arXiv subjects

T. Taniguchi

Publications and source records attributed to T. Taniguchi.

At least 55 records · Page 3Linked to original sources

arXiv2025

Enhanced Terahertz Thermoelectricity via Engineered van Hove Singularities and Nernst Effect in Moiré Superlattices

Thermoelectric materials, long explored for energy harvesting and thermal sensing, convert heat directly into electrical signals. Extending their application to the terahertz (THz) frequency range opens opportunities for low-noise, bias-free THz detection, yet conventional thermoelectrics lack the sensitivity required for practical devices. Thermoelectric coefficients can be strongly enhanced near van Hove singularities (VHS), though these are usually difficult to access in conventional materials. Here we show that moiré band engineering unlocks these singularities for THz optoelectronics. Using 2D moiré structures as a model system, we observe strong enhancement of the THz photothermoelectric response in monolayer and bilayer graphene superlattices when the Fermi level is tuned to band singularities. Applying a relatively small magnetic field further boosts the response through the THz-driven Nernst effect, a transverse thermoelectric current driven by the THz-induced temperature gradient. Our results establish moiré superlattices as a versatile platform for THz thermoelectricity and highlight engineered band structures as a route to high-performance THz optoelectronic devices.

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arXiv2025

Evidence for a Superfluid-to-solid Transition of Bilayer Excitons

One of the most spectacular properties associated with Bose-Einstein condensation (BEC) is superfluidity in which the system exhibits zero viscosity and flows without dissipation. The superfluid phase has been observed in wide ranging Bosonic systems spanning naturally occurring quantum fluids, such as liquid helium, to engineered platforms such as bilayer excitons and cold atom systems. Theoretical works have proposed that interactions could drive the BEC ground state into another exotic phase that simultaneously exhibits properties of both a crystalline solid and a superfluid - termed a supersolid. Identifying a material system, however, that hosts the predicted BEC solid phase, driven purely by interactions and without imposing an external lattice potential, has remained elusive. Here we report observation of a superfluid to insulator transition in the layer-imbalanced regime of bilayer magneto-excitons. Mapping the transport behavior of the bilayer condensate as a function of density and temperature, suggests that the insulating phase is an ordered state of dilute excitons, stabilized by dipole interactions. The insulator melts into a recovered superfluid upon increasing the temperature, which could indicate that the low temperature solid is also a quantum coherent phase.

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arXiv2025

The carbon nanotube gatemon qubit

Gate-tunable transmon qubits are based on quantum conductors used as weak links within hybrid Josephson junctions. These gatemons have been implemented in just a handful of systems, all relying on extended conductors, namely epitaxial semiconductors or exfoliated graphene. Here we present the coherent control of a gatemon based on a single molecule, a one-dimensional carbon nanotube, which is integrated into a circuit quantum electrodynamics architecture. The measured qubit spectrum can be tuned with a gate voltage and reflects the quantum dot behaviour of the nanotube. Our ultraclean integration, using a hexagonal boron nitride substrate, results in record coherence times of 200ns for carbon nanotube-based qubits. Furthermore, we investigate its decoherence mechanisms, thus revealing a strong gate dependence and identifying charge noise as a limiting factor. On top of positioning carbon nanotubes as contenders for future quantum technologies, our work paves the way for studying microscopic fermionic processes in low-dimensional quantum conductors.

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arXiv2025

Enhanced shot noise in graphene quantum point contacts with electrostatic reconstruction

Shot noise measurements in quantum point contacts are a powerful tool to investigate charge transport in the integer and fractional quantum Hall regime, in particular to unveil the charge, quantum statistics and tunneling dynamics of edge excitations. In this letter, we describe shot noise measurements in a graphene quantum point contact in the quantum Hall regime. At large magnetic field, the competition between confinement and electronic interactions gives rise to a quantum dot located at the saddle point of the quantum point contact. We show that the presence of this quantum dot leads to a $50-100~\%$ increase in the shot noise, which we attribute to correlated charge tunneling. Our results highlight the role played by the electrostatic environment in those graphene devices.

