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T. Taniguchi

Publications and source records attributed to T. Taniguchi.

At least 91 records · Page 5Linked to original sources
arXiv2024

Anomalous terahertz photoconductivity caused by the superballistic flow of hydrodynamic electrons in graphene

Light incident upon materials can induce changes in their electrical conductivity, a phenomenon referred to as photoresistance. In semiconductors, the photoresistance is negative, as light-induced promotion of electrons across the band gap enhances the number of charge carriers participating in transport. In superconductors, the photoresistance is positive because of the destruction of the superconducting state, whereas in normal metals it is vanishing. Here we report a qualitative deviation from the standard behavior in metallic graphene. We show that Dirac electrons exposed to continuous wave (CW) terahertz (THz) radiation can be thermally decoupled from the lattice by 50~K which activates hydrodynamic electron transport. In this regime, the resistance of graphene constrictions experiences a decrease caused by the THz-driven superballistic flow of correlated electrons. We analyze the dependencies of the negative photoresistance on the carrier density, and the radiation power and show that our superballistic devices operate as sensitive phonon-cooled bolometers and can thus offer a picosecond-scale response time. Beyond their fundamental implications, our findings underscore the practicality of electron hydrodynamics in designing ultra-fast THz sensors and electron thermometers.

cond-mat.mes-hall↗
arXiv2024

Spin-orbit proximity in MoS$_2$/bilayer graphene heterostructures

Van der Waals heterostructures provide a versatile platform for tailoring electronic properties through the integration of two-dimensional materials. Among these combinations, the interaction between bilayer graphene and transition metal dichalcogenides (TMDs) stands out due to its potential for inducing spin-orbit coupling (SOC) in graphene. Future devices concepts require the understanding the precise nature of SOC in TMD/bilayer graphene heterostructures and its influence on electronic transport phenomena. Here, we experimentally confirm the presence of two distinct types of SOC, Ising (1.55 meV) and Rashba (2.5 meV), in bilayer graphene when interfaced with molybdenum disulphide, recognized as one of the most stable TMDs. Furthermore, we reveal a non-monotonic trend in conductivity with respect to the electric displacement field at charge neutrality. This phenomenon is ascribed to the existence of single-particle gaps induced by the Ising SOC, which can be closed by a critical displacement field. Remarkably, our findings also unveil sharp peaks in the magnetoconductivity around the critical displacement field, challenging existing theoretical models.

cond-mat.mes-hall↗
arXiv2024

Atomic and electronic structure of defects in hBN: enhancing single-defect functionalities

Defect centers in insulators play a critical role in creating important functionalities in materials: prototype qubits, single-photon sources, magnetic field probes, and pressure sensors. These functionalities are highly dependent on their mid-gap electronic structure and orbital/spin wave-function contributions. However, in most cases, these fundamental properties remain unknown or speculative due to the defects being deeply embedded beneath the surface of highly resistive host crystals, thus impeding access through surface probes. Here, we directly inspected the atomic and electronic structures of defects in thin carbon-doped hexagonal boron nitride (hBN:C) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Such investigation adds direct information about the electronic mid-gap states to the well-established photoluminescence response (including single photon emission) of intentionally created carbon defects in the most commonly investigated van der Waals insulator. Our joint atomic-scale experimental and theoretical investigations reveal two main categories of defects: 1) single-site defects manifesting as donor-like states with atomically resolved structures observable via STM, and 2) multi-site defect complexes exhibiting a ladder of empty and occupied mid-gap states characterized by distinct spatial geometries. Combining direct probing of mid-gap states through tunneling spectroscopy with the inspection of the optical response of insulators hosting specific defect structures holds promise for creating and enhancing functionalities realized with individual defects in the quantum limit. These findings underscore not only the versatility of hBN:C as a platform for quantum defect engineering but also its potential to drive advancements in atomic-scale optoelectronics.

cond-mat.mtrl-sci↗
arXiv2024

Ultra-low carrier density superconducting bolometers with single photon sensitivity based on magic-angle twisted bilayer graphene

