arXiv subjects

Takuya Iwasaki

Publications and source records attributed to Takuya Iwasaki.

18 recordsLinked to original sources

arXiv2026

Spin-coherence characterization of boron vacancy defects in hexagonal boron nitride with broadband microwave pulses

Negatively charged boron vacancy (VB-) defects in hexagonal boron nitride (hBN) are promising for nanoscale-proximity quantum sensing. To evaluate their performance, it is important to characterize the spin coherence times T2* and T2. In this study, we realized sub-GHz Rabi oscillations of VB- using an isotopically enriched hBN thin film directly stamped onto a narrow gold wire. Using these strong microwave pulses, we performed Ramsey interference and Hahn echo measurements. The Ramsey interference signal showed Gaussian-like decay, yielding T2* = 13.8 ns. The Hahn echo measurement gave T2 = 108.7 ns and a stretch factor of α= 1.25. These results experimentally clarify the spin coherence properties of VB- and provide an effective method for evaluating the coherence of spin defects in van der Waals thin films with broad resonance linewidths.

cond-mat.mes-hall↗

arXiv2025

Hexagonal boron nitride/bilayer graphene moiré superlattices in the Dirac-material family: energy-band engineering and carrier doping by dual gating

We review the fabrication and transport characterization of hexagonal boron nitride (hBN)/Bernal bilayer graphene (BLG) moiré superlattices. Due to the moiré effect, the hBN/BLG moiré superlattices exhibit an energy gap at the charge neutrality point (CNP) even in the absence of a perpendicular electric field. In BLG, the application of a perpendicular electric field tunes the energy gap at the CNP, which contrasts with single-layer graphene and is similar to the family of rhombohedral multilayer graphene. The hBN/BLG moiré superlattice is associated with non-trivial energy-band topology and a narrow energy band featuring a van Hove singularity. By employing a dual-gated device structure where both the perpendicular displacement field and the carrier density are individually controllable, systematic engineering of the energy-band structure can be achieved. The data presented here demonstrate the universality and diversity in the physics of hBN/BLG moiré superlattices.

cond-mat.mes-hall↗

arXiv2025

Systematic investigation of dynamic nuclear polarization with boron vacancy in hexagonal boron nitride

Dynamic nuclear polarization (DNP) using the boron vacancy ($\mathrm{V_B^-}$) in hexagonal boron nitride (hBN) has gained increasing attention. Understanding this DNP requires systematically investigating the optically detected magnetic resonance (ODMR) spectra and developing a model that quantitatively describes its behavior. Here, we measure the ODMR spectra of $\mathrm{V_B^-}$ in $\mathrm{h}^{10}\mathrm{B}^{15}\mathrm{N}$ over a wide magnetic field range, including the ground state level anti-crossing (GSLAC), and compare them with the results of the Lindblad-based simulation that considers a single electron spin and three neighboring $^{15}\mathrm{N}$ nuclear spins. Our simulation successfully reproduces the experimental spectra, including the vicinity of GSLAC. It can explain the overall behavior of the magnetic field dependence of the nuclear spin polarization estimated using the Lorentzian fitting of the spectra. Despite such qualitative agreement, we also demonstrate that the fitting methods cannot give accurate polarizations. Finally, we discuss that symmetry-induced mechanisms of $\mathrm{V_B^-}$ limit the maximum polarization. Our study is an essential step toward a quantitative understanding of DNP using defects in hBN and its quantum applications.

cond-mat.mes-hall↗

arXiv2024

Systematic characterization of nanoscale $h$-BN quantum sensor spots created by helium-ion microscopy

The nanosized boron vacancy ($V_\mathrm{B}^-$) defect spot in hexagonal boron nitride ($h$-BN) is promising for a local magnetic field quantum sensor. One of its advantages is that a helium-ion microscope can make a spot at any location in an $h$-BN flake with nanometer accuracy. In this study, we investigate the properties of the created nanosized $V_\mathrm{B}^-$ defect spots by systematically varying three conditions: the helium-ion dose, the thickness of the $h$-BN flakes, and the substrate on which the $h$-BN flakes are attached. The physical background of the results obtained is successfully interpreted using Monte Carlo calculations. From the findings obtained here, a guideline for their optimal creation conditions is obtained to maximize its performance as a quantum sensor concerning sensitivity and localization.

