

Space-Charge-Limited van der Waals Spin Transistor

Integrating semiconducting and magnetic materials could combine transistor-like operation with nonvolatility and enable architectures such as logic-in-memory. Here, we employ correlated electrical transport and scanning nitrogen-vacancy (NV) center magnetic imaging to elucidate a spin transistor concept that amalgamates vertical and lateral hopping transport inside a 2D antiferromagnetic semiconductor, mechanistically distinct from vertical tunneling devices. Our device, based on a monolayer-bilayer junction in CrSBr, displays giant, gate-tunable magnetoresistance driven by the dual action of electrostatic doping on space-charge-limited lateral conduction and interlayer exchange coupling. Moreover, we visualize a field-trainable, layer-sharing effect that selects between coherent or domain-wall reversal at the spin-flip transition, enabling multilevel, memristive conductance states. Our layer-dependent space charge mechanism for convergent electrical and magnetic control opens opportunities to address limitations in contemporary computing.

cond-mat.mes-hall↗

arXiv2026

Van Hove singularity-driven giant Nernst signal in twisted double bilayer graphene

Twisted graphene layers host van Hove singularities (vHSs), peaks in the electronic density of states, thought to drive exotic phases in moiré materials, but their effect on thermal transport has remained unclear. Here we show that vHSs in twisted double bilayer graphene (tDBLG) generate an unusually large Nernst signal-the transverse voltage produced by a longitudinal temperature gradient in a magnetic field. The pronounced Nernst peaks at the vHSs of the conduction and valence bands of tDBLG are tunable by an electric field with a maximum value of $\sim 40$ $μV K^{-1} T^{-1}$ at $\sim 1$ $K$, which is comparable to the best-known Nernst materials. Our theoretical calculations show that the large enhancement of the Nernst signal arises from the Lifshitz transitions around the vHSs. These findings establish the Nernst effect as a sensitive probe of Fermi-surface topology in moiré materials, and identify a universal thermoelectric signature of van Hove singularities.

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