The superconducting (SC) state of magic-angle twisted bilayer graphene (MATBG) shows exceptional properties, as it consists of an unprecedentedly small electron (hole) ensemble of only ~ 10 power 11 carriers per square centimeter, which is five orders of magnitude lower than in traditional superconductors. This results in an ultra-low electronic heat capacity and kinetic inductance of this truly two-dimensional SC, and provides record-breaking key parameters for a variety of quantum sensing applications, in particular in thermal sensing and single photon detection (SPD), which traditionally exploit thermal effects in nanostructured superconducting thin films. In this work, we systematically study the interaction of the superconducting state of MATBG with individual light quanta. We discover full destruction of the SC state upon absorption of a single infrared photon even in a 16 square micrometer sized device, which showcases its exceptional bolometric sensitivity. Upon voltage biasing close to its critical current, we further show that this non-optimized device can be used as a SPD, whose click-rate is proportional to the number of absorbed photons, following Poissonian statistics. Our work offers insights into the MATBG-photon interaction and shows up pathways to use low-carrier density graphene-based superconductors as a new platform for developing revolutionary new quantum devices and sensors.

cond-mat.mes-hall↗
arXiv2024

Circular THz ratchets in a 2D-modulated Dirac system

We report on the observation of the circular ratchet effect excited by terahertz laser radiation in a specially designed two-dimensional metamaterial consisting of a graphene monolayer deposited on a graphite gate patterned with an array of triangular antidots. We show that a periodically driven Dirac fermion system with spatial asymmetry converts the a.c. power into a d.c. current, whose direction reverses when the radiation helicity is switched. The circular ratchet effect is demonstrated for room temperature and a radiation frequency of 2.54 THz. It is shown that the ratchet current magnitude can be controllably tuned by the patterned and uniform back gate voltages. The results are analyzed in the light of the developed microscopic theory considering electronic and plasmonic mechanisms of the ratchet current formation.

cond-mat.mes-hall↗
arXiv2024

Terahertz ratchet in graphene 2D metamaterial formed by a patterned gate with an antidot arrayd

We report the observation of the terahertz-induced ratchet effect in graphene-based two-dimensional (2D) metamaterials. The metamaterial consists of a graphite gate patterned with an array of triangular antidots placed under a graphene monolayer. We show that the ratchet current appears due to the noncentrosymmetry of the periodic structure unit cell. The ratchet current is generated owing to the combined action of a spatially periodic in-plane electrostatic potential and a periodically modulated radiation electric field caused by near-field diffraction. The magnitude and direction of the ratchet current are shown to be controlled by voltages applied to both back and patterned gates, which change the lateral asymmetry, carrier type and density. The phenomenological and microscopic theories of ratchet effects in graphene-based 2D metamaterials are developed. The experimental data are discussed in the light of the theory based on the solution of the Boltzmann kinetic equation and the calculated electrostatic potential profile. The theory describes well all the experimental results and shows that the observed ratchet current consists of the Seebeck thermoratchet contribution as well as the linear contribution, which is sensitive to the orientation of the radiation electric field vector with respect to the triangles.

cond-mat.mes-hall↗
arXiv2024

Electric field tunable edge transport in Bernal stacked trilayer graphene

This letter presents a non-local study on the electric field tunable edge transport in an hBN-encapsulated dual-gated Bernal stacked (ABA) trilayer graphene across various displacement fields ($D$) and temperatures ($T$). Our measurements revealed that the non-local resistance ($R_{NL}$) surpassed the expected classical ohmic contribution by a factor of at least two orders of magnitude. Through scaling analysis, we found that the non-local resistance scales linearly with the local resistance ($R_{L}$) only when the $D$ exceeds a critical value of $\sim0.2$ V/nm. Additionally, we observed that the scaling exponent remains constant at unity for temperatures below the bulk-band gap energy threshold ($T<25$ K). Further, the value of $R_{NL}$ decreases in a linear fashion as the channel length ($L$) increases. These experimental findings provide evidence for edge-mediated charge transport in ABA trilayer graphene under the influence of a finite displacement field. Furthermore, our theoretical calculations support these results by demonstrating the emergence of dispersive edge modes within the bulk-band gap energy range when a sufficient displacement field is applied.

cond-mat.mes-hall↗
arXiv2023

Quantum Transport and Spectroscopy of Two-dimensional Perovskite/Graphene Interfaces

Quantum transport properties in molecularly thin perovskite/graphene heterostructure are experimentally investigated by Shubnikov-de Hass (SdH) oscillation and photo-resistance spectroscopy. We find an efficient charge transfer between the perovskite nanosheets and graphene, with a high hole concentration in graphene of up to $\rm \sim 2.8 \times 10^{13}\ cm^{-2}$. The perovskite layer also increases Fermi velocity lowering the effective mass of graphene from expected $\rm \sim 0.12\ m_e$ to $\rm \sim 0.08\ m_e$. Combining magneto-resistance and density functional theory calculations, we find that the carrier density in graphene significantly depends on the perovskite termination at the interface, affecting the charge transfer process and leading to a coexistence of regions with different doping. We also investigate the photo-response of the SdH oscillation under illumination. Using photo-resistance spectroscopy, we find evidence of photo-assisted transport across the perovskite layer between two graphene electrodes mediated by hot carriers in perovskite. Our results provide a picture to understand the transport behavior of 2D perovskite/graphene heterostructure and a reference for the controlled design of interfaces in perovskite optoelectronic devices.