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arXiv2025

Milli-Tesla Quantization enabled by Tuneable Coulomb Screening in Large-Angle Twisted Graphene

The electronic quality of graphene has improved significantly over the past two decades, revealing novel phenomena. However, even state-of-the-art devices exhibit substantial spatial charge fluctuations originating from charged defects inside the encapsulating crystals, limiting their performance. Here, we overcome this issue by assembling devices in which graphene is encapsulated by other graphene layers while remaining electronically decoupled from them via a large twist angle (~10-30°). Doping of the encapsulating graphene layer introduces strong Coulomb screening, maximized by the sub-nanometer distance between the layers, and reduces the inhomogeneity in the adjacent layer to just a few carriers per square micrometre. The enhanced quality manifests in Landau quantization emerging at magnetic fields as low as ~5 milli-Tesla and enables resolution of a small energy gap at the Dirac point. Our encapsulation approach can be extended to other two-dimensional systems, enabling further exploration of the electronic properties of ultrapure devices.

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arXiv2025

Revealing electron-electron interactions in graphene at room temperature with the quantum twisting microscope

The Quantum Twisting Microscope (QTM) is a groundbreaking instrument that enables energy- and momentum-resolved measurements of quantum phases via tunneling spectroscopy across twistable van der Waals heterostructures. In this work, we significantly enhance the QTMs resolution and extend its measurement capabilities to higher energies and twist angles by incorporating hexagonal boron nitride (hBN) as a tunneling dielectric. This advancement unveils previously inaccessible signatures of the dispersion in the tunneling between two monolayer graphene (MLG) sheets, features consistent with a logarithmic correction to the linear Dirac dispersion arising from electron-electron (e-e) interactions with a fine-structure constant of alpha = 0.32. Remarkably, we find that this effect, for the first time, can be resolved even at room temperature, where these corrections are extremely faint. Our results underscore the exceptional resolution of the QTM, which, through interferometric interlayer tunneling, can amplify even subtle modifications to the electronic band structure of two-dimensional materials. Our findings reveal that strong e-e interactions persist even in symmetric, nonordered graphene states and emphasize the QTMs unique ability to probe spectral functions and their excitations of strongly correlated ground states across a broad range of twisted and untwisted systems.

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arXiv2025

The Interacting Energy Bands of Magic Angle Twisted Bilayer Graphene Revealed by the Quantum Twisting Microscope

Electron interactions in quantum materials fundamentally shape their energy bands and, with them, the material's most intriguing quantum phases. Magic angle twisted bilayer graphene (MATBG) has emerged as a model system, where flat bands give rise to a variety of such phases, yet the precise nature of these bands has remained elusive due to the lack of high-resolution momentum space probes. Here, we use the quantum twisting microscope (QTM) to directly image the interacting energy bands of MATBG with unprecedented momentum and energy resolution. Away from the magic angle, the observed bands closely follow the single-particle theory. At the magic angle, however, we observe bands that are completely transformed by interactions, exhibiting light and heavy electronic character at different parts of momentum space. Upon doping, the interplay between these light and heavy components gives rise to a variety of striking phenomena, including interaction-induced bandwidth renormalization, Mott-like cascades of the heavy particles, and Dirac revivals of the light particles. We also uncover a persistent low-energy excitation tied to the heavy sector, suggesting a new unaccounted degree of freedom. These results resolve the long-standing puzzle in MATBG - the dual nature of its electrons - by showing that it originates from electrons at different momenta within the same topological heavy fermion-like flat bands. More broadly, our results establish the QTM as a powerful tool for high-resolution spectroscopic studies of quantum materials previously inaccessible to conventional techniques.