cond-mat.mes-hall↗

arXiv2024

Gapped Dirac materials and quantum valley currents in dual-gated hBN/bilayer-graphene heterostructures

In gapped Dirac materials, the topological current associated with each valley can flow in opposite directions creating long-range charge-neutral valley currents. We report valley currents in hBN/bilayer-graphene heterostructures with an energy gap, which is tunable by a perpendicular electric (displacement) field in a dual-gated structure. We observed significant nonlocal resistance, consistent with the scaling theory of the valley Hall effect. In the low-temperature limit, the nonlocal resistance approaches a saturated value near the "quantum limit," indicating the emergence of quantum valley currents.

cond-mat.mes-hall↗

arXiv2024

Bilayer Graphene on hexagonal Boron Nitride and the Family of quantum MetaMaterial

We report on the fabrication and characterization of dual-gated hexagonal boron nitride (hBN)/bilayer-graphene (BLG) superlattices. Due to the moire effect, the hBN/BLG superlattice harbors an energy gap at the charge neutral point (CNP) and the satellites even without a perpendicular electric field. In BLG, moreover, the application of a perpendicular electric field tunes the energy gap, which contrasts with the single-layer graphene (SLG) and is linked to the family of rhombohedral multilayer graphene. Therefore, the hBN/BLG superlattice is accompanied with non-trivial energy-band topology and a narrow energy band with van Hove singularities. By the dual gating, systematic engineering of the energy-band structure can be performed and the carrier concentration is fine-tunable. This review is an extended version of the talk based on ref. [1], which is also a supplementary to the ref. T. Iwasaki, Y. Morita, K. Watanabe, T. Taniguchi, Phys. Rev. B106, 165134 (2022) and Phys. Rev. B109, 075409 (2024). The data show the universality and diversity in the physics of the hBN/BLG superlattices.

cond-mat.mes-hall↗

arXiv2024

Magnetotransport properties in van-der-Waals \textit{\textbf{R}}Te$_{3}$ (\textit{\textbf{R}} = La, Ce, Tb)

Rare-earth tritellurides are van-der-Waals antiferromagnets which have been attracting attention as materials not only with high mobility, but also with various states such as superconductivity under high pressure, incommensurate charge-density-wave (CDW) phase, and multiple antiferromagnetic phases. In this work, we performed longitudinal resistivity and Hall resistivity measurements simultaneously in exfoliated $R$Te$_{3}$ ($R$ =La, Ce, Tb) thin film devices, in order to investigate the influence of magnetic ordering on transport properties in van-der-Waals magnetic materials. We have obtained carrier mobility and concentration using a two-band model, and have observed an increase in carrier mobility in the antiferromagnets CeTe$_{3}$ and TbTe$_{3}$ due to the magnetic transition. Especially in CeTe$_{3}$, the carrier concentration has changed drastically below the magnetic transition temperature, suggesting the interaction between the CDW and antiferromagnetic phases. In addition, the analysis of the Shubnikov-de Haas oscillations in CeTe$_{3}$ supports the possibility of Fermi surface modulation by magnetic ordering. This research will pave the way not only for spintronic devices that take advantage of high mobility, but also for the study of the correlation between CDW and magnetism states in low-dimensional materials.

cond-mat.str-el↗

arXiv2024

Effects of Marangoni and drag forces on the transition from vapor-rich to air-rich bubbles

In this study, we investigated the formation of air-rich microbubbles through local photothermal heating of non-degassed water. When non-degassed water is locally heated, vapor-rich bubbles are initially formed. These bubbles have a maximum radius of approximately 9 μm and stabilize while oscillating and exhaling air-rich bubbles. However, when the exhaled bubbles fuse and grow, they revert to vapor-rich bubbles on the heat source. The vapor-rich bubbles are then exposed to a large amount of air, causing them to transition to air-rich bubbles. In this paper, the motion of the exhaled air-rich bubble is explained by the drag force owing to the flow and the Marangoni force owing to the temperature gradient acting on the bubble. Because the drag force is proportional to the bubble radius, and the Marangoni force is proportional to the square of the bubble radius, the larger the bubble, the stronger the effect of the Marangoni force. Thus, as the bubbles grow, they are drawn toward the heat source against the flow created by the vapor-rich bubbles. These results are useful for a better understanding of bubble growth and determining the conditions for the stable formation of vapor-rich bubbles in non-degassed water.