cond-mat.mes-hall↗
arXiv2023

High-mobility compensated semimetals, orbital magnetization, and umklapp scattering in bilayer graphene moire superlattices

Twist-controlled moire superlattices (MS) have emerged as a versatile platform in which to realize artificial systems with complex electronic spectra. Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) form an interesting example of the MS that has recently featured a set of unexpected behaviors, such as unconventional ferroelectricity and electronic ratchet effect. Yet, the understanding of the BLG/hBN MS electronic properties has, at present, remained fairly limited. Here we develop a multi-messenger approach that combines standard magnetotransport techniques with low-energy sub-THz excitation to get insights into the properties of this MS. We show that BLG/hBN lattice alignment results in the emergence of compensated semimetals at some integer fillings of the moire bands separated by van Hove singularities where Lifshitz transition occurs. A particularly pronounced semimetal develops when 8 electrons reside in the moire unit cell, where coexisting high-mobility electron and hole systems feature a strong magnetoresistance reaching 2350 % already at B=0.25 T. Next, by measuring the THz-driven Nernst effect in remote bands, we observe valley splitting, pointing to an orbital magnetization characterized by a strongly enhanced effective g-factor of 340. Last, using THz photoresistance measurements, we show that the high-temperature conductivity of the BLG/hBN MS is limited by electron-electron umklapp processes. Our multi-facet analysis introduces THz-driven magnetotransport as a convenient tool to probe the band structure and interaction effects in vdW materials and provides a comprehension of the BLG/hBN MS.

cond-mat.mes-hall↗
arXiv2023

Electrical noise spectroscopy of magnons in a quantum Hall ferromagnet

Collective spin-wave excitations-magnons-in a quantum Hall ferromagnet are promising quasi-particles for next-generation spintronics devices, including platforms for information transfer. Detection of these charge-neutral excitations relies on the conversion of magnons into electrical signals in the form of excess electrons and holes, but if these signals are equal the magnon detection remains elusive. In this work, we overcome this shortcoming by measuring the electrical noise generated by magnons. We use the symmetry-broken quantum Hall ferromagnet of the zeroth Landau level in graphene to launch magnons. Absorption of these magnons creates excess noise above the Zeeman energy and remains finite even when the average electrical signal is zero. Moreover, we formulate a theoretical model in which the noise is generated by equilibration (partial or full, depending on the bias voltage) between edge channels and propagating magnons. Our model, which agrees with experimental observations, also allows us to pinpoint the regime of ballistic magnon transport in our device.

cond-mat.mes-hall↗
arXiv2023

Nanoparticle Stressor-Induced Single-photon Sources in Monolayer WS$_2$ Emitting into a Narrowband Visible Spectral Range

A van der Waals heterostructure containing an atomically thin monolayer transition-metal dichalcogenide as a single-photon emitting layer is emerging as an intriguing solid-state quantum-photonic platform. Here, we report the utilization of spin-coating of silica nanoparticles for deterministically creating the spectrally isolated, energetically stable, and narrow-linewidth single-photon emitters in ML-WS$_2$. We also demonstrate that long-duration low-temperature annealing of the photonic heterostructure in the vacuum removes the energetically unstable emitters that are present due to fabrication-associated residue and lead to the emission of single-photons in a <25 nm narrowband visible spectral range centered at $\sim$620 nm. This work may pave the way toward realizing a hybrid-quantum-photonic platform containing a van der Waals heterostructure/device and an atomic-vapor system emitting/absorbing in the same visible spectral range.

physics.app-ph↗
arXiv2023

180$^\circ$-twisted bilayer ReSe$_2$ as an artificial noncentrosymmetric semiconductor

We have fabricated a 180$^\circ$-twisted bilayer ReSe$_2$ by stacking two centrosymmetric monolayer ReSe$_2$ flakes in opposite directions, which is expected to lose spatial inversion symmetry. By the second harmonic generation and angle-resolved photoemission spectroscopy, we successfully observed spatial inversion symmetry breaking and emergent band dispersions. The band calculation shows the finite lifting of spin degeneracy (~50 meV) distinct from natural monolayer and bilayer ReSe$_2$. Our results demonstrate that the spin-momentum locked state, which leads to spintronic functions and Berry-curvature-related phenomena, can be realized even with the stacking of centrosymmetric monolayers.