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arXiv2025

Weak localization as probe of spin-orbit-induced spin-split bands in bilayer graphene proximity coupled to WSe$_2$

Proximity coupling of bilayer graphene (BLG) to transition metal dichalcogenides (TMDs) offers a promising route to engineer gate-tunable spin-orbit coupling (SOC) while preserving BLG's exceptional electronic properties. This tunability arises from the layer-asymmetric electronic structure of gapped BLG, where SOC acts predominantly on the layer in contact with the TMD. Here, we present high-quality BLG/WSe$_2$ devices with a proximity-induced SOC gap and excellent electrostatic control. Operating in a quasi-ballistic regime, our double-gated heterostructures allow to form gate-defined p-n-p cavities and show clear weak anti-localization (WAL) features consistent with Rashba-type SOC. At lower hole densities, a transition to weak localization (WL) is observed, signaling transport through a single spin-split valence band. These findings - in agreement with calculations - provide direct spectroscopic evidence of proximity-induced spin-split band in BLG and underscore the potential of BLG/TMD heterostructures for spintronics and spin-based quantum technologies.

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arXiv2025

Twist-Angle-Controlled Anomalous Gating in Bilayer Graphene/BN Heterostructures

Anomalous gating effects-such as gate ineffectiveness and pronounced hysteresis-have been observed in graphene-based systems encapsulated in boron nitride (BN) and linked to a possible ferroelectric state. However, their origin, stability, and reproducibility remain under debate. Here, we present charge transport experiments in dual-gated, dynamically rotatable van der Waals heterostructures based on bilayer graphene encapsulated in BN. Remarkably, the angular degree of freedom acts as an ON/OFF switch for the anomalous gating response. We show that the angular alignment between the two BN layers -- not the presence of a moiré superlattice with graphene -- is the key parameter governing these effects. The relevant alignment between the two BN layers, to observe the anomalous gating effect at room temperature, lies between 15 deg and 45 deg, with no evidence of the expected 60 deg periodicity. Both gate ineffectiveness and hysteresis are highly sensitive to small angular changes, which we classify into three distinct regimes. Our results clarify the conditions necessary to reproduce these phenomena and pave the way for theoretical investigation of their microscopic origins.

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arXiv2025

AC-Stark Spectroscopy of Interactions between Moiré Excitons and Polarons

We use nonlinear pump-probe spectroscopy to study optical excitations in a charge-tunable MoSe$_2$\WS$_2$ moiré heterostructure. An intense red-detuned laser pulse creates a photonic dressing of the material by introducing a large virtual population of excitons or exciton polarons in a deep moiré potential. By measuring the resulting ac Stark effect with a weak resonant laser pulse, we gain access to the nature and mutual interactions of the elementary optical excitations. At charge neutrality, our measurements reveal that different exciton resonances, associated with confinement of their center-of-mass motion in the moiré potential, have a significant spatial overlap. The resulting short-range interactions manifest themselves as a density-dependent blueshift for same-valley excitons and bound biexciton states for opposite-valley excitons. The attractive polaron resonance that appears upon injection of electrons into the heterostructure shows a contrasting behavior: Here, we observe an electron-density-independent light shift and a clear pump-power-dependent saturation. These features are equivalent to that of an ensemble of independent two-level emitters and indicate a breakdown of the Fermi-polaron picture for optical excitations of electrons subject to a strong moiré potential. Our work establishes an experimental approach to elucidate the elementary optical excitations of semiconductor moiré heterostructures, providing a solid ground for the spectroscopy of correlated electronic and excitonic states in such materials.

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arXiv2025

Thermopower probing emergent local moments in magic-angle twisted bilayer graphene

Recent experiments on magic-angle twisted bilayer graphene (MATBLG) have revealed the formation of flatbands, suggesting that correlation effects are likely to dominate in this system. Yet, a global transport measurement showing distinct signatures of strong correlations like local moments arising from the flatbands is missing. Utilizing thermopower as a sensitive global transport probe for measuring entropy, we unveil the presence of emergent local moments through their impact on entropy. Remarkably, in addition to sign changes at the Dirac point ($ν= 0$) and full band filling ($ν= \pm 4$), the thermopower of MATBLG demonstrates additional sign changes at the location, $ν_{cross} \sim \pm 1$, which do not vary with temperature from $5K$ to $\sim 60K$. This is in contrast to sensitive temperature-dependent crossing points seen in our study on twisted bilayer graphene devices with weaker correlations. Further, we have investigated the effect of magnetic field ($B$) on the thermopower, both $B_{\parallel}$ and $B_{\perp}$. Our results show a $30\%$ and $50\%$ reduction, respectively, that is consistent with suppression seen in the layered oxide due to the partial polarization of the spin entropy. The observed robust crossing points, together with suppression in a magnetic field, cannot be explained solely from the contributions of band fermions; instead, our data is consistent with the dominant contribution arising from the entropy of the emergent localized moments of a strongly correlated flatband.