physics.flu-dyn↗

arXiv2023

Magnetic field imaging by hBN quantum sensor nanoarray

Placing a sensor close to the target at the nano-level is a central challenge in quantum sensing. We demonstrate high-spatial-resolution magnetic field imaging with a boron vacancy (V$_\text{B}^-$) defects array in hexagonal boron nitride with a few 10 nm thickness. V$_\text{B}^-$ sensor spots with a size of (100 nm)$^2$ are arranged periodically with nanoscale precision using a helium ion microscope and attached tightly to a gold wire. The sensor array allows us to visualize the magnetic field induced by the current in the wire with a spatial resolution beyond the diffraction limit. Each sensor exhibits a practical sensitivity of $73.6~μ\text{T/Hz}^{0.5}$, suitable for quantum materials research. Our technique of arranging V$_\text{B}^-$ quantum sensors periodically and tightly on measurement targets will maximize their potential.

cond-mat.mes-hall↗

arXiv2022

Dual-gated hBN/bilayer-graphene superlattices and the transitions between the insulating phases at the charge neutrality point

We report on transport properties in dual-gated hexagonal boron nitride (hBN)/bilayer-graphene (BLG) superlattices. Here, BLG is nontwisted, i.e., plain. This paper focuses on the charge neutrality point (CNP) for a plain BLG. Under a perpendicular magnetic field, transitions between two insulating phases at the CNP are detected by varying a displacement field with the study on the resistance-temperature characteristics and the magnetoresistance. This work opens avenues for exploring the global phase diagram of the hBN/BLG superlattices beyond the CNP.

cond-mat.mes-hall↗

arXiv2022

Plasmon triggered ultrafast operation of color centers in hBN layers

High-quality emission centers in two-dimensional materials are promising components for future photonic and optoelectronic applications. Carbon-enriched hexagonal boron nitride (hBN:C) layers host atom-like color-center (CC) defects with strong and robust photoemission up to room temperature. Placing the hBN:C layers on top of Ag triangle nanoparticles (NPs) accelerate the decay of the CC defects down to 46 ps from their reference bulk value of 350 ps. The ultrafast decay is achieved due to the efficient excitation of the plasmon modes of the Ag NPs by the near field of the CCs. Simulations of the CCs/Ag NP interaction present that higher Purcell values are expected, although the measured decay of the CCs is limited by the instrument response. The influence of the NP thickness to the Purcell factor of the CCs is analyzed. The ultrafast operation of the CCs in hBN:C layers paves the way for their use in demanding applications, such as single-photon emitters and quantum devices.

physics.optics↗

arXiv2021

Electron transport in folded bilayer-bilayer graphene/hexagonal boron nitride superlattices under high magnetic fields

Employing graphene as a template, we fabricate moiré superlattices by stacking bilayer or folded bilayer-bilayer graphene (BLG or fBBLG) and hexagonal boron nitride (hBN), i.e., hBN/BLG/hBN or hBN/fBBLG/hBN stacks, with a small twist angle between the graphene and one of the two hBN layers. Because of the modulation due to the hBN, higher-generation Dirac points can emerge with a narrow bandwidth and van Hove singularities. In the moiré superlattice devices, we can therefore access the higher-generation Dirac points by in-situ gate tuning. This study is based on our previous paper (Appl. Phys. Express 13, 035003 (2020)). Here we show more extended data by applying high magnetic fields up to ~24 T. We also comment on the temperature dependence of the resistivity and magnetoresistance with reference to the 'plain' BLG data for a comparative study.

cond-mat.mes-hall↗

arXiv2020

Direct growth of germanene at interfaces between van der Waals materials and Ag(111)