cond-mat.mes-hall↗
arXiv2023

Diamond quantum magnetometer with dc sensitivity of < 10 pT Hz$^{-1/2}$ toward measurement of biomagnetic field

We present a sensitive diamond quantum sensor with a magnetic field sensitivity of $9.4 \pm 0.1~\mathrm{pT/\sqrt{Hz}}$ in a near-dc frequency range of 5 to 100~Hz. This sensor is based on the continuous-wave optically detected magnetic resonance of an ensemble of nitrogen-vacancy centers along the [111] direction in a diamond (111) single crystal. The long $T_{2}^{\ast} \sim 2~\mathrm{μs}$ in our diamond and the reduced intensity noise in laser-induced fluorescence result in remarkable sensitivity among diamond quantum sensors. Based on an Allan deviation analysis, we demonstrate that a sub-picotesla field of 0.3~pT is detectable by interrogating the magnetic field for a few thousand seconds. The sensor head is compatible with various practical applications and allows a minimum measurement distance of about 1~mm from the sensing region. The proposed sensor facilitates the practical application of diamond quantum sensors.

quant-ph↗
arXiv2023

Charge transfer and asymmetric coupling of MoSe$_2$ valleys to the magnetic order of CrSBr

Van der Waals (vdW) heterostructures composed of two-dimensional (2D) transition metal dichalcogenides (TMD) and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in non-magnetic TMDs. Here, we report magneto-photoluminescence (PL) investigations of monolayer (ML) MoSe$_2$ on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe$_2$, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley $g$-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first principles calculations suggest that MoSe$_2$/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.

cond-mat.mes-hall↗
arXiv2023

Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1$T$-TaS$_2$ due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS$_2$ devices. Here, we demonstrate the fabrication of nanothick verti-lateral $H$-TaS$_2$/1$T$-TaS$_2$ heterostructures in which the number of endotaxial metallic $H$-TaS$_2$ monolayers dictates the number of resistance transitions in 1$T$-TaS$_2$ lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

cond-mat.mtrl-sci↗
arXiv2023

Self-aligned hybrid nanocavities using atomically thin materials

Two-dimensional (2D) van der Waals layered materials with intriguing properties are increasingly being adopted in hybrid photonics. The 2D materials are often integrated with photonic structures including cavities to enhance light-matter coupling, providing additional control and functionality. The 2D materials, however, needs to be precisely placed on the photonic cavities. Furthermore, the transfer of 2D materials onto the cavities could degrade the cavity quality $(Q)$ factor. Instead of using prefabricated PhC nanocavities, we demonstrate a novel approach to form a hybrid nanocavity by partially covering a PhC waveguide post-fabrication with a suitably-sized 2D material flake. We successfully fabricated such hybrid nanocavity devices with hBN, WSe$_2$ and MoTe$_2$ flakes on silicon PhC waveguides, obtaining $Q$ factors as high as $4.0\times10^5$. Remarkably, even mono- and few-layer flakes can provide sufficient local refractive index modulation to induce nanocavity formation. Since the 2D material is spatially self-aligned to the nanocavity, we have also managed to observe cavity PL enhancement in a MoTe$_2$ hybrid cavity device, with a cavity Purcell enhancement factor of about 15. Our results highlights the prospect of using such 2D materials-induced PhC nanocavity to realize a wide range of photonic components for hybrid devices and integrated photonic circuits.

physics.optics↗
arXiv2023

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

cond-mat.mes-hall↗
arXiv2023

Enhancement of electron magnetic susceptibility due to many-body interactions in monolayer MoSe$_2$

Employing the original, all-optical method, we quantify the magnetic susceptibility of a two-dimensional electron gas (2DEG) confined in the MoSe$_2$ monolayer in the range of low and moderate carrier densities. The impact of electron-electron interactions on the 2DEG magnetic susceptibility is found to be particularly strong in the limit of, studied in detail, low carrier densities. Following the existing models, we derive the value of $g_0 = 2.5 \pm 0.4$ for the bare (in the absence of the interaction effects) $g$-factor of the ground state electronic band in the MoSe$_2$ monolayer. The derived value of this parameter is discussed in the context of estimations from other experimental approaches. Surprisingly, the conclusions drawn differ from theoretical ab-initio studies.

cond-mat.mes-hall↗