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arXiv2025

Impact of the angular alignment on the crystal field and intrinsic doping of bilayer graphene/BN heterostructures

The ability to tune the energy gap in bilayer graphene makes it the perfect playground for the study of the effects of internal electric fields, such as the crystalline field, which are developed \Reb{when other layered materials are deposited on top of it}. Here, we introduce a novel device architecture allowing a simultaneous control over the applied displacement field and the crystalline alignment between two materials. Our experimental and numerical results confirm that the crystal field and electrostatic doping due to the interface reflect the 120$^{\circ}$ symmetry of the bilayer graphene/BN heterostructure and are highly affected by the commensurate state. These results provide an unique insight into the role of twist angle in the development of internal crystal fields and intrinsic electrostatic doping in heterostructures. Our results highlight the importance of layer alignment, beyond the existence of a moiré superlattice, to understand the intrinsic properties of a heterostructure.

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arXiv2025

Non-Reciprocal Current-Phase Relation and Superconducting Diode Effect in Topological-Insulator-Based Josephson Junctions

Josephson junctions (JJ) are essential for superconducting quantum technologies and searches of self-conjugate quasiparticles, pivotal for fault-tolerant quantum computing. Measuring the current-phase relation (CPR) in JJ based on topological insulators (TI) can provide critical insights into unconventional phenomena in these systems, such as the presence of Majorana bound states (MBS) and the nature of non-reciprocal transport. However, reconstructing CPR as a function of magnetic field in such JJs has remained experimentally challenging. Here, we introduce a platform for precise CPR measurements in planar JJs composed of NbSe$_2$ and few layer thick Bi$_2$Se$_3$ (TI) as a function of magnetic field. When a single flux quantum $Φ_\mathrm{0}$ threads the junction, we observe anomalous peak-dip-shaped CPR behaviour and non-reciprocal supercurrent flow. We demonstrate that these anomalies stem from the edge-amplified sloped supercurrent profile rather than MBS signatures often invoked to explain puzzles emerging near $Φ_\mathrm{0}$ in TI-based JJ. Furthermore, we show that such a supercurrent profile gives rise to a previously overlooked, robust and tunable Josephson diode effect. These findings establish field-dependent CPR measurements as a critical tool for exploring topological superconducting devices and offer new design principles for non-reciprocal superconducting electronics.

cond-mat.supr-con↗

arXiv2025

Probing the flat-band limit of the superconducting proximity effect in Twisted Bilayer Graphene Josephson junctions

While extensively studied in normal metals, semimetals and semiconductors, the superconducting (SC) proximity effect remains elusive in the emerging field of flat-band systems. In this study we probe proximity-induced superconductivity in Josephson junctions (JJs) formed between superconducting NbTiN electrodes and twisted bilayer graphene (TBG) weak links. Here the TBG acts as a highly tunable topological flat-band system, which due to its twist-angle dependent bandwidth, allows to probe the SC proximity effect at the crossover from the dispersive to the flat-band limit. Contrary to our original expectations, we find that the SC remains strong even in the flat-band limit, and gives rise to broad, dome shaped SC regions, in the filling dependent phase diagram. In addition, we find that unlike in conventional JJs, the critical current Ic strongly deviates from a scaling with the normal state conductance GN. We attribute these findings to the onset of strong electron interactions, which can give rise to an excess critical current, and also work out the potential importance of quantum geometric terms as well as multiband pairing mechanisms. Our results present the first detailed study of the SC proximity effect in the flat-band limit and shed new light on the mechanisms that drive the formation of SC domes in flat-band systems.