Germanene, a two-dimensional honeycomb germanium crystal, is grown at graphene/Ag(111) and hexagonal boron nitride (h-BN)/Ag(111) interfaces by segregating germanium atoms. A simple annealing process in N2 or H2/Ar at ambient pressure leads to the formation of germanene, indicating that an ultrahigh-vacuum condition is not necessary. The grown germanene is stable in air and uniform over the entire area covered with a van der Waals (vdW) material. As an important finding, it is necessary to use a vdW material as a cap layer for the present germanene growth method since the use of an Al2O3 cap layer resulted in no germanene formation. The present study also proved that Raman spectroscopy in air is a powerful tool for characterizing germanene at the interfaces, which is concluded by multiple analyses including first-principles density functional theory calculations. The direct growth of h-BN-capped germanene on Ag(111), which is demonstrated in the present study, is considered to be a promising technique for the fabrication of future germanene-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Light-Assisted and Gate-Tunable Oxygen Gas Sensor based on Rhenium Disulfide (ReS2) Field-Effect Transistors

Gas sensors based on transition metal dichalcogenides (TMDCs) have attracted much attention from a new perspective involving light-assisted or gate-voltage operation. However, their combined roles as regards the gas sensing performance and mechanism have not yet been understood due to the lack of controlled studies. This study systematically investigates the oxygen sensor performance and mechanism of few-layer-thick rhenium disulfide (ReS2) field-effect transistors (FETs) under light illumination and gate biasing. As a result, a combination of light illumination and positive gate voltage enhanced the device responsivity over 100% at a 1% oxygen concentration, that is, the approach achieved a practical sensitivity of 0.01% ppm-1, which outperform over most of the reports available in the literature. Furthermore, the fabricated devices exhibited long-term stability and stable operation even under humid conditions, indicating the ability of the sensor device to operate in a real-time application. These results contribute to the development of versatile tunable oxygen sensors based on TMDC FETs.

physics.app-ph↗

arXiv2020

Single-Carrier Transport in Graphene/hBN Superlattices

Graphene/hexagonal boron nitride (hBN) moiré superlattices have attracted interest for use in the study of many-body effects and fractal physics in Dirac fermion systems. Many exotic transport properties have been intensively examined in such superlattices, but previous studies have not focused on single-carrier transport. The investigation of the single-carrier behavior in these superlattices would lead to an understanding of the transition of single-particle/correlated phenomena. Here, we show the single-carrier transport in a high-quality bilayer graphene/hBN superlattice-based quantum dot device. We demonstrate remarkable device controllability in the energy range near the charge neutrality point (CNP) and the hole-side satellite point. Under a perpendicular magnetic field, Coulomb oscillations disappear near the CNP, which could be a signature of the crossover between Coulomb blockade and quantum Hall regimes. Our results pave the way for exploring the relationship of single-electron transport and fractal quantum Hall effects with correlated phenomena in two-dimensional quantum materials.

cond-mat.mes-hall↗

arXiv2019

Observation of superconductivity in bilayer graphene/hexagonal boron nitride superlattices

A class of low-dimensional superconductivity (SC), such as most of "atomic-layer" SCs, has survived only under certain circumstances, implying a role of the substrate. Moreover, in some recent SC discoveries at heterogeneous interfaces, SC was buried in bulk solids and ex situ. Genuine atomic-layer SC is difficult to access. Here we report a novel route to atomic-layer SC in graphene superlattices. Our device comprises stacked non-twisted bilayer graphene (BLG) and hexagonal boron nitride (hBN), i.e., hBN/BLG/hBN Moiré superlattices. Upon in situ electrostatic doping, we observe an SC dome with a critical temperature up to $T_{\rm{BKT}} = 14 \rm{K}$, corresponding to the confinement of vortices. We believe that SC via doping Dirac materials is ubiquitous in condensed matter and that this study paves a way toward the design of a new SC family.

cond-mat.supr-con↗

arXiv2019

Topological Valley Currents in Bilayer Graphene/Hexagonal Boron Nitride Superlattices

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moiré superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moiré superlattice with broken inversion symmetry leads to a hot spot with Berry curvature accumulating at the charge neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 $\text{k}Ω$, which obeys a scaling relation. This nonlocal resistance evolves from the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way to developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking.

cond-mat.mes-hall↗

arXiv2019

Fabry-Pérot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts

We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry-Pérot (FP) resonances due to np'n (pn'p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p-n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices.

cond-mat.mes-hall↗