cond-mat.supr-con↗

arXiv2025

Spin-valley polarization control in WSe$_2$ monolayers using photochemical doping

We report on the influence of a photochemical doping method on the spin-valley polarization degree ($P_{c}$) of excitons in WSe$_2$ monolayers. By varying the carrier density and transitioning from an excess of electrons (n-type) to an excess of holes (p-type), we observe a non-monotonic dependence of $P_{c}$ on the doping level. Using controlled, single-shot photochlorination steps, we unveil this non-monotonic behavior, with $P_{c}$ reaching a minimum value of less than 10$\%$ at 78 K near the charge neutrality point, while increasing by a factor of three at a hole density of $5 \times 10^{11} \,\mathrm{cm^{-2}}$. The impact of the doping on $P_{c}$ is explained using a phenomenological model that accounts for various mechanisms influencing exciton polarization dynamics, including exciton-carrier scattering processes and exciton-to-trion conversion rates. Among these, exciton-carrier collisions emerge as the dominant mechanism driving the observed variations in $P_{c}$, while the exciton effective lifetime remains nearly independent of doping. These findings highlight the potential of photochemical methods for investigating valley physics and for effectively tuning the exciton polarization degree in transition metal dichalcogenide monolayers.

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arXiv2025

Brightened emission of dark trions in transition-metal dichalcogenide monolayers

The optical emission spectra of semiconducting transition-metal dichalcogenide monolayers highlight fascinating recombination processes of charged excitons (trions). When charge tunable WSe$_2$ monolayers are moderately doped with electrons, a strong luminescence peak emerges just below the well-understood spectral lines associated with the recombination of negatively charged bright and dark trions. Despite previous investigations, its origin remains elusive. Here, we demonstrate that this luminescence peak is the result of electron-electron assisted recombination that brightens the dark trion emission. Supporting evidence for this second-order recombination process comes from identifying the brightened emission of positively charged dark trions when the monolayer is electrostatically doped with holes. Remarkably, the discovered hole-hole assisted luminescence peak emerges in the near infrared, about 500 meV below the well-studied spectral region of excitons and trions. In magneto-photoluminescence experiments we find that the $g$-factor of this new transition ($g$ = +4) has an opposite sign compared to the well-known $g$-factor of neutral or charged excitons. This allows us to propose a mechanism of brightening of the positively charged dark trion involving the $Γ$ valence band.

cond-mat.mes-hall↗

arXiv2024

Emission and Coherent Control of Levitons in Graphene

Flying qubits encode quantum information in propagating modes instead of stationary discrete states. Although photonic flying qubits are available, the weak interaction between photons limits the efficiency of conditional quantum gates. Conversely, electronic flying qubits can use Coulomb interactions, but the weaker quantum coherence in conventional semiconductors has hindered their realization. In this work, we engineered on-demand injection of a single electronic flying qubit state and its manipulation over the Bloch sphere. The flying qubit is a Leviton propagating in quantum Hall edge channels of a high-mobility graphene monolayer. Although single-shot qubit readout and two-qubit operations are still needed for a viable manipulation of flying qubits, the coherent manipulation of an itinerant electronic state at the single-electron level presents a highly promising alternative to conventional qubits.

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arXiv2024

Josephson junctions based on ultraclean carbon nanotubes

We present a technique for integrating ultraclean carbon nanotubes into superconducting circuits, aiming to realize Josephson junctions based on one-dimensional elementary quantum conductors. This technique primarily involves depositing the nanotube in the final step, thus preserving it from the inherent contaminations of nanofabrication and maintaining contact solely with superconducting electrodes and a crystalline hBN substrate. Through transport measurements performed in both the normal and superconducting states, we demonstrate that our method yields high-quality junctions with Josephson energies suitable for quantum device applications, such as carbon nanotube-based superconducting qubits